Experimental and Theoretical Exploration of ESIPT in a Systematically Constructed Series of Benzimidazole Based Schiff Base Probes: Application as Chemosensors

Abstract

Herein, we have utilized 2-(2-hydroxyphenyl)benzimidazole (HBI) to synthesize 3-(1H-benzoimidazol-2-yl)-2-hydroxy-5-methyl-benzaldehyde (HBIA) followed by three Schiff bases by using -ortho (H₂ BIo), -meta (H₃ BIdm) and -para (H₂ BIp) substituted amino benzoic acids and studied their photophysical properties. We have successfully derived molecular structures of HBI, HBIA and H₃ BIdm which reveals that in HBI and HBIA, the phenolic -OH is intramolecularly hydrogen bonded with sp² N of benzimidazole group whereas in H₃ BIdm, it is hydrogen bonded with imine C=N of Schiff base moiety, which is responsible for different solid state emission properties of the reported compounds. Extensive experimental and theoretical studies show that for all three Schiff bases, in solution due to activation of C=N isomerization, ESIPT operates through benzimidazole site and displays different emission from the solid state. Furthermore, H₂ BIo, H₃ BIdm and H₂ BIp selectively sense Cu²⁺ in semi aqueous medium with nano-molar detection limit and in HuH-7 cells through the inhibition of ESIPT of process.