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Spectroscopy and Photochemistry; General Theory
Molecular Packing and Singlet Fission: The Parent and Three Fluorinated 1,3-Diphenylisobenzofurans Eric A. Buchanan, Ji#í Kaleta, Jin Wen, Saul H. Lapidus, Ivana Cisarova, Zdenek Havlas, Justin Johnson, and Josef Michl J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03875 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 18, 2019
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The Journal of Physical Chemistry Letters
Molecular Packing and Singlet Fission: The Parent and Three Fluorinated 1,3Diphenylisobenzofurans
Eric A. Buchanan,¶ Jiøí Kaleta,¶,§ Jin Wen,§ Saul H. Lapidus,‡ Ivana Císaøová,Z Zdenìk Havlas,§ Justin C. Johnson,† and Josef Michl¶,§
Department of Chemistry, University of Colorado, Boulder, Colorado 80309, United States; § Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nám. 2, 16610 Prague 6, Czech Republic; ‡ Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Ave., Bldg. 433/D002, Argonne, IL 60439; Z Department of Inorganic Chemistry, Faculty of Science, Charles University, Hlavova 2030, 12840 Prague, Czech Republic, † National Renewable Energy Laboratory, Golden, Colorado 80401, United States ¶
ABSTRACT Crystal structures, singlet fission (SF) rate constants, and other photophysical properties are reported for three fluorinated derivatives of 1,3-diphenylisobenzofuran and compared with those of the two crystal forms of the parent. The results place constraints on the notion that the effects of molecular packing on SF rates could be studied separately from effects of chromophore structural changes by examining groups of chromophores related by weakly perturbing substitution if their crystal structures are different. The results further provide experimental evidence that dimer-based models of SF are not sufficiently general and that trimer- and possibly even higher oligomer-based or many-body models need to be formulated.
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Singlet fission (SF)1,2,3,4,5,6,7 converts a singlet into two triplet excitations, providing an opportunity8,9 to overcome the Shockley-Queisser limit10 on the maximum efficiency of solar cells. It only proceeds efficiently in a handful of mostly impractical materials, and better ones are needed urgently. Both the nature of the chromophores and their packing11,12,13,14,15,16,17,18,19,20 are crucial, and here we address the latter. Experimental information on desirable packing consists of observations on covalent dimers21,22,23,24,25,26,27,28 and materials available as multiple polymorphs.11,12,13,14,20 Future design of efficient SF materials by crystal engineering or by synthesis of covalent dimers could benefit from comparison of derivatives of a chromophore17,29,30 that leave the molecular photophysics nearly intact but affect packing. In an effort to test this notion we use fluorine substitution, uncommon in SF studies,31 and compare 1,3-diphenylisobenzofuran (1)32,33,34 with 1-F1, 1-F5, and 1-F10 (Chart 1). The polymorphs13 1á and 1â differ strongly in SF properties, promising sensitivity to packing. For experimental and computational procedures, see Supporting Information.
Chart 1. Structural formulas and molecular axes.
Isolated Molecules. In toluene solution, the observed S0-Sn, S1-Sn, and T1-Tn absorption and S1-S0 emission spectra, fluorescence quantum yields ÖF, lifetimes of S1 (ôF) and T1 (ôT), ground state bleach (S0-Sn), and stimulated S1-S0 emission of the fluorinated species (Chart 135) are similar to those of 136 (Table 1 and Figures 1 and S1 - S3A). For 1-F1, whose substituent hardly affects the aryl twist angles, there is virtually no difference. For 1-F5 and 1-F10, carrying ortho fluorine substituents that increase one or both twists, S1 excitation energy increases by ~1500 and ~2800 cm-1, respectively, ôF is shorter, and the intense S1-Sn band near 14300 cm-1 is red-shifted and weaker (Table S1). S1 lifetimes from transient absorption roughly match those from fluorescence decay (ôF) and are mostly monoexponential, with some additional activity up to ~10 ps, possibly due to adjustment of phenyl twist angles after excitation. Vibrational structure is clearer in fluorescence of 1 than in absorption. This has been attributed to a different twist in S0 and S1.36 In S0 con and dis rotamers are present and contribute different and overlapping Franck-Condon envelopes. In S1 only one effectively planar rotamer exists. Computations (Table S1) suggest that this difference also exists in 1-F1 and 1-F5, and is smaller in 1-F10.
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The Journal of Physical Chemistry Letters
In the following, we use the intersection of normalized absorption and fluorescence spectra for the S0-S1 energy difference. The S0-T1 energy difference is known only for 1, with almost identical values from bracketing sensitization in solution and electron energy loss spectroscopy on the solid36 (we use the latter). For 1-F1, 1-F5, and 1-F10, we shift the value for 1 by the amount calculated with the B3LYP version of density functional theory (DFT), which is also used to calculate the reorganization energy ë for SF.
Figure 1. Fluorescence (A) and absorption (B): 1, 1-F1, 1-F5, and 1-F10 in toluene.
