Heavy-atom effect on radiative and radiationless transitions in charge-transfer complexes

The quantum yields of fluorescence and phosphorescence and decay times of fluorescence were measured for a series of charge-transfer complexes of hexamethylbenzene (an electron donor) with various electron acceptors containing heavy (halogen) atoms and the rate constants of different radiative and radiationless transitions were determined. It was found that radiative T₁ → S₀ transition, i.e. the phosphorescence, is strongly enhanced by the heavy-atom effect. The mechanism of the heavy-atom enhancement of charge-transfer phosphorescence is discussed and it is concluded that increasing spin- orbit mixing of radiative T₁ → S₀ charge-transfer transition with singlet-singlet, locally excited transition of acceptor molecule is responsible for the observed effects.