External heavy-atom spin-orbit coupling—The T1 vibronic activity of naphthalene in p-dihalogenated benzene host crystals

Abstract

The phosphorescence of naphthalene in p-dibromobenzene, α-phase p-dichlorobenzene, and γ-phase p-dichlorobenzene host crystals has been studied at high resolution and 4.6°K. The T1 absorption of naphthalene in a p-dibromobenzene host crystal at 4.6°K was recorded by the use of excitation spectroscopy. With the exception of the b3gv17 fundamental (which is nearly degenerate with the agv1 fundamental and probably borrows intensity from it) only vibrations of ag symmetry are active in the naphthalene T1 absorption and emission. This vibronic activity indicates that the external spin-orbit coupling mechanism operative in the heavy-atom-containing chemically mixed crystal is different from the internal spin-orbit coupling mechanism active in the T1 of naphthalene in a deuteronaphthalene crystal. This enhancement of only totally symmetric modes indicates that the external spin-orbit coupling induced by heavy atoms is purely electronic in nature. The internal heavy atom effect is shown to enhance the totally symmetric modes of the parent naphthalene and to vibronically enhance the out-of-plane halogen modes. The external spin-orbit coupling mechanism induced by heavy atoms arises from a mixing of several singlet states with the naphthalene T1. This mixing could involve singlet states of either the naphthalene molecule or the surrounding host molecules.