In this study a series of donor-acceptor type fluorescent molecules were synthesised and there photophysical properties were investigated spectroscopically. The molecules are 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 diphenyl amine fragment acts as the donor and a class of heterocyclic groups such as benzoimidazole, benzoxazole, benzothiazole act as the acceptor. It is found that the photophysical properties of benzoimidazole based molecule BIDP behaves differently, 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 reduced solvatochromic effect compared to the other two. This anomalous behaviour is justified by the lowest change in dipole moment of BIDP on going from ground to the excited state, which results in reducing 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 1H-imidazole atom in BIDP with weakly electron donating -Methyl group and strongly electron withdrawing tert-Butyloxycarbonyl group showed significant change in the photophysical property. 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.

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