Photoinduced electron transfer reaction center models

Photoinduced electron transfer reactions have been studied in the dyads 44(6), 45(9), and 46(11). The 6-bond porphyrin-quinone dyad 44(6) serves as a model for aspects of ET in the photosynthetic reaction center, since congeners of these chromophores are present in the center as cofactors (Figure 37). Photoinduced ET in... 

Most of the interest in mimicing aspects of photosynthesis has centered on a wide variety of model systems for electron transfer. Among the early studies were experiments involving photoinduced electron transfer in solution from chlorophyll a to p-benzoquinone (21, 22) which has been shown to occur via the excited triplet state of chlorophyll a. However, these solution studies are not very good models of the in vivo reaction center because the in vivo reaction occurs from the excited singlet state and the donor and acceptor are held at a fixed relationship to each other in the reaction-center protein. 

In the natural photosynthetic reaction center, ubiquinones (QA and QB), which are organized in the protein matrix, are used as electron acceptors. Thus, covalently and non-covalently linked porphyrin-quinone dyads constitute one of the most extensively investigated photosynthetic models, in which the fast photoinduced electron transfer from the porphyrin singlet excited state to the quinone occurs to produce the CS state, mimicking well the photo synthetic electron transfer [45-47]. However, the CR rates of the CS state of porphyrin-quinone dyads are also fast and the CS lifetimes are mostly of the order of picoseconds or subnanoseconds in solution [45-47]. A three-dimensional it-compound, C60, is super-... 

Synthetic carotenoids incorporated into multicomponent sutructures such as triad 8 have been used as spectroscopic labels to follow the flow of triplet energy and the spin dynamics in model photosynthetic reaction centers. Excitation of the porphyrin moiety in 8 yields C- P-C,.q which decays by photoinduced electron transfer to yield - "- , . This state rapidly evolves into the final charge separated state, C -P-(Liddell et al., 1997). The fact that both electron transfer steps in 8 occur even in a glassy matrix at 77K has made it possible to observe some unusual... 

Among the systems proposed as models for the photosynthetic reaction center, supramolecular assemblies in which Ru(II)-polypyridine complexes and 4,4 -bipyridinium units are held together noncovalently in threaded and interlocked structures have been extensively studied [43, 82-88]. In such assemblies, connections between the molecular components rely on charge transfer interactions between the electron acceptor bipyridinium units and aromatic electron donor groups (Fig. 3). For instance, in the various pseudorotaxanes formed in acetonitrile solution at 298 K by the threading of cyclophane 4 + by the dioxybenzene-containing tethers of 192+ (Fig. 17) [84], an efficient photoinduced electron... 

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