Acceptor-Donor-Acceptor Systems Based on
Indacenodithiophene-Extended Tetrathiafulvalene

Christina Schøttler, Célia Noël, Laura Le Bras, Hayley Melville, Olivier Alévêque, Arthur David, Olivier Segut, Maxime Rémond, Eric Levillain, Philippe Blanchard, et al.

► To cite this version:

Christina Schøttler, Célia Noël, Laura Le Bras, Hayley Melville, Olivier Alévêque, et al. Acceptor-Donor-Acceptor Systems Based on Indacenodithiophene-Extended Tetrathiafulvalene. European Journal of Organic Chemistry, 2025, 28 (13), pp.e202401413. 10.1002/ejoc.202401413. hal-05055160

HAL Id: hal-05055160

https://univ-angers.hal.science/hal-05055160v1

Submitted on 4 May 2025

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Acceptor-Donor-Acceptor Systems Based on
Indacenodithiophene-Extended Tetrathiafulvalene

Christina Schøttler,[a, b] Célia Noël,[c] Laura Le Bras,[c] Hayley Melville,[d] Olivier Alévêque,[d] Arthur H. G. David,[d] Olivier Segut,[d] Maxime Rémond,[d] Eric Levillain,[d] Philippe Blanchard,[*][d] and Mogens Brøndsted Nielsen*[a]

[a] Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen Ø (Denmark); [b] Sino-Danish College (SDC), University of Chinese Academy of Sciences Beijing (China); [c] Chrono-environnement, Université de Franche-Comté, 16 route de Gray, 25000 Besançon (France); [d] Univ Angers, CNRS, MOLTECH-ANJOU, SFR MATRIX, 2 Bd Lavoisier, 49045 Angers Cedex (France)

Corresponding authors: [philippe.blanchard@univ-angers.fr](mailto:philippe.blanchard@univ-angers.fr); [mbn@chem.ku.dk](mailto:mbn@chem.ku.dk)

**Abstract**

Here, we present a series of acceptor-donor-acceptor (A-D-A) conjugated molecules incorporating an indacenodithiophene-extended tetrathiafulvalene (IDT-TTF) as the central electron-donating moiety, functionalized with various lateral electron acceptors. A key synthetic step was the Vilsmeier-Haack formylation of the IDT-TTF, providing the dialdehyde in quantitative yield. This compound was further used for the incorporation of dicyanovinyl and indanedione acceptors. UV-vis absorption spectroscopy of the A-D-A compounds in solution revealed a broad absorption in the visible spectrum which is further red-shifted when proceeding from solutions to thin films. Their cyclic voltammograms in solution showed a multi-redox behavior – undergoing both oxidation and reduction events. Notably, the oxidation of these compounds unveiled the formation of intermolecular mixed-valence and π-dimer species, especially strong in the case of the indanedione-containing derivative. Computational calculations helped us to identify the electronic transitions involved in the main absorption bands of these A-D-A compounds displaying different behaviors depending on the acceptor attached to the IDT-TTF core. Finally, the potential of these compounds as donors in organic solar cells was investigated and the best performing solar cell exhibited a modest yet promising power conversion efficiency of 0.87%.
**Introduction**

Tetrathiafulvalene (TTF) is a redox-active molecule that undergoes two reversible one-electron oxidations, generating first a radical cation TTF⁺ and ultimately a dication TTF²⁺ with two 6π aromatic 1,3-dithiolium rings (Figure 1). TTF and derivatives have been widely explored in materials endowed with electronic properties and supramolecular chemistry.[1] The redox properties of TTF have been finely tuned by the introduction of π-conjugated systems instead of the central double bond, as illustrated in Figure 1, resulting in so-called extended TTFs.[2]

Indacenodithiophene (IDT) is a planar π-conjugated system consisting of five fused rings (Figure 1). It has been widely explored as building-block in electron donor materials for organic solar cells.[3] A few TTFs with IDT as the extending moiety have previously been reported, first by Shao and Zhao in 2009[4] and more recently by some of us[5] (compounds I and II in Figure 1, respectively). Both of these IDT-TTFs showed oxidations during cyclic voltammetry (CV) measurements, with three irreversible oxidations for compound I and two reversible ones for compound II, with the latter strongly resembling what has been reported for the more extensively studied indenofluorene-extended TTFs.[6] In addition, indenofluorene-extended TTFs, possessing benzenic analogous structures compared to IDT-TTFs, were included in donor-acceptor scaffolds which were integrated in dye-sensitized solar cells affording good power conversion efficiency (PCE).[7] We have also recently found that the reactivity of indenofluorene-extended TTFs towards singlet molecular oxygen can be reduced upon introduction of electron-withdrawing substituents at the core, hence improving organic photovoltaic device stability.[8]

In this context, we became interested in the synthesis and characterization of acceptor-donor-acceptor (A-D-A) conjugated molecules based on a central IDT-TTF as electron-donating unit. These A-D-A type molecules are known to provide relevant materials for high performance organic solar cells[9] due to i) their broad absorption spectrum in the visible spectrum, ii) the fine-tuning of their HOMO and LUMO energy levels, and iii) their good charge transport properties.

Herein we describe the synthesis and optoelectronic properties of four IDT-TTFs (compounds 1-4, see Figure 1). We introduced branched thioalkyl chains on the dithiole rings to increase solubility of the A-D-A molecules. Formylation of IDT-TTF 1 to the dialdehyde 2 was a key
step for subsequent introduction of the electron-accepting groups in compounds **3** and **4**, i.e. dicyanovinyl and 1,3-indandione, respectively.

<figure><img src="image_1.png" /><figcaption>**Figure 1.** Overview of relevant structural motifs, previously reported **IDT-TTFs**, and the four **IDT-TTFs** presented in this work.</figcaption></figure>

# Results and Discussion

## Synthesis

The synthesis of the two **IDT-TTFs 1** and **2**, and the further functionalization with electron-accepting groups to A-D-A compounds **3** and **4**, are shown in Scheme 1. The synthetic route started with a Horner-Wadsworth-Emmons reaction between indacenodithiophene dione[10] **5** and phosphonate **6**,[11] in a similar manner compared to previously reported procedures for **IDT-TTFs**,[5a,6] providing **IDT-TTF 1** in 76% yield. Remarkably, the dialdehyde **2** was obtained in quantitative manner by a regioselective Vilsmeier-Haack formylation, and final Knoevenagel condensations with either malononitrile or 1,3-indandione afforded the A-D-A compounds **3** and **4** in excellent yields: Quantitative yield for **3** and 85% yield for **4**. Compounds **2**, **3** and **4**
were characterized by means of high-resolution mass spectrometry (HRMS), infrared, ¹H and ¹³C NMR spectroscopies. However, for compound **3**, a ¹³C NMR spectrum could not be obtained on account of its poor solubility in organic solvents. Thermogravimetric analysis (Figures S34-S35) was performed on A-D-A compounds **3** and **4** and demonstrated their stability at rather high temperatures (T_d = 286 °C for **3** and T_d = 289 °C for **4**).

<figure><img src="image_2.png" /><figcaption>**Scheme 1.** Synthesis of compounds **1-4**.</figcaption></figure>
Electronic Properties

The optical properties of compounds **1-4** were investigated by UV-vis absorption spectroscopy in solution and as thin films, and the results are summarized in Table 1 and spectra in Figure 2.

