Vibronically assisted sub-cycle charge transfer at a non-fullerene acceptor heterojunction

Domain: chemistry
Published: 2026-03-05
Editorial outcome: major_revision

Abstract
Excited-state charge transfer underpins organic photovoltaics, photocatalysis and photodetection, but is traditionally thought to require large energy offsets and strong donor–acceptor coupling that can limit device performance. Here, we investigate through-space polymer non-fullerene-acceptor based model heterojunctions in which a perylene diimide acceptor is covalently tethered to a low-band gap polymer donor. These systems feature an exceptionally small energy offset (<100 meV) between frontier orbitals, with weak donor–acceptor coupling in the Franck–Condon region. We nevertheless achieve a charge-transfer timescale of ~18 fs. This ultrafast charge-transfer is accompanied via the launch of coherent wavepackets along a high-frequency vibrational coordinate (26 fs period) on the non-fullerene acceptor’s potential energy surface. We uncover specific polymer-centered driving vibrational modes that enable such rapid charge-transfer rates, by mixing Frenkel exciton and charge-transfer states following photoexcitation. Our results demonstrate that ultrafast charge-transfer can be achieved—ultimately limited by high-frequency vibrational periods—even in the absence of large energy offsets or strong ground-state coupling.

Introduction
Photoexcited charge transfer (CT) plays a pivotal role in organic photovoltaics (OPV)1–3, photosynthesis4 and photocatalysis5–7. In such light-harvesting applications, dissociation of excitons via electron (or hole) transfer to an electron acceptor (or donor) allows for the eventual generation of free charges, used to generate photocurrent or drive redox chemistry1,2,4,6,8.

One key design principle for improving the performance of light-harvesting devices is to enhance the CT rate at the heterojunction between the donor and acceptor molecules. This ensures that photo-generated excitons form free charges faster than they radiatively or non-radiatively recombine. A key question is what ultimately controls the rate of CT. Typically for polymer-fullerene-based heterojunctions, large energetic offsets between frontier orbitals (≥500 meV) have enabled ultrafast CT on sub-100 fs timescales9–11. However, such large offsets limit the maximum achievable open-circuit voltage in photovoltaics or increase the overpotential in photocatalysis. Recent developments in non-fullerene acceptors12 (NFAs) have demonstrated that fast (~100–250 fs) CT can occur even with low driving forces13–15—but this is not universally observed16. In many cases, similar systems with low energetic offsets exhibit much slower dynamics, extending from tens of ps to ns16–20. This inconsistency points to a gap in understanding which factors govern the ultimate rate of CT when the energetic offset is small, and how we can achieve the fastest possible electron transfer in that regime.

Traditionally, the rate of CT in light-harvesting devices is believed to be primarily governed by the electronic coupling and energy offset between the frontier orbitals between donor and acceptor molecules21,22. Recent studies on symmetry-breaking CT (SBCT) in covalent dimers and ordered aggregates have shown that strong electronic coupling can enable ultrafast, low-energy-loss CT23–25. However, the role of vibrational modes—whether they assist, limit, or modulate the CT process in weakly coupled organic heterojunctions—remains poorly understood. Understanding this interplay is essential for identifying the fundamental speed limits of photoinduced charge separation in molecular heterojunction-based photovoltaic devices.

Here, we show that CT in weakly coupled organic heterojunctions occurs on sub-20 fs timescales—approaching the half-period of high-frequency molecular vibrations—without requiring large energetic offsets or strong ground-state electronic coupling. Using synthetically defined donor–acceptor model heterojunctions, we demonstrate that spatial alignment of interfacially active vibrational modes enables vibronically driven, ballistic electron transfer. Ultrafast spectroscopy reveals electron transfer as fast as ~18 fs with accompanying coherent vibrational wavepackets on the acceptor, while quantum dynamical simulations complemented with experiments identify polymer-centered vibrational modes that mix Frenkel exciton (FE) and CT states, providing a mechanistic origin for this behavior.

Results
Model organic heterojunctions
We present a fundamental study of CT dynamics in a model heterojunction composed of donor and acceptor chromophores covalently tethered together. As shown in Fig. 1a, b, asymmetric and planar NFA, a perylene diimide (PDI) derivative26, is covalently attached to a low-bandgap polymer (Ref-P), via non-π-conjugating alkyl chains in a “through-space” co-facial manner. Two different arrangements of the donor polymer and acceptor chromophores are explored, with the PDI unit at the face of either the electron-rich (BDT27) or electron-deficient (DPP27) moiety of the polymer backbone, which we will refer to as TS-P2 (acceptor-on-electron rich system) and TS-P3 (acceptor-on-electron deficient system) architectures, respectively (Fig. 1b, for synthetic protocols see “Methods” and ref. 28). These model systems allow us to isolate the dynamics of the CT process, which can be masked via exciton and charge-transport processes in OPV thin-films or by molecular diffusion in photocatalytic systems29–32. We emphasize that this study aims at elucidating the fundamental interfacial processes governing initial charge-separation dynamics, rather than optimizing device-level power-conversion efficiency. The model heterojunctions also allow us to probe the effect of the relative spatial orientation of different parts of the chromophores with respect to each other, something that would be impossible to study within a conventional OPV blend or photo-redox active heterojunction nanoparticle system5,6 with disordered arrangements of chromophores.

Recently, it was demonstrated that in these model systems the microscopic orientation between donor and acceptor chromophores could significantly influence CT energies28 and that the CT rates varied from slow (>1 ps) to fast (temporally unresolved), approximately below the instrument response of the measurements (150 fs). However, the fundamental molecular-level origin of the dramatic differences in CT rates between different configurations remains unknown, and the broader question of what ultimately governs the rate of CT in organic heterojunctions remains unanswered.

Sub-vibrational period charge transfer
To study the CT dynamics of these model heterojunctions, we begin by employing pump-probe optical spectroscopy. As depicted in Fig. 1c, d The low band-gap Ref-Polymer has an absorption maximum at 750 nm, whereas the PDI electron acceptor has its peak absorption at 550 nm. The origin of the different intensities of the 0–0 and 0–1 peaks in polymer backbone chromophore absorption spectra has been discussed previously28, and we believe that these differences predominantly arise from a greater degree of conformational disorder in TS-P3 relative to TS-P2. This separation in absorption features between the polymer backbone chromophore and PDI means that our sub-12 fs red pump pulse (Fig. 1e) excites only the donor polymer backbone, while the green pump pulse excites the PDI moiety predominantly due to its preferential absorption. The absorbance of the model heterojunctions and Ref-P measured with photothermal deflection spectroscopy (PDS) is presented in Fig. 1f.

We monitor the initially photo-excited polymer excitons with an infrared probe pulse after selectively pumping the polymer backbone chromophore with 700 nm narrowband optical pulses (pulse width 150 fs). As shown in Fig. 2a, the transient absorption (TA) spectra of the Ref-P polymer (turquoise green spectra) show a negative transmission signal with broad spectral coverage (1220–1580 nm), which can be assigned to the photoinduced absorption (PIA) of the polymer backbone neutral S1 to Sn state. The transient spectra of the TS-P2 (violet) and TS-P3 (orange) show an additional feature around 1250–1400 nm along with the neutral chromophore S1 → Sn PIA of the backbone chromophore. This new blue-shifted spectral feature can be assigned to the hole absorption of the polymer (as confirmed by blending the Ref-P with the high (380 meV) PCBM/C60 LUMO energy offset acceptor, see Supplementary Note 3). The transient kinetic traces show attenuation of the neutral S1 state population of the polymer backbone, indicating electron transfer to the PDI unit, as shown in Fig. 2b. While TS-P2 exhibits electron transfer on a sub-picosecond timescale, TS-P3 undergoes rapid quenching of its neutral excitonic state almost concurrently with photoexcitation—within the instrument response (~150 fs)—rendering the process temporally unresolved. In both TS-P3 and TS-P2, the hole wavefunction of the lowest energy excited state is predominantly localized on the DPP segment28, resulting in a greater spatial separation between the electron (localized on PDI) and hole for the case of TS-P2. This geometry in TS-P2 leads to weaker electronic coupling and consequently a slower, configuration-dependent CT process compared to the TS-P3 architecture28. To investigate how fast the CT is in TS-P3, we excited the polymer backbone chromophore with a sub-12 fs 700 nm centered broadband red pulse (Fig. 1e, Supplementary Note 4), and the photoexcited species were probed with a temporally delayed broadband visible pulse, as depicted in Fig. 2d–f. As shown in Fig. 2d, for Ref-P, we observe ground state bleach (maxima at 760 nm), stimulated emission (SE) (820 nm), and a negative transmission signal around 900 nm, corresponding to the PIA of the S1 state of the polymer backbone chromophore. These features exhibit minimal temporal dynamics in the very early time frame (20–40 fs) as expected for Ref-P.

In contrast, as shown in Fig. 2e, TS-P3 displays significant early time (20–40 fs) dynamics. The SE band and ground state bleach band of the polymer backbone chromophore are rapidly quenched within the first few temporal frames (<40 fs) due to the generation of an underlying negative differential transmission (ΔT/T) signal. This negative signal is attributed to the absorption of the PDI radical anion, the photoproduct of the ultrafast electron transfer from the polymer backbone, which has a broad absorption spectrum across the probe wavelengths (illustrated by the blue shaded transmission spectra of the PDI radical anion, for more details see Supplementary Note 5). Turning to TS-P2, as shown in Fig. 2f, the dynamics of the spectral features at the very early time frames (20–40 fs) are silent like in Ref-P. The photogeneration of the PDI radical anion happens in a much slower timescale (376 fs, Supplementary Fig. 42) in TS-P2 compared to TS-P3, Fig. 2c. The slower electron transfer rate is consistent with the previously described 150 fs IR-probed TA results where the TS-P3 dynamics remained unresolved. To quantify the ultrafast CT dynamics in TS-P3, kinetic analyses were performed after isolating the coherent artifact (Supplementary Fig. 41 and Note 17), which yielded a fast component of 18.1 ± 3.1 fs, corresponding to a conservative upper

Methods

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors report studies of charge transfer process in donor-acceptor (DA) structures comprising a polymer labelled as Ref-P as donor and a non-fullerene-based acceptor, perylene diimide (PDI). PDI is covalently linked via a non-conjugated chain to the electron rich (TS-P2) and electron deficient (TS-P3) side of the polymer. The main idea of the paper is to demonstrate that ultrafast electron transfer times can also be reached in DA systems with small energy offsets. This contrasts the “common knowledge” that large DA energetic offsets are beneficial for an efficient electron transfer and may by beneficial for increasing the open-circuit voltage in photovoltaic devices.\nThe authors have used temporally short ~12 fs red and green pump pulses to perform transient absorption (TA) spectroscopy. For TS-P3, the TA spectra show femtosecond dynamics which have been assigned to ultrafast electron transfer from Ref-P to PDI. TA spectra recorded with reduced time resolution have been reported in a recent N. Chem publication by some of the authors. The TA measurements are complemented by careful sample characterization (as reported in the SI) and preliminary quantum chemical calculations. All in all, the work is detailed and thorough.\nSeveral questions arise that need to be addressed before a recommendation about publication of the manuscript can be made.\n1) TS-P3 contains the PDI acceptor unit attached to electron deficient side of polymer. Under excitation with red pump pulses this results in fast TA dynamics (Fig. 2e) while the TS-P2, in which PDI is attached to the electron rich side, shows dynamics on a much slower picosecond time scale. Very little said about the dynamics in TS-P2. This should be explained in more detail.\n2) It remains rather unclear how the electron transfer time of 15 fs is deduced from the data in Fig. 2e. It is not clear how the dynamics of the donor and acceptor population are deduced from these data. A global fit and, at least, a rate equation model to describe the time evolution of the spectra is necessary.\n3) It is stated in the abstract that “…specific driving modes enable such CT rates by mixing single Frenkel exciton states CT state…”. I find it surprising that no evidence for such ultrafast vibronic couplings are seen in the TA spectra in Fig. 2e. Since the time resolution is claimed to be faster than any relevant vibrational mode in the system, I would expect that coherent wavepacket motion modulates the TA spectra as seen by other groups in TA spectroscopy. If the time resolution is indeed as high as claimed by the authors, the gradual evolution of the TA spectra with time seems to speak against the relevance of vibronic couplings. A much more in-depth discussion of the role of vibronic couplings on the TA spectra in Fig. 2e seems necessary.\n4) The abstract claims “… unprecedented CT rates…”. This statement is incorrect, s., e.g., R. Huber et al., JPCB 106, 6494 (2002) and others.\n5) No explanation has been given for microscopic origin of the observed fast electron transfer to non-covalently linked PDI electron acceptor. Is it through space to the stacked PDI moiety?\n6) The assignment of 1283 cm-1 observed in TS-P3 is confusing. The authors have assigned the 1283 cm-1 mode to a C-C-stretching mode of the PDI radical anion since they observed a 1295 cm-1 mode in the Raman spectra of an isolated PDI molecule. In the next paragraph they argue that this mode is an excited state mode of PDI because its frequency is 1283 and not 1295 cm-1 as observed in isolated molecule. They support this argument by different control measurements in Fig. 2b. Honestly speaking, all these measurements seem to show (to me) pretty much the same lineshape of the Fourier transform band in the 1280 cm-1 region. I do not think that a distinction between ground and excited state mode can be made on the basis of these measurements. For this, much more detailed control measurements are needed. Such a distinction can be achieved, for instance by analyzing the phase of the vibrational modulation of the TA spectra. A clear and convincing demonstration that the 1283 cm-1 mode is an excited state vibration of PDI seems necessary to make the paper publishable.\n7) The authors find evidence for quite rapid electron transfer dynamics and substantial couplings of the electronic excitations to high frequency modes. This strongly suggests that nonadiabatic dynamics resulting from vibronic couplings are important for the electron transfer rate and yield. Theoretical models that describe these dynamics should obviously take such nonadiabatic couplings and the effects of vibronic decoherence on the resulting quantum dynamics into account.\nWhen discussing their results, the authors speak of “ballistic electron transfer”, “”conventional” field-driven coherence” and “reaction-driven coherences” and “impulsive forces” generating vibrational coherences. These are complicated concepts, partly borrowed from classical mechanics. These concepts are used without proper explanation, let alone definition, of the terms. This makes it very difficult for a reader to understand what the authors want to say. Even more, the use of such oversimplified, quasi-classical concepts may also be highly misleading.\nTheoretical models that can properly account for the effects of vibronic couplings on charge transfer dynamics in donor-acceptor systems are well developed (Tretiak, Prezhdo, Bredas, …). Also, the decoherene phenomena can readily be described using, e.g., Redfield or Lindblad master equations. I strongly suggest to attempt such a modelling in order to support the conclusions that are drawn from the experiments.\nOther minor comments:\nThere seems to be significant contribution of cross-phase modulations close to time zero especially seen from crosscuts extending over 100 fs. Therefore, it becomes important to show the subtraction scheme especially for analysing data in 20-40 fs waiting time.\nPage 10, line 238, the reference to figure, should read Figure 3b?\nPage 3, line 63, factors that govern", "reviewer_2": "The manuscript by Ghosh, et al. details the excited-state dynamics of model polymer/non-fullerene acceptor heterojunctions monitored with high-time resolution broadband transient absorption spectroscopy. The model systems are comprised of a dyketopyrrolopyrrole-benzodithiophene (DPP-DBT) polymer donor and a perylene diimide (PDI) acceptor in two different configurations: one with the PDI closely interacting with the electron-rich BDT unit (TS-P2), and the other where it is more closely associated with the electron-deficient BDP moiety (TS-P3). They observe sub-picosecond charge separation in TS-P3, while for TS-P2 they show charge separation in <15 fs, which is less than the vibrational period of the high-frequency Raman-active transitions available. The results are high quality and the analysis is sophisticated, and the understanding proposed will be of significant interest to the field. While the proposed explanation is compelling, there are numerous unanswered questions that need to be addressed before this work can be considered publishable in Nature Communications.\nMajor issues:\n-The authors central claim is that charge transfer here is fast while still directly occurring from the local exciton state of the polymer and not an optically accessible charge-transfer state. They have performed photothermal deflectance spectroscopy measurements and DFT calculations to rule out this possibility. However, the photothermal deflectance spectroscopy was performed under quite different conditions (i.e. in a high polarity environment and in a solvent where aggregation is discouraged), and so this is not conclusive. The DFT calculations are also prone to functional- and basis set-specific artifacts that could miss such interactions. Generally, the UV/vis spectra usually give some indication of donor-acceptor interactions that give rise to new CT bands, but these data (Figure 1c) are not discussed in a meaningful way in this context. Specifically, there are substantial differences between the absorption spectra of TS-P2 and the constituent Ref-P/PDI spectra that are not discussed. The splitting and red-shifting of the low-energy polymer absorption band could easily indicate aggregation of the polymer backbone or charge-transfer interactions between the PDI and BDT. Similarly, though less drastically, the absorption of TS-P3 shows broadening of the vibronic progressions of both PDI and the polymer transitions. These differences in spectra lead to questions about aggregation and disorder that should be addressed: how rigid are these structures and what role does flexibility and overall disorder have on the dynamics? Are the polymer molecular weights (including those within the distribution) sufficient to allow additional interactions with the PDI unit? A discussion of the length of the polymer in the main text would add to the rigor of the discussion of the absorption spectrum. But moreover a better discussion of the UV/vis spectra and whether the clear differences in the spectra are indicative of ground state electronic interactions should be given.