Fluorescence characteristic and molecular interaction in the excited state of 1,2,3,4-tetrahydroquinoline

Abstract

Fluorescence spectra of 1,2,3,4-tetrahydroquinoline (THQ) in binary and ternary mixtures in nonpolar and hydrogen bonding solvents at different temperatures are investigated. A probable case of partial protonation of THQ at 300 K due to hydrogen bonding in the excited state with ethanol is reported. At 77 K, there is no significant hydrogen bonding interaction between these molecules. Interaction between the n-orbital of triethylamine (TEA) and the π-electron system of excited THQ results in the formation of a CT-type complex which causes slow quenching of the fluorescence of THQ at 300 K. This quenching is not observed at 77 K because restriction of molecular orientation at 77 K prevents the formation of such complex In pyridine, the fluorescence is strongly quenched both at room temperature and at 77 K. This has been attributed to π-electronic delocalization interaction between the fluorescer THQ and the nonfluorescing quencher pyridine. Possibility of conformational change is suggested. Rate constants associated with suitable reaction scheme for depletion of excited state are estimated. It is indicated that the quenching of fluorescence may be largely diffusion controlled.