<table><caption>**Table 1.** UV-vis absorption data of compounds **1-4** in CH₂Cl₂ at 20 °C (absorption maxima λmax and molar absorptivities ε; sh = shoulder).</caption><thead><tr><th>Compound</th><th>λ<sub>max</sub> [nm] (ε [10<sup>3</sup> M<sup>-1</sup> cm<sup>-1</sup>]); solution</th><th>λ<sub>max</sub> [nm]; thin film</th></tr></thead><tbody><tr><td>1</td><td>484 (51), 358 (28), 349 (26) sh, 304 (32) sh, 290 (37)</td><td>643 sh, 549 sh, 493 sh, 462, 357</td></tr><tr><td>2</td><td>555 (38), 522 (26), 479 (24), 440 (40), 399 (42), 325 (20) sh, 293 (37)</td><td>584, 540, 499, 454 sh, 403</td></tr><tr><td>3</td><td>645 (9.1), 550 (39), 513 (32), 480 (38), 450 (28) sh, 316 (23), 263 (34), 248 (43)</td><td>721 sh, 602 sh, 549, 497 sh, 455 sh</td></tr><tr><td>4</td><td>559 (61), 493 (51), 468 (47) sh, 334 (33), 268 (53)</td><td>677, 622, 570 sh, 505 sh</td></tr></tbody></table>

Comparing the IDT-TTF **1** with the dialdehyde **2**, the extension of the π-system in **2** results in the appearance of several absorption maxima between 350 and 600 nm and a significant redshift of the longest-wavelength absorption maximum of 71 nm (from 484 nm for **1** to 555 nm for **2**). Further functionalization of the system with two dicyanovinyl units led to an additional broad charge transfer band, centered at 645 nm in compound **3**. In the case of compound **4**, the introduction of 1,3-indandione produced a significant broadening of the lowest-energy absorption band with a maximum at 559 nm and a low-energy absorption onset at around 800 nm. Absorption spectra in different solvents were recorded for the three A-D-A compounds **2**, **3**, and **4**. A small shift in the longest-wavelength absorption maximum was observed for all compounds, indicating a minor charge-transfer character for these transitions (see Figures S17-S20 in the Supporting Information, SI).

The optical properties of compounds **1-4** were also investigated as thin films. The normalized UV-vis absorption spectra of the films are depicted in Figure 2 and the related data are presented in Table 1. Compared to its absorption in solution, compound **1** shows a blueshift of 22 nm for the longest-wavelength absorption maximum in the thin film and the appearance of shoulders at lower energy. The change from solution to thin-films led to a red-shift of the absorption spectra for compounds **2** and **3** while the effect was more pronounced in the case of compound
4. Moreover, strong aggregation from the flat IDT-TTFs core led to a drastic broadening of the absorption spectra in thin films.

<figure><img src="image_3.png" /><figcaption>**Figure 2.** UV-vis absorption spectra for compounds **1-4** in CH<sub>2</sub>Cl<sub>2</sub> (solid lines) and as thin films (dashed lines).</figcaption></figure>

The HOMO energies of compounds **1-4** were determined from the characterization of thin films by photoelectron yield spectroscopy in air (PYSA), while their optical band gaps were determined by the absorption onset at low energy. The LUMO energy levels were calculated by adding the optical band gap values to the HOMO energies. The HOMO energy levels are presented in Figure 3 along with the band gaps and LUMO energy levels (vide infra). **IDT-TTF 1** has the highest HOMO energy level at -5.04 eV, indicating that it is the strongest donor of the four compounds which is to be expected as it has only the electron-donating dithiafulvene
groups and no electron-accepting groups. Formylation of **IDT-TTF 1** to the dialdehyde **2** resulted in a lowering of the HOMO energy level by 0.29 eV. The dicyanovinyl groups of compound **3** had no further effect on the HOMO energy level compared to the dialdehyde **2**, while the 1,3-indandione accepting groups of compound **4** resulted in a higher HOMO energy level at -5.20 eV. The band gap was decreased by 0.19 eV by formylation of **IDT-TTF 1** to the dialdehyde **2** (2.09 eV for **1** and 1.90 eV for **2**). Functionalization of the system with either the dicyanovinyl or the 1,3-indandione accepting groups resulted in a decrease in band gap, to 1.59 eV for dicyanovinyl compound **3** and 1.66 eV for 1,3-indandione compound **4**. Regarding the LUMO energy levels, the dicyanovinyl compound **3** had the lowest LUMO level at -3.74 eV, indicating that it is the strongest acceptor of the four compounds, followed by the 1,3-indandione compound **4** with a LUMO level at -3.54 eV. **IDT-TTF 1** had the highest LUMO energy level at -2.95 eV.

<figure><img src="image_4.png" /><figcaption>**Figure 3.** HOMO/LUMO energy levels (numbers at bottom and top, respectively) and band gaps for compounds **1-4**. The HOMO energy levels were determined by PYSA, and the band gaps were determined from the absorption onset of thin films of the compounds.</figcaption></figure>
The electrochemical properties of compounds **1-4** were further investigated by cyclic voltammetry, and the potentials against ferrocene (Fc/Fc$^+$) are summarized in Table 2. Cyclic voltammograms of compounds **1-4** (in CH$_2$Cl$_2$ with 0.1 M Bu$_4$NPF$_6$ as supporting electrolyte) are shown in Figure 4. Two reversible oxidations were observed for both **IDT-TTF 1** and dialdehyde **2**, with the first oxidation at +0.25 V for **1** and at +0.41 V for **2**, indicating that replacing hydrogen atoms with electron-withdrawing formyl groups renders the first oxidation significantly more difficult (by 150 mV). The second oxidation, however, only became slightly more difficult upon formylation (by 40 mV; from +0.49 V for **IDT-TTF 1** to +0.55 V for dialdehyde **2**). Interestingly, a broadening of the first oxidation wave compared to the second one was observed for dialdehyde **2**, suggesting the possible formation of different species after generation of the radical cation state, as discussed below. No reductions were observed for **IDT-TTF 1**, while dialdehyde **2** exhibited one reversible reduction at -1.76 V. The electrochemical behavior of **IDT-TTF 1** is in agreement with what has previously been reported for a similar **IDT-extended TTF** with hexyl chains instead of the branched 2-ethylhexyl chains of **IDT-TTF 1**.[5a]

<table><caption>**Table 2.** Electrochemical data from deconvolution of cyclic voltammograms of compounds **1-4** in CH$_2$Cl$_2$ (+0.1 M Bu$_4$NPF$_6$); potentials in volts vs Fc/Fc$^+$.</caption><thead><tr><th>Compound</th><th><i>E</i><sub>ox</sub> [V]</th><th><i>E</i><sub>red</sub> [V]</th></tr></thead><tbody><tr><td>1</td><td>+0.25, +0.49</td><td>N/A</td></tr><tr><td>2</td><td>+0.41, +0.55</td><td>-1.76</td></tr><tr><td>3</td><td>+0.43</td><td>-1.29</td></tr><tr><td>4</td><td>+0.18, +0.47, +0.66</td><td>-1.30</td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Cyclic voltammograms of compounds **1-4** in CH<sub>2</sub>Cl<sub>2</sub> (0.5 mM for **1** and **2**, 0.1 mM for **3** and **4**); supporting electrolyte: 0.1 M Bu<sub>4</sub>NPF<sub>6</sub>, scan rate: 100 mV/s. All potentials are depicted against the Fc/Fc<sup>+</sup> redox couple.</figcaption></figure>

