Effects of Crystal Morphology on Singlet Exciton Fission in

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Effects of Crystal Morphology on Singlet Exciton Fission in Diketopyrrolopyrrole Thin Films Patrick E Hartnett, Eric A. Margulies, Catherine M Mauck, Stephen A. Miller, Yilei Wu, Yi-Lin Wu, Tobin J. Marks, and Michael R. Wasielewski J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b10565 • Publication Date (Web): 28 Jan 2016 Downloaded from http://pubs.acs.org on February 4, 2016

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The Journal of Physical Chemistry

Effects of Crystal Morphology on Singlet Exciton Fission in Diketopyrrolopyrrole Thin Films Patrick E. Hartnett1, Eric A. Margulies1, Catherine M. Mauck1, Stephen A. Miller1, Yilei Wu1, Yi-Lin Wu,1 Tobin, J. Marks1,2* and Michael R. Wasielewski1* 1

Department of Chemistry and the Argonne-Northwestern Solar Energy Research Center,

2

Department of Materials Science and Engineering and the Materials Research Center,

Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA

Abstract Singlet exciton fission (SF) is a promising strategy for increasing photovoltaic efficiency, but in order for SF to be useful in solar cells it should take place in a chromophore which is air-stable, highly absorptive, solution processible, and inexpensive. Unlike many SF chromophores, diketopyrrolopyrrole (DPP) conforms to these criteria, and here we investigate SF in DPP for the first time. SF yields in thin films of DPP derivatives, which are widely used in organic electronics and photovoltaics, are shown to depend critically on crystal morphology. Timeresolved spectroscopy of three DPP derivatives with phenyl (3,6-diphenylpyrrolo[3,4-c]pyrrole1,4(2H,5H)-dione, PhDPP), thienyl (3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)dione,

TDPP),

and

phenylthienyl

(3,6-di(5-phenylthiophen-2-yl)pyrrolo[3,4-c]pyrrole-

1,4(2H,5H)-dione, PhTDPP) aromatic substituents in 100-200 nm thin films, reveals that efficient SF occurs only in TDPP and PhTDPP (τSF = 220 ± 20 ps), despite the fact that SF is most exoergic in PhDPP. This result correlates well with the greater degree of π-overlap and closer π-stacking in TDPP (3.50 Å) and PhTDPP (3.59 Å) relative to PhDPP (3.90 Å), and demonstrates that SF in DPP is highly sensitive to the electronic coupling between adjacent 1 ACS Paragon Plus Environment


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chromophores. The triplet yield in PhTDPP films is determined to be 210 ± 35% by the singlet depletion method and 165 ± 30% by the energy transfer method, showing that SF is nearly quantitative in these films and that DPP derivatives are a promising class of SF chromophores for enhancing photovoltaic performance.

INTRODUCTION Singlet exciton fission (SF) is a spin-allowed process in which a singlet exciton splits into two triplet excitons on adjacent molecules.1 Although SF was first proposed as a mechanism for delayed fluorescence in anthracene in 1965, it has been the subject of increasing recent research interest because of its potential to enhance the efficiency of photovoltaic devices.2-5 The design of a molecular chromophore that undergoes SF efficiently presents a significant challenge that requires optimization of both energetics and intermolecular electronic coupling. Efficient SF requires that the singlet excited state energy (ES) be greater than or equal to twice the triplet excited state energy (ET) for the process to be energetically favorable.1 Additionally, the electronic coupling between neighboring chromophore molecules must be optimized to allow SF to proceed more rapidly than other excited state decay pathways.6 To date, two classes of molecules have been proposed as potential candidates for SF, biradicaloids and large alternant hydrocarbons, the latter of which, including polyacenes, have been the most extensively studied family of SF molecules.4, 7-12 The polyacenes, in particular tetracene, pentacene, and their derivatives, have been shown to undergo SF in high yields; however

many

derivatives

are

oxidatively

unstable

under

ambient

conditions.13

Diphenylisobenzofuran, a molecule with significant biradical character, has also demonstrated 200% triplet yields due to SF;7 however films of diphenylisobenzofuran convert to a polymorph

2 ACS Paragon Plus Environment

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with low SF yields in a matter of several days.4,

10

More robust chromophores such as

perylenediimides as well as some charge transfer polymers and naturally occurring carotenoids have also been shown to undergo SF with varying yields.1, 4, 14-16 SF is of interest for photovoltaic applications because in principle it can be used to overcome the Shockley-Queisser (SQ) power conversion efficiency (PCE) limit. The SQ PCE limit for single junction photovoltaic devices is 33.7%, which can be exceeded by using multi-exciton generation (MEG).17-18

