Ultrafast Singlet Exciton Fission Dynamics in 9,10-Bis(phenylethynyl

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C: Physical Processes in Nanomaterials and Nanostructures

Ultrafast Singlet Exciton Fission Dynamics in 9,10(Bisphenylethynyl)anthracene Nanoaggregate and Thin Film Biswajit Manna, Amitabha Nandi, and Rajib Ghosh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05260 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 25, 2018

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The Journal of Physical Chemistry

Ultrafast Singlet Exciton Fission Dynamics in 9,10(Bisphenylethynyl)anthracene Nanoaggregate and Thin Film Biswajit Manna1, Amitabha Nandi1, 2 and Rajib Ghosh1* 1

Radiation and Photochemistry Division, Bhabha Atomic Research Center, Mumbai 400085, India 2

Homi Bhabha Nation Institute, Training School Complex, Anushakti Nagar, Mumbai, 400094, India ABSTRACT Possibility of overcoming Shockley−Queisser limit of organic solar cell (OSC) efficiency by multi exciton generation through singlet exciton fission has attracted significant research interest in recent time. Herein we show that 9,10-(bisphenylethynyl)anthracene (BPEA), an ethynyl derivative of anthracene and widely used fluorescence molecular probe exhibits efficient singlet exciton fission process in solid state. Steady state and time resolved emission experiments carried out on nanoaggregate and thin film of BPEA reveals orders of magnitude reduction in emission yields and singlet lifetime as compared to near unity emission yield and long emission lifetime in solution. Femtosecond and nanosecond resolved transient absorption studies unraveled exciton-exciton annihilation(at high excitation fluence) and singlet exciton fission to be the dominant relaxation process with fission yield of about 74 ± 6 %. High singlet fission yield with long triplet lifetime (about 30 µs) of BPEA in thin film and aggregate form makes this material an interesting candidate for further study in OSC application. 1. INTRODUCTION Potential applications of organic semiconducting materials in opto-electronics and photovoltaics have attracted continued research interest on solid state photophysics to unravel structure and morphology dependence of exciton relaxation behavior.1,2 Possibility of overcoming Shockley−Queisser limit of photovoltaic efficiency by singlet fission(SF) induced multiple exciton generation has gained great research impetus in last few years.1,3-8 SF is a photophysical process in which singlet excited state of a chromophore interact with a nearby ground state molecule to form two triplet states. Extracting charge carriers from these triplet pairs generated from single photon absorption is shown to boost efficiency of solar photovoltaics significantly.9-11 The singlet exciton fission process in organic material is limited by the 1 ACS Paragon Plus Environment

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conditions of strong molecular coupling and energy conservation law i.e. the singlet energy must be greater or equal to the double of lowest triplet energy (2×ET ≤ ES).8 Several polyacene derivatives (e,g, tetracene, pentacene rubrene, perylenebisimides), isobenzofurans, thiophene, diketopyrrolopyrrole, caretenoid derivatives, where the thermodynamic and kinetic conditions for SF is satisfied and has been discovered to exhibit efficient SF process.12-35 A great deal of theoretical and experimental research have been undertaken to underpin the singlet fission mechanism, to provide design principle for synthesis of singlet fissile materials. 36-41 However, the number of SF material discovered so far is limited and library of SF materials is continuously expanding.8,9 Herein we report for the first time, a well known, commercially available anthracene derivative, known as 9,10-bis(phenylethynyl) anthracene (BPEA, refer to Scheme 1 for structure), which exhibit singlet exciton fission upon photoexcitation of nanoaggregate or thin film samples. In solution phase, BPEA absorbs in blue region of visible light and provides strong green emission with the emission yield of near unity.42,43 Because of high fluorescence yield, the molecule has been extensively used as fluorescence probe in chemical and biological medium.44-47 BPEA nanostructures and crystals have been extensively studied in organic electronic devices because of remarkably high charge carrier mobility.48 However, detailed studies on photoinduced exciton dynamics is not reported. We show that in aggregate state or in thin film, strong intermolecular interaction provides necessary coupling to facilitate efficient SF. From historical perspective, we note that SF was first identified in anthreacene crystal about five decades ago, even though SF yield is very small due to endothermic nature of the process.49 As theoretically predicted for other derivatives, phenylethynyl substituents in BPEA seem to perturb the singlet and triplet energy levels to make the SF process thermodynamically feasible.50 Indeed, in solution phase, experimental report suggests singlet and triplet levels are at about 2.6 eV and 1.3 eV, respectively.47,51-53 However, in solution, inefficient spin orbit coupling makes ISC mediated triplet relaxation path insignificant.47 In solid state, strong intermolecular interaction seems to provide necessary coupling, as we identified efficient SF in thin film and nanoaggregate of BPEA by steady state and ultrafast optical spectroscopy.