One reversible oxidation at +0.43 V was observed for dicyanovinyl compound **3**, only 30 mV higher than the first oxidation potential for dialdehyde **2**; it was not possible to further oxidize **3** as it proved unstable at higher potentials. One irreversible reduction was observed for **3** at -1.29 V. A very similar irreversible reduction was detected for compound **4** at -1.30 V. Remarkably, compound **4** showed three reversible oxidation waves at +0.18 V, +0.47 V, and +0.66 V. At this stage, we assumed that the first two distinct waves could be both associated to
the oxidation of **4** into **4**⁺ and corresponded to the successive formation of mixed-valence (4)₂⁺ and π-dimer species (4⁺)₂ hence leading to a strong splitting of the first oxidation wave of **4** into **4**⁺ in two waves, while the third corresponded to the oxidation of the radical cation **4**⁺ into dication 4²⁺. Based on this hypothesis that will be discussed below, the oxidation potential of **4** into **4**⁺ is situated at +0.33 V in the middle of the two split first oxidation waves. Thus, the first oxidation potentials of compounds **1-4** measured in solution follow the trend observed from the HOMO energy levels estimated by PYSA on thin films (*vide supra*), with **IDT-TTF 1** as the strongest donor followed by compound **4** and then compounds **2** and **3** being close in donor ability (with the same HOMO energy levels and only a small difference of 30 mV in first oxidation potentials). The highest LUMO level of **IDT-TTF 1** deduced from PYSA and absorption spectroscopy explains the absence of electrochemical reduction wave in cyclic voltammetry. The reduction potentials for compounds **2-3** recorded by cyclic voltammetry are also in agreement with the relative acceptor strengths suggested by the LUMO energy levels of Figure 3; however, the difference in reduction potential measured in solution between compounds **3** and **4** is insignificant (10 mV), whereas there is a more significant difference in LUMO energy levels (0.20 eV) measured from thin-films.

Further investigation was pursued to understand the specific electrochemical behavior of
the oxidation of compounds **2** and **4** in their radical cations, a broadening of the first oxidation
wave for **2** and a significant splitting for the formation of **4**⁺ being observed. Thus, the
deconvolution of cyclic voltammograms confirmed the broadening of the wave associated with
the oxidation process in radical cation for compound **2** and the significant splitting for that of
compound **4** (Figure S24). Both compounds led to the formation of mixed-valence and π-dimer
species during the first oxidation process, followed by that of dication species at higher
oxidation potentials.[6] The broadening observed for compound **2** suggests weaker
intermolecular interactions compared to compound **4**. In contrast, compound **4** showed a
significant splitting of the first oxidation wave, indicating strong intermolecular electronic
coupling between the redox-active centers, leading to stable mixed-valence and π-dimer states.
Remarkably, in the case of **4**, the splitting of the first oxidation process is more pronounced
than those observed intermolecularly for extended TTFs.[12] It is, however, similar to those
detected when studying covalently linked indenofluorene-extended TTF dimers.[13]

Spectroelectrochemical measurements were performed to investigate the species produced
during the oxidation of compounds **2** and **4**. These measurements, along with principal
component analysis (Figures S36-S37), allowed real-time monitoring of the spectroscopic
signatures, confirming the formation of π-dimer species in both compounds, as evidenced by distinctive optical signatures in the near-infrared (NIR) region beyond 1100 nm, followed by the appearance of dication species exhibiting a characteristic absorption band between 800 and 1200 nm centered at *ca.* 1000 nm (Figure 5). Although no NIR signature of mixed-valence species was detected for both **2** and **4** within the range covered by our detector (350 to 1400 nm), a clear bleaching of the absorption bands of **4** centred at *ca.* 600 nm was observed (Figure 5b) followed by the appearance of the optical features of the π-dimer species beyond 1100 nm. This bleaching is the consequence of the formation of **4**⁺ and mixed-valence species.

<figure><img src="image_6.png" /><figcaption>**Figure 5.** Absorption map of the spectroelectrochemical experiments for (a) dialdehyde **2** and (b) indanedione-containing IDT-TTF **4**; supporting electrolyte: 0.1 M Bu₄NPF₆, scan rate: 10 mV/s, 1 frame = 1 s.</figcaption></figure>

Electrochemical simulations using a square scheme provided insights into the formation of mixed-valence, π-dimer, and dication species during the two-step oxidation processes (see SI for details). The square scheme model helped to explain the observed splitting and broadening behavior by simulating the interplay between different oxidation states (Figures S38-S39), highlighting the role of electronic coupling and molecular geometry in stabilizing these intermediates. The strong splitting observed for compound **4**, which contains electron-withdrawing indanedione groups, suggests a more stable formation of mixed valence/π-dimer species compared to compound **2**, influenced by both electronic and steric factors. The presence of electron-accepting π-extended indanedione platforms A may favor intermolecular interactions as it has been described in non-fullerene acceptors of A-D-A structure where A···A interactions dominate the intermolecular arrangements.[14]
Computations

To gain a deeper insight into the optoelectronic properties of compounds **1-4**, (TD-)DFT calculations have been performed. After an optimization step, the vertical absorption transitions were computed. All the details of the computational approach are gathered in the SI. As the alkyl chains R of the dithiol rings are poorly involved within the electronic transitions, they have been replaced by methyl groups (Figure S29). The four structures were found to be quite rigid, and they remain planar throughout the optimization process, allowing a full conjugation of the molecules. There is, however, a subtle structural difference between compounds **2**, **3**, and **4** with a slight opening of the structure due to the relative orientation of either the aldehyde, dicyanovinyl or 1,3-indandione with respect to the neighboring thiophene for **2**, **3**, and **4** respectively (Figure 6). Compound **4** is the most open one with an angle of 132°.

<figure><img src="image_7.png" /><figcaption>**Figure 6.** Representation of the optimized structures of compounds **1-4**. The angle illustrating the opening of the structure, θ, has been highlighted via a ball representation of the constitutive atoms. The values of both angles (for both sides of the molecule) have also been reported.</figcaption></figure>

There is good agreement between the calculated and experimental UV-vis absorption properties both in terms of position of the peaks and their intensities (Table 3 and Figure S30). For **1**, the
first $\lambda_{max}$ is equal to 484 nm while $\lambda_{calc}$ is found at 489 nm. For **2**, **3**, and **4**, there is a larger bathochromic deviation for the first main absorption peak with $\Delta\lambda$ equal to 15, 44, and 16 nm, respectively. Those deviations remain lower than 0.2 eV (common admitted threshold for TD-DFT calculations[15]) with values equal to 0.06, 0.12, and 0.06 eV for **2**, **3**, and **4** respectively. For all the compounds, the calculations reproduce nicely the different experimental maxima of the spectra with a maximum $\Delta\lambda$ of 30 nm ($\Delta E$ = 0.25 eV) and 45 nm ($\Delta E$ = 0.25 eV) for the transitions $S_0 \rightarrow S_6$ and $S_0 \rightarrow S_5$ of **2** and **3**. Due to the conjugation of the molecules, all the absorption bands can be attributed to $\pi \rightarrow \pi^*$ transitions. A representation of all the molecular orbitals involved in the main absorption bands are presented in SI (Figure S31). Two types of behavior have been observed with whether a transition that remain on the TTF/TTF or thiophene/thiophene axis or a "mixed" transition with electronic density moving from one axis to another one (Figure S32). For **1**, **2**, and **3**, the first most important absorption band always corresponds to a $S_0 \rightarrow S_1$ excitation and is ascribed to a HOMO/LUMO transition. It is a mixed transition with the electronic density going from the thiophene/thiophene axis to the TTF/TTF axis. Noteworthy is the fact that the electronic delocalization is increasing when going from **1** to **3**, especially due to the thiophene substitution, which is at the origin of the higher $\lambda_{max}$ observed for **3**. For **4**, the first most important arises from a $S_0 \rightarrow S_2$ excitation and is ascribed to a HOMO-1/LUMO transition, localized on the thiophene/thiophene axis.

