Ratiometric fluorescence sensing ability of 2-(2′-hydroxyphenyl)benzimidazole and its nitrogen substituted analogues towards metal ions

Excited state intramolecular proton transfer (ESIPT) based ratiometric fluorescence sensing of 2-(2′-hydroxyphenyl)benzimidazole (HPBI) and its nitrogen substituted analogues viz. 2-(2′-hydroxyphenyl)-3H-imidazo[4,5-b]pyridine (HPIP-b) and 2-(2′-hydroxyphenyl)-1H-imidazo[4,5-c]pyridine (HPIP-c) towards different metal ions was studied. The normal to tautomer fluorescence intensity ratio of HPBI increases as high as 143 and 92 folds in presence of Cu2+ and Fe3+, respectively. The ratio of HPIP-b reaches maximum up to 4.7 fold in presence of Co2+ and that of HPIP-c up to 11.6 fold in presence of Pd2+. The increase in intensity ratio of the normal emission in presence of metal ions is attributed to inhibition of ESIPT process. The crystal structure of 1:2 Ni2+ complex of HPIP-b indicates that the metal ion coordinates through imidazole nitrogen and phenolic oxygen of the fluorophores.