Hydrogen Bonding Effect on the Fluorescence of π-Electron System

Abstract

Abstract 1. The absorption and fluorescence spectra of α-, β- naphthols and related compounds were measured in various mixed solvents in connection with the hydrogen bonding phenomena. The observed spectral shifts of absorption and fluorescence spectra, which were almost the same (∼30 Å), could be solely attributed to the hydrogen bonding effect. 2. It was found that the fluorescence intensity becoms stronger by hydrogen bonding when dioxane and acetic acid esters other than ethylchloroacetate are used as the proton acceptors. In the case of β-naphthol, it was established that the rate of inner quenching is decreased by hydrogen bonding. When ethylchloroacetate was used as a proton acceptor, fluorescence quenching was observed, which is also to be attributed to hydrogen bonding. 3. For β-naphthol, the measurement of the relative intensity established the fact that a new and different equilibrium state of hydrogen bonding, which shifts somewhat toward association, is reached during the life time of the excited state. 4. The mechanism of hydrogen bonding was discussed on the basis of the theory of electron migration, and it was concluded that the observed extra-stabilization of the hydrogen bond in the excited state as compared to the ground state, is mainly due to the increase of electron migration in the former state.