THE TRIPLET STATE IN STILBENE cistrans PHOTOISOMERIZATION JACK SALTIEL*, D. W. L. CHANG, E. D. MEGARITY, A. D. ROUSSEAU, P. T. SHANNON, B. THOMAS and A. K. URIARTE Department of Chemistry, The Florida State University, Tallahassee, Florida 32306, USA ABSTRACT Stilbene triplets are further characterized by their interaction with quenchers. A previously proposed model for stilbene triplet decay and quenching, together with rate constants for excitation transfer from indeno[2,1-a]indene triplets to azulene, is used to account for the temperature dependence of the azulene effect on stilbene photoisomerization and is shown to lead to erroneous predictions concerning the lifetime and geometry of stilbene triplets. This conclusion is based on oxygen quenching experiments suggesting a twisted geometry for stilbene triplets in benzene solution with a lifetime of —120 ns at 30°. Azulene quenching experiments show that bromine substitution at the para position of stilbene markedly increases 'S — T intersystem crossing efficiency while substitution at the meta position does not enhance this process at 30°. These results are consistent with the fact that in contrast to the para position, the meta position is a near node in the highest occupied and lowest unoccupied molecular orbitals of stilbene. INTRODUCTION The geometry, lifetime and reactivity of stilbene triplets have to a large measure been inferred from experiments in which the stilbenes function as acceptors or donors of triplet excitation13. Such approaches will be the subject of the first part of this paper. Knowledge concerning the efficiency of stilbene triplet interactions with a variety of quenchers has been used in determining the extent to which these triplets participate in photoisomeri- zation following direct stilbene excitation16. In the second part of this paper quenching methods will be applied in determining whether bromine atom substitution influences the efficiency of intersystem crossing in the stilbenes. THE LIFETIME AND GEOMETRY OF STILBENE TRIPLETS IN SOLUTION The interaction of stilbene triplets with quenchers Under triplet (sensitized) excitation conditions stilbene photoisomerization has been proposed to occur by decay from twisted triplets, 3p13' , which lie lower in energy than cisoid triplets but are nearly isoenergetic with transoid 559