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Scheme 1: Structure of the BPEA molecule 2. EXPERIMENTAL SECTION 9,10-bis(phenylethynyl)anthracene (BPEA) (purity > 97%) was obtained from Aldrich and purified by vacuum sublimation. Platinum octaethylporphyrin (purity > 98%), Polyvinyl alcohol (PVA) (99% Hydrolyzed) and HPLC grade tetrahydrofuran (THF) were purchased from Sigma Aldrich and used without further purification. Nanopure water was used in sample preparation and spectroscopic experiments of nanoaggregates Nanoaggregates were synthesized using reprecipitation method.54-56 500 µl of 1.5 mM THF solution of BPEA injected in 10 ml aqueous solution of PVA (1 mg/ml) in stirring condition.49,50 PVA acts as stabilizer of dispersed nanoaggregates in aqueous solution. Morphological characterization of nanoaggregates were performed by dynamic light scattering (Malvern 4800 Autosizer having 7132 digital correlator) and atomic force microscopy (NTMDT, Solver Model) experiments. Thin film of BPEA was prepared by thermal vapour deposition (SMART COAT 3 from Hind High Vac. India) on a transparent glass under high vacuum (1 × 10-6 Milibar base pressure) condition. Approximately 50 nm thick film was deposited on glass substrate kept at room temperature with a deposition rate of 0.2 Å- 0.5Å.

The X-ray diffraction pattern of vacuum

dried nanoaggregate and thin film samples were recorded on a Agilent Super Nova X-ray diffractometer using Cu-kα radiation (λ = 1.5418 Å). The UV-Visible absorption and photoluminescence spectra were recorded using a JASCO (model: V-670) spectrophotometer and Horiba JobinYvon (Model: Fluorolog-3), spectrofluorometer, respectively.

Time Correlated Single Photon Counting (TCSPC) 3 ACS Paragon Plus Environment


measurement (IBH, U.K.) was employed to measure photoluminescence decay kinetics after exciting the sample with 405 nm light. Sub picosecond resolved fluorescence and transient absorption experiments were carried out using fluorescence upconversion (FOG-100, CDP corporation, Russia) and ultrafast transient absorption (ExciPRO, CDP corporation Russia) spectrometers coupled to an femtosecond amplified laser system (Amplitude Technologies, France). The details of the experimental set up is described elsewhere.55,57 Thin film deposited glass substrate was mounted on a rotating cell and aqueous dispersed nanoaggregate sample was put in a 1 mm thick rotating cell. The samples were excited at 390 nm. Transient absorption (fluorescence) was recorded at the magic angle polarization of the pump and probe (gate) pulse. Transient absorption spectra were corrected for wavelength dependent group velocity dispersion. Temporal kinetics were fitted with a sum of exponential convoluted with the instrument response function. Nanosecond flash photolysis experiments were performed using a Laser Kinetic Spectrometer (Edinburgh Instruments, UK, model LP920). Sample were excited by a 7 ns NdYAG laser either at 532 nm (second harmonic) or at 355 nm (third harmonic) and probed with a pulsed xenon lamp. All spectroscopic measurements were carried out in ambient condition and at room temperature. 1. RESULTS AND DISCUSSIONS The average diameter of the BPEA nanoaggregates prepared by reprecipitation method was measured to be about 170 nm by DLS method (Figure 1A). AFM analysis (Figure 1B) confirmed that diameter of the particles to be in the range of 150 to 200 nm and height of about 20 (±5) nm. The XRD pattern (Figure S1, supporting information section) of the vacuum dried nanoaggregate and thin film exhibit similar pattern like power XRD data of BPEA in crystalline form.58 Good match of XRD patterns of the sample with the single crystal data suggests that only one polymorph dominates in the sample. We note that XRD of nanoaggregate exhibits broad background scattering as compared to sharper peaks in thin film. This indicates contribution of amorphous phase in nanoaggregate prepared by reprecipitation method.


