On the viscosity effects in proton transfer systems. Structure and photophysics of 2-(2′-hydroxy-5′-methylphenyl)-3,3-dimethylindole

Abstract

Temperature effects on fluorescence quantum yields and lifetimes of 2-(2′-hydroxy-5′-methylphenyl)-3,3-dimethylindole (HBDMI) and 2-(2′-hydroxyphenyl)benzoxazole (HBO) in the series of hydrocarbons were studied. The results obtained allowed us to examine the role of the viscosity in the electronically excited proton transfer (PT) systems. The large-amplitude torsional motions operate in the initially excited and PT forms. The X-ray determination of the crystal structure revealed that HBDMI is distorted from planarity in the crystalline state. We propose that solvent frictional forces effectively hinder the formation of the coplanar structure required for the barrierless proton transfer in the excited state.