Singlet Fission and Excimer Formation in Disordered Solids of Alkyl

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Singlet Fission and Excimer Formation in Disordered Solids of Alkyl-Substituted 1,3-Diphenylisobenzofurans Paul I. Dron, Josef Michl, and Justin C. Johnson J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b07362 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 22, 2017

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The Journal of Physical Chemistry

Singlet Fission and Excimer Formation in Disordered Solids of Alkyl-Substituted 1,3Diphenylisobenzofurans

Paul I. Dron,† Josef Michl, †,ǁ,* Justin C. Johnson§,*

†

Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80301-

0215, United States ǁ Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nám. 2, 16610 Prague 6, Czech Republic §

National Renewable Energy Laboratory, Golden, Colorado 80401, United States

Abstract We describe the preparation and excited state dynamics of three alkyl derivatives of 1,3diphenylisobenzofuran (1) in both solutions and thin films. The substitutions are intended to disrupt the slip-stacked packing observed in crystals of 1 while maintaining the favorable energies of singlet and triplet for singlet fission (SF). All substitutions result in films that are largely amorphous, as judged by the absence of strong X-ray diffraction peaks. The films of 1 carrying a methyl in the para position of one phenyl ring undergo SF relatively efficiently (≥ 75% triplet yield, ΦT), but more slowly than thin films of 1. When the methyl is replaced with a t-butyl, kinetic competition in the excited state favors excimer formation rather than SF (ΦT = 55%). When t-Bu groups are placed in both meta positions of the phenyl substituent, SF is slowed down further and ΦT = 35%. Keywords: singlet fission, thin film, photophysics, triplet, excimer 1 ACS Paragon Plus Environment


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I. Introduction Singlet fission (SF) is a process in which a chromophore in an excited singlet state (S1) interacts with a nearby ground state (S0) chromophore to yield two triplet states (T1) on the two chromophores.1-2 Research on SF has been pursued for over half a century, starting with the invocation of the phenomenon to account for low fluorescence quantum yields of anthracene3 and tetracene4 crystals. Interest has resurged in the past decade as researchers have realized that biexciton generation in SF might enhance photocurrents in photovoltaic applications, especially for photoexcitation above 2 eV.5-8 1,3-Diphenylisobenzofuran9 (1) was predicted to be a suitable chromophore for SF.10 It has two crystalline polymorphs, called α-1 and β-1, that have dramatically different triplet yields.11-13 The more thermodynamically stable form, β-1, undergoes rapid excimer formation that quenches SF, whereas α-1 shows very high values of ΦT approaching the theoretical limit of 2.14 The crystal structures of the two polymorphs of 1 exhibit similar slipped π-stacking interactions and also longer range order. This confirmed that subtle distinctions in mutual dispositions may have a large effect on the matrix element for SF,1,

15,16

on the competing processes (i.e., excimer

formation), or both. In the pursuit of “designed SF,” namely control of inter-chromophore coupling via synthetic means to promote SF over other excited state pathways, our groups have explored dimers of 1.1718

To date we have investigated linearly coupled dimers of 1 in solution, all of which have quite

low values of ΦT. In general, these studies confirm theoretical results regarding SF: (i) SF yields should be low in linearly-linked systems based on poor HOMO-LUMO intermolecular overlap contributions to the SF matrix element;19 (ii) charge transfer states play a significant role in promoting triplet formation in these systems.20