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15

% Contribution (Intensity)

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10

5

0 100

200 300 Size (nm)

400

A

B

Figure 1: (A) Particle size distribution of BPEA nanoaggregates obtained from DLS measurement, (B) AFM image of the BPEA nanoaggregates drop-casted on freshly cleaved mica. Figure 2A shows steady state absorption spectra of nanoaggregate (NA) and thin film (TF) of BPEA along with monomer spectrum measured in THF solution. Absorption spectra of NA and TF exhibit significant broadening as compared to molecular absorption in solution. The lowest energy absorption band is red shifted by about 35 nm and 60 nm for NA and TF samples, respectively. This indicates strong intermolecular interaction in solid phase which perturbs excitonic transition. Dominant J type of interaction along the long axis of the molecule (i.e., along phenylethynyl chain), as seen in the crystal structure of BPEA renders red shifted absorption band for lowest energy singlet excitonic transition. In solution, the phenylethynyl ring can freely rotate and thus may exist in various conformations which blurs the sharp vibronic feature of absorption spectrum typical of anthracene derivatives. However, aggregation induced planarization of the phenylethynyl groups may also contribute to absorption red shift. Greater extent of broadening and red shift of the absorption edge in film possibly originates from better molecular packing offering stronger intermolecular interaction in TF.



(A)

60 nm 35 nm

THF solution Nanoaggregate Thin film

THF solution Nanoaggregate Thin film

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Time (ns) Figure 2: The steady state UV-Visible absorption (A), Room temperature photoluminescence spectra (B) and photoluminescence decay(C) recorded after excitation with 405 nm light for BPEA in tetrahydrofuran THF solution, NA dispersed in water and thin film. Inset shows temporal profile recorded at 530 nm with BPEA NA sample using fluorescence up-conversion technique.

The room temperature photoluminescence measurement (Figure 2B) shows that in TF and NA, emission spectra are red shifted as compared to that of solution. Interestingly, emission 6 ACS Paragon Plus Environment

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in solid state is strongly quenched (about fifty fold) as compared to strong emission in solution. Relative to near unity emission yield of BPEA in solution,42,43 the photoluminescence yield of BPEA in NA and TF is only about 2% (Table 1).

Strong emission quenching in solid state

suggests that intermolecular interaction opens up new channel for nonradiative deactivation of singlet excitons created by photoexcitation. Time resolved photoluminescence measurement (Figure 2C) were carried out to determine excited state decay lifetime. In accordance with steady state measurement, photoluminescence decay kinetics of BPEA in NA and TF decays orders of magnitude faster than molecular emission decay in solution. As compared to 3.2 ns lifetime59 of molecular emission decay, major decay of emission in NA and TF occurs within instrument response function of our TCSPC set up (i.e. about 150 ps). Table 1: Photophysical parameters of BPEA in THF solution, NA and TF. Sample

Φfl

τ (amplitude)

BPEA Solution

1

3.18 ns (100)

BPEA NA

0.018

20 ps (98)

