Novel quadruple fluorescence of donor–acceptor benzoylthiourea derivatives

Abstract

Three donor–acceptor benzoylthiourea derivatives were characterized structurally by FTIR and NMR methods. Electronic spectra were investigated by UV absorption and steady fluorescence methods. Quadruple fluorescence bands on the fluorescence spectra were observed and re-assigned. Differently from the previous assignment for benzanilide (BA), the fluorescence bands with emission maxima of 480–524 nm were attributed to ESIPT (excited state intermolecular proton transfer). The fluorescence bands in the region 420–460 nm were explained by the TICT (twisted intramolecular charge transfer) model. Two short-wavelength fluorescence emissions at about 300 and 360 nm were assigned to the S2 state of two rotating isomers characteristic of the nature of the local excited (LE) state. The proposed mechanism for the emission was quantitatively supported by MP2 and CASSCF calculations.