On the origin of fluorescence quenching of pyridylindoles by hydroxylic solvents.

Three isomeric 4'-pyridyl-substituted indoles, with the substituent in positions 2, 3 and 7, reveal strong fluorescence in aprotic solvents, both polar and nonpolar, whereas the emission is strongly quenched in water and alcohol solutions. Both viscosity and alcohol acidity play a role in efficient excited state deactivation. The process becomes faster for more acidic alcohols. It can be slowed down by increasing viscosity, which indicates that the proton movement is accompanied by large amplitude motions in the hydrogen-bonded complex. Quenching is not observed upon formation of solvates in which pyridylindoles act as hydrogen bond donors. The experimental results, combined with calculations can be explained by a model which assumes excited state protonation of the pyridine nitrogen atom, followed by twisting of the pyridyl group, leading to a low-energy structure. An alternative mechanism is also considered, in which the excited state proton transfer is accompanied by electron transfer from water or alcohol into a half-filled orbital of the chromophore, which leads to a conical intersection of the S(1) and S(0) energy surfaces.