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, respect ively) has been studied using steady-state and time-resolved emission spectroscopy at various temperatures and by semiempirical quantum chem ical methods. For both of them two distinct ground-state rotamers I an d II respectively responsible for the ''normal'' and the ''tautomer'' emission have been detected. In hydrocarbon solvents at room temperatu re and at 77 K the tautomer emission predominates over the normal emis sion for both HPBI and HBO. This indicates that rotamer II, responsibl e 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 sugge sted that due to the increased solvation, the more polar rotamer I bec omes stabler than II for HPBI in alcohol and the substantial temperatu re variation is due to the change in the population of the two rotamer s with temperature. From the detailed temperature variation in alcohol ic 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 f ormation 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 distr ibution of the rotamers in the ground state (S-0) and the first excite d 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 ca lculation indicates that the barrier for the interconversion of the tw o rotamers is too high in the excited state (S-1 and T-1) for free int erconversion.