\n-It is not clear how these results translate into solid-state design rules for better organic photovoltaic (OPV) device performance. Rapid charge separation from polymers to non-fullerene acceptors, including PDI is not novel and not inherently reliant on the mechanism proposed here. While this mechanistic approach will certainly be interesting to the community, it is not clear how these effects can be leveraged in devices. Better contextualization of these results is necessary to increase their impact and utilization by the community. Related, there are significant examples in Nature where fast, high yielding charge transfer occurs with minimal energy loss, such as the special pair in photosynthesis. This strategy has even been adapted in artificial systems for OPVs utilizing symmetry-breaking charge transfer [J. Am. Chem. Soc. 2015, 137, 16, 5397–5405]. A difference, of course, is that in those systems there are frequently indications of strong electronic coupling in the ground state, except in the case of null-aggregates which are known to exhibit excited-state symmetry breaking. In those and related cases, the role of vibrations has been investigated [J. Am. Chem. Soc. 2022, 144, 34, 15539–15548, Nat. Chem. 14, 786–793 (2022)], but this context is lacking here. As such, the notion that energy loss and charge transfer rates are \"fundamental barriers\" are overstatements.\n-The ultrafast nature of the charge transfer in TS-P3 being driven by the 1512 cm-1 polymer mode is intriguing, so the natural question is how general is this conclusion? The authors state that this mode is localized mostly in the DPP unit, which seems consistent with the difference in dynamics between TS-P2 and TS-P3, however, this is not emphasized in the discussion. Indeed, it This is disappointing, as it may point towards the generality of this mechanism and its use in designing more efficient charge transfer systems. This element should be discussed more. Does the solvent play any specific role, either through inertial or dielectric stabilization or through coupling to the solute modes? Such effects would not be present in the solid-state device so this potential effect is worth clarification.\n-The CW Raman spectra results for PDI where the 1283 cm-1 mode is absent are compelling evidence for the lack of ground-state excitation of the PDI. However, the signal-to-noise ratio of these data is not particularly high and there may be indications of this missing feature close to the noise floor.\n-Can the authors perform a short-time Fourier transform on the transient absorption data to see how the vibrational modes behave in time [J. Am. Chem. Soc. 2022, 144, 15539−15548]? This may better help demonstrate their proposed mechanism.\nMinor issues:\n-The abstract is very vague and should refer to the real types of systems being discussed in the text.\n-Figure 2a: \"S0-->Sn\" absorption is presumably \"S1-->Sn\" absorption as stated on page 7.\n-Figure 2b: What is the origin of the relatively slow rise in the Ref-P signal? This is also present to a lesser extent in the TS-P2 data.\n-Figure 2d-f: It would be helpful to show a \"zero-line\" to guide the eye for when the transient absorption signals cross change sign.\n-Figure 3b: what is the range of integration for the Fourier transform data?\n-Page 9, line 214: \"TP-P3\" is presumably \"TS-P3\"\n-Page 10/Supplementary Note 5: The agreement of the transient absorption spectrum with the spectrum of the chemically reduced PDI is not great. What is the origin of this disagreement? Aggregation is known to cause such broadening and shifting of anion spectra. Its use as supporting evidence is undermined by the lack of agreement and discussion.\n-Page 21, the figure call in the methodology section is incomplete."}, "author_response": "Response to the reviewers’ comments\nReviewer #1 (Remarks to the Author):\nThe authors report studies of charge transfer process in donor-acceptor (DA) structures\ncomprising a polymer labelled as Ref-P as donor and a non-fullerene-based acceptor,\nperylene diimide (PDI). PDI is covalently linked via a non-conjugated chain to the electron\nrich (TS-P2) and electron deficient (TS-P3) side of the polymer. The main idea of the paper\nis to demonstrate that ultrafast electron transfer times can also be reached in DA systems\nwith small energy offsets. This contrasts the “common knowledge” that large DA\nenergetic offsets are beneficial for an efficient electron transfer and may by beneficial for\nincreasing the open-circuit voltage in photovoltaic devices. The authors have used\ntemporally short ~12 fs red and green pump pulses to perform transient absorption (TA)\nspectroscopy. For TS-P3, the TA spectra show femtosecond dynamics which have been\nassigned to ultrafast electron transfer from Ref-P to PDI. TA spectra recorded with\nreduced time resolution have been reported in a recent N. Chem publication by some of\nthe authors. The TA measurements are complemented by careful sample\ncharacterization (as reported in the SI) and preliminary quantum chemical calculations.\nAll in all, the work is detailed and thorough. Several questions arise that need to be\naddressed before a recommendation about publication of the manuscript can be made.\nResponse: We sincerely thank the reviewer for their careful reading of our manuscript\nand their constructive feedback. We are particularly encouraged by the positive remarks\nthat “the TA measurements are complemented by careful sample characterization (as\nreported in the SI) and preliminary quantum chemical calculations. All in all, the work is\ndetailed and thorough.” The acknowledgement of the thoroughness of our\nmeasurements and analyses is highly motivating. In the following, we respond point-by-\npoint to the reviewer’s questions.\n1) TS-P3 contains the PDI acceptor unit attached to electron deficient side of polymer.\nUnder excitation with red pump pulses this results in fast TA dynamics (Fig. 2e) while the\nTS-P2, in which PDI is attached to the electron rich side, shows dynamics on a much\n1\nslower picosecond time scale. Very little said about the dynamics in TS-P2. This should\nbe explained in more detail.\nResponse: We agree with the reviewer that our earlier discussion of the TS-P2 dynamics\nrequired further elaboration. We have accordingly revised both the main text and the\nSupplementary Information.\nAs the reviewer correctly noted, in TS-P3 the PDI acceptor is attached to the electron-\ndeficient DPP unit, whereas in TS-P2 it is linked to the electron-rich BDT unit. However in\nthe charge transfer excited states for both the TS-P2 and TS-P3 the hole is more localised\n(PR = 2) on the DPP fragment. This electronic distribution previously established in the\nh\nref1 leads to large separation of the electron (PDI) in the TS-P2. Consequently, charge\ntransfer in TS-P2 proceeds via a longer-range, configuration-dependent pathway,\nresulting in slower kinetics.\nThe central question of this study is why charge transfer in TS-P3 occurs exceptionally\nrapidly for an organic system and whether vibrational coupling plays a decisive role. In\nthis context, TS-P2 serves as a critical control system: its slower charge-transfer\ndynamics provide a benchmark for interpreting the vibrational features in TS-P3—\nparticularly in assessing whether the 1283 cm ¹ mode corresponds to an off-resonant\nground-state wavepacket or to an wavepacket that is related to the charge-transfer\n⁻\nexcited state.\nTo quantify the charge-transfer rate under ‘red’ pump excitation, we analysed the TS-P2\ntransient absorption data after removing coherent artefacts (Supplementary Fig. 42). The\ndifferential trace is well described by a mono-exponential decay (red fit), yielding τ = 376\nfs—consistent with a slower, weakly coupled electron-transfer process with respect to\nthe faster electron transfer in TS-P3 (sub-15 fs). This new analysis and the associated\nmechanistic explanation have been incorporated into the revised manuscript as detailed\nbelow.\nAction:\nRevised Main Manuscript:\nIn the section which describes the IR-probed 150fs resolved TA analysis is done:\n2\n“While TS-P2 exhibits electron transfer on a sub-picosecond timescale, TS-P3 undergoes\nrapid quenching of its neutral excitonic state almost concurrently with photoexcitation—\nwithin the instrument response (~150 fs)—rendering the process temporally unresolved.\nIn both TS-P3 and TS-P2, the hole wave function of the lowest energy excited state is\npredominantly localized on the DPP segment1, resulting in a greater spatial separation\nbetween the electron (localized on PDI) and hole for the case of TS-P2. This geometry in\nTS-P2 leads to weaker electronic coupling and consequently a slower, configuration-\ndependent CT process compared to the TS-P3 architecture.1.”\nIn the section which describes the ultrafast ‘red’ pump excitation:\n“Turning to TS-P2, as shown in the Figure 2f, the dynamics of the spectral features at the\nvery early time frames (20-40 fs) are silent like in Ref-P. The photogeneration of the PDI\nradical anion happens in much slower time scale (376 fs, SI fig. 42) in TS-P2 compared to\nTS-P3, Figure 2c. The slower electron transfer rate is in consistent with the previously\ndescribed 150 fs IR-probed TA results where the TS-P3 dynamics remained\nunresolved.….”\nRevised Supplementary Information\nSupplementary Note 16. Extraction of the ultrafast charge-transfer dynamics in model\nheterojunction\nSupplementary Fig. 42 | Difference trace of TS-P2 after removal of the coherent artefact fitted with a mono-\nexponential decay (red line), giving a time constant τ = 376 fs.\n3\n2) It remains rather unclear how the electron transfer time of 15 fs is deduced from the\ndata in Fig. 2e. It is not clear how the dynamics of the donor and acceptor population are\ndeduced from these data. A global fit and, at least, a rate equation model to describe the\ntime evolution of the spectra is necessary.\nResponse: We thank the reviewer for this insightful suggestion. Global analysis is indeed\na rigorous and widely used method in time-resolved spectroscopy, enabling reliable\nextraction of kinetic and population-transfer information—provided the underlying\nsystem has well-separated, thermally populated states. However, in our present case,\nthe earliest spectral dynamics (<50 fs) fall within the coherent regime: here, excitonic and\ncharge-transfer (CT) states are strongly vibronically mixed and evolve as superposed\nwavepackets rather than independent populations. This rapid quantum coherence and\ncoupling prevents the spectra from being represented as sums of time-independent\nbasis functions, so conventional global fitting or rate-equation models cannot faithfully\ndescribe these ultrafast processes2. To address this valuable comment, we have clarified\nour methodology and added new supporting analysis in the Supplementary Information\n(SI Fig. 41). In the revised manuscript, we directly extracted the differential kinetics of the\ndonor and acceptor features to determine the characteristic ET timescale. As shown in\nSupplementary Fig. 41a, the raw ΔT/T traces of TS-P3 and Reference exhibit identical\ncoherent artefacts around time zero (< 20 fs, vertical line in Fig. 41), but diverge\nimmediately afterward (≥ 20 fs), with TS-P3 showing an additional ultrafast decay absent\nin the reference. Subtracting the reference trace isolates the CT-specific component\n(Supplementary Fig. 41b), which is well described by a mono-exponential decay with τ =\n13.9 ± 1 fs. This value represents the experimentally resolvable upper limit of the ET\nprocess, approaching our 11 fs instrument response function, and therefore indicates\nthat electron transfer completes within one vibrational period of the 1512 cm ¹ DPP-\nlocalized mode. This interpretation is further supported by the 100 fs-resolved transient\n⁻\nabsorption kinetics (Fig. 2b), which track solvation and dielectric relaxation. In Ref-P,\npronounced sub-ps solvation dynamics are observed, and these are partially retained in\nTS-P2 (the slower-ET analogue). In contrast, TS-P3 exhibits no such relaxation signature\nand instead decays directly into the product state immediately after photoexcitation. This\n4\nbehaviour is consistent with a near-instantaneous charge transfer completed within one\nvibrational cycle of the 1512 cm ¹ DPP-localized mode.\n⁻\nWe believe that incorporating this analysis strengthens the manuscript and makes the\ninterpretation clearer. To reflect the reviewer’s suggestion, we have also added an explicit\nnote in the main text stating that the sub-15 fs value corresponds to the upper limit of the\nET timescale derived directly from differential spectral kinetics, rather than a fitted\npopulation-transfer rate. We thank the reviewer for prompting this addition, which\nsignificantly improves the clarity of the analysis.\nAction:\nMain Manuscript:\nTo quantify the ultrafast CT dynamics in TS-P3, kinetic analyses were performed after\nisolating the coherent artefact (Supplementary Fig. 41 and Note 17), which yielded a\nmono-exponential decay with τ = 14.9 ± 1 fs, establishing an upper limit for the electron-\ntransfer process. Such sub-15 fs charge transfer is markedly faster than in conventional\nOPV systems, where CT times are typically well above 15fs, often by more than few order\nof magnitude3–6\nRevised Supplementary Information\nSupplementary Note 17: Extraction of the ultrafast charge-transfer dynamics in\nmodel heterojunction\nTo further substantiate the sub-15 fs charge transfer (CT) dynamics inferred from the\ntransient absorption spectra (Fig. 2e, main text), we compared the time-domain kinetics\nof TS-P3 with those of the reference polymer (Ref-P), as shown in Supplementary Fig. 41.\nPanel a shows the averaged ΔT/T traces of TS-P3 (blue) and Ref-P (orange) at 790–820 nm,\nwhere the acceptor (PDI) absorption dominates7. Both datasets exhibit identical\ncoherent artefacts around time zero (< 20 fs), arising from cross-phase modulation and\npump–probe overlap. Beyond this temporal region, however, the TS-P3 trace displays a\npronounced rapid decay that is completely absent in Ref-P, indicating an additional\nultrafast process linked to charge separation. It is also important to note that beyond ~20\n5\nfs there is no further spectral evolution in Ref-P (Fig. 2d), confirming that the contribution\nof coherent artefacts or cross-phase modulation is minimal after this time (vertical\ndotted line in Supplementary Fig. 41a). To isolate this CT-specific contribution, the\nReference trace was subtracted from the TS-P3 signal. The resulting differential kinetics\n(Supplementary Fig. 41b) were fit with a mono-exponential decay function, yielding a\ntime constant of τ = 14.9 ± 1 fs. This value represents the experimentally resolvable\nupper limit of the CT timescale, approaching the 11 fs instrument response function. The result\nimplies that electron transfer in TS-P3 completes essentially within a single vibrational\nperiod of the 1512 cm ¹ DPP-localised mode. In the following Supplementary Note 21,\nwe corroborate these experimental observations with quantum-dynamical calculations\n⁻\nwhich show the presence of a DPP-localized mode at approximately 1500 cm-1 strongly\ncoupled with the electron transfer.\nSupplementary Fig. 41| (a) Transient absorption kinetics (ΔT/T) of TS-P3 (blue) and Ref-P (orange) averaged over 790–\n820 nm. The vertical dashed line marks time zero (< 20 fs), corresponding to the coherent artefact region. (b) Difference\ntrace (TS-P3 – Ref-P) fitted with a mono-exponential decay (red line), giving a time constant τ = 14.9 ± 1 fs.\nSuch near-instantaneous transfer is consistent with the vibronically coherent regime\ndiscussed in the main text and is further supported by the 100 fs-resolved transient\nabsorption dynamics (Fig. 2b, main text), where slower sub-picosecond solvation\nrelaxation is evident in Ref-P and TS-P2 but absent in TS-P3. Together, these findings\nconfirm that the < 15 fs component represents the coherent electron transfer event rather\nthan subsequent solvent or structural relaxation.\n6\n3) It is stated in the abstract that “… specific driving modes enable such CT rates by mixing\nsingle Frenkel exciton states CT state …”. I find it surprising that no evidence for such\nultrafast vibronic couplings are seen in the TA spectra in Fig. 2e. Since the time resolution\nis claimed to be faster than any relevant vibrational mode in the system, I would expect\nthat coherent wavepacket motion modulates the TA spectra as seen by other groups in\nTA spectroscopy. If the time resolution is indeed as high as claimed by the authors, the\ngradual evolution of the TA spectra with time seems to speak against the relevance of\nvibronic couplings. A much more in-depth discussion of the role of vibronic couplings on\nthe TA spectra in Fig. 2e seems necessary.\nResponse: We thank the reviewer for raising this important point. In many systems,\nvibronic coupling is indeed readily identified through clear oscillatory modulation of\ntransient absorption features via a Franck–Condon–type detection mechanism. This\nresults in periodic red and blue shift of the peak position. However it is noteworthy that\nunder a Herzberg–Teller detection pathway8–10, wavepacket motion primarily modulates\nthe oscillator strength. As a result in such detection mechanism which is often found in\norganic molecules8–10 , oscillatory spectral blue and red shift is not present, which is a\ndiagnostic feature of the nonc-Condon type Herzberg–Teller coupling8–10. In such cases\nthe vibrational coherence is mainly detected from the kinetic profile of individual probe\nwavelength.\nNevertheless, upon close inspection, the stimulated emission band of the polymer\nbackbone in Fig. 2e exhibits oscillatory behaviour before it evolves into the PDI radical-\nanion absorption signature. To aid the reader, we have added a visual guide to highlight\nthis feature. We also highlight the temporal oscillation in the time domain data in the\nfigure 3a in the manuscript. We have also added the two-dimensional representation of\nthe data where the oscillatory component of the spectral features are clear\n(Supplementary Fig. 9.2). In addition, we now include a dedicated Supplementary\nSection (Supplementary Note 7) with detailed discussion of vibronic coupling as the\nreviewer suggested, including appropriate citations due to word count constrain in the\nmain manuscript. We believe this significantly improved the revised manuscript. We also\nhighlight that our newly performed quantum dynamics simulations (Supplementary Note\n7\n21, revised SI) show that the DPP–BDT backbone chromophore is strongly coupled to\nseveral high-frequency vibrational modes (including the 1531 cm ¹ mode that drives\nultrafast electron transfer) to the FE and CT excitons and the polymer cation. These\n⁻\nresults corroborate the experimental evidence of strong vibronic coupling observed\nunder impulsive excitation.\nAction:\nRevised Supplementary Information\nSupplementary Note 7: Additional discussion on the vibronic coupling of the model\nheterojunction\nHere we discussed the vibronic coupling of model heterojunctions. In the electronic\nexcited state (S ) manifold, the vibrational ground state is represented as and\n1\nthe first excited vibrational states are . After impulsive photoexc\n𝜓\nita𝑖=ti0o\n(\n𝑛\n𝑥\n)\nby a\nbroadband laser pulse from the electron\n𝜓\nic𝑖= g1r\n(\n𝑜\n𝑢\n)\nnd state, multiple vibrational states will\nbe populated, leading to, a newly generated non-stationary states which can be\nrepresented as:\n−𝑖𝜔0𝑡 −𝑖𝜔1𝑡\n𝜓W(h𝑟e,r𝑡e) = 𝑐𝑖 , =0(𝑡),𝑒 corre𝜓sp 𝑖= o 0 n(d𝑟)s +to𝑐 t 𝑖= h 1 e( 𝑡c)o𝑒ntribu𝜓ti 𝑖 o = n 1 (o𝑟f) t+he⋯ each vibrational states to the\nnon-eig\n𝑐\ne𝑖n= 0sta 𝑐𝑖t=e1.