Thiacalix[4]arene-functionalized molecular clusters involving the Keggin-type PM₄Mo₈ (M = Co, Ni) motif: Electrochemical and photothermal conversion properties

Herein, four new thiacalixarene-functionalized polyoxometalate clusters, [Na_2.5(H₂O)_2.5Co₄(TC4A)PMo₈O₃₂][(CH₃)₂NH₂]_4.5(CH₃OH)(DMF)_1.5(H₂O) (PCo₄Mo₈ (1), H₄TC4A = p-tert-butylthiacalix[4]arene), [H₄Ni₄(TC4A)PMo₈O₃₂][(CH₃)₂NH₂]₃(DMF)₄(CH₃OH)₂(H₂O)₂ (PNi₄Mo₈ (2)), and [M₄(TC4A)PMo₈VO₃₃][N(CH₃)₄]₂[HN(CH₃)₃][(CH₃)₂NH₂](DMF)₂(CH₃OH)₅ (M = Co, PCo₄Mo₈V (3); M = Ni, PNi₄Mo₈V (4)), were synthesized under solvothermal conditions and characterized using single-crystal X-ray diffraction. These four clusters were found to contain Keggin-type PM₄Mo₈ motifs, exhibiting redox properties similar to those of PMo₁₂O₄₀³⁻ (PMo₁₂), superior structural stability, and the ability to electrocatalytically reduce IO₃⁻. The substitution of the four metal ions, along with the capping with the TC4A ligand and VO unit, considerably modulated the visible-light absorption of the clusters, enhancing the photothermal conversion in the solid-state and organic solutions. The efficiency of the photothermal conversion revealed that the absorption of visible light relies on electron transfer pathways, which were elucidated through band gap analysis, density functional theory calculations, and X-ray photoelectron spectroscopy.