<table><caption>**Table 3.** Experimental ($\lambda_{max}$ in nm) and calculated ($\lambda_{calc}$ in nm) UV-vis absorption properties of compounds **1-4**. For each calculated transition reported the corresponding oscillator strength (f) and description in terms of molecular orbitals have been provided.</caption><thead><tr><th>Compound</th><th>λ<sub>max</sub></th><th>λ<sub>calc</sub></th><th>f</th><th>Description</th><th>Attribution</th></tr></thead><tbody><tr><td rowspan="2">1</td><td>484</td><td>489</td><td>0.70</td><td>H → L</td><td>S<sub>0</sub> → S<sub>1</sub></td></tr><tr><td>358</td><td>353</td><td>0.53</td><td>H → L+2</td><td>S<sub>0</sub> → S<sub>5</sub></td></tr><tr><td rowspan="5">2</td><td>555</td><td>570</td><td>0.46</td><td>H → L</td><td>S<sub>0</sub> → S<sub>1</sub></td></tr><tr><td>479</td><td>463</td><td>0.68</td><td>H-1 → L</td><td>S<sub>0</sub> → S<sub>2</sub></td></tr><tr><td>440</td><td>418</td><td>0.84</td><td>H → L+1</td><td>S<sub>0</sub> → S<sub>4</sub></td></tr><tr><td>399</td><td>369</td><td>0.60</td><td>H-1 → L+1</td><td>S<sub>0</sub> → S<sub>6</sub></td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="3">3</td><td>645</td><td>689</td><td>0.17</td><td>H → L</td><td>S<sub>0</sub> → S<sub>1</sub></td></tr><tr><td>550</td><td>546</td><td>1.71</td><td>H-1 → L</td><td>S<sub>0</sub> → S<sub>3</sub></td></tr><tr><td>480</td><td>445</td><td>1.08</td><td>H → L+2</td><td>S<sub>0</sub> → S<sub>5</sub></td></tr><tr><td rowspan="2">4</td><td>559</td><td>575</td><td>2.15</td><td>H-1 → L</td><td>S<sub>0</sub> → S<sub>2</sub></td></tr><tr><td>493</td><td>448</td><td>1.03</td><td>H → L+2</td><td>S<sub>0</sub> → S<sub>5</sub></td></tr></tbody></table>
Organic Solar Cell Fabrication and Characterization

The potential of these IDT-TTF-derivatized A-D-A compounds in organic electronics was
preliminary explored by investigating compounds 2-4 as small-molecule donors for organic
solar cells. All experimental conditions and results are summarized in Table S3 in the SI. Those
molecular donors were combined with PC71BM (HOMO = -5.90 eV, LUMO = -3.90 eV) or
Y6 (HOMO = -5.65 eV, LUMO = -4.10 eV), as fullerene and non-fullerene electron-
acceptors,[16] in solution-processed bulk heterojunction organic solar cells (BHJ OSCs). The
highest power conversion efficiency (PCE) was obtained for inverted BHJ OSCs where the 1,3-
indandione-based compound 4 was combined with the acceptor Y6 in a 1:1.2 ratio with the
following architecture: Glass/ITO/ZnO/4:Y6/MoO3/Ag (Figure 7a).[17] The active layer was
spin-coated from a chlorobenzene solution with 3% of 1,3,5-trichlorobenzene as additive, and
the resulting films were subsequently thermally annealed. The atomic force microscopy images
(Figure S33) of the photoactive layer on glass substrates display an amorphous surface with an
average roughness of 3.58 nm. The absorption spectrum of the film of the blend (Figure 7b)
shows good optical complementarity, with a greater contribution from Y6 (λmax = 847 nm) than
from compound 4. The best performing device had a PCE of 0.87% associated to a short-circuit
current density (Jsc) of 3.95 mA cm-2, an open-circuit voltage (Voc) of 0.50 V and a fill factor
(FF) of 44%. (Figure 7c). Despite this relatively low value of PCE, this result can be considered
encouraging as A-D-A molecules with an extended IDT-TTF as the central part have never
been studied, to the best of our knowledge, as materials in organic solar cells. The contributions
of the donor and acceptor to the generation of the photocurrent was estimated by recording an
external quantum efficiency (EQE) spectrum (Figure 7d). Interestingly, the contribution of
compound 4 to the generation of photocurrent is slightly higher than the contribution of Y6, in
contrast to its lower contribution to the overall absorption of light in the film (Figure 7b). Thus,
the donor and acceptor show a more balanced contribution than would be expected from the
UV-vis absorption spectrum of the thin film.
<figure><img src="image_8.png" /><figcaption>**Figure 7.** Organic solar cells with compound **4** as molecular donor. a) Architecture of the solar cells. b) UV-vis absorption spectrum of spin-coated film obtained from a solution of **4** and **Y6** (1.2:1) in CHCl₃ (10 mg mL⁻¹). c) J/V curves in the dark and under illumination of the best solar cell. d) EQE spectrum of the best solar cell.</figcaption></figure>

**Conclusion**

In summary, an indacenodithiophene-extended tetrathiafulvalene (IDT-TTF) was synthesized along with a series of conjugated acceptor-donor-acceptor type molecules based on this core. All compounds showed absorptions in the full visible region in solution and as thin films, and the HOMO/LUMO energy levels and band gaps indicated that the functionalization with electron-withdrawing groups increases the electron-accepting ability of IDT-TTF, while decreasing the electron-donating ability. Furthermore, these compounds undergo from one to two reversible oxidations in which the first oxidation can be broadened or split on account of the formation of intermolecular cationic mixed-valence and π-dimer species. Remarkably, the IDT-TTF functionalized with peripheral indanedione moieties exhibited a remarkably strong splitting of its first oxidation (0.28 V). No reductions were observed for the IDT-TTF itself, while the acceptor-donor-acceptor compounds each underwent one reduction process reversible
when the accepting group is an aldehyde, but otherwise irreversible. Thus, we have shown that
an **IDT-TTF** functionalized with various electron-accepting groups retains the ability to be
reversibly oxidized while the electron-accepting groups also introduce reducibility of the
compounds, resulting in donor-acceptor compounds, and strengthen their self-association in
oxidation. Computational calculations revealed that the electronic transitions associated with
the most important absorption maxima can stay localized on the TTF/TTF or
thiophene/thiophene axis, or can move from one axis to the other, depending on the acceptor
attached to the **IDT-TTF** core. Finally, the potential of these acceptor-donor-acceptor
compounds with an **IDT-TTF** core as donor materials in organic solar cells was investigated.
A modest yet promising PCEmax of 0.87% was obtained for the best performing device.
However, the best performing compound **4** showed moderate solubility in chlorobenzene,
which makes the formation of thin layers tedious, and therefore the fabrication of photovoltaic
devices complicated. In this context, more soluble IDT-TTF based compound are needed.
Future work will focus on further structural modifications to increase device performances.

## Acknowledgements

Sino-Danish College (SDC) is acknowledged for financial support. The supercomputer
facilities of the Mésocentre de calcul de Franche-Comté are acknowledged. We thank Dr.
Clément Cabanetos (CNRS) for fruitful discussions. We also acknowledge Benjamin Siegler,
Ingrid Freuze, and Christian Brou from the SFR MATRIX for their help with the compound
characterization.