\nThe time-dependent molecular polarization can be described as -\n𝑃W(h𝑡e)r=e ⟨𝜓 is( 𝑟th,𝑡e) d|𝜇ip|𝜓ol(e𝑟 ,m𝑡)o⟩ment operator.\n𝜇\n𝑃(𝑡)\n−𝑖𝜔0𝑡 −𝑖𝜔1𝑡 −𝑖𝜔0𝑡 −𝑖𝜔1𝑡\n= ⟨𝑐𝑖=0(𝑡)𝑒 𝜓𝑖=0(𝑟)+𝑐𝑖=1(𝑡)𝑒 𝜓𝑖=1(𝑟)|𝜇|𝑐𝑖=0(𝑡)𝑒 𝜓𝑖=0(𝑟)+𝑐𝑖=1(𝑡)𝑒 𝜓𝑖=1(𝑟)⟩\n∗ −𝑖(𝜔1−𝜔0)𝑡 ∗ 𝑖(𝜔1−𝜔0)𝑡\nThe macroscopic pola𝑃r(iz𝑡a)t=ion𝜇 0 c 1 a(n𝑐0 b𝑐1 e𝑒 represente+d 𝑐a 1 s𝑐 0 𝑒 ) , where is the\nnumber of molecules11. Hence, the vibrational coherenc\n𝑃\ne𝑁 g\n(\n𝑒\n𝑡\n𝑛\n)\n𝑒\n=\n𝑁\n𝑃\n(𝑡\n)\n𝑁\n𝑃\n(𝑡\n)\n.\nHence, the vibrational coherenc\ne g\ne\nra\nte\nd\nb\ny\nthe superp\nosition\nof two vibrational states can oscillate with . The oscillatory time\n|𝑒𝑠⟩\n(𝜔1−𝜔0) = 𝜔\n8\ndependent change in the macroscopic polarization will vanish if the energetic bandwidth\nof the excitation laser source is lower than .\n|𝑒𝑠⟩\n𝜔\nSupplementary Fig. 9.1 | Two kind of resonant probing mechanism: a, Frank-condon model and b, non-Condon\nHerzberg Teller model\nVibrational coherence can be detected through two complementary mechanisms. In the\nFranck–Condon (FC) picture (supplementary fig. 9.1a), nuclear motion periodically\ndisplaces the excited-state potential energy surface, leading to periodic red–blue shifts\nof electronic transitions in the transient absorption spectra. This results in a\ncharacteristic modulation of peak positions, which is readily observed when spectral\nfeatures are well isolated. In contrast, in the Herzberg–Teller (HT) mechanism8–10,\nvibrational motion modulates the electronic transition dipole moment or interstate\ncoupling, producing oscillatory intensity changes in the transient features rather than\npronounced spectral shifts. As a result of that if the vibrational coherence detection\nmechanism is not HT type mechanism then it is possible to detect the oscillatory spectral\nmovement is possible.\nNuclear wavepacket motion in the vibration co-ordinate can be used as a probe for\nexciton-vibrational coupling12. In such impulsive excitation, the amount of the oscillation\nis dependent on the electron−phonon coupling strengths are represented by a set of\nparameters [Δ, S, λ]. Δ is a dimensionless displacement of the normal coordinate (as\ndiscussed above), S is the Huang−Rhys parameter, and λ is the reorganization energy.\nBoth S and λ exclusively dependent on the displacement (Δ) as follows:\n9\n;\n2\n𝛥\n𝑆 = 2 𝜆 = ℏω𝑆\nwhere ω is the frequency of the optical phonon. λ ( Δω) can be calculated from impulsive\nvibrational spectroscopy data using ℏ , where OD is the optical density of\nd𝑂𝐷\nthe sample and is the amplitud𝐴e𝑂 o𝑆𝐶f t=he( o𝑑s𝜔ci)ll𝛥at𝜔ions. of any vibrational modes\ncan be obtained\n𝐴\nby𝑂 𝑆fi𝐶tting the residuals to a damped sine fu\n𝐴\nn𝑂c𝑆t𝐶ion. Relative between\ndifferent modes for comparison purposes, can be obtained from the\n𝐴\n𝑂re𝑆𝐶lative FFT\namplitudes.\nThe wavelength-resolved transient absorption dynamics of Ref-P and TS-P3 are shown in\nSupplementary Fig. 9.2. These data display clear oscillatory profiles, consistent with\nexciton–vibrational coupling. The maps also highlight distinct excited-state behaviours\nshaped by polymer architecture, with TS-P3 exhibiting modified spectral features on the\nultrafast timescale that can be attributed to the presence of the through-space PDI units.\nTransient absorption map of the Ref-P polymer, showing the temporal evolution of its photoexcited states. b,\nTransient absorption map of the TS-P3 space polymer, highlighting the influence of through-space PDI units on the excited-state\ndynamics. c,d, corresponding zoomed-in versions Ref-P(c) and TS-P3 (d).\nEffective Huang–Rhys factor from band-integrated IVS: The high-frequency IVS\ncontribution (1000–1500 cm ¹) was isolated by band-pass filtering the TA residuals after\nbackground subtraction as shown in supplementary figure 9.3. An effective modulation\n⁻\ndepth was defined as the RMS amplitude of the band-passed residual divided by\nband\nthe smooth TA background at the same probe wavelength. For Herzberg–Teller detection\n𝑚\nwe used ; for Franck–Condon detection we used , where\neff band eff band\nis the r\n𝑆\nepre\n≈\nse\n𝑚\nntative half-width and the band center. M\n𝑆\nea\n≈\nsu\\𝑚\nred a\nΓ\nm‾/ p\n(\n2\nℏ\n𝜔\n‾)\nitudes were\nΓ\nc‾orrected for the 12 fs Gaussian IRF usin\n𝜔\n‾\ng\n( fs). For a quick\n2 2\n𝐻(𝜔) = exp [−𝜔 𝜎 /2] 𝜎 = 5.1\n11\nmode-resolved estimate, FFT peak areas within ±10 cm ¹ of each mode were used to\napportion into via weights FFT area, followed by mode-specific IRF corrections.\neff ⁻\nReported\n𝑆\nvalues\n𝑆\ni thus represe\nw\ni\nnt\n∝\nconservative (band-integrated) lower bounds on the\nhigh-frequency reorganization. From this analysis method we estimate effective for\n𝑆\nhigh freq. modes (1000-2000 cm-1) is 0.52 and for the 1515 (±10) cm-1 mode the 𝑆e 𝑖 ff is\n0.08 which is comparable to the previously reported BDT-DPP polymers13,14.\n𝑆\nTo connect the vibronic coupling observed in the ultrafast photophysics, we carried out\nquantum dynamics simulations using the LVC Hamiltonian for the DPP–BDT and PDI\nfragments detailed in the supplementary note 21. This approach allows us to track in real\ntime how nuclear motion along specific normal modes influences the evolution of the FE\nand CT states and the formation of the charge-separated configurations. By explicitly\npropagating the coupled electron–nuclear wavepacket, the simulations reveal the\nvibrational modes that most efficiently drive population transfer between these\nmanifolds (supplementary fig. 51). These simulations clearly show that several high-\nfrequency modes of the DPP-BDT are strongly coupled with both the exciton as well as\nthe cationic states(supplementary fig. 50).\ntransient absorption kinetics (green) and the corresponding moving-average background (red, 200 fs window). b,\nResiduals obtained after subtracting the exponential kinetic fit (orange, top) and band-pass fit (blue, bottom), revealing\ncoherent oscillations associated with vibrational modes. c, Fourier transforms (FFT) of the respective residuals,\nshowing dominant vibrational modes in the 400–1600 cm ¹ range. d, Comparison between the raw FFT spectrum\n(orange) and the time-resolution–corrected FFT (grey dashed), highlighting sharpening of vibrational peaks after\ncorrection. ⁻\nMain manuscript\n13\n(Revised) figure 2e\n4) The abstract claims “… unprecedented CT rates…”. This statement is incorrect, s., e.g.,\nR. Huber et al., JPCB 106, 6494 (2002) and others.\nResponse: We thank the reviewer for pointing this out. We recognise the pioneering work\nof Huber and co-workers, for example:\n Huber, R. et al., J. Phys. Chem. B106, 6494–6499 (2002).\n Huber, R. et al., Chem. Phys.285, 39–45 (2002).\nThe study reported sub-10 fs electron transfer times in dye/semiconductor oxide\ncolloidal systems. We agree that these timescales are comparable to the sub-cycle\ncharge transfer observed in our work. However, as the reviewer notes, those hybrid\norganic–inorganic systems benefit from large dielectric constants that facilitate free\ncharge formation. By contrast, our system is a purely organic donor–acceptor\nheterojunction, in which such sub-15 fs ultrafast transfer in underexplored to the best of\nour knowledge.\nAction: In recognition of this point, we have revised the abstract to replace\n“unprecedented CT rates” with “ultrafast CT rates in purely organic systems.”\n14\n5) No explanation has been given for microscopic origin of the observed fast electron\ntransfer to non-covalently linked PDI electron acceptor. Is it through space to the stacked\nPDI moiety?\nResponse: We thank the reviewer for raising this point. Because the PDI is connected via\na non-conjugated tether, electron transfer cannot proceed through a ‘through-bond’\npathway but must occur via a ‘through-space’ mechanism in both TS-P2 and TS-P3, as\nthe reviewer suggested. This distinguishes our constructs from previously reported\nmodel heterojunctions such as polymers with pendant donor/acceptor groups15–18 or\nblock copolymers19–21, where through-bond interactions dominate.\nTo further probe the microscopic origin of CT, we also examined a physical mixture of Ref-\nP and PDI at the same overall composition. As shown in the SI (originally included for a\ndifferent purpose), this mixture does not display the PDI radical-anion signatures or\naccelerated bleach decay that are observed in TS-P3. Instead, its kinetics closely\nresemble Ref-P alone, consistent with negligible ET. This control confirms that stochastic\nencounters between Ref-P and PDI do not yield efficient femtosecond CT, and that the\npre-organised geometry enforced by the covalent tether is essential for enabling through-\nspace coupling.\nTo address the reviewer’s question regarding whether the donor–acceptor interaction in\nTS-P3 is mediated through the non-conjugated linkers or arises from direct through-\nspace coupling, we performed additional quantum chemical calculations using a\ndiabatization approach on two model systems. These include (a) the full TS-P3 dimer,\nwhich retains the alkyl linkers connecting the DPP–BDT unit to the PDI, and (b) a truncated\nversion in which these linkers were removed (supplementary fig.). As detailed in\nsupplementary note 21, both models yield essentially identical electronic couplings, and\nthe corresponding diabatic Hamiltonians are reported in supplementary table 5. This\ndemonstrates that the linkers do not play a role in the coupling mechanism and confirms\nthat the donor–acceptor interaction is dominated by through-space, rather than through-\nbond, coupling.\n– TS-P3: PDI is placed adjacent to the DPP moiety, where close co-facial proximity\ncombines with strong vibronic coupling to the ~1512 cm ¹ DPP-localised mode.\n⁻\n– TS-P2: PDI is tethered near the BDT unit, where the through-space interaction is weaker\nand no equivalent vibrational driving mode is engaged, resulting in slower ~376 fs\ntransfer.\nAction:\nMain text:\n“Because the PDI is attached via a non-conjugated linker, CT in TS-P3 occurs through\nspace. This is supported by the absence of CT in a Ref-P + PDI mixture (Supplementary\nNote 16) and by electronic coupling calculations performed on TS-P3 with and without\nthe linkers (Supplementary Note 21), which produce nearly identical FE-CT couplings\n(supplementary table 5), confirming that linkers do not mediate the interaction. Thus, the\nultrafast <15 fs CT arises uniquely from the tethered through-space geometry and its\ncoupling to the DPP-localised vibrational mode.”\nSupplementary Information\nSupplementary Note 16: Microscopic Origin of Charge Transfer\nBecause the PDI is connected to the polymer via a non-conjugated alkyl tether, charge\ntransfer cannot proceed through a through-bond pathway but should occur via through-\nspace interactions between the polymer donor backbone and the PDI acceptor. This\ndesign distinguishes our ‘through space’ constructs from other model heterojunctions,\nsuch as polymers with pendant donor/acceptor15–18 groups or block copolymers19–21,\nwhere through-bond coupling is significant.\nAdditional evidence for the ‘structurally predefined through-space’ nature of the transfer\ncomes from a control experiment in which Ref-P and PDI were physically mixed at the\nsame overall composition. As shown in the supplementary figure 20, this mixture does\nnot exhibit the characteristic PDI radical-anion signatures or accelerated polymer bleach\n16\ndecay observed in TS-P3. Instead, its kinetics closely resemble Ref-P alone, indicating\nnegligible charge transfer. This confirms that random intermolecular encounters do not\nsupport efficient femtosecond CT, and that the covalent tether is essential for enforcing\ndonor–acceptor ‘through-space’ proximity and dynamic CT-LE mixing through the\nvibrational modes.\nTo evaluate whether donor–acceptor coupling in TS-P3 is transmitted through the alkyl\nlinkers or arises from through-space interactions, we conducted diabatization\ncalculations on two variants of the model system: (a) the full TS-P3 dimer with all non-\nconjugated linkers included, and (b) a truncated structure with these linkers removed\n(supplementary fig. 49). The results of these quantum-dynamics-based calculations are\npresented in supplementary note 21, and the corresponding diabatic Hamiltonians are\ncompiled in supplementary table 5. In both cases, the extracted electronic couplings are\neffectively identical, indicating that the linkers do not contribute to the coupling pathway.\nThese findings confirm that the donor–acceptor interaction in TS-P3 is governed by\nthrough-space rather than through-bond coupling.\nThe difference between TS-P2 and TS-P3 therefore lies in the attachment site:\n TS-P3: PDI lies adjacent to the electron-deficient DPP moiety, enabling close co-\nfacial geometry. Strong vibronic coupling to the ~1512 cm ¹ which is a DPP-\nlocalised vibrational mode dynamically mix singlet (Frenkel exciton, FE) and CT\n⁻\nstates following photoexcitation, allowing for CT to be achieved on timescales\nmuch faster than would be expected when considering purely static electronic\ncoupling interactions.\n TS-P2: PDI is tethered near the electron-rich BDT moiety, where spatial overlap\nand vibronic driving are weaker, leading to a slower ~376 fs transfer.\nTogether, these results show that through-space coupling is the operative mechanism in\nboth systems, but vibronic enhancement at the DPP site makes CT in TS-P3 exceptionally\nfast.\nPDI maintaining the same number of PDI content per BDT-DPP unit as TS-P3 (excitation by the 700 nm centred\nbroadband pulse) :a, Transient absorption (ΔT/T) map. b, Transient absorption spectra (ΔT/T) plotted against probe\nwavelength (nm) for different pump-probe delay. c, Transient absorption kinetics extracted from the 790-830 nm. d,\nProbe wavelength resolved IVS map. The red dotted line highlights the position of the 1283 cm-1.\n** We have not reproduced the supplementary table 5, supplementary note 21 and\nsupplementary figure 49 here to avoid repeat as that is included in the action of the\npoint number 7 made by the reviewer.\n6) The assignment of 1283 cm-1 observed in TS-P3 is confusing. The authors have assigned\nthe 1283 cm-1 mode to a C-C-stretching mode of the PDI radical anion since they\nobserved a 1295 cm-1 mode in the Raman spectra of an isolated PDI molecule. In the next\nparagraph they argue that this mode is an excited state mode of PDI because its\nfrequency is 1283 and not 1295 cm-1 as observed in isolated molecule. They support this\n18\nargument by different control measurements in Fig. 2b. Honestly speaking, all these\nmeasurements seem to show (to me) pretty much the same lineshape of the Fourier\ntransform band in the 1280 cm-1 region. I do not think that a distinction between ground\nand excited state mode can be made on the basis of these measurements. For this, much\nmore detailed control measurements are needed. Such a distinction can be achieved,\nfor instance by analyzing the phase of the vibrational modulation of the TA spectra. A clear\nand convincing demonstration that the 1283 cm-1 mode is an excited state vibration of\nPDI seems necessary to make the paper publishable.\nResponse:\nWe thank the reviewer for raising this important point and acknowledge that our original\ndescription was not sufficiently clear. The observation of a 1295 cm ¹ mode in the cw-\nRaman spectrum of isolated PDI was used to identify the 1283 cm ¹ vibrational\n⁻\ncoherence as structurally originating from the PDI moiety, since the polymer backbone\n⁻\nhas no mode in the 1275–1325 cm ¹ region. However, as the reviewer rightly points out,\nthe small frequency shift alone cannot serve as definitive evidence that the 1283 cm ¹\n⁻\ncoherence arises from the excited state, and we have revised the manuscript to clarify\n⁻\nthis.\nWe agree that in the full-range stacked comparison, the Fourier transform bands in the\n1280–1300 cm ¹ region appear broadly similar, which may have given the impression that\nno clear distinction can be made. To address this, we now provide a zoomed-in,\n⁻\nnormalised, and overlapped comparison in the 1280–1300 cm ¹ window in the revised SI\n(Supplementary Fig. 43), which highlights reproducible differences between the 1295\n⁻\ncm ¹ ground-state Raman/IVS modes of isolated PDI and the 1283 cm ¹ feature observed\n⁻\nin TS-P3. The 1291 cm ¹ ground-state vibrational coherence and the 1295 cm ¹ cw-\n⁻ ⁻\nRaman mode both show symmetric lineshapes, with the latter displaying a slight high-\n⁻ ⁻\nenergy shoulder. In contrast, the 1283 cm ¹ feature in TS-P3 is clearly shifted, broadened,\nand asymmetric. All Fourier transforms were performed using the same time-domain\n⁻\nwindow (40–1700 fs) to ensure an unbiased comparison of lineshapes.\nWe agree with the reviewer that early-time phase analysis of oscillations can provide a\ndiagnostic to distinguish between ground- and excited-state coherences. In principle,\n19\ncosine-like wavepackets (ISRS in the ground state) have phase mπ, whereas sine-like\nwavepackets (excited-state launch) have phase (2m+1)π/2, requiring precision on the\norder of τ/4 to discriminate. For the 1283 cm ¹ mode (τ ≈ 26 fs, τ/4 ≈ 6.5 fs), this distinction\nis technically challenging: the quarter-period is comparable to the 11 fs IRF, and most\n⁻\nimportantly the early-time window is further convoluted by coherent artefacts, cross-\nphase modulation, and other polymer backbone Franck–Condon modes (Fig. 3a), all of\nwhich bias phase retrieval. For this reason, phase analysis is applied to slower modes\n(100–500 fs period), as in the 106 fs mode reported by Rather et al. (Nat. Chem. 202122)\nor 416 fs mode reported by Kong et al (Chem. Sci, 202423). Moreover, the analysis of our\nresults shows that the 1283 cm ¹ coherence is not launched directly by the light field but\nis instead driven by the ultrafast charge transfer. In such cases, theory and simulations\n⁻\npredict additional phase behaviour: a π/2 shift relative to the reactant coherence (which\ncould superficially resemble a ground-state ISRS signature) and a π shift relative to other\nFranck–Condon active modes. This scenario is consistent with the behaviour described\nby Rather et al. (Nat. Chem. 202122). However, such analyses have only been\ndemonstrated for lower-frequency modes with long periods, and are not technically\nreliable for the present high-frequency regime.\nAs the early time few cycle phase analysis is challenging for this particular case, we\nprovide two conclusive evidence that we have revised in the main and SI that shows the\nmode is not a ground state mode. If it was a ground state mode it would be generated by\nthe ISRS mechanism (often referred as off-resonant IVS24) which we now highlight\nproperly in the revised manuscript.\n We analysed the spectral dependence (figure 3c, main) of the vibrational\ncoherence of the 1283 cm-1 mode. A more important diagnostic is that the 1283\ncm ¹ coherence in TS-P3 is spectrally localised to the absorption window of the\nPDI radical anion (650–850 nm), and not to the neutral PDI bleach-tail nor having\n⁻\na flat spectral profile throughout. This indicates that the oscillation is resonantly\ndetected through the excited-state CT absorption, rather than through ground-\nstate Raman scattering. We therefore assign the 1283 cm ¹ coherence to the PDI\nradical anion which is formed during ultrafast charge transfer. Such deep-red\n⁻\n(650-850 nm) spectral dependence can not be explained if the vibrational\n20\ncoherence is generated on the ground state. We have revised Fig. 3c to include a\ncomparison between the PDI radical-anion feature extracted from the TA data\nalong with the chemically reduced PDI radical-anion absorption, plotted with the\nspectral dependence of the 1283 cm ¹ mode\n⁻\n We also note that TS-P2 and a physical mixture of Ref-P and PDI, both of which\n⁻\ncontain the same amount of PDI, do not show the 1283 cm ¹ feature. This absence\nindicates that the mode is not generated on the ground state of PDI via an ISRS\n⁻\nmechanism, but is instead associated with the radical anion formed in TS-P3\nthough fast electron transfer reaction.