Controlling narrowband phosphorescence and delayed fluorescence by aggregate size

Domain: neuroscience
Published: 2026-04-09
Editorial outcome: accept

Abstract
Exciton coherence is a phenomenon involving collective electronic transition dipole moments of
molecular aggregates with interesting photophysical behaviors such as superradiance and
subradiance in J- and H-type aggregates, respectively. Although singlet aggregate excitons have
been extensively studied, the understanding of triplet exciton coherence in terms of specific
conditions is still lacking due to limited experimental observations. Here, by synthesizing model
organic compounds of fluorene monomer, dimer, trimer, and polymer, we systematically studied
their photoluminescence in both solution and aggregation states. Strong triplet exciton coherence
is present in polyfluorene aggregates, including nanoparticles, microparticles, and thin films, and
is manifested either as HJ-aggregate phosphorescence or delayed fluorescence depending on the
size of these aggregates. Notably, in the phosphorescence state, the polyfluorene polymer exhibits
an unusually sharp atomic-spectrum-like emission band with a narrow full width at half maximum
(FWHM) of only 0.05 eV (14 nm) and the longest lifetime of 0.63 s. This study shows that the size
of organic aggregates is essential in dictating organic exciton dynamics in the solid state and holds
importance for the development of advanced optoelectronic technologies.

Introduction
The concept of excitons was first introduced by Yakov Frenkel in 1931 in the context of how
excitation energy transforms into heat in solids1. Today Frenkel excitons describe bound states of
an electron and a hole on the same molecule within the crystal2. When a Frenkel exciton is
generated, the oscillating dipole on the molecule may also cause resonant electromagnetic
disturbances in neighboring ground-state molecules3. Coherence can thus arise from a concerted
manner of coupling of transition dipole moments (\mu) among all perturbed molecules, enabling
delocalized excitonic states and highly efficient wave-like exciton migratory pathways1.
Coherence in molecular excitons plays a pivotal role in energy transport both in natural light-
harvesting systems and in organic optoelectronic devices4, 5.

Over the past several decades, the field of molecular excitons has witnessed significant
theoretical advancements6. For example, Michael Kasha’s early molecular exciton model
developed in the 1950s is still qualitatively used to explain spectroscopic behaviors within
molecular aggregates, such as superradiance in head-to-tail J-aggregates and subradiance in side-
by-side H-aggregates, which is described by the Hamiltonian6: H = E +D +
\sum J |m⟩⟨n| (Equation 1), where E , D and J are the energy terms for monomer exciton,
m,n M m,n
gas-to-solid phase change, and coupling between neighboring molecules, respectively7.
Recently, Frank Spano’s reviews summarized more fine-grained theoretical frameworks for
understanding how coherence manifests in UV–Vis and fluorescence spectral features, particularly
emphasizing the critical role of vibronic coupling (the interaction between electronic excitations
and molecular vibrations) in determining the photophysical properties of molecular aggregates7.
These reviewed studies explore how various interactions influence the relative intensity ratios of
e.g., the 0–0 and 0–1 vibronic peaks in the spectra of various molecular aggregates beyond Kasha’s
point dipole model, such as disorder and orbital overlap7. For example, the spectra of J-aggregates
are characterized by a dominant 0–0 peak while H-aggregates show an enhanced 0–1 peak, which
is described by the Frenkel–Holstein Hamiltonian7: H = E +D +\sum J |m⟩⟨n|+
FH M m,n m,n
\omega \sum b † b +\omega \sum {\lambda(b † +b )+\lambda2}|n⟩⟨n| (Equation 2), where the last two new energy
vib n n n vib n n n
terms describe how vibration frequencies and nuclear relaxation influence the spectral profile7.