BPEA TF

0.020

22 ps (98)

Using fluorescence upconversion set up having temporal resolution of about 200 fs, the main component of photoluminescence decay lifetime was measured to be about 20 ps (Inset of figure 2C). This suggests in TF and NA, singlet excitons mainly relax by a nonradiative decay channel. However, we observed an ultrafast component in the photoluminescence decay measured by fluorescence up-conversion technique which depends on the excitation intensity. At higher excitation density, ultrafast component of photoluminescence decay becomes faster and suggests a bimolecular decay mechanism of the singlet excitons. At higher exciton density created under high excitation fluence, diffusion of excitons followed by exciton-exciton annihilation is known to contribute to decay of the excitons.54,55,60 By varying the excitation intensity and probing exciton annihilation kinetics, diffusion properties of excitons in organic semiconductor can be estimated. 54, 55, 60. In presence of bimolecular exciton - exciton annihilation along with unimolecular decay of excitons, decay kinetics can be represented by the following equation:54 7 ACS Paragon Plus Environment


=

(1)

Where, [S1]t, [S1]0 are the singlet exciton density at time t and 0; k1 represents the annihilation free decay rate constant; k2' = k2[S1]0 where k2 is exciton – exciton annihilation rate constant. As transient photoluminescence is proportional to exciton density, photoluminescence decay signal follows the same trend and can be represented by the following equation, =

+

(2)

Here, It and I0 are the photoluminescence intensity at time t and 0 and R represent the residual signal. Figure 3A shows the ultrafast component of transient photoluminescence decay of BPEA NA sample at different excitation fluence along with the fit function given in equation 2. The fit parameters are given in Table S1 in supporting information (SI). Exciton – exciton annihilation rate constant (k2) was obtained from the slope of k2ʹ vs [S1] 0 plot (Figure 3B). Similar experiment and analysis also carried out with the TF samples the experimental data are provided in the SI section (See Figure S2 and Table S2).

Eneregy/ Pulse

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0.9 Th

[S1]0 =

0.6 0.18 x 1020 cm-3

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Figure 3: (A) Temporal profiles recorded for 530 nm emission after photoexcitation with 390 nm light having different energy/pulse for BPEA NA. The black lines show the fit functions for those 8 ACS Paragon Plus Environment

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temporal profiles. Inset: Temporal profiles recorded at 9µJ/Pulse for longer time period. (B) k2' vs. [S1] 0 plot . Table 2: Exciton diffusion parameters estimated through analysis of excitation intensity dependent exciton – exciton annihilation kinetics. Sample

k2

Ra

D

τ

LD

NA

2.8 × 10-9 cm3 s-1

1.1 nm

10.1 × 10-4cm2. s-1

18 ps

3.3 nm

TF

2.32 × 10-9 cm3 s-1

1.29 nm

7.2 × 10-4cm2. s-1

20 ps

2.9 nm

The exciton density at the onset of annihilation has been utilized to estimate the annihilation radius (Ra), (see SI section Calculation S1 and S2)54,55 and singlet exciton diffusion coefficient(D) was determined using equation 3. The diffusion coefficient is found to be comparable to that of previously reported for anthracene. Further, diffusion length was estimated by equation 4. In absence of annihilation, unimolecular decay lifetime of singlet excitons estimated for both cases and found to have lifetime of 18 ps and 20 ps for NA and TF, respectively. The singlet exciton diffusion parameters for both forms are found to be comparable. The short lifetime of singlet exciton causes very short exciton diffusion length of ~3 nm. It is important to note that while charge carrier mobility of BPEA nanocrystal is remarkably high47 and consequently exhibits potential application in organic transistors61, exciton diffusion length in nanoaggregate is limited to a few nanometer, largely due to ultrafast nonradiative exciton deactivation within a few tens of picoseconds. k2 = 8πDRa

(3)

LD = (6Dτ)1/2

(4)