Specifically, SF is the molecular analogue of MEG with a carrier

multiplication factor of 2, and offers a theoretical PCE limit of 44.4%, when the SF chromophore is paired with a second chromophore having an excited singlet state energy close to ET of the SF chromophore.17 To date, several SF chromophores have been incorporated in photovoltaic devices designed to take advantage of SF, with pentacene being particularly successful due to its fast SF rate and its low oxidation potential, making it a suitable electron donor in a variety of systems. For instance, an organic photovoltaic (OPV) device utilizing pentacene as the donor material and C60 as the acceptor material achieved an external quantum efficiency over 100%.19 Additionally, a hybrid inorganic-organic solar cell, in which triplet excitons generated in pentacene by SF undergo charge separation with PbS nanocrystals, resulted in collectable photocurrent, and device efficiencies approaching 5% could be achieved using PbSe nanocrystals.20-21 While these systems clearly demonstrated the potential of SF in photovoltaic systems, they are limited by thin optimal thicknesses of the SF chromophore layer. Therefore, one way to address this limitation would be to develop more strongly absorptive SF chromophores.4 Based on the AM1.5 solar spectrum, the theoretically most efficient chromophore for enhancing photovoltaic performance via SF should have a singlet excited state energy near 2.2



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Chart 1. Structures of the Molecules Discussed Here.

eV and a triplet excited state energy near 1.1 eV. This material could then be paired with a chromophore having a singlet excited state energy of 1.1 eV.4,

17

One intriguing class of

chromophores that satisfies these SF energetic requirements are diketopyrrolopyrroles (DPPs), which, unlike the vast majority of singlet fission chromophores, are not based on polycyclic aromatic cores. DPP (Chart 1) is a commonly used chromophore in organic electronics and photovoltaics because it strongly absorbs visible light, and is chemically robust, inexpensive, straightforward to modify synthetically, stable under ambient conditions, and a p-type semiconductor with good hole mobility.22-27 Janssen and coworkers reported the triplet state energy of 3,6-bis(thien-2-yl) DPP (TDPP) to be 1.1 eV in toluene as determined by the energy transfer method.28 Given that the singlet excited state energy of TDPP lies at 2.2 eV, this suggests that the energetic requirement for SF is satisfied in TDPP. Indeed, we recently reported rapid photo-induced triplet exciton formation in thin films of 3,6-bis(5-phenylthien-2-yl)-DPP (PhTDPP) and have proposed SF as a possible mechanism.29 Given that most other reported SF chromophores are expensive and tedious to synthesize, more weakly absorptive, unstable towards oxidation or phase changes, and/or have low triplet yields, and considering that DPP is solution-processible, and a good p-type semiconductor, DPP clearly has the potential for developing more efficient devices based on SF.


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In this contribution, we explore SF phenomena in DPP derivatives by examining the photophysics of TDPP, PhTDPP, and 3,6-diphenyl-DPP (PhDPP) in thin films, as they represent a range of common substitutions in DPP chromophores. Single crystal x-ray diffraction structures of the three molecules reveal that both TDPP and PhTDPP form closely-packed columns with π-π stacking distances of ~3.5 Å, while PhDPP packs more loosely with a π-π stacking distance of ~3.9 Å. There is also a significant decrease in the degree of π-π overlap in PhDPP, versus that in TDPP and PhTDPP, due to the different slip angles in the crystal structures. It will be shown below that these packing geometries are consistent with the observation of rapid triplet exciton formation by SF in TDPP and PhTDPP, but not in PhDPP. These results demonstrate that SF in DPP is very sensitive to the electronic coupling between adjacent chromophores. Furthermore, the high triplet exciton yields observed in thin films of TDPP and PhTDPP demonstrate that DPP derivatives are capable of efficient SF, and offer promising design rules for new families of chromophores to enhance organic photovoltaic performance. EXPERIMENTAL Materials Synthesis. PhDPP, TDPP, and palladium octabutoxyphthalocyanine (PdPc) were synthesized and purified according to reported literature procedures.30-31 PhTDPP was synthesized from 3,6-bis(5-bromothien-2-yl)-2,5-bis(2-ethylhexyl)-DPP32 as described below. Phenylboronic acid was purchased from Sigma Aldrich. Synthesis of 3,6-bis(5-phenylthien-2-yl)diketopyrrolopyrrole (PhTDPP). A mixture of 3,6bis(5-bromothien-2-yl)-2,5-bis(2-ethylhexyl) DPP (20 mg, 0.030 mmol) and phenylboronic acid (40 mg, 0.33 mmol) and K2CO3 (90 mg, 0.65 mmol) in toluene (5 mL), ethanol (0.5 mL), and water (0.5 mL) was purged with N2 for 15 min, after which Pd(PPh3)4 (5 mg, 0.004 mmol) was



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added and the mixture was purged with N2 for an additional 10 min. The reaction mixture was then heated at 60 °C overnight. The crude mixture was next cooled to room temperature, poured into water (25 mL), and extracted with CH2Cl2 (2 × 25 mL). The organic layer was separated, dried over Na2SO4, filtered, and the solvents were removed under vacuum and purified by silica gel chromatography (1:1 CH2Cl2:hexanes) to give the product as a purple solid (18.5 mg, 93%). 1

H NMR (400 MHz, CDCl3) δ 8.95 (d, J = 4.1 Hz, 2H), 7.67 (d, J = 7.6 Hz, 4H), 7.49–7.31 (m,

