Abstract
Studies were carried out on the photochemical redox reactions of a series of structurally related inner-sphere Cu(II)/dicarboxylate complexes in aqueous solution (N2-purged), using steady-state illumination. Cu(I) quantum yields (313 nm) of these systems were characterized with respect to the effects of copper(II) speciation, mainly considering the effect of the dicarboxylate structure on the Cu(I) quantum yield. For each solution composition, which corresponds to a unique Cu(II) speciation, the Cu(II)-based molar absorptivity (εCu(II), M-1 cm-1) and the quantity ΦCu(I)εCu(II) (where ΦCu(I) represents the experimental Cu(I) quantum yield) were determined from experiments. This experimental information (εCu(II) and ΦCu(I)εCu(II)) was explicitly combined with the calculated equilibrium Cu(II) speciation (based on critically reviewed thermodynamic data) in a quantitative model to determine molar absorptivities (εCuL) and Cu(I) quantum yields (ΦCu(I),CuL) of the CuL complexes, where L represents the dicarboxylate ligand. For Cu(dicarboxylate)0, the observed relative reactivity of Cu(I) quantum yields at 313 nm (ΦCu(I),CuL) varies by 50-fold and is affected by the dicarboxylate structure as follows: oxalate (0.42) > succinate (0.10) ≫ maleate (0.008). This trend in relative reactivity parallels the trend in expected relative stability of the carbon-centered radicals derived from decarboxylation of the carboxylate (i.e., acyloxyl) radicals formed in the initial photoinduced ligand-to-metal charge (electron) transfer reaction of the Cu(II)/dicarboxylate complex.