Photoactive Membranes

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, 

DPL Intermediate acceptors (alloxazine or vitamine Kj), when inserted into the membrane, increase 4 . Two-step photoactivation of electron transfer. (ZnC12TPyP+) = 100 (AF) 63 

DPL Two-step photoactivation of electron transfer at simultaneous excitation of both photosensitizers in the membrane 64,65 

DHP Mn(III) reduction to Mn(II) is conjugated with DHP membrane oxidation. No electron transfer without PVS 70 

DHP - Only indirect evidence for transmembrane electron transfer was obtained 78 

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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. 

Only a few recent studies have dealt with the use of photoactive membranes prepared by immobilization of titania particles in polymeric membranes or deposition of porous titania coatings. " They were concerned with photooxidation applications like antifouhng or elimination of small organic molecules that cannot be stopped by conventional membrane treatments, but which are very harmful to the environment, like VOCs. 

Both PSI and PSII are necessary for photosynthesis, but the systems do not operate in the implied temporal sequence. There is also considerable pooling of electrons in intermediates between the two photosystems, and the indicated photoacts seldom occur in unison. The terms PSI and PSII have come to represent two distinct, but interacting reaction centers in photosynthetic membranes (36,37) the two centers are considered in combination with the proteins and electron-transfer processes specific to the separate centers. 

Systems that contain the photocatalyst as a suspended powder or colloid are not convenient in continuous flow or circulation arrangements in which the photoactive system is held in place and can readily be removed for regeneration. CdS can be produced or immobilized in a Nafion membrane together with finely dispersed platinum... 

Regarding the design of PET across membranes, the systems studied can be divided into two large groups (i) — the systems with the photosensitizer embedded into vesicle membrane and (ii) - the systems with the photosensitizer located outside the membrane. These two types of membranes can be named as photoactive and photopassive, respectively. 

PS photo current One-quantum PET across membrane. Zn-TPP-Ac is 30 times more photoactive than ZnTPP 96... 

H]azidopine (51), a radioactive photoactive DHPs calcium channel blocker, photolabels Pgp in membrane vesicles from KBC1 cells. This pho-... 

FIGURE 25.14 Schematic representation of the coupled separation and photodegradation of VOCs in the treatment of wastewater using a photoactive low-ultrafiltration membrane. 

Ceramic membranes are the most often used asymmetric membranes. When the separative layer, which is usually in contact with the feed, is also photoactive, irradiation must be applied on this top layer. A second configuration can also be considered. It consists of a conventional asymmetric membrane without photoactive separative layers but with a photoactive coating deposited on the surface of the grains of the support. In this case, the irradiation is apphed on the opposite side of the membrane, in contact with the permeate. Such a configuration could be used for instance in the final treafment of wastewater with a low-ultrafiltration membrane which provides retention of colloids and macromolecules, whereas small unretained molecules like VOCs would be photo-oxidized on the other side of the membrane (Figure 25.14). 

As seen in Table 7.1 and 7.2, there are, in addition to these two examples, several other photoactive cells in which we have identified cubic membranes. In the case of the mitochondrial cubic membrane in Tupaia gUs we can speculate that the use of an isotropic structure allows efficient capture of the incoming light, which has to pass the giant mitochondria before reaching the outer segment. Similar reasons may guide the choice of a cubic membrane system in the lens of certain scaleworms. 

Ammonium salts with two different alkyl chains were prepared directly via subsequent alkylations of dimethylamine with primary bromides and crystallization. Commercial hexadecyl-methylamine can be conveniently applied in the same way in order to convey functionality to cationic synkinons. A recent example describes subsequent alkylations with a small functional and a long-chain primary bromide (Scheme 2.4). A-acylated / -phenylenediamine was also alkylated at the second nitrogen atom which had two different alkyl chains, with or without extra functionality . After deacylation, this head group can be diazotized or coupled oxidatively with various heterocycles in water (Scheme 2.4). Photoactive and coloured membrane surfaces are thus obtained. Phenylene-diamine, pyridine and in particular A-methyl-4,4-bipyridinium chloride are relatively weak nucleophiles. Substitution of bromides is slow and the more reactive iodides can rarely be obtained commercially, but the selection of nitromethanes as solvent for bromide substitution is of great help as well as the addition of sodium iodide to enforce a Finkelstein reaction or a combination of both. 

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