8H), 4.06 (dt, J = 11.2, 5.9 Hz, 4H), 1.93 (m, 2H), 1.45–1.21 (m, 16H), 0.94–0.79 (m, 12H). 13C NMR (126 MHz, CDCl3) δ 161.77, 149.68, 139.93, 136.80, 133.21, 129.18, 128.86, 128.83, 126.17, 124.51, 108.23, 46.01, 39.26, 30.37, 28.59, 23.72, 23.13, 14.09, 10.61. MALDI-TOF (m/z) [M]- = 676.373 (calcd for C42H48N2O2S2: 676.316). Sample Preparation. Solution samples were dissolved in CH2Cl2 at a concentration where the absorbance at the excitation wavelength was approximately 1. Thin film samples were vapor deposited at a pressure of 10–6 Torr onto glass slides with the crucible held at 250 °C and the target held at 90 °C to promote the formation of uniformly crystalline films. The films were then placed in a glass petri dish above a reservoir of CH2Cl2 and were solvent vapor annealed for 3 h to further enhance their crystallinity. The samples were then dried under vacuum for 30 min prior to measurements. The thicknesses of the vapor deposited thin films were determined by profilometry using a Veeco Dektak 150 surface profiler. The thickness of each film was measured three times in three different locations, and the average thickness and standard deviation are reported. Samples for triplet sensitization were prepared by first dissolving mixtures of PhTDPP/PdPc (20:1 w/w), TDPP/PdPc(5:1 w/w), or polystyrene/PdPc (20:1 w/w) in


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chloroform at c = 20 mg/mL. The solutions were then spin-coated on a glass coverslip and dried under vacuum. These samples were not annealed in an attempt to prevent phase segregation. Steady-State Spectroscopy. Steady-state optical absorption spectra of solution samples were measured on a Shimadzu UV-1800 spectrometer. Scatter-corrected absorbances of the thin films were measured using a Perkin Elmer LAMBDA 1050 UV/Vis/NIR spectrometer equipped with an integrating sphere and transmission, reflectance, and corrected absorbance spectra of each film are provided in the Supporting Information (Figure S13). Steady-state photoluminescence spectra were measured in right angle mode with a HORIBA Nanolog spectrofluorimeter equipped with an integrating sphere (Horiba Quanta–φ) for absolute photoluminescence quantum yield determination. The concentration of the air-equilibrated sample solutions was adjusted to obtain absorption values A < 0.1 at the excitation wavelength. One centimeter path length square optical Suprasil Quartz (QS) cuvettes were used for measurements at room temperature of dilute solutions in CH2Cl2. Electrochemistry. Cyclic voltammetry and differential pulse voltammetry were performed using a CH Instruments Model 622 electrochemical workstation on 1 mM solutions of PhDPP, TDPP, PhTDPP, and PdPc in anhydrous CH2Cl2 with 100 mM tetrabutylammonium hexafluorophosphate as the supporting electrolyte. All measurements were done using a platinum disc working electrode, a platinum wire counter electrode, and a silver wire quasi-reference electrode. All redox potentials were referenced to ferrocene/ferrocenium as an internal standard and are reported vs. saturated calomel electrode (SCE). X-Ray Crystallography. Crystals of PhTDPP were grown by slow diffusion of methanol vapor into a solution of the compound in p-xylene. The crystal was mounted on a polymer loop with Paratone oil and the data was collected at 100 K on a Bruker Kappa APEX II CCD



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diffractometer equipped with a CuKα IµS microfocus source with MX Optics. The data were absorption corrected using SADABS. The structure was solved using SHELXT and refined using SHELXL using Olex2 software. All non-H atoms were refined anisotropically and H atoms refined isotropically and constrained to ideal geometries. The structure has been deposited in the Cambridge Crystallographic Data Centre database (CCDC 1052591). Calculations. Triplet energies of PhDPP, TDPP, and PhTDPP were calculated in Q-Chem using (U)B3LYP/6-31+G*. The ground state energy and the lowest energy triplet state were calculated separately and subtracted to give the triplet excited state energy in the gas phase. The ground-state biradical character of PhDPP, TDPP, and PhTDPP, as well as those of the pentacene, tetracene, and diphenylisobenzofuran was analyzed by their natural orbital occupation numbers in the LUMO according to the method of Kudo et al33-35 at the level of CASSCF(2,2)/631G//RB3LYP/6-31G* in ORCA.

The occupation numbers are summarized in the

supplementary material (Table S10). Femtosecond Transient Absorption Spectroscopy (fsTA). Low-fluence transient absorption experiments were performed as follows. The fundamental output (1040 nm, 4.5 W, 350 fs) of a direct diode pumped 100 kHz amplifier (Spirit 1040-4, Spectra Physics) is divided with a beam splitter into two beam paths. White light continuum probe pulses (480-1100 nm) are created by focusing the smaller fraction (0.50 W) to a ~40 µm spot size in a 5 mm thick undoped yttrium aluminum garnet (YAG) crystal. The larger fraction (4.0 W) is used to drive a noncollinear optical parametric amplifier (Spirit-NOPA-3H, Spectra Physics) which creates visible pump pulses (500-800 nm, ~75 fs). Note that although the NOPA output bandwidth is tunable and much shorter pulses are possible, relatively narrow bandwidth pump pulses are used here to minimize the spectral window affected by scattered pump light. This, combined with the chirp


