Solvent Effect on the Photoinduced Intramolecular Charge-Transfer and Redox Potentials of Three Difuranones

We report solvent-dependent excited state properties of three difuranone derivatives with a quinoidal backbone by steady-state and lifetime fluorescence measurements and theoretical calculations. Remarkable bathochromic shifts in fluorescence with diminished intensity indicate the occurrence of strong intramolecular charge-transfer transitions in high polar solvents. Cyclic voltammetric redox potentials reveal an interesting variation of biradical characters of the compounds with increasing solvent polarity. Solvent polarity also significantly modulates the energy levels of the charge-transfer (CT) states, as observed from the combined analyses of redox potentials and photophysical data via the Rehm-Weller equation. When high polar solvents favor forward CT by a more exoergic driving force and stabilize the charge-separated states, the reverse CT process diminishes. Estimated free energies of activation for CT suggest that high polar solvents lessen the activation barrier. Calculated excited state energies of the compounds at the CAM-B3LYP/6-31+G* level fulfill the primary conditions required for singlet fission, a process that can substantially increase the efficiency of solar cells, and the crystal packing for compound 1 also reveals a favorable geometry for singlet fission.