To date, nearly all molecular exciton studies are focused on the spin singlet states, for the
obvious reason that the foundation of the theory calls for a large enough transition dipole moment2,
3, 8. Although organometallic molecules exhibit a mixed luminescent state, their \mu is substantial
due to a strong heavy atom effect that lends singlet \mu to the triplet one9. For a nearly pure triplet
exciton, its \mu \approx 0 (J \approx 0) and according to Equation 1, the delocalized exciton reduces to a
m,n
monomeric one, suggesting that aggregation exerts no influence on triplet excitons and thus the
phosphorescence decay7. In a conjugated polymer system, however, the negligible J
m,n
term can
be replaced by a coupling term V due to the presence of very efficient nearest-neighbor Dexter
m,n
electron exchange, i.e., H = E +D +\sum V |m⟩⟨n|+\omega \sum b † b +\omega \sum {\lambda(b † +
FH T m,n m,n vib n n n vib n n
b )+\lambda2}|n⟩⟨n| (Equation 3)10. For small molecular crystals, the coupling V
n m,n
is typically very
small (as the Pauli exclusion places a distance limit between two closed-shell structures), with an
estimated value of < 0.1 eV11. Nonetheless, such an interaction is expected to be larger among
neighboring molecules within a conjugated polymer chain, to the point where it can overcome the
localization caused by vibrations12.

Therefore, triplet exciton coherence in conjugated polymers such as polyfluorene should
manifest in emission spectra that possess well-resolved vibronic bands13-34, where the change in
relative intensity ratio of vibronic bands is expected to be similar to the singlet case given the
similarity in their respective Hamiltonians7, 35. Here, we systematically investigate triplet exciton
coherence using fluorene monomer (MFL, Fig. 1a), covalent dimer (DFL), and covalent trimer
(TFL) as model compounds due to their well-resolved vibronic peaks and the absence of
significant spin–orbit coupling pathways36. It is found that aggregation leads to the formation of a
special type of HJ-aggregate, which may enhance both S -to-T and T -to-S (first singlet excited
1 1 1 0
state: S , first triplet excited state: T , ground state: S ) simultaneously to allow for the observation
1 1 0
of phosphorescence induced by exciton coherence37. We also show that aggregation size plays a
critical role in tuning aggregate phosphorescence. Specifically, narrowband phosphorescence is
only observed in mesoscopic aggregates such as gels and microfibers with an average size of \mu m.
In solution or smaller nanoaggregates (less than 400 nm), delayed fluorescence (DF, Fig. 1c, d)
dominates due to efficient 1D intra-chain triplet–triplet exciton annihilation (TTA); in larger bulk
aggregates, however, phosphorescence is severely quenched due to excessive electronic coupling,
or aggregation-caused quenching (ACQ). In the carefully tuned, aggregation-induced mesoscopic
organogel phase (Fig. 1b), however, a balance is achieved between enhanced 2D or 3D interchain
exciton migration, which dilutes local exciton concentration and thus mitigates TTA, and reduced
ACQ of the triplet exciton. This balance provides the optimal conditions for observing narrowband
phosphorescence (Fig. 1b, d).

Results
The monomer MFL is commercially available while the other two oligomers (DFL and TFL)
and polymer (PFL) were synthesized via published methods (Supplementary Figs. 1 and 2). All
small molecular fluorene derivatives were rigorously purified using column chromatography. The
MFL, DFL, and TFL molecules are characterized by proton nuclear magnetic resonance (1H-NMR,
Supplementary Figs. 3, 6, and 9), carbon nuclear magnetic resonance (13C-NMR, Supplementary
Figs. 4, 7, and 10), gas chromatography–mass spectrometry (GC–MS, Supplementary Figs. 5, 8,
and 11), and single crystal X-ray diffraction (XRD, Fig. 2e and Supplementary Table 1). The PFL
polymer is characterized by 1H-NMR (Supplementary Fig. 12), gel permeation chromatography
(GPC, Supplementary Fig. 13 and Supplementary Table 2), thermogravimetric analysis–
differential scanning calorimetry (TGA–DSC, Supplementary Fig. 14), and XRD. The number-
average molecular weight (M ) of PFL is determined to be 36.6 kDa with a polydispersity index
n
(PDI) of 2.5, which is typical for stepwise polymerization methods38, 39. All four samples, MFL,
DFL, TFL, and PFL, in optically dilute solutions at concentrations of 5×10-5 M, 2×10-5 M, 1×10-5
M, and 5×10-8 M, respectively, are first analyzed with UV–Vis absorption spectroscopy
(Supplementary Fig. 15). These solutions exhibit absorption bands at 240-310 nm, 270-360 nm,
273-400 nm, and 320-430 nm, with absorption maxima at 264 nm (4.70 eV), 326 nm (3.80 eV),
346 nm (3.58 eV), and 390 nm (3.18 eV), respectively, suggesting expanded \pi-conjugation with
increased repeating units. The steady-state photoluminescence (PL, Fig. 2a) spectra of the four
samples were systematically investigated in 2-methyltetrahydrofuran (2-mTHF) solutions, with
concentrations of MFL, DFL, and TFL set at 1 × 10-4 M, and PFL at 1 × 10-7 M to yield good
signal-to-noise ratios for all measurements. For MFL, the fluorescence emission is centered at 315
nm (\tau = 3.6 ns, Supplementary Fig. 16 and Supplementary Table 3), with two vibronic bands at
304 nm (0–0) and 326 nm (0–2) at room temperature.

