Multifrequency time-resolved EPR (9.5GHz and 95GHz) on covalently linked porphyrin-quinone model systems for photosynthetic electron transfer: effect of molecular dynamics on electron spin polarization

Covalently linked porphyrin-quinone model systems for photosynthetic electron transfer were examined by using time-resolved electron paramagnetic resonance (TREPR) at intermediate magnetic field and microwave frequency (0.34T/9.5GHz, X-band) and high field and frequency (3.4T/95GHz, W-band). The paramagnetic transients studied were the light-induced spin-correlated radical pair states of the donor-acceptor complex in polar solvents below the melting point and in the soft glass phase of a liquid crystal. It is shown that the systems form strongly exchange-coupled radical pairs, whose TREPR lineshapes are determined mainly by fast electron recombination together with both spin-lattice relaxation and modulation of the exchange interaction. Below the melting point the spin-lattice relaxation rate naturally slows down, but that of the spin on the quinone site is still of the order of 106 s-1. Most probably this is due to contributions from spin-rotation interaction, and dependent on the molecular orientation with respect to the magnetic field. This relaxation anisotropy is related to anisotropic motion of the quinone site in the solvent cage. The results allow conclusions to be drawn concerning the molecular dynamics and flexibility of the systems. To yield longlived radical pair states that would mimic photosynthetic electron transfer, the two mechanisms described, modulation of exchange and spin-rotation interactions, have to be suppressed by reducing the molecular flexibility of the complex.