On the Relationship Between the Rates of the Radiative and Radiationless Deactivation of the Lowest Triplet State of Aromatic Compounds

Orlandi and Siebrand [1] have shown that radiationless transitions in molecules are subject to an electronic propen­ sity rule which does not refer directly to the states involved in the transition but instead to the radiative transition with which it competes. In a series of related molecules higher radiative rate constants are accompanied by higher rate con­ stants for the competing radiationless transition, i.e. radia­ tive and non-radiative rate constants are to some degree correlated. Since the Orlandi-Siebrand rule operates inde­ pendently of the Franck-Condon principle, application of the rule first requires elimination of Franck-Condon effects, e.g. by extrapolating rate constants to zero energy gaps [2]. In this note the Orlandi-Siebrand rule is examined with regard to the Tt -> S0 transition of some aromatic com­ pounds with the intention to contribute to the evaluation of the rule as a heuristic principle in molecular triplet spectros­ copy.