Article pubs.acs.org/JPCA
Excitation Localization/Delocalization Isomerism in a Strongly Coupled Covalent Dimer of 1,3-Diphenylisobenzofuran Joel N. Schrauben,† Akin Akdag,§ Jin Wen,∥ Zdenek Havlas,∥ Joseph L. Ryerson,†,‡ Millie B. Smith, Josef Michl,‡,∥ and Justin C. Johnson*,† †
National Renewable Energy Laboratory, Golden, Colorado 80401, United States Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, Colorado 80309, United States § Department of Chemistry, Middle East Technical University, 06800 Ankara, Turkey ∥ Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 16610 Prague, Czech Republic ‡
S Supporting Information *
ABSTRACT: Two isomers of both the lowest excited singlet (S1) and triplet (T1) states of the directly para, para′-connected covalent dimer of the singlet-fission chromophore 1,3diphenylisobenzofuran have been observed. In one isomer, excitation is delocalized over both halves of the dimer, and in the other, it is localized on one or the other half. For a covalent dimer in solution, such “excitation isomerism” is extremely rare. The vibrationally relaxed isomers do not interconvert, and their photophysical properties, including singlet fission, differ significantly.
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INTRODUCTION
species directly leading to two coexisting isomeric species with different degrees of delocalization appears to be uncommon. Delocalization and charge transfer in coupled chromophore systems have arisen as important topics in understanding and optimization of SF.11−13 Our groups have been focusing on biradicaloids as candidate chromophores for SF, and particularly on 1,3-diphenylisobenzofuran (1),14 which was proposed as a potential SF chromophore on the basis of state energy considerations.15 Whereas thin films of 1 exhibited 200% efficient triplet formation when crystallized in a particular polymorph,16−18 weakly coupled covalently bound dimers of 1 showed solution triplet quantum yields of less than 10%.19 Covalently bound tetracene dimers with an “edge-to-edge” or “face-to-face” geometry undergo SF in nonpolar solvents with similarly low efficiencies,20−22 although those with stronger coupling via an ethynyl linker have shown much higher triplet yields.23,24 This behavior is similar to that of functionalized pentacene dimers, which in solution have exhibited efficient SF characterized through the interchromophore distance dependence25 and the interplay between SF and TT annihilation in dimers constructed with para, meta, and ortho linking geometries26 and in a T-shaped dimer.27 In the present study, we investigate dimer 4 (Figure 1), in which the attachment occurs as a para−para′ link between the phenyl groups of neighboring chromophores of 1. Calculations have shown that coupling through the para positions of the
Interchromophore coupling in covalently bound dimers has been investigated for several decades for its role in stimulating excitonic effects,1 intramolecular charge transfer,2 and more recently, singlet fission (SF).3,4 For dimers in which the chromophores are connected through a single C−C bond, the torsional landscape can be complex, with shallow minima and maxima separating planar (conjugated) and twisted (nonconjugated) geometries. As multichromophoric molecular systems are constructed (e.g., from dimers to oligomers or dendrimers),5−7 increases in rigidity and length tend to reduce torsional bond angle disorder and promote the formation of coupled chromophore units that sustain primarily delocalized excitations. The balance between localization and delocalization can be quite delicate, and mixtures of delocalized and localized species may appear in seemingly homogeneous samples.8−10 For example, ground-state isomerization has been observed by single-molecule fluorescence in biperyleneimide molecules bound to a polymer host.8 Static disorder in the environment produces the delocalized versus localized behavior evident in emission spectra, although fluctuations incite “switching” from one form to the other. Studies of stilbene dimers in solution have shown that two dominant ground-state conformers exist, one with a planar geometry and one twisted.9 The planar dimer exhibits red-shifted absorption and an enhanced emission rate compared with the twisted dimer, which undergoes isomerization in the excited state on a 5−30 ps time scale. Although these examples set the stage for the general notions addressed here, our observation of excitation from a single ground-state © XXXX American Chemical Society
Received: January 25, 2016 Revised: April 25, 2016
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Figure 1. (A) 1,3-Diphenylisobenzofuran (1) and its dimers. 2 and 3 were examined in a previous report,19 and 4 is the subject of this report. Molecular axis directions are defined for the monomer. (B) Possible major (left) anti and (right) syn conformers of 4. The major conformers are derived through a 180° rotation about the dihedral angle α or β. (C) The torsion angles α, β, and γ for 4.
enantiomers, with phenyl twists disrotatory). The known crystal structure14 contains only the C2 conformer, but in solution all are computed to be about equally present at equilibrium. The energies and other properties of the C2 and Cs conformers are nearly identical, except for the shape of the Franck−Condon envelope of the first transition.14 In dimer 4 there are five single bonds that would be expected to be twisted by similar amounts in either sense (±20° or ±160° if one of the connected rings is five-membered and ±35° or ±145° if both are six-membered) by analogy to the monomer, producing up to 4 5 conformers, counting enantiomers. The actual number will be smaller because of the symmetries present. For instance, only the values of ±20° need to be considered for the angle γ, and some of the structures are meso, reducing the total number significantly. Once again, we would expect the properties of all of these conformers to be very similar. This expectation was confirmed by the results of DFT and TD-DFT B3LYP/SVP geometry optimizations, which were performed for 32 distinct conformers of 4 (Table S1 in the Supporting Information). We found that the ground-state conformer energies differed by less than 0.4 kcal/mol and their first triplet excitation energies by less than 0.2 kcal/mol. The interconversion of the conformers is expected to be as easy as in 1. A diligent search for further ground-state conformers did not reveal any, and the use of dimethyl sulfoxide (DMSO) solvent in the COSMO approximation31 had a negligible effect.
