Nonradiative deactivation of the lowest triplet state of naphthalene and its hydroxy derivatives at 77 K

Abstract

The validity of an inductive resonance theory of energy transfer from the T 1→S 0 transition dipole to overtone vibrations of molecular groups containing H and D atoms is experimentally tested for a series of compounds whose conjugation systems are similar in size. To this end, by using kinetic, spectral, and luminescent methods (measurements of the phosphorescence decay times, phosphorescence spectra, ratios between the quantum yields of phosphorescence and fluorescence at 77 K, total quantum yields of fluorescence at 293 K, and ratios between the quantum yields of fluorescence at 293 and 77 K), the deactivation processes of the lowest excited T 1 and S 1 states of seven emitting centers (naphthalene, its hydroxy and dihydroxy derivatives, and their monoanions) in solutions in ethanol-h 6, ethanol-d 6, and their 2: 1 mixtures with diethyl ether are studied. For all the compounds studied, the rate constants k r of the radiative T 1→S 0 transition and the changes in the overlap integrals of the spectra of phosphorescence and absorption of overtones of CH stretching vibrations are determined. The rate constants of energy transfer k dd(CH) from the T 1→S 0 transition dipole to the stretching vibrations of the CH bonds are calculated without regard for the changes in the localization and orientation of this transition dipole in the compounds under study. The contribution of an individual CH group k nr(CH) to the total rate constant of nonradiative deactivation of the T 1 state averaged over the CH groups of the naphthalene ring system is ascertained. A good correlation between the changes in the constants k nr(CH) and k dd(CH) in the series of the hydroxy derivatives of naphthalene is found, which is indicative of the inductive resonance mechanism of the energy degradation of the T 1 state. The deviations from proportionality between the changes in these constants upon passing from naphthalene to its hydroxy derivatives, which correlate with a marked increase in the radiative constant k nr of the hydroxy derivatives in comparison with naphthalene, indicate changes in the strength and localization of the T 1→S 0 transition dipole moment and in its orientation with respect to the plane of the molecule that occur due to introduction of a heteroatom, oxygen, whose lone pair of electrons enters into conjugation with the πelectrons of the naphthalene ring system.