Photosynthesis, artificial donor-acceptor separation

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. 

Figure 4.17 Schematic representation of an ideal system for artificial photosynthesis. The fundamental elements are present a light harvesting system, a triad for charge separation (D—P—A, Donor—Primary acceptor—Acceptor), a...

Recently a number of covalently linked porphyrin-quinone systems such as IS (Malaga et al., 1984) or 16 (Joran et al., 1984) have been synthesized in order to investigate the dependence of electron-transfer reactions on the separation and mutual orientation of donor and acceptor. These systems are also models of the electron transfer between chlorophyll a and a quinone molecule, which is the essential charge separation step in photosynthesis in green plants. (Cf. Section 7.6.1.) Photoinduced electron transfer in supra-molecular systems for artificial photosynthesis has recently been summarized (Wasielewski, 1992). 

The theoretical grounds of ET processes are of basic importance to understand natural photosynthesis and to tailor artificial photosynthetic devices. Marcus theory [10,31], summarized in Eq. (6), provides the essence of ET and charge separation. It states that the ET rate constant between a donor and an acceptor depends exponentially on the distance, d, separating the redox components, and on the free-energy change, AG°, and the reorganization energy, 1, associated with the ET process. 

---