Ground and excited-state's intramolecular proton transfer in 2-(2'-hydroxyphenyl)iminomethylbenzimidazole and 2-(2'-hydroxy-4-bromophenyl)iminomethylbenzimidazole

2-(2'-hydroxyphenyl)iminomethylbenzimidazole(1) and its derivative 2-(2'-hydroxy-4-bromo-phenyl)iminomethylbenzimidazole (2), have been sythesised and their ground and excited-state properties were studied. These compounds show ground-state keto-enol tautomerism. The bromo-substituent in compound 2 influence the ratio of the two conformers favoring the enol-form, due to the reduction of imine-nitrogen's basicity. Absorption spectra are solvent dependent, and exhibit a blue shift from non-polar to polar solvents. In protic solvents the low energy absorption maxima, correlate with the acidity of the solvent. In the excited-state compounds 1 and 2, exhibit dual emissions in the near UV and visible regions. The origin of these emissions have been assigned to arise from the enol-form and the excited state intramolecular proton transfer (ESIPT) (keto-form), respectively. ESIPT emissions maxima are sensitive to the solvent, showing a considrable shift to higher energy as the polarity is increased.