Triplet state isomerizations

If a common intermediate were not involved in the triplet state isomerization of stilbene, the sum of d>c T and T-.0 could have any value from zero to two. 

Scheme 3.2 A schematic presentation of triplet-state isomerization in retinal. The pathways of isomerization are shown by arrows, whose lengths are proportional to the values of quantum yield [8].

Table 3.2 lists the relevant spectral and kinetic parameters determined by the SVD and global-fitting analyses of time-resolved data matrices. The Tn <— T, absorption maxima of the set of triplet speties are very similar to one another except for that of the 13 -cis T1 a fact which caused difficulty in the analysis. The lifetimes of the cis T, speties could be determined only as an average, although the lifetime of the all-trans T, could be easily determined. Fortunately, however, the time constants of CTI in the T, state could be determined successfully for the set of cis isomers. The ranking of the rate of triplet-state isomerization parallels that of the quantum yield of isomerization per triplet speties generated, the latter... 

Transient absorption spectroscopy has been a conventional technique for studying the configurational (conformational) changes that are reflected in the Tn <— T, optical transition. Picosecond to microsecond time-resolved, pump and probe measurement has been widely used to examine the triplet-state isomerizations as far as each ris-T, and the all-trans-Tj species are selectively observed. Even when their wavelengths are very similar to one another, the SVD and global-fitting analysis can successfully identify a set of triplet species appearing in different time-scales, when a correct kinetic model is built. 

More recently, Tokumaru and collaborators found that the O-methyl ether of an aromatic ketone oxime, 2-acetylanthracene oxime O-methyl ether, upon irradiation in benzene underwent one-way Z, -isomerization through a quantum chain process in the triplet state (Scheme 7). The kinetics of the triplet-state isomerization and the potential energy surface of the triplet state were studied by transient spectroscopy. A quantum yield as high as 22 at a concentration of 1.35 x 10 M was obtained. According to their calculation, the Tj potential-energy curves of the substrate are in agreement with the rotation mechanism for one-way isomerization. ... 

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