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We continue our pursuit of understanding the role of inter-chromophore coupling in SF with disordered solids, a distinct class of chromophore assemblies. Disordered solids have possible advantages over crystalline solids in terms of ease and cost of processing and relative insensitivity of the photophysics to exact microstructure/morphology. SF in disordered solids was first reported in the amorphous films of diphenyltetracene.21 Triplet rise kinetics were identified and associated with both prompt (~ 2 ps) and delayed (> 50 ps) SF events. A model was formulated in which the prompt SF occurred when photoexcitations arose near molecular pairs positioned favorably for SF. The delayed component was hypothesized to arise from singlet exciton diffusion from unfavorable to favorable sites. Efficient SF in amorphous triisopropylsilylethynyl (TIPS) pentacene has also been shown.22 Polycrystalline thin films of 1 have several shortcomings: (i) the structure can change spontaneously under ambient conditions, and the thermodynamically stable form has a low triplet yield; (ii) the material sublimes at ambient pressure and mild heating; and (iii) 1 cannot be handled or studied under air, as it undergoes oxidation processes that open the furan ring.23-24 Disordered films alleviate the sensitivity to structure and can be more stable under ambient conditions. The compounds 1-Me, 1-tBu, and 1-(tBu)2, (Chart 1) provide a unique opportunity to study the relationship between chromophore coupling and photophysics in disordered aggregates of 1. We observe that the ΦT values are reduced as bulky substituents are added, emphasizing that preserving strong interchromophore interactions is extremely important for achieving SF yields in derivatives of 1. This contrasts with pentacene systems that seem to undergo efficient SF almost regardless of the type and strength of interchromophore coupling, presumably because of higher exothermicity.25



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Chart 1. A. Structures of 1,3-diphenylisobenzofuran (1) and alkylated derivatives.

II. Experimental General. All solvents were reagent grade (Aldrich). Those used in the spectroscopic measurements, toluene (TOL) and chloroform (CF), were degassed with nitrogen for at least 30 min prior to being brought into the glovebox. Films were prepared under inert conditions by drop-casting solutions (~3-5 mg/mL) of the compounds in a 50:50 mixture of toluene and chloroform onto 1” diameter sapphire substrates. The films were sealed in inert atmosphere by sandwiching a Viton o-ring between the substrate with the film and a second sapphire substrate in an optical mount.

General Procedure for the Synthesis of Alkylated 1,3-Diphenylisobenzofurans. n-Butyllithium (1.6 M, 7 mL, 11.2 mmol) is added under argon to a solution of aryl bromide (10.6 mmol) in dry THF at -50 °C. After 2 h of stirring under argon at -50 °C, 3phenylphthalide (2 g, 9.5 mmol) is added. The stirred reaction mixture is kept under argon for two more hours. Acetic anhydride (1 mL) is added, the reaction mixture is refluxed for 10 min, the reaction mixture is concentrated to dryness, and ethyl acetate (50 mL) is added. The organic phase is extracted with saturated aqueous NaCl (2 x 75 mL), concentrated to dryness and chromatographed on silica gel with petroleum ether.


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Scheme 1. Synthesis of alkyl derivatives of 1.

1-Me: 1H NMR (benzene-d6, 300 MHz, δ): 7.87-7.81 (m, 4H), 7.67-7.61 (m, 2H), 7.267.21 (m, 2H), 7.10-7.03 (m, 3H), 6.80-6.74 (m, 2H), 2.14 (s, 3H); 13C NMR (benzene-d6, 75 MHz, δ): 144.7 (1C), 143.9 (1C), 138.0 (1C), 136.9 (1C), 132.7 (1C), 130.4 (1C), 130.2 (1C), 129.9 (2C), 129.2 (2C), 126.9 (1C), 125.4 (1C), 125.3 (2C), 125.2 (2C), 125.1 (1C), 120.7 (1C), 120.5 (1C), 21.3 (1C); MS (ESI+) (m/z): 284.1199 (M+, ∆ = -0.7 ppm); IR (KBr, cm-1): 3056 (m), 3026 (m), 2914 (m), 2856 (m), 1672 (w), 1629 (w), 1597 (s), 1548 (m), 1512 (w), 1495 (s), 1447 (s), 1300 (w), 1205 (m), 1188 (w), 1174 (m), 1150 (w), 1104 (m), 1074 (m), 1027 (w), 1001 (w), 995 (m), 972 (m), 941 (m), 910 (m), 818 (s), 804 (m), 763 (s), 748 (w), 742 (s), 720 (w), 696 (s), 656 (s), 641 (w), 618 (w), 609 (w), 576 (w), 554 (w), 499 (m), 430 (m); UV-Vis (toluene): 23190 cm-1 (ε = 17910 M-1·cm-1), 24150 cm-1 (ε = 22715 M-1·cm-1), 30770 cm-1 (ε = 7935 M-1·cm-1), 31950 cm-1 (ε = 8267 M-1·cm-1), 37170 cm-1 (ε = 30300 M-1·cm-1), 38170 cm-1 (ε = 29440 M-1·cm-1); m.p. 92 – 94 °C. Anal. Calcd. for C21H16O: C, 88.70; H, 5.67. Found: C, 88.47; H, 5.85. 1-tBu: 1H NMR (benzene-d6, 300 MHz, δ): 7.90-7.85 (m, 4H), 7.75-7.70 (m, 1H), 7.657.60 (m, 1H), 7.39-7.34 (m, 2H), 7.27-7.21 (m, 2H), 7.09-7.03 (m, 1H), 6.83-6.75 (m, 2H), 1.26 (s, 9H); 13C NMR (benzene-d6, 75 MHz, δ): 156.24 (1C), 141.03 (2C), 138.04 (1C), 135.47 (1C), 5 ACS Paragon Plus Environment


