X- K-band transient EPR of the light induced radical ion pairs in photosynthetic model systems

The light induced electron transfer in the complex 2-tetraalkylphenylenediamine-Zinc porphyrin-2-naphthoquinone (TAPD-ZnP-NQ) is investigated using transient EPR at X-band (9 GHz) and K-band (24 GHz). Spin polarized spectra are presented for the radical pair TAPD^+•NQ^-• which is generated by light excitation of the complex at 30 K in 2-methyl-tetrahydrofuran and the liquid crystal BDH E7. In the liquid crystal, the complex is partially ordered and the spectra depend on the orientation of the sample with respect to the magnetic field. A qualitative interpretation of the spectra on the basis of the coupled, correlated radical pair (CCRP) model is presented. The spectra of the partially oriented samples clearly show that: (i) the exchange interaction, J, makes a substantial contribution to the coupling between the radicals and is of ihe same order of magnitude as the dipole-dipole coupling (ii) when this coupling is taken into account, the experimental spectra are consistent with the expected geometry of the complex. This system is one of the few cases in which the two couplings are of similar magnitude and allows their influence on the polarization pattern to be investigated. The value of J≈0.1 mT obtained from the spectra is at least two orders of magnitude larger than that found in the analogous radical pair, P^+• Q^-• in photosynthetic reaction centres, although the average distance between the spins in the two systems differ at most by a factor of 1.5. Possible reasons for this difference are discussed.