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## Entry for the Table of Contents

<figure><img src="image_9.png" /></figure>

Indacenodithiophene-extended tetrathiafulvalene (IDT-TTF) cores were employed to develop acceptor-donor-acceptor chromophores. These compounds demonstrated extensive visible-light absorption and complex redox properties, including π-dimer and mixed-valence formations. Computational studies revealed tunable electronic transitions, while preliminary solar cell tests led to promising efficiency, showcasing IDT-TTF's as a versatile donor unit for organic electronics.
Table of Contents

<table><tr><td>Experimental Procedures</td><td>S1</td></tr><tr><td>NMR Spectroscopy</td><td>S4</td></tr><tr><td>IR Spectroscopy</td><td>S8</td></tr><tr><td>UV-Vis Absorption and Emission Spectroscopy</td><td>S9</td></tr><tr><td>Electrochemistry</td><td>S13</td></tr><tr><td>HOMO/LUMO Energy Levels</td><td>S16</td></tr><tr><td>Computational details</td><td>S18</td></tr><tr><td>Organic Solar Cells</td><td>S20</td></tr><tr><td>Atomic Force Microscopy (AFM)</td><td>S21</td></tr><tr><td>Thermogravimetric Analysis (TGA)</td><td>S22</td></tr><tr><td>Spectroelectrochemistry</td><td>S23</td></tr><tr><td>References</td><td>S24</td></tr></table>
# Experimental Procedures

## General methods

Anhydrous THF was obtained by distillation over Na/benzophenone, and anhydrous Et₂O was obtained from a solvent drying system (SPBT-1 Benchtop Solvent Purification System from LC Technology Solutions Inc.). Anhydrous ClCH₂CH₂Cl and DMF were used as purchased without further drying.

Purification by column chromatography was carried out using silica gel (particle size 63-200 µm). TLC was carried out on aluminum sheets covered with silica gel coated with fluorescent indicator. NMR spectra were obtained on a Bruker 300 MHz Avance III spectrometer (300 MHz for ¹H) or 500 MHz Avance III HD spectrometer (500 MHz for ¹H and 126 MHz for ¹³C). Deuterated chloroform (CDCl₃, ¹H = 7.26 ppm, ¹³C = 77.16 ppm) or deuterated benzene (C₆D₆, ¹H = 7.16 ppm, ¹³C = 128.39 ppm) were used as solvents and internal references. Chemical shifts are referenced to the ppm scale and coupling constants are expressed in Hertz (Hz). High resolution mass spectrometry (HRMS) was performed on a JEOL JMS-700 B/E.

Photoemission yield spectroscopy in air (PYSA) measurements were recorded using a Riken Keiki PYSA spectrometer (Model AC-2) with power settings of 5 nW. Thermogravimetric analysis (TGA) was performed with a Netzsch TG 209 F3 tarsus®. Fourier-transform infrared (FT-IR) spectroscopy was performed on a Bruker VERTEX 70 spectrometer equipped with a Bruker Platinum ATR.

Atomic force microscopy (AFM) experiments were performed using the Nano-Observer device (CSInstrument, Les Ulis, France). The topographic images were obtained at room temperature in tapping mode using ACTA-50 SPM Probe (AppNano, USA). Images were processed with the Gwyddion free SPM data analysis software (v2.64, Czech Metrology Institute, Brno, Czech Republic).

## Absorption and emission spectroscopy

UV-Vis absorption spectra were recorded on a Shimadzu UV-1800 UV-Vis spectrophotometer using a quartz cell (path length 1 cm) in CH₂Cl₂, CH₃Cl, or PhCl, or on an Agilent Cary UV-Vis spectrophotometer in PhMe or mixtures of CH₂Cl₂ and MeCN. Thin films were prepared on glass slides, previously cleaned with soapy water, deionized water, acetone and ethanol, using a spin coater by depositing 80 µL of a solution of the compound (concentration 10 mg/mL in CHCl₃, 1500 rpm for 60 s). The spectra of thin films were recorded on a Shimadzu UV-1800 UV-Vis spectrophotometer in the 1000-330 nm range. The extinction coefficient at each wavelength was determined from 3 individually prepared solutions as the average. Fluorescence was measured on a Shimadzu RF-6000 spectrophotometer using a quartz cell (path length 1 cm). Quantum yields were measured on a Jasco FP-8500 spectrophotometer equipped with an ILF-835 integration sphere.

## Electrochemistry

Cyclic voltammetry experiments were carried out at room temperature under Ar in a glove box with a Bio-Logic SAS SP-150 potentiostat with scan rates 100, 200, 500, and 1000 mV/s and concentrations of 0.5, 0.1, 0.05, and 0.01 mM for compounds **1** and **2**, and concentrations of 0.1, 0.05, and 0.01 mM for compounds **3** and **4** due to poor solubility in the electrolyte solution. A silver wire immersed in a 0.1 M Bu₄NPF₆ solution in CH₂Cl₂ separated from the analyte by a fritted glass was used as a pseudo reference electrode, a Pt wire was used as the counter electrode, and a Pt electrode (a disk of 2 mm diameter) was used as the working electrode. Measured potentials were referenced to the ferrocene/ferrocenium (Fc/Fc⁺) redox couple, measured before and after the experiment. A 0.1 M solution of Bu₄NPF₆ in CH₂Cl₂ was used as electrolyte.

## Synthesis

Compounds 5[1] and 6[2] were synthesized according to literature and modified literature procedures.

## Spectroelectrochemistry

Time-resolved spectroelectrochemistry was performed using the already described home self-made cell described elsewhere.[3] A distance of 25-50 µm between the surface of the electrode and the optical window was typically used in our experiments. Electrochemical measurements were carried out using a platinum wire counter electrode and a silver wire as a quasi-reference electrode with a Biologic SP-300 potentiostat driven by the EC-Lab software including ohmic drop compensation. Experiments were recorded in dry HPLC-grade CH₂Cl₂ with TBAPF₆ (electrochemical grade, Fluka) as supporting electrolyte. All solutions were prepared and transferred into the spectroelectrochemical cell in a glove box containing dry, oxygen-free (<1 ppm) argon, at room temperature. Spectrophotometric measurements were carried out with a homemade bench composed of different PRINCETON INSTRUMENTS modules (light sources, fibers, monochromators, spectroscopy camera and software). To start the two experiments at the same time, the two benches are synchronized with TTL signals.
## Synthesis of compound 1

<figure><img src="image_10.png" /></figure>

A suspension of 5 (496 mg, 1.69 mmol) in dry THF (100 mL) was cooled to -78 °C. In a separate flask, a solution of 6 (3.40 g, 6.79 mmol) in dry THF (100 mL) was cooled to -78 °C before LiHMDS (8 mL, 8 mmol, 1.0 M in toluene) was added, resulting in a color change from red to orange. The solution was stirred at -78 °C for 30 min before it was allowed to warm to rt and stirred for 18 h. Then aqueous $NH_4Cl$ (sat., 20 mL) was added, followed by water (80 mL), before the mixture was extracted with $CH_2Cl_2$ (4 x 100 mL). The combined organic phases were dried over $MgSO_4$, filtered, and the solvent was removed under reduced pressure. The residue was purified by flash column chromatography twice (SiO$_2$, 1) 40% $CH_2Cl_2$/heptane; 2) 20% $CH_2Cl_2$/heptane), yielding 1 (1.34 g, 0.129 mmol, 76%) as a dark red solid. $R_f$ = 0.23 (20% $CH_2Cl_2$/heptane). ¹H NMR (500 MHz, $C_6D_6$): $δ$ 7.94 (s, 2H), 7.29 (d, $J$ = 5.0 Hz, 2H), 6.90 (d, $J$ = 5.0 Hz, 2H), 2.88 - 2.72 (m, 8H), 1.61 - 1.55 (m, 4H), 1.50 - 1.20 (m, 32H), 0.95 - 0.83 (m, 24H) ppm. ¹³C NMR (126 MHz, $C_6D_6$): $δ$ 142.7, 140.7, 136.8, 136.6, 133.0, 129.5, 128.4, 126.1, 121.7, 119.6, 114.7, 40.9, 40.0, 39.9, 32.6, 32.5, 29.2, 29.1, 25.7, 23.4, 14.4, 14.4, 11.2, 11.0 ppm; 3 $sp^3$ signals missing, presumably due to overlap. HRMS (MALDI) $m/z$ = 1042.2969 [M$^+$], calcd for $C_{54}H_{74}S_{10}^+$ = 1042.2992.

