Mimicking photosynthetic process

There are several photocatalysts mimicking hydrogenase activity that are not based on metalloporphyrin systems. Among them there are mixed-valence complexes of rhodium or iridium, [41] as well as complex systems encompassing photosensitizers (eg ruthenium complexes) attached to a catalytic bimetallic centre [43], The design of more sophisticated systems approaches that of photosynthetic processes [44],... 

Even a relatively simple bacterial photosynthetic system is very complex and its synthetic imitation is a challenging task. Mimicking of the natural photosynthetic process requires synthetic models of all the crucial components and linking them together into a working molecular assembly. All the elements (antenna, charge separation, and reaction centres) may involve transition metals. Application of metal complexes facilitates mimicking of this complex chemical system due to rich and versatile photochemical processes typical for transition metal complexes (see section 6.4 in Chapter 6) [48]. 

The ability of membranes to compartmentalize reagents and control the permeation of chemical species may also allow the control of electron transfer in a more sophisticated way within the aggregate bilayer [86]. Photosynthetic processes occur specifically in membranes [87] (thylakoid membranes) so there is continuous interest in mimicking these phenomena with synthetic vesicles [86]. Though a large amount of information is available on the components of biological systems that operate electron transport, the actual mechanism of the process is far from being understood in detail. 

The second article also deals with PET in arranged media, however, this time by discussing comprehensively the various types of heterogeneous devices which may control supramolecular interactions and consequently chemical reactions. Before turning to such applications, photosynthetic model systems, mainly of the triad type, are dealt with in the third contribution. Here, the natural photosynthetic electron transfer process is briefly discussed as far as it is needed as a basis for the main part, namely the description of artificial multicomponent molecules for mimicking photosynthesis. In addition to the goal to learn more about natural photosynthetic energy conversion, these model systems may also have applications, which, for example, lie in the construction of electronic devices at the molecular level. 

Two general approaches of modeling photosynthesis can be considered, one involving mimicking the functions of the photosynthetic reaction center by means of synthetic analogs [25-27]. To this extent the synthesis of linked multicomponent donor-acceptor assemblies could lead to charge separation by means of sequential ET processes as outlined in Eqs. (1) to (4), where S is the light-active component and A and D represent electron acceptor and donor units, respectively. 

Figure 4.15 (a) Mimicking of natural photoinduced processes for designing optobioelectronic systems. Schematic configurations Photo-bioelectrochemical cells based on photoanodes consisting of photosynthetic reaction centers (b) A PS I-based cell, (c) A PS Il-based cell. 

Stemming from their multifarious roles in natural processes, [metallo] porphyrins have found numerous applications in artificial systems aimed at mimicking important biological functions. Many different metalloporphyrins have been designed in order to accomplish specific tasks and, in particular, novel approaches have been used to assemble several porphyrins into a cluster. This ability to concentrate metalloporphyrins into a supramolecular assembly is of special relevance in that it takes us one step closer to constructing practical devices. This chapter will attempt to review the progress made in the assembly of porphyrin derivatives into supramolecular systems and will describe the aptitude of such assemblies to photosensitize particular reactions. The work described here is primarily concerned with trying to reproduce, under controlled conditions, some of the important features of photosynthetic reaction center complexes. 

Typically, those ferrocenyl-porphyrins have been explored in photoinduced electron transfer and charge separation processes mimicking the activity of the photosynthetic system, as well as in the development of catalysts for multielectron transfer reactions. Ferrocenyl substituents were shown to enhance the electrocat-alytic activity of cobalt [41, 42], iron [43], and copper [44—46] porphyrins for tetraelectronic reduction of dioxygen to water. 

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