Chromophoric assembly

From a functional point of view, this important property can be readily built into low molecular weight chromophore assemblies acting as artificial reaction centers (coordination compounds, the population of CT states is directly related to the concept of light-induced charge separation in photosynthesis. Whenever such CT states are photoreactive and lead to the formation of the same kind of permanent redox products as observed in photosynthesis, the most essential features of the primary light reactions have been successfully duplicated. In a more strict sense, this is of course only true, if actinic red or NIR-light of comparable wavelength is absorbed by both the natural and artificial photosynthetic systems. 

Fig. 19.10. Schematic diagram of nanostructured-semiconductor-redox chromophore assembly for electrochromic applications.

The promotion of one electron to an excited state upon light absorption generates an electron-hole pair in the frontier orbital system of the porphyrinic chro-mophore. This electron-hole pair is an exciton, and its generation introduces a nonpermanent (transitory) dipole in the chromophore. The energy transfer process is strongly related to the interactions between excitons in multiple chromophore assemblies. 

Another way of templating the chromophore assembly, based on the copper(I) assisted synthesis of catenanes and rotaxanes developed by Dietrich-Buchecker and Sauvage in the 1980s, has led to the rotaxanes 118-120 represented in Figure 13.62. In this case, excitation of either the zinc or the gold porphyrins leads to photoinduced charge transfer to generate [(ZnP)2] -AuP . 

Figure 13.67. As with most of the approaches in this section, the design of the assembly very closely resembles the features of LHi and LH2 natural systems. Aggregates of zinc bacteriochlorins are trapped in a lecithin or Triton-X bilayer to generate chromophore assemblies with spectroscopic behavior that resembles that of the bacteriochlorophyll c aggregates in the antennae systems The aim of the work is indeed to mimic the function of chlorosomes. Chlorosomes are aggregates of BChl c, d, and e that are embedded in lipid monolayers.

The amazing structure with precise molecular dignment and reaction mechanism of energy and electron transfer in natural photosynthesis undoubtedly affords one of the most promising examples for photochemically active supramo-lecular systems. In the following section, recent approaches involving molecular systems linked by covalent bonds and chromophoric assemblies will be surveyed. 

C. Excited Energy Transfer In Chromophoric Assemblies and Supramolecular System... 

Among intensive studies on linked molecular systems, chromophoric assemblies, and supramolecular systems, most efforts have been focused on electron transfer processes in those systems [460,468,470-478,501]. As described in Section in.B, the inverted region predicted by the Marcus theory on electron transfer was found in the linked molecular system having donor and acceptor connected with the appropriate spacer group to verify the theory [479,480,488,490,530,540,733-735]. The inverted region was also observed in various linked molecular systems having porphyrins andmetalloporphyiins as shown in Fig. 53 (25,26) [481-484]. 

On the basis of extensive challenging approaches for chromophoric assemblies and supramolecular systems, supramolecular devices such as photoresponsive organic conductors and photochemically acdvated molecular wire have been proposed. 

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