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132.63 (2C), 130.34 (2C), 130.16 (2C), 129.93 (1C), 129.81 (2C), 129.71 (2C), 129.62 (1C), 128.63 (1C), 125.48 (2C), 34.91 (1C), 31.02 (3C); MS (ESI+) (m/z): 326.1663 (M+, ∆ = -2.5 ppm); IR (KBr, cm-1): 3054 (m), 2961 (s), 2901 (m), 2864 (m), 1790 (w), 1661 (m), 1626 (w), 1598 (s), 1546 (w), 1512 (m), 1497 (vs), 1475 (w), 1449 (s), 1410 (w), 1393 (w), 1362 (s), 1315 (w), 1299 (w), 1268 (s), 1208 (m), 1194 (w), 1159 (w), 1116 (w), 1102 (m), 1070 (m), 1014 (w), 1001 (w), 993 (m), 973 (m), 942 (m), 902 (w), 833 (vs), 762 (vs), 749 (m), 702 (w), 689 (s), 657 (s), 639 (w), 628 (w), 610 (m), 584 (w), 570 (w), 553 (w), 542 (m), 489 (w), 429 (m); UV-Vis (toluene): 22930 cm-1 (ε = 16900 M-1·cm-1), 24095 cm-1 (ε = 22181 M-1·cm-1), 30770 cm-1 (ε = 8540 M-1·cm-1), 31950 cm-1 (ε = 8890 M-1·cm-1), 37170 cm-1 (ε = 31460 M-1·cm-1), 38170 cm-1 (ε = 30760 M-1·cm-1); m.p. 47-49 °C. Anal. Calcd. for C24H22O: C, 88.31; H, 6.79. Found: C, 88.14; H, 7.04. 1-(tBu)2: 1H NMR (benzene-d6, 300 MHz, δ): 7.98-7.92 (m, 4H), 7.72-7.60 (m, 1H), 7.51 (s, 1H), 7.25-7.19 (m, 3H), 7.06-7.02 (m, 1H), 6.89-6.75 (m, 2H), 1.34 (s, 18H);

13

C NMR

(benzene-d6, 75 MHz, δ) 151.72 (2C), 145.91 (1C), 144.06 (1C), 132.34 (1C), 131.81 (1C), 129.23 (2C), 126.98 (1C), 125.45 (1C), 125.30 (1C), 125.20 (2C), 122.79 (1C), 122.59 (1C), 121.79 (1C), 120.64 (1C), 120.61 (1C), 120.32 (1C), 35.10 (2C), 31.60 (6C); MS (ESI+) (m/z): 382.2290 (M+, ∆ = -1.8 ppm); IR (KBr, cm-1): 3086 (w), 3056 (m), 3039 (m), 2965 (s), 2952 (w), 2903 (m), 2865 (m), 1936 (w), 1793 (w), 1592 (vs), 1545 (w), 1499 (s), 1476 (m), 1460 (w), 1451 (s), 1425 (w), 1393 (s), 1377 (w), 1363 (s), 1315 (w), 1276 (m), 1250 (s), 1202 (m), 1149 (w), 1132 (w), 1073 (m), 1026 (w), 1008 (m), 993 (m), 950 (m), 909 (m), 899 (w), 881 (s), 861 (s), 846 (s), 827 (w), 778 (w), 765 (vs), 740 (w), 702 (s), 694 (vs), 671 (vs), 629 (w), 606 (s), 542 (w), 531 (w), 489 (w), 430 (m); UV-Vis (toluene): 22940 cm-1 (ε = 15900 M-1·cm-1), 24200 cm-1 (ε = 23360 M-1·cm-1), 30860 cm-1 (ε = 8540 M-1·cm-1), 31950 cm-1 (ε = 8800 M-1·cm-1), 37300 6 ACS Paragon Plus Environment

