Photoredox polypyridine complexes

PHOTOREDOX AND SENSITIZATION PROCESSES INVOLVING TRANSITION METAL POLYPYRIDINE COMPLEXES... 

In this chapter three topics related to photosensitization and photocatalysis of polypyridyl complexes are discussed types of excited states encountered and "tuning" of their properties, light-induced electron and energy transfer reactions and their applications. Concepts are illustrated with select examples, with references made only to mononuclear complexes. A comprehensive review of the photophysics and photochemistry of various polypyridine complexes has been published recently [13]. In the accompanying chapter, Scandola et al. discuss extension of the photophysics and photoredox chemistry to polynuclear systems. 

In experimental studies of photoinduced electron transfer reactions, the free energy dependence of the quenching of a particular metal complex (e.g, Ru(bpy)3 + ) by a series of structurally related quenchers of known properties is used to confirm the calculated potential of the excited state couple and to estimate its exchange rate. For a given quencher a graded series of polypyridine complexes with different substituted bpy/phen ligands can also be used equivalently. These provide a set of experimentally measured data to check the observation of normal and inverted behaviour predicted by Marcus theory under certain conditions. Rate constants for back electron transfer of photoredox products have been measured in a similar manner in some cases and these were also subjected to analysis. 

A number of sensitized photoredox processes based on the photolysis of Re carbonyl polypyridine complexes in the presence of amines have been identified [56-58] ... 

The progress was painstakingly slow but in recent years there has been tremendous progress in the design of highly efficient sensitized photoelectrochemical cells. Principles and performance features of these photovoltaic cells form the topic for discussion in this chapter. Several topics related to materials considered in this chapter are treated in more detail in other chapters, e.g, principles of photoredox reactions (Balzani and Maestri), their applications in polypyridine complexes (Kalyanasundaram) and polynuclear complexes as light-harvesting units (Scandola et al.). 

Indeed, photoredox catalysis with Ru polypyridine complexes has emerged as a powerful tool for redox reactions including formation of carbon-carbon bonds based on oxidation of sp C-H bonds via single-electron-transfer (SET) processes. Results that are closely related to those shown in Schemes 33,34, and 35, where the carbon-carbon bond formation resulted from the benzyUc sp C-H oxidative activation in the presence of BuOOH, have been recently reported for the regioselective functionalization of tetrahydroisoquinolines with cyanide and a variety of nucleophiles arising from ketones, nitroalkanes, allyltrimethylsilane, silyl enol ethers, 1,3-dicarbonyl compounds under photocatalytic conditions [67-70] as illustrated in Scheme 62 [67]. Other applications of Ru(bipy)3Cl2 in photocatalytic cycUzation reactions involving carbon-carbon btmd formation have appeared [71, 72]. 

Abstract Photoredox catalysis by well-known nithenium(II) polypyridine complexes is a versatile tool for redox reactions in synthetic organic chemistry, because they can effectively catalyze single-electron-transfer (SET) processes by irradiation with visible light. These favorable properties of the catalysts provide a new strategy for efficient and selective radical reactions. Salts of tris(2,2 -bipyridine)mthenium (II), [Ru(bpy)3], were first reported in 1936. Since then, anumber of works related to artificial photosynthesis and photofunctional materials have been reported, but only limited efforts had been devoted to synthetic organic chemistry. Remarkably, since 2008, this photocatalytic system has gained importance in redox reactions. In this chapter, we will present a concise review of seminal works on ruthenium photoredox catalysis around 2008, which will be followed by our recent research topics on trifluoromethylation of alkenes by photoredox catalysis. 

Polypyridine ruthenium (II) chelate complex [Ru (Bpy) ] is known to participate in a photoredox reaction on excitation with visible light, coupled with the... 

Noteworthy is the presence in this pathway of Trpl91, which is the center of radical formation in CCP compound I as discussed in Section IV. Using flash photolysis it has been possible to directly observe intracomplex electron transfer from ferrocyt c to compounds I and II of CCP in the preformed complexes. In one set of experiments (108), cyt c was covalently derivatized at surface lysine residues with a Ru-polypyridine photoredox reagent (Ru). Photoinduced intracomplex electron transfer was observed following rapid reduction of cyt c by Ru ... 

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