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 r eactions of triplet uroporphyrin were determined separately in aqueous solution using a photoacoustic technique. The formation of triplet zi nc uroporphyrin was accompanied by a 1.5 +/- 0.1 Angstrom(3) per molec ule decrease in volume, but only about 0.4 +/- 0.2 Angstrom(3) for the free base uroporphyrin. Formation and decay of singlet oxygen via pho tosensitization with the above porphyrins both proceeded on the micros econd time scale in H2O but were temporally well separated in D2O. The volume decrease on formation of singlet oxygen was similar to 0.4 Ang strom(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, T his electrostriction can be calculated by the Drude-Nernst equation wi th radii of 5.5 Angstrom (porphyrin cation) and 2.5 Angstrom (quinone anion). Combining the measured enthalpy change (-0.37 eV) with the dif ference in redox potentials of these compounds (-0.94 eV) yields an en tropy change of 0.58 eV for the recombination of the ions. The large v alue of the entropy change indicates that entropy cannot be neglected in explaining electron transfer reactions. This study shows that photo acoustics allows a direct measure of this parameter, which is otherwis e difficult to obtain.