Kurreck (2000) 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 ® eld and microwave frequency (0:34 T/9:5 GHz, X-band) and high ® eld and frequency (3:4 T/95 GHz, 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 10 6 s ¡1 . Most probably this is due to contributions from spin± rotation interaction, and dependent on the molecular orientation with respect to the magnetic ® eld. 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¯exibility of the systems. To yield long-lived 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¯exibility of the complex.
We investigate the effect of stochastic modulation of the exchange interaction .J on singlet (S)-triplet (T) transitions in radical pairs. These transitions limit the lifetime of the photo-generated radical pairs in covalently linked porphyrin-quinone systems that have been developed for biomimetic modeling of photosynthetic electron transfer processes. In order to explain transient electron paramagnetic resonance (EPR) results in different magnetic fields, i.e., with X-band ((0.34 T)/ (9.5 GHz)) and W-band ((3.4 T)/(95 GHz)) time-resolved EPR, we have to assume that J x is modulated over a range of 20000 G, which is wide enough that S is temporarily almost degenerate with To as well as with T_,. This large modulation of J x is caused by restricted rotational diffusion of the quinone subunit with respect to the porphyrin subunit. However, because of the small interradical distance of about 1.0-1.4 nm, the radical pair is continuously kept in the strong coupling limit and, therefore, we observe only EPR transition between the triplet sublevels. We find an approximation to solve the stochastic Liouville equation valid for rotational diffusion on an intermediate time scale, i.e., the diffusion rate DR is smaller than the singlet electron recombination rate K s -109 s-', but larger than the ST transition rates K o , K_ i < 106 s-'•
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