## Synthesis of compound 2

<figure><img src="image_11.png" /></figure>

A mixture of dry 1,2-dichloroethane (20 mL) and dry DMF (1 mL) was cooled in an ice-water bath before oxalyl chloride (1 mL, 12 mmol) was added dropwise, resulting in precipitation of a white solid. The mixture was stirred cold for 45 min, after which a solution of 1 (507 mg, 0.486 mmol) in dry 1,2-dichloroethane (50 mL) at rt was added dropwise to the cold mixture via cannula over 15 min. The reaction mixture was then allowed to warm to rt and stirred for 18 h after which it was quenched with $NaOH$ (1 M, 50 mL), stirred for 1 h to neutralize, and then diluted with $CH_2Cl_2$ (50 mL). The phases were separated, and the aqueous phase was extracted with $CH_2Cl_2$ (2 x 50 mL), and the combined organic phases were dried over $MgSO_4$, filtered, and the solvent removed under reduced pressure, yielding 2 (548 mg, 0.499 mmol, quantitative yield) as a dark red solid. ¹H NMR (500 MHz, $C_6D_6$): $δ$ 9.76 (s, 2H), 7.52 (s, 2H), 7.33 (s, 2H), 3.13 - 3.02 (m, 8H), 1.82 - 1.70 (m, 4H), 1.62 - 1.30 (m, 32H), 1.18 - 0.83 (m, 24H) ppm. ¹³C NMR (126 MHz, $C_6D_6$): $δ$ 181.7, 146.9, 145.7, 142.4, 140.3, 136.7, 132.2, 130.8, 128.4, 117.0, 114.7, 41.4, 41.4, 40.2, 40.1, 32.8, 32.7, 29.3, 29.3, 26.0, 25.9, 23.6, 23.5, 14.6, 14.5, 11.6, 11.2 ppm; 1 $sp^2$-C signal missing, presumably due to overlap. HRMS (MALDI) $m/z$ = 1098.2885 [M$^+$], calcd for $C_{56}H_{74}O_2S_{10}^+$ = 1098.2896. IR $v_{max}$/cm$^{-1}$ 1651 (C=O).
## Synthesis of compound 3

<figure><img src="image_12.png" /></figure>

A solution of 2 (200 mg, 0.182 mmol) and malononitrile (603 mg, 9.13 mmol) in CH₂Cl₂ (50 mL) was placed under an Ar atmosphere. Then Et₃N (0.05 mL) was added, and the reaction mixture was stirred at rt for 6 days before the solvent was removed under reduced pressure, and the residue was washed with EtOH, yielding 3 (217 mg, quantitative yield) as a dark solid. ¹H NMR (300 MHz, CDCl₃): δ 7.79 (s, 2H), 7.72 (s, 2H), 7.32 (s, 2H), 3.03 (d, J = 6.2 Hz, 8H), 1.69 - 1.62 (m, 4H), 1.49 - 1.43 (m, 8H), 1.36 - 1.29 (m, 16H), 0.99 - 0.87 (m, 32H) ppm. Not sufficiently soluble for ¹³C NMR. HRMS (MALDI) *m/z* = 1194.3138 [M⁺], calcd for C₆₂H₇₄N₄S₁₀⁺ = 1194.3121. IR *vmax/cm⁻¹* 2212 (CN).

## Synthesis of compound 4

<figure><img src="image_13.png" /></figure>

A solution of 2 (202 mg, 0.184 mmol) and 1,3-indandione (270 mg, 1.85 mmol) in CH₂Cl₂ (50 mL) was placed under an Ar atmosphere. Then Et₃N (0.05 mL) was added, and the reaction mixture was stirred at rt for 6 days before the solvent was removed under reduced pressure, and the residue was washed with EtOH, yielding 4 (213 mg, 85%) as a dark solid. ¹H NMR (300 MHz, CDCl₃): δ 7.79 (s, 2H), 7.65 (d, J = 7.4 Hz, 2H), 7.57 (d, J = 7.4 Hz, 2H), 7.44-7.33 (m, 6H), 7.23 (s, 2H), 3.15 (2 × d, J = 6.8 Hz, 8H), 1.79 - 1.71 (m, 4H), 1.66 - 1.59 (m, 8H), 1.42 - 1.33 (m, 16H), 1.05 - 0.88 (m, 32H) ppm. ¹³C NMR (126 MHz, CDCl₃): δ 189.7, 189.0, 141.7, 140.5, 139.9, 137.8, 136.1, 134.2, 133.7, 132.8, 123.1, 122.0, 117.3, 114.9, 60.5, 42.1, 40.9, 39.7, 32.3, 32.2, 28.9, 28.9, 25.5, 25.4, 23.1, 14.2, 11.0, 11.0 ppm; 7 sp² and 3 sp³ carbon signals missing, presumably due to overlap. HRMS (MALDI) *m/z* = 1354.3417 [M⁺], calcd for C₇₄H₈₂O₄S₁₀⁺ = 1354.3420. IR *vmax/cm⁻¹* 1716 (C=O) and 1674 (C=O).
NMR Spectroscopy

<figure><img src="image_14.png" /><figcaption>Figure S1. ¹H NMR spectrum (500 MHz, C₆D₆) of compound 1.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Figure S2. ¹³C NMR spectrum (126 MHz, C₆D₆) of compound 1.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>Figure S3. ¹H NMR spectrum (500 MHz, C₆D₆) of compound 2.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>Figure S4. ¹³C NMR spectrum (126 MHz, C₆D₆) of compound 2.</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>Figure S5. ¹H NMR spectrum (300 MHz, CDCl₃) of compound 3.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>Figure S6. ¹H NMR spectrum (300 MHz, CDCl₃) of compound 4.</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>Figure S7. ¹³C NMR spectrum (126 MHz, CDCl₃) of compound 4.</figcaption></figure>
IR Spectroscopy

<figure><img src="image_21.png" /><figcaption>Figure S8. IR spectrum of compound 2.</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>Figure S9. IR spectrum of compound 3.</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>Figure S10. IR spectrum of compound 4.</figcaption></figure>
UV-Vis Absorption and Emission Spectroscopy

<table><caption>Table S1. Summary of optical properties in CH₂Cl₂ (absorption maxima λ_max, molar absorptivities ε, absorption onset wavelengths λ_onset, emission maxima λ_em, and fluorescence quantum yields Φ_f). Shoulders determined visually from the absorption spectrum.</caption><thead><tr><th>Compound</th><th>λ<sub>max</sub> [nm] (ε [10<sup>3</sup> M<sup>-1</sup> cm<sup>-1</sup>])<br>solution (* = shoulder)</th><th>λ<sub>max</sub> [nm]<br>film (* = shoulder)</th><th>λ<sub>onset</sub> [nm]<br>film</th><th>λ<sub>em</sub> [nm]<br>solution</th><th>Φ<sub>f</sub> [%]<br>solution</th></tr></thead><tbody><tr><td>1</td><td>484 (51), 358 (28), 349 (26)*,<br>304 (32)*, 290 (37)</td><td>643*, 549*, 493*,<br>462, 357</td><td>594</td><td>N/A</td><td>N/A</td></tr><tr><td>2</td><td>555 (38), 522 (26), 479 (24),<br>440 (40), 399 (42), 325 (20)*,<br>293 (37)</td><td>584, 540, 499, 454*,<br>403</td><td>651</td><td>617</td><td>4</td></tr><tr><td>3</td><td>645 (9.1), 550 (39), 513 (32),<br>480 (38), 450 (28)*, 316 (23),<br>263 (34), 248 (43)</td><td>721*, 602*, 549,<br>497*, 455*</td><td>780</td><td>N/A</td><td>N/A</td></tr><tr><td>4</td><td>559 (61), 493 (51), 468, (47)*,<br>334 (33), 268 (53)</td><td>677, 622, 570*, 505*</td><td>745</td><td>N/A</td><td>N/A</td></tr></tbody></table>

