atic S tudy of the In tra - annu lar In ternal H eavy - atom Effect

Phosphorescence in ethanol and n-pentane at 77 K of the benzologues 1-8 of furan, thiophene, selenophene and tellurophene has been investigated. The rate constants of both the radiative (A.PT) and non-radiative (A:GT) deactivation of the lowest triplet state correlate linearly with X cl C2 where ck denotes the Hückel AO coefficients in the HOMO of the carbon atoms bound to the hetero-atom and ( is the spin-orbit coupling constant of the hetero-atom present. The linear correlation observed between A.PT and kGT is an example for the Orlandi-Siebrand rule. The influence of an external heavy-atom perturber (methyl iodide) on phosphorescence lifetimes and the vibrational structure of phosphorescence spectra is the more efficient the less efficient is the internal perturbation caused by the intra-annular heavy-atom.