Viologen vesicles

The recombination between Ru(bipy)j and viologen radicals nearby the inner surface of the vesicle is found to be strongly inhibited as compared to homogeneous solution, presumably due to a fast extraction of hydrophobic viologen radicals into the depth of the membrane. 

It proves possible to anchor catalysts of H2 evolution to the outer and inner surface of the vesicle membrane. These catalysts are finely dispersed (10-20 A in diameter) metal Pt or Pd particles formed via reduction of appropriate salts in vesicle suspension (see [15, 16] and refs, therein). Among the viologen-type electron carriers a promising one is p-bis (1,2,5-triphenyl-4-pyridil)benzene which possesses reduction potential low enough for water reduction at neutral pH. Recently, using this mediator we succeeded in H2 evolution conjugated with PET... 

On sonication, surfactants (4, 5) form vesicles which are polymerized by an initiator or by UV irradiation across either their bilayers or their head groups depending on the position of the double bond (Fig. 8). The polymeric vesicles are stable for extended periods even in 25 % C2H5OH. Efficient charge separation has been realized in such chemically disymmetrical polymerized vesicles. Photoexcitation of Ru(bpy)2 + placed on the outside of the vesicle resulted in the formation of long-lived reduced viologens on the inside. 

Fig. 8. Proposed formation of polymeric surfactant vesicles in which the outer redox-active viologen groups have been removed by nucleophilic cleavage of the ester bond...

Dissymmetrical SUVs can be formed by limiting reactions to the outer surfaces of polymerized surfactant vesicles (Fig. 39). Chemical dissymmetry has been created, for example, in polymerized vesicles prepared from surfactants containing ester-linked viologen moieties in their headgroups. Cleavage of the... 

Finally, a number of studies have employed viologens in the bilayer, either added as an amphiphilic reagent to DPPC,340 or as the sole component of the vesicle walls.337,341 342 In the latter cases, double bonds in the hydrocarbon chain of the viologen have been polymerized to give added stability and rigidity to the bilayer structure. 

Table 2. Rate constants of transmembrane electron transfer (k,) by viologen radical cations, their recombination with Ru(bpy)j+ in homogeneous solution (k ) and inside the vesicle cavity (kr and kb) and efficiencies of spatial charge separation (t ) for viologens with various substituents [201]...

Fig. 5. Viologen-mediated electron transport across the vesicle membrane (a) — scheme of the process (b) — variations of viologen radical cation concentration in the presence (/) and the absence (2) of 0.75 mol/1 K3Fe(CN)6 in the inner cavity. Sodium dithionite concentration is 0.01 mol/1. The molar ratio viologen lipid =1 20...

Hurst and co-workers [172, 180-185] extensively studied the mechanisms of dark electron transfer by alkylmethylviologen radical cations across the membranes of DHP vesicles. The experiments carried out and the results obtained are quite similar to those described above. In particular, it has been demonstrated that during transmembrane electron transfer in steady-state conditions viologen radical cations migrate from one side of the membrane to the other with the rate constant kt = 2 x 10-2 s-1. 

Sometimes electron transfer via the transport of electron carrier and via electron exchange reactions occur simultaneously. Co-existence of both these channels was observed for dark electron transfer across the viologen-containing vesicle membrane [169, 201]. To illustrate this let us turn back to the experiments shown schematically in Fig. 5 a. In accordance with the reaction sequence (34)-(37) and... 

Cationic vesicles have been used to accomplish charge separation (Mon-serrat and Gratzel, 1981). The photosensitiser was a water-soluble porphyrin and electron acceptor was a modified, water-soluble viologen. The porphyrin photo-reduced the viologen which in its reduced form is lipid soluble but water insoluble. Consequently, the reduced species enters the vesicle. So effective is the charge separation that multimer formation of the reduced species in the vesicle can be observed. Another method which has been employed is to immobilise donors and acceptors on the surface of latex particles (Frank et al., 1979). 

DHP vesicles. Top left, surface adsorption at the membrane interface top right, intercalation within the bilayer as monomeric units bottom, aggregation within the membrane bilayer. The average separation distance of the DHP head groups is 7.5 A 14a] and the distance between the quaternary nitrogen atoms of the viologen is 7.0 A. 

Net reaction between hydrophilic oxidants and reductants separated by bilayer membranes has been convincingly demonstrated in numerous asymmetrically organized vesicle and planar bilayer systems [2a-c]. Particularly compelling examples are those in which electrogenic electron transport is accompanied by net translocation of lipophilic cations or anions, as is required to maintain electroneutrality. For example, MV + occluded within the inner aqueous phase of DHP vesicles can be completely reduced to the MV+ radical cation by 8204 ion in the bulk phase, but only if an equal or greater amount of viologen is initially present in the external environment [40]. Neither MV-+ nor 8204 is membrane permeable. The reaction is biphasic, with rapid bimolecular reduction of external being followed by... 

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