At 77 K, the fluorescence emission slightly narrowed with an increased lifetime of 5.2 ns (Fig.
2b, c and Supplementary Table 4) presumably due to inhibited non-radiative processes. Notably,
a new phosphorescence band emerged at ~ 420-520 nm (\lambda = 459 nm). For DFL, the room-
max
temperature fluorescence emission is redshifted to 340-470 nm, with a dominant 0–1 peak at 379
nm (\tau = 0.95 ns); cooling to 77 K, however, results in a reduced lifetime of \tau = 0.86 ns; similarly
for TFL, lifetime at emission maximum (\lambda = 391 nm, \tau = 0.81 ns) is also reduced (\tau = 0.71 ns)
max
with a significant intensity enhancement (Supplementary Fig. 17). The reduction in lifetime and
concomitant increase in intensity at 77 K suggests intramolecular J-dimer and J-trimer formation
for DFL and TFL, respectively. For the PFL polymer, the emission maximum at 415 nm (\tau = 0.43
ns) is red-shifted to 442 nm (\tau = 0.34 ns) with significantly enhanced intensity at 77 K. Although
the reduction in lifetime does suggest intramolecular J-aggregate as found for DFL and TFL, the
nearly vanished 0–0 transition at 77 K suggests predominant H-type aggregation.

To understand the behavior of the polymer emission at 77 K, we first examined the molecular
packing of the three small molecules based on single-crystal X-ray diffraction (XRD) data (Fig.
2e), and cross-examined the spectroscopic changes in fluorescence emission (Supplementary Figs.
18 and 19) for these crystals at room temperature and 77 K, respectively. All crystal emission
spectra exhibit substantial enhancement of the 0–0 band when the temperature is lowered from
298 K to 77 K, which is likely due to the inhibited thermal population from the lowest 0–0 state to
higher vibronic levels. The much stronger 0–0 band compared to the 0–1 band for the monomer
and dimer crystals suggest J-aggregates, which are also consistent with a fluorescence lifetime
reduction at a lower temperature. The trimer, on the other hand, exhibits increased lifetime at 77
K with the 0–1 band being the most intense, which suggests H-aggregate characteristics. Based on
analysis of photoluminescence and XRD results, a reasonable explanation is shown in Fig. 2d. If
intermolecular interactions favor the face-to-face aryl-aryl \pi-stacking geometry, it then would lead
to H-aggregates; however, if polymer mainchains adopt an extended or linear conformation (in the
\beta phase36, 40), intramolecular J-aggregates would also be present since the PFL main chain can be
thought of as a covalently bound J-aggregate of fluorene monomers. Such sites harboring both J-
and H-aggregates are referred to as HJ-aggregates, as has been both theoretically and
experimentally examined by Spano and Lupton37, 41, 42, respectively.