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in the continuum probe, results in an instrument response function of around 100-150 fs over all probe wavelengths. For all experiments, pump and probe polarizations are set at the magic angle with respect to one another. Because exact knowledge of the pump laser spot size is critical to the triplet yield calculations in this study, a 1.00 mm diameter pinhole was placed in the beam paths 2.5 mm in front of the sample position. As the pump beam is focused by a relatively long 750 mm focal length lens, the spot size at the sample position is to a good approximation equal to the pinhole diameter. To minimize photoexcitation densities, all samples are irradiated with only 10 nJ pump pulses as measured through the pinhole. Samples are kept under vacuum inside a cryostat (VPF-100, Janis Research) at room temperature throughout the duration of the experiments to minimize photochemical degradation. After passing through the sample, the continuum probe is spectrally dispersed inside a modified SPEX 270m monochromator equipped with a 600 groove/mm grating. A two inch diameter silver mirror is placed in the beam path prior to the exit slit, which directs the dispersed probe onto a CMOS linear image sensor (S10112-512Q, Hamamatsu). Signal differencing is achieved by modulating the pump beam prior to the sample position at 476 Hz. The signal was averaged for 12 s at each pump-probe delay time, which results in a typical baseline noise of ~5 x 10-6 ∆A. Nanosecond Transient Absorption Spectroscopy (nsTA). The film samples were evacuated to 10-3 Torr in a Janis VPF-100 cryostat at room temperature to minimize potential sample degradation by air. Nanosecond transient absorption experiments were performed by exciting the sample with 7 ns, 1.6 mJ, 532 nm pulses using the frequency-tripled output of a Continuum Precision II 8000 Nd-YAG laser pumping a Continuum Panther OPO. The probe pulse, generated using a xenon flashlamp (EG&G Electro-Optics FX-200), and pump pulse were overlapped on the sample which was placed behind a 1.00 cm diameter aperture used to precisely



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control the spot size. Kinetic traces were observed from 390-800 nm every 5 nm using a 416 nm long-pass filter (above 420 nm), a monochromator, and photomultiplier tube (Hamamatsu R928) with high voltage applied to only 4 dynodes. Kinetic traces were recorded with a LeCroy Wavesurfer 42Xs oscilloscope interfaced to a data system using a custom LabVIEW program (LabVIEW v. 8.6.1). Spectra were constructed from the single wavelength kinetic traces taken every 5 nm. Each kinetic trace is representative of an average of 150 shots. To increase the signal-to-noise ratio of the spectral profiles, 5-10 ns segments of data are averaged, and the median time is reported as the time of the spectral slice. In the case of PhTDPP, the kinetics were also measured every 1 nm from 390 nm to 500 nm in order to better resolve the transient feature centered at 428 nm. RESULTS AND DISCUSSION Structural Characterization. The molecular structures of the three DPP derivatives are shown in Figure 1. All three molecules are based on the same DPP core, but are substituted with

a

b

c

3.89 Å

4.32 Å

2.12 Å

0.13 Å

3.55 Å

3.34 Å

3.50 Å

3.90 Å

3.59 Å

Figure 1. Comparisons of the crystal structures of PhDPP (a), TDPP (b), and PhTDPP (c). Top view (upper row) and side view (lower row). The structures of PhDPP and TDPP were taken from previously reported coordinates.30


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different aromatic groups, PhDPP by phenyl groups, TDPP by thiophenes, and PhTDPP by 5phenylthiophenes. Since these aromatic substituents are π-conjugated with the DPP core, they have a significant effect on the electronic structure of the molecules as well as on their crystal packing. TDPP and PhDPP are solubilized with N-n-hexyl chains, which should allow the molecules to pack as closely as possible. PhTDPP, on the other hand, was synthesized using branched 2-ethylhexyl side chains because the n-hexyl analogue is poorly soluble. The spatial relationship between the neighboring DPP chromophores in the solid state was characterized by single crystal X-ray diffraction, and the crystal parameters for PhTDPP are summarized in Table S1, while the structures of PhDPP and TDPP were reported previously.30 All three molecules pack in infinite π-stacking columns with a variable degree of displacement

a

b

c

Figure 2. Lattice packing of PhDPP (a), TDPP (b), and PhTDPP (c). All three molecules pack in infinite π-stacked columns, but while the columns of PhDPP and TDPP pack in a herringbone motif, PhTDPP packs in a lamellar motif. The structures of PhDPP and TDPP were taken from previously reported coordinates.30

between adjacent molecules (Figure 1). Columns of PhDPP and TDPP pack in a herringbone arrangement, whereas PhTDPP forms lamellar stacks (Figure 2). The herringbone arrangement of PhDPP and TDPP allows for an interaction between the aromatic protons of one column with the carbonyl groups of adjacent columns.36-37 Additionally, T-shaped edge-to-face aromatic



