Vesicle photosensitized electron transfer

Surfactant vesicles constitute a very flexible medium for the support of semiconductors. Semiconductor particles can be localized at the outer, the inner, or at both surfaces of single-bilayer vesicles (Fig. 102). Each of these arrangements has certain advantages. Semiconductor particles on outer vesicle surfaces are more accessible to reagents and can, therefore, undergo photosensitized electron transfer more rapidly. Smaller and more monodispersed CdS particles can be prepared and maintained for longer periods of time in the interior of vesicles than in any other arrangement... 

The separation of photoproducts formed in photosensitized electron transfer reactions is essential for efficient energy conversion and storage. The organization of the components involved in the photo-induced process in interfacial systems leads to efficient compartmentalization of the products. Several Interfaclal systems, e.g., lipid bllayer membranes (vesicles), water-in-oil mlcroemulslons and a solid SIO2 colloidal Interface, have been designed to accomplish this goal. 

The photosensitized oxidation of DPB in vesicles in the first mode gave 1 and 2 as the unique products (Fig. 22). We believe that these products are derived from the singlet-oxygen pathway. In contrast, the photosensitized oxidation of DPB in vesicles in the second mode only produced the electron-transfer-mediated products 1,2,3, and 5 (Fig. 22). No singlet-oxygen products were detected. These observations demonstrate, once again, that one can control the selectivity in pho-... 

The proposed mechanism of electron transfer across Chl-containing membranes of vesicles in A // Chi // D (i.e. for systems containing Chi as a photosensitizer in the membrane and donor, D, and acceptor, A, particles outside and inside the vesicle, respectively) and D // Chi //A systems was outlined in early papers [42,43,... 

Experimentally, electron transfer across vesicle membranes with an asymmetrically embedded photosensitizers was first observed in System 17 of Table 1. Katagi et al. [64, 65] succeeded in embedding a photosensitizer (ZnC18TMPyP3+) into the bilayer membrane both uniformly and selectively in its outer monolayer, i.e. asymmetrically. In the latter case no electron transfer across the membrane took place until the other photosensitizer (ZnTPP) was introduced into the membrane uniformly. The proposed mechanism of electron transfer involved two photochemical steps ... 

For the systems with photoactive membranes discussed in the previous section the photosensitizer embedded into the vesicle membranes not only participated in photochemical and dark redox reactions with substances which are located in water phases on both sides of the membrane, but also served as the carrier of the electron across the membrane. In the presence of the appropriate electron carrier which is capable of penetrating through the membrane core it is also possible to perform electron transfer between membrane-separated water phases when photosensitizers are located in these phases rather than in the membrane. Membranes containing no photosensitizers can be called photopassive ones since no photophysical and photochemical processes occur in them, and their role is only to (i) provide electron transfer from one water phase to the other leading to the formation of spatially separated oxidant and reductant and (ii) to suppress recombination reactions. 

In natural photosynthesis the quinones are widely used as electron carriers. Unfortunately, the low values of cpc in the reaction of quinones with 3Ru(bpy) + make the direct use of these important electron carriers rather inefficient. However, introduction of the electron carrier Rh(bpy) + into the inner volume of the vesicle in addition to photosensitizer Ru(bpy) +, provides much more efficient electron transfer from 3Ru(bpy) + to a quinone embedded into the membrane. This was found for System 25 of Table 1. 

Two celebrated early investigations of transmembrane oxidation-reduction were interpreted in terms of direct electron exchange between redox partners bound at the opposite vesicle interfaces. One involved apparent reduction of diheptylviologen [( 7)2 V +] in the inner aqueous phase of phosphatidylcholine liposomes by EDTA in the bulk phase that was mediated by membrane-bound amphiphilic Ru(bpy)3 + analogs the ruthenium complexes acted as photosensitizers and were presumed to function as electron relays by undergoing Ru(II)-Ru(III) electron exchange across the bilayer [105]. The other apparently involved direct electron transfer between photoexcited Ru(bpy)3 + and bound at the opposite interfaces of asym-... 

An intramolecular excited state proton transfer occurs on irradiation of hypericin 12gi36-i39 Excitation of hypericin in lipid vesicles results in excited state regioselective transfer of a proton to the substrate from one of the peri-hydroxyl groups . Hypericin in its triplet state reacts with reducing agents to afford a long-lived transient presumed to be the resultant radical anion ". Both electron donors and acceptors can quench the fluorescence of hypericin . A detailed review of the reactions of the photosensitizer hypericin has been published. Some of the work described dealt with its photochemical deprotonation in the excited state. ... 

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