Ultrafast Transient Absorption Study: To unravel the nonradiative decay mechanism of singlet excitons in BPEA nanoaggregate and thin film, ultrafast transient absorption measurements were carried out. The transient absorption spectra of BPEA in THF solution, shown in Figure S3 in the SI section, exhibit a negative band corresponding to ground state bleach in 400-470 nm and stimulated emission in 470-520 nm region and a positive excited state absorption (ESA) band associated with the S1Sn transition in 520-650 nm region. At initial few picosecond time delays, ESA band displays small red shift with increase in intensity 9 ACS Paragon Plus Environment


attributed to vibrational cooling and planarization of the phenylethynyl rings in the S1 state. The transient spectrum does not evolve further and only decays in intensity in several nanosecond timescale, consistent with long lifetime of the S1 state (3.1 ns). 650 nm

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0-6 ps

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Figure 4: (A) Transient absorption spectra recorded at different delay times for BPEA NA after excitation with 390 nm laser pulse having 10µJ/Pulse energy (B) temporal profiles along with fitted data recorded at 650 nm, 508 nm and 475 nm, respectively. Table 3: Fitting Parameters of the temporal profiles recorded at different wavelengths for BPEA NA. Wavelength (nm)

τ1 (a1)

τ2 (a2)

τ3 (a3)

475

------

20 ps (24)

Long (>300 ps) (-17.5)

508

1.2 ps (66)

24 ps (-56)

Long (>300 ps) (24.5)

650

0.9 ps (-54.5)

25 ps (-12.5)

-----


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Upon aggregation the excited state dynamics of this molecule completely changes (Figure 4). Following photoexcitation by 390 nm femtosecond laser pulse, nanoaggregate exhibits broad ESA band of S1 excitonic state extended for 530-750 wavelength region with maximum at 650 nm (figure 4A). The ground state bleach (GSB) signal which is expected to appear in the entire 400-520 nm region, only partly appear in 500-520 nm region and 400-420 wavelength region due to overlap with positive ESA signal in this region. The ESA signal in 550-750 nm region is purely associated with S1 state absorption and proportional to the S1 excitonic population. Major intensity of this ESA signal promptly decays in first 4-5 ps timescale followed by slow decay in 100 ps timescale. The temporal kinetics recorded at 650 nm exhibits biexponential decay with lifetime of about 1 ps and 25 ps (Figure 4B). The first decay component is observed to be dependent upon the excitation intensity and is attributed to exciton exciton annihilation process. This is consistent with transient photoluminescence decay kinetics described in previous section. The second component represents lifetime of the excitonic state in annihilation free regime. The time constant matches well with that obtained from transient photoluminescence experiment. The decay time of the S1 excitonic state thus appears to be almost two orders of magnitude faster than the S1 decay time in solution phase. The mechanistic insight to the exciton decay path is obtained from the transient spectral evolution in bleach region. Following ground state bleach (GSB) recovery in first few picoseconds due to exciton-exciton annihilation induced ground state repopulation, GSB signal grows in intensity up to 100 ps (Figure 4A). Concurrently, ESA signal at 480 nm region also exhibits rise in intensity during this time scale. The transient species formed in 100 ps does not decay in 1 ns (which is temporal window of our transient spectrometer) and thus represents a long lived species. Growth of GSB and concomitant rise in ESA at 480 nm region clearly suggests that more ground state population is excited by interaction with the singlet excitons. This kind of transient spectral evolution in molecular solids in crystalline, thin film or aggregate is explained by singlet exciton fission process where one singlet exciton share its energy with a ground state molecule to generate two triplet excitons.8,