phenyl groups results in the largest values of the electron transfer integral but increases the endothermicity of SF by lowering the energy of the S1 state much more than the energy of the T1 state.28,29 Experimentally, the excitation localization/ delocalization isomerism observed here for 4 in solution enriches the photophysical picture of SF by providing two unique pathways to (TT): one that is strongly inhibited as a result of a reduction in the S1 energy upon delocalization and another that can proceed through localized states that have more favorable energies for SF.
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RESULTS
Calculated Ground-State Conformations. The heterocyclic core of the basic chromophore 1 is planar, but in the ground state its two phenyl substituents are twisted ∼20° out of plane to minimize the interference of their ortho hydrogen atoms with those of the core. The degree of twist is a little smaller than the ∼35° in the archetypical standard, biphenyl, because one of the rings is five-membered and the steric interference is reduced. The barrier to interconversion by rotation around the single bond has not been measured or calculated, but there is little doubt that it is less than that in biphenyl (2−3 kcal/mol30). Twists of the two phenyl substituents around single bonds produce up to 22 conformers, but the symmetry equivalence of the two bonds reduces the number from four to three: one conformer is of Cs symmetry (meso, with phenyl twists conrotatory), and two are of C2 symmetry (a pair of B
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Table 1. Optimized State Energies and Geometries for 4-syn and 4-anti in the Gas Phase and in DMSO (Energies in kcal/mola and Angles in deg) gas phase state
energy
α
β1
DMSO β2
γ1
S0 S1* S0S1b T1* S0T1b 1 (TT) 5 (TT) = Q
0.0 51.4 60.8 35.7 38.0 63.6 66.4
(18.0) (21.3) (12.5) (13.3) (22.2) (23.2)
31.7 13.6 28.1 0.6 24.7 23.3 32.1
16.3 4.2 19.9 0.4 25.3 2.2 1.8
16.3 4.2 2.6 0.4 2.7 2.0 1.8
17.8 9.9 25.6 9.7 27.5 3.3 0.0
S0 S1* S0S1b T1* S0T1b 1 (TT) 5 (TT) = Q
0.0 51.2 60.8 35.6 37.6 63.4 66.2
(17.9) (21.3) (12.5) (13.2) (22.2) (23.2)
31.6 14.8 28.8 0.5 24.5 23.2 31.5
166.3 175.5 159.3 179.3 154.4 177.8 175.7
16.6 4.2 1.3 0.6 2.8 1.9 4.0
17.8 9.8 25.7 9.6 26.9 1.5 0.6
γ2
energy
4-syn 17.8 9.9 5.7 9.7 2.5 3.2 0.0 4-anti 17.6 9.8 4.8 9.6 2.5 1.4 0.1
α
β1
β2
γ1
γ2
0.0 45.3 59.1 35.1 37.6 56.1 58.9
[−6.9] [−13.0]c [−8.6]c [−7.5] [−7.3] [−14.5] [−14.4]
29.9 2.8 27.0 1.3 21.7 23.3 32.1
12.8 0.2 20.9 0.3 24.5 2.2 1.8
12.8 0.2 0.3 0.3 1.8 2.0 1.8
16.0 6.2 25.1 7.2 26.7 3.3 0.0
16.0 6.2 1.1 7.2 0.9 3.2 0.0
0.0 45.3 59.1 35.1 37.4 54.4 57.2
[−7.0] [−12.9] [−8.7] [−7.5] [−7.2] [−16.0] [−15.9]
28.8 1.5 27.6 0.1 21.9 23.2 31.5
170.2 179.8 158.2 180.0 155.4 177.8 175.7
14.6 0.2 0.2 0.0 0.9 1.9 4.0
16.7 5.3 25.0 6.0 25.6 1.5 0.6
16.0 5.3 0.2 6.0 0.8 1.4 0.1
a
DFT energies (B3LYP/SVP without and with COSMO, DMSO (ε = 46.7)) relative to the ground state. Values in square brackets are the solvent stabilization energies. Energies in 103 cm−1 are given in parentheses. bLocalized S1 and T1 states (S0S1 and S0T1). The geometries were optimized by TD-HF/SVP, and the energies were calculated by TD-B3LYP/SVP. cThe S1* and S0S1 energies are 51.4 and 61.5 kcal/mol, respectively, in toluene (ε = 2.4).
Figure 2. (A, B) Comparison of the calculated geometries of the delocalized S1* and localized S0S1 states for 4-syn (TD-B3LYP/SVP and TD-HF/ SVP, respectively). (C, D) Comparison of the calculated geometries of the delocalized T1* and localized S0T1 states for 4-anti (B3LYP/SVP and HF/SVP, respectively).
