What is the influence of a ‘heavy atom’ on the decay mechanisms of the lowest triplet state of aromatic hydrocarbons?

The various decay mechanisms of the lowest triplet state of a-halogenated naphthalenes have been investigated by means of polarized light spectroscopy. Contrary to pure aromatic hydrocarbons, the halogenated derivatives exhibit strong in-plane polarization in the 00-transition of the phosphorescence, which is enhanced more rapidly than the out-of-plane polarized intensity if one increases the atomic number of the substituent. The various features of the polarized emission spectra are related to a second-order Herzberg-Teller coupling. This type of interaction can provide intensity for the 00-transition via out-of-plane promoting modes. It varies with the one centre spin-orbit integral, thus explaining the strong enhancement with the heavy atom. It accounts for a deuterium isotope effect in the radiative rate of the 00-transition and in the degree of polarization.