To further elucidate the electronic properties underlying the photophysical behavior of MFL,
DFL, and TFL, we performed detailed calculations of their excited-state energies and spin–orbit
coupling (SOC) matrix elements, as summarized in Fig. 3 and Supplementary Table 5. Taking the
ground-state singlet (S ) as the energy reference (0 eV), the first excited singlet states (S ) were
0 1
calculated to be 4.50 eV for MFL, 3.63 eV for DFL, and 3.33 eV for TFL (Fig. 3a, c), while the
corresponding first triplet states (T ) were found at 2.95 eV, 2.55 eV, and 2.41 eV (Fig. 3b, c),
1
respectively. This progressive decrease in both S and T energies from monomer to trimer is
1 1
consistent with the experimentally observed red-shift in fluorescence and phosphorescence
emission as the conjugation length increases (Fig. 3 c, Fig. 2a, and Fig. 4a). Analysis of the relevant
energy gaps and SOC values reveals further insights into their emission properties. For MFL, the
S –T gap is only 0.14 eV with a SOC matrix element of 0.05 cm-1, and the S –T gap is 0.69 eV
1 5 1 2
(SOC = 0.03 cm-1). For DFL, the S –T energy gap narrows to 0.60 eV, with an increased SOC
1 2
value of 0.19 cm-1, while TFL shows a similar S –T gap of 0.61 eV and a SOC of 0.11 cm-1 (Fig.
1 2
3c). The relatively large S –T energy gaps in DFL and TFL, compared with the S –T gap in MFL,
1 2 1 5
despite their somewhat higher SOC values, are likely responsible for their weaker
phosphorescence intensities, since efficient intersystem crossing (ISC) depends on both strong
SOC and small energy gaps between singlet and triplet states. The electron (blue)–hole (green)
distributions in the S and T states of MFL, DFL, and TFL were analyzed at the optimized ground-
1 1
state geometry using the Multiwfn program (Fig. 3d). The spatial separation and distribution
patterns of electron and hole densities reveal differences in exciton delocalization across the three
molecules. In particular, TFL exhibits more extended distributions, suggesting enhanced
conjugation and thus stronger interchain electronic interactions compared to MFL and DFL. These
calculated trends in electronic structure and SOC, illustrated in Fig. 3, directly correlate with the
experimental observations of emission efficiency and spectral shifts, supporting the notion that
increased conjugation length influences both the energy landscape and exciton dynamics in these
fluorene-based systems.

Next, we experimentally investigated delayed emission spectra of these four samples in 2-mTHF
glasses at 77 K (Fig. 4a) at a concentration of 1×10-4 M. The dimer DFL and trimer TFL have 1,2-
diiodoethane (EID) added to enhance triplet-state emission, with a molar ratio to solvent of 1:10;
the weak signals from these two samples without heavy-atom presence (Supplementary Fig. 20)
are likely because intramolecular J dimer and trimer do not favor intersystem crossing compared
to fluorescence decay. The MFL glass shows phosphorescent emission in the range of 410-640 nm,
peaking at 459 nm (0–1, \tau = 5.7 s, Fig. 4b and Supplementary Fig. 21). The dimer DFL also shows
a 0–1 phosphorescent emission maximum at 560 nm (\tau = 0.14 s), where the reduction in lifetime
is expected as the heavy-atom co-solvent (10%) is present43. However, TFL shows a 0–0
phosphorescent emission maximum at 539 nm (\tau = 0.23 s). The PFL polymer exhibits an unusual
atomic spectrum-line like band with a maximum at 597 nm (FWHM = 0.05 eV, \tau = 0.63 s),
accompanied by a single shoulder band at 656 nm. Given that PFL fluorescence spectrum exhibits
four vibronic bands at room temperature and three at 77 K, the two-band spectral feature suggests
that the main band is most likely positioned at 0–1 (the new 0–0 band for the polymer aggregate),
instead of 0–0. The intensity ratios of the 0–0 and 0–1 phosphorescence peaks for the samples (0–
1/0–2 for PFL, Supplementary Figs. 22-25) is summarized in the bar chart in Fig. 4c. It is evident
that this ratio gradually increases as the number of the repeating units increases, reaching its peak
value in PFL, suggesting growing character of J-aggregates phosphorescence.

