NONVERTICAL TRIPLET EXCITATION TRANSFER TO ARYLALKENE ACCEPTERS - FURTHER EVIDENCE THAT DOUBLE-BOND TORSION IS UNIMPORTANT

Time-resolved kinetic absorption spectrophotometry and photoacoustic calorimetry have been employed in a study of the kinetics and energetics of triplet excitation transfer to a series of arylalkene accepters. Both the cis and trans isomers of beta-methylstyrene are able to achieve significant relaxation in the triplet state. Nonvertical behavior is observed only for the cis isomer, for which single-bond torsions are expected to contribute to the measured relaxation. A comparison of the behavior of tetraphenylallene and its model, 1,1-diphenylethylene, is also revealing. Relaxation through double bond torsion in the allene triplet state leads to an ''allyl-vinyl'' biradical of particularly low energy. Measured energy transfer rate constants for the two molecules display a close correspondence. These results provide further compelling evidence for a recently proposed model for the nonvertical excitation transfer process dominated by single bond torsional modes.