Charge transfer and proton transfer reactions in the excited hydrogen bonded complex in non-polar solvents

Abstract

Fluorescence spectral measurements in benzene solution have revealed that the strong hydrogen bond between β-naphthol and triethylamine in the equilibrium ground state and the excited Franck-Condon state shifts to an ion-pair in the excited equilibrium state, from which fluorescent transition occurs. In benzene, the shift of the fluorescence spectrum due to the ion-pair formation is about 4000 cm−1. In cyclohexane, the shift of the fluorescence spectrum due to hydrogen bonding is about 2700 cm−1. No such shifts occur, however, when acetic acid esters or acetonitrile are used as proton acceptors. The observed phenomena are interpreted as due to a strong charge transfer from the nitrogen non-bonding orbital of triethylamine to the -OH antibonding orbital of naphthol in the Franck-Condon excited state, which is followed by an almost complete proton transfer from oxygen to nitrogen in the excited equilibrium state. In cyclohexane, the ion-pair formation as in benzene solution is rather difficult. Nevertheless, owing to the strong charge transfer, the equilibrium -OH distance becomes longer and the force constant of OH stretching becomes smaller in the excited state than in the ground state. This circumstance is indicated by the observed broadness of the fluorescence band. In the case of acetic acid esters or acetonitrile, no such strong charge transfer interaction exists. The rate constants for the hydrogen bond formation and dissociation in the excited state have been estimated from the observed fluorescence intensity at an appropriate wavelength in the case of the β-naphthol-triethylamine system.