<figure><img src="image_24.png" /><figcaption>Figure S11. Normalized absorption (black) and emission (red, λ<sub>ex</sub> = 540 nm) spectra of compound 2 in CH₂Cl₂.</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>Figure S12. UV-vis absorption spectrum of compound 1 as a film, with the tangents used to determine the onset wavelength shown in red.</figcaption></figure>
<figure><img src="image_26.png" /><figcaption>Figure S13. UV-vis absorption spectrum of compound **2** as a film, with the tangents used to determine the onset wavelength shown in red.</figcaption></figure>

<figure><img src="image_27.png" /><figcaption>Figure S14. UV-vis absorption spectrum of compound **3** as a film, with the tangents used to determine the onset wavelength shown in red.</figcaption></figure>

<figure><img src="image_28.png" /><figcaption>Figure S15. UV-vis absorption spectrum of compound **4** as a film, with the tangents used to determine the onset wavelength shown in red.</figcaption></figure>
<figure><img src="image_29.png" /><figcaption>Figure S16. Normalized UV-vis absorption spectra of compound **2** in CH<sub>2</sub>Cl<sub>2</sub> and PhMe.</figcaption></figure>

<figure><img src="image_30.png" /><figcaption>Figure S17. Normalized UV-vis absorption spectra of compound **2** in different mixtures of CH<sub>2</sub>Cl<sub>2</sub> and MeCN (ratios given as CH<sub>2</sub>Cl<sub>2</sub>:MeCN). Solutions were prepared by dissolving compound **2** in CH<sub>2</sub>Cl<sub>2</sub> and then adding MeCN; at a ratio of 1:5, precipitation of compound **2** upon addition of MeCN was observed, resulting in deviation from the baseline as observed in the spectrum. Due to the precipitation, larger ratios of MeCN were not investigated.</figcaption></figure>
<figure><img src="image_31.png" /><figcaption>Figure S18. Normalized UV-Vis absorption spectra of compound 3 in CH₂Cl₂, PhCl, CHCl₃, and PhMe.</figcaption></figure>

<figure><img src="image_32.png" /><figcaption>Figure S19. Normalized UV-Vis absorption spectra of compound 4 in CH₂Cl₂, PhCl, CHCl₃, and PhMe.</figcaption></figure>
Electrochemistry

<figure><img src="image_33.png" /></figure>

Figure S20. Cyclic voltammograms of compounds **1** and **2** at different concentrations in CH₂Cl₂; supporting electrolyte: 0.1 M Bu₄NPF₆, scan rate: 100 mV/s. All potentials are depicted against the Fc/Fc⁺ redox couple.

<figure><img src="image_34.png" /></figure>

Figure S21. Cyclic voltammograms of compounds **1** and **2** at different scan rates at 0.5 mM in CH₂Cl₂; supporting electrolyte: 0.1 M Bu₄NPF₆. All potentials are depicted against the Fc/Fc⁺ redox couple.
<figure><img src="image_35.png" /><figcaption>Figure S22. Cyclic voltammograms of compounds **3** and **4** at different concentrations in CH₂Cl₂; supporting electrolyte: 0.1 M Bu₄NPF₆, scan rate: 100 mV/s. All potentials are depicted against the Fc/Fc⁺ redox couple.</figcaption></figure>

<figure><img src="image_36.png" /><figcaption>Figure S23. Cyclic voltammograms of compounds **3** and **4** at different scan rates at 0.1 mM in CH₂Cl₂; supporting electrolyte: 0.1 M Bu₄NPF₆. All potentials are depicted against the Fc/Fc⁺ redox couple.</figcaption></figure>
<figure><img src="image_37.png" /><figcaption>Figure S24. Deconvoluted voltammograms of compounds **2** and **4** in CH<sub>2</sub>Cl<sub>2</sub> at 0.5 mM and 0.1 mM, respectively; supporting electrolyte: 0.1 M Bu<sub>4</sub>NPF<sub>6</sub>, scan rate: 100 mV/s. All potentials are depicted against the Fc/Fc<sup>+</sup> redox couple.</figcaption></figure>
## HOMO/LUMO Energy Levels

The HOMO energy level of each compound was determined by photoelectron yield spectroscopy in air (PYSA) measurements on thin films. The band gap of each compound was determined from the UV-vis absorption spectrum of thin films.

$$ \Delta E^{\text{HOMO-LUMO}} = 1240/\lambda_{\text{onset}} \text{ eV} ; \quad E^{\text{LUMO}} = E^{\text{HOMO}} + \Delta E^{\text{HOMO-LUMO}} $$

<table><caption>Table S2. Summary of HOMO-LUMO energy levels and band gaps of compounds 1-4.</caption><thead><tr><td>Compound</td><td>HOMO level</td><td>LUMO level</td><td>Band gap</td></tr></thead><tbody><tr><td>1</td><td>-5.04</td><td>-2.98</td><td>2.06</td></tr><tr><td>2</td><td>-5.33</td><td>-3.43</td><td>1.90</td></tr><tr><td>3</td><td>-5.33</td><td>-3.74</td><td>1.59</td></tr><tr><td>4</td><td>-5.20</td><td>-3.54</td><td>1.66</td></tr></tbody></table>

<figure><img src="image_38.png" /><figcaption>Figure S25. PYSA spectrum of the thin film of compound 1.</figcaption></figure>

<figure><img src="image_39.png" /><figcaption>Figure S26. PYSA spectrum of the thin film of compound 2.</figcaption></figure>
<figure><img src="image_40.png" /><figcaption>Figure S27. PYSA spectrum of the thin film of compound 3.</figcaption></figure>

<figure><img src="image_41.png" /><figcaption>Figure S28. PYSA spectrum of the thin film of compound 4.</figcaption></figure>
## Computational details

All the DFT calculations have been performed with Gaussian09[4] software. We used the PBE0[5] exchange-correlation functional along with the 6-311+G(d,p) basis set. The solvent was considered through an implicit solvation model, namely the PCM model[6] with dichloromethane. Each optimization was followed by a frequency calculation to ensure the structures are in a true minimum of the potential energy surface. The optical properties were computed with and without the alkyl chains (replaced by a methyl group). The results are presented in Figure S28. Considering the lateral alkyl chains or not has a slight impact on the spectrum, the shift between the maxima is negligible. For example, the first main absorption band is equal to 489 nm without lateral alkyl chains and to 490 nm with the lateral alkyl chains.