62

We note that second component is independent of excitation intensity as

expected in the annihilation free temporal regime (Figure S4 in SI section). As detailed in the next section, the long lived transient spectrum is characterized to be triplet state. Our ultrafast transient spectral study reveals that exciton – exciton annihilation (at high excitation density) 11 ACS Paragon Plus Environment


followed by singlet fission is the major relaxation channel of the singlet excitons created upon photoexcitation of BPEA nanoaggregate.36, 63 The transient spectral dynamics and timescale was observed to be similar in vapour deposited thin film of BPEA (Figure S5 and table S3 in SI section). Thus, singlet fission in BPEA is activated of solid phase, mainly promoted by strong intermolecular interaction, as in solution phase, exciton fission dynamics was not observed. In solid crystalline state, intermolecular interaction provides necessary coupling for fission process.64 The energetic criterion for singlet exciton fission i.e. 2×ET ≤ ES seems to be fulfilled in BPEA. Historically, signature of singlet fission was first observed in anthracene crystal.48 However, the singlet fission yield in anthracene crystal is very small due to endothermic nature of the process. In BPEA, the biphenylethynyl substituents possibly perturb the electronic states to fulfill the energetic criterion of fission process. Quantum chemical calculation on anthracene derivatives by Bhattacharyya.et. al. have revealed that ethynyl substituent perturb singlet and triplet levels towards favorable energies to make fission process thermodynamically feasible.50, Experimentally, BPEA based triplet-triplet annihilation (reverse process of singlet fission) mediated fluorescence upconversion system is reported to be less efficient and is attributed to lowering of triplet levels.51 Characterization of triplet state:

BPEA in solution exhibits near unity emission

quantum yield and triplet population is immeasurably small.47 To record the triplet spectrum and triplet extinction coefficient of BPEA in solution, sensitization experiment was performed by platinum-octaethylporphyrin (PtOEP). PtOEP is chosen as the sensitizer as it is known to efficiently transfer energy to anthracene derivatives and this property had been extensively exploited in TTA-upconversion system.65,66 In a mixture of BPEA and PtOEP in THF solution, PtOEP was selectively excited at 532 nm by a nanosecond laser. PtOEP undergoes intersystem crossing to triplet state in a few picosecond time scale and decays in hundred microsecond timescale.67,68 Hence energy transfer from triplet PtOEP (3PtOEP*) to BPEA could be monitored by probing the nanosecond to a few microsecond transient spectral evolution. At 10 ns delay, transient spectrum of 3PtOEP* is characterized by triplet absorption at 423 nm and ground state bleach at 540 nm (Figure S6 in SI section). After 2 microsecond, due to triplet energy transfer to BPEA, both bleach and ESA of 3PtOEP* decay with concurrent ESA increase at 440-500 nm region with absorption maximum at ~480 nm, which is attributed to absorption of 3BPEA*. We


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note that triplet absorption by 3BPEA* measured in solution closely matches with the transient absorption of NA and TF of BPEA measured in femtosecond experiment. We further confirmed triplet nature of the long lived transient in NA and TF of BPEA by recording nanosecond transient absorption spectra at different delay times using nanosecond flash-photolysis technique. We have excited the nanoaggregate and thin film samples at 355 nm by ns laser and recorded the transient absorption spectra in the µs time domain (Figure 5 and figure S7 in SI section). Both NA and TF exhibits ESA band in the 440-490 nm wavelength region along with a bleach signal at 530 nm and the lifetime of this transient is measured to be about 35 (±5) µs. The microsecond spectrum nicely matches with the picosecond transient spectrum (Figure S8 in SI section) recorded in ultrafast study and clearly support our assignment of triplet formation in BPEA NA and TF in tens of picosecond timescale.