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cm-1 (ε = 28660 M-1·cm-1), 38300 cm-1 (ε = 28970 M-1·cm-1); m.p. 129 – 131 °C. Anal. Calcd. for C28H30O: C, 87.91; H, 7.90. Found: C, 88.00; H, 7.88. Steady State Optical Spectroscopy. Optical extinction spectra were collected using a Varian Cary 500 Scan UV-Vis-NIR spectrophotometer (600 nm/min). The baseline was collected in dual-beam mode with blank substrates in both the sample and reference beam paths. Sample data were referenced against a blank substrate in the reference beam path. No additional accounting for scattering or reflection was made with this instrument. Steady-state fluorescence was measured with a Horiba Jobin Yvon Model FL-1039/40 Fluorolog, a Horiba Jobin Yvon iHR320 spectrograph, and a Horiba Jobin Yvon SPEX Instruments S.A. Group Spectrum One G35 CCD camera. Solution fluorescence quantum yield (ΦF) was measured using 1 in cyclohexane as a reference (ΦF = 0.95).9 Quantum yields in films were determined using an integrating sphere with a 405 nm light emitting diode as the excitation source. Time-resolved Spectroscopy. Time-resolved photoluminescence was measured using a supercontinuum fiber laser (Fianium, SC-450-PP) operating at 10 MHz as the excitation source. The excitation energy used was 23810 cm-1. A streak camera (Hamamatsu C10910-04) was employed to measure spectra as a function of time delay from 400-900 nm.

The instrument

response function (IRF) depends on the time window size, but at 1 ns range the IRF is approximately 100 ps. Decays were fitted using a multiexponential function convoluted with the IRF, which was measured by collecting the scattered excitation beam from a Ludox solution or a roughened quartz slide. Ultrafast transient absorption was measured on solutions and films at an excitation energy of 25000 cm-1. The excitation pulses were produced by a Coherent Libra regeneratively amplified 7 ACS Paragon Plus Environment


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Ti:Sapphire laser with ~ 4 W, 1 kHz, ~100 fs output at 800 nm; about 30% of this is directed into a TOPAS-C optical parametric amplifier that is capable of producing wavelengths from 300 to 2600 nm as the excitation source. The excitation beam is attenuated and introduced into an Ultrafast Systems Helios spectrometer, and ~100 µJ of the 800 nm Libra output is also directed into the Helios, passing along a multi-pass delay stage that can afford ~5 ns of pump-probe delay, then focused onto a continuously moving CaF2 crystal to produce broadband visible spectrum (300-850 nm), used as the probe beam. The probe is passed through a neutral density filter, where a fraction is picked off to be used as a reference to account for fluctuations in probe intensity. The pump and probe beams are overlapped at the sample at a spot of ~250 µm diameter. Typical pulse energies range from 20-100 nJ. The probe is polarized at the magic angle with respect to the pump beam. The excitation is modulated at 500 Hz through an optical chopper to record both pump on and pump off spectra. CCD arrays of points are used for detection of both the probe and reference, and the transient signal is calculated in the Helios software. Typical acquisitions scan the entire 5 ns of the delay stage using 200 points with exponential time spacing, using several forward and reverse scans to average while monitoring for sample degradation. Background and chirp corrections were carried out using the Surface Explorer software (Ultrafast Systems), and other data manipulations and plotting were carried out using Igor Pro. For triplet sensitization and excimer/triplet lifetime experiments the transient kinetics were probed using an EOS spectrometer (Ultrafast Systems). Excitation was at 25000 cm-1 and 200500 nJ pulse energy. An electronic delayed continuum source provides the probe for transient absorption spectra with ~100 ps resolution to delay times of hundreds of µs.