Calculated Singly Excited States: The First Excited Singlet and the First Triplet. DFT calculations were performed for selected syn and anti conformers (Tables 1 and S1−S6). Similarly as in 1, in the optimized gas-phase and DMSO structures of the excited states, most of the twist angles β and γ are much closer to zero than in the S0 ground state. The angle α decreases greatly as well, and as a result, the number of conformer minima that are surrounded by significant barriers in the excited states is considerably reduced relative to S0. A casual search for an optimized geometry of the excited singlet state of either 4-syn or 4-anti leads to a minimum in which both halves of the dimer are excited (Figure S1). We use the symbol S1* for this nearly planar state with symmetrically delocalized excitation. However, optimization searches starting
For further discussions of the ground states, we have selected two conformers of 4, one of the “syn” type, with the furan oxygen atoms on the same side of the dimer, and one of the “anti” type, with the furan oxygens on opposite sides of the dimer, as represented in Figure 1B. The major conformers of the dimers are accessible through a 180° rotation about the dihedral angle α or β. Tables 1, S1, and S2 contain the calculated energies and geometries of the singlet and triplet states of these different major conformers. The conformers were selected from the conformation stability test by the PBE/ def2-SVP method.32 We reoptimized the most stable conformers at the B3LYP/SVP level, and the vibrational frequencies were tested in the ground state for the conformers in Table 1. C
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Figure 3. Spin density (α minus β spin) of the T1* and S0T1 states of 4-anti (B3LYP/SVP).
exactly true for 5(TT) in 4. The energies of the three TT states differ for two reasons:3 (i) the energies of the double-triplet configurations differ in their electron repulsion terms, and (ii) the 3(TT) and 1(TT) configurations generally interact fairly strongly with other nearby triplet and singlet configurations, whereas other quintet configurations tend to be too far away in energy for similar interactions with 5(TT). The calculations show that 1(TT) is stabilized with respect to 5(TT) by nearly 3 kcal/mol in both 4-syn and 4-anti. This leaves the 1(TT) state above the S0S1 state by less than 3 kcal/mol, a value that we consider to be less than the calculation uncertainty and thus consistent with the possibility of singlet fission at room temperature. For all of the major conformers, the S1* state is significantly stabilized relative to both 5(TT) and 1(TT). For 4syn and 4-anti, S1* is more stable by ∼12 kcal/mol and unlikely to undergo fission to form the 1(TT) state at room temperature. Measured Ground and Excited Singlet and Triplet Spectra. Dimer 4 was prepared and purified as reported elsewhere.34 The UV−vis absorption and fluorescence spectra are shown in Figure 4. Solvatochromism is clearly present, with a shift of ∼750 cm−1 as the solvent polarity is increased from cyclohexane (CH) to DMSO. 4 also displays drastically different fluorescence spectra in CH, toluene (Tol), and DMSO, with the fluorescence maximum shifting by more than 2000 cm−1 across the series. In addition to the large red shift, the fluorescence of 4 in DMSO is also significantly broadened compared with that of 4 in CH and Tol. The calculated transitions are plotted along with the experimental spectra. We note that the TD-DFT methodology used herein to compute the transition energies in these dimers produces charge-transfer states at artificially low energies that must be ignored.35,36 For example, two nearly degenerate locally excited S0−S1 transitions are anticipated in these dimers, but four low-energy transitions are calculated. The spurious additional low-energy chargetransfer transitions have low calculated oscillator strengths and do not affect the predicted spectra. In CH and Tol, the fluorescence profile of 4 is excitationwavelength-dependent. As the excitation energy is increased, additional intensity arises at higher energy (Figure S3). The apparent absorption edges for fluorescence detected at 20 410 and 22 200 cm−1 are shifted from the ground-state absorption onset by ∼500 and ∼1500 cm−1, respectively. In DMSO, the fluorescence spectra are almost invariant with respect to excitation energy. In all three solvents, ΦF decreases as the excitation energy is increased from the absorption peak toward 25 600 cm−1 (Table 2). As shown in Figure 4C, the T1−Tn spectrum of 4 collected at early times following direct photoexcitation is red-shifted by about 2000 cm−1 from that of 1, which was obtained through
at certain unsymmetrical geometries of 4-syn lead to a higherenergy local minimum similar to that found in 2 and 3,19 in which only one half of the dimer is excited, a situation we label as S0S1. In this isomer, the central twist angle α and the twist angles β1 and γ1 in the unexcited half of the molecule are much larger than the angles β2 and γ2 in the excited half and are also larger than the same angles in the S1* isomer. In 4-anti, an analogous minimum with localized excitation was found only when the Hartree−Fock method was used. This behavior seems to reflect the tendency of the DFT method to favor delocalized structures, whereas the HF method favors localized structures.33 The difference in the gas-phase energies of the more stable S1* species with delocalized excitation and the less stable S0S1 species with localized excitation is about 9 kcal/mol when calculated by the TD-DFT method and about 8 kcal/mol when evaluated by the TD-HF method. In the lowest triplet state of 4-anti, the situation is very similar. Two minima can again be found, one corresponding to