Curiously, we also noted a much higher-energy, delayed emission (\Delta t = 1.0 ms) between 430-
525 nm with three vibronic bands at 445, 476, and 507 nm, which closely matches the spectral
profile of PFL fluorescence (Supplementary Fig. 26), which is assigned as delayed fluorescence.
Considering that the observed energy gap between the blue and red peaks is sufficiently large (>
0.5 eV), the prominent delayed emission is most likely due to TTA44. To substantiate the
hypothesis, we measured the decay kinetics of the presumable phosphorescence band at 597 nm
(Supplementary Fig. 27). As the blue peak ceases to exist beyond 50 ms, we first measured the
decay kinetics for the first 50 ms, which can be fitted using a simple second-order kinetics model
according to 1/[3An*] = 2k t+1/[3An*] (R2 > 0.99, Fig. 4d and Supplementary Table 6)44;
TTA 0
the calculated rate constant is k = 2.57 × 10−4 s-1. Beyond 50 ms, the decay follows simple
TTA
single-exponential first-order kinetics with a half-life of 630 ms. Although the phosphorescence
decay channel is present at all times, the much faster TTA process clearly dominates the first 50
ms, yielding pseudo second-order kinetics.

During measurement, we noted that the thawed PFL 2-mTHF solution remained immobile and
glass-like at 298 K. Upon closer examination, we found that the freezing process in liquid nitrogen
transformed the PFL solution into an organic gel at a concentration of 10-4 M. Control experiments
in other solvents confirmed that the freezing-thawing process induces gelation exclusively in 2-
mTHF (at 10-4 M or 10-5 M), whereas no gelation was observed in dichloromethane (DCM) or
toluene (Supplementary Figs. 28-30). This observation prompted us to investigate how molecular
aggregation influences the phosphorescence induced by exciton coherence. To probe the
concentration-dependent aggregate size, we then performed dynamic light scattering (DLS)
measurements of PFL in 2-methyltetrahydrofuran at different concentrations, where it is shown
that significant PFL aggregates start to form at > 10 -6 M with a size distribution between 120-1400
nm and an average size of 377 nm (Fig. 4e). At a concentration of 10-5 M, the distribution is
narrower, ranging from 200-900 nm, with an average size of 388 nm. At 10-4 M, the aggregate size
distribution is between 400-2500 nm with an average size of nearly 1 \mu m. We then examined the
dependence of delayed emission spectra on aggregate size (Fig. 4f). At a concentration of 10-7 M,
the well-dissolved polymer solution is dominated by a delayed fluorescence band at 443 nm (\tau
DF
= 1.2 ms, Supplementary Fig. 31); a very weak phosphorescence emission peak is observed at 593
nm (\tau = 0.52 s). As the concentration increases to 10-6 M, the phosphorescence peak also starts to
rise relative to delayed fluorescence. The blue fluorescence is visible for approximately 0.02 s after
turning off the light source (Fig. 4g). At a concentration of 10-5 M, phosphorescence emission is
observed from 570-680 nm, with the peak at 597 nm (\tau = 0.69 s), accompanied by small emission
peaks at 657 nm and 646 nm, with intensity decreasing sequentially. Additionally, delayed
fluorescence was observed between 440 and 525 nm, with a main peak at 474 nm (\tau = 4.0 \mu s). A
comparable peak was also seen at 449 nm, along with a broad shoulder at 500 nm. The visible
afterglow transitions from purple to red and lasts for approximately 0.4 s.

Finally, we demonstrate that photoluminescence spectroscopy can be an important method to
study the influence of microscopic morphologies of organic optoelectronic materials, which may
be difficult to discern via other characterization methods. Differential thermogravimetric (DTG,
Fig. 5a) analysis shows that all PFL samples begins to lose significant weight near 400 ℃, with
the most rapid loss around 450 ℃, ceasing by 500 ℃. The film prepared from dichloromethane
(DCM) displays a more pronounced DTG peak and a lower residual mass (42.1% at 500 ℃)
compared to films from 2-methyltetrahydrofuran (2-mTHF, 47.5%) and untreated samples
(50.2%). Notably, the 2-mTHF film exhibits gradual weight loss from 100-300 ℃, likely due to
moisture retention and incomplete solvent removal. Powder X-ray diffraction (PXRD) patterns
(Fig. 5b) for all PFL forms display broad, low-intensity diffraction features centered at 2\theta \approx 20°,
indicating low crystallinity and suggesting that the exciton dynamics are governed primarily by
amorphous aggregate morphologies rather than long-range order. Raman spectroscopy (Fig. 5c)
reveals minimal spectral differences among the PFL samples. All exhibit a prominent peak at 1605
cm-1 (fluorenyl ring stretching) and several weaker peaks (1257, 1280, 1306, 1348 cm-1) associated
with C–C stretching between phenyl rings45. The films display slightly narrower and more intense
peaks, which we attribute to enhanced electronic polarizability from extended conjugation45.