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interactions are observed in PhDPP between neighboring columns.38 In the case of PhTDPP, there are also interactions between the thiophene 4-protons and the carbonyl groups of adjacent molecules (Figure S2). The phenyl rings in PhDPP are twisted at an angle of 32° relative to the DPP core, whereas the aromatic units in TDPP and PhTDPP are substantially more co-planar as a result of reduced steric repulsion and the hydrogen-bonding between the thiophene 3-proton and the DPP carbonyl oxygen.30, 39 Thus, the thiophenes in TDPP are twisted at an angle of 11°, whereas the thiophene and phenyl rings in PhTDPP are twisted by 2.7° and 6.7° relative to the DPP core, respectively. The large torsional angle in PhDPP is likely responsible for the large π-π stacking distance of 3.90 Å observed in PhDPP, compared to interplanar distances of 3.50 Å and 3.59 Å in TDPP and PhTDPP, respectively, as well as the decrease in π-π overlap observed in PhDPP (Figure 1). The π-π stacking distance in PhTDPP is slightly larger than that of TDPP despite both having a high degree of planarity. This can be explained by the more sterically-encumbered branched side chains of PhTDPP which may push the molecules further apart. In-plane and out-of-plane thin film XRD spectra are shown in the Supporting Information (Figure S1). In films of PhDPP and TDPP the feature corresponding to the (100) plane is greatly enhanced in the out-of-plane scattering pattern, while in films of PhTDPP the (001) reflection is enhanced. In addition, the peaks corresponding to the (002) planes of the in-plane scattering patterns of PhDPP and TDPP are enhanced, while the reflection corresponding to the (010) plane is enhanced in films of PhTDPP. In all three molecules this result suggests that the crystals are oriented such that the side chains are normal to the substrate. The reason that different scattering planes are enhanced in PhTDPP films relative to those of PhDPP and TDPP films is that PhDPP and TDPP crystallize in the P21/c space group, while PhTDPP crystallizes


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in the P-1 space group. These results are significant because they not only demonstrate that the reported crystal structures are maintained in the thin films, but that all three molecules are oriented similarly with respect to the substrate. Steady State Optical Characterization. The steady state absorption and fluorescence spectra of PhDPP, TDPP, and PhDPP are shown in Figure 3 and their photophysical properties are summarized in Table 1. It can be clearly seen from the CH2Cl2 solution absorption spectra that the excited singlet state energy decreases with increasing conjugation length and that the twist about the phenyl ring attachment axis results in a blue shift in PhDPP relative to TDPP as well as a loss of vibronic structure. The absorption coefficients of the molecules in solution increase as the absorption maxima redshift with PhTDPP having the strongest absorbance (Table 1). All three DPP derivatives exhibit high fluorescence quantum yields in CH2Cl2 (Table 1). This result is significant in that it shows that there is no intrinsic pathway for rapid singlet excited state quenching in these molecules, and therefore little to no triplet state yield in solution for any of the molecules. This suggests that any triplet state formation in the film must be a result of some intermolecular process such as SF. We also confirmed the relative stability of PhTDPP to

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Figure 3: Steady state absorption and emission spectra of CH2Cl2 solutions (black) and absorption spectra of vapor deposited thin films (red) of PhDPP (220 ± 20 nm, a), TDPP (182 ± 8 nm, b), and PhTDPP (128 ± 2 nm, c).


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photooxidation and found it to be more stable than TIPS-pentacene, which is a commonly used air-stable pentacene derivative (Figure S3). A new red-shifted absorption feature appears in all thin film spectra; this feature is most distinct in the PhTDPP film, while there is only a small shift in the absorption maximum of PhDPP. In the thin solid films the fluorescence of all three molecules is significantly quenched; however, PhDPP fluoresces far more strongly than do the other derivatives. The fluorescence quantum yields of TDPP and PhTDPP in the thin films could not be determined due to their weak fluorescence, while PhDPP has a fluorescence quantum yield of 8.9 ± 0.3%, suggesting that there is an additional mechanism of fluorescence quenching for TDPP and PhTDPP that is not present in PhDPP. Table 1. Photophysical Properties of PhDPP, TDPP, and PhTDPP

a

b

Fluorescence quantum yield in solutions. Fluorescence quantum yield in thin films (PhDPP, 220 ± 20 c

nm; TDPP, 182 ± 8 nm; PhTDPP, 128 ± 2 nm). Average energy of the absorption and emission d

maxima. V vs SCE. *From Patil et al.30

Triplet Energy and Biradical Character Calculations.

In order for SF to be

thermodynamically favorable, the lowest triplet excited state energy of the molecule must be less than or equal to half of its lowest singlet excited state energy. The triplet state energies of all three DPPs were estimated by density functional theory (DFT) at the (U)B3LYP/6-31+G* level of theory in the gas phase. The calculated triplet state energy ET,calc = 1.03 eV for TDPP is less than half of ES,solution = 2.24 eV, and is similar to the reported estimate of ET,solution = 1.1 eV.28 The calculated energies for the PhDPP and PhTDPP triplet states are ET,calc = 1.21 and 1.02 eV,


