Intramolecular heavy-atom effect in the photophysics of organic molecules

This review systematizes the data on the effect of intramolecular spin–orbit perturbations on the photophysics of organic molecules introduced by heavy atoms (the intramolecular, or 'internal', heavy-atom effect). The focus is on two classes of compounds: (1) halogenated derivatives of aromatic and heteroaromatic compounds and (2) porphin derivatives (halogenated porphyrins and metalloporphyrins). The enhancement of radiative and nonradiative intercombination transitions under the heavy-atom effect is responsible for fluorescence quenching and the shortening of phosphorescence duration. Photophysical investigations show that the transition probabilities in different channels of deactivation of excited electronic states exhibit specific dependence on the structure of heavy-atom bearing molecules. This allows one to acquire information on the spin–orbit interaction in molecules and their electronic structure, and to control the parameters of chromophores and luminophors in optical systems.