Application of the transient grating technique to the study of the energetics and dynamics of electron transfer reactions: separation of an ion pair into free ions

The enthalpy of formation of the free ions generated by the electron transfer between benzophenone in the lower triplet state and 1,4-diazabicyclo [2,2,2] octane in acetonitrile has been measured using transient thermal phase grating. This enthalpy is smaller by more than 0.14 +/- 0.08 eV than the enthalpy of formation of the geminate ion pair obtained from a previous ps thermal phase grating investigation. Therefore, the dissociation of a geminate ion pair into free ions in endothermic. However, separation is exergonic if translational entropy in taken into account. A small and positive value of C, the correction term in the Rehm-Weller equation, is obtained if this term is considered as a free energy. The principles of the transient grating technique as a tool for investigating photoinduced processes in solution are briefly reviewed. (C) 1998 Elsevier Science S.A.