Article pubs.acs.org/JPCA

Singlet Exciton Fission in Thin Films of tert-Butyl-Substituted Terrylenes Samuel W. Eaton,†,‡ Stephen A. Miller,†,‡ Eric A. Margulies,†,‡ Leah E. Shoer,†,‡ Richard D. Schaller,†,§ and Michael R. Wasielewski*,†,‡,§ †

Department of Chemistry and ‡Argonne−Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, United States § Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439-4803, United States S Supporting Information *

ABSTRACT: Two terrylene chromophores, 2,5,10,13-tetra(tertbutyl)terrylene (1) and 2,5-di(tert-butyl)terrylene (2), were synthesized and studied to determine their singlet exciton fission (SF) efficiencies. Compound 1 crystallizes in one-dimensional stacks, whereas 2 packs in a slip-stacked, herringbone pattern of dimers motif. Strongly quenched fluorescence and rapid singlet exciton decay dynamics are observed in vapor-deposited thin films of 1 and 2. Phosphorescence measurements on thin films of 1 and 2 show that SF is only 70 meV endoergic for these chromophores. Femtosecond transient absorption experiments using low laser fluences on these films reveal rapid triplet exciton formation for both 1 (τ = 120 ± 10 ps) and 2 (τ = 320 ± 20 ps) that depends strongly on film crystallinity. The transient absorption data are consistent with formation of an excimer state prior to SF. Triplet exciton yield measurements indicate nearly quantitative SF in thin films of both chromophores in highly crystalline solvent-vapor-annealed films: 170 ± 20% for 1 and 200 ± 30% for 2. These results show that significantly different crystal morphologies of the same chromophore can both result in high-efficiency SF provided that the energetics are favorable.

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been pointed out.29−32 Although both perylene33,34 and perylene-3,4:9,10-bis(dicarboximide) (PDI)35 have been shown to undergo SF in the solid state, the former requires substantial energy beyond its first excited singlet state, whereas SF in the latter is about 0.2 eV endoergic and occurs in τ = 180 ± 10 ps to give a 140% triplet exciton yield.35 Terrylene, the longer analogue of perylene, has a high fluorescence quantum yield and reasonably good photostability and has thus been used primarily as a fluorescent probe molecule in single-molecule spectroscopy.36−38 However, it has not achieved wider application due to its relatively low solubility in organic solvents.39,40 To circumvent these problems, Müllen and co-workers synthesized 2,5,10,13-tetra(tert-butyl)terrylene (1) as a more soluble alternative,41−43 which is also photostable and possesses a low excited singlet state (S1) energy similar to that of unsubstituted terrylene.42 Results from TD-DFT calculations place the relevant states for SF at nearly ideal energies (E(S1) = 2.29 eV, E(T2) = 2.33 eV, E(T1) = 1.10 eV, 2E(T1) − E(S1) = ΔESF = −0.13 eV) and satisfy both energy requirements for generating high triplet yields (E(T2) > E(S1) ≥ 2E(T1)).31

INTRODUCTION Singlet exciton fission (SF), a spin-allowed process by which a singlet exciton is energetically down-converted to two independent triplet excitons, provides a way to dramatically improve the performance of organic photovoltaics (OPVs) and perhaps inorganic photovoltaics as well.1,2 This may be achieved by pairing a high band gap SF material with a low band gap absorber, where the SF material harvests high-energy photons and generates a pair of triplet excitons isoenergetic with the low band gap excitons, thereby minimizing energy loss.3,4 The net increase in photocurrent has been calculated to boost the Shockley−Queisser limit from 33 to 45% in an ideal system.4 This realization has triggered a resurgence of interest in materials previously known to undergo SF, such as tetracene,5−11 pentacene,9,12−17 and polyenes,18−20 as well as a search for new molecules capable of SF.5,21−24 Although polyacenes and polyenes form an excellent foundation for SF research, expanding the number of SF chromophores is important for gaining a greater understanding of the SF mechanism, as well as identifying high-performance absorbers for solar cell applications. To this end, theoretical studies have been critical in guiding new SF materials discovery25−29 and have established the basic criteria for high-efficiency SF. The importance of di- and tetra-radical character in the π electronic structure of polycyclic aromatic hydrocarbons, including the rylene family of chromophores, on SF performance has also © 2015 American Chemical Society

