Hydrogen-Bonding-Induced Phenomena in Bifunctional Heteroazaaromatics

Ground and excited state processes induced by hydrogen bond formation are discussed for a family of heterocyclic compounds which possess both a proton donor (pyrrole NH group) and an acceptor (pyridine-type nitrogen). Excited state double proton transfer and rapid S0 ← S1 internal conversion are observed only for molecules capable of forming cyclic, multiply hydrogen-bonded complexes. If the 1:1 cyclic, doubly hydrogen-bonded solvate is present in the ground state, the phototautomerization occurs even in rigid solvents at low temperatures. Internal conversion process requires solvent rearrangement and, therefore, does not proceed in a rigid environment. Another type of fluorescence quenching was also detected, involving photoinduced electron transfer from an excited chromophore to an aromatic hydrogen-bonded acceptor, such as pyridine. In molecules consisting of proton donor and acceptor units linked by a single bond, syn−anti rotamerization caused by hydrogen bonding is observed.