Photochemistry of N-benzylideneanilinium cations in concentrated sulphuric acid solutions

The photoreversible E reversible arrow Z isomerization of a series of protonated N-benzylideneanilines (X-C6H5CH=NC6H5-Y; X=H, N(CH3)(2); Y=H, NO2) was investigated in concentrated sulphuric acid solution. The UV absorption spectra of the E isomers are practically independent of substituents on benzene rings and suggest a planar conformation for protonated N-benzylideneanilines. The UV spectra of Z isomers were obtained from photostationary state conditions at two wavelengths. Also the quantum yields of E --> Z and Z --> E photoisomerization are not markedly affected by the ring substituents and are compatible with a photoreversible process occurring through a common excited state.