Received: March 20, 2015 Revised: April 9, 2015 Published: April 9, 2015 4151

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evaporator. The approximate film thickness was monitored in situ using an in-chamber quartz-crystal microbalance. Amorphous films of 1 and 2 were evaporated onto room temperature (22 °C) ArrayIt Super Clean 2 glass substrates (3 × 10−6 Torr, 2.0 Å/s) and stored in sealed vials under nitrogen at −18 °C to impede crystallization. Crystalline films of 1 and 2 were produced by solvent vapor annealing the amorphous films by exposure to toluene vapor in a sealed vial for 24 h followed by drying under vacuum (10−3 Torr) overnight to remove residual solvent. Film thicknesses were confirmed by profilometry using a Veeco Dektak 150 surface profiler with 25 μm diameter stylus and were 100 nm for 1 and 140 nm for 2. Grazing incidence Xray diffraction (GIXRD) measurements were performed using a Rigaku ATX-G thin film diffraction workstation in grazing incidence geometry (0.2° incident angle). All GIXRD diffractograms were background subtracted using blank glass substrates. Steady-State Optical Characterization. Solution-phase absorbance spectra and extinction coefficients were measured with a Shimadzu 1800 spectrophotometer. Film transmittance (T) and total reflectance (R) spectra were acquired with a PerkinElmer LAMBDA 1050 UV/vis/NIR spectrophotometer equipped with an integrating sphere, and the true absorbance (A) spectra were calculated by A = −log(T + R). Fluorescence spectra were measured with a PTI QuantaMaster 1 singlephoton spectrofluorometer in right-angle configuration, whereas solution and solid-state fluorescence quantum yields were measured with a Horiba FluoroMax-4 outfitted with an integrating sphere. For phosphorescence measurements, the samples were loaded into an evacuated (10−7 Torr) cryostat and were excited with a 532 nm continuous wave laser (5 mW). Emitted light was passed through a silicon filter, sent to a 0.3 m spectrograph, and imaged on a Princeton Instruments OMA-V cooled InGaAs photodiode array. Integration times on the detector were typically 60 s. Transient Optical Characterization. Low-fluence femtosecond transient absorption (fsTA) experiments were performed using a high repetition rate instrument. The fundamental output (1040 nm, 4.5 W, 350 fs) of a commercial direct diode-pumped 100 kHz amplifier (Spirit 1040-4, Spectra Physics) was divided into two beam paths with a beam splitter. White light continuum probe pulses (480−1100 nm) were 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) was used to drive a noncollinear optical parametric amplifier (Spirit-NOPA-3H, Light Conversion) to create visible pump pulses (500 nm, 75 fs, 0.25 μJ). Although the NOPA output bandwidth is tunable, smaller bandwidth pump pulses were utilized to minimize the spectral window affected by scattered pump light. This, combined with the chirp in the continuum probe pulse, results in an instrument response of 100−150 fs over all probe wavelengths. For all experiments, the pump and probe polarizations were set at magic angle with respect to one another. To minimize photoexcitation densities, all samples were irradiated with 10 nJ pump pulses focused to a relatively large spot size (1.4 mm fwhm diameter). After passing through the sample, the continuum probe was spectrally dispersed inside a modified SPEX 270m monochromator equipped with a 600 grooves/mm grating. A 2 in. diameter silver mirror was placed in the beam path prior to the exit slit, which directs the dispersed probe onto a CMOS linear image sensor (S10112512Q, Hamamatsu). Signal differencing was achieved by

In addition to energetics, the SF rate depends on the intermolecular electron repulsion integrals, which, in turn, depend on orbital overlap.2 Thus, intermolecular distance and orientation in the solid state have a significant impact on SF rates. The large steric bulk of the peripheral tert-butyl groups in 1 was originally anticipated to increase the distance between adjacent chromophores43 and therefore decrease this coupling, resulting in less than ideal SF performance. Thus, we decided to prepare 2,5-di(tert-butyl)terrylene (2), which was predicted to have similar optical properties as 1 and reasonable solubility, yet would allow for a significantly higher degree of intermolecular π orbital overlap in a slip-stacked geometry in the solid. Here we report the synthesis as well as structural and optical characterization of 1 and 2 in both solution and vapordeposited thin solid films. X-ray crystal structures of both compounds are reported and related to their thin film structures. Solvent vapor annealing was used to modify the degree of crystallinity in the solid films. Femtosecond transient absorption spectroscopy was used to determine the ultrafast excited-state dynamics of singlet exciton decay and triplet exciton formation in the films, where it was determined that an intermediate singlet excimer state precedes formation of the triplet excitons. Triplet exciton yields were determined using nanosecond transient absorption spectroscopy to identify processes competing with SF. The SF rates and yields were found to depend on both the crystal structure of the chromophore and its degree of crystallinity in the film.

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EXPERIMENTAL SECTION Synthesis. The synthesis and characterization of 1 and 2 are described in the Supporting Information. The final products were purified by gradient sublimation (1, 300 °C, 3 × 10−6 Torr; 2, 275 °C, 3 × 10−6 Torr). Crystallography. Single crystals of 1 and 2 were both grown by slow diffusion of ethanol into an ether solution. These crystals were mounted with Paratone oil on glass capillaries and placed in the nitrogen cold stream (100 K) of a Bruker AXS APEX2 diffractometer equipped with a chargecoupled device (CCD) detector and a Cu Kα microfocus source with MX optics. Both crystals contained a minor twin component, which was taken into account during the solution process. All data were corrected for absorption via TWINABS. Structures were solved and refined using SHELXTL.44 Molecular offsets were determined using the coordinate rotation method outlined in the Supporting Information. Thin Film Fabrication and Characterization. Films were evaporated using a Denton Vacuum DV502-A vacuum thermal 4152

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Figure 1. Molecular packing of 1: dimer top view (left), dimer side view (center), and bulk side view (right).

Figure 2. Molecular packing of 2: dimer top view (left), dimer side view (center), and bulk side view (right).