Table 1. Photophysics in toluene solution (energy units: 103 cm!1). Cmpd
S06S1a
S16S0b
S16Snc
ÖFd
ôF/nse
T16Tnf
ôT/ìsg
ëh
S06T1i
ESFj
136
24.20 (22.75)
22.07
21.2
0.99
4.7
22.0
>200
3.12
11.40
0.05
1-F1
24.44 (22.80)
22.28
20.7
0.94
5.0
22.0
>200
3.34
11.38
-0.04
1-F5
25.58 (23.80)
23.20
20.7
0.88
3.1
22.3
>200
3.24
12.20
0.60
1-F10
27.10 (25.05)
24.52
20.4
0.86
2.7
21.7
>200
3.48
13.07
1.14
a
Maximum of the first absorption band. In parentheses, intersection of absorption and emission curves. b The first fluorescence peak. c Prominent band in S1 absorption. d Fluorescence quantum yield. e Fluorescence lifetime. f Prominent band in triplet absorption (sensitized with anthracene). g Triplet lifetime. h B3LYP reorganization energy for SF. i Triplet excitation energy from B3LYP shift relative to 1. j SF endoergicity expected in the absence of intermolecular interactions (S1 excitation energy from the intersection of absorption and emission curves).
Solids. Crystal structures (Figure 2) of 1 and 1-F1 are almost identical, with parallel planes of slip-stacked isobenzofuran cores separated by 3.45 Å. In 1-F5 the z axes (Figure 1) of stacked neighbors are antiparallel and the C6F5 and C6H5 rings of neighbors stack. The isobenzofuran plane is almost parallel with that of the nearest C6H5 group, 3.41 Å apart. The packing of 1-F10 allows a strong interaction of the C6F5 rings with neighboring isobenzofuran cores, which are not mutually parallel. The C6F5 groups on neighbors are nearly parallel and are stacked 3.26 Å apart. The z axes of slip-stacked nearest neighbors are mutually nearly orthogonal, with the z axis of one along the y axis of the other.
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Figure 2. X-Ray diffraction crystal structures of (A) 1á,13 (B) 1â,13 (C) 1-F1, (D) 1-F5, and (E) 1-F10.
Figure S5 shows all neighbor pairs that can be excised from the crystal lattices of 1á, 1â, 1F1, 1-F5, and 1-F10, along with their SF electronic matrix elements T2 and excimer stabilizations Eexc computed by the method of ref. 5. The most strongly interacting pairs are shown in Figure 3.
Figure 3. Neighbor pairs in crystals calculated to interact the most strongly (data for 1 from ref. 13).
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The Journal of Physical Chemistry Letters
Table 2. Photophysical properties of thin solid films (energy units: 103 cm!1). No.
S06S1a
S16S0b
S16Snc
ÖFd
ÖT e
ôF/ns (%)f
kSF/ns%1g
T16Tnh
1á36
21.25
20.36
20.9
0.10
1.4±0.2533,37
1.0 (52); 2.7 (42)
56±6
21.5
1â36
20.7±0.313,33
20.25
20.4
0.59
0.1±0.05
1.3 (54); 3.3 (46)
10±4
20.8
1-F1
21.30
20.36
20.8
0.10
1.3 ± 0.15
1.0 (33); 2.8 (60)
50±4
21.5
1.4 (42); 4.9 (54)
48±8
21.7
0.6 (85); 3.5 (5)
91±11
23.1
1-F5
21.95
20.74
21.0
0.040
1-F10
22.88
21.80
20.7
0.035
i
1.5 ± 0.3
a
Absorption onset (intercept of baseline with linearly extrapolated rising slope; absorption bands are distorted by scattering). b The first peak in fluorescence. c Prominent band in excited singlet absorption. d Fluorescence quantum yield. e Triplet yield. f The two most important contributions to multiexponential fluorescence lifetime at the emission peak (% contribution in parentheses). g Mean values and standard deviations were obtained from measurement on several films (on each film, five measurements on different spots). Decay time of S1 is equal to the rise time of triplet. Assuming SF is the dominant process, its rate constant is the inverse of the decay time. h Prominent band in triplet absorption. i Triplet yield not determined due to spectral overlap.
Drop-cast thin films of 1-F1, 1-F5 and 1-F10 on glass (Figures S3B and S4) show sparse X-ray diffraction patterns similar to those of 1,13 indicating strong texturing (Figure S6). The dominant (001) reflection for 1 is also found for 1-F1, with the molecules oriented with the z axis (Chart 1) perpendicular to the substrate. For 1-F5 the dominant peak is due to the (10-1) plane, suggesting that z is tilted away from the surface normal. 1-F10 shows primarily peaks due to the (x10) planes but others as well, indicating a more isotropic distribution of crystallites. Table 2 and Figure 4 show the photophysical properties of the thin films. There was considerable uncertainty in the S1 excitation energy in 1â when it was evaluated in different ways,13,33 and the same is undoubtedly true in all five solids. Still, the absorption and fluorescence peak positions are ordered as in solution (Table 1). The fluorescence of 1-F10 shows a second strong peak separated by 1900 cm-1, assigned to excimer emission. Fluorescence decay is multiexponential for all thin films (Figure 4). An instrument-limited phase (< 100 ps) is followed by at least two other decay components with lifetimes from 1 to 15 ns. Similar kinetics for 1 were previously attributed to SF followed by delayed fluorescence.37 Here, too, we expect the fast component (100 ps); B: 1-F1; C: 1F5; D: 1-F10.
The SF rate constants,
