EXCITED-STATE INTRAMOLECULAR PROTON-TRANSFER IN 2-(2'-HYDROXYPHENYL)BENZIMIDAZOLE AND 2-(2'-HYDROXYPHENYL)-BENZOXAZOLE - EFFECT OF ROTAMERISM AND HYDROGEN-BONDING

An excited-state intramolecular proton transfer (ESIPT) process in 2-(2'-hydroxyphenyl)benzimidazole and -benzoxazole (HPBI and HBO, respectively) has been studied using steady-state and time-resolved emission spectroscopy at various temperatures and by semiempirical quantum chemical methods. For both of them two distinct ground-state rotamers I and II respectively responsible for the ''normal'' and the ''tautomer'' emission have been detected. In hydrocarbon solvents at room temperature and at 77 K the tautomer emission predominates over the normal emission for both HPBI and HBO. This indicates that rotamer II, responsible for the tautomer emission, is intrinsically stabler than rotamer I, which causes the normal emission. In alcoholic glass at 77 K for HPBI a dramatic enhancement of the normal emission is observed. It is suggested that due to the increased solvation, the more polar rotamer I becomes stabler than II for HPBI in alcohol and the substantial temperature variation is due to the change in the population of the two rotamers with temperature. From the detailed temperature variation in alcoholic medium the ground-state energy difference between rotamers I and II is determined. In dioxane-water mixtures it is observed that with the addition of water the quantum yield of the normal emission increases, which is ascribed to the inhibition of the ESIPT process due to the formation of an intermolecular hydrogen bond involving water. CNDO/S-CI calculations were performed optimizing the ground: state geometry by the AM1 method. Details of the energy, dipole moment, and charge distribution of the rotamers in the ground state (S-0) and the first excited singlet state (S-1) and the barrier for the interconversion of I and II in S-0, S-1, and first excited triplet state are discussed. The calculation indicates that the barrier for the interconversion of the two rotamers is too high in the excited state (S-1 and T-1) for free interconversion.