Quantum yields as high as 120 were achieved for triplet-sensitized photoisomerizations of several Dewar
benzene reactants, R, to the corresponding
benzene products, P. Considera
ble chain amplification is maintainedeven at high conversion. All relevant rate constants of this triplet chain reaction were extracted from laserflash photolysis plus steady-state photolysis experiments. The crucial rate constant
ka for adia
batic isomerizationof the triplet reactant to triplet product (R*
![](/images/entities/rarr.gif)
P*) cannot
be directly measured
because it is so large relativeto the
bimolecular rate of R* formation via sensitization. However,
ka was o
btained indirectly using a cage/encounter complex model to analyze the competition
between the dissociation of encounter pairs with thesensitizer, e.g., S/R*
![](/images/entities/rarr.gif)
S + R*, and the in-cage processes, S/R*
![](/images/entities/rarr.gif)
S/P*
![](/images/entities/rarr.gif)
S*/P, in nonviscous and viscoussolvents. These measurements yielded
ka values of (~4-9) × 10
9 s
-1, which suggests that only a small (~3kcal/mol) energy
barrier exists along the potential energy surface from R* to P*. Steady-state data indicatedthat the chain-terminating rate constant R*
![](/images/entities/rarr.gif)
R is negligi
bly small, an ideal condition for chain amplification.Triplet energy transfer from a series of sensitizers to the Dewar
benzene derivatives shows a nonclassicalfalloff in rate constants with decreasing sensitizer triplet energy, suggesting energy transfer to thermally distortedconfigurations having lower singlet-triplet energy gaps. As a result of distorted geometries of R* and P*,the chain-propagating energy transfer from P* to R proceeds with a rate constant of only ~2 × 10
7 M
-1 s
-1,despite strong exothermicity. The isomerization reaction can release over 100 kcal/kcal of a
bsor
bed photonsdue to the high-energy content of the reactant together with the large chain length.