Photoinduced charge separation systems

This review article attempts to summarize and discuss recent developments in the studies of photoinduced electron transfer in functionalized polyelectrolyte systems. The rates of photoinduced forward and thermal back electron transfers are dramatically changed when photoactive chromophores are incorporated into polyelectrolytes by covalent bonding. The origins of such changes are discussed in terms of the interfacial electrostatic potential on the molecular surface of the polyelectrolyte as well as the microphase structure formed by amphiphilic polyelectrolytes. The promise of tailored amphiphilic polyelectrolytes for designing efficient photoinduced charge separation systems is afso discussed. 

Microenvironmental effects such as electrostatic or hydrophobic ones provided by molecular assemblies or polymers on the photoinduced charge separation and electron relay are described in this chapter together with the solid phase and macroheterogene-ous photoinduced charge separation system constructed with polymer solids. 

Recently, photochemical and photoelectrochemical properties of fullerene (Cto) have been widely studied [60]. Photoinduced electron-transfer reactions of donor-Qo linked molecules have also been reported [61-63]. In a series of donor-Cfio linked systems, some of the compounds show novel properties, which accelerate photoinduced charge separation and decelerate charge recombination [61, 62]. These properties have been explained by the remarkably small reorganization energy in their electron-transfer reactions. The porphyrin-Qo linked compounds, where the porphyrin moieties act as both donors and sensitizers, have been extensively studied [61, 62]. 

The second example (Figure 9) of orbital symmetry effects concerns photoinduced charge separation in the norbornylogous dyads, 5(7) and 6(7), which is symmetry forbidden in the former system but symmetry allowed in the latter. In these systems, orbital symmetry effects are more pronounced than in 2(8) and 4(8), amounting to a 28-fold modulation in the magnitude of Vj201... 

Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. This process is involved in many organic photochemical reactions. It plays a major role in photosynthesis and in artificial systems for the conversion of solar energy based on photoinduced charge separation. Fluorescence quenching experiments provide a useful insight into the electron transfer processes occurring in these systems. 

The ionic field of micells increases the efficiency of photoinduced charge separation. Laser flash photolysis showed a longer lifetime of the e formed by irradiation of a donor molecule (D pyrene, perylene etc.) solubilized in anionic micells such as sodium lauryl sulfate (SDS) than in a non-micell systems 19b). This is why the e is repulsed by the anionic field at the micellar surface into the bulk solution (Eq. (10)). 

In such vesicle systems, the electrons are transported through the membrane. Electron carriers such as quinones or alloxazines in the vesicle wall enhance remarkably the rate of photoinduced charge separation. The vesicle system shown in Fig. 6 contains the surfactant Zn-porphyrine complex (ZnC12TPyP) in the wall 23). 

More recently, the second-generation molecular shuttle 374+ (Fig. 13.32) was designed and constructed.38 The system is composed of two devices a bistable redox-driven molecular shuttle and a module for photoinduced charge separation. In the stable translational isomer, the electron-accepting cyclophane 124+, which is confined in the region of the dumbbell delimited by the two stoppers Tj and T2, encircles the better electron donor tetrathiafulvalene (TTF) station. 

Fig. 1.1 Photoinduced charge separation by (a) semiconductor particle and (b) a system composed of sensitizer with electron donor and/or acceptor.

Such a photoinduced charge separation can proceed effectively provided an electric field (potential gradient) has been established at the position where the primary photoexcitation takes place. In general, a potential gradient can be produced at the interface between two different substances (or different phases). For example, a very thin (ca. 50 A) lipid membrane separating two aqueous solutions inside the chloroplasts of green plants is believed to play the essential role in the process of photosynthesis, which is the cheapest and perhaps the most successful solar conversion system available. 

Thus, a number of processes may take place within supramolecular systems, modulated by the arrangement of the components excitation energy migration, photoinduced charge separation by electron or proton transfer, perturbation of optical transitions and polarizabilities, modification of redox potentials in ground or excited states, photoregulation of binding properties, selective photochemical reactions, etc. 

A design of photocatalytic systems to perform water cleavage is most often based on the following approach. The first stage of the cleavage reaction involves a photoinduced charge separation, i.e. electron transfer from the excited photosensitizer, S, to an acceptor, A ... 

Specifically, energy- and charge-transfer properties of several different molecular-wire systems have been studied within the framework of photoinduced charge separation and solar-energy conversion. Up front, the conductance behavior of wire-like molecules was of particular interest. Such features have been carefully examined in view of possible applications in the fields of molecular electronics and/or photovoltaic devices. Among the tested systems, 7t-conjugation played a crucial role. 

The photocurrent generation in the present system is initiated by photoinduced charge separation from the porphyrin excited singlet state (1H2P /H2P+ = -0.7 V vs. NHE) [78] in the dendrimer to C60 (C60/Cf>0 = -0.2 V vs. NHE) [78] in the porphyrin dendrimer-C60 complex rather than direct electron injection to conduction band of Sn02 (0 V vs. NHE) system [91] The reduced C60 injects electrons into the Sn02 nanocrystallites, whereas the oxidized porphyrin (H2P/H2P+ = 1.2 V vs. NHE) [78] undergoes electron-transfer reduction with iodide (I3 /I = 0.5 V vs. NHE) [78] in the electrolyte system [91]. 

It would appear that the possibilities of obtaining photoinduced charge separation with trinuclear systems containing the Ru(bpy)22+ chromophoric unit are bound to the possibility of (i) using two bridges entailing different degrees of adiabaticity for primary recombination and secondary electron transfer (electronic... 

As reported earlier (Oevering et al., 1987 Warman et al., 1986) intramolecular electron transfer following photoexcitation occurs for all values of n and in a variety of solvents. This electron transfer results in quenching of the typical dimethoxynaphthalene fluorescence for l(n) as compared to that for the isolated donor 2. Determination of the rate constant (k) of this photoinduced charge-separation was achieved in a variety of solvents (Oevering et al., 1987) by comparison of the lifetime of the residual donor fluorescence in l(n) for n= 8, 10, 12 with that of the reference system 2 via eq.(2) ... 

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