Hydrogen Bonding Effect on the Fluorescence of π-Electron System

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.

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

“…Some aspects of this effect on the fluorescence of naphthols were studied by the present authors [3]. Nagakura and Gouterman [l] have demonstrated that both ~-and fl-naphthols form a quite strong hydrogen bond with triethylamine (TEA) in the excited Franck-Condon state and the equilibrium ground state.…”

“…Furthermore, the same circumstance holds when the hydrogen bond is formed between nitrogen heterocycles as proton acceptors and fluorescers, and proton donors such as phenol, naphthol, aniline and pyrrole [8]. In both of these cases, the hydrogen bond is closely related to the 7r-electron systems of the donors and acceptors, and the observed strong fluorescence quenching has been interpreted as due to the delocalization of ~-electrons through the hydrogen bond, in the excited electronic state [3,8]. The lowest ionization potentials of TEA and pyridine are 7.50 ev (n-orbital) [9] and 9.26 ev (Tr-orbital) [10], respectively.…”

“…On the other hand, it is well known that light absorption causes rather strong change in some molecular properties in the excited electronic state, such as in the dipole moment, ionic dissociation constant and hydrogen bonding ability [3][4][5][6]. In the case of naphthols, the magnitude of dipole moment [5] and the hydrogen bonding ability [3] are greater in the first excited singlet state than in the ground state and the ionic dissociation constant in the sense : 138 N. Mataga and Y. Kaifu is much larger in the first excited singlet state compared with that in the ground state [4].…”

“…In the case of naphthols, the magnitude of dipole moment [5] and the hydrogen bonding ability [3] are greater in the first excited singlet state than in the ground state and the ionic dissociation constant in the sense : 138 N. Mataga and Y. Kaifu is much larger in the first excited singlet state compared with that in the ground state [4]. These facts have been ascribed to the increased migration of ~r-electrons on the oxygen to the vacant 7r-MO's of naphthalene ring in the excited electronic state [3][4][5]. In view of the above facts, one may expect an extraordinarily strong hydrogen bond or ion-pair formation which can be regarded as a limiting form of a strong hydrogen bond, in the excited equilibrium state of the naphthol-TEA system in non-polar solvent.…”

“…Our primary object is to study this possibility by fluorescence spectral measurements. The fluorescence of naphthols, naphthylamines and carbazole has been found to be strongly quenched by hydrogen bonding when pyridine is used as proton acceptor [3,7]. Furthermore, the same circumstance holds when the hydrogen bond is formed between nitrogen heterocycles as proton acceptors and fluorescers, and proton donors such as phenol, naphthol, aniline and pyrrole [8].…”