Role of Higher Triplet States in the Anthracene-Sensitized Photoisomerization of Stilbene and 2,4-Hexadiene

Article abstract of DOI:10.1021/ja00347a002

Irradiation of benzene solutions of anthracene (A) in the presence of varying amounts of trans,trans-2,4-hexadiene (tt) leads to trans-cis photoisomerization of the diene.Ratios of tc/cc are close to those expected for a high triplet energy donor.The anthracene-sensitized diene photoisomerization is not quenched significantly by cis-stilbene, though pronounced c -> t stilbene photoisomerization is observed.Conversely, the diene quenches the anthracene-sensitized cis-stilbene photoisomerization.In air-saturated solutions where the lowest triplet of anthracene, ³A^*, is quenched by oxygen and is thus unavailable for interactions with the diene, diene photoisomerization to tc is slightly enhanced, as are tc/cc ratios.The upper limit for the rate constant of ³A^* quenching in benzene by tt, determined by flash-kinetic spectroscopy at room temperature, is 1.2₅E3 M^-1 s^-1.When reported values for the interaction of A excited singlets, ¹A^*, with the diene and A are used, kinetic analysis of the isomerization quantum yields leads to the conclusion that triplet excitation transfer to the diene is almost exclusively from a higher A triplet, ³A^**, while ³A^* is responsible for stilbene photoisomerization.The data require that formation of the two triplets from ¹A^* be mainly sequental: ¹A^* -> ³A^** -> ³A^*.The estimated effective lifetime of ³A^** is ca. 30 ps.The effect of oxygen on the formation of cc suggests that quenching of ¹A^* by oxygen leads in part to adiabatic formation of ³A^**.This pathway may account for the inefficiency in singlet oxygen formation associated with ¹A^* quenching.