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RESULTS AND DISCUSSION Synthesis. The syntheses of 1 and 2 were based primarily on preparations reported by Müllen and co-workers with minor changes (Supporting Information, Scheme S1).42 Notably, the final ring-closing step was accomplished using potassium in refluxing THF rather than AlCl3.50 This approach afforded cleaner final products in higher yields prior to gradient sublimation. Crystallography. Compound 1 packs in the P21/n space group with a unit cell containing two translationally inequivalent molecules as shown in Figure 1. The molecules each have slight core twists of 8.56° and 8.26° between the central and terminal naphthalene subunits. The adjacent cores are separated by 3.68 Å, significantly greater than the π-stacking distance found in α-perylene (3.46 Å)51 or quaterrylene (3.41 Å).52 Additionally, the molecules are offset by 2.67 and 0.35 Å on the longitudinal and transverse axes, respectively, and rotated by 31.7°. This rotation results from the steric bulk of the tert-butyl groups and allows for overall closer π−π contact between adjacent molecules. The rotation alternates between adjacent molecules to form extended 1-D stacks in a hexagonal packing arrangement. SF may be either helped or hindered by the increased π−π stacking distance because the increased interchromophore π−π stacking distance reduces the overall orbital overlap favorable for SF.1,2 Compound 2 adopts a lower symmetry space group, P1, with a unit cell also containing two translationally inequivalent molecules as shown in Figure 2. Instead of twists, however, the cores exhibit longitudinal bends of 14.25° and 14.01°. These

chopping the pump beam prior to the sample position at 476 Hz. The average total exposure time per data point in the experiments is 12 s, which results in a typical baseline noise of ca. 5 × 10−6 ΔA. Kinetic analyses of the fsTA three-dimensional data sets were performed using singular value decomposition followed by global fitting to a kinetic rate model. The fitting program was written in MATLAB45 and used a Levenberg− Marquardt algorithm to fit the data to the kinetic model shown in eqs 1 and 2: d[Ex] = −k SF[Ex] dt

(1)

d[T] 1 = 2k SF[Ex] − k TR [T] 1 dt

(2)

Ex is the singlet excimer state preceding SF, and T1 is the triplet exciton product. Solution-phase fsTA measurements were made by exciting the sample with 525 nm, 120 fs excitation pulses at 1 kHz repetition rate using an instrument described previously.46,47 Fluorescence lifetimes were measured by exciting the samples with 415 nm, 100 fs pulses at an 820 kHz repetition rate and analyzing the fluorescence with a Hamamatsu C4780 Streakscope.48 Nanosecond transient absorption (nsTA) was performed using 7 ns, 525 nm, 1.55 mJ/pulse laser pulses with a large spot size (1.0 cm fwhm diameter) from an instrument described previously.49 All film samples were held in an evacuated cryostat (VPF-100, Janis Research, 10−3 Torr) at room temperature to avoid oxygeninduced degradation. 4153

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Figure 3. GIXRD of thin films of 1 (A) and 2 (B) with assigned Miller indices. Annealing increases film crystallinity in both cases, but more so for 2. Annealed film patterns also match well with the simulated patterns (blue), indicating that the film packing matches the acquired single-crystal structure.

Figures S3−S5), which showed that half of the crystallites were oriented with the (001) plane within ∼6° of being parallel to the substrate plane. Although the texture analysis above provides the orientations of the crystallite relative to the substrate, it does not provide the more optically relevant molecular orientationsue. These may be calculated from the molecular orientation within the single-crystal structure using the rotation matrix described in the Supporting Information. The angles between the longitudinal axes of the two translationally inequivalent molecules and the c-axis were found to be 3.9° and 4.1°, respectively, or nearly parallel. Combined with the c-axis orientation (∼6°), the diffraction data show that the molecular longitudinal axes are oriented to within ∼10° of being perpendicular to the substrate place. As the optical experiments reported here are conducted with the probe beam normal to the film substrates, this has two implications: (1) only a small fraction of the molecules within each film will be sampled, thus resulting in a lower optical density; and (2) transitions polarized along the transverse axis will be sampled preferentially over those polarized along the longitudinal axis. Solution Optical Characterization. Absorption and fluorescence spectra of 1 and 2 in toluene are shown in Figure 4. Both compounds exhibit the characteristic vibronic progression observed in lower rylene analogues. The spectra differ negligibly in peak position with 1 bathochromically

bends result from the formation of slipped dimers between adjacent molecules with displacements of 4.35 and 1.47 Å along the longitudinal and transverse axes, respectively. The displacements allow for a smaller π-stacking distance of 3.31 Å, as the steric bulk of the tert-butyl groups does not induce the rotation observed in 1. The dimers pack in a herringbone arrangement very similar to that found in α-perylene.51 One might predict more favorable SF in 2 entirely on the basis of packing; close π−π contact ensures initial exciton delocalization, whereas the significant slip on the longitudinal axis helps reduce the electron repulsion integral difference, thus favoring SF.2 However, αperylene has been shown to form excimers at a rate competitive with SF.33,34 Due to the similarity in structure and packing, a similar process may be viable in 2. Film Structural Characterization. Both annealed (1an, 2an) and unannealed (1un, 2un) films were characterized by grazing-incidence X-ray diffraction. Unannealed films 1un and 2un diffract poorly compared to the annealed films, demonstrating that rapid deposition results in predominately amorphous rather than polycrystalline domains (Figure 3, red). The peak area ratio provides a qualitative measure of relative crystalline volume: Vab = Ia/Ib, where Ia and Ib are the total integrated intensities of the two diffractograms and Vab is the crystalline volume ratio.53 Applying this relationship results in ratios of 1.49 and 10.2 for 1 and 2, respectively, and demonstrates that although solvent vapor annealing can be an effective way to increase film crystallinity, the method is material dependent. The substantially greater diffraction from 2an suggests that 2 has a lower intrinsic barrier to crystallization than 1. This is not surprising, as the closer π−π stacking observed in the crystal structure of 2 would imply much stronger intermolecular forces are available to drive crystallization. The experimental film diffractograms were also compared to diffractograms simulated from the single-crystal structures. Film 1an exhibits a series of weak diffraction peaks, which can be assigned to a range of Miller indices (Figure 3A, black); the range of indexed peaks and good agreement with simulated powder pattern suggest the diffracting crystallites are randomly oriented. In contrast, film 2an has only four measurable diffraction peaks (Figure 3B, black), which can be assigned to the {00l} family of reflections (Figure S2). This is characteristic of a highly textured film and was confirmed by synchrotronbased grazing-incidence wide-angle X-ray scattering (see

