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 the 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 z 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.

Abstraet. The donor-acceptor complex, 2-tetraalkylphenylenediamine-zinc porphyrin-2-naphthoquinone (TAPD-ZnP-NQ), is studied using transient EPR at K-band (24 GHz). Spin polarized spectra of the radical pair TAPD+'NQ -" and of the triplet state TAPD-3ZnP-NQ are observed simultaneously following light exeitation of the complex at 30 K in ~ozen solutions 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. The orientational distribution function is obtained independently by simulating the spectra of the triplet state and assuming that the principal axes of the order matrix coincide with those of the inertia tensor as has been found for solutes in nematie phases. The spectra of TAPD+'NQ -" are analyzed on the basis of the coupled correlated radical pair (CCRP) model using ah estimate of the geometry based on the X-ray crystal structures of the components and the independently obtained orientational distribution function. It is shown that the experimental speetra are eonsistent with the assumed geometry and that the exchange coupling, J, is approximately 0.1 mT. This value is at least two orders of magnitude larger than that found in the analogous radical pair P+'Q-" in photosynthetic reaction centres. This difference is rationalized in terms of different mechanisms for the exchange coupling.