\nNow in the revised main text we have highlighted this two point to establish the vibrational\ncoherence at 1283 cm-1 is generated not from ground state ISRS mechanism. To further\nclarify this assignment, we have now included a decision-tree summary in the\nSupplementary Information (SI Fig. 44) that collates all experimental and computational\nevidence.\nWe would like to highlight that in light with this comment, we computed the mode-\nspecific gradients that define the linear vibronic coupling (LVC) Hamiltonian using the\n𝛼\nnormal modes of is\n𝜆\no𝑘lated DPP–BDT and PDI fragments at the TS-P3 monomer geometry\n(as detailed in supplementary note 21). Ground-state gradients were obtained from the\nharmonic QM Hessian, and excited-state gradients for the locally excited, cationic, and\nanionic states were constructed within the Vertical Gradient (VG) approximation as\nimplemented in FCclasses3.0. This approach reveals that several high-frequency DPP–\nBDT modes exhibit large displacements, with the 1531 cm ¹ mode acting as the dominant\ndriving coordinate. The PDI modes, in contrast, begin to evolve only after the CT states\n⁻\nare populated via this fast electron transfer, in agreement with the experimental\nobservations (supplementary figure 51). This is especially clear for the PDI mode at 1299\ncm-1, whose frequency coincides with what has been found experimentally\n(corresponding to experimental frequency 1283 cm-1). In our impulsive vibrational\nspectroscopy measurements, the vibrational coherence along the 1283 cm ¹ coordinate\nappears only after electron transfer, fully consistent with the simulations, which identify\n⁻\nthis mode as a PDI-anion vibration activated following CT-state formation.\nRevised main manuscript excerpt\nPage-10\nWe can assign this newfound vibrational coherence to a high-frequency C-C\n𝛽\nstretching mode of the PDI acceptor moiety, since we observe a mode at similar\n𝜔\nfrequency regime for an isolated PDI molecule with CW Raman spectroscopy (1295 cm-\n1, blue spectra, Figure 3b(4)) and off-resonant IVS (1291 cm-1, grey spectra, Figure 3b(4)).\nIn addition, the polymer backbone chromophore has no mode in this region 1275–1325\ncm ¹ further confirmed by vibronic simulations with displaced harmonic oscillator model\n(supplementary fig. 50).\n⁻\nPage-11\nFirst, we rule out the possibility that wavepacket is an off-resonant wavepacket\n𝛽\nlaunched on the ground state surface of PDI, i.e. the possibility that the ultrashort optical\n𝜔\npulse, off-resonant with the electronic transition of PDI…….…..In contrast, the vibrational\ncoherence at 1283 cm ¹ ( ), obtained by resonant impulsive excitation of the polymer\n𝛽\nbackbone in TS-P3, shows its strongest amplitude at probe wavelengths between 650\n⁻ 𝜔\nand 850 nm (black trace, Fig. 3c, top). This spectral envelope closely matches the PDI\nradical-anion feature retrieved from the transient absorption data (red trace, Fig. 3c, top),\nand is further qualitatively complemented by the absorption profile of the chemically\nreduced PDI species (blue trace, Fig. 3c, top).\n⁻\nPage-13\n“See Supplementary Fig. 44 for a decision-tree summary of the evidence supporting the\nassignment of the 1283 cm ¹ coherence.”\n⁻\nPage-14\nTo further explore these experimental observations, we constructed a linear vibronic\ncoupling model for a representative TS-P3 monomeric unit, including the most relevant\nvibrational modes that couple to the localized (diabatic) FE state on the DPP–BDT unit as\nwell as to two CT states (in which the electron is transferred from DPP–BDT to PDI).\nperformed full quantum dynamics simulations with the multi-layer multi-configurational\ntime-dependent Hartree (ML-MCTDH) method25. The simulations reveal that the PDI\nvibrational mode at 1299 cm ¹—coincident with the experimentally observed feature\n𝛽\nin Fig. 3b—begins to evolve only after the CT states become populated through ultrafast\n⁻ 𝜔\nelectron transfer (see Supplementary Fig. 51). In contrast, the DPP–BDT polymer modes\nstart oscillating immediately at time zero and act as driving modes for the CT as we\ndiscuss below. We note in passing that several groups have attempted similar excited\nstate dynamics simulations, using not only full quantum dynamics approaches but also\nmixed quantum–classical molecular dynamics26–28.\nRevised supplementary information excerpt\nSupplementary Note 18. Further discussion on the assignment of the 1283 cm ¹\nVibrational Coherence in TS-P3\n⁻\nThe assignment of the 1283 cm ¹ vibrational coherence (VC) observed in TS-P3 requires\ncareful consideration, as it lies close to the 1295 cm ¹ Raman-active mode of isolated\n⁻\nPDI. CW-Raman and off-resonant impulsive vibrational spectroscopy (IVS) of isolated\n⁻\nPDI show a 1295 cm ¹ mode (and a 1291 cm ¹ coherence in IVS), both with symmetric\nlineshapes (Supplementary Fig. 43). By contrast, the 1283 cm ¹ feature in TS-P3 is\n⁻ ⁻\nshifted, broadened, and asymmetric. The polymer backbone has no modes in the 1275–\n⁻\n1325 cm ¹ region, confirming that this feature originates from the PDI moiety. Below, we\ncollate experimental, spectroscopic, and theoretical evidence to demonstrate that this\n⁻\nfeature arises from the PDI radical anion generated during ultrafast charge transfer (CT),\nrather than from a ground-state impulsive Raman (ISRS) response.\n The 1283 cm ¹ coherence is tightly confined to the spectral region corresponding\nto the PDI radical-anion absorption (650–850 nm; Fig. 3c) and is absent from the\n⁻\nneutral PDI bleach tail. This spectral localisation shows that the oscillation is\nresonantly detected through the excited-state charge-transfer (CT) absorption,\n23\nrather than arising from ground-state Raman pathways. In particular, the\npronounced deep-red localisation is inconsistent with ground-state ISRS, which\nwould produce a broadband response extending across the neutral bleach. We\nhave revised the figure 3c to incorporate the absorption of the PDI radical anion\nfeature retrieved from TA for the comparison.\n TS-P2, which contains the same PDI concentration as TS-P3, shows no 1283 cm ¹\ncoherence, thereby ruling out any assignment to vibrational coherence generated\n⁻\nby impulsive Raman scattering in the ground-state manifold. (Figure 3b(2), main)\n A physical mixture of Ref-P and PDI with the same composition as TS-P3 likewise\nshows no 123 cm ¹ coherence, and its kinetics closely match those of Ref-P\nalone. This further rules out an origin in vibrational coherence generated by\n⁻\nimpulsive Raman scattering in the ground-state manifold of TS-P3.\n(Supplementary Fig. 20)\nTo aid interpretation, we provide a decision-tree schematic (Supplementary Fig. 44) that\nsummarises the combined evidence. Together, these results confirm that the 1283 cm ¹\ncoherence in TS-P3 arises from the PDI radical anion on the excited-state CT surface, and\n⁻\nnot from ground-state ISRS.\nspectrum of TS-P3 (orange) with cw-off-resonant Raman (blue) and off-resonant IVS (gray) spectra of isolated PDI.\nWhile the 1291–1295 cm ¹ ground-state Raman/IVS modes of PDI exhibit symmetric lineshapes, the 1283 cm ¹ feature\nin TS-P3 is distinctly shifted, broadened, and asymmetric. All Fourier transforms were performed over the same time\nwindow (40–1700 fs) to e⁻nable unbiased comparison. ⁻\nsummarising experimental and computational evidence for the microscopic origin of the 1283 cm ¹ mode.\n⁻\n⁻\nTo quantify vibronic interactions in TS-P3, we computed the mode-specific gradients\n𝛼\nthat define the linear vibronic coupling (LVC) Hamiltonian in Eq. S21.4 as explained mo\n𝜆\nre𝑘\nin detail in Supplementary Note 21, using the normal modes of isolated DPP–BDT and PDI\nfragments evaluated at the TS-P3 monomer geometry. Ground-state gradients were\nobtained from the harmonic QM Hessian, while excited-state gradients for the locally\nexcited, cationic, and anionic states were constructed within the Vertical Gradient (VG)\napproximation (see supplementary note 21). This framework reveals that several high-\n26\nfrequency DPP–BDT modes exhibit large displacements, with the 1531 cm ¹ mode acting\nas the principal driving coordinate in the reduced model. The PDI modes, in contrast,\n⁻\nbegin to evolve only after the CT states are populated via this fast electron transfer, in\nagreement with the experimental observations (supplementary figure 51). This is\nespecially clear for the PDI mode at 1299 cm-1, whose frequency coincides with what has\nbeen found experimentally. In our impulsive vibrational spectroscopy measurements,\nthe vibrational coherence along the 1283 cm ¹ coordinate appears only after electron\ntransfer, fully consistent with the simulations, which identify this mode as a PDI-anion\n⁻\nvibration activated following CT-state formation.\n7) The authors find evidence for quite rapid electron transfer dynamics and substantial\ncouplings of the electronic excitations to high frequency modes. This strongly suggests\nthat nonadiabatic dynamics resulting from vibronic couplings are important for the\nelectron transfer rate and yield. Theoretical models that describe these dynamics should\nobviously take such nonadiabatic couplings and the effects of vibronic decoherence on\nthe resulting quantum dynamics into account. When discussing their results, the authors\nspeak of “ballistic electron transfer”, “conventional field-driven coherence” and\n“reaction-driven coherences” and “impulsive forces” generating vibrational coherences.\nThese are complicated concepts, partly borrowed from classical mechanics. These\nconcepts are used without proper explanation, let alone definition, of the terms. This\nmakes it very difficult for a reader to understand what the authors want to say. Even more,\nthe use of such oversimplified, quasi-classical concepts may also be highly misleading.\nTheoretical models that can properly account for the effects of vibronic couplings on\ncharge transfer dynamics in donor-acceptor systems are well developed (Tretiak,\nPrezhdo, Bredas, …). Also, the decoherene phenomena can readily be described using,\ne.g., Redfield or Lindblad master equations. I strongly suggest to attempt such a\nmodelling in order to support the conclusions that are drawn from the experiments.\nResponse:\nthe ultrafast charge separation process in TS-P3 as observed in our experiments.\nFollowing the reviewer’s suggestion, we performed additional simulations. In particular,\n(i) we confirmed the reviewer expectation of strong vibronic couplings of the exciton and\nrelated CT states with a number of modes localized both on DPP-BDT polymer (see for\nTable S6 and Fig. S51); (ii) we built a suitable ab-initio parametrized Linear Vibronic\nCoupling (LVC) Hamiltonian representing the TS-P3 systems in order to perform full\nquantum dynamics using the gold-standard method for such a task, that is Multi-\nConfigurational Time Dependent Hartree (MCTDH)29–33.\nWe have described the set-up of the model Hamiltonian and the outcome of our\nQuantum Dynamics (QD) simulations in a new Supplementary note in the SI titled:\nExcited-state quantum dynamics. There, we first explain how we have built the LVC\nHamiltonian. For the electronic part of the Hamiltonian, we have computed the lowest\ndiabatic states of a representative TS-P3 monomer unit that shows a Frenkel exciton state\non the DPP-BDT unit and two lower energy CT (DPP-BDT →PDI) states where the electron\ntravels from DPP-BDT to the PDI. For the vibrational part we included the most relevant\nmodes for the dynamics among which the high frequency driving mode (reported already\nin the main text), which couples both to the exciton and the cation of the polymer,\nand\nthe modes of the PDI. Our simulations nicely show that the modes localized on the PDI\nstart moving following the ultrafast population transfer from the DPP-BDT to the PDI.\nThe results show an ultrafast population transfer from the DPP-BDT occurring in\napproximately ~75 fs when the energy off-set between LE and CT states is set to about\n200 meV which is consistent with what we found from our photoluminescence7 and ER-\nEIS measurements (supplementary note 1). We note our simulated timescales are still\nslower than those observed experimentally, and the reason might be related to several\nsimplification that are necessary to make the QD feasible on such a complex system, as\nfor instance: the approximation of using the same gradients for different CT states, the\nreduced number of nuclear degrees of freedom and the absence of non-local electron-\nvibrational couplings. Despite these shortcomings we believe our new simulations\nprovided additional important insights as we explain in the text below helpful to further\nsupport our experimental data.\nsimulations using either full quantum26,27 as well as mixed quantum-classical molecular\ndynamics26,28 and we have acknowledged this amount of work in our revision by citing a\nfew of these works on page 14.\nWe thank the reviewer for highlighting the need for clearer contextualisation of technical\nconcepts. We have now revised the manuscript accordingly. Guided by our new\nsimulation results, we have expanded the explanations of these concepts and improved\nthe transitions surrounding their introduction. These changes appear on pages 14, and\n19, where we now define the terms more explicitly and cite the relevant literature.\nAction: The action is the creation of an additional Supplementary note as explained\nbelow.\nSupplementary Note 21: Excited-state Quantum dynamics\nLinear Vibronic Coupling Hamiltonian for TS-P3\nThe linear vibronic coupling Hamiltonian used to describe the ultrafast dynamics of the TS-P3\npolymer is written as a sum of electronic, vibrational and electron-vibronic Hamiltonians:\n(S21.1)\n𝑒𝑙 𝑣𝑖𝑏 𝑒𝑙−𝑣𝑖𝑏\n𝐻 = 𝐻 +𝐻 + 𝐻\nThe electronic Hamiltonian ( ) describes the various electronic excitations of the system in\n𝑒𝑙\nthe diabatic representation. These include localized excitations (FE) on the DPP–BDT polymer\n𝐻\nbackbone and charge-transfer (CT) excitations between the DPP–BDT polymer and the PDI\nunit. More specifically,\n= (S21.2)\n𝑒𝑙\nHere, the diagonal elements c𝐻o rresp𝐻 o F n E d+ to𝐻 t L h E e − C v T ertical energies of the diabatic excitations,\nwhile the off-diagonal elements, , represent the excitonic couplings that describe\ninteractions between the FE and CT states included in the model. The electronic Hamiltonian\n𝑉𝑘𝑙\nis constructed using a diabatization procedure starting from the adiabatic states of the TS-P3\npolymer, as detailed in the next paragraph.\n𝑣𝑖𝑏\nof the M (mass-weighted) dimensionless coordinates and their conj\\𝐻\nu gate momenta of the\nmodes of the TS-P3 system represented using:\n𝑞𝛼 𝑝𝛼\n(S21.3)\n𝑣𝑖𝑏 𝑣𝑖𝑏 𝑣𝑖𝑏 1 2 1 2\n𝐻 = 𝐾 + 𝑉 = ∑𝜔𝛼 𝑝𝛼 + ∑𝜔𝛼 𝑞𝛼\n2 𝛼 2 𝛼\nWhere is the frequency of the specific hierarchically selected mode (considering ).\nNote th\n𝜔\nat𝛼 a dimensionless coordinate and related momenta are fo\\𝛼\nund from the an\\ℏ\nal\n=\nogue\nquantity with dimensions by: 𝑞𝛼 and 𝑞, 𝛼 respectively.\n𝜔𝛼 𝑚𝑣𝛼\n𝑞𝛼 = √ ℏ √𝑚𝑥𝛼 𝑝𝛼 = √𝑚ωℏ\nThe electronic excitations, i.e., the formation of FE states and CT states, are strongly coupled\nto high-frequency vibrational modes as shown also by our impulsive vibrational spectroscopy\nexperiments. Such electron-vibrational interactions are presented by the following\nHamiltonian:\n(S21.4)\n𝑒𝑙−𝑣𝑖𝑏 𝐹𝐸−𝑣𝑖𝑏 𝐶𝑇−𝑣𝑖𝑏\n𝐻 =𝐻 + 𝐻\nThe coupling between these quantized vibrational modes and the LE and CT excitations are\nincorporated into our Hamiltonian Eq. S21.1 by using a quantum displaced harmonic oscillator\nas explained the next paragraph and done by other groups.34,35\nThe first term represents the coupling between FE states (for instance those on the TS polymer)\nand a given mode is:\n𝛼\n𝑁𝐿𝐸𝑁𝑒𝑥𝑐 (S21.5)\n𝐹𝐸−𝑣𝑖𝑏 𝛼,𝐿𝐸\n𝐻 = ∑ ∑ 𝜆𝑘 𝑞𝛼 |𝐹𝐸𝑘⟩⟨𝐹𝐸𝑘|\n𝑘=1 𝛼\nSimilarly, the coupling between CT states and the nuclear degrees of freedom is given by:\n𝑁𝐶𝑇𝑁𝑐𝑎𝑡 𝑁𝐶𝑇𝑁𝑎𝑛𝑖 (S21.6)\n𝐶𝑇−𝑣𝑖𝑏 𝛼,cat 𝛼,ani\n𝐻 = ∑ ∑𝜆𝑘 𝑞𝛼 |𝐶𝑇𝑘⟩⟨𝐶𝑇𝑘| +∑ ∑ 𝜆𝑘 𝑞𝛼 |𝐶𝑇𝑘⟩⟨𝐶𝑇𝑘|\n𝑘=1 𝛼 𝑘=1 𝛼\nrepresent the gradients of the excited potential energy surface (excitonic, cationic\n𝛼,(LE,ani,cat)\nor anionic states of the CT) at the equilibrium geometry of the ground state and are referred to\n𝜆𝑘\nas first-order intra-state electronic-vibrational coupling constants (which related also to the\ndimensionless shift by ).36 In Eq. S21.6, we make the assumption that the CT\n𝛼 𝛼, 𝛼\n𝑔𝑘 𝜆𝑘 = 𝑔𝑘ℏ𝜔𝛼 30\nstates are simply given by the formation of cationic state on the DPP-BDT polymer with and\nthe anion on the PDI unit.\nDiabatization and Electronic Hamiltonian\nTo carry out the quantum dynamics simulations, we first constructed the electronic Hamiltonian\nin Eq. S21.4 in the diabatic representation. This was achieved performing electronic structure\nexcited state calculations on a reduced model comprising a single polymer (DPP-BDT) unit\nand a single PDI molecule. The use of this simplified model was motivated by computational\nefficiency and by the fact that the adiabatic states of this reduced system (supplementary fig.\nS48) are representative of those in larger polymers (supplementary fig. 36). This is also shown\nin supplementary table 3, with the most important difference being the number of states with\nCT character in the same energy window (which is higher for longer polymer systems).\noptimized TS-P3 monomer structure, along with the corresponding hole–particle natural transition orbitals.\nSupplementary Table 3 | Excitation energies (in eV) and their associated oscillator strengths for the TS-P3 dimer and\nmonomer, as computed at the TDDFT SRSH LC-ωhPBE/6-311G(d,p) level of theory.\nTS-P3 dimer TS-P3 monomer\nState Energy Osc. Str. State Energy Osc. Str.\n(character) (eV) (character) (eV)\nS1 (CT1) 1.23 0.009 S1 (CT1) 1.36 0.018\nS2 (CT2) 1.33 0.025 S2 (CT2) 1.96 0.002\nS3 (CT3) 1.68 0.020 S3 (FE) 2.15 0.376\nS4 (CT4) 1.81 0.009\nS5 (CT5) 1.87 0.003\nS6 (FE) 1.94 1.776\n31G(d,p) level of theory, features one bright excitation localized on the polymer and two lower-\nenergy dark states corresponding to charge-transfer (CT) states from the polymer donor to the\nPDI acceptor. To evaluate the electronic couplings between the diabatic LE and intramolecular\nCT states, we employed an adiabatic-to-diabatic transformation using the multi-state fragment\nexcitation difference–fragment charge difference (MS-FED-FCD) method. The MS-FED-FCD\nmethod extends and unifies the capabilities of previously developed property-based\ndiabatization schemes, namely the two-state fragment excitation difference (FED)37 and\nfragment charge difference (FCD)38,39 approaches, which have been described in detail\nelsewhere40–43. The method partitions the system into two fragments, a donor and an acceptor\n(in our case the DPP-BDT unit and the PDI, respectively), and, using suitable additional\noperators, transforms the adiabatic Hamiltonian of the dimer (comprising two or more adiabatic\nstates) into a diabatic representation yielding the electronic Hamiltonian in Eq. S21.2. This\ntransformation enables direct evaluation of the excitonic couplings between LE and CT states\nfrom the diabatic Hamiltonian matrix.