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 CoII-centered ligand [CoIIW11O39]10–. The eight complex series,
[CoII(MxOHy)­W11O39](12–x−y)– (MxOHy = VIVO, CrIII(OH2), MnII(OH2), FeIII(OH2), CoII(OH2), NiII(OH2), CuII(OH2), ZnII(OH2)), of which six are reported for the first time,
was synthesized starting from [CoIIIW11O39]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 [CoIIW12O40]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 [CoIIW12O40]6– while more than tripling
the absorbance at 400 nm.

基于钴(II)中心配体[CoIIW11O39]10–的一系列异质双金属过渡金属取代多氧金属酸盐（TMSPs）已被合成。该系列包含八个复杂化合物，[CoII(MxOHy)­W11O39](12–x−y)–（其中MxOHy = VIVO, CrIII(OH2), MnII(OH2), FeIII(OH2), CoII(OH2), NiII(OH2), CuII(OH2), ZnII(OH2)），其中六个为首次报道，它们均由[CoIIIW11O39]9–合成，并通过光谱学、电化学和计算技术进行研究，以评估取代过渡金属对金属至多氧金属酸盐电荷转移（MPCT）跃迁的光动力学影响。这些双金属配合物均表现出比完全配合物[CoIIW12O40]6–更高的可见光吸收能力，并展现出与完全配合物相同的MPCT跃迁，但它们在激发态的寿命较短（在水中为亚300皮秒）。寿命的缩短可归因于配位水合配体的非辐射弛豫、由于负电荷增加而与阳离子的相互作用增强，以及能隙定律，其中阴离子电荷似乎是最关键的单因素。最令人鼓舞的结果来自铬和铁取代的系统，它们在400纳米处的吸光度增加了三倍以上，同时保持了激发态寿命至少为[CoIIW12O40]6–的50%。”
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