Electron and Energy Transfer Properties

Sliwka HR, Melp TB, Foss BJ, Abdel-Hafez SH, Partali V, Nadolski G, Jackson H, and Lockwood SE. 2007. Electron- and energy-transfer properties of hydrophilic carotenoids. Chemistry—A European Journal 13(16) 4458 1466. 

The study of photoinduced ET in covalently linked donor-acceptor assemblies began with comparatively simple dyad systems which contain a transition metal center covalently linked to a single electron donor or acceptor unit [26]. However, work in this area has naturally progressed and in recent years complex supramolecular assemblies comprised of one or more metal complexes that are covalently linked to one or more organic electron donors or acceptors have been synthesized and studied [27-36]. Furthermore, several groups have utilized the useful photoredox properties of transition metal complexes to probe electron and energy transfer across spacers comprised of biological macromolecules such as peptides [37,38], proteins [39,40], and polynucleic acids [41]. 

In this report, we will describe some of our studies aimed at (i) obtaining new inorganic photosensitizers by second-sphere modification of known ones, and (ii) assembling photosensitizer units with other molecular components in discrete, covalently bound supramolecular structures. Studies of type (i), besides their intrinsic interest, have some relevance to the problem of how the properties of a photosensitizer are modified by inclusion in a supramolecular structure. Systems of type (ii) would be useful to study the basic processes of intramolecular electron and energy transfer involved in the performance of molecular photochemical devices. 

This review is not intended to provide an exhaustive picture of photoinduced electron (and energy) transfer in covalently linked inorganic systems. It should demonstrate, however, that this is an active and attractive field of research. Coordination compounds, because of their intrinsic combinatory nature, offer wide possibilities in terms of tailoring spectroscopic, redox, and photophysical properties of molecular components. The continuing progress in chemical synthesis provides a variety of... 

There is no limit to the number of photochromic systems possible. The systems discussed are excellent candidates for integration into solid-state devices because nearly all retain their photochromic properties in the absence of solvent. The organization of these systems in tandem with other molecular systems is being pursued. For the switching applications many of these systems have much too slow a turnover rate to be explored as working devices. That is unless the connectivity in these systems can be increased. In the meantime, photochromic systems will probably be explored as possible optical memory devices. The most promising switches are those based on the much faster processes of electron and energy transfer. We will now examine research in these areas. 

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. 

Attempts to better understand natural photosynthesis and to construct artificial models continue to be important areas of modern photochemistry. The photophysical properties of certain bacteriochlorophylls and carotenes have been reported while the mechanisms of electron and energy transfer in natural... 

Detailed electron- and energy-transfer studies have been carried out, on the copper(I) complexes, on the demetalated species and, in some cases, on complexes obtained by exchanging the copper center for another cationic metal (Li, Ag" "). The photophysical properties have been studied in different solvents, however, being solvents of similar polarity, the results for different systems can be considered comparable. Time-resolved measurements allowed us to analyze in detail several electron and energy transfer processes occurring in cascade in some of the highly multicomponent catenanes and rotaxanes of the present article. In addition, it has been possible to identify very distinct conformers in solution, some of them containing two chromophores located close to one another and others with remote chromophores. 

An understanding of the structure and properties of excited states in the photochemistry of coordination compounds is of fundamental importance. Furthermore, monitoring electron and energy transfer is vital for a wide range of applications. TRIR spectroscopy often provides key information on the excited states of coordination compounds, in particular those containing CO or CN ligands, since these groups act as direct probes of the electron density at the metal center. 

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