On the other hand, SEM imaging provides direct visualization of the morphological diversity
among the samples. First, films prepared from DCM exhibit a smooth, planar morphology
punctuated by concave defects (~ 2 \mu m wide), within which a network of nanofibers (< 100 nm
diameter) forms a multilayered mesh (Fig. 5e, f and Supplementary Fig. 32). This ordered structure
likely facilitates coherent interchain interactions, as evidenced by a pronounced enhancement of
phosphorescence relative to delayed fluorescence at 77 K (Fig. 5d). The planar arrangement and
extended \pi-conjugation in these mesoscopic aggregates support efficient exciton migration and
suppress triplet–triplet annihilation (TTA), creating optimal conditions for observing narrowband
phosphorescence, consistent with the aggregate-size-dependent effects discussed in the abstract
and introduction. Secondly, films prepared from 2-mTHF display a less uniform surface with fine
wrinkles and undulations (hundreds of nm to several \mu m wide, Fig. 5h, i and Supplementary Fig.
33), reflecting less ordered aggregation due to the solvent’s higher boiling point and slower
evaporation, thus favoring delayed fluorescence over phosphorescence in the emission spectrum
at 77 K (Fig. 5g). This demonstrates how less ordered, smaller aggregates shift the balance toward
TTA-driven delayed fluorescence, in agreement with our findings on the control of exciton
dynamics by aggregate size. Thirdly, PFL bulk solids are characterized by a highly porous,
honeycomb-like structure with elliptical pores (~ 2 \mu m diameter) and a network of nanofibers
(hundreds of nm thick) forming an irregular, multilayered matrix (Fig. 5k, l and Supplementary
Fig. 34). The abundance of sub-micron pores and the highly disordered arrangement promote
nonradiative decay pathways and exciton quenching (Fig. 5j).

Methods
Materials. 9,9-Dimethylfluorene (MFL, 98%), Tetrakis(triphenylphosphine)palladium (99%),
Dimethylformamide (DMF, \ge 99.5%), 2,7-bis-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-
dioctylfluorene (98%), and 1,2-diiodoethane (EID, 98%) were acquired from Energy Chemical
Co., Ltd. and used as received (MFL was purified before measurements). 2-Bromo-9,9-
dimethylfluorene (98%) and 9,9-Dioctyl-2,7-dibromofluorene (97%) were acquired from Bide
Pharmatech Ltd. and used as received. 2-(9,9-Dimethyl-9H-fluoren-2-yl)-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (98%) and 2,7-dibromo-9,9-dimethyl-9H-fluorene (98%) were acquired from
Shanghai Haohong Biopharmaceutical Technology Co., Ltd. and used as received. Other reagents
and solvents were acquired from Sinopharm Chemical Reagent Co., Ltd. and used as received.
The water was deionized with a Milli-Q SP reagent water system (Millipore) to a specific
resistivity of 18.2 M\Omega\cdot cm. All compounds used in this work were purified by column
chromatography with silica gel. Analytical thin layer chromatography (TLC) was performed using
glass plates coated with silica gel (0.25 mm). Column chromatography was performed using silica
gel 60 (230-400 mesh, Merck) with the indicated solvents.

