Journal of the American Chemical Society p. 2530 - 2538 (1983)
Update date:2022-08-15
Topics:
Saltiel, Jack
Townsend, David E.
Sykes, Alan
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.
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