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III. Results Steady-state Solution Spectra. The spectroscopic properties of solutions of 1-Me, 1tBu, and 1-(tBu)2 resemble those of 1 quite closely (Figure 1). The first absorption peak is red shifted by 50 to 100 cm-1 with respect to its position in 1. The presence of distinct but strongly overlapping absorption spectra of two degenerate conformers (disrotary (DIS) and conrotary (CON) relative twisting of phenyl groups9) broadens the absorption bands. In the excited state such twisting is nearly absent and as a result only one conformer is effectively present, and the observed fluorescence bands are sharpened. No shift is detected in the peak of the fluorescence among the three derivatives. The peak molar absorption coefficient for each of the substituted derivatives is approximately equal to that of 1 (23400 ± 500 M-1 cm-1). The fluorescence quantum yields are all equal within experimental error: ΦF = 95±4%.

Figure 1. Quantitative absorption and normalized fluorescence spectra for alkylated derivatives of 1 in toluene solution. Dashed lines are absorption and fluorescence of 1 in toluene. 9 ACS Paragon Plus Environment


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Transient Spectroscopy of Solutions.

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Time-resolved fluorescence kinetics were

obtained after weak pulsed excitation of the solutions at the peak of the lowest absorption band (Figure S1). The transients could be fitted, regardless of detected wavelength, with a single exponential decay function, showing lifetimes of 4.8 ± 0.1 ns, 4.9 ± 0.1 ns, and 5.0 ± 0.1 ns for 1-Me, 1-tBu, and 1-(tBu)2, respectively. Transient absorption spectra and kinetics for 1-Me match those of 1 faithfully (Figure S2). An excited state lifetime of 5.2 ± 0.3 ns is derived from global fits. For 1-tBu and 1-(tBu)2 the spectra also match those of 1, but good fits require a biexponential function. A weak 30-50 ps component, 10-20% of the full decay amplitude, precedes a dominant slower component, which is 4.8 ± 0.3 ns and 4.6 ± 0.4 ns for 1-tBu and 1(tBu)2, respectively. The faster decay component is most likely related to structural relaxation after photoexcitation, possibly mediated by solvent. This will be discussed further below. Triplets in Solution. Triplet absorptions of all derivatives in solution were identified in a flash photolysis experiment (excitation at 28200 cm-1) through triplet sensitization measurements with anthracene in TOL (Figure S3). The triplet spectra match those of 1 in solution, as well as triplet spectra of 1 in thin films.9, 11, 13 In the absence of triplet sensitization no triplets are observed for solutions of 1-Me, 1-tBu and 1-(tBu)2. Photophysics of Thin Films. The absorption spectrum for the thin film of 1-(tBu)2 is unshifted compared with that of solution 1-(tBu)2, Figure 2. The drop-cast thin films were between 100-200 nm thick based on the average absorption compared with thermally evaporated films of 1 of known thickness. The spectra of films of 1-Me and 1-tBu are each shifted by about 200 cm-1 from that of their respective solutions. Broadening is apparent in films of 1-Me and 1tBu, whereas films of 1-(tBu)2 have roughly the same absorption width as the solution. The peak of the fluorescence from all films is strongly red-shifted from the absorption peak, maximizing 10 ACS Paragon Plus Environment

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near 17000 cm-1. For 1-Me the fluorescence band profile is largely featureless. For both 1-tBu and 1-(tBu)2 a blue-shifted shoulder is apparent, peaking near 20000 cm-1, in addition to the main band.

Figure 2. (a) Extinction and (b) fluorescence of drop-cast films of 1-Me, 1-tBu, and 1-(tBu)2. Absorption and fluorescence for a film of α−1 1 is shown as a dashed curve (from ref 14). Enhanced extinction above 25000 cm-1 in 1-Me and 1-tBu films results from light scattering.

