Diversity of excited state deactivation paths in heteroazaaromatics with multiple intermolecular hydrogen bonds

Abstract

AbstractTransient absorption and luminescence studies were performed for several heteroazaaromatic molecules which possess both hydrogen bonding donor and acceptor groups, and which are capable of forming cyclic, double intermolecular hydrogen bonds. In protic solvents, these systems exhibit complex photophysics, manifested by a strong quenching of the fluorescence. In dipyrido[2,3‐a:3′,2′‐i]carbazole and 7,8,9,10‐tetrahydropyri‐do[2,3‐a]carbazole, photoinduced double proton transfer is observed in the first excited singlet state, leading to tautomeric species which emit strongly Stokes‐shifted fluorescence. The large and temperature‐independent rate of this reaction is observed for the case in which a cyclic complex with an alcohol molecule is formed already in the ground state. Another excited state depopulation channel, internal conversion caused by proton movement in the cyclic complex, is observed in alcohol solutions of 2‐(2′‐pyridyl)‐indoles and pyrido[2,3‐a]carbazole. In these cases, formation of cyclic, doubly hydrogen‐bonded structures occurs in the excited state and involves solvent rearrangement. The rate of this process changes with solvent viscosity and becomes negligible at low temperatures, when the viscosity is high. Fast internal conversion has also been observed in hydrogen‐bonded complexes with azaaromatic bases. Its mechanism can be explained by intermolecular charge transfer interactions. In alcohol solutions of 2‐(2′‐pyridyl)indoles, transient absorption results point to yet another channel of excited state deactivation – photoionization, occurring faster than S1 decay.