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respectively, less than or close to half of their respective lowest singlet excited state energies, ES,solution = 2.50 and 2.04 eV. Although the exact relationship between ES,film and ET,film in the thin-film samples is complicated by the red-shift of the singlet excited-state energy, based on the DFT calculations and given the small changes in the film absorption spectra, PhDPP is expected to be the most energetically favorable molecule for SF among these three derivatives. Furthermore, an interesting trend revealed by the calculations is that the SF exoergicity in this series decreases as the singlet excited state energy decreases. This could explain why SF has not yet been observed in DPP, since the DPP-based materials commonly used in organic electronics typically have much lower excited singlet state energies than does PhTDPP.25, 40-44 Biradicaloid molecules are proposed to be efficient SF chromophores.4 Considering the potential biradical character in DPPs, we examined this property by analyzing the ground-state natural orbital occupation numbers (NOON) by following Kudo’s method at the level of CASSCF(2,2)/6-31G//RB3LYP/6-31G*.35 The LUNO occupancies for PhDPP, TDPP, and PhTDPP are found to be 0.075, 0.072, and 0.069 respectively, suggesting that DPP has a level of biradical character similar to that of other singlet fission chromophores (Table S10). Transient Absorption Spectroscopy. Femtosecond transient absorption (fsTA) spectroscopy of the DPP derivatives in CH2Cl2 solution shows that all three form singlet excited states that absorb beyond 650 nm in PhDPP and TDPP, and beyond 700 nm in PhTDPP, and which decay monoexponentially to their respective ground states (Figure S4). The singlet excited state lifetimes of PhDPP, TDPP, and PhTDPP in solution are 6.9 ± 0.2 ns, 6.0 ± 0.1 ns, and 4.7 ± 0.1 ns, respectively. Note, however, that markedly different excited state dynamics are observed in the thin films of these molecules (Figure 4). Transient absorption experiments were performed



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Figure 4. fsTA spectra (a-c) and species-associated spectra (d-f) from global data fitting of thin films of PhDPP (220 ± 20 nm, a,d), TDPP (182 ± 8 nm, b,e), and PhTDPP (128 ± 2 nm, c,f).

using a 1 mm excitation spot size and 10 nJ/pulse excitation energy to minimize excitation density and avoid singlet-singlet annihilation effects. The spectra of the three molecular thin films (Figures 4a-c) and their kinetics were analyzed by singular value decomposition (SVD) and global fitting to a kinetic model to give species-associated spectra (Figures 4d-f) and kinetic parameters (Figure S5). The PhDPP film spectra (Figures 4a,d) were fit to a model where the initial singlet exciton decays to an excimer state in 42 ± 2 ps, which is accompanied by a decrease in stimulated emission and a broadening of the excited state absorption spectrum. This state then decays monoexponentially to the ground state with a lifetime of 1290 ± 80 ps. There is no indication of the formation of a triplet exciton or any other long-lived species. The fsTA spectra of PhTDPP thin films following photo-excitation are dominated by a ground state bleach and a very weak absorption at 800 nm, which decay monoexponentially in τ = 220 ± 20 ps to yield the triplet state, as indicated by the positive absorption features at 560 nm


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and 600 nm, resulting from overlap of the triplet-triplet absorption with the ground state bleach. Similarly, the corresponding spectra of TDPP thin films display a weak absorption at 700 nm, which undergoes a bi-exponential decay to the triplet state (590 nm and 625 nm) with a major component (τ = 220 ± 50 ps, 80%) and a minor component (τ = 70 ± 10 ps, 20%). The TDPP spectra are best fit to a model in which two singlet exciton populations decay at different rates. This type of behavior has been observed before and can be explained by disorder in the film, possibly arising from crystal grain boundaries or amorphous regions which are difficult to detect using standard techniques.45 The assignment and characterization of the triplet exciton absorption spectra will be discussed in detail below. The observation of triplet excitons in TDPP and PhTDPP but not in PhDPP, together with the greater π-π stacking distance and decreased π-π overlap in PhDPP relative to the other DPP materials, is indicative of SF in TDPP and PhTDPP thin films. This observation suggests that SF in the present DPP series is highly sensitive to interchromophore electronic coupling and may be less sensitive to energetics, since PhDPP is expected to be the most energetically favorable molecule of the series for SF, but it is the only one for which SF is not observed. This hypothesis is further supported by the fact that when dissolved in a polystyrene matrix, both TDPP and PhTDPP form excimer states with red-shifted fluorescence spectra (Figures S5, S6).46-47 Excimer formation in the polystyrene matrix suggests that the DPP molecules are aggregated; however, as demonstrated by the small shift in steady state absorption relative to CH2Cl2 solution spectra, the aggregates are likely small and disordered. The fact that the excimer state forms in the polystyrene matrix and is not accompanied by triplet exciton formation implies that not only strong electronic coupling, but the particular molecular geometry obtained in crystalline TDPP and PhTDPP is essential for triplet formation. It also shows that the



