Donor-acceptor photosynthetic model

Gust, D, and Moor, T.A. (eds) (1989) Covalently linked donor-acceptor photosynthetic model systems. Tetraedron 45 (special issue)... 

In artificial photosynthetic models, porphyrin building blocks are used as sensitisers and as electron donors while fullerenes are used as electron acceptors. Triads, tetrads, pentads and hexads containing porphyrins and Qo have been reported in the literature (see the Further Reading section). 

Reaction of 217 with Cjq leads to the amino-protected porphyrin-fulleropyrroli-dine, which can easily be deprotected to the corresponding amine [229, 277]. By further functionalization via amide coupling an easy access to extended donor-acceptor systems is possible. A carotene-porphyrin-fullerene triad was prepared by reaction of the amine with the appropriate carotene acid chloride. The motivation for the synthesis of all these donor-acceptor systems is the attempt to understand and imitate the photosynthetic process. On that score, a model for an artificial photosynthetic antenna-reaction center complex has been achieved by attaching five porphyrin cores in a dendrimer-like fashion to the fullerene [242]. 

Several approaches to artificial photosynthesis involve the mimicking of membranes to effect charge separation. An easy extension of the micellar effects described above to systems amenable to study as photosynthetic models can be encountered in the charge separation derived on synthetic vesicles or membranes (275). Sonic dispersal of long chain ammonium halides, phosphates, sulfonate, or carboxylates produces prolate ellipsoidal vesicles with long term stabilities which can entrain and trap molecules in their compartments. With donor-acceptor photosystems, four physical arrangements about the vesicle are important, Fig. 6. 

Photosynthetic model systems have recently been exhaustively reviewed elsewhere [5, 6, 218] and a number of results are given in the latest literature [219-224]. The attention of the researchers is focused on topics such as electron-transfer chain and energy dissipation within models (the first step is the transfer of an electron from a metallotetrapyrrole moiety yielding a cation radical) the dependences of the electron-transfer rate constant on the driving force of the process distance and mutual orientation of donor and acceptor sites influences of membranes and medium (solvent) properties, etc. 

One of the simplest kinds of photosynthetic model systems which may be envisaged is a solution of a photochemically active pigment and an additional electron donor or acceptor. The following sorts of reactions can be studied ... 

In conclusion the molecules l(n) consitute a series of model systems that not only provides donor-acceptor separations similar to those found in the photosynthetic unit, but also mimics the high rate and the temperature independence of the primary steps in photosynthesis. 

As demonstrated in this chapter, the binding of metal ions to maclocyclic ligands (e.g., porphyrins) results in the change in both the thermodynamic and dynamic properties of ET reactions of metalloporphyrins. Excellent models of the photosynthetic reaction center were developed by the appropriate choice of combination of metal ions and macrocyclic ligands. The lifetimes of the CS states in models of photosynthetic reaction center composed of electron donors and acceptors also were controlled by binding of metal ions to radical anions of electron acceptor moieties in the electron donor-acceptor hnked molecules. The control of ET processes by coordination of metal ions to the dyads led us to develop a unique fluorescence sensor for the ion. The binding of metal ions to radical anions of electron acceptors results in acceleration of thermal ET reactions, which would otherwise be impossible to occur. Such effects of metal ions to enhance the ET... 

Two general approaches of modeling photosynthesis can be considered, one involving mimicking the functions of the photosynthetic reaction center by means of synthetic analogs [25-27]. To this extent the synthesis of linked multicomponent donor-acceptor assemblies could lead to charge separation by means of sequential ET processes as outlined in Eqs. (1) to (4), where S is the light-active component and A and D represent electron acceptor and donor units, respectively. 

The most simple kind of photosynthetic model systems consists of a chromophore linked to an electron acceptor or donor. For example, the porphyrinquinone diad 1 represents a model of the primary [32] photosynthetic reaction center. Various diads have been synthesized and their photophysical properties have recently been reviewed [33]. The general strategy of multistep ET in triad and multifunctionalized molecular assemblies provides a useful means for maximizing the quantum yields of charge-separated states and the lifetime of the redox intermediate states. Evidently, various molecular assemblies following this strategy have been exploited. 

In order to observe the important structure-dependent anisotropic spin-spin interactions, such as the dipolar interaction, D, within radical pairs and to prevent spin lattice relaxation from destroying the spin polarization, it is necessary to examine the radical pairs in the solid state at low temperatures. Photosynthetic model systems based on chlorophyll or porphyrin electron donors have the interesting, but unfortunate property that the efficiency of their light-initiated, singlet state electron transfer reactions is negligibly low whenever they are dissolved in solid solutions. Stated more precisely, the decay rates of chlorophyll and porphyrin excited singlet states are much faster than the rates of electron transfer from these donors to most electron acceptors in the solid state. 

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