the more stable delocalized T1* state and the other to the less stable localized S0T1 state, in which the central angle α is again considerably twisted (to ∼25°). In 4-anti the energy difference is ∼2 kcal/mol when evaluated by either the DFT or HF method. Inclusion of a solvent in the calculation has only minor effects. In DMSO, the S1* and T1* geometries of 4-syn are nearly identical and almost perfectly planar, while in the gas phase T1* is calculated to be more planar than S1*. The same is true for 4-anti. Figure 2 shows a comparison of the geometries of the delocalized S1* and localized S0S1 states of 4-syn and of the geometries of the delocalized T1* and localized S0T1 states of 4-anti. Figure 3 presents the calculated spin densities of the T1* and S0T1 states of 4-anti, clearly showing the localization of spin density in S0T1 on one half of the dimer. Calculated Doubly Excited States: Quintet 5(TT) and Singlet 1(TT). The optimized geometries of the major conformers of 4 in these states were obtained in the gas phase, and the geometries of 4-syn were also obtained in DMSO solution. They show values of the dihedral angle α similar to those found in the optimized ground-state geometry (Table 1). In both the 5(TT) and 1(TT) states, the geometry of each half strongly resembles that of 1 in its T1 state, as expected for a double-triplet state. It is nearly perfectly planar, and the distortion from overall planarity is due to the central ∼30° twist at α. A calculation for 3(TT) would not be easy to perform with standard software and was not attempted, but the results can be expected to be similar. In the first approximation, the excitation energies of the three TT states would be expected to equal twice the excitation energy of the T1 state of 1, which was determined to be ∼33 kcal/mol by electron energy loss spectroscopy.14 This is nearly D
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produced by direct excitation is identical to that measured by anthracene sensitization of 4 (the red trace in Figure 4C). The transient absorption spectrum of the intermediate species T1 is red-shifted from that of 1, and by itself, spectral comparison does not easily confirm its identity. As is shown below, its long lifetime (∼50 μs) and the lack of spectral correlation with other reasonable alternatives (i.e., a charge-transfer species) allow us to assign the intermediate species as the localized S0T1 state and the long-lived species as the delocalized T1* triplet. These assignments are further supported by the good alignment of the observed absorption bands with the energies of the calculated transitions of the two species (Figure 4). The triplet formation onset for 4 in CH and Tol is blueshifted by more than 500 cm−1 relative to the absorption spectrum (Figure 5), indicating that additional energy beyond
Figure 4. (A) UV−vis absorption and (B) fluorescence spectra of 1 (blue) and 4 (purple) in CH (solid), Tol (dotted), and DMSO (dashed). The bars represent the vertical gas-phase B3LYP/SVP S0−Sn transition energies and oscillator strengths (n = 0−20) at the optimized ground-state geometries of the conformer 4-syn (xpolarized, green; y-polarized, red). See Figure 1A for axis definitions. (C) Triplet absorption spectra of 4 (purple) in Tol from direct excitation at 28 200 cm−1 and 1 (blue) in Tol from anthracene sensitization. For 4-syn the red and green bars are for T1*−Tn* transitions while the light-blue bars are for S0T1−S0Tn y-polarized transitions; the calculated results for the syn and anti conformers are nearly identical (see Table S4). The anthracene-sensitized T1−Tn spectrum is shown in red.
Figure 5. Triplet action spectra of 4 in CH, Tol, and DMSO. The plots compare the triplet absorption amplitude as a function of excitation energy (open circles with error bars) to the absorptance spectrum (absorptance = 1 − T; solid lines). The energy offset of the triplet action spectrum from the onset of absorptance (Δ in Table 3), when observed, is highlighted using dashed lines.
the absorption onset is required for the observation of triplet excited state absorption, consistent with the notion that the localized S0T1 state does not form from the lowest-energy excited singlet isomer, S1*. We postulate that instead the S0T1 state forms from the localized S0S1 state, which can only be accessed with additional excitation energy above the absorption onset; this hypothesis is further discussed below. In DMSO,
sensitization. At much later times, the shape of the T1−Tn spectrum for 4 changes significantly, and the peak shifts to about 15 000 cm−1. This slowly developing T1−Tn spectrum
Table 2. Measured Photophysical Parameters of 4 in Various Solvents solvent
ν̃abs/103 cm−1
CH
22.12
Tol
21.74
DMSO
21.37
ΦFa (exc/cm−1) 0.44 0.46 0.53 0.39 0.42 0.55 0.51 0.59 0.77
(25000) (20000) (25000) (19600) (25000)
kF/ns−1 (%)b
ΦTc
Δ/cm−1 d
1.22 (66) 0.27 (34)
0.01
550
0.98 (85) 0.45 (15)
0.03
890
0.52(91) 0.26 (9)
0.01
0
(20830)
With excitation at the peak of the lowest-energy absorption band, unless otherwise noted. bFrom time-resolved fluorescence measurements, with excitation near the peak of the lowest-energy absorption band. cFrom flash photolysis measurements comparing the triplet amplitude by direct excitation near the peak of the lowest-energy absorption band to that produced by sensitization using triplet anthracene. dObserved shift in the triplet action spectrum from the onset of absorption. a
E
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Figure 6. Ultrafast transient absorption spectra of 4 in Tol and DMSO after excitation at 25 600 cm−1 (top row) and 20 800 cm−1 (bottom row). The ground-state absorption spectra (black dashed lines) and steady-state fluorescence spectra (dashed pink lines) are included.
