Highly Efficient Triplet Chain Isomerization of Dewar Benzenes: Adiabatic Rate Constants from Cage Kinetics
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- 作者：Paul B. Merkel ; Yeonsuk Roh ; Joseph P. Dinnocenzo ; Douglas R. Robello ; Samir Farid
- 刊名：Journal of Physical Chemistry A
- 出版年：2007
- 出版时间：February 22, 2007
- 年：2007
- 卷：111
- 期：7
- 页码：1188 - 1199
- 全文大小：193K
- 年卷期：v.111,no.7(February 22, 2007)
- ISSN：1520-5215
文摘
Quantum yields as high as 120 were achieved for triplet-sensitized photoisomerizations of several Dewarbenzene reactants, R, to the corresponding benzene products, P. Considerable 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 adiabatic isomerizationof the triplet reactant to triplet product (R* 
P*) cannot be directly measured because it is so large relativeto the bimolecular rate of R* formation via sensitization. However, ka was obtained indirectly using a cage/encounter complex model to analyze the competition between the dissociation of encounter pairs with thesensitizer, e.g., S/R* S + R*, and the in-cage processes, S/R* S/P* S*/P, in nonviscous and viscoussolvents. These measurements yielded ka values of (~4-9) × 109 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* R is negligibly 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 × 107 M-1 s-1,despite strong exothermicity. The isomerization reaction can release over 100 kcal/kcal of absorbed photonsdue to the high-energy content of the reactant together with the large chain length.
P*) cannot be directly measured because it is so large relativeto the bimolecular rate of R* formation via sensitization. However, ka was obtained indirectly using a cage/encounter complex model to analyze the competition between the dissociation of encounter pairs with thesensitizer, e.g., S/R* S + R*, and the in-cage processes, S/R* S/P* S*/P, in nonviscous and viscoussolvents. These measurements yielded ka values of (~4-9) × 109 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* R is negligibly 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 × 107 M-1 s-1,despite strong exothermicity. The isomerization reaction can release over 100 kcal/kcal of absorbed photonsdue to the high-energy content of the reactant together with the large chain length.