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intermolecular coupling favorable for excimer formation may not be the same as that which is favorable for singlet fission. This could provide an additional explanation for why PhDPP undergoes excimer formation so rapidly while not undergoing singlet fission. It is also worth noting that DPP is a “push-pull” type chromophore with intramolecular charge-transfer (CT) character, and efficient SF has been reported for molecules with strong CT character.14 Although the fsTA spectra do not provide direct evidence of charge separation (the DPP cation absorbs strongly between 700 and 800 nm),29 CT interactions may play a role in determining the relative rates of SF in these molecules. The implications of the CT character in DPP derivatives are currently being investigated. Triplet Characterization. To confirm that the long-lived features observed in the transient absorption spectra of TDPP and PhTDPP thin films result from triplet formation and not from any other long-lived species, such as excimers or free charge carriers, the triplet-triplet absorption spectra were obtained by triplet sensitization in thin films. The thin film triplet exciton spectra were obtained using PdPc as the triplet sensitizer. PdPc was chosen for its rapid, quantitative intersystem crossing yield (τ = 7.6 ± 0.1 ps, ET = 1.24 eV), narrow and isolated absorption at 730 nm, and the fact that it cannot undergo photoinduced charge transfer with TDPP or PhTDPP (c.f., the redox potentials in Table 1 and Eox = 0.42 V and Ered = -1.01 V vs SCE for PdPc).31, 48 The DPP triplet absorption spectrum overlaps that of PdPc on the timescale of the experiment so the sensitized spectrum (Figure S10d,e) resembles a linear combination of the PdPc triplet spectrum (obtained through fsTA of PdPc in polystyrene, Figure S10b,c) and the DPP triplet spectrum. The triplet spectra of the DPP derivatives alone were reconstructed by subtracting the residual PdPc triplet absorption (Figure 5).


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The useful wavelength range for comparing the PhTDPP and TDPP sensitized spectra to the long-lived species in the pure films is limited to approximately 530-680 nm by pump scatter

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Figure 5. Solid state triplet sensitization of PhTDPP (a), and TDPP (b). The black traces show the fsTA transient of PdPc doped films 2 ns after excitation, the red traces show the triplet transient spectra of pure DPP films 4 ns after excitation, and the blue traces show the sensitized DPP triplet transient spectra which were reconstructed by subtracting residual PdPc triplet (green traces) from the transient spectra of the doped films.

from PdPc excitation at the red spectral edge and from direct excitation of the DPP derivatives at the blue spectral edge. Although the pure films are all vapor-deposited to ensure that they are homogenous and crystalline, the sensitized film was spin-coated to allow PdPc doping into the DPP films. The absorption spectra of the doped films (Figure S10) exhibit a linear combination of the absorption spectra of the vapor deposited films and PdPc in solution suggesting that the DPP assumes a similar packing morphology upon spin coating and vapor deposition, and that the PdPc is isotropically distributed in the films, and not strongly interacting with the DPP matrix. The triplet sensitization of PhTDPP yields a spectrum which very closely matches the observed transient absorption spectrum between 550 nm and 650 nm at late times (Figure 5a). In the case of the TDPP films, the spectra match between 550-580 nm, but the overall overlap is not as close. Notably, it is necessary to increase the concentration of PdPc in this film to increase the signal-to-noise ratio of the subtracted spectrum, suggesting that the intensity of the TDPP triplet-



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triplet absorption is much weaker than that of PhTDPP. This renders sensitization more difficult and may contribute to the observed spectral differences. Due to the similarity between the tripletsensitized spectra and the long-lived species observed in the pure DPP derivative films, it is concluded that the long-lived species is in fact a triplet rather than an excimer or a long-lived, charge-separated state. Triplet Yield Analysis. The most telling characteristic of SF is that it can afford triplet exciton yields of up to 200% per absorbed photon. The exceptionally close match between the sensitized triplet and the intrinsic triplet spectra of PhTDPP enables determination of the triplet extinction coefficient, which, in turn, can be used to determine the triplet exciton yield in the films. However, the small differences in peak position and shape between the sensitized and intrinsic TDPP spectra result in somewhat larger errors in measuring the triplet yield this way. Additionally, in the case of TDPP the ∆ε of the triplet is small, which makes it difficult to determine an accurate yield. For these reasons, we focus on determining the PhTDPP triplet exciton yield using the energy transfer and the singlet depletion methods, both of which have been described in detail by Carmichael and Hug.49 A modified version of the singlet depletion method has recently been used to determine the triplet exciton yield due to SF in polycrystalline 2,5,8,11-tetraphenylperylene-3,4:9,10-bis(dicarboximide), pentacene, and palladium mesotetraphenylporphyrin thin films.15 An important step in determining the triplet yield through either of the above methods is to calculate the excitation density (ξ) of the sample. This is accomplished using eq. 1, where E is the

=

( )


(1)

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pump pulse energy, λ is the pump wavelength, K = 1/hc = 5.034 × 10-15 J-1 nm-1, A is the absorbance of the sample at the pump wavelength, l is the sample thickness, and a is the spot size of the pump pulse. The excitation density can then be used to determine the expected ground state bleach for a given triplet exciton yield, and conversely, the triplet exciton yield based on the measured ground state bleach. The singlet depletion method is based on the assumption that the absorption maxima in the ground state absorption spectrum do not line up exactly with peaks in the triplet excited state absorption spectrum. With this assumption, the singlet ground state spectrum can be added into the transient absorption spectrum, which is composed of the ground-state bleach and the excited triplet state absorption, to determine the triplet-triplet absorption spectrum. By comparing the amount of the singlet ground state spectrum that must be added to the transient absorption spectrum to yield a reasonable triplet-triplet absorption spectrum with the amount of the singlet excited state initially produced, the triplet excited state yield can be estimated. The transient absorption spectrum of PhTDPP has a sharp bleach centered at 428 nm. This feature was chosen for the analysis because the singlet depletion method is most accurate when there is a sharp transient feature. Transient absorption spectra of the PhTDPP thin film at 100 ns following photoexcitation with various amounts of the ground state absorption spectrum added are shown in Figure 6. Based on the assumptions of this method, the true triplet-triplet absorption spectrum should display a linear shape in the region centered around 428 nm. At a 100 ns delay, this linear profile is attained when 120 ± 20% of the expected ground state bleach is added to the transient spectrum. Since some of the triplet population returns to the ground state within the first 100 ns following excitation, the actual triplet yield in the film is much higher. The maximum triplet yield can be calculated by extrapolating the triplet absorption changes obtained by nsTA (Figure