Time-resolved fluorescence data on a sub-ns timescale for films of alkylated 1 are shown in Figure 3. Fluorescence decays at longer times are shown in Figure S4. All emission lifetimes are multiexponential, and the average decay times increase from 1-Me to 1-tBu to 1-(tBu)2. The initial decay is instrument limited for 1-Me (< 90 ps), 250 ps for 1-tBu, and 750 ps for 1-(tBu)2. The ratio of the amplitude of the fast decay compared with all slower decays is 10:1 for 1-Me, 1:1 for 1-tBu, and 1:2 for 1-(tBu)2. Results of fits can be found in Table S1. The time-resolved fluorescence spectra show an increase in red-shifted fluorescence with increased delay time (Figure S5). The shift is most pronounced for 1-tBu and 1-(tBu)2 films and persists beyond the



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instrument response time of 100 ps but saturates within 300 ps. For 1-Me, the shift is small and saturates within 100 ps.

Figure 3. Fluorescence decay profiles for films of alkylated 1. The curves were generated by integrating fluorescence in a band from 18000-21000 cm-1. Instrument response is shown as the dashed curve.

Transient absorption (TA) measurements were performed on all films at 25000 cm-1 excitation. From studies on films of 1,11, 14 spectral features associated with the populations in S1, T1, and an excimer states can be identified for each film, Figure 4. These include a broad excited state absorption (ESA) near 15000 cm-1, a narrower ESA at 20500 cm-1, and stimulated emission at 19000 cm-1. Triplet absorption occurs near 21500 cm-1, and is observed as an apparent shift of the S1 ESA peak at longer times. Excimer absorption is best probed at 18000 cm-1 and is uncovered as S1 ESA and stimulated emission decrease.


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Figure 4. Transient absorption spectra after 25000 cm-1 excitation for (a) 1-Me, (b) 1-tBu, and (c) 1-(tBu)2 sliced at the delay times indicated in the legend.

Kinetic analysis of the 1-Me film TA data reveal at least two stages of excited state evolution after photoexcitation. The first stage, occurring in 17 ps (τexc), results in S1 decay to a second species that resembles the excimer. We note that the absorption features do not match the radical cation and anion spectra obtained previously (Figure S6). The second stage occurs in approximately 294 ps (τT) and involves further decay of S1 into the excimer as well as into the triplet. The slowest time component, lasting much longer than 5 ns, is dominated by triplet absorption but does contain a small amount of excimer absorption (Figure 5a). For 1-tBu films these stages are repeated but with altered time constants (35 ps for τexc, 483 ps for τT). Also, the relative amount of excimer detected at long times is larger than in 1-Me. The 1-(tBu)2 dynamics are slowed further, with τexc = 76 ps and τT = 832 ps. The relative amount of excimer remaining at long times is larger than that of 1-tBu. For 1-Me, a small relative amplitude of triplet absorption is detected during the first stage of evolution (Figure 5b); however, 1-tBu and 1(tBu)2 films show no rise until after 50 ps. 13 ACS Paragon Plus Environment


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Figure 5. (a) Slow component decay associated spectra for thin films, normalized to the peak of the triplet at 21500 cm-1. (b) Unnormalized kinetics for the triplet in thin films as detected by probing at the T1-T5 absorption position (21900 cm-1).

The triplet yields ΦT were measured by comparing the initial bleach strength to the maximum triplet amplitude, as in ref 11. No net molecular orientation was assumed, as the absence of peaks in XRD suggests no long-range order. The ΦT for 1-Me is found to be 75 ± 20%. ΦT for 1-tBu is determined to be 55% ± 18%, and for 1-(tBu)2 it is 34% ± 15%. Given that a small amount of crystallinity was detected for 1-Me, some crystalline regions with possible net molecular orientation are present. ΦT determined assuming an entirely amorphous film is a lower limit in that case, with an upper limit of 105% if full crystallinity and net crystallite orientation similar to that in thin films of 1 are assumed.

The excimer and triplet lifetimes were investigated with TA spectroscopy in the ns-µs range (Figure S7). The excimer lifetimes are 13 ns and 6 ns for 1-Me and 1-tBu, respectively. The 14 ACS Paragon Plus Environment

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triplet lifetimes are highly nonexponential and significant noise prevented a full analysis, but the primary decay times are 0.4 µs and 1.8 µs for 1-Me and 1-tBu, respectively. The 1-(tBu)2 film photodegraded during the experiment, therefore kinetic data were unreliable. Table 1. Photophysical data for films of alkylated 1. Compd.