In addition,

comparison of triplet spectrum obtained from sensitization experiment and microsecond transient spectrum of nanoaggregate (figure S9 in SI section) shows excellent match in triplet absorption band region (bleach band region does not exactly match due to changes in absorption spectrum from solution to solid state). This further substantiates the long lived transient spectrum is characteristics of triplet exciton and not reminiscent of any signal generated from spurious thermal effect. τ =32 µs

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Figure 5: Transient absorption spectra recorded at different delay times for BPEA NA after excitation with 355 nm light. Inset: temporal profiles along with fitted data recorded at 475 nm. The picosecond transient spectral evolution having the special feature of growth in GSB signal concomitant with ESA growth in 480 nm region characterized to be long lived triplet is proposed as a definite proof of singlet fission mediated triplet exciton generation in BPEA thin film and NA. An important checkpoint of SF induced efficient triplet formation is measurement of triplet yield. Triplet yield of more than 100 % is expected in case of SF induced triplet formation provided SF yield is more than 50%. Estimation of triplet yield requires knowledge about the molar absorption coefficient of singlet and triplet state at the monitoring wavelength. Due to strong overlap of ground state absorption and ESA of singlet exciton with triplet exciton absorption band, some reasonable assumptions were introduced to estimate the triplet yield. As described in supporting information (Calculation S3 in SI section) and triplet yields in NA and TF of BPEA are estimated to be about 137 (± 5) % and 158 (± 5) % respectively. This corresponds to SF yield of about 69% and 79% in NA and TF. Slightly lower triplet yield in nanoaggregate sample as compared to that in thin film is attributed to presence of amorphous phase in nanoaggregate samples prepared by reprecipitation method which decreases SF efficiency slightly.. 5. CONCLUSION In conclusion, we have discovered efficient singlet fission in solid state of a well known commercial fluorescent dye 9, 10 bis-phenylethynylanthracene (BPEA). Present work explored singlet exciton diffusion and exciton fission in BPEA in NA dispersed in water and vapour deposited TF form. It is revealed NA and TF of BPEA undergo efficient singlet fission in a few tens of picosecond to generate long lived triplet state in contrast to negligible triplet yield and near unity fluorescence quantum yield in solution. Presence of two phenylethynyl substituents at 9, 10 position of anthracene possibly alter the singlet and triplet levels to make fission process energetically feasible and strong intermolecular interaction in solid state provides necessary electronic coupling for SF which is absent in solution phase. Generation of long lived triplet excitons in high yield by SF mechanism make this material potential candidate for solar photovolatics. Further study toward in-depth mechanistic understanding of SF process (for


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example, effect of molecular packing, polymorphism or molecular substituent) is warranted to exploit BPEA derived chromophores in solar photovolaics. ABBREVIATIONs BPEA: 9,10-(bisphenylethynyl)anthracene, OSC: organic solar cell, TF: thin film; NA: nanoaggregate; PVA: polyvinyl alcohol, THF: tetrahydrofuran, TCSPC: Time Correlated Single Photon Counting, AFM: atomic force microscopy, DLS: dynamic light scattering, XRD: X-ray Diffraction, TTA: triplet-triplet annihilation, SF: singlet fission, ISC: intersystem crossing; ESA: excited state absorption; SE: stimulated emission,; GSB: ground state bleach, PtOEP: platinumoctaethylporphyrin ASSOCIATED INFORMATION SUPPORTING INFORMATION XRD data for BPEA nanoaggregate and thin film, Supporting transients spectral data, and Calculations related to exciton diffusion and fission parameters AUTHOR INFORMATION *Corresponding author: Dr. Rajib Ghosh, E-mail: [email protected], [email protected] Note: Authors declare no competing financial interest. ACKNOWLEDGEMENTS Funding from department of atomic energy is gratefully acknowledged. The authors gratefully acknowledge the kind help of Dr. V. Sudarshan and Dr. R. Ganguly of Chemistry Division, BARC for AFM and DLS measurements, respectively. We are also thankful to B. G. Vats of Fuel Chemistry Division, BARC, Mumbai for helping in XRD measurement of the samples. Authors gratefully acknowledge support and encouragement from Dr. S. Nath, Dr. H. Pal and Dr. P. D. Naik of RPC division, BARC.



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TOC Graphic


Ultrafast Singlet Exciton Fission Dynamics in 9,10-Bis(phenylethynyl

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