Photoinduced processes energy transfer

Photosynthesis is a natural energy conversion system that converts solar energy into chemical energy, the primary processes of which are a cascade of photoinduced excitation energy transfer to the reaction center GiC) and the subsequent rapid electron transfer to generate charge-separated state. Over the past decades, a series of significant results have been obtained for the smdy of photosynthesis (1) The stracture of the protein subunits in the photosynthetic RC has been ascertained. 

Metal complexes of bipyridine and phenanthroline units are of interest for photophysical and photochemical investigations since they are suitable tools for the examination of photoinduced intramolecular energy transfer Deeper understanding of these processes will help in the development of efficient systems for the conversion of solar into chemical energy In this context we are trying to develop heterooligometallic complexes of the ligands 6 and 17. 

Many processes in living organisms are closely linked to energy transfer and to charge transfer complexes. Therefore, studies of the properties of PCSs are important in solving certain problems of bioenergetics, enzymatic catalysis, photoinduced carcinogenesis, etc. 

Use simple molecular orbital diagrams to show the various processes of photoinduced energy transfer and photoinduced electron transfer. 

Fig. 9 Schematic representation of the self-assembled 2 Nd3+ [Ru(bpy)2(CN)2]. Curved arrows represent photoinduced energy transfer processes. For more details, see text...

Class 3 fluorophores linked, via a spacer or not, to a receptor. The design of such sensors, which are based on molecule or ion recognition by a receptor, requires special care in order to fulfil the criteria of affinity and selectivity. These aspects are relevant to the field of supramolecular chemistry. The changes in photophysical properties of the fluorophore upon interaction with the bound analyte are due to the perturbation by the latter of photoinduced processes such as electron transfer, charge transfer, energy transfer, excimer or exciplex formation or disappearance, etc. These aspects are relevant to the field of photophysics. In the case of ion recognition, the receptor is called an ionophore, and the whole molecular sensor is... 

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. 

Fig. 16. Representation of photoinduced energy transfer and electron transfer processes involved in supramolecular photochemistry. Generation of R S, RS, R+S", or R-S+ may be followed by a chemical reaction.

Fig. 17. Molecular recognition-dependent photochemical molecular devices. Photochemical processes such as energy transfer (ET) or photoinduced electron transfer (PeT) may be induced via association of two (or more) complementary units, each bearing a component of the device the complementary units may be as small as heterocyclic bases or as large as antigen-antibody conjugates.

In this chapter we have described the photophysics and photochemistry of C6o/C70 and of fullerene derivatives. On the one hand, C6o and C70 show quite similar photophysical properties. On the other hand, fullerene derivatives show partly different photophysical properties compared to pristine C6o and C70 caused by pertuba-tion of the fullerene s TT-electron system. These properties are influenced by (1) the electronic structure of the functionalizing group, (2) the number of addends, and (3) in case of multiple adducts by the addition pattern. As shown in the last part of this chapter, photochemical reactions of C60/C70 are very useful to obtain fullerene derivatives. In general, the photoinduced functionalization methods of C60/C70 are based on electron transfer activation leading to radical ions or energy transfer processes either by direct excitation of the fullerenes or the reaction partner. In the latter case, both singlet and triplet species are involved whereas most of the reactions of electronically excited fullerenes proceed via the triplet states due to their efficient intersystem crossing. 

The study of photoinduced processes in Ir(III)-based arrays has been exploited since facile synthetic methods for the preparation of Ir(III)-polyimine complexes became available. Polynuclear complexes containing, in addition to Ir(III) centers, either Os(II)-, Ru(II)-, Re(II) or Cu(I)-polyimine units linked by different bridging ligands, displayed a rich variety of photoinduced energy transfer processes. In Fig. 22 are reported some representative cases of early studies. 

An extension of this study revealed that following selective excitation of the Ir(III)-based unit at 355 nm, rather than that of the porphyrin units at 532 nm, i.e., when the Ir center acts as a photosensitizer, the type of ensuing photoinduced process can be switched in the case of array (a), Fig. 27, from the electron transfer discussed above, to energy transfer [87]. 

Switching systems based on photochromic behavior,I29 43,45 77-100 optical control of chirality,175 76 1011 fluorescence,[102-108] intersystem crossing,[109-113] electro-chemically and photochemical induced changes in liquid crystals,l114-119 thin films,170,120-1291 and membranes,[130,131] and photoinduced electron and energy transfer1132-1501 have been synthesized and studied. The fastest of these processes are intramolecular and intermolecular electron and energy transfer. This chapter details research in the development and applications of molecular switches based on these processes. 

Concepts of Photoinduced Electron and Energy Transfer Processes Across Molecular Bridges... 

At this point, we have to consider yet another very important process that is likely to go hand in hand with photoinduced electron transfer reactions—the electronic energy transfer. 

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