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c. i Bull. SOC. Chim. Belg., 71, p. 641 (1962) - ...

EFFECT OF TEMPERATURE AND VISCOSITY ON THE TRUE FIRST ORDER DECAY

OF THE TRIPLET STATE OF AROMATIC MOLECULES

George PORTER and Louis J. STIEF Department of Chemistry, University of Shefield, Shefield, England

The heavy atom effect on the radiationless decay of the triplet state of aromatic molecules has been demonstrated (1) for anthracene and its halo-substituted derivatives. In the case of 9,lO dibromoanthra- cene, it was demonstrated that the first order rate constant for the decay of the triplet was independent of viscosity between 1 and 190 centipoise. Use of this compound enables a study of the effect of temperature to be unambiguously determined since accompanying changes in solvent viscosity have no effect on the unimolecular decay of the triplet and bimolecular quenching processes are relatively unimportant for the triplet state of this molecule. Such an investigation has been made at temperatures between 155 and -135OC (2). There is a small, positive activation energy of 0.5 kcal. The temperature effect alone is, however, not sufficient to account for the very long life- time of the triplet observed in rigid media (glass at -196OC). For 9,lO dibromoanthracene, the triplet lifetime observed at - 196OC is at least ten times longer than that calculated on the basis of the measured temperature coefficient. There is evidently a strong effect of viscosity on the unimolecular triplet to singlet transition at very high viscosities. In the high viscosity region, there is no observable effect of temperature on the triplet lifetime.

Communique a la Socikte Chimique de Belgique le 18 juin 1962

(1) M.Z. HOFFMAN and G. PORTER, Proc. Roy. SOC. (in press). (2) G. PORTER and L.J. STIEF, Nature, 195, 991 (1962).