Photochemical Conversion Models

A model proposed for photochemical conversion of solar energy 11,14) is shown in Fig. 3. The system is made of a photoreaction couple, two kinds of electron mediator, and reduction as well as oxidation catalysts. It is designed to share the necessary functions among the various compounds because it would be difficult for one single compound to bear all the functions. A single component carrying out the total conversion would of course be the best system. 

Pi and P2 are the photochemical reaction center serving also as light-harvesting unit. They can be two kinds of compounds or a single compound (P) such as a metal complex. The photoreaction center must have a strong absorption in the visible region. Tt and T2 are the electron mediators which take out photochemically separated charges rapidly to prevent back reactions. C and C2 are the reduction and oxidation 

In the water photolysis system, the potential of Q should be lower than —0.41 V and that of C2 higher than 0.82 V. For the proton reduction, two electron process (Eq. (4), E0 = —0.41 V) is much more favorable than the stepwise reaction in which the first step (Eq. (5)) 

In the photochemical conversion model (Fig. 3), the most serious problem is the undesired and energy-consuming back electron transfer (shown as dotted arrows) as well as side electron transfer, e.g., the electron transfer from (Q) to (T2)ox. It is almost impossible to prevent these undesired electron transfers, if the reactions are carried out in a homogeneous solution where all the components encounter with each other freely. In order to overcome this problem, the use of heterogeneous conversion systems such as molecular assemblies or polymers has attracted many researchers. The arrangement of the components on a carrier, or the separation of the Tj—Q sites from the T2—C2 ones in a heterogeneous phase must prevent the side reactions of electron transfer. 

The possibilities of the two kinds of back electron transfer can be diminished to one by selecting the reaction components. When the excited state of P is quenched oxidatively by T, the only possible back electron transfer is from (T,)red, to (P)M. On the contrary, when P is quenched reductively by T2, the back electron transfer to be considered is only from (P)rcd to (T2)ox. In either case, the back electron transfer can be prevented by a molecular design based on reaction dynamics. For the 

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In this review article, the functions of polymers and molecular assemblies for solar energy conversion will be described including photochemical conversion models, elemental processes for the conversion such as charge separation, electron transfer, and catalysis for water decomposition, as well as solar cells. 

An alternative approach, using semiconductors as light-driven electron donors, has been demonstrated in model systems (Gratzel 1982 Nikhandrov et al. 1988). These are more stable than the photosystems, but show lower photochemical conversion efficiencies owing to short-circuiting of reducing equivalents. The presently used... 

Fig. 3. A model system for photochemical conversion of solar energy. R2 Reducing agent, R, Oxidizing agent C2 Oxidation catalyst Cj Reduction catalyst T2, T, Electron mediators P2-Pj = P Photoreaction center...

Since the reduction potential of MV2+/MV is low enough (—0.44 V at pH 7) to reduce protons, the presence of platinum as a catalyst in the solution containing MV 7 brings about hydrogen formation. Scheme 1 is a typical model of photo-induced charge separation and electron relay to yield H2. It also represents the half reaction cycles of the reduction site for the photochemical conversion shown in Fig. 3. 

L. Rizzuti and A. Brucato, Recent Developments in Heterogeneous Photoreactor Modelling, in Photochemical Conversion and Storage of Solar Energy, E. Pelizzetti and M. Schiavello, Eds., Kluwer Academic Publishers, Dordrecht, 1991, p. 561, and literature cited therein. 

Although water photolysis is the simplest photochemical conversion system, carbon dioxide reduction is still an attractive research subject as a synthetic model for C02 reduction in photosynthesis. There are numerous reports on chemical and photochemical C02 reduction,245 but it is not the aim of this chapter to review these works. 

The lower part of Figure 3.1 shows a simplified model of the excited states. Only two excited states are represented, but each represents a set of actual levels. The lifetimes of all these levels are assumed to be very short in comparison of those of the two excited states, and form the cross section for absorption of one photon by the trans and the cis isomers, respectively. The cross sections are proportional to the isomers extinction coefficients, y is the thermal relaxation rate it is equal to the reciprocal of the lifetime of the cis isomer (x ). tc and ct are the quantum yields (QYs) of photoisomerization they represent the efficiency of the trans->cis and cis—>trans photochemical conversion per absorbed photon, respectively. They can be calculated for isotropic media by Rau s method, which was adapted from Fisher see Appendix A) for anisotropic media, they can be calculated by a method described in this chapter. Two mechanisms may occur during the photoisomerization of azobenzene derivatives—one from the high-energy 7C-7t transition, which leads to rotation around the azo group, i.e., - M=N-double bond, and the other from the low-energy transition, which... 

FIGURE 9.2 Simplified model of the molecular states. Only two excited states have been represented, but each of them may represent a set of actual levels We only assume that the lifetime of all these levels is very short. cross sections for absorption of one photon by a molecule in the trans or the ds state, respectively. is the thermal relaxation rate. and are the quantum yields of photoisomerization and represent the probability per absorbed photon of the photochemical conversion. 

To summarize, in the present section, we have demonstrated that the stilbene photoisomerization is a training area, a relatively simple and convenient model reaction for a thorough investigation of detailed mechanisms of photochemical reactions and factors affecting the photochemical conversion rate. Theoretical and experimental data in this area vdll pave the way for practical application of stilbenes as switching materials and biophysical probes (Chapter 10). 

The latest development is now to combine continuous photochemistry with microstmctured equipment. Only very recenfly photochemical conversions in microreactors have received a considerable amount of attention due to the problem often encountered in conventional photoreactors that the distribution of radiation is inhomogeneous in the reaction zone. During the scale-up process, such inhomogeneities often require intensive modeling and design considerations usually on the basis of photon transport models [66], and such models have been, for example, developed for biomedical and analytical purposes [67]. The problem of the intensity distribution in a reactor is illustrated in Figure 3.10. It is obvious that spatial restriction of the irradiation zone in a microphotoreactor to a... 

To understand the fundamental photochemical processes in biologically relevant molecular systems, prototype molecules like phenol or indole - the chromophores of the amino acids tyrosine respective trypthophan - embedded in clusters of ammonia or water molecules are an important object of research. Numerous studies have been performed concerning the dynamics of photoinduced processes in phenol-ammonia or phenol-water clusters (see e. g. [1,2]). As a main result a hydrogen transfer reaction has been clearly indicated in phenol(NH3)n clusters [2], whereas for phenol(H20)n complexes no signature for such a reaction has been found. According to a general theoretical model [3] a similar behavior is expected for the indole molecule surrounded by ammonia or water clusters. As the primary step an internal conversion from the initially excited nn state to a dark 7ta state is predicted which may be followed by the H-transfer process on the 7ia potential energy surface. 

Apart from the possible use of polymerized vesicles as stable models for biomembranes (Sect. 4) there may be a variety of different applications. Polymerized surfactant vesicles have been proposed to act as antitumor agents on a cellular level33 in analogy to the action of the immune system of mammals against tumor cells 85). Polymerized vesicles open the door to chemical membrane dissymmetry 22) which in turn, may lead to enhanced utility in photochemical energy transfer84 (solar energy conversion, artificial photosynthesis). The utilization of unpolymerized lipo-... 

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