TOL ΦF/ %

Film ΦF / %

Film ΦT / %

Film τT / ps

Film τexc / ps

1-Me

99

1.9

75-105a

294

17

1-tBu

91

2.9

55

483

36

1-(tBu)2

99

1.7

35

832

75

a

The range represents calculated values assuming an entirely amorphous film to entirely oriented crystallites.

Annealing.

Mild thermal annealing at 100°C produced no discernible change in the

properties of the 1-tBu or 1-(tBu)2 films, which showed no peaks in the XRD powder pattern, Figure S8.

Temperatures above 100°C resulted in degradation or sublimation of films.

However, the 1-Me film became more crystalline upon mild annealing. The change in the absorption spectrum upon annealing was minor, but the peak of the fluorescence red-shifted by 1000 cm-1. In transient absorption, features associated with the excimer formed more rapidly (τexc = ~13 ps) than in as-cast films, but no triplets were observed. Despite repeated efforts at crystallization, no suitable crystals could be grown for obtaining crystal structures.

Discussion Effect of Alkyl Substitution. The photophysics of 1-Me, 1-(tBu)2, and 1-(tBu)2 in solution is very similar to that of 1. The absorption spectra are broadened by the presence of conformers, CON and DIS, which have different relative strengths of 0-0 and 0-1 vibronic features in the S015 ACS Paragon Plus Environment


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S1 absorption band. The overlapping spectra lead to similar broadening for all of the alkylated derivatives of 1. The spectral shapes and decay times of fluorescence are also quite similar among the series. Nearly monoexponential decay is observed in transient absorption but with a small component on the 5-30 ps timescale that may be associated with untwisting of the phenyl rings upon photoexcitation. Despite the spectral similarities, it is clear that the substituted groups add bulk and thus steric hindrance that disrupts the natural proclivity of 1 to spontaneously form crystalline layers when solvent is removed. 1-Me films are partially crystalline as cast (Figure S8), but 1-tBu and 1(tBu)2 films are resistant to crystallization, even after mild thermal annealing. The result is that films are amorphous or nearly so, and the absorption spectra are similar to those in solution but with slight red-shifting and broadening, Figure 2. In a dynamic sense, the heterogeneous film environments allow excitations to sample the different sites in the film through energy transfer, and the lower energy regions will be preferentially populated over time. This effect can be seen in the time-resolved fluorescence spectra, which exhibit an enhanced red-shifted component that arises on a timescale commensurate with excimer formation determined from transient absorption (Figure S5). Fluorescence collected under steady-state conditions is dominated by the lower energy emission, which is evidenced by the strongly red-shifted and broadened fluorescence spectra compared to films of 1 (Figure 2b). The low energy species are assigned as excimers, with support from the observation of characteristic transient absorption near 18000 cm1

(Figure 5a). The nature of the intermolecular geometries that gives rise to these excimers is

difficult to assess given the lack of structural information. Although fits to the rise of excimer species using an exponential model are satisfactory, the time constants derived from such fits probably represent a range of excimer formation times that


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reflects the density of excimer-like geometries in the film.

The average distance between

chromophores is likely to grow as substituents are added, and with the increased distance should come slower energy migration and longer excimer formation times. The expected trend is reflected in data in Table 1, where the excimer formation time τexc increases from 17 ps to 75 ps with the increasing size of substituted groups. If a strict kinetic competition between excimer formation and SF is assumed, we should expect to observe triplets rise on the same time scale as excimers, but instead triplets rise more slowly. The exception is 1-Me, in which 10-15% of the total triplet amplitude rises in less than 50 ps. Nonetheless, the observation of excimer formation well in advance of the majority of triplet formation suggests two possible models of excited state behavior: (i) the excimer serves as an intermediate between the localized S1 and TT, or (ii) there is a heterogeneous distribution of intermolecular arrangements (e.g., α-type, β-type, amorphous) such that excited state evolution occurs through several pathways in parallel. These models and the associated kinetic schemes are depicted in Figure 6. Model (i) has been suggested in other SF-active systems,26-27 but the quickly formed excimer states are likely lower in energy than initially formed S1, rendering SF significantly endothermic.28 Moreover, at longer delay times, excimers decay in parallel with triplets rather than in series.