Figure 4. UV−vis absorption (solid) and fluorescence (dashed) spectra of 1 and 2 in toluene. Fluorescence excitation was at 525 nm, and the fluorescence spectra are normalized to the absorption spectra. 4154

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because raising the sample temperature to 270 K results in the loss of this feature. The lowest energy peak at 1358 nm results from emission from the sample tube as it is neither temperature- nor sample-dependent (Figure S8). Determining whether the phosphorescence originates from monomeric chromophores or aggregates is important for calculating the energetics of SF because E(T1) may be lower in the aggregates because the triplet state may be stabilized by interactions with neighboring molecules. The phosphorescence spectrum shown in Figure 5 most likely arises only from monomeric chromophores as the low-temperature ground-state absorption spectrum reveals no changes that could be attributed to aggregate formation (e.g., band broadening, redshifting, new bands). Varying sample concentration or solvent did not result in a measurable spectral shift (Figure S9), suggesting that 2 remains monomeric. Solution-phase triplet sensitization of 1 and 2 with anthracene provided unambiguous spectral information on the tert-butylterrylene Tn ← T1 transitions (Figure 6). Exciting anthracene codissolved with 1 or 2 in toluene rapidly generates the anthracene T1 state (1.85 eV), which energy transfers to the much lower energy terrylene T1 state over the course of microseconds. Two excited-state absorption features were identified at 548 nm (2.26 eV) and 508 nm (2.44 eV) corresponding roughly to the T5 ← T1 (2.34 eV) and T6 ← T1 (2.40 eV) transitions predicted by theory.31 These data, as well as the solution-phase fsTA data, confirm that although the Tn ← T1 transitions and ground-state bleach overlap strongly, clear spectral tags are still available to track the triplet-state dynamics. Thin Film Optical Characterization. Steady-state absorbance spectra of the four films are shown in Figure 7. Unannealed films 1un and 2un both exhibit red-shifting and broadening of their absorption spectra compared to solution, as well as a decrease in vibronic transition ratio (Figure 7, black). However, the spectra are overall quite similar despite the widely differing molecular packing arrangements found in the singlecrystal structures. This behavior is characteristic of molecular solids with poor intermolecular electronic coupling, suggesting the majority of molecules within the films are disordered with minimal intermolecular interaction.12 Solvent annealing increases film crystallinity as observed by the GIXRD measurements, and the post-deposition treatment also results in substantial restructuring of the absorption spectra. For the 1an film, the absorbance peaks sharpen and a new shoulder emerges at 620 nm (Figure 7A, red). This sharpening, while indicative of a narrower distribution of crystalline states, does not indicate substantially increased electronic overlap, which is supported by the relatively large π−π stacking distance found in the crystal structure. For the 2an film, a more sizable change is observed upon solvent vapor annealing (Figure 7B, red). An approximately 80% decrease in total absorbance is observed particularly in the low-energy bands. The decrease is likely due to reorientation of the transition dipoles within the film. Because the S1 ← S0 transition dipole is oriented along the longitudinal molecular axis, reorientation of the molecular domains would result in a decrease in absorbance for normally incident light. Additionally, transition dipoles oriented along the transverse axis, such as the T5 ← T1 transition,56 are more pronounced due to the alignment between dipole moment and electric field vector of the probe light. The S1 energies of both 1an and 2an are 1.93 eV, and their T1 energies are 1.00 eV, so that SF is only slightly endoergic at 70 meV. The fluorescence quantum yields of all

shifted by 2 nm relative to 2 and in vibronic transition oscillator strength with 2 having a 4% higher (0,0)/(1,0) transition ratio. The fluorescence spectra of 1 and 2 exhibit similar trends. The solution-phase excited-state lifetimes of 1 (3.5 ± 0.2 ns) and 2 (3.6 ± 0.2 ns) measured by fsTA spectroscopy (Figure S6) agree well with the measured fluorescence lifetimes of 3.49 ± 0.02 ns and 3.51 ± 0.03 ps, respectively. This is also in close agreement with the excited-state lifetime of 1 reported by Koch and Müllen (3.8 ns).54 Besides S1 decay, a persistent signal is observed at 550 nm (2.25 eV), which is attributed to the T5 ← T1 absorption band (2.34 eV) because a triplet transition having a similar energy has been calculated for terrylene.31 Some appreciable intersystem crossing is not entirely unexpected as the calculated T2 energy (2.33 eV31) is predicted to be close to the experimental S1 energy for terrylene (2.18 eV), and the quantum yield of fluorescence for both compounds is less than unity (1, 0.79 ± 0.05; 2, 0.71 ± 0.05). Assuming that fluorescence and intersystem crossing are the only excited-state decay pathways, upper limits for the intersystem crossing rates and the intrinsic radiative rates may be estimated from the fluorescence quantum yields and fluorescence rates (see the Supporting Information). These calculations yield minimum intersystem crossing time constants of 17 ± 4 and 12 ± 3 ns for 1 and 2, respectively. On the basis of these estimates, spin−orbit intersystem crossing should not be significantly competitive with triplet formation by ultrafast SF. Triplet-State Characterization. Phosphorescence spectra of 1 and 2 were measured in dilute solid solutions to corroborate the calculated T1 energies (Figure 5). As nearly