Instrumentation and Sample Preparation. NMR spectra were recorded on a Bruker AV400 NMR
spectrometer at room temperature, 400 MHz for 1H and 101 MHz for 13C. Chemical shifts of NMR
spectra are reported in ppm relative to the signals corresponding to the residual protio chloroform.
Electron Ionization (EI) mass spectra were recorded on a Thermo-Fisher Gas Chromatography Q
Exactive Orbitrap Mass Spectrometer. Gel permeation chromatography (GPC) was recorded on
an Agilent 1260 Infinity II GPC/SEC system. Thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) were recorded on a TA Instruments Discovery SDT 650. Ultraviolet–
visible (UV–Vis) absorption spectra of solutions were recorded on a PERSEE TU-1901 UV–vis
spectrometer. The steady-state emission spectra were recorded on a Horiba FluoroMax-4
spectrofluorometer (Horiba Scientific), using a vertically mounted 150-W ozone-free cw xenon
arc lamp as the light source. The delayed emission spectra were also recorded on a Horiba
FluoroMax-4 spectrofluorometer, using a 10-W xenon flash lamp as the excitation source. The
lifetime data were acquired with a FluoroHUB TCSPC. Fluorescence lifetime data were acquired
with a 1 MHz LED laser with the excitation peak at 285 nm (NanoLED-280). Phosphorescence
lifetime data were acquired with a 1 MHz LED laser with the excitation peak at 281 nm
(SpectraLED-280), 344 nm (SpectraLED-340). The fluorescence lifetime values were extracted
from the decay curves by iterative reconvolution fitting, which accounted for the instrument
response function (IRF). Lifetime data were analyzed with Data Station v6.6 (Horiba Scientific).
Powder X-ray diffraction patterns were recorded on a Multifunctional Rotating-anode X-ray
Diffractometer - Rigaku SmartLab. Single crystal samples were obtained by slow evaporative
crystallization of compounds in acetonitrile or chloroform solutions. Single crystal data were
collected at room temperature using a Rigaku Oxford Diffraction XtaLAB PRO 007HF (Cu) with
graphite monochromated Cu-K\alpha radiation (\lambda = 1.54184 \mathring{A}). The PFL film was prepared by
dissolving PFL (10-5 M) completely in 2-methyltetrahydrofuran (mTHF) or dichloromethane
(DCM) through gentle heating. The solution was then rapidly cooled to 77 K to induce gelation.
The resulting organogel was centrifuged to remove the majority of the solvent and subsequently
dried in a vacuum oven at 30 ℃ to obtain the film. Scanning electron microscope (SEM) images
used for morphology study were obtained via ZEISS GeminiSEM 500 schottky field emission
scanning electron microscope.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors have addressed all my concerns, I have no more questions now.I suggest its publication.", "reviewer_3": "The authors have well addressed the comments and the revised manuscript at this stage is acceptable.", "reviewer_4": "I am not an expert on polymer aggregates in general or on fluorene aggregates specifically on which there appears to be a large volume of previous literature, but I can comment on a number of the issues raised by reviewer 2. In particular Most of Reviewer 2's comments could be simply resolved through improved and less hyped language. In particular the desire to retain \"coherent phosphorescence\" is, in my view, inappropriate. This is an ensemble experiment and the emission is certainly incoherent regardless of whether the emitting states are delocalized or not. Similarly the word \"modulating\" in the title is not likely the best/clearest choice. \"Influencing\" or even \"controlling\" would be clearer. Similarly \"ultra-narrowband\" is a relative term that requires context.\nAs the other reviewers feel the paper is significant I would be comfortable recommending publication if the authors remove the excessive hype throughout, and come up with a better more factually accurate title and conclusion."}, "author_response": "We thank the reviewer for the constructive and objective evaluation of our manuscript. In accordance with the recommendations, we have systematically revised the manuscript to eliminate any perceived hype. First, we have revised the title to \"Controlling Narrowband Phosphorescence and Delayed Fluorescence by Aggregate Size\". Throughout the text, we have replaced instances of \"modulating\" with more objective terms such as \"controlling\", \"influencing\", or \"tuning\" to accurately describe the effect of aggregate size on exciton dynamics. Furthermore, to accurately describe the underlying physics without mischaracterizing the emission field, we have systematically removed the phrases \"phosphorescence coherence\" and \"coherent phosphorescence\" from the abstract, main text, and conclusion. Instead, we now use \"phosphorescence induced by exciton coherence\" to strictly denote the spatial delocalization of the exciton, and we use the factual term \"narrowband phosphorescence\" when describing the optical phenomenon itself. Additionally, taking the comment on relative terms into account, we have removed the prefix \"ultra-\" and simply use \"narrowband\" throughout the manuscript to maintain strict objectivity. We believe these thorough and targeted revisions have successfully stripped away the excessive hype, resulting in a much more rigorous, objective, and scientifically accurate manuscript."}]

Decision letter