\nHere, we applied the MS-FED-FCD diabatization in combination with TDDFT at the LC-\nωhPBE/6-311G(d,p) level of theory to both the monomer model TS-P3 system and the dimer\nTS-PX reported in the main text and supplementary note 15. Our simulations (see\nsupplementary table 4) confirm that the electronic couplings between LE and CT states are\nsmall (i.e., 15-28 meV) in both cases, consistent with our experimental observations of weak\nexcitonic effects influencing the optical absorption. Therefore, the ultrafast charge separation\nobserved is not driven by strong excitonic couplings as it is the case for other system in the\nliterature44–46. As discussed in the main text and shown by full quantum dynamics below, even\nin the presence of such weak electronic interactions, rapid charge separation occurs, driven by\nvibronic couplings and energy alignment between LE and CT states.\nSupplementary Table 4 | Diabatic excitation energies (in eV) for the TS-P3 dimer and monomer, as computed with the multi-\nstate fragment excitation difference–fragment charge difference (MS-FED-FCD) method on top of a TDDFT SRSH LC-\nωhPBE/6-311G(d,p) calculation. In the table there are also reported the electronic coupling (in meV) between the Frenkel\nexciton (FE) and the various CT states.\nTS-P3 dimer TS-P3 monomer\n(meV) coupling\n(meV)\nCT-1 1.23 15 CT-1 1.35 15\nCT-2 1.33 -25 CT-2 1.96 28\nCT-3 1.68 10 FE 2.20 /\nCT-4 1.81 -1\nCT-5 1.87 3\nFE 1.96 /\nSupplementary Table 5: Diabatic Hamiltonian of the TP-P3 monomer with and without linkers. All the elements are in\neV.\nTS-P3 mon. without\nTS-P3 mon. linkers\nLE CT2 CT1 FE CT2 CT1\nFE 2.196 0.015 0.028 2.211 0.015 0.027\nCT2 0.015 1.930 0.000 0.015 1.986 0.000\nCT1 0.028 0.000 1.346 0.027 0.000 1.355\nOur diabatic Hamiltonian shows that, as we move from low-energy charge-transfer (CT) states\n(e.g., CT1) to higher-energy CT states, the excitonic coupling decreases in both monomer and\ndimer polymers. This behaviour arises because the electron and hole become more spatially\nseparated, as illustrated in supplementary fig. 48 and 36. The larger separation leads to a weaker\nCoulomb attraction, which in turn results in a higher energy for the long-range CT states.\nWe\nalso note that the energy offset between FE and CT states in our simulations depends on the\npolymer size. Specifically, the energy offset decreases as the polymer length increase as we\nshow in Table S4.\nBDT unit and the PDI\n.\nAs a further note, to understand whether the linkers connecting the DPP-BDT unit to the PDI\naffect the excitonic properties of the aggregate, we also performed the diabatization in the same\nTS-P3 model system where the non-conjugated linkers between donor and acceptor were\nremoved (Supplementary Fig. 49). The corresponding Hamiltonians are reported in\nsupplementary table S5. Both cases yield similar electronic couplings, confirming that the non-\nconjugated linkers play a negligible role and that the interactions are primarily through-space\nin nature.\nNormal mode analysis and Vibronic interactions\nAs observed in our experiments, the vibronic interactions play an important role in the ultrafast\ndynamics of the TS-P3 systems. To estimate the strength of these interactions, we computed\nthe gradients along the normal mode coordinates appearing in Eq. S21.5 and S21.6 using\n𝛼\nthe normal modes of the isolated structures if DPP-BDT and PDI fragments taken at the\n𝜆𝑘\noptimized TS-P3 monomer geometry in its ground state in the gas phase (see Fig. S48). These\ngradients, used to set-up the LVC Hamiltonian in Eq. S21.4, are computed analytically\nemploying a (quantum) harmonic oscillator model. This approach allows us to compute the\n35\nground state potential energy surface (PES) in the harmonic approximation using the QM\nhessian and frequency at the isolated fragment geometries.\nTo build the excited-state PES, which in our case, refer to the locally excited state and the\ncationic state of the DPP-BDT molecule and the anionic state the PDI molecule, respectively,\nwe adopt the Vertical Gradient (VG) approach as implemented in the FCclasses3.0\nsoftware.47,48. Within this approximation final-state PES is assumed to have the same normal\nmodes and frequencies as the initial (ground) state potential. The VG accounts only for the\neffect of the dimensionless displacement , for each mode from the equilibrium position.\nThese displacements are related to the gra𝑔d𝛼ients by: 𝛼 for a given k\n𝛼 𝛼,(LE,ani,cat) 𝛼\nstate. The frequencies are multiplied by a fact\\𝜆\no𝑘r 0.94\n𝜆\n9\n𝑘\nas done fo\n=\nr\n𝑔\nth𝑘e\nℏ\nT\\𝜔\nS𝛼-P3 dimer in\nsupplementary fig. 36.\nThe gradients calculated in this way for DPP-BDT LE and cationic states as well as for the\nanionic PDI states are reported in Fig. S50. The relaxation energy of each mode can be\nestimated as well and it is reported in the same Figure.\n𝛼2\n𝑟𝑒𝑙 𝑔𝑘\n𝜆𝛼 = 2 ℏ𝜔𝛼\nWe note that, in general, the gradients of different CT states are not identical. However, for\nsimplicity, we assume here that all CT states have the same gradients, obtained by summing\nthe gradients of the anionic and cationic contributions as shown in Eq. 21.6. This approximation\ncould be relaxed by computing the gradient numerically for each CT state.35,49 However, this is\nnot practical, as it would require diabatizing the states for each normal mode of the TS-P3\nmonomer, which contains a large number of modes. For this reason, we rely on the simpler\napproach described above.\nThese simulations clearly show that several high-frequency modes of the DPP-BDT are\nstrongly coupled with both the exciton as well as the cationic states. In particular, the mode at\n1531 cm-1 (at a frequency that agrees with the mode observed in IVS, 1519 cm-1) shows the\nlargest displacement and can be considered the driving mode of this reduced model.\nConcerning the PDI, our simulations show a strong coupling between the anion and the modes\naround ~1200-1300 cm-1. This observation support the fact that the vibrational coherence along\nthe mode at 1283 cm-1 is generated upon the electron transfer. As we discuss below, these modes\nare clearly involved in the excited state dynamics of the system which support the experimental\nobserved in the impulsive vibrational spectroscopy.\n(middle panels) of the PES of exciton and cation of DPP-BDT and anion of PDI, respectively as well as related relaxation\nenergies (bottom panels) as explained in the text.\nQuantum dynamics\nNuclear wave-packet propagations were carried out using the MCTDH method29–33, as\nimplemented in the QUANTICS code.50 The wavepackets were propagated for 200 fs in steps of\n0.5 fs. As the number of modes is sizable, we relied on multi-Layer (ML) extension of\nMCTDH.33 We adopted the variable mean field scheme with a Runge-Kutta integrator of order\n5 and an accuracy threshold of 10−7. Different numbers of single-particle functions (SPFs) for\neach layer as well as a different number of modes were tested for convergence of the results.\nNotably, considering all normal modes for each diabatic state of TS-P3 monomer is still too\ndemanding. To reduce the number of modes included in the Hamiltonian (Eq. S21.1), we\nselected a total of 36 most important modes (18 for the DPP-BDT and 18 for the PDI), with the\nlargest contribution to the total relaxation energy. The selected modes and their corresponding\ngradients are reported in Table 6.\nDPP-BDT excitonic and cation state as well as the PDI anionic state\nDPP-BDT PDI\nExciton cation anion\n(cm1) (eV) (eV) (eV𝛼) (eV) (cm1) (eV) (e\\𝑟\nV\n𝑒𝑙\n)\n15𝜔3 𝛼 1.1 0𝛼.1543 0𝑟𝑒.0𝑙627 0.1𝜆197 0𝑟𝑒.0𝑙378 15𝜔9 𝛼 8.8 0𝛼.1388 0.𝜆0 𝛼 486\n𝜆 𝜆𝛼 𝜆𝛼 𝜆\n1361.5 0.1050 0.0327 -0.1092 0.0353 1373.0 -0.1077 0.0341\n1726.0 0.0214 0.0011 0.0876 0.0179 1353.5 0.0882 0.0232\n14.0 0.0045 0.0059 0.0078 0.0173 387.5 -0.0342 0.0122\n179.5 -0.0023 0.0001 -0.0240 0.0129 1579.4 0.0595 0.0090\n472.3 0.0222 0.0042 -0.0347 0.0103 1753.0 0.0606 0.0084\n175.6 0.0074 0.0013 0.0210 0.0101 1255.2 0.0500 0.0080\n1066.2 0.0496 0.0093 0.0502 0.0095 1044.2 0.0451 0.0079\n64.7 -0.0013 0.0001 -0.0119 0.0089 526.5 0.0292 0.0065\n40.5 0.0090 0.0081 -0.0081 0.0065 1298.1 -0.0439 0.0060\n607.1 -0.0170 0.0019 -0.0347 0.0080 1743.0 0.0477 0.0053\n1224.2 0.0125 0.0005 0.0467 0.0072 1296.7 0.0407 0.0052\n1438.9 -0.0401 0.0045 0.0487 0.0066 1431.3 0.0361 0.0037\n619.1 -0.0134 0.0012 -0.0309 0.0062 226.9 0.0136 0.0033\n690.4 0.0261 0.0040 0.0317 0.0059 802.9 0.0138 0.0010\n1496.2 0.0464 0.0058 -0.0240 0.0015 707.0 0.0120 0.0008\n1443.2 0.0323 0.0029 -0.0417 0.0049 22.6 -0.0021 0.0008\n1109.5 -0.0159 0.0009 -0.0350 0.0045 1736.9 -0.0157 0.0006\nThe results of our quantum dynamics (QD) trajectories are reported in Fig. 51. The first\nimportant observation is that a fast population transfer is recovered when using the Hamiltonian\ndefined in supplementary table 5 with and energy off-set between the FE state and the CT1 and\nCT2 of 0.850 eV and 0.583, respectively as found for our model TS-P3 monomer system (see\nyellow line in supplementary fig. 51). However, this transfer remains slower than that observed\nexperimentally and an important reason for that is that as discussed and shown before in Table\nS4, longer polymers show a reduced offset between FE and CT sates. Thus, we tested the\nsensitivity of the electron transfer with respect to such an offset, by decreasing the energy\ndifference between LE -CT2(1) linearly by factor 2. We find that the rate of this ultrafast\ntransfer is strongly dependent on such an energy offset between the LE and CT states. The\ntransfer gets faster when it becomes barrierless.\nWe note that when using an offset value comparable to the experimental estimate (around 200\nmeV), the transfer rate decreases to ~75fs, thus closing the gap with the experimental\nmeasurements. Remaining discrepancies might be attributed to the approximation of using the\nsame gradients for different CT states, the reduced number of nuclear degrees of freedom and\nthe absence of non-local electron-vibrational couplings.\nDespite these differences, our simulations clearly show the rapid generation of CT states that\nget populated on an ultrafast time scale. More importantly, they reveal that the fast-driving\nmode of the DPP-BDT polymer at 1531 cm⁻¹, together with contribution from the other\nvibrational modes, drives the initial ultrafast transfer. The PDI modes, in contrast, begin to\nevolve only after the CT states are populated via this fast electron transfer, in agreement with\nthe experimental observations. This is especially clear for the PDI mode at 1299 cm-1, whose\nfrequency coincides with what has been found experimentally. In our impulsive vibrational\nspectroscopy measurements, the vibrational coherence along the 1283 cm⁻¹ coordinate appears\nonly after electron transfer, fully consistent with the simulations, which identify this mode as a\nPDI-anion vibration activated following CT-state formation.\ncharacteristic time scale of population transfer for different energy offsets between the L and CT2(1) states. b) Excited-state\npopulation for an energy offset of approximately 200 meV between the FE and CT states, as observed experimentally. c)\nAverage wavepacket position as a function of time. This shows that the high-frequency DPP-BDT mode begins oscillating\nimmediately with a period of ~20 fs, whereas the PDI modes (particularly the one at 1299 cm⁻¹) start oscillating only after the\npopulation transfer has occurred.\nOther minor comments:\nThere seems to be significant contribution of cross-phase modulations close to time zero\nespecially seen from crosscuts extending over 100 fs. Therefore, it becomes important to\nshow the subtraction scheme especially for analysing data in 20-40 fs waiting time.\nResponse: We thanks the reviewer for pointing this out. We have addressed this point\nwhile addressing the question number 2.\nphase modulation before kinetic fitting which is included in the supplementary\ninformation note 17.\nPage 10, line 238, the reference to figure, should read Figure 3b?\nResponse: We thank the reviewer for noticing this error.\nAction: The reference has been corrected to “figure 3b” from figure 4b in the revised\nmanuscript.\nPage 3, line 63, factors that govern\nResponse: We thank the reviewer for pointing this mistake out.\nAction: The text has been corrected from “factors that governs” to “factors that govern”\nin the revised manuscript.\nReviewer #2 (Remarks to the Author):\nThe manuscript by Ghosh, et al. details the excited-state dynamics of model\npolymer/non-fullerene acceptor heterojunctions monitored with high-time resolution\nbroadband transient absorption spectroscopy. The model systems are comprised of a\ndyketopyrrolopyrrole-benzodithiophene (DPP-DBT) polymer donor and a perylene\ndiimide (PDI) acceptor in two different configurations: one with the PDI closely interacting\nwith the electron-rich BDT unit (TS-P2), and the other where it is more closely associated\nwith the electron-deficient BDP moiety (TS-P3). They observe sub-picosecond charge\nseparation in TS-P3, while for TS-P2 they show charge separation in <15 fs, which is less\nthan the vibrational period of the high-frequency Raman-active transitions available. The\nresults are high quality and the analysis is sophisticated, and the understanding\nproposed will be of significant interest to the field. While the proposed explanation is\n41\ncompelling, there are numerous unanswered questions that need to be addressed before\nthis work can be considered publishable in Nature Communications.\nResponse: We thank the reviewer for their thoughtful and constructive evaluation of our\nmanuscript. We are especially encouraged by the positive remarks that “The results are\nhigh quality and the analysis is sophisticated, and the understanding proposed will be of\nsignificant interest to the field.” We appreciate that the reviewer recognises the novelty of\nour model donor–acceptor systems and the mechanistic insights into ultrafast charge\nseparation that we provide. Their acknowledgement of the sophistication of our analysis\nand the potential impact of our findings on the wider community is highly motivating.\nMajor issues:\nTo address the queries point by point we have divided the first comment into multiple\nsub-comments.\n-The authors central claim is that charge transfer here is fast while still directly occurring\nfrom the local exciton state of the polymer and not an optically accessible charge-\ntransfer state. They have performed photothermal deflectance spectroscopy\nmeasurements and DFT calculations to rule out this possibility.\nWe agree with the reviewer that one of the key output of this work is formation of the CT\nstate in ultrafast sub-vibrational period timescale directly from the local exciton state of\nthe polymer without accessing a optically accessible charge transfer state. The key\nsupporting evidence for the information is as reviewer mentioned\n Absence of the sub-bandgap CT transition revealed by the photothermal\ndeflection spectroscopy (PDS)\n By exciting the sample with broadband red pump when the CT transitions are in\nturn ~102 weaker than the singlet LE transition.\n Additionally we don’t observe any PDI-specific mode during the cw-Resonant\nRaman spectroscopy with 785 nm excitation. (figure 3b(5))\n We also don’t observed any PDI-specific mode at the PDI ground state bleach\nregime (fig. 3b(6)). We only observed polymer backbone modes.\ndirect excitation of a charge-transfer (CT) state does not occur, in the decision summary\ntable in the supplementary information 44 which is added in the revised manuscript.\nHowever, the photothermal deflectance spectroscopy was performed under quite\ndifferent conditions (i.e. in a high polarity environment and in a solvent where aggregation\nis discouraged), and so this is not conclusive.\nResponse: We thank the reviewer for raising this important point and agree that solution-\nphase PDS differs from the more commonly implemented film-based measurements,\nparticularly with respect to aggregation and local morphology. In our study, we\nintentionally performed PDS in chloroform solution—following the methodology of Sun\net al. (Nature 615, 830–835, 2023)—to ensure full consistency with our transient\nabsorption and impulsive vibrational experiments, which were all carried out in the same\nsolvent environment.\nWe acknowledge that film-based PDS could provide complementary information;\nhowever, it would also introduce additional inter-chain aggregation- and morphology-\ndependent variables that could obscure the specific donor–acceptor orientation effects\nwe aimed to isolate. For this reason, we selected the solution-phase configuration to\nprovide a cleaner, internally consistent comparison between the other experiments such\nas IVS, TA performed on the two model heterojunction studied here.\nAction:\nThe following part has been added in the method section of the main manuscript.\nRevised main manuscript excerpt\n“Photothermal deflection spectroscopy (PDS) was performed in chloroform solution,\nfollowing the approach of Sun et al. (Nature 615, 830–835, 2023). We chose the solution-\nphase method to ensure consistency with the solvent environment used in our transient\nabsorption and vibrational experiments, and to avoid additional aggregation or\nmorphology effects that can complicate interpretation in thin-film measurements.”\ncould miss such interactions.\nResponse: We agree with the reviewer that DFT has its own drawbacks, especially when\nused for excited-state calculations, and might miss some important effects depending\non the choice of the exchange-correlation functional. However, in this work, we were\ninterested in a qualitatively accurate description of the CT and Frenkel excitonic states,\nand for that reason, we employed a state-of-the-art scheme, which uses a range-\nseparated hybrid functional (specifically designed for the characterization of long-range\neffects as CT states, which are not fully captured by hybrid functional like B3LYP) and\nintroduces screening effects to mimic the environment. We are thus confident that the\ninteractions the reviewer is referring to are well included in our method. Regarding the\nbasis set, for excited-state calculations we used a 6-311G(d,p) basis set, which is a quite\nlarge basis set, even considering the size of our system (> 250 atoms). Our data suggests\nthat oscillator strengths of the charge-transfer (CT) transitions are roughly 800 times\nweaker than those of the lowest Frenkel excitonic transitions, supporting the assignment\nof the 1283 cm ¹ vibrational coherence as not originating from any directly excitable CT\ntransition, as shown below.\n⁻\nAction:\nSupplementary Fig. 2.2| Calculated electronic transitions of the DPP-BDT dimer of the TS-P3 architecture. Asterisked mode\ncorresponds to the Frenkel exciton (FE). excited-state calculations is performed using 6-311G(d,p) basis set with range-\nseparated hybrid functional (specifically designed for the characterization of long-range effects as CT states) which further\nreveal oscillator strengths of the charge-transfer (CT) transitions are roughly 800 times weaker than those of the lowest\nasterisked Frenkel excitonic transitions.\nGenerally, the UV/vis spectra usually give some indication of donor-acceptor interactions\nthat give rise to new CT bands, but these data (Figure 1c) are not discussed in a\n44\nmeaningful way in this context. Specifically, there are substantial differences between\nthe absorption spectra of TS-P2 and the constituent Ref-P/PDI spectra that are not\ndiscussed. The splitting and red-shifting of the low-energy polymer absorption band\ncould easily indicate aggregation of the polymer backbone or charge-transfer\ninteractions between the PDI and BDT. Similarly, though less drastically, the absorption\nof TS-P3 shows broadening of the vibronic progressions of both PDI and the polymer\ntransitions. These differences in spectra lead to questions about aggregation and\ndisorder that should be addressed: how rigid are these structures and what role does\nflexibility and overall disorder have on the dynamics? Are the polymer molecular weights\n(including those within the distribution) sufficient to allow additional interactions with the\nPDI unit? A discussion of the length of the polymer in the main text would add to the rigor\nof the discussion of the absorption spectrum. But moreover a better discussion of the\nUV/vis spectra and whether the clear differences in the spectra are indicative of ground\nstate electronic interactions should be given.