<figure><img src="image_42.png" /><figcaption>Figure S29. Comparison of calculated spectra of 1 without lateral alkyl chains (full line) and with lateral alkyl chains (dotted line). The convolution has been obtained with a FWHM = 0.3 eV.</figcaption></figure>

<figure><img src="image_43.png" /><figcaption>Figure S30. Experimental (black; recorded in CH₂Cl₂) and calculated (colored) UV-vis absorption spectra of compounds 1-4. The calculated optical properties have been represented as sticks and also as a convolution with a FWHM = 0.3 eV.</figcaption></figure>
<figure><img src="image_44.png" /><figcaption>Figure S31. Representation of the molecular orbitals involved in the main transitions reported in Table 3 of compounds 1-4. Isodensity=0.0004 a.u.</figcaption></figure>

<figure><img src="image_45.png" /><figcaption>Figure S32. Illustration of the orientation of the electronic density with the thiophene-thiophene axis (top) and TTF-TTF axis (bottom).</figcaption></figure>
# Organic Solar Cells

## Preparation of inverted organic solar cells: Glass/ITO/ZnO/donor:acceptor/MoO₃/Ag

ITO-covered glass substrates were cleaned with deionized water and deconex followed by deionized water, then acetone, and finally ethanol by ultrasound sonication for 15 min before drying and cleaning for 15 min in a UV-ozone cleaner. A solution of ZnO-precursor was prepared by adding ethanolamine (54 µL) and zinc acetate (196 mg) to absolute ethanol (6 mL), and the mixture was stirred and heated to 45 °C for 2 h. ZnO films were obtained by spin-coating the solution onto the ITO substrates under rotation at 2000 rpm for 60 s, followed by thermal annealing in an oven at 180 °C for 15 min. Then, a solution of the chosen acceptor (Y6 or PC71BM) and either 2, 3, or 4 with a ratio of D:A w/w%: 1:1.2 at 15.4 mg mL⁻¹ in chlorobenzene with 3% 1,3,5-trichlorobenzene was stirred at 100 °C for 1 h and was spin-coated hot onto the substrates at 1500 rpm for 60 s. The resulting thin films were thermally annealed for 5 min at 110 °C under an inert atmosphere in a glovebox. A mask defining disks of 0.28 cm² surface area was introduced and the substrates were then transferred to a chamber under vacuum (ca. 10⁻⁶ mbar) for the thermal deposition of thin films of MoO₃ (7 nm) and Ag (100 nm). The resulting solar cells were tested using a Oriel Sol3A solar simulator from Newport Instruments equipped with a Xenon lamp (AM1.5 conditions under an incident light of 100 mW/cm²), a Keithley source-meter and an in-house software.

<table><caption>Table S3. Summary of results from the organic solar cells.</caption><thead><tr><th>Entry</th><th>Donor</th><th>Acceptor</th><th>V<sub>oc</sub> (V)</th><th>J<sub>sc</sub> (mA/cm<sup>2</sup>)</th><th>PCE<sub>max</sub> (%)</th><th>FF (%)</th><th>PCE<sub>avg</sub> (%)</th><th>Number of cells</th></tr></thead><tbody><tr><td>1</td><td>4</td><td>Y6</td><td>0.50</td><td>-3.95</td><td>0.87</td><td>44</td><td>0.71 ± 0.18</td><td>18</td></tr><tr><td>2</td><td>4</td><td>PC<sub>71</sub>BM</td><td>0.58</td><td>-2.55</td><td>0.77</td><td>51.8</td><td>0.41 ± 0.26</td><td>18</td></tr><tr><td>3</td><td>3</td><td>Y6</td><td>0.14</td><td>-2.16</td><td>0.11</td><td>36.3</td><td>0.03</td><td>18</td></tr><tr><td>4</td><td>2</td><td>Y6</td><td>0.17</td><td>-2.14</td><td>0.14</td><td>37.2</td><td>0.04</td><td>18</td></tr></tbody></table>
Atomic Force Microscopy (AFM)

The topographic images at 5 µm show a smooth homogeneous and amorphous surface with an average roughness (RMS) of 5.64 and 3.58 nm respectively for thin films of the photoactive layers **2:Y6** (1:1.2) and **4:Y6** (1:1.2) on glass. The thin film prepared from the photoactive layer **3:Y6** (1:1.2) shows bigger agglomerates of several hundreds of microns and a higher average roughness than the two other surfaces at 7.57 nm.

<figure><img src="image_46.png" /><figcaption>Figure S33. AFM images of different photoactive layers **2:Y6** (1:1.2), **3:Y6** (1:1.2), and **4:Y6** (1:1.2).</figcaption></figure>
<figure><img src="image_47.png" /><figcaption>Thermogravimetric Analysis (TGA)</figcaption></figure>

Figure S34. TGA of compound 3 recorded under nitrogen at 10 °C min⁻¹. Decomposition temperature corresponding to a 5% weight loss: T_d = 286 °C.

<figure><img src="image_48.png" /></figure>

Figure S35. TGA of compound 4 recorded under nitrogen at 10 °C min⁻¹. Decomposition temperature corresponding to a 5% weight loss: T_d = 289 °C.
Spectroelectrochemistry

<figure><img src="image_49.png" /><figcaption>Figure S36. 3D spectra showing the extracted principal components during the spectroelectrochemical measurements of 2 in CH₂Cl₂ with TBAPF₆ (0.1 M) as supporting electrolyte confirming the formation of (a) π-dimer and (b) dication species. 1 frame is equal to 1 s.</figcaption></figure>

<figure><img src="image_50.png" /><figcaption>Figure S37. 3D spectra showing the extracted principal components during the spectroelectrochemical measurements of 4 in CH₂Cl₂ with TBAPF₆ (0.1 M) as supporting electrolyte confirming the formation of (a) π-dimer and (b) dication species. 1 frame is equal to 1 s.</figcaption></figure>
# Cyclic Voltammetry Simulations

## Square scheme for the first oxidation process:

$$ \begin{array}{ccc} M & \overset{E_1}{\rightleftharpoons} & M^+ + e^- \\ \updownarrow K_{MV} & & \updownarrow K_{\pi D} \\ MV^+ & \overset{E_2}{\rightleftharpoons} & \pi D^{2+} + e^- \end{array} $$

Nernst equation: $E_1 + a \ln(\frac{M^-}{[M]}) = E_2 + a \ln(\frac{\pi D^{2+}}{[MV^+]})$ with $a = \frac{RT}{F}$

Consequently, $\Delta E = E_2 - E_1 = a \ln(\frac{K_{MV}}{K_{\pi D}})$ and if $K_{MV} > K_{\pi D}$, $\Delta E > 0$ and $E_2 > E_1$

## Second oxidation process: $M^+ \overset{E_3}{\rightleftharpoons} M^{2+} + e^-$

$M$ corresponds to neutral molecules (either 2 or 4); $M^+$ corresponds to radical cations; $MV^+$ corresponds to mixed-valence species; $\pi D^{2+}$ corresponds to $\pi$-dimers; $M^{2+}$ corresponds to dications.

## DigiElch parameters:

Diffusion: Semi-Infinite 1D; Geometry: Planar; Area (cm²): 0.07
Ru (Ohm): 0; Cdl (F): 0; Temp. (K): 293; v (V/s): 0.1; D (cm²/s) = 1 10⁻⁶ for all species

* For compound 2:

Splitting process: $E_1$ (V) = 0.875; $\alpha$ = 0.5; $k_s$ (cm/s) = 0.05 and $E_2$ (V) = +0.915; $\alpha$ = 0.5; $k_s$ (cm/s) = 0.05
Second reduction: $E_3$ (V) = +1.000; $\alpha$ = 0.5; $k_s$ (cm/s) = 0.05
Chemical reactions: $K_{MV} = 10^{+4}$; $k_f = 10^{+6}$ and $K_{\pi D} = 2.11 10^{+3}$; $k_f = 10^{+6}$

* For compound 4:

Splitting process: $E_1$ (V) = 0.830; $\alpha$ = 0.5; $k_s$ (cm/s) = 0.05 and $E_2$ (V) = +1.000; $\alpha$ = 0.5; $k_s$ (cm/s) = 0.005
Second reduction: $E_3$ (V) = -1.150; $\alpha$ = 0.5; $k_s$ (cm/s) = 0.005
Chemical reactions: $K_{MV} = 10^{+6}$; $k_f = 10^{+9}$ and $K_{\pi D} = 1.34 10^{+3}$; $k_f = 10^{+9}$
<figure><img src="image_51.png" /><figcaption>Figure S38. Simulation of the CV of compound 2 at different concentrations.</figcaption></figure>

<figure><img src="image_52.png" /><figcaption>Figure S39. Simulation of the CV of compound 4 at a concentration of 10⁻⁴ M.</figcaption></figure>
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