Figure 5. Emission spectrum of 2 in MCH at 80 K. The shoulder (red) centered at 1239 nm is due to phosphorescence, whereas the major peak (green) and broad baseline (blue) originate from sample fluorescence and sample glass fluorescence, respectively.

identical spectra were obtained for 1 and 2, only those for 2 are shown. Upon excitation in methylcyclohexane (MCH) at 80 K, three distinct emission peaks are observed. When the emission filtering scheme was changed to allow higher energy photons to reach the detector (two 1000 nm and one 850 nm long-pass filters in place of a silicon filter), the high-energy peak extended into a broad emission band (Figure S7). This is similar to observations made of tetracene aggregates analyzed in a PMMA matrix, where the authors attributed the broad, high-energy emission band to delayed fluorescence.55 Therefore, we also assign the complete, broad high-energy peak observed at 1119 nm to residual fluorescence. We assign the shoulder centered at 1239 nm (E(T1) = 1.00 eV) to terrylene phosphorescence 4155

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Figure 6. Triplet sensitization of 1 (A) and 2 (B) by the anthracene triplet state in toluene.

Figure 7. Absorption spectra of 1 (A) and 2 (B) in unannealed (black) and annealed (red) thin films, normalized to the unannealed spectra. Solidstate E(S1) values of 2.01 eV (1un), 1.93 eV (1an), 1.95 eV (2un), and 1.93 eV (2an) were estimated at 10% peak height of each low-energy band.

Figure 8. Fluorescence spectra of 1 (A) and 2 (B) in unannealed (black) and annealed (red) thin films.

with decreasing pump fluence and remain unchanged at 10 nJ/ pulse (∼1 × 1017 excitons/cm3). This decrease in decay rate correlates with the decreasing probability of singlet−singlet annihilation. This threshold also agrees with similar studies that analyze singlet−singlet annihilation in organic semiconductors.21,57 A common feature among all film fsTA data is an ultrafast decay component of the ground-state bleach (Figure 10). This decay occurs in less than a picosecond and, considering its sign, could correspond to excited-state absorption growth, vibrational relaxation, or loss of stimulated emission. Becausee the excited-state absorption feature from 630 to 800 nm does not

films are ≤0.01; thus, their fluorescence spectra are relatively weak (Figure 8). The broad, red-shifted peaks suggest that the emission derives largely from excimers within the film. Thin Film Transient Absorption Spectroscopy. fsTA was also used to probe the excited-state processes within the films. The fsTA apparatus outlined in the Experimental Section affords us an excellent signal-to-noise ratio from 510 to 800 nm with very low excitation densities. This is especially important for avoiding singlet−singlet annihilation, which decreases the initial excited-state population and complicates the kinetic analysis. An example power study on 2un (Figure 9) demonstrates that the excited-state dynamics slow significantly 4156

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and environments are present within the films. To illustrate this, the film kinetics were analyzed by singular value decomposition and global fitting to decay-associated spectra (Figure 11B,F). Although this approach neglects bimolecular events, it is valid because the excimer states formed within the first picosecond should be relatively localized with short diffusion lengths. The similarity between the decay-associated spectra supports the idea that individual excited-state populations, although spectrally similar, decay much differently depending on their unique environments. Both long-time (≫ 8 ns) traces are composed of the Tn ← T1 transitions overlapping with the ground-state bleaches. This assignment is further supported by their hundreds of nanoseconds to microsecond lifetimes as measured by nanosecond TA spectroscopy (see below). The positive features near 555 nm match well with the strong T5 ← T1 transition identified from the triplet sensitization experiment in Figure 6. However, the SF rate is unclear from the fsTA spectra because the strongly overlapping features and the existence of multiple populations preclude a direct kinetic analysis. The fsTA spectra of the 1an film are shown in Figure 11C. Although spectrally similar to both of the unannealed films, the 1an film exhibits an absorptive feature at 566 nm akin to the sensitized triplet Tn ← T1 spectrum. Performing singular value decomposition and a species-associated global kinetic fit using the kinetic model in eqs 1 and 2 yields the singlet- and tripletstate spectra (Figure 11D). This simple kinetic model captures all of the excited-state dynamics, except for the ultrafast excimer formation. The major 120 ± 10 ps component is assigned to SF of the rapidly formed excimer state. The observed rate is reasonable for SF considering the process is only slightly endoergic in film 1an (ΔESF = 70 meV), especially when compared to PDI (180 ± 10 ps, ΔESF = 200 meV).35 The more crystalline annealed 2an films also exhibit significantly different behavior compared to the unannealed 2un films (Figure 11G). Singular value decomposition and species-associated global kinetic analysis reveal the singlet and triplet exciton spectra (Figure 11H), demonstrating again that in the singlet−singlet annihilation-free regime only a simple kinetic model is required to fit the data. At early times, the ground-state bleach is nearly absent due to the substrateinduced orientation of the transition dipole moments within the film; however, the ultrafast stimulated emission decay is still present at 590 nm. Another consequence of the strongly oriented chromophores in the film is that a strong positive feature corresponding to the triplet state is observed to grow near 545 nm with a time constant of 320 ± 20 ps. The remaining excited-state absorption features decay at a similar rate to leave only the overlapping T5 ← T1/ground-state bleach spectrum, only now with a much stronger contribution from the triplet−triplet transition resulting from the preferential orientation of its transition dipole in the film. Triplet Dynamics and Yield. Nanosecond transient absorption (nsTA) spectroscopy was performed to determine the triplet exciton dynamics and the overall triplet yield. The transient spectra (Figure S10) all closely match the long-time species observed in the fsTA spectra with no discernible spectral shifting over the lifetime of the triplet. The time constants and amplitudes from the kinetic fits are shown in Table 1. These long-lived population dynamics are comparable with those of other SF molecules such as PDI and pentacene.35 Although fsTA spectroscopy reveals rapid singlet decay to form triplet, it does not completely rule out alternative singlet

