MECHANICALLY INDUCED RADICAL PAIR FORMATION IN PORPHYRIN QUINONE AND RELATED DONOR-ACCEPTOR MIXTURES - UNUSUAL STABILITY AND ZERO-FIELD SPLITTINGS

The mechanochemical activation of polycrystalline mixtures of porphyri ns and quinones (as well as some other donors and quinones) was found to produce large concentrations of triplet radical pairs with unusual stability in the solid state matrix. High-field/high-frequency (2 mm b and) EPR experiments allowed observation of low-temperature spin polar ization and determination of the sign of the zero-field splitting para meters D. They appeared to be positive, in contrast to most chemically or photochemically produced donor-acceptor radical pairs, for which t he point dipole approximation (D < 0) holds. To explain the experiment al results, a matrix stabilization mechanism of exciplexes in the cour se of mechanochemically initiated electron (or hydrogen) transfer is p ostulated.