\nResponse: For TS-P2 and TS-P3, the spectrum can be divided into the 450–570 nm (2.2–\n2.7 eV) region, where the PDI chromophore absorbs, and the 570–800 nm (1.5–2.2 eV)\nregion, which corresponds with absorption from the conjugated polymer backbone. Both\nRef-P2 and TS-P3 have absorption maxima at ~760 nm (~1.63 eV) with a vibronic shoulder\nat ~690 nm (~1.80 eV). The absorption spectrum of TS-P2 shows an inversion of the “0–0”\nand “0–1” intensities, such that the maximum is found at ~690 nm with a smaller peak at\n~760 nm. We have previously studied the origin of the inversion of these absorption peaks\nwhere we reported the initial synthesis paper1. As shown in Fig 5 and 6 of the\nsupplementary information in this manuscript1, the different ratios of these two peaks\nwas captured using our xtb-ensemble approach which included vibronic contributions.\nImportantly, the difference in ratios of the two absorption bands was dictated by the\nconformational changes to the polymer backbone and not any intermolecular (or\nalternatively same polymer chain intramolecular) aggregation effects. Indeed we also\nfound that TS-P3 showed greater conformational disorder than TS-P2 (Fig 7 – SI of that\npaper1). In addition to this, we also reported the concentration dependent UV-Vis and we\ndid not observe an substantial changes further suggesting that this is not an\nintermolecular effect. Finally, we note that when planning the synthesis of these\n45\nmaterials, we introduced the bulky chain on the rear of the PDI to discourage any\nintramolecular PDI stacking. The reviewer asks an interesting question with respect to\nthe role of flexibility and disorder on the dynamics. Currently this work is ongoing and\nsubsequent generations of these materials are being designed to possess similar\nenergetics + distance but different levels of rigidity but these materials are extremely\ncomplex to synthesize and beyond the scope of this investigation.\nAction:\nWe have added a sentence in the main text saying\n“The origin of the different intensities of the 0-0 and 0-1 peak in the UV-Vis absorption\nspectra have been discussed previously (reference1) and we believe they predominantly\narise for a greater degree of conformation disorder in TS-P3 relative to TS-P2.”\n-It is not clear how these results translate into solid-state design rules for better organic\nphotovoltaic (OPV) device performance. Rapid charge separation from polymers to non-\nfullerene acceptors, including PDI is not novel and not inherently reliant on the\nmechanism proposed here. While this mechanistic approach will certainly be interesting\nto the community, it is not clear how these effects can be leveraged in devices. Better\ncontextualization of these results is necessary to increase their impact and utilization by\nthe community. Related, there are significant examples in Nature where fast, high yielding\ncharge transfer occurs with minimal energy loss, such as the special pair in\nphotosynthesis. This strategy has even been adapted in artificial systems for OPVs\nutilizing symmetry-breaking charge transfer [J. Am. Chem. Soc. 2015, 137, 16, 5397–\n5405]. A difference, of course, is that in those systems there are frequently indications of\nstrong electronic coupling in the ground state, except in the case of null-aggregates which\nare known to exhibit excited-state symmetry breaking. In those and related cases, the role\nof vibrations has been investigated [J. Am. Chem. Soc. 2022, 144, 34, 15539–15548, Nat.\nChem. 14, 786–793 (2022)], but this context is lacking here. As such, the notion that\nenergy loss and charge transfer rates are \"fundamental barriers\" are overstatements.\nhas greatly strengthened the conceptual framing and broader impact of our manuscript.\nWe fully agree that positioning our results alongside the well-established framework of\nsymmetry-breaking charge transfer (SBCT) and related vibronic-coupling phenomena\nprovides valuable context. Seminal contributions from Kim, Wasielewski, Würthner,\nThompson and co-workers (J. Am. Chem. Soc. 2015, 137, 5397; Nat. Chem. 2022, 14,\n786; J. Am. Chem. Soc. 2022, 144, 15539) have elegantly shown that strong ground-state\nelectronic coupling, and in some cases null-aggregate configurations, can enable the\ngeneration of charge-transfer states with minimal energy loss. In contrast, our study\nextends this mechanistic concept to weakly coupled polymer–acceptor heterojunctions,\nwhich lack measurable ground-state coupling or aggregate order. The model system is\ndeliberately designed using a low-bandgap push–pull polymer and a non-fullerene\nacceptor with minimal energetic offset, yielding regio-selective and exceptionally rapid\n(< 15 fs) electron transfer. Here, the transfer proceeds through coherent mixing of the\nlocally excited (LE) and charge-transfer (CT) states in the excited manifold, rather than\nthrough ground-state delocalisation. In TS-P3, selective coupling between the donor’s\nhigh-frequency (1512 cm ¹) DPP-localised vibration and the electronic gap drives this\nsub-vibrational-period transfer, demonstrating that coherence-mediated charge transfer\n⁻\ncan occur even in electronically disordered systems. We believe this observation\ncomplements and broadens the SBCT paradigm—showing that mode-specific vibronic\ncoupling can be purposefully engineered to promote ultrafast, low-loss charge\nseparation in realistic solid-state environments where strong static coupling is absent.\nIn response to the reviewer’s valuable feedback, we have revised both the Introduction and\nDiscussion of the results to explicitly link our findings to the SBCT framework, highlight\ntheir relevance for OPV material design, and acknowledge the pioneering work of the\nabove-cited studies. We have also refined the phrasing from “fundamental barriers” to\n“prevailing limitations” to maintain a balanced perspective. We are grateful to the\nreviewer for prompting these additions, which have notably improved the clarity, rigour,\nand contextual significance of the paper.\nAction\n47\n The Introduction has been revised to clearly contextualise the symmetry-breaking\ncharge transfer (SBCT) mechanism in relation to efficient charge-transfer (CT)\nstate formation with minimal energy loss.\n The Discussion now explicitly connects the present findings to the broader SBCT\nand vibronic-coupling literature, with appropriate citations to key studies.\n The phrase “fundamental barriers” has been rephrased as “prevailing limitations”\nto maintain a balanced and precise interpretation.\nRevised main manuscript excerpt\nIntroduction section: Traditionally, the rate of charge transfer in light harvesting devices\nis believed to be primarily governed by the electronic coupling and energy offset between\nthe frontier orbitals between donor and acceptor molecules51,52. Recent studies on\nsymmetry-breaking CT (SBCT) in covalent dimers and ordered aggregates have shown\nthat strong electronic coupling can enable ultrafast, low-energy-loss CT44–46.However, the\nrole of vibrational modes—whether they assist, limit, or modulate the charge transfer\nprocess in weakly coupled organic heterojunctions—remains poorly understood.\nUnderstanding this interplay is essential for identifying the fundamental speed limits of\nphotoinduced charge separation in molecular heterojunction based photovoltaic\ndevices.\nDiscussion section: In contrast, the FE energy plunges when moving along the 1529 cm-1\nmode leading to (avoided) crossings with the lower-lying CT excitations and a massive\nreorganization of the oscillator strength, see Figure 4b. In a way, this is expected since\nthis vibrational mode has a large amplitude over the regions of the backbone in close\ncontact with the PDI acceptors. Thus, the theoretical calculations support the view that\nthe 1529 cm-1 mode drives the electron transfer from the polymer backbone to the nearby\nPDI units, by causing a mixing of the singlet exciton and CT PES surfaces. This is\nillustrated in Figure 4c. Hence it is this driving mode that undergoes a large reduction of\nintensity in the IVS spectra. This mechanism is conceptually analogous to the vibronic\ncoherence driven coupling between locally excited and charge-transfer states reported\nin symmetric molecular dimers showing symmetry-breaking charge transfer (SBCT)45,46,\n48\nbut here it operates within an asymmetric, weakly coupled heterojunction which lacks\nmeasurable ground state coupling.\nConclusion section: ………where our results suggest that previously established design\nrules about the tradeoff between energy loss and charge transfer rates may not represent\nfundamental barrier prevailing limitations to improve further materials design strategies.\n-The ultrafast nature of the charge transfer in TS-P3 being driven by the 1512 cm-1 polymer\nmode is intriguing, so the natural question is how general is this conclusion? The authors\nstate that this mode is localized mostly in the DPP unit, which seems consistent with the\ndifference in dynamics between TS-P2 and TS-P3, however, this is not emphasized in the\ndiscussion. Indeed, it This is disappointing, as it may point towards the generality of this\nmechanism and its use in designing more efficient charge transfer systems. This element\nshould be discussed more. Does the solvent play any specific role, either through inertial\nor dielectric stabilization or through coupling to the solute modes? Such effects would\nnot be present in the solid-state device so this potential effect is worth clarification.\nResponse:\nWe thank the reviewer for this thoughtful and constructive comment. We agree that\nassessing the generality and environmental dependence of the 1512 cm ¹–driven charge-\ntransfer (CT) mechanism is crucial for evaluating its broader relevance to organic\n⁻\nphotovoltaic (OPV) systems.\nWe would like to emphasize that our present study is aimed at understanding the\nfundamental interfacial processes that determine the initial charge-separation\ndynamics, rather than optimizing device-level power conversion efficiency. Modern OPV\nheterojunctions typically consist of low–bandgap push–pull polymers paired with non-\nfullerene acceptors, giving rise to multiple interfacial configurations. This structural\nheterogeneity is deliberately mimicked in our two model heterojunctions, TS-P2 and TS-\nP3. As the reviewer correctly notes, the distinct behaviour of these two systems\ndemonstrates the robustness of our conclusions: in TS-P3, where the PDI acceptor is\npositioned adjacent to the electron-deficient DPP unit, the DPP-localised 1512 cm ¹\n⁻\nmode couples directly to the electronic gap, driving sub-15 fs electron transfer. In\ncontrast, in TS-P2, where the DPP moiety is spatially separated from the PDI, this vibronic\ncoupling is much weaker, resulting in slower (≈ 376 fs) charge transfer. Thus, the\ngenerality of the mechanism lies in this regio-specific control — when the driving\nvibrational mode is co-localised with the donor–acceptor interface, ultrafast charge\ntransfer is universally enabled. As the reviewer suggested this element is discussed in the\nrevised manuscript.\nRegarding the influence of the solvent, all measurements were performed in a non-\ncoordinating, weakly polar medium. Exploring solvent- and linker-dependent effects is\nan active direction of our ongoing work, involving variations in both the solvent polarity\nand the length of the non-conjugated tether between donor and acceptor. These results\nwill be presented in a forthcoming publication. However, it is worth emphasizing that this\nstudy focuses on the fundamental role of interfacial arrangement in governing charge-\ntransfer dynamics using well-defined model heterojunctions. While these systems are\ndesigned to emulate the key electronic configurations found in real OPV interfaces,\nreplicating such precisely controlled molecular geometries in device-scale architectures\nlies beyond the scope of the present work. This clarification has been added to the\nrevised manuscript.\nAction:\nRevised main manuscript excerpt\nIntroduction section: ……..which we will refer to as TS-P2 (‘acceptor-on-electron rich\nsystem’) and TS-P3(‘acceptor-on-electron deficient system’) architectures, respectively\n(Figure 1b, for synthetic protocols see supplementary methods and ref1). These model\nsystems allow us to isolate the dynamics of the CT process, which can be masked via\nexciton and charge-transport processes in OPV thin-films or by molecular diffusion in\nphotocatalytic systems53–56. We emphasize that this study aims at elucidating the\nfundamental interfacial processes governing initial charge-separation dynamics, rather\nthan optimising device-level power-conversion efficiency.\nthe generality of this vibronic-driving mechanism. When the high-frequency DPP-\nlocalised vibration (1512–1529 cm ¹) coincides with the donor–acceptor interface, as in\nTS-P3, it directly modulates the CT gap, enabling sub-15 fs electron transfer. In TS-P2,\n⁻\nwhere this mode is spatially displaced from the PDI acceptor, the coupling is weaker and\nCT slows markedly (≈ 376 fs). This demonstrates a general design rule: co-localising\ninterfacially active vibrational modes with the electronic coupling region enables\nultrafast CT even in weakly coupled systems.\n-The CW Raman spectra results for PDI where the 1283 cm-1 mode is absent are\ncompelling evidence for the lack of ground-state excitation of the PDI. However, the\nsignal-to-noise ratio of these data is not particularly high and there may be indications of\nthis missing feature close to the noise floor.\nResponse: We thank the reviewer for this helpful comment and for recognising that the\nabsence of the 1283 cm ¹ mode in the cw-Raman spectrum supports the lack of direct\nground-state excitation of any charge-transfer complex involving the PDI moiety. We have\n⁻\nrepeated the 785 nm excitation cw-Raman measurements, and the unsmoothed raw\ndata are now included in Supplementary Fig. 45. We agree that the signal-to-noise ratio\nis inherently lower than for 532 nm excitation, as expected from the 1/λ⁴ dependence of\nRaman scattering efficiency (≈5× weaker at 785 nm) and the presence of a fluorescence\nbackground. Nevertheless, the new measurements—performed under multiple\nexperimental configurations—consistently show no detectable 1283 cm ¹ feature within\nthe experimental noise limits, reinforcing our original conclusion. This finding is further\n⁻\nsupported by the 532 nm cw-Raman data, where strong PDI modes are clearly observed\neven at 5% PDI loading suggesting a strong Raman cross-section of that mode when it is\nresonantly excited. This supports that any ground-state CT feature at 785 nm would be\nwell above the current detection limit if it existed. To aid clarity, we have also included a\ndecision-tree summary in the Supplementary Information (supplementary Fig. 44) that\n51\noutlines all experimental and analytical steps used to assign the 1283 cm ¹ mode to the\nexcited-state PDI radical anion.\n⁻\nAction:\nRevised main manuscript excerpt\nUtilizing continuous wave resonance Raman with 785 nm excitation on TS-P3, we do not\nobserve the (1283 cm-1) mode (Figure 3b(5)) , even though the PDI modes—including\n𝛽\nthe analogous 1291 cm ¹ band—exhibit strong Raman cross-sections when excited\n𝜔\nresonantly (Supplementary Note 19).\n⁻\nRevised supplementary information excerpt\nSupplementary Note 19: Additional cw-Raman Analysis of PDI and TS-P3 Derivatives\nTo verify whether the 1283 cm ¹ mode observed in the time-resolved measurements\ncould originate from directly excitable PDI species (any ground-state charge-transfer (CT)\n⁻\ncomplex), we performed comparative continuous-wave (cw) Raman spectroscopy using\nboth 532 nm (resonant with PDI) and 785 nm excitation which should be resonant o any\nground state CT complex if exists.\nThe spectra of TS-P3, TS-P3-5 %, and TS-P3-10 % (where PDI loading is reduced to 5 %\nand 10 %, respectively, please see Supplementary Fig. 7 for chemical structure) exhibit\npronounced resonance-enhanced bands associated with the PDI chromophore (marked\nby red asterisks). Even at 5 % loading, the PDI modes strongly dominate over the polymer\nbackbone features (black asterisks), confirming the strong Raman cross-section of PDI\nmodes under resonant excitation. The strong enhancement of these bands at very low\nconcentrations demonstrates the high sensitivity (Raman cross section) of the cw-\nRaman experiment to PDI vibrations when resonantly excited.\nUnder 785 nm excitation, TS-P3 spectra show markedly weaker overall Raman\nintensities, as expected for off-resonant conditions where the Raman scattering\nefficiency scales as 1 / λ⁴ (≈5× weaker at 785 nm than 532 nm). The signal-to-noise ratio\nis further reduced because of the fluorescence background. Importantly, no Raman\nfeature is detected near 1283 cm ¹ within the 3σ noise level, even in the TS-P3 spectrum\n⁻\n52\n(black trace, Fig. 45). The noise level was estimated from the flat baseline region (1700–\n2000 cm ¹), confirming no significant band at 1283 cm ¹ with an amplitude > 3σ.\n⁻ ⁻\nThe combination of (i) strong, clearly detectable PDI modes under resonant excitation\neven at 5 % PDI content and (ii) the complete absence of any detectable feature near\n1283 cm ¹ at 785 nm off-resonant excitation provides compelling evidence that no\nground-state CT complex or directly excitable PDI species exists with an appreciable\n⁻\ncross-section near 785 nm. If such a complex were present, the 1283 cm ¹ mode would\nbe expected to under these conditions.\n⁻\nThese results therefore confirm that the 1283 cm ¹ vibrational coherence observed in the\ntransient absorption experiments originates not direct excitation of the PDI involved CT\n⁻\ncomplex.\nFigure 45. cw-Raman spectra of PDI and TS-P3 under 532 nm and 785 nm excitation. Resonant 532 nm excitation\n(blue/orange) shows strong PDI modes (red asterisks), even at 5 % loading, while off-resonant 785 nm excitation\n(black) shows no detectable 1283 cm ¹ feature within the 3σ noise level (estimated from 1700–2000 cm ¹). This\nconfirms the absence of any directly excitable ground-state CT complex.\n⁻ ⁻\n53\n-Can the authors perform a short-time Fourier transform on the transient absorption data\nto see how the vibrational modes behave in time [J. Am. Chem. Soc. 2022, 144,\n15539−15548]? This may better help demonstrate their proposed mechanism.\nResponse: We thank the reviewer for this valuable and constructive suggestion. We\nperformed a time-domain analysis of the vibrational modes after applying narrow-band\nbandpass filtering. The results reinforce our earlier finding that the 1512 cm ¹ mode—\npreviously identified as the driving vibration for electron transfer from CW-resonant\n⁻\nRaman, resonant IVS, and quantum-chemical analyses—exhibits a markedly faster\ndephasing time. Following the reviewer’s recommendation, we also conducted a short-\ntime Fourier transform (STFT) analysis using an approach consistent with the cited\nreference. The TS-P3 IVS data reveal a clear lag in the appearance of the 1283 cm ¹ mode,\nsupporting its assignment as a vibration generated by the electron-transfer process\n⁻\nrather than by direct impulsive photoexcitation. Owing to space constraints, a detailed\ndescription of these analyses is provided in a dedicated section of the Supplementary\nInformation, which is now cited in the main manuscript.\nAction:\nRevised main manuscript excerpt\n“The short time FT analysis (Supplementary Fig. 47) shows a clear temporal lag in the\nappearance of the mode, confirming that it arises from charge-transfer dynamics\n𝛽\nrather than direct impulsive excitation. ”\n𝜔\n“This is further supported by the faster decoherence of the 1507 cm ¹ mode in TS-P3\nrelative to Ref-P, whereas the other Franck–Condon-active modes exhibit comparable\n⁻\ndephasing times in both systems (supplementary figure 46) in inverse FT analysis.”