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

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...