PHOTOOXIDATION OF ZINC TETRAPHENYLPORPHYRIN BY BENZOQUINONE - A FOURIER-TRANSFORM ELECTRON-PARAMAGNETIC RESONANCE INVESTIGATION

Fourier transform electron paramagnetic resonance (FT-EPR) has been used to study the photooxidation of zinc tetraphenylporphyrin (ZnTPP) by benzoquinone (BQ). A method was developed for the analysis of the time dependence of the FT-EPR spectra, following pulsed laser excitation of ZnTPP, that gives rate constants for the electron-transfer process as well as for the spin-polarization evolution in paramagnetic species involved in the reaction. It is found that the rate constant of electron transfer from photoexcited triplet ZnTPP to BQ in ethanol is concentration dependent. The data yield a value for the electron-transfer-reaction radius of triplet ZnTPP, the limiting rate constant that would obtain for [BQ] --> 0, and the radical pair lifetime. In addition, values are found for the spin-lattice relaxation times of the BQ- anion radical and triplet porphyrin precursor and for electron transfer from BQ- to BQ.