Substituent effect on the solvatochromic behaviour of benzoimidazole based donor–acceptor type fluorescent molecules

In this study, a series of donor–acceptor type fluorescent molecules were synthesised and their photophysical properties were investigated spectroscopically. The molecules were 4-(1H-benzo[d]imidazol-2-yl)-N,N-diphenylaniline [BIDP], 4-(benzo[d]oxazol-2-yl)-N,N-diphenylaniline [BODP] and 4-(benzo[d]thiazol-2-yl)-N,N-diphenylaniline [BTDP] where the diphenyl amine fragment acts as the donor and a class of heterocyclic groups such as benzoimidazole, benzoxazole, and benzothiazole act as the acceptor. It is found that the photophysical properties of the benzoimidazole based molecule BIDP are different, where increasing polarity of the medium results in less Stokes shift of the emission band as compared to the other two. This effect is reflected on the CIE 1931 diagram of these molecules, where BIDP shows a reduced solvatochromic effect compared to the other two. This anomalous behaviour is justified by the lowest change in the dipole moment of BIDP on moving from the ground to the excited state, which results in the reduction of the solvatochromic effect. Structurally back donation of charge from the benzoimidazole fragment to the benzene ring may prevent facile charge transfer from the donor diphenylamine part. Judiciously designed molecules with substitution of a 1H-imidazole atom in BIDP with a weakly electron-donating methyl group and a strongly electron-withdrawing tert-butyloxycarbonyl group showed a significant change in the photophysical properties. These new molecules upon substitution will give new insights about the charge transfer reaction of heterocycle-based donor–acceptor type molecules in the excited state.