DIVERSITY OF EXCITED-STATE DEACTIVATION PATHS IN HETEROAZAAROMATICS WITH MULTIPLE INTERMOLECULAR HYDROGEN-BONDS

Transient absorption and luminescence studies were performed for sever al heteroazaaromatic molecules which possess both hydrogen bonding don or and acceptor groups, and which are capable of forming cyclic, doubl e intermolecular hydrogen bonds. In protic solvents, these systems exh ibit complex photophysics, manifested by a strong quenching of the flu orescence. In dipyrido [2,3-a:3',2'-i]carbazole and 7,8,9,10-tetrahydr opyrido[2,3-a]carbazole, photoinduced double proton transfer is observ ed in the first excited singlet state, leading to tautomeric species w hich emit strongly Stokes-shifted fluorescence. The large and temperat ure-independent rate of this reaction is observed for the case in whic h a cyclic complex with an alcohol molecule is formed already in the g round state. Another excited state depopulation channel, internal conv ersion caused by proton movement in the cyclic complex, is observed in alcohol solutions of 2-(2'-pyridyl)-indoles and pyrido[2, 3-a]carbazo le. In these cases, formation of cyclic, doubly hydrogen-bonded struct ures occurs in the excited state and involves solvent rearrangement. T he rate of this process changes with solvent viscosity and becomes neg ligible at low temperatures, when the viscosity is high. Fast internal conversion has also been observed in hydrogen-bonded complexes with a zaaromatic bases. Its mechanism can be explained by intermolecular cha rge transfer interactions. In alcohol solutions of 2-(2'-pyridyl)indol es, transient absorption results point to yet another channel of excit ed state deactivation -photoionization, occurring faster than S-1 deca y.