Transition Metal Substitution Effects on Metal-to-Polyoxometalate Charge Transfer

A series of hetero-bimetallic transition metal-substituted polyoxometalates (TMSPs) were synthesized based on the Co^II-centered ligand [Co^IIW₁₁O₃₉]^10–. The eight complex series, [Co^II(M^xOH_y)­W₁₁O₃₉]^{(12–x−y)–} (M^xOH_y = V^IVO, Cr^III(OH₂), Mn^II(OH₂), Fe^III(OH₂), Co^II(OH₂), Ni^II(OH₂), Cu^II(OH₂), Zn^II(OH₂)), of which six are reported for the first time, was synthesized starting from [Co^IIIW₁₁O₃₉]^9– and studied using spectroscopic, electrochemical, and computational techniques to evaluate the influence of substituted transition metals on the photodynamics of the metal-to-polyoxometalate charge transfer (MPCT) transition. The bimetallic complexes all show higher visible light absorption than the plenary [Co^IIW₁₂O₄₀]^6– and demonstrate the same MPCT transition as the plenary complex, but they have shorter excited-state lifetimes (sub-300 ps in aqueous media). The decreased lifetimes are rationalized on the basis of nonradiative relaxation due to coordinating aqua ligands, increased interaction with cations due to increased negative charge, and the energy gap law, with the strongest single factor appearing to be the charge on the anion. The most promising results are from the Cr- and Fe-substituted systems, which retain excited-state lifetimes at least 50% of that of [Co^IIW₁₂O₄₀]^6– while more than tripling the absorbance at 400 nm.