Energy and Electron Transfer Reactions

This article is intended to review the published work on the photochemistry and photophysics of osmium complexes that has appeared in the literature over the past several years. We have attempted to cover, albeit somewhat selectively, literature dating back to the year 2000. A variety of reviews pertaining to particular aspects of osmium photophysics and photochemistry were published prior to 2000. A few reviews discuss the photophysical behavior of primarily monometallic Os complexes in solution [1,2]. Several earlier reviews discuss light induced energy and electron transfer reactions involving osmium complexes in much of this work the Os complex is not the chro-mophore [3-6]. Finally, one review exists discussing the photochemistry of Os carbonyl complexes [7]. 

The electronic coupling between an initial (reactant) and a final (product) state plays a key role in many interesting chemical and biochemical photoinduced energy and electron transfer reactions. In excitation (or resonance) energy transfers (EET or RET) [1,2], the excitation energy from a donor system in an electronic excited state (D ) is transferred to a sensitizer (or acceptor) system (A). Alternatively, in photoinduced electron transfers (ET) [3,4], a donor (D) transfers an electron to an acceptor (A) after photoexcitation of one of the components (see Figure 3.50). 

D.M. Kaschak, J.T. Lean, C.C. Waraksa, G.B. Saupe, H. Usami, T.E. Mallouk, Photoin-duced Energy and Electron Transfer Reactions in Lamellar Polyanion/Polycation Thin Pilms Toward an Inorganic Leaf , J. Am. Chem. Soc., 121, 3435 (1999)... 

More useful mechanistic information is obtained from intramolecular electron-transfer reactions if the kinetics for the electron-transfer step can be isolated from the effects of diffusion. The main stimulus for making such studies is the urge to design systems that mimic some of the essential features of the photosynthetic reaction centre complex and much attention has focussed on the study of porphyrin-based photoactive dyads. Thus, a series of N-alkylporphyrins linked to a quinolinium cation has been synthesized and found to display a rich variety of photoreactions. The singlet excited state of the quinolinium cation operates in both intramolecular energy- and electron-transfer reactions while the excited singlet state of the porphyrin transfers an electron to the appended quinolinium cation. Several new porphyrin-quinone dyads have been studied,including cyclophane-derived systems where the reactants are held in a face-to-face orienta-... 

Measurements of the oxidation potentials yielded results which reflected the close proximity of the two phthalocyanine planes. The first two oxidation potentials (one-electron oxidations from each phthalocyanine ring) were 100 mV apart, suggesting the delocahzation of the cation radical over the two phthalocyanines jt-electronic framework. This behavior shows some similarity to that of the porphyrin dimers, and is expected to favor energy- and electron-transfer reactions. Another characteristic feature of this phthalocyanine dimer is that its fluorescence quantum yields are almost the same as those of the corresponding monomers (0.45, 0.26, and 0.76 for Zn(OBu), Zn(f-Bu), and Mg(f-Bu) phthalocyanines, respectively). Such a highly fluorescent phthalocyanine dimer has never been reported before. 

Abstract The approach based on the copper(I)-templated synthesis of porphyrin catenanes and rotaxanes developed by the authors group is here reviewed. Zn(II) porphyrins and gold(III) porphyrins were chosen as electron donors and electron acceptors, respectively, to constitute the electro- and photoactive parts of the present systems. The processes—energy and electron transfer reactions—occiuring in the interlocked structures upon light absorption in the presence or absence of Cu(I) are presented, their rates and efficiencies critically compared and discussed with respect to properties of the components and of the ensemble. A detailed examination of differences and analogies in photoreactivity between the present and closely related systems reported by others is presented. 

An intriguing series of electro- and photoactive porphyrin catenanes and rotaxanes, obtained by the authors through copper(I)-templated synthesis, is reviewed in the sixth chapter by Lucia Flamigni, Valerie Heitz, and Jean-Pierre Sauvage. The photo-induced processes - energy and electron transfer reactions - occurring in the interlocked structures upon light absorption are discussed in detail and critically compared to closely related systems reported by others. 

The electronic effects in energy and electron transfer reactions, including excited state systems, have been discussed in a review by Endicott. The trends observed in the rate constants for the quenching of the doublet E) excited state of [Cr(bpy)3] by a series of organochromium complexes, [Cr(H20)5R], indicate an outer-sphere electron transfer mechanism. The different reactivity patterns found for the oxidations of [(H20)Co([14]aneN4)R] complexes by [Ru(Bpy)3] and [ Cr(bpy)3] point to electron and energy transfer mechanisms, respectively. The reductive quenching of [ Cr(bpy)3] by Fe produces [Cr(bpy)3], which also quenches the excited state in the absence of added... 

Even though energy and electron transfer reactions using complexes such as [Ru(bpy)3] have been shown to occur with significant success, the effici cy of any bimolecular en gy or electron transfer reaction is limited by the necessity for the light absorber to come into contact with a suitable quencher during the excited state of the light absorber. These processes are also limited by back election transfer. These facts have sparked the interest in polymetallic supramolecular systems. 

Ru(bpy) " in Silica-Titania. Tris(2,2 -bipyridine)ruthenium(II), Ru(bpy)3 ,has received the considerable attention because of its unique properties such as strong luminescence, moderate excited-state lifetime, energy and electron transfer reactions and chemical stability (Kalyanasundaram, 1992). The luminescent excited state of Ru(bpy)3 isassigned to the metal-to-ligand charge-transfer (MLCT) state. The luminescence properties are very sensitive to polarity and viscosity of solvent because of the MLCT character. When Ru(bpy)3" is excited, solvent reorientation aroimd the excited state molecules occurs to stabilize the MLCT excited state with a large dipole moment. Therefore, a blue shift of the luminescence is induced when motion of the solvent molecules is restricted. 

A point that must be stressed is that an electronically excited state is a species with quite different properties compared with those of the ground state molecule. Therefore, both the thermodynamic and kinetic aspects of photoinduced energy and electron transfer reactions must be carefully examined. 

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