Anomalous dual fluorescence of benzanilide

The absorption and fluorescence spectra of benzanilide and N-methylbenzanilide have been investigated in solution and low-temperature glasses and assigned with the aid of ZINDO calculations. The anomalous dual fluorescence observed by previous workers has been assigned to the two lowest singlet states of benzanilide. The lower energy n,π^* state is populated by excitation in the long-wavelength tail of the absorption band. Its fluorescence is readily detected in low-temperature glasses and at room temperature in aromatic solvents. Large NMR solvent-induced shifts provide evidence for ground-state complex formation of benzanilide with aromatic solvents. The higher energy π,π^* state is populated by excitation of an allowed transition. It undergoes twisting about the amide C-N bond to form a fluorescent twisted intramolecular charge-transfer state with a maximum of 520 nm in benzene solution. In rigid glasses twisting to form the twisted charge-transfer state cannot occur, and the π,π^* state undergoes internal conversion to the lower energy n,π^* state.