Photosynthetic systems mimics

An important class of porphyrins is that constituted by confor-mationally distorted porphyrins, which mimic the non-planar geometry of the porphyrins present in photosynthetic systems.89 Obtainment of such non-planar distortions is associated with the introduction into the macrocyclic frame of proper crowding substituents, which therefore not only cause structural distortion but also affect, through their electronic effects, the redox potentials. A typical case is that constituted by [Cun(OETPP)] (OETPP = 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin), the saddle-distorted molecular structure of which is illustrated in Figure 56.102... 

This chapter focuses on the progress and challenges in the field of photocatalysis as applied towards the water splitting reaction (Eq. 1.). More specifically, homogeneous molecule based systems that mimic the natural photosynthetic system are examined for their potential to drive reaction 1. A number of molecules, including porphyrins, metalloporphyrins and phthalocyanines,17 transition metal complexes of Ru, Os, Re, Rh, Pt, Cu,811 and acridine and flavin derivatives,1214 have been examined as the chormophores and sensitizers for light driven processes. 

This controlled dye array can be mimicked using thin film preparation techniques. The LB method is the technique most suited to layering various dye molecules in a desired sequence and at desired distances. Figure 6.8 shows hetero-type LB films that can mimic the dye array in the photosynthetic system of a cyanobacterium. Detailed time-resolved fluorescence spectra upon... 

Figure 6.8. LB film as a mimic of the dye array in the photosynthetic system of a cyanobacterium...

This chapter reviews the work ofthe last five to six years on paramagnetic states of carotenoids using electron magnetic resonance. Mainly radical cation and neutral molecular triplet states are treated. Part of this ch te deals with paramagnetic states of carotenoids in model systems. These have been synthesized in order to mimic both electron and energy transfer processes in the natural photosynthetic systems. Consequently, the electron magnetic resonance (EMR) spectroscopy of carotenoid triplet and radical states yields important information about their photochemistry. Finally, the EMR spectroscopy on carotenoid radicals is reviewed. It serves to establish the database on their intrinsic properties which is necessary for the analysis of carotenoid radicals in vivo. 

Compared with photosynthesis, which photochemical water splitting crudely mimics, the latter has all the elements of the system moving randomly in solution. Here, the various elements of the reaction are free to collide with each other and so lose their energy, or become involved in wasteful side reactions that end up consuming them. Photosynthetic systems, on the other hand, have all the active ingredients bound to cell membranes and positioned so that all the photophysical events (e.g. photon capture and electronic excitation) and their consequences (e.g. electron relay and water photooxidation) occur as quickly and as efficiently as possible. An... 

The use of multiple hydrogen bonding also allows for the assembly of higher-order supramolecular structures that begin to mimic the complexity of biological arrays. In the bacterial photosynthetic system, many antenna... 

The assembly of molecular components for light harvesting and charge separation in artificial photosynthetic systems is of current interest. To mimic the multistep electron transfer in natural systems, Imahori et al. prepared a mixed SAM that combines an artificial antenna system (pyrene) with an artificial reaction centre (porphyrin), to examine the possibihty of photoin-duced energy transfer in two-dimensional assemblies (Fig. 29, 20) [27,147]. The ratio of porphyrin pyrene in the mixed SAM estimated from the absorption spectra on the gold surface is significantly lower than that of the solution from which the SAM was prepared. The strong tz-tz interaction of... 

Moreover, supra-molecular systems involving crown ethers, fullerene and k-extended systems have been achieved that can mimic the photosynthetic process [9-14]. The fullerene Qo has been used successfully as an electron acceptor in the construction of model photosynthetic systems [9], the r-extended systems, such as porphyrins [12], phthalocyanines [13], r-extended tetrathiafulvalene (w -exTTF) derivatives [9,10], which are utilized as electron donors, while the crown ethers act as a bridge between the electron donor and acceptor. In the absorption spectrum of the complexes, the absorption maxima are associated experimentally and theoretically with the formation of charge-transfer states [14-16]. Consequently, these supramolecular systems have potential for applications in photonic, photocatalytic, and molecular optoelectronic gates and devices [9-14]. As a result, the study of the conformations and the complexation behavior of crown ethers and their derivatives are motivated both by scientific curiosity regarding the specificity of their binding and by potential technological applications. 

A mimic system of a photosynthetic apparatus was realized by solubihzing C-phy-cocyanin and zinc phthalocyanine in reversed micelles of tween-80 dispersed in cyclohexane [63]. 

There have been many attempts to mimic some features of photosynthesis with abiotic systems for purposes of artificial solar energy conversion. Ideally a fuel, e.g., H2, is formed through a photosynthetic process. Photolysis of water is a highly endergonic process ... 

Li and coworkers49 reported a molecular motion of /1-carotene and a carotenopor-phyrin dyad (composed of a porphyrin, a trimethylene bridge and a carotenoid polyene) in solution. Internal rotational motions in carotenoid polyenes and porphyrins are of interest because they can mediate energy and electron transfer between these two moieties when the pigments are joined by covalent bonds. Such internal motions can affect the performance of synthetic model systems which mimic photosynthetic antenna function,... 

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