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b

Figure 6: Triplet yield analysis by the singlet depletion method. The expected bleach based on the excitation density is shown in navy blue, the transient absorption of a PhTDPP film at 100 ns is shown in black, and the same spectrum with various amounts of bleach added in are also shown (a). The same curves with a line between 420 nm to 435 nm subtracted is included to more clearly show that the point at which the sharp bleach centered at 428 nm becomes linear is when the triplet yield is 120% at 100 ns or 210 % at 1 ns (b).

S8) back to 1 ns (~5x the SF time constant), which gives a triplet exciton yield of 210 ± 35%. This value is a measure of the number of triplets formed for each excitation in the film and suggests that SF is nearly quantitative in PhTDPP films. The theoretical maximum possible triplet yield through SF in these systems is 200% which suggests that the assumptions of the singlet depletion method may lead to somewhat artificially high yields in this case. Specifically, if the region of the triplet absorption spectrum investigated in Figure 5 were concave down rather than linear the method would be expected to overestimate the triplet yield. The triplet yield of PhTDPP films estimated by the singlet-depletion method was corroborated using the energy transfer method. The triplet ∆ε for PhTDPP thin films was measured relative to the known ∆ε of the PdPc triplet as described in the Supporting Information. Using the experimentally derived value of ∆ε for 3*PhTDPP (∆ε = 13,500 cm-1 M-1 at 590 nm), the triplet concentration can be determined and consequently, by comparison to the excitation density, the triplet yield per excitation can also be determined. Using this method the triplet yield for the PhTDPP film is determined to be τSF = 165 ± 30% at 2 ns after 22 ACS Paragon Plus Environment

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photoexcitation, slightly lower than the yield obtained using the singlet depletion method but confirming that SF occurs in high yield in films of PhTDPP. CONCLUSIONS Analysis of the transient absorption spectra of DPP thin films shows that SF produces a high triplet exciton yield in TDPP and PhTDPP thin films, but not in PhDPP thin films or in disordered TDPP and PhTDPP aggregates. These conclusion are supported by the observation that there is no detectable triplet formation in CH2Cl2 solutions, in contrast to the rapid triplet formation occurring in τSF = 220 ± 50 ps in TDPP films and τSF = 220 ± 20 ps in PhTDPP films. The triplet yield resulting from SF in PhTDPP thin films is estimated by both the singlet depletion and the energy transfer methods and found to be 210 ± 35% and 165 ± 30%, respectively. The triplet yield of TDPP cannot be determined in the same way due to the weak transient signal. The reason that the signal is weaker in films of TDPP than in films of PhTDPP is unclear but it may result from the lack of vibronic structure in the TDPP thin film as the most intense absorptions in the PhTDPP triplet spectrum correspond with the sharper vibronic structure in the ground state absorption. It is likely however, that triplet formation proceeds via a similar mechanism in these films since the driving force for SF and the triplet formation rate are very similar in the two materials. The X-ray crystal structures of TDPP and PhTDPP show that these chromophores are closely π-π stacked in the solid state with 3.50 Å and 3.59 Å interplanar distances, respectively. In contrast, the X-ray crystal structure of PhDPP shows that the molecular planes are further apart with a π-π stacking distance of 3.90 Å, which is a plausible explanation for why SF does not occur in PhDPP films.

Furthermore, when disordered

aggregates of TDPP and PhTDPP are formed in polystyrene, excimer formation rather than triplet formation is observed. This suggests that SF in DPP derivatives is very sensitive to



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interchromophore electronic coupling, so that carefully controlling the relative orientations of adjacent DPP chromophores should maximize SF rates and yields. The combination of the high triplet yield in DPP demonstrated here, the strong optical absorption, electron donating, and ptype semiconducting properties of DPP derivatives, and the fact that they are air stable and solution processible, suggest that DPP is potentially useful for enhancing solar cell performance as many of these properties are uncommon among SF chromophores.

ASSOCIATED CONTENT Supporting Information Additional information on transient absorption analysis and triplet yield analysis are given in the supplementary material as well as X-ray characterization and computational data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (M.R.W.) *E-mail: [email protected] (T.J.M.)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, DOE under grant no. DE-FG02-99ER14999 (M.R.W.). This work made use of the J.B.Cohen X-Ray Diffraction Facility supported by the MRSEC program


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Effects of Crystal Morphology on Singlet Exciton Fission in

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