Photoredox reactions, redox potentials

An Aside Redox Potentials Involved in Photoredox Reactions... 

A photoredox reaction is a redox reaction that occurs after electronic excitation of one or several reaction partners. By electronic excitation of a molecule or a solid such as a semiconductor a strongly reducing photoelectron and a strongly oxidizing photohole may be formed (Fig. 1). The ionization potential of an electronically excited species decreases, and the electron affinity increases approximately by the excitation energy. 

An application of photoredox systems for fuel production from light encounters three major problems summarized in Fig. 3.2. First, sensitizer/relay couples have to be found which are suitable from the viewpoint of light absorption and redox potentials and undergo no chemical side reactions in the oxidation state of interest. The sensitizer... 

Consideration of the nature of the LMCT transitions, redox energetics, and photoredox behaviour of transition-metal ammine complexes has allowed Endicott and co-workers18 to propose new models for the potential energy surfaces describing their photoredox reactions. These models have been used to discuss the differences in photoreactivity of [Co(NH3)5Br]2+ and [Co(NH3)5-N03]2+.21 These differences are ascribed to (i) more Co-radical bonding in the... 

Ir Aryl halides (X=I, Br) were believed to be unsuitable substrates for the visible light photoredox chemistry because of their high redox potentials. However, when the reaction with Arl was carried out in benzene in the presence of the Ir complex [Cp IrHCl]2,(Cp =q -pentamethylcyclopentadienyl) as catalyst and KOBu at 80°C, the biaryls were obtained in a 43-73% yield [69]. In addition, in the photostimulated reaction with Ir(ppy)j (tris(2-phenylp3Tidine)iridium) as catalyst, biphenyls were obtained at room temperature (rt) from Arl or within the range it to 90°C from ArBr (Eq. 9.18) [70] ... 

A redox reaction between an excited state and suitable reagents can be described like a normal redox reaction as an electron transfer between an occupied energy level and a vacant one. The principle of a photoredox reaction is shown in Fig. 1.3 for a homogeneous system using an energy level scheme for the excited molecule and the potential reaction partners. In the excited state one electron is transferred to a higher electronic level which was previously vacant... 

Whether or not a given photoredox reaction can occur spontaneously depends on the free energy change AG for the process. In simple terms, Weller s expression relates the free energy change AG with the excited state energy and redox potentials of the redox couples involved ... 

The proposed catal) ic cycle includes the formation of a chiral Breslow intermediate from NHC and a reactive iminium ion via photoredox catalysis (Scheme 7.9). In this reaction, oxidation of the Breslow intermediate or NHC by [Ru(bpy)3] " and terminal oxidants could be problematic, because of the similar redox potentials of this catalyst scaffold and tertiaiy amines. To suppress such unproductive pathways, careful choice of NHC with proper steric and electronic properties and the use of a weak oxidant are important. 

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