Figure 7. (A, B) Room-temperature ns−μs transient spectra of 4 in Tol after excitation at 28 170 cm−1 for (A) 12 μM and (B) 2 μM solutions. Red = 1 μs; blue = 100 μs. (C) Observed rates of S0T1 formation (black squares) as a function of concentration.
stimulated emission features to the red of 20 000 cm−1 match those observed in the steady-state fluorescence experiment (Figure 4) that are assigned to S1*. Stimulated emission features at positions higher than 20 000 cm−1, likely containing contributions from S0S1, overlap too strongly with the bleach for a definitive assignment. In Tol, the singlet excited state absorptions and stimulated emission decay after several nanoseconds with the concomitant rise of a broad absorption feature matching that observed at early delay times in the flash photolysis experiment and assigned to S0T1. For 25 600 cm−1 excitation, a change in the shape of stimulated emission can be observed (more strength at higher energies) at delay times longer than 2 ns, consistent with a high residual fraction of the longer-lived S0S1 states. For 20 800 cm−1 excitation, with which S0S1 is not produced, no such shift in the stimulated emission occurs with increasing time delay. The absorptive features to the red of 16 700 cm−1 are assigned as excited-state absorption within the singlet manifold, although a clear assignment to S0S1 and S1* is impossible because of overlap between broad bands. The same spectral features are observed at early delay times for excitation of nonpolar solutions of 4 at 20 800 cm−1; however, at later times triplets are only very weakly present. In a DMSO solution of 4 excited at 25 600 cm−1, the stimulated emission is red-shifted over the first 30 ps, leaving a broad and unstructured band at longer time delays. Excitation at 20 800 cm−1 yields similar bands at early times but with much less pronounced shifting compared with 25 600 cm−1 excitation. Because of the strength and breadth of stimulated emission, triplet absorption features, if present, are obscured to delays of 4.6 ns.
triplet formation from 4 tracks the absorption onset, although a secondary rise at about 2000 cm−1 above the onset is detected. Fluorescence Decay Kinetics. Time-resolved fluorescence measurements for 4 in Tol and CH reveal nearly monoexponential decays at excitation energies near the absorption onset with a rate constant (∼0.9 ns−1) roughly 4−5 times larger than for 1. At higher excitation energies the decays become clearly biexponential (Figure S4A), with an added slower decay component that is closer to that of 1 (∼0.4 ns−1). With the assumption of the presence of a slow and a fast fluorescence emission, a global fit to a series of time-resolved fluorescence spectra collected using different excitation energies results in decay-associated spectra (Figure S4B). The relative amplitude of the slow component, Aslow, is significantly reduced at 200 K compared with the value at room temperature (Figure S4C). This observation argues against the possibility that the dual emission behavior is due to an impurity not detected by NMR or optical characterization after repeated purification steps. We tentatively assign the slower emission to S0S1 and the fast emission to S1*. Justification for this assignment will be discussed below. In DMSO the fluorescence decays are monoexponential at low (19 200 cm−1) and high (25 000 cm−1) excitation energies and biexponential at intermediate energies. For low-energy excitation, the rate constant decreases with increasing solvent polarity (kF = 1.22 ns−1 in CH, 0.98 ns−1 in Tol, and 0.52 ns−1 in DMSO; see Table 2). Transient Absorption. The TA spectra of 4 are shown in Figure 6. The ground-state bleach, centered at 22 200 cm−1 in Tol and ∼21 300 cm−1 in DMSO, appears to overlap with stimulated emission in the 16600−20000 cm−1 range in Tol and extends much further into the red in DMSO. The F
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Figure 8. (A) Schematic diagram of singlet potential energy surfaces for 4. (B) Kinetic scheme for 4, with the identical scheme involving S1S0 omitted for clarity. Population of T1* from T1S0 and S0T1 occurs via energy transfer (dashed lines). The distinction between intersystem crossing and singlet fission for the process described by kT is discussed in the text.
emission modes arose from static disorder due to interactions with the medium9 or to the existence of two minima on the ground-state potential,10 in which case photoexcitation of the distinct species produces photophysics consistent with either localization or delocalization. The particular case of a dimer proposed here, involving a broad ground-state minimum associated with two distinct fluorescent isomers in the first excited singlet state, one with localized and the other with delocalized excitation, appears not to have been observed before. Considering the temperature-dependent fluorescence data (Figure S4), which provide an estimate of the fraction of S0S1 relative to the total singlet population, we arrive at an effective activation energy of 800 ± 150 cm−1 (2.5 kcal/mol), assuming an Arrhenius relationship. Because no minima of significantly different energy were found on the S0 surface in spite of a diligent computational search, we interpret this activation energy as being due to motion along a configurational coordinate away from the ground-state energy minimum (see the schematic in Figure 8A). The observation of a continuum of effective energy separations up to about 1500 cm−1 from the fluorescence excitation experiments (Figure S3B) suggests that the vertical transition necessary to populate S0S1 can occur in any number of geometries along this coordinate. The reduction in the blue-shifted and slower-decaying component of fluorescence at low temperatures (Figures S3C and S4C) provides evidence of restricted motion and a limited range of accessible dimer geometries, reducing the probability of vertical transitions to the portion of the first excited singlet surface from which molecules relax to the S0S1 state. We assume that the peak of the fluorescence from S0S1 corresponds to a vertical transition