Supramolecular systems, energy, electron transfer

The most prevalent photoinduced processes in supramolecular and interfacial systems are electron transfer, energy transfer and nuclear motion, such as proton transfer and isomerization. Before discussing these processes, it is important to outline the fundamental properties of electronically excited states. 

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. 

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. 

In a supramolecular system, electronic energy transfer, as depicted in Fig. 3.1, can be viewed as a radiationless transition between two locally, electronically excited states. Similarly to the electron transfer described above, we deal with two different states. Nevertheless, these states are local excitations lacking any charge transfer. 

Based on their easily tunable photophysical and redox properties, transition metal complexes are versatile components to be used in the construction of photochemical molecular devices. The studies presented in this article show that the combination of the Ru(bpy)22+ photosensitizer and cyanide bridging units allows the synthesis of a variety of polynuclear systems that exhibit interesting photochemical properties. Depending on the nature of the attached metal-containing units, supramolecular systems can be obtained that undergo efficient photoinduced intramolecular energy or electron transfer processes. 

Much more complex functions can be achieved by assembling molecules into supramolecular systems. Upon excitation with chemical species, electrons, and photons, suitably designed supramolecular systems can indeed perform a variety of useful functions related to energy- and electron-transfer processes and to mechanical movements. 

Other researchers have investigated intramolecular energy and electron transfer in supramolecular species based on ruthenium(II) polypyridine systems 153). These supramolecular species could perform the same function as light harvesting structures found in the photosynthetic apparatus. 

For a more exhaustive discussion on photo-induced energy and electron transfer in supramolecular systems, please refer to Refs. (5-7). 

It should be noted that the 1,3-dimethoxybenzene and 2-naphthyl chromophoric units contained in the branches of the dendrimer are not involved in metal coordination. In some way, they belong to a second coordination sphere. If is considered a large metal complex, the absorption and emission bands of the 1,3-dimethoxybenzene and 2-naphthyl chromophoric units can formally be classified as LC. However, can be more properly viewed as a supramolecular (multicomponent) species (12). In such species, each chromophoric imit displays its own absorption spectrum since there is no appreciable interactions among them in the ground state, but in the excited state even weak interactions can cause intercomponent energy or electron-transfer processes. This kind of reasoning can also be applied to all the other systems discussed in this chapter. 

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