Mechanisms and salt effects in photoredox and quenching process involving cobalt (III) complexes




Cai, Lezhen

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The novel complexes [special characters omitted] and [special characters omitted] were prepared and characterized. Photoredox quantum yields for the formation of [special characters omitted] from the above compounds were measured on irradiation at 360 nm to be 0.065, 0.082, 0.0088 and 0.0040 respectively. With added thiocyanate a significant increase in ΦCo2+ occurred. This can be modeled in two ways; (i) scavenging of thiocyanate radical from an initial caged radical pair giving 6-25 ps estimates for the lifetime of the latter species; (ii) photolysis of a thiocyanate/complex ion pair, giving formation constants of 0.19, 0.09, 0.08 and 0.05 for the complexes [special characters omitted] and [special characters omitted] respectively. Subnanosecond laser flash photolysis studies showed evidence for the formation of [special characters omitted]. The effects of added electrolytes and of viscosity on the formation and decay of [special characters omitted] were also investigated. To help to distinguish between the above two mechanisms, the zero-charged novel complex [special characters omitted] (tacn = 1,4,7-triazacyclononane) was synthesized and characterized. It is thermally stable in aqueous/DMSO solution, but on irradiation at 360 nm undergoes parallel photosubstitution to form DMSO and aqua-substituted products with an overall quantum yield of 0.012. The product yields increase linearly with added thiocyanate. For a 1 M thiocyanate solution, the quantum yield for disappearance of the starting complex rose to 0.022 and a small redox yield of 0.0008 was found. Under these same conditions, ns laser flash photolysis at 355 nm revealed a transient absorption owing to [special characters omitted], which was produced with a quantum yield of 0.036. These results are interpreted in terms of scavenging of radical pair species by thiocyanate ion followed by back electron transfer to give a photosubstituted product, and a radical pair quantum yield of 0.29 and lifetime of 12 ps was derived. The emission of [special characters omitted] (where pop = μ-pyrophosphite-P,P’) can be quenched by the complexes [special characters omitted] (where X = [special characters omitted]) only in the presence of electrolytes. The salt effects have been studied using the salts MCl, M'Cl2, or [special characters omitted] (where M, M’ and R represent alkali, alkaline earth metals, and alkyl respectively, with n = 0-3), and [special characters omitted]. For 0.5 M cation concentration, second-order quenching rate constants kq lie in the range [special characters omitted]. For the different quencher complexes used, kq decreases in the order [special characters omitted]. The oxidative quenching products [special characters omitted] are observed, and their quantum yields are 0.083 and 0.027 respectively for the reaction of [special characters omitted] with [special characters omitted] and [special characters omitted] in 0.5 M KCl / pH2 solution. The quenching occurred by atom transfer (dominant) and electron transfer (minor) for quencher [special characters omitted] or [special characters omitted], while only electron transfer was observed for [special characters omitted] and [special characters omitted] quenchers. The quenching efficiency of the cobalt complexes increases with electrolyte concentration and specific cation effects are observed in the kq with the following trends Li+ < Na+ < K+ < Cs+: Mg2+ < Ca2+ < Sr2+ < Ba2+; NH4+ < MeNH3+ < Me2NH2+ < Me3NH+: Et3NH+ < Et2NH2+ < EtNH3+: n-PrNH3+ < EtNH3+ < MeNH3+.



Cobalt compounds, Synthesis, Complex compounds