Toward Designed Singlet Fission: Electronic States and Photophysics of 1,3-Diphenylisobenzofuran

Abstract

Single crystal molecular structure and solution photophysical properties are reported for 1,3-diphenylisobenzofuran (1), of interest as a model compound in studies of singlet fission. For the ground state of 1 and of its radical cation (1(+*)) and anion (1(-*)), we report the UV-visible absorption spectra, and for neutral 1, also the magnetic circular dichroism (MCD) and the decomposition of the absorption spectrum into purely polarized components, deduced from fluorescence polarization. These results were used to identify a series of singlet excited states. For the first excited singlet and triplet states of 1, the transient visible absorption spectra, S(1) --> S(x) and sensitized T(1) --> T(x), and single exponential lifetimes, tau(F) = approximately 5.3 ns and tau(T) = approximately 200 micros, are reported. The spectra and lifetimes of S(1) --> S(0) fluorescence and sensitized T(1) --> T(x) absorption of 1 were obtained in a series of solvents, as was the fluorescence quantum yield, Phi(F) = 0.95-0.99. No phosphorescence has been detected. The first triplet excitation energy of solid 1 (11,400 cm(-1)) was obtained by electron energy loss spectroscopy, in agreement with previously reported solution values. The fluorescence excitation spectrum suggests an onset of a nonradiative channel at approximately 37,000 cm(-1). Excitation energies and relative transition intensities are in agreement with those of ab initio (CC2) calculations after an empirical 3000 cm(-1) adjustment of the initial state energy to correct differentially for a better quality description of the initial relative to the terminal state of an absorption transition. The interpretation of the MCD spectrum used the semiempirical PPP method, whose results for the S(0) --> S(x) spectrum require no empirical adjustment and are otherwise nearly identical with the CC2 results in all respects including the detailed nature of the electronic excitation. The ground state geometry of 1 was also calculated by the MP2, B3LYP, and CAS methods. The calculations provided a prediction of changes of molecular geometry upon excitation or ionization and permitted an interpretation of the spectra in terms of molecular orbitals involved. Computations suggest that 1 can exist as two nearly isoenergetic conformers of C(2) or C(s) symmetry. Linear dichroism measurements in stretched polyethylene provide evidence for their existence and show that they orient to different degrees, permitting a separation of their spectra in the region of the purely polarized first absorption band. Their excitation energies are nearly identical, but the Franck-Condon envelopes of their first transition differ to a surprising degree.

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

“…[8][9][10][11][12][13], and recent results suggest SF yields can approach 200% in carotenoid aggregates and isobenzofuran thin films. [14][15][16] SF has also been observed in covalent dimer molecules, albeit with much lower yields. [17][18][19] In order to optimize the SF process, an improved physical understanding of the process and how it depends on molecular structure is required.…”

Ultrafast internal conversion in a low band gap polymer for photovoltaics: experimental and theoretical study

“…1 To avoid or somehow limit this loss, several approaches have been proposed, including tandem or multi layer solar cell architectures [2][3][4][5][6][7][8][9] or exploiting the singlet fission process. [10][11][12][13][14][15][16] A complete understanding of the IC processes could suggest better ways to optimize the energy conversion efficiency in OPV materials, for instance exploiting hot charge dissociation mechanisms at organic donor/acceptor interfaces. [17][18][19][20][21][22] Traditionally fast IC processes have been described in terms of intramolecular vibrational energy redistribution, where the energy initially stored into few Franck-Condon (FC) modes is redistributed into many vibrational modes by anharmonic couplings.…”

Covalent Dimers of 1,3-Diphenylisobenzofuran for Singlet Fission: Synthesis and Electrochemistry

“…This has been established in the case of 1, where the only observed differentiating feature between the conrotatory and disrotatory conformation is a small difference in the shape of the vibrational envelope of the first electronic absorption band. 11 We have verified this expectation further by locating all 32 conformational minima in the ground states of each of the dimers 4 -6 and finding that their calculated highest occupied molecular orbital (HOMO from DFT) and lowest unoccupied molecular orbital (LUMO from DFT) energies are identical within 0.01 eV. 23 In this work, we considered 10 conformers for each compound, spanning the conformational space corresponding to rotation about single bonds linking the two monomers.…”

“…6,7,8 Irradiation produced up to two triplet excitations per absorbed photon, implying that both the chromophore itself and the inter-2 chromophore coupling in one of the two known modifications of the solid are favorable. 9,10 Although the high transparency of 1 in most of the visible region of the spectrum 11 and its sensitivity to the combination of air and light preclude any practical use in a solar cell, it is of considerable interest as a model compound in fundamental studies of the still only poorly understood nature of the singlet fission process. One of the issues that are currently most intriguing is the design of efficient inter-chromophore coupling.…”