\nRevised supplementary information excerpt\nSupplementary Note 20: Narrowband filtered inverse FFT and short time FFT (STFT)\nanalysis:\nResonant Raman spectra of the Ref-P, TS-P2, TS-P3 at the high frequency regime (excitation wavelength, 785 nm). b,\nIntegrated Fourier transform IVS spectra for the probe wavelength range, λ = 890-920 nm which corresponds to the\npolymer backbone chromophore S1→Sn photo-induced absorption c,d,The transient absorption signal in the 890–930\nnm probe range was first detrended using a 200 fs moving-average window to isolate coherent residuals for TS-P3(c)\nand Ref-P(d). The residual trace was then narrow-band filtered in the frequency domain within two wavenumber\nwindows (1400–1450 cm ¹ and 1500–1550 cm ¹) by zeroing all other spectral components and performing an inverse\nfast Fourier transform (iFFT). This procedure retrieves the oscillatory components associated with individual\nvibrational modes, while t⁻he analytic-signal (Hi⁻lbert transform) envelope provides their temporal dephasing behaviour.\nThe faster decay of the 1500–1550 cm ¹ component compared with the 1400–1450 cm ¹ band indicates a shorter\ncoherence lifetime of the mode driving electron transfer in TS-P3.\n⁻ ⁻\nWe analysed the photoinduced absorption region (890–920 nm) corresponding to the S₁\n→ S transition in the polymer, as presented in the main text. As shown in Figure 3d (main\nfigure) and reproduced in Supplementary Figure 46a, a significant reduction in the FFT\nₙ\namplitude of the 1507 cm ¹ mode is observed for TS-P3 compared with TS-P2 and Ref-P.\nIn contrast, the CW-resonant Raman spectra of all three systems exhibit nearly identical\n⁻\n55\nvibrational features (Figure 3e, reproduced as Supplementary Figure 46b). This indicates\nthat the loss of intensity in the 1507 cm ¹ mode for TS-P3 originates from vibrational\ndecoherence processes occurring within the excited-state manifold, consistent with\n⁻\nprevious observations in small-molecule systems in solution22,57.\nTo further examine the time-domain behaviour of these modes, we applied a narrow-\nband frequency mask and retrieved the corresponding temporal profiles via inverse FFT,\nfollowing the procedure described in Ref58. The resulting data are shown in\nSupplementary Figures 46c (TS-P3) and 46d (Ref-P). The dephasing times extracted for\nTS-P3 are 1182 fs in the 1400–1450 cm ¹ region and 870 fs in the 1500–1550 cm ¹ region,\nwhile for Ref-P the corresponding values are 1139 fs and 1041 fs, respectively. These\n⁻ ⁻\nresults demonstrate that the 1512 cm ¹ mode in TS-P3 exhibits a faster dephasing rate\nthan in Ref-P, whereas the 1428 cm ¹ spectator mode shows comparable dephasing\n⁻\ntimes in both systems. This is consistent with the quantum chemical calculations\n⁻\nreported in Fig. 4 of the main text. The accelerated decoherence of the 1512 cm ¹ mode\nin TS-P3 supports its role as a driving mode in the electron-transfer process. Such rapid\n⁻\ndephasing of a driving vibrational mode is often attributed to nuclear nesting effects. We\nfurther note that this mode likely possesses an even faster component, as the electron\ntransfer occurs on a sub-15 fs timescale; however, the early-time dynamics are obscured\nby coherent artefacts and cross-phase modulation, which introduce high-frequency\noscillatory contributions.\nWe performed a narrow-band time-domain analysis and a short-time Fourier transform\n(STFT58) following the approach in ref58, using a 200 fs window with 20 fs overlap. The STFT\nspectrogram (Supplementary Fig. 47) shows a clear temporal lag in the appearance of\nthe 1283 cm ¹ mode, confirming that it arises from charge-transfer dynamics rather than\ndirect impulsive excitation. Complementary inverse FFT analysis of the 1400–1450 cm ¹\n⁻\nand 1500–1550 cm ¹ windows reveals faster dephasing of the 1512 cm ¹ DPP-localised\n⁻\nmode in TS-P3 (τ ≈ 870 fs) relative to Ref-P (τ ≈ 1040 fs), while the spectator 1428 cm ¹\n⁻ ⁻\nmode remains unchanged. These results reveal the distinct temporal evolution of\n⁻\nvibrational coherences, showing the 1283 cm ¹ mode as a product-state vibration\ngenerated upon charge transfer, and the analysis further support our assignment of the\n⁻\n1512 cm ¹ mode as the driving coordinate for the ultrafast electron-transfer process.\n⁻ 56\nFigure 47. Short-time FT (STFT) analysis of vibrational dynamics in TS-P3 at 890–930 nm probe window. (a)\nTransient absorption time trace (red) with its 200 fs moving-average background (blue) and the extracted residual\noscillations (orange). The 200 fs moving average method is used for subtracting the background to remove the low-\nfrequency component from the time domain data. The residual isolates coherent vibrational motion superimposed on\nthe electronic response. (b) Fast Fourier transform (FFT) of the residual signal showing distinct vibrational modes\nbetween 200 and 1800 cm ¹. (c) Short-time Fourier transform (STFT) spectrogram revealing the temporal evolution of\nthese modes. The dashed line marks the 1283 cm ¹ feature, characteristic of the mode activated by electron-transfer\ndynamics. The colour scale⁻ represents oscillation power (a.u.).\n⁻\nMinor issues:\n-The abstract is very vague and should refer to the real types of systems being discussed\nin the text.\nResponse: We thank the reviewer for this valuable comment. The abstract has been\nrevised to be more specific and representative of the studied systems:\n Specified the system as through-space polymer–non-fullerene (PDI–BDT–DPP)\nheterojunctions.\n57\n Added details on precisely controlled geometry, weak donor–acceptor coupling,\nand <100 meV energy offset.\n Identified a 25 fs high-frequency vibrational mode on the PDI acceptor’s potential\nenergy surface.\n Indicated that this DPP-centred mode drives ultrafast charge transfer through\nvibronic coupling.\nWe hope these revisions make the abstract clearer, more informative, and directly\naligned with the content of the manuscript.\nAction:\nRevised abstract:\nExcited-state charge transfer (CT) plays a crucial role in organic photovoltaics,\nphotocatalysis, and photodetection applications. Traditionally, rapid CT is thought to\nrequire a large energy offset and strong electronic coupling between donor and acceptor\nmolecules —conditions that maybe detrimental to overall device performance. The role\nof vibrations in driving CT and how this might ultimately govern rates of charge transfer in\norganic heterojunctions is still an open question. Here, we investigate through-space\npolymer–non-fullerene model heterojunctions in which a perylene diimide (PDI) acceptor\nis covalently tethered to a low-bandgap push–pull (BDT-DPP) polymer donor. These\nsystems feature precisely controlled intermolecular geometry and an exceptionally small\nenergy offset (<100 meV) between frontier orbitals, with weak donor–acceptor coupling\nin the Franck–Condon region. Despite the low-energy offset, we achieve a CT timescale\nless than 15 fs, which is accompanied via the launch of coherent wavepackets along the\ncoordinate of a high frequency mode of period 25fs, on the PDI acceptor’s potential\nenergy surface. Using evidence from steady-state and ultrafast electronic and vibrational\nspectroscopy, combined with electronic structure calculations we identify the specific\nDPP-centered driving modes that enable such unprecedented CT rates in purely organic\nsystems, by mixing the singlet Frenkel exciton state and CT state potential energy\nsurfaces following photoexcitation. Our results show that it is possible to achieve very\n58\nrapid CT, limited ultimately by the vibrational time-period of high-frequency modes,\ndespite the lack of large energy offsets or strong coupling between donor and PDI\nacceptor units in the ground state of the system. This has important implications for the\ndesign of organic materials for organic photovoltaic, photocatalysis and optoelectronics.\n-Figure 2a: \"S0-->Sn\" absorption is presumably \"S1-->Sn\" absorption as stated on page 7.\nResponse: We thank the reviewer for pointing out this mistake and apologise for the\noversight.\nAction: The label in Figure 2a has been corrected from “S₀ → S ” to “S₁ → S ” in the revised\nmanuscript.\nₙ ₙ\n-Figure 2b: What is the origin of the relatively slow rise in the Ref-P signal? This is also\npresent to a lesser extent in the TS-P2 data.\nResponse: We thank the reviewer for this comment. We assign the slow sub picosecond\nrise in the Ref-P signal to the dielectric relaxation or excited state solvation. The\nassignment is in supported by the push-pull dyes (monomeric analogues of the Ref-P)\nas seen before (ref59).\nAction:\nRevised supplementary information excerpt (Supplementary Note 17)\nSuch near-instantaneous transfer is consistent with the vibronically coherent regime\ndiscussed in the main text and is further supported by the 100 fs-resolved transient\nabsorption dynamics (Fig. 2b, main text), where slower sub-picosecond solvation\nrelaxation is evident in Ref-P and TS-P2 but absent in TS-P3. Together, these findings\nconfirm that the < 15 fs component represents the coherent electron transfer event rather\nthan subsequent solvent or structural relaxation.\n59\n-Figure 2d-f: It would be helpful to show a \"zero-line\" to guide the eye for when the\ntransient absorption signals cross change sign.\nResponse: We thank the reviewer for this helpful suggestion.\nAction: A zero-line has been added to Figures 2d–f in the revised manuscript to guide the\neye when the transient absorption signals cross through zero.\n-Figure 3b: what is the range of integration for the Fourier transform data?\nshould be stated explicitly.\nAction: We have clarified in the revised manuscript that the Fourier transform in Figure\n3b was performed over the range 40–1700 fs, and this information has been added to the\nmain text figure caption.\n-Page 9, line 214: \"TP-P3\" is presumably \"TS-P3\"\nResponse: We thank the reviewer for pointing out the typo.\nAction: We have corrected the ‘TP-P3’ to ‘TS-P3’.\n-Page 10/Supplementary Note 5: The agreement of the transient absorption spectrum\nwith the spectrum of the chemically reduced PDI is not great. What is the origin of this\ndisagreement? Aggregation is known to cause such broadening and shifting of anion\nspectra. Its use as supporting evidence is undermined by the lack of agreement and\ndiscussion.\nResponse: We thank the reviewer for highlighting this important point. We agree that the\nspectral differences between the transient absorption (TA) features and the chemically\nreduced PDI . reference require further clarification. As reported by other groups60,\naggregation of PDI-. anions typically leads to pronounced spectral broadening\n⁻\naccompanied by a 200–300 meV blue shift. In contrast, the chemically reduced PDI in our\nstudy shows peaks at 714 nm and 797 nm, whereas the transient PDI . features observed\nin TS-P3 appear at 734 nm and 825 nm—corresponding to moderate red-shifts of ≈ 46\n⁻\nmeV and 52 meV, respectively, together with some broadening.\nThe opposite direction of this shift indicates that aggregation is not the dominant origin\nof the discrepancy. Instead, the red-shift most likely arises from local environmental\neffects within the polymer–PDI heterojunction—specifically, electrostatic stabilization of\nthe PDI anion by the adjacent polymer hole (reflecting differences in counter-ion\ndistribution and solvation) and dielectric confinement in the nanoscale mixed phase.\nAdditionally, static conformational disorder within the polymer matrix may introduce a\ndistribution of effective electron–hole distances, contributing to the observed\nbroadening of the PDI anion band in the TA spectra.\nImportantly, following the reviewer’s helpful suggestion, we have now reassessed the\ncomparison used to discuss the 1283 cm ¹ mode. We directly compare the spectral\ndistribution of this vibrational coherence with the PDI feature retrieved from the\n⁻\ntransient absorption data, which shows excellent correspondence in both position and\n⁻\nshape (Supplementary Fig. 6b). The chemically reduced PDI spectrum is now retained\nonly as a qualitative reference. We thank the reviewer for prompting this improvement,\n⁻\nwhich has substantially strengthened the discussion in Supplementary Note 5 and\nclarified the excited-state origin of the 1283 cm ¹ mode.\n⁻\nAction:\nMain manuscript:\n“In contrast, the vibrational coherence at 1283 cm-1 ( ) obtained by resonant impulsive\n𝛽\nexcitation of the polymer backbone in TS-P3 shows its strongest amplitude at probe\n𝜔\nwavelengths between 650-850 nm (black spectra, Figure 3c, top). This maximum\namplitude closely resembles the absorption of the PDI radical anion (orange spectra,\nFigure 3c, top).”\nThe section above is replaced by-\n“In contrast, the vibrational coherence at 1283 cm ¹ ( ), obtained by resonant\n𝛽\nimpulsive excitation of the polymer backbone in TS-P3, shows its strongest amplitude at\n⁻ 𝜔\nprobe wavelengths between 650 and 850 nm (black trace, Fig. 3c, top). This spectral\nenvelope closely matches the PDI radical-anion feature retrieved from the transient\nabsorption data (red trace, Fig. 3c, top), and is further qualitatively complemented by the\nabsorption profile of the chemically reduced PDI species (blue trace, Fig. 3c, top).”\n⁻\nas a function of probe wavelength while resonantly exciting TS-P3 with the ‘red impulsive pump’. The orange filled blue\ndotted linearea represents the absorbance of PDI radical anion and the red line corresponds to the PDI anion\nabsorption retrieved from TA (supplementary note XX). (bottom) the FT amplitude for the vibrational coherence at 1295\ncm-1 mode as a function of probe wavelength while off-resonantly exciting PDI with the ‘red impulsive pump’.\nSupplementary Note 5:\nIn Supplementary Fig. 6, the differential transmission signal (ΔT/T) for the TS-P3 system\nis presented at 20 fs and 40 fs, represented by the solid dark blue and green curves,\nrespectively. As noted in Fig. 2d (main text), the Ref-P system exhibits no dynamics on\nthis ultrafast timescale. Therefore, the difference Δ(ΔT/T) = ΔT/T(20 fs) - ΔT/T(40 fs) should\npredominantly reflect the absorption of the PDI radical anion., provided there is minimal\nspectral contribution from other features, such as the hole polaron associated with the\npolymer backbone. It is important to note that the PDI radical anion spectrum\nqualitatively retrieved from the transient absorption (TA) data (red curve, Supplementary\nFig. 6) is both broadened and red-shifted relative to the chemically reduced PDI reference\n(blue shaded area, Supplementary Fig. 6a–b). The chemically reduced PDI exhibits\nabsorption peaks at 714 nm and 797 nm, whereas the transiently generated PDI features\nin TS-P3 appear at 734 nm and 825 nm—corresponding to moderate red-shifts of\n⁻\napproximately 46 meV and 52 meV, respectively, accompanied by noticeable broadening.\nThis red-shift likely originates from several factors. First, the chemical structure differs:\nin TS-P3 the PDI is covalently linked to the polymer backbone via methoxy-\ndimethylamine or 1,3-dimethoxypropane tethers (Supplementary Fig. 6c), which can\nmodify the frontier orbital energies compared with the isolated PDI-OMe reference.\nSecond, the charge-stabilizing environment is distinct. In the chemically reduced\nsample, the PDI anion is generated in a polar solvent using\ntetrakis(diethylamino)ethylene, where the counter-ions are fully solvated and spatially\nseparated. In contrast, within TS-P3 the electron transfer produces a tightly bound\n63\ncharge-transfer (CT) state in which the hole polaron on the polymer backbone acts as a\nproximal counter-charge. The resulting local electrostatic stabilization, partial solvation,\nand reduced dielectric screening can shift the PDI absorption to lower energy.\n⁻\nFinally, the broader spectral profile observed in the TA-retrieved spectrum can be\nattributed to static conformational disorder within the polymer matrix, which introduces\na distribution of effective electron–hole separations and hence a spread in local\nstabilization energies. Collectively, these effects explain the moderate red-shift and\nbroadening of the transient PDI spectrum relative to the chemically reduced reference.\n⁻\nAs the chemically reduced PDI spectrum serves only as a qualitative reference, we now\ndirectly compare the spectral distribution of the 1283 cm ¹ vibrational coherence (black\n⁻\ncurve, Supplementary Fig. 6b) with the PDI absorption feature retrieved from the\n⁻\ntransient absorption data (red curve). This comparison reveals a markedly better\n⁻\ncorrespondence, both in spectral position and in overall envelope shape, confirming that\nthe coherence is resonantly detected through the transiently generated PDI radical anion\nrather than through the ground-state PDI. Notably, the nodal behaviour near the peak of\nthe PDI absorption band arises from a phase inversion of the vibrational coherence\nsignal, as expected for modulation of the excited-state charge-transfer absorption. This\n⁻\nagreement provides strong evidence that the 1283 cm ¹ mode originates from the PDI\nradical anion formed upon ultrafast charge transfer.\n⁻\nSupplementary Fig. 6| Spectral characterization of the PDI radical anion: (a) The blue shaded are with dotted line\ncorresponds to the absorption spectra of chemically reduced PDI radical anion. (b) The PDI anion feature derived from\nTA and from chemical reduction is overlaid with the spectral intensity profile of the 1283 cm-1 mode (c) The PDI\nstructure that is used for the chemical reduction.\n64\n-Page 21, the figure call in the methodology section is incomplete.\nResponse: We thank the reviewer for pointing out the error in citing the SHG-FROG\ncharacterization in the methodology section.\nAction: The figure call has been corrected from “Supplementary Fig. xx12” to\n“Supplementary Fig. 5” on Page 21 of the methodology section in the revised manuscript.\nReferences\n1. Royakkers, J. et al. 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Comparison of vertical and adiabatic harmonic\napproaches for the calculation of the vibrational structure of electronic spectra.\nPhysical Chemistry Chemical Physics14, 13549 (2012).\n48. Cerezo, J. & Santoro, F. FCclasses3 : Vibrationally resolved spectra simulated at\nthe edge of the harmonic approximation. J Comput Chem44, 626–643 (2023).\n-\n49. Aarabi, M. et al. Quantum-Classical Protocol for Efficient Characterization of\nAbsorption Lineshape and Fluorescence Quenching upon Aggregation: The Case\nof Zinc Phthalocyanine Dyes. J Chem Theory Comput\nhttps://doi.org/10.1021/acs.jctc.3c00446 (2023) doi:10.1021/acs.jctc.3c00446.\n50. Worth, G. A. Quantics: A general purpose package for Quantum molecular\ndynamics simulations. Comput Phys Commun248, 107040 (2020).\n51. Kippelen, B. & Brédas, J. L. Organic photovoltaics. Energy and Environmental\nScience vol. 2 251–261 Preprint at https://doi.org/10.1039/b812502n (2009).\n52. Janssen, R. A. J. & Nelson, J. Factors limiting device efficiency in organic\nphotovoltaics. Advanced Materials vol. 25 1847–1858 Preprint at\nhttps://doi.org/10.1002/adma.201202873 (2013).\n53. Eastham, N. D. et al. Hole-Transfer Dependence on Blend Morphology and Energy\nLevel Alignment in Polymer: ITIC Photovoltaic Materials. Advanced Materials30,\n(2018).\n54. Zheng, Z. et al.Efficient Charge Transfer and Fine-Tuned Energy Level Alignment in\na THF-Processed Fullerene-Free Organic Solar Cell with 11.3% Efficiency.\nAdvanced Materials29, (2017).\n55. Menke, S. M., Luhman, W. A. & Holmes, R. J. Tailored exciton diffusion in organic\nphotovoltaic cells for enhanced power conversion efficiency. Nat Mater12, 152–\n157 (2013).\n56. Yuan, J. et al. Single-Junction Organic Solar Cell with over 15% Efficiency Using\nFused-Ring Acceptor with Electron-Deficient Core. Joule3, 1140–1151 (2019).