X-BAND AND K-BAND TRANSIENT EPR OF THE LIGHT-INDUCED RADICAL-ION PAIRS IN PHOTOSYNTHETIC MODEL SYSTEMS

The light induced electron transfer in the complex 2-tetraalkylphenyle nediamine-Zinc porphyrin-2-naphthoquinone (TAPD-ZnP-NQ) is investigate d using transient EPR at X-band (9 GHz) and K-band (24 GHz). Spin pola rized spectra are presented for the radical pair TAPD(+.)NQ(-.) which is generated by light excitation of the complex at 30 K in 2-methyl-te trahydrofuran and the liquid crystal BDH E7. In the liquid crystal, th e complex is partially ordered and the spectra depend on the orientati on of the sample with respect to the magnetic field. A qualitative int erpretation of the spectra on the basis of the coupled, correlated rad ical pair (CCRP) model is presented. The spectra of the partially orie nted samples clearly show that: (i) the exchange interaction, J, makes a substantial contribution to the coupling between the radicals and i s of the same order of magnitude as the dipole-dipole coupling (ii) wh en this coupling is taken into account, the experimental spectra are c onsistent with the expected geometry of the complex. This system is on e of the few cases in which the two couplings are of similar magnitude and allows their influence on the polarization pattern to be investig ated. The value of J approximate to 0.1 mT obtained from the spectra i s at least two orders of magnitude larger than that found in the analo gous radical pair, P(+.)Q(-.), in photosynthetic reaction centres, alt hough 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.