Photoacoustic evaluation of volume and entropy changes in energy and electron transfer. Triplet state porphyrin with oxygen and naphthoquinone-2-sulfonate

Both the enthalpy and volume changes in energy and electron transfer reactions of triplet uroporphyrin were determined separately in aqueous solution using a photoacoustic technique. The formation of triplet zinc uroporphyrin was accompanied by a 1.5 +/- 0.1 Angstrom(3) per molecule decrease in volume, but only about 0.4 +/- 0.2 Angstrom(3) for the free base uroporphyrin. Formation and decay of singlet oxygen via photosensitization with the above porphyrins both proceeded on the microsecond time scale in H2O but were temporally well separated in D2O. The volume decrease on formation of singlet oxygen was similar to 0.4 Angstrom(3). Electron transfer to napthoquinone-2-sulfonate from triplet zinc uroporphyrin took place with a rate constant of 6 x 10(8) M(-1) s(-1), and a volume contraction of -4.4 Angstrom(3) per pair of ions, This electrostriction can be calculated by the Drude-Nernst equation with radii of 5.5 Angstrom (porphyrin cation) and 2.5 Angstrom (quinone anion). Combining the measured enthalpy change (-0.37 eV) with the difference in redox potentials of these compounds (-0.94 eV) yields an entropy change of 0.58 eV for the recombination of the ions. The large value of the entropy change indicates that entropy cannot be neglected in explaining electron transfer reactions. This study shows that photoacoustics allows a direct measure of this parameter, which is otherwise difficult to obtain.