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ESIPT-inspired benzimidazolyl substituted fluorescein dyes were synthesized. PH-sensitivity was determined by the photophysical property measured at a physiological possible pH range. Fluorescence quantum efficiency values were calculated independently at two different emissions. A rational relationship is defined between fluorescence quantum efficiency and calculated HOMO energy.

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Novel 2-(1H-benzimidazol-2-yl)-5-(N,N-diethylamino) phenol, 2-(1,3-benzoxazol-2-yl)-5-(N,N-diethylamino) phenol, 2-(1,3-benzothiazol-2-yl)-5-(N,N-diethylamino) phenol and their derivatives have been synthesized from p-N,N-diethyl amino salicylaldehyde with different substituted o-phenylenediamine or o-aminophenol or o-aminothiophenol and their photo-physical properties were studied. Effects of solvent polarity in the absorption-emission properties of synthesized compounds were investigated. All these compounds shows excited state intra-molecular proton transfer pathway having single absorption and dual emission characteristics. The fluorescent compounds were characterised by FT-IR, (1)HNMR, (13)C NMR and Mass spectral analysis. TGA analysis showed these compounds are thermally stable up to 200 °C.

An efficient and mild protocol has been developed for the preparation of benzimidazoles, benzoxazoles, and benzothiazoles from reactions of aldehydes with o-substituted aminoaromatics in the presence of catalytic amount of Indion 190 resin. Short reaction time, ambient conditions, simple work-up procedure, high yield, easy availability, reusability, and use of an eco-friendly catalyst are some of the striking features of the present protocol.