Substituent Effect on the Excited-State Dynamics of Benzo[b]phospholium Salts Investigated Using Picosecond Time-Resolved Spectroscopy

Abstract

Abstract Photophysical and photochemical properties of 1-methyl-1,2-diphenylbenzo[b]phospholium iodide (diPh-I) and 1-methyl-1,2,3-triphenylbenzo[b]phospholium iodide (triPh-I) were investigated by obtaining time-resolved fluorescence and the transient absorption spectra with sub-picosecond time-resolution. Unusually, fluorescence lifetimes of the triphenyl-substituted compound in various solvents were approximately 100 ps, which is several times shorter than the lifetime of diPh-I. Quantum chemical calculations suggest that the excited-state planarization of the dihedral angle between the benzo[b]phospholium backbone and the phenyl group at Cα possibly contributes to the non-radiative relaxation of triPh-I. In the case of diPh-I, the fluorescence excitation spectrum obtained in dichloromethane depended on the concentration, suggesting that diPh-I forms molecular aggregates in the ground state. The transient absorption spectra of diPh-I showed that, at various concentrations of dichloromethane, the dissociation dynamics of aggregation to produce the monomeric form in the excited state occurred over several picoseconds.