Figure 9. fsTA power study on film 2un shows increasing excited-state lifetime with decreasing pump pulse energy. Excitation was at 525 nm, and decay was monitored at 710 nm.

Figure 10. Early time bleach decays for all four tert-butyl terrylene films from fsTA. Kinetics fit biexponetially at the center of the lowest energy bleach band (580−590 nm).

exhibit the same decay and excitation at the band edge (625 nm) does not eliminate these rapid bleach dynamics, the decay is assigned to loss of stimulated emission rather than vibrational relaxation. All other excited-state features remain unchanged, however, suggesting a sub-picosecond, nonradiative transition from the initial singlet to an intermediate state. Similar behavior was recently reported in solution-phase aggregates of PDI, where the authors attributed this observation to excimer-state formation.58 The authors hypothesized that this same process may occur with other perylene-based chromophores on the basis of their similarities in electronic structure.59 Because rapid excimer formation has also been observed in perylene single crystals,33 we extend this argument to terrylene-based chromophores 1 and 2 because it is consistent with their observed ultrafast dynamics in the films. fsTA spectra for the four films are shown in the left column of Figure 11. In general, all films exhibit ground-state bleach features between 510 and 630 nm and broad, featureless excited-state absorption bands from 630 to 800 nm. Assuming that all further dynamics proceed from the initial excimer state, we will focus on the picosecond and longer time regime. The time-dependent fsTA spectra of 1un and 2un (Figure 11A,E) appear both spectrally and kinetically similar, which follows from the similarity between their steady-state absorbance spectra and overall lack of crystallinity. Single-wavelength kinetic fits required multiple components, regardless of the feature, suggesting that a variety of molecular conformations 4157

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Figure 11. FsTA spectra (left column) with decay- and species-associated spectra (right column, see text) of thin films of 1 and 2: (A, B) 1un (1.0 × 1017 excitons/cm3); (C, D) 1an (1.2 × 1017 excitons/cm3); (E, F) 2un (3.9 × 1016 excitons/cm3); (G, H) 2an (3.7 × 1016 excitons/cm3).

and assuming that the calculated or measured solution-phase extinction coefficients do not change in the solid. As the triplet transitions overlap strongly with the ground-state bleaches for 1 and 2, these methods are not suitable for quantifying triplet yields. We recently reported a method based on quantifying the amount of ground-state bleach needed to reconstruct the T−T absorption spectrum to determine the triplet yield for systems with strongly overlapping absorption features,35 which we apply again here (see details in Figures S11−S14). An example of the use of this method for 1an is shown in Figure 12A, and the resulting triplet yields for all four films are displayed in Figure 12B. In general, films of 2 have higher triplet exciton yields than do films of 1, suggesting that either

Table 1. nsTA Kinetic Decay Fits Measured at the GroundState Bleach Minima (580−590 nm) τ 1 (ns) 1un 1an 2un 2an

146 113 96 74

± ± ± ±

2 2 2 2

(80%) (86%) (83%) (91%)

τ 2 (μs) 1.23 3.23 0.85 2.94

± ± ± ±

5 (14%) 6 (14%) 0.03 (13%) 0.13 ns (9%)

τ 3 (μs) >4 (6%) >4 (4%)

exciton decay pathways and is thus not a complete argument for high-yield SF. Alternative methods for quantifying triplet yield have been reported,12,16,22,60 but all rely on tracking triplet growth kinetics at wavelengths independent from other features 4158

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Figure 12. Example of ground-state (red) addition to the nsTA spectrum (black) to yield the true triplet absorption (blue) for 1an (A); calculated triplet yields for unannealed and annealed thin films of 1 and 2 (B).

