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 porphyrins 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 band) EPR experiments allowed observation of low-temperature spin polarization and determination of the sign of the zero-field splitting parameters D. They appeared to be positive, in contrast to most in chemically or photochemically produced donor—acceptor radical pairs, for which the point dipole approximation (D<0) holds. To explain the experimental results, a matrix stabilization mechanism of exciplexes in the course of mechanochemically initiated electron (or hydrogen) transfer is postulated.