Separation of the exchange and dipolar contributions to the spin-spin coupling in the donor-acceptor complex TAPD-ZnP-NQ

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 excitation of the complex at 30 K in frozen 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 nematic phases. The spectra of TAPD^+̇NQ^-̇ are analyzed on the basis of the coupled correlated radical pair (CCRP) model using an 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 spectra are consistent 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.