the closer π−π distance or the slip-stacked packing of 2 relative to 1 favors SF. For 2un, low crystallinity does not have as great an impact as it does in 1un because strong π−π interactions likely favor dimerization to yield SF-active sites. Finally, increasing crystallinity results in higher triplet exciton yields for both compounds. As discussed earlier, a more crystalline film may provide a higher density of SF-capable sites, thus increasing overall triplet exciton production. However, on the basis of the triplet decay time constants in Table 1, increasing crystallinity also results in more rapid triplet−triplet annihilation. This is likely due to the increase in exciton diffusion length afforded by larger crystalline domains, which results in a higher probability of annihilation. The SF triplet yields in films of both 1an and 2an are very high despite the significant differences in crystal morphology of 1 and 2. In contrast, our previous work on 2,5,8,11-tetraphenylPDI shows that its SF yield is much more sensitive to the interchromophore orientation enforced by crystal morphology. The interchomophore geometry determines the electronic coupling matrix elements for the SF process, yet does not take into account the energetics of the process. In the case of terrylenes 1 and 2, SF is endoergic by 70 meV, whereas it is 200 meV endoergic for 2,5,8,11-tetraphenyl-PDI. Given that in a formal sense SF can be viewed as a double electron transfer process, it is perhaps not surprising that a Marcus-like rate versus coupling and free energy dependence may determine SF rates.1,2 Excimers and the Effects of Intermolecular Coupling on SF. The sub-picosecond loss of stimulated emission observed in the fsTA spectra is evidence for rapid formation of an excimer intermediate. The broad red-shifted fluorescence emission spectra from the films provide further evidence of excimer formation. SF from an excimer intermediate has recently been reported for TIPS-pentacene to explain both solution-phase SF as well as concentration-dependent fluorescence.16 To describe the kinetic processes within thin films of 1 and 2, a simple energy-level diagram may be constructed that incorporates this excimer state as an intermediate (Figure 13). Although the rate of each process depends additionally on the crystallinity of the chromophore within the film, this scheme provides a general model that can be used to interpret the kinetic processes within the films. For the two annealed films of 1an and 2an, fsTA spectroscopy reveals that triplet exciton formation by SF occurs in nearly quantitative yield, indicating negligible

Figure 13. Energy-level diagram depicting kinetic processes following photoexcitation to the S1 state. kR, radiative decay (3.5 ± 0.1 ns); kEX‑EM, excimer radiative decay (>7 ns); kEX, excimer formation (50 ns).

competition from alternative decay pathways. The difference in observed SF time constants for film 1an (120 ± 10 ps) and film 2an (320 ± 20 ps) depends on two factors: the singlet- and triplet-state energies and the relative intermolecular coupling. As demonstrated recently by Yost et al.,61 SF is quite sensitive to both of these parameters and can result in SF rates spanning orders of magnitude. Because the SF energetics for films of both 1an and 2an are the same, their SF time constants directly reflect intermolecular electronic coupling as well as film crystallinity. Although the two tert-butyl groups within 2 successfully induced a slip-stacked dimer structure, the resulting herringbone pattern of dimers may not be optimal for SF. Computational studies will be required to determine the exact nature of the relative inter- and intradimer couplings relevant to SF. Crystallinity has proven exceptionally important for clarifying the fsTA kinetic analysis and for increasing the ultimate SF rate in both 1 and 2. The less crystalline samples demonstrate that not all relative geometries undergo rapid SF; a significant fraction of the excited singlet-state population remains past 500 ps in both unannealed films. Whereas SF has been shown to occur rapidly in some amorphous polyacenes, the proposed mechanism relies on exciton diffusion to SF “hot-spots”, where the process is rapid.21,62 As shown above, however, rylenebased materials have a tendency to rapidly form excimer states, which are more localized in the solid, thus making high SF yields via a diffusion-based SF mechanism less likely. 4159