\n57. Yoneda, Y. et al. Vibrational Dephasing along the Reaction Coordinate of an\nElectron Transfer Reaction. J Am Chem Soc143, 14511–14522 (2021).\n58. Hong, Y., Schlosser, F., Kim, W., Würthner, F. & Kim, D. Ultrafast Symmetry-\nBreaking Charge Separation in a Perylene Bisimide Dimer Enabled by Vibronic\n(2022).\n59. Liu, Z. et al. Intersystem Crossing in Acceptor-Donor-Acceptor Type Organic\nPhotovoltaic Molecules Promoted by Symmetry Breaking in Polar Environments.\nJournal of Physical Chemistry Letters13, 10305–10311 (2022).\n60. Li, H. & Wenger, O. S. Photophysics of Perylene Diimide Dianions and Their\nApplication in Photoredox Catalysis. Angewandte Chemie - International Edition\n61, (2022).\nReviewer #1 (Remarks to the Author):\nAfter the first round of reviewing, the manuscript has been improved substantially. The\nauthors have responded to the comments and suggestions of both reviewers. They have\nadded new measurements and new data analysis. This is very helpful and much\nappreciated.\nIn particular, I have enjoyed seeing the excited state quantum dynamics simulations\nwhich are now added as a new section in the Supplementary Information.\nAll in all, the extensive revisions made by the reviewers have supported the case of the\nauthors quite strongly. I think that the manuscript is now much closer to being accepted\nfor publication in Nature Communications.\nI want to ask the authors to fully address the following points before recommending\npublication of this article.\nResponse: We thank the reviewer for their careful re-evaluation of the manuscript and\nfor their thoughtful and positive assessment. We are pleased that the reviewer considers\nthe manuscript to be substantially improved following revision, and that the additional\nmeasurements, data analysis, and the newly added excited-state quantum dynamics\nsimulations in the Supplementary Information are regarded as valuable and informative.\nWe appreciate the reviewer’s recognition that these extensive revisions have\nstrengthened the overall case for the work.\nWe address each of the remaining points raised by the reviewer in detail below.\na) I very much appreciate the details that are now given about the data analysis (SFig 41).\nI must say that I have my doubts about the claim ET time of 14.9 +/- 1 fs. I understand that\nthe subtraction of coherent artefacts is challenging and U see that this has been done\ncarefully. Yet, I think that red curve in 41b will also describe the data if an ET time of 20 or\n25 fs is assumed (depending on how one takes into the account remaining “artefacts”. I\n1\nthink that it is fair that the ET is fast (< 25 fs or so) but I do not think that the ET time can be\nextracted with one fs precision. I request to change this.\nResponse: We thank the reviewer for raising this important point. We agree that the\nextracted ultrafast electron-transfer timescale can be sensitive to the temporal window\nused in the analysis. As shown in the figure, when different upper time limits are\nconsidered, the fast time constant is distributed between approximately 12.8 fs and 20.1\nfs.\nThis variation arises because, following the rapid electron-transfer event, a weak slower\nsignal component is present, which we attribute to dielectric relaxation of the newly\nformed charged species. To account for this effect more rigorously, we have extended the\nfitting window to 2000 fs. After careful subtraction of the coherent artefact, the kinetics\nwere analysed using a bi-exponential model. This yields a fast component of 18.1 ± 3.1\nfs, with a conservative upper bound of 21.2 fs, fully consistent with the reviewer’s\nsuggestion that the electron transfer is ultrafast (< 25 fs) but cannot be determined with\none-femtosecond precision. Importantly, this refinement does not alter the physical\ninterpretation of the results: the electron-transfer process remains faster than the period\nof the coherently generated vibrational mode (~26 fs), and therefore still occurs within\nthe vibrationally coherent regime discussed in the manuscript. We have revised the\nrelevant sections of the main text and Supplementary Information accordingly and now\nreport the bi-exponential analysis throughout.\nAction:\nThe resulting differential kinetics (Supplementary Fig. 41b) were fit with a mono-exponential\ndecay function, yielding a time constant of τ = 14.9 ± 1 fs. This value represents the\nexperimentally resolvable upper limit of the CT timescale, approaching the 11 fs instrument\nresponse function. The resulting differential kinetics (Supplementary Fig. 41b) were analysed\nusing a biexponential model comprising a dominant ultrafast component and a much weaker\nslower contribution. The fast component is attributed to electron transfer from the polymer\nbackbone to the PDI acceptor, while the slower component likely reflects secondary processes\nfollowing charge separation, such as dielectric or structural relaxation of the newly formed\ncharge-separated state. This analysis yields a fast characteristic timescale of 18.1 ± 3.1 fs,\ncorresponding to a conservative upper bound of 21.2 fs for the electron-transfer process.\nImportantly, this analysis supports the physical interpretation of the results: the electron-\ntransfer process is faster than the period of the coherently generated vibrational mode (~26 fs),\nconfirming that charge transfer occurs on a sub-vibrational timescale.\nSupplementary Fig. 41 | b, Difference trace (TS-P3 – Ref) fitted with a dominant ultrafast decay component (red line),\ncorresponding to an electron-transfer timescale of ~18 fs (upper bound ~21 fs).\nMain excerpt\nAll mentions of the electron-transfer timescale in the main text have been revised for\nconsistency with the updated analysis. For example:\n… which yielded a fast component of 18.1 ± 3.1 fs, corresponding to a conservative upper\nbound of ~21.2 fs for the electron-transfer process a mono-exponential decay with τ = 14.9 ±\n1 fs, establishing an upper limit for the electron-transfer process\n3\nb) I guess that the time axis in SI Fig. 4 should be “ps” – not “fs” ;-)\nResponse: We thank the reviewer for identifying this error. The time axis in\nSupplementary Fig. 4 was incorrectly labelled; the correct unit is ps rather than fs.\nAction: This has now been corrected in the revised Supplementary Information.\nc) The title in SFig. 49 should probably read “with/without linkers”.\nResponse: We thank the reviewer for noting this typographical error.\nAction: The title of Supplementary Fig. 49 has been corrected to read “with/without\nlinkers.”\nd) I acknowledge the discussion about Herzberg Teller couplings and agree qualitatively\nwith what is said. Yet, I think that a much more careful analysis of the (possibly short-\nlived) excited state vibrations is needed before conclusions can be drawn. I would\ntherefore suggest not to make too many strong statements about the origin of these\nvibrational coherences.\nResponse: We thank the reviewer for this important point and agree that caution is\nrequired when interpreting the origin of short-lived excited-state vibrational coherences.\nIn the revised manuscript, we no longer make definitive statements assigning the\nobserved coherences to a specific Herzberg–Teller coupling mechanism. Instead, the\ndiscussion is framed more conservatively, emphasising that the observations are\nconsistent with vibronic coupling without uniquely establishing a single microscopic\norigin.\nIn addition, we have included further time–frequency analyses (narrowband-filtered\ninverse FFT and short-time Fourier transform) in the revised Supplementary Information\n(Supplementary Note 20) to better characterise the vibrational coherence, while\nexplicitly noting the limitations of such methods at the earliest time delays. Those\nanalysis further supported the mechanism that is proposed. As suggested by the\n4\nreviewer, we have moderated the strength of several mechanistic statements in the main\ntext.\nWe hope that these revisions adequately address the reviewer’s concern.\nAction: Relevant changes in the main text are highlighted in red.\nSince the authors have revised the manuscript very thoroughly, I gladly recommend\npublication of this article after these minor points have been carefully addressed.\nWe hope that the remaining minor points raised by the reviewer have now been fully\naddressed, and we sincerely thank the reviewer for their careful reading and constructive\nfeedback.\nReviewer #2 (Remarks to the Author):\nThe authors have made significant efforts to address the reviewer comments, and the\nmanuscript is much improved as a result. The manuscript is suitable for publication in\nNature Communications.\nResponse: We thank the reviewer for their careful re-evaluation of the manuscript and\nfor their positive assessment. We are pleased that the revisions have addressed the\nreviewer’s concerns and that the manuscript has been substantially improved as a result.\nWe appreciate the reviewer’s time and constructive feedback, which have strengthened\nthe clarity and quality of the work.\n5"}, {"round": 2, "reviewer_comments": {"reviewer_1": "After the first round of reviewing, the manuscript has been improved substantially. The authors have responded to the comments and suggestions of both reviewers. They have added new measurements and new data analysis. This is very helpful and much appreciated.\nIn particular, I have enjoyed seeing the excited state quantum dynamics simulations which are now added as a new section in the Supplementary Information.\nAll in all, the extensive revisions made by the reviewers have supported the case of the authors quite strongly. I think that the manuscript is now much closer to being accepted for publication in Nature Communications.\nI want to ask the authors to fully address the following points before recommeding publication of this article.\na) I very much appreciate the details that are now given about the data analysis (SFig 41). I must say that I have my doubts about the claim ET time of 14.9 +/- 1 fs. I understand that the subtraction of coherent artefacts is challenging and U see that this has been done carefully. Yet, I think that red curve in 41b will also describe the data if an ET time of 20 or 25 fs is assumed (depending on how one takes into the account remaining “artefacts”. I think that it is fair that the ET is fast (< 25 fs or so) but I do not think that the ET time can be extracted with one fs precision. I request to change this.\nb) I guess that the time axis in SI Fig. 4 should be “ps” – not “fs” ;-)\nc) The title in SFig. 49 should probably read “with/without linkers”.\nd) I acknowledge the discussion about Herzberg Teller couplings and agree qualitatively with what is said. Yet, I think that a much more careful analysis of the (possibly short-lived) excited state vibrations is needed before conclusions can be drawn. I would therefore suggest not to make too many strong statements about the origin of these vibrational coherences.\nSince the authors have revised the manuscript very thoroughly, I gladly recommend publication of this article after these minor points have been carefully addressed.", "reviewer_2": "The authors have made significant efforts to address the reviewer comments, and the manuscript is much improved as a result. The manuscript is suitable for publication in Nature Communications."}, "author_response": "Response to the reviewers’ comments\nReviewer #1 (Remarks to the Author):\nAfter the first round of reviewing, the manuscript has been improved substantially. The\nauthors have responded to the comments and suggestions of both reviewers. They have\nadded new measurements and new data analysis. This is very helpful and much\nappreciated.\nIn particular, I have enjoyed seeing the excited state quantum dynamics simulations\nwhich are now added as a new section in the Supplementary Information.\nAll in all, the extensive revisions made by the reviewers have supported the case of the\nauthors quite strongly. I think that the manuscript is now much closer to being accepted\nfor publication in Nature Communications.\nI want to ask the authors to fully address the following points before recommending\npublication of this article.\nResponse: We thank the reviewer for their careful re-evaluation of the manuscript and\nfor their thoughtful and positive assessment. We are pleased that the reviewer considers\nthe manuscript to be substantially improved following revision, and that the additional\nmeasurements, data analysis, and the newly added excited-state quantum dynamics\nsimulations in the Supplementary Information are regarded as valuable and informative.\nWe appreciate the reviewer’s recognition that these extensive revisions have\nstrengthened the overall case for the work.\nWe address each of the remaining points raised by the reviewer in detail below.\na) I very much appreciate the details that are now given about the data analysis (SFig 41).\nI must say that I have my doubts about the claim ET time of 14.9 +/- 1 fs. I understand that\nthe subtraction of coherent artefacts is challenging and U see that this has been done\ncarefully. Yet, I think that red curve in 41b will also describe the data if an ET time of 20 or\n25 fs is assumed (depending on how one takes into the account remaining “artefacts”. I\n1\nthink that it is fair that the ET is fast (< 25 fs or so) but I do not think that the ET time can be\nextracted with one fs precision. I request to change this.\nResponse: We thank the reviewer for raising this important point. We agree that the\nextracted ultrafast electron-transfer timescale can be sensitive to the temporal window\nused in the analysis. As shown in the figure, when different upper time limits are\nconsidered, the fast time constant is distributed between approximately 12.8 fs and 20.1\nfs.\nThis variation arises because, following the rapid electron-transfer event, a weak slower\nsignal component is present, which we attribute to dielectric relaxation of the newly\nformed charged species. To account for this effect more rigorously, we have extended the\nfitting window to 2000 fs. After careful subtraction of the coherent artefact, the kinetics\nwere analysed using a bi-exponential model. This yields a fast component of 18.1 ± 3.1\nfs, with a conservative upper bound of 21.2 fs, fully consistent with the reviewer’s\nsuggestion that the electron transfer is ultrafast (< 25 fs) but cannot be determined with\none-femtosecond precision. Importantly, this refinement does not alter the physical\ninterpretation of the results: the electron-transfer process remains faster than the period\nof the coherently generated vibrational mode (~26 fs), and therefore still occurs within\nthe vibrationally coherent regime discussed in the manuscript. We have revised the\nrelevant sections of the main text and Supplementary Information accordingly and now\nreport the bi-exponential analysis throughout.\nAction:\nThe resulting differential kinetics (Supplementary Fig. 41b) were fit with a mono-exponential\ndecay function, yielding a time constant of τ = 14.9 ± 1 fs. This value represents the\nexperimentally resolvable upper limit of the CT timescale, approaching the 11 fs instrument\nresponse function. The resulting differential kinetics (Supplementary Fig. 41b) were analysed\nusing a biexponential model comprising a dominant ultrafast component and a much weaker\nslower contribution. The fast component is attributed to electron transfer from the polymer\nbackbone to the PDI acceptor, while the slower component likely reflects secondary processes\nfollowing charge separation, such as dielectric or structural relaxation of the newly formed\ncharge-separated state. This analysis yields a fast characteristic timescale of 18.1 ± 3.1 fs,\ncorresponding to a conservative upper bound of 21.2 fs for the electron-transfer process.\nImportantly, this analysis supports the physical interpretation of the results: the electron-\ntransfer process is faster than the period of the coherently generated vibrational mode (~26 fs),\nconfirming that charge transfer occurs on a sub-vibrational timescale.\nSupplementary Fig. 41 | b, Difference trace (TS-P3 – Ref) fitted with a dominant ultrafast decay component (red line),\ncorresponding to an electron-transfer timescale of ~18 fs (upper bound ~21 fs).\nMain excerpt\nAll mentions of the electron-transfer timescale in the main text have been revised for\nconsistency with the updated analysis. For example:\n… which yielded a fast component of 18.1 ± 3.1 fs, corresponding to a conservative upper\nbound of ~21.2 fs for the electron-transfer process a mono-exponential decay with τ = 14.9 ±\n1 fs, establishing an upper limit for the electron-transfer process\n3\nb) I guess that the time axis in SI Fig. 4 should be “ps” – not “fs” ;-)\nResponse: We thank the reviewer for identifying this error. The time axis in\nSupplementary Fig. 4 was incorrectly labelled; the correct unit is ps rather than fs.\nAction: This has now been corrected in the revised Supplementary Information.\nc) The title in SFig. 49 should probably read “with/without linkers”.\nResponse: We thank the reviewer for noting this typographical error.\nAction: The title of Supplementary Fig. 49 has been corrected to read “with/without\nlinkers.”\nd) I acknowledge the discussion about Herzberg Teller couplings and agree qualitatively\nwith what is said. Yet, I think that a much more careful analysis of the (possibly short-\nlived) excited state vibrations is needed before conclusions can be drawn. I would\ntherefore suggest not to make too many strong statements about the origin of these\nvibrational coherences.\nResponse: We thank the reviewer for this important point and agree that caution is\nrequired when interpreting the origin of short-lived excited-state vibrational coherences.\nIn the revised manuscript, we no longer make definitive statements assigning the\nobserved coherences to a specific Herzberg–Teller coupling mechanism. Instead, the\ndiscussion is framed more conservatively, emphasising that the observations are\nconsistent with vibronic coupling without uniquely establishing a single microscopic\norigin.\nIn addition, we have included further time–frequency analyses (narrowband-filtered\ninverse FFT and short-time Fourier transform) in the revised Supplementary Information\n(Supplementary Note 20) to better characterise the vibrational coherence, while\nexplicitly noting the limitations of such methods at the earliest time delays. Those\nanalysis further supported the mechanism that is proposed. As suggested by the\n4\nreviewer, we have moderated the strength of several mechanistic statements in the main\ntext.\nWe hope that these revisions adequately address the reviewer’s concern.\nAction: Relevant changes in the main text are highlighted in red.\nSince the authors have revised the manuscript very thoroughly, I gladly recommend\npublication of this article after these minor points have been carefully addressed.\nWe hope that the remaining minor points raised by the reviewer have now been fully\naddressed, and we sincerely thank the reviewer for their careful reading and constructive\nfeedback.\nReviewer #2 (Remarks to the Author):\nThe authors have made significant efforts to address the reviewer comments, and the\nmanuscript is much improved as a result. The manuscript is suitable for publication in\nNature Communications.\nResponse: We thank the reviewer for their careful re-evaluation of the manuscript and\nfor their positive assessment. We are pleased that the revisions have addressed the\nreviewer’s concerns and that the manuscript has been substantially improved as a result.\nWe appreciate the reviewer’s time and constructive feedback, which have strengthened\nthe clarity and quality of the work.\n5"}, {"round": 3, "reviewer_comments": {"reviewer_1": "The presentation of the material has been improved and I gladly support publication of the manuscript."}, "author_response": ""}]

Decision letter