A scenario in which a

subpopulation of excimers and (TT) species exist in equilibrium cannot be ruled out but is difficult to test experimentally. Model (i) is also inconsistent with the observed anticorrelation between the amplitude of the excimer rise and the eventual triplet yield. Although excimers form the most rapidly in 1-Me films, the amplitude of the associated absorptive signal is relatively small, which leaves almost half of the population remaining in localized S1 and able to undergo SF if a site with appropriate intermolecular coupling is populated. Excimers form more slowly in 1-tBu and 1-(tBu2), but the



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amplitude of the signal is large, leaving little available population remaining in S1 states to undergo SF. These trends confirm that multiple excited state decay pathways exist for each type of film and that the proportion of population traversing the desired pathway (i.e., SF) is largest with the smallest perturbation to the structure of 1 (i.e., 1-Me). Mutual dispositions of molecular pairs that most resemble α-type pairs may exist in the highest density in 1-Me films (represented by yellow boxes in Figure 6), and diffusion to these sites leads to triplets via SF. The crystallites observed in such films, Figure S9, may be evidence of the relative proportion of α-type sites that are SF-active.

Indeed, fewer such crystallites are observed for 1-tBu and 1-tBu2 films.

However, short-range structure cannot be revealed with the characterization techniques employed here.

Annealing 1-Me produces crystallinity and stronger interchromophore

interactions, which serve primarily to increase the excimer formation rate constant kexc and to reduce the available population in S1 to undergo SF. Our past studies indicate that thermal annealing produces a high density of β-type sites, which lead to faster excimer formation.13 The triplet formation times are increased for all of the films of derivatives compared with films of α−1. With an assumed random orientation of molecules in the film, the likelihood of the favored interchromophore interactions for SF occurring at the initial site of photoexcitation is small, and thus we observe at most 10-15% of the total population of triplets rise within the ~20 ps SF time measured for α-1 films. The delayed rise of triplets suggests that the effective SF time observed in disordered films more likely reflects the period of time it takes for an excitation to diffuse to a site appropriate for fast SF, 1/kdiff, than it does the intrinsic SF rate corresponding to specific intermolecular dispositions, 1/kSF. This diffusion time is related to the diffusion coefficient and the density of SF-active sites. Although we can assume that the exciton diffusion coefficient is reduced as the steric bulk of substituents increases, the trends in excimer and triplet 18 ACS Paragon Plus Environment

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formation times will also reflect variations in the density of sites where excimers or triplets are likely to form. Disentangling the two effects is difficult, given the lack of structural information.

Figure 6. Homogeneous (i) and two-site (ii) models for excited state dynamics in alkylated films of 1. States are ground state (S0), localized singlet (S1), excimer (Exc), and triplet (T1). Conclusions We have investigated a series of DPIBF derivatives that form disordered solids upon casting films from solution. The triplet yield is highest in the semi-crystalline films of 1-Me, and is systematically reduced for derivatives with bulkier substituents. The excited state dynamics in all cases reveals a strong tendency for excimer formation on a 20-100 ps timescale. This behavior is in contrast to other systems that appear to have more generous allowance of variations in intermolecular coupling that still produce fast SF (e.g. pentacene).29-30 These observations lead us to conclude that for systems in which SF is not exoergic or optimal SF couplings are not very large, ensuring a high density of optimized mutual dispositions and thus 19 ACS Paragon Plus Environment


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optimized SF rates (i.e., by making dimers or by inducing crystallinity) is paramount to achieving high triplet yields. Otherwise, formation of low energy species in the solid films (i.e., excimer formation) presents a strongly competitive pathway with triplet formation. Casting films of dimers with pre-determined intermolecular dispositions for fast SF may be a useful path toward producing higher triplet yields in amorphous solids of DPIBF.

Acknowledgements This material is based on work supported by the U. S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences, and Geosciences.

J.C.J

acknowledges DE-AC36-08GO28308 with NREL and J.M. acknowledges DOE DE-SC0007004. J.M. also acknowledges support from the Grant Agency of the Czech Republic, GA15-19143S.

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Singlet Fission and Excimer Formation in Disordered Solids of Alkyl

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