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  • Picosecond Transient Thermal Phase Grating Study of a Photoinduced Electron Transfer Reaction in Solution
    E. Vauthey and A. Henseler
    Journal of Physical Chemistry, 99 (21) , 1995, p8652-8660
    DOI:10.1021/j100021a033 | unige:2986 | Abstract | Article PDF
A study of the dynamics and thermodynamics of the photoinduced electron transfer reaction between benzophenone and 1 ,Cdiazabicycl0[2.2.2]octane in acetonitrile using picosecond transient thermal phase grating spectroscopy is reported. Two heat releases were observed within the time window of the experiment (0-4 ns): a fast one corresponding to the formation of a benzophenone lower triplet state and a slow one due to the electron transfer reaction. The observed dynamics is in agreement with earlier studies using transient absorption spectroscopy. The quenching of benzophenone in the first singlet excited state at high concentration of quencher and the very rapid decay of the resulting singlet geminate ion pair are confirmed. The enthalpy of formation of the geminate ion pair was determined by comparing the amounts of heat released in the fast and slow processes. The electron transfer is more exergonic by 0.23 eV than calculated from the Rehm- Weller equation, assuming spherical ions with point charges in their centers. The difference is tentatively ascribed to the electrostatic interaction within the ion pair.
A study of the isomerisation dynamics of 3,3′-diethylthiacarbocyanine in the excited state in alkanenitriles as well as in the ground state in both alkanenitriles and n-alcohols, using ps transient grating spectroscopy and flash photolysis is reported. The intrinsic activation energy for isomerisation has been determined from isoviscosity plots. In the ground state, the activation energy amounts to 12.9 kcal/mol in both nitriles and alcohols, while it is equal to 3.0 kcal/mol in the excited state. The reduced isomerisation rate constant shows a fractional dependence on the solvent viscosity, with a fractional exponent α of 0.20 and 0.38 for the ground state isomerisation in alcohols and nitriles and of 0.65 for the excited state isomerisation. The data have been analysed within the framework of the Kramers theory using different models for friction. The best agreement was obtained with the Grote-Hynes model, which takes into account the frequency dependence of the friction. The solvent dependence of α for the ground state isomerisation is accounted for by a different average value of the infinite frequency shear modulus in alcohols and nitriles. The barrier height dependence of α is explained by a decrease of the barrier's top frequency by going from the ground to the excited state.
An application of transient thermal phase grating spectroscopy with near-infrared probing to the determination of free ion yield in photoinduced electron transfer reactions is presented. The model system is 9,10-dicyanoanthracene with various electron donors in acetonitrile. The measured yields are in good agreement with those obtained from photoconductivity, when the latter are correctly calibrated. The method is explained and its advantages and limitations are discussed.

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Eric Vauthey

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