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(2) Smith, M. B.; Michl, J. Singlet Fission. Chem. Rev. 2010, 110, 6891−6936. (3) Nozik, A. J.; Miller, J. Introduction to Solar Photon Conversion. Chem. Rev. 2010, 110, 6443−6445. (4) Hanna, M. C.; Nozik, A. J. Solar Conversion Efficiency of Photovoltaic and Photoelectrolysis Cells with Carrier Multiplication Absorbers. J. Appl. Phys. 2006, 100, No. 074510. (5) West, B. A.; Womick, J. M.; McNeil, L. E.; Tan, K. J.; Moran, A. M. Ultrafast Dynamics of Frenkel Excitons in Tetracene and Rubrene Single Crystals. J. Phys. Chem. C 2010, 114, 10580−10591. (6) Burdett, J. J.; Gosztola, D.; Bardeen, C. J. The Dependence of Singlet Exciton Relaxation on Excitation Density and Temperature in Polycrystalline Tetracene Thin Films: Kinetic Evidence for a Dark Intermediate State and Implications for Singlet Fission. J. Chem. Phys. 2011, 135, 214508. (7) Grumstrup, E. M.; Johnson, J. C.; Damrauer, N. H. Enhanced Triplet Formation in Polycrystalline Tetracene Films by Femtosecond Optical-Pulse Shaping. Phys. Rev. Lett. 2010, 105, 257403. (8) Burdett, J. J.; Bardeen, C. J. Quantum Beats in Crystalline Tetracene Delayed Fluorescence Due to Triplet Pair Coherences Produced by Direct Singlet Fission. J. Am. Chem. Soc. 2012, 134, 8597−8607. (9) Chan, W.-L.; Berkelbach, T. C.; Provorse, M. R.; Monahan, N. R.; Tritsch, J. R.; Hybertsen, M. S.; Reichman, D. R.; Gao, J.; Zhu, X. Y. The Quantum Coherent Mechanism for Singlet Fission: Experiment and Theory. Acc. Chem. Res. 2013, 46, 1321−1329. (10) Wilson, M. W. B.; Rao, A.; Johnson, K.; Gélinas, S.; di Pietro, R.; Clark, J.; Friend, R. H. Temperature-Independent Singlet Exciton Fission in Tetracene. J. Am. Chem. Soc. 2013, 135, 16680−16688. (11) Birech, Z.; Schwoerer, M.; Schmeiler, T.; Pflaum, J.; Schwoerer, H. Ultrafast Dynamics of Excitons in Tetracene Single Crystals. J. Chem. Phys. 2014, 140, No. 114501. (12) Ramanan, C.; Smeigh, A. L.; Anthony, J. E.; Marks, T. J.; Wasielewski, M. R. Competition between Singlet Fission and Charge Separation in Solution-Processed Blend Films of 6,13-Bis(triisopropylsilylethynyl)pentacene with Sterically-Encumbered Perylene-3,4:9,10-bis(dicarboximide)s. J. Am. Chem. Soc. 2011, 134, 386− 397. (13) Chan, W.-L.; Ligges, M.; Jailaubekov, A.; Kaake, L.; Miaja-Avila, L.; Zhu, X. Y. Observing the Multiexciton State in Singlet Fission and Ensuing Ultrafast Multielectron Transfer. Science 2011, 334, 1541− 1545. (14) Rao, A.; Wilson, M. W. B.; Albert-Seifried, S.; Di Pietro, R.; Friend, R. H. Photophysics of Pentacene Thin Films: The Role of Exciton Fission and Heating Effects. Phys. Rev. B: Condens. Matter 2011, 84, No. 195411. (15) Wilson, M. W. B.; Rao, A.; Ehrler, B.; Friend, R. H. Singlet Exciton Fission in Polycrystalline Pentacene: From Photophysics toward Devices. Acc. Chem. Res. 2013, 46, 1330−1338. (16) Walker, B. J.; Musser, A. J.; Beljonne, D.; Friend, R. H. Singlet Exciton Fission in Solution. Nat. Chem. 2013, 5, 1019−1024. (17) Wilson, M. W. B.; Rao, A.; Clark, J.; Kumar, R. S. S.; Brida, D.; Cerullo, G.; Friend, R. H. Ultrafast Dynamics of Exciton Fission in Polycrystalline Pentacene. J. Am. Chem. Soc. 2011, 133, 11830−11833. (18) Wang, C.; Tauber, M. J. High-Yield Singlet Fission in a Zeaxanthin Aggregate Observed by Picosecond Resonance Raman Spectroscopy. J. Am. Chem. Soc. 2010, 132, 13988−13991. (19) Wang, C.; Berg, C. J.; Hsu, C.-C.; Merrill, B. A.; Tauber, M. J. Characterization of Carotenoid Aggregates by Steady-State Optical Spectroscopy. J. Phys. Chem. B 2012, 116, 10617−10630. (20) Lanzani, G.; Cerullo, G.; Zavelani-Rossi, M.; De Silvestri, S.; Comoretto, D.; Musso, G.; Dellepiane, G. Triplet-Exciton Generation Mechanism in a New Soluble (Red-Phase) Polydiacetylene. Phys. Rev. Lett. 2001, 87, No. 187402. (21) Roberts, S. T.; McAnally, R. E.; Mastron, J. N.; Webber, D. H.; Whited, M. T.; Brutchey, R. L.; Thompson, M. E.; Bradforth, S. E. Efficient Singlet Fission Discovered in a Disordered Acene Film. J. Am. Chem. Soc. 2012, 134, 6388−6400.

CONCLUSIONS We have reported the structural and photophysical characterization of a pair of tert-butyl-substituted terrylene chromophores in solution and in thin solid films. The chromophores were successfully crystal engineered to adopt two different solid-state packing arrangements, which affect the chromophore/substrate interactions as well as both the steady-state optical properties and the ultrafast dynamics in thin films. We have also shown that the SF process is only slightly endoergic in thin films of both 1 and 2 and most likely occurs through an excimer intermediate state. However, conversion of singlet to triplet excitons via SF was shown to occur on a range of time scales, which were both chromophore- and morphologydependent. Despite complete convolution of the ground-state and triplet-state spectra, the triplet yield was determined to be near 200% in crystalline films of both chromophores when probed in a singlet−singlet annihilation-free regime. Future work will include further investigation of the excimer intermediate and its role in SF as well as fabrication of solar cells because terrylenes meet many of the requirements (high extinction coefficient, photostability, processability) of a useful SF chromophore.

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ASSOCIATED CONTENT

* Supporting Information S

Experimental details including synthesis, crystallographic parameters, and coordinate rotation procedure, grazing incidence X-ray scattering, intersystem crossing rate calculations, phosphorescence control spectra, nsTA transient spectra and kinetic fits, and triplet yield analysis for each film. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(M.R.W.) E-mail: [email protected]. Phone: (847) 467-1423. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, U.S. Department of Energy (DOE), under Grant DEFG02-99ER14999 (M.R.W.). S.W.E. acknowledges support from the International Materials Institute for Solar Energy and Environment funded by the National Science Foundation under Grant DMR-0843962. Powder X-ray diffraction measurements were performed at the J. B. Cohen X-ray Diffraction Facility supported by the Materials Research Science and Engineering Centers (MRSEC) program of the National Science Foundation (Grant DMR-0520513) at the Materials Research Center of Northwestern University. This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences User Facility, under Contract DE-AC0206CH11357.

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REFERENCES

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