to the ground state, and we add the 800 cm−1 energy separating the two geometries on the ground-state surface, defined as ΔEg, to the ∼1500 cm−1 difference in the onsets of fluorescence excitation spectra, defined as ΔEe, to arrive at the energy difference between S0S1 and S1*. This experimental estimate (∼2300 cm−1) is similar to the calculated difference of ∼3000 cm−1. Some population transfer from S0S1 to S1* might be expected and apparently does occur on a subnanosecond time scale (Figure S7), analogous to dynamic planarization or other forms of excited-state isomerization in oligomer and dimer systems.5 The intramolecular motions along the reaction coordinate that permit the electronic excitation to populate the localized S0S1 state are likely to involve a combination of twisting at the central twist angle and skeletal vibrations that convert the symmetrical ground-state geometry with largely localized double bonds into an unsymmetrical geometry in which one
In all of the solvents, the formation of S0T1 occurs between 1 and 10 ns upon excitation at 25 000 cm−1 (Figure S5). In contrast, long-lived features observed upon 20 800 cm−1 excitation are extremely weak. In dilute solutions S0T1 decays primarily to the ground state with a lifetime of ca. 100 μs, while in more concentrated solutions it decays with near unit efficiency to the delocalized T1* species identified through triplet sensitization experiments (cf. Figure 7A; the isosbestic point is near 16 000 cm−1). Through fitting of individual traces at specific wavelengths associated with the two triplet species, the S0T1 decay and T1* rise were found to occur at equal rates within the experimental uncertainty, which was roughly 10%. For a global fit at all wavelengths, a common rate constant, kobs, was assumed for the processes and was found to be ∼1 × 104 s−1 below an S0S0 concentration of ∼5 μM. A strong dependence of kobs on concentration is evident from 5 to 22 μM (Figure 7C). The overall triplet yield determined from excitation near the peak of the ground-state absorption band varies from roughly 0.03 (Tol) to 0.01 (DMSO). The yields of both S0T1 and T1* are reduced with decreasing temperature (Figure S6). The trends are identical above 275 K, consistent with near-unity population transfer, but below 275 K, where diffusion becomes inhibited as the solution is cooled (Figure S6), the trends diverge such that S0T1 has a higher yield than T1*.
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DISCUSSION Singlet Excited State Properties. The steady-state and time-resolved fluorescence (Figures S3 and S4 and Table 2) show that the excited singlet state is depopulated by fluorescence when excitation occurs near the absorption onset, whereas a second depopulation channel opens at higher excitation energies. The fluorescence excitation spectra of emission collected at 19 600 and 22 200 cm−1 are shifted by more than 1000 cm−1 from each other, which aligns with the shift of the two bands from each other by 1000−1500 cm−1 from a global fit to the fluorescence decay. One band has better-defined vibronic features, most likely as a result of coupling of the ground to excited state transition with ring skeletal modes, and is consistent with a higher-energy localized excited state (S0S1). The other is red-shifted and broadened with apparently weaker vibronic coupling (S1*). The two bands combine to give the observed steady-state spectrum with excitation that populates both species, except very near the absorption onset. The assignment of the species responsible for the dual fluorescence behavior is supported by the results of our calculations and has precedent in prior dimer studies that reported two disparate modes of emission similar to those observed here.9,10 In those other cases, the observation of two G
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The Journal of Physical Chemistry A
considered the origin of dual fluorescence, this is considered unlikely since a computational search for it revealed none. Figure 8A depicts this situation, which accounts for the excitation-energy-dependent observations in the steady-state fluorescence (Figure S3), time-resolved fluorescence (Figure S4), and ultrafast transient absorption data (Figure 6) as well as the computational result that there are separate minima on the potential surface, one at a delocalized geometry (S1*) and the other at a localized geometry (S0S1), that are separated by ∼9 kcal/mol (3000 cm−1) in energy. The energy ordering of S0T1 and T1* and the triplet energy transfer inferred from experiments matches that of similarly planar or twisted oligomers.37 Using the kinetic scheme in Figure 8B, we carried out global fitting of the ultrafast transient absorption data (eqs S2 and Table 3). Under the assumption of a negligible rate of internal
of the isobenzofuran moieties has “aromatic” character with nearly equal C−C bond lengths (Figure 2). Triplet Formation Mechanism in Nonpolar Solvents. From the observed transient spectra, it is clear that the triplet formation mechanism in 4 is distinct from that in the more weakly coupled dimers of 1. First, the long-lived photoinduced absorption spectrum, also observed upon triplet sensitization, is unique (Figure 4). Comparison with calculated spectra supports an assignment to a triplet state, T1*, in which excitation is delocalized over both halves of the dimer. The delocalized triplet is formed through an intermediate that decays into T1* in about 50 μs, much more slowly than the intermediate radical cation−anion pair (C+A−) does in 2 and 3. In nonpolar solutions, 4 produces triplets only upon excitation at energies more than 500 cm−1 above the absorption onset (Figure 5). It is very unlikely that this intermediate is an excited singlet state, since its conversion to the ground state should then take no more than 100 ns, and it is equally unlikely that it is a short-lived ground-state singlet isomer of 4, since its encounter with a molecule of 4 in its ground state sensitizes the latter to the T1* state. This process slows down as the concentration and temperature decrease, in line with its