Theoretical study on counter anion- and solvent-dependent fluorescence quenching mechanism of 2-phenylbenzo[b]phospholium salts

•Environment-dependent fluorescence quenching mechanism of 2-phenylbenzo[b]phospholium salts was theoretically analyzed.•Several stable structures of ion pairs are obtained in the solution phase.•Absorption and fluorescence processes are characterized by ππ* local excitation.•Fluorescence quenching can be explained by the relaxation to the charge transfer state.•In the electronic excited states, the drastic change in solvation structure is observed by the 3D-RISM-SCF method. Recently reported highly fluorescent 2-phenylbenzo[b]phospholium salt shows an interesting photochemical property. While the counter anion and solvent dependency indicate that the salt is mainly responsible for the absorption and fluorescence spectra, the fluorescence decay clearly depends on the counter anion. In the present study, the detailed mechanism is elucidated using quantum chemical computation and its hybrid with statistical mechanics called 3D-RISM-SCF. Based on the careful exploration of plausible structures of the ion pairs in the methanol and dichloromethane solution, we identify the structure of the charge transfer state, the key on the photoprocess.