The Recombination Dynamics of the Radical Pair P+H− in External Magnetic and Electric Fields

Abstract

SummaryWhen electron transfer to the primary quinone is blocked, the radical pair P+H− (P: primary donor, H: bacteriopheophytin at the A-branch) recombines on the 10 ns time scale either to the ground state P or, after hyperfine-induced singlet-triplet-mixing, to the triplet state 3P*. An external magnetic field hinders singlettriplet-mixing, thus reducing the yield of 3P* and slowing the recombination of P+H−. Magnetic field dependent measurements of the recombination dynamics allow the determination of the recombination rates ks and kT and the exchange interaction J. From these parameters free energies, electronic matrix elements and reorganization energies relevant for the fast charge separation and slow charge recombination processes in the reaction center can be determined. In many cases, recombination data constitute the sole experimental access to such parameters, which constitute the basis for the theoretical treatment of electron transfer processes.In this review, results obtained on quinone-depleted reaction centers of Rhodobacter sphaeroides, Rhodobacter capsulatus and Chloroflexus aurantiacus are discussed in the context of (i) the similarity of reaction centers from different organisms, (ii) the mechanism of primary charge separation, (iii) the distinction between structural and energetic effects of genetic alterations of the reaction center, and (iv) the effects of an external electric field, which shifts the energy of the charge separated states. Furthermore, different recombination dynamics observed in transient absorption and delayed fluorescence reveal an inhomogeneous broadening of radical pair energies in the reaction center. This energetic broadening allows us to understand a variety of phenomena: (a) the observed multiphasic electron transfer kinetics, (b) the unexpectedly weak electric field effects on the fluorescence and (c) the discrepancies of energetics determined by delayed fluorescence and transient absorbance measurements. As a consequence, absorption measurements are better suited to determine the average of the energetic distribution of the radical pair P+H−.