GROUND-STATE AND EXCITED-STATE TAUTOMERISM IN 2-(3'-HYDROXY-2'-PYRIDYL)BENZIMIDAZOLE

The tautomerism of 2-(3'-hydroxyl-2'-pyridyl)benzimidazole (HPyBI) in the ground and first excited singlet states was investigated by UV-vis absorption spectroscopy, by steady-state and time-resolved fluorescence spectroscopy, and by semiempirical (AM1) calculations (including exploration of the potential energy surface for the interconversion process and the effect of solvation). We conclude that ground-state HPyBI is enolic in nonaqueous solvents, whereas in water the keto and enol forms have almost the same energy and are present in equilibrium. In all the solvents considered, practically the only excited tautomer observed is the keto form, which is created from the enol form by ultrafast intramolecular proton transfer.