bimolecular and diffusion-controlled nature. At low concentrations, the decay of the intermediate is no longer coupled to the production of T1*, since intersystem crossing (ISC) to the ground state outcompetes the sensitization process. Taken along with the lack of spectral correlation, these observations rule out the possibility that the intermediate is simply C+A−, and instead, it is likely S0T1. Calculations indeed predict that in addition to the nearly planar fully delocalized triplet T1*, the lowest triplet potential energy surface contains two symmetryrelated additional minima, S0T1 and T1S0, with a twisted molecular structure and triplet excitation localized on one or the other half of the dimer. The absorption spectrum of the intermediate observed for 4 is shifted from T1−Tn for 1 in a manner that agrees with the calculated shift between the strongest transitions, S0T1−S0Tn for 4 and T1−Tn in 1 (20 300 cm−1 in Table S4 vs 22 800 cm−1 as in ref 14). The behavior is also consistent with the biexciton T1T1, which would possess a time-dependent mixture of singlet, triplet, and quintet character. At higher concentrations where sensitization prevails, the kinetics of the disappearance of the intermediate and the rise of triplet absorption cannot be used to support the presence or absence of T1T1. However, the peak extinction coefficient of the intermediate is similar to that of the nearly identical T1 absorption peak of the monomer 1 (∼3 × 104 M−1 cm−1 in both cases), making it unlikely that the intermediate is the biexciton T1T1, which should exhibit approximately twice the absorption intensity. It is possible that T1T1 forms initially but converts to S0T1 within the first ∼25 ns, where strong excited singlet absorption is also present in the spectrum, masking any spectral evolution involving triplets. A kinetic scheme that describes the experimental observations is shown in Figure 8B. The excitation energy that needs to be absorbed by ground-state 4 in order to populate S0T1 is higher by at least 500 cm−1 than the onset of absorption. We believe that the need for extra energy could arise from a process in which S0T1 is formed only after proceeding through a singlet state that is higher in energy than the relaxed singlet S1*, such as S0S1, and that this higher-energy singlet then preferentially populates S0T1 over T1*. The possible existence of a stable twisted conformer in the ground state could also explain the behavior, but as was already discussed above when we
Table 3. Values from Global Fitting of Ultrafast Transient Absorption of 4 (Rate Constants Have Units of ns−1 Unless Otherwise Noted) solvent
kF (480 nm)
kT (480 nm)a
kF (390 nm)
kT (390 nm)a
kETb
CH Tol DMSO
1.22 0.98 0.52
∼0 ∼0 0.26
0.33 0.45 0.46
0.10 0.27 0.17
4.1
a
Estimated rate constant for S0T1 formation. bEstimated rate constant for T1* formation (in M−1 ns−1) through bimolecular triplet energy transfer from S0T1 to the ground state to form T1*, as measured by flash photolysis.
conversion to the ground state, S0S1 undergoes fluorescence with kF = 0.45 ns−1 and S1* fluoresces with kF = 0.98 ns−1 (from the minority and majority components in time-resolved fluorescence at 25 000 cm−1; Table 2). The scheme contains a parameter describing the [S0S1]/([S1*] + [S0S1]) fraction, and for 25 600 cm−1 excitation of 4 in Tol its value is about 0.2, consistent with the fractions derived from the amplitudes of the fast and slow fluorescence decay components. The fit also indicates that S0T1 is formed with kT = 0.26 ns−1. If S0S1 could be exclusively excited, the kinetic scheme and the rate constants predict a yield of S0T1 of 30%, which is much higher than the overall ΦT of 3−5% determined from flash photolysis. The fast S0T1 formation and its relatively high yield may be indicative of SF, despite the absence of strong evidence for the formation of a T1T1 species. Direct formation of a triplet state through ISC is much less than 1% efficient and is 2 orders of magnitude slower in isolated 1.14 Excitation at 20 800 cm−1 in time-resolved fluorescence and absorption measurements reveals a monoexponential decay of S1* with a rate constant of 1 ns−1 (cf. ∼0.22 ns−1 in 1), indicating an increase in oscillator strength engendered by delocalization. Indeed, the calculated oscillator strength of the lowest significant singlet transition of 4 (1.389; Table S3) is nearly double that of 2 (∼0.8).19 Some internal conversion (kIC) evidently also occurs to explain the less than unity ΦF value. The near planarity of S1* may enhance the coupling necessary for SF, but the reduced energy of this delocalized species is unfavorable for the process energetically. In addition to the energy barrier presented by the stabilization of S1* with respect to 2 × T1*, the faster radiative decay from S1* and competitive nonradiative decay may further reduce the likelihood of forming triplets from S1* via SF. H
DOI: 10.1021/acs.jpca.6b00826 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A Triplet Formation in DMSO. The bathochromic shift in the absorption and fluorescence spectra in DMSO may be due to partial planarization of the S0 geometry in the polar solvent compared with the gas phase (Table 1), resulting in a reduced singlet−singlet transition energy. According to Table 1, the transition energy to the S0S1 state is reduced by 1.7 kcal/mol upon going from the gas phase to DMSO, whereas the transition energy to the more polarizable S1* state is reduced by 6.1 kcal/mol. Solutions of 4 in DMSO show a shift in the stimulated emission intensity toward lower energies over the first ∼20 ps, followed by a decay of these features on a nanosecond time scale (Figure 6). We ascribe this rapid initial kinetics as conversion of S0S1 into S1*. Since the latter is stabilized significantly upon solvation in DMSO, the barrier that hinders the escape of population from S0S1 to S1* in nonpolar solvents may be reduced or removed altogether. Some indication of heterogeneous behavior, as in the appearance of biexponential decays at some wavelengths in Figure S4, may be attributed to other ground-state geometries due to conformations or aggregation that exists in DMSO but is less likely in other solvents. Global fitting of the transient absorption data was performed for 4 in DMSO using the same model as for 4 in nonpolar solvents but ignoring the first 30 ps after photoexcitation in which spectral shifting occurs. The rate constant for triplet formation from S1* is independent of the excitation energy, in agreement with the triplet action spectrum, and radiative decay from S1* occurs with kF = 0.52 ns−1. For analysis of 4 in DMSO at early time delays, we used singular value decomposition (SVD)38 to extract the kinetics of spectral shifting. Two SVD components describe the spectrotemporal data with only minor residuals and are correlated with a fast and a slow time constant. The slow component is essentially monoexponential with a decay time of 0.8 ns (Tol) or 1.5 ns (DMSO), and the spectra in these two solvents are similar (Figure S7), being dominated by stimulated emission. The fast-component spectra possess both a positive feature (15000−18000 cm−1), representing stimulated emission gain, and a negative feature (18000−21000 cm−1), representing stimulated emission loss. The fast component has a stronger spectral weight in DMSO (0.16) compared with Tol (0.03). In Tol, the short time scale associated with stimulated emission shifting is 0.5 ps, while in DMSO the process is biexponential with time constants of 2.1 and 17 ps. The loss of the more structured and blue-shifted stimulated emission and concomitant rise in the red-shifted featureless stimulated emission is consistent with S0S1 to S1* population transfer. The increased prominence of the transfer in DMSO supports the hypothesis of a reduced barrier between S0S1 and S1* in highly polar solvents, resulting in the observed steady-state dominance of the solvent-stabilized S1* state (e.g, Figure 4). In Tol the shifting is minor and occurs only at very early time delays, probably in competition with vibrational cooling. Excitation Localization/Delocalization Isomerism. Dimers like 4 with a single C−C bond bridge would be expected to have a high degree of conformational flexibility, with a torsional potential landscape containing many energetically equivalent shallow minima, and calculations indeed show little variation in the conformer ground-state energies and other properties as the torsional angles are adjusted by 180° turns. However, the two kinds of concurrent photophysical behavior observed for 4 cannot be attributed to this conformational richness in the ground state since all of the ground-state
conformers are calculated to have essentially identical properties. Instead, this behavior is due to the existence of isomerism on the singlet and triplet excited state surfaces. On the lowest excited singlet hypersurface, this isomerism is attributed to species at two distinct types of local minima: (i) the unsymmetrical structures S0S1 and S1S0, in which electronic excitation is localized in one or the other half of the dimer by strong bond length distortions and the interaction between the two halves is reduced by increased twisting of the central bond, with the bond length distortions converting the alternating bond lengths of the ground-state isobenzofuran moiety to the nearly even bond lengths characteristic of its excited state; and (ii) the symmetrical structure S1*, in which electronic excitation is delocalized over both dimer halves while their coupling is enhanced by the near planarity of the central bond. The observation of both the localized and delocalized excitation in the same dimer molecule is highly unusual but not unprecedented.9,10 Access to ground-state geometries appropriate for photoexcitation to S0S1, as opposed to the more generally accessible S1*, can be stimulated or suppressed thermally. Perhaps even more striking is the persistence of the two long-lived isomeric excited species in the triplet manifold. Localized triplets in supramolecular systems are common,37 while delocalized triplets are much rarer39 and in some cases observed only in higher-lying triplet states (Tn, n > 1).40 In 4, the less stable localized triplet species S0T1 and the more stable delocalized triplet species T1* do not directly interconvert at an observable rate in spite of their long lifetimes, and they must be separated by a significant energy barrier. They behave independently, and each decays to the ground state by ISC. Their formal interconversion is accomplished only by energy transfer from the higher-energy S0T1 state to a ground-state molecule, which is thereby promoted to the lower-energy T1* state. The molecular features that control the competition between localization and delocalization in an excited state of a dimer are clearly the same as those that control the formation of a largeradius exciton versus a small-radius (self-trapped) exciton in an infinite chain through the exciton−phonon coupling constant g = S/B. A large site-distortion energy S favors localization, and a large exciton band half-width B (strong intersite conjugation) favors delocalization.41−44 When the two tendencies are nearly balanced, an opportunity arises for a small potential energy barrier to endow the two forms with independent existence. This situation appears to be rare, but it is encountered in 4, whereas in 2 and 3 the conjugation is too weak and only the species with localized excitation, S0S1 and S0T1, correspond to mimima on the lowest excited singlet and triplet potential energy surfaces.
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CONCLUSIONS We have measured photophysical processes in a directly coupled dimer of 1, finding data indicative of a delocalized excitation and a unique mechanism of triplet formation that is enhanced in one excited-state geometry that maintains an S1 energy similar to that of 1. The flow of population between the localized and delocalized excited singlets is modulated by solvent polarity. The localized singlet produces a localized triplet with relatively high efficiency (up to 30%) but is always present in a smaller concentration than the delocalized singlet, which yields triplets less efficiently (
