Synthesis and Study of Photothermal Properties of a Mixed-Valence Nanocluster CuI/CuIIwith Strong near-Infrared Optical Absorption Supported by 4-tert-Butylcalix[4]arene Ligand

The first mixed-valence nanocluster CuI/CuIIwith the highest percentage of CuIIions was synthesized by using 4-tert-butylcalix[4]arene (Calix4), with the formula DMF2⊂[(CO3)2-@CuII6CuI3(Calix4)3Cl2(DMF)5(H3O)]•DMF (1), as a photothermal nanocluster. Its structure was characterized using single-crystal X-ray diffraction, Fourier-transform infrared spectroscopy, and powder X-ray diffraction. In addition, the charge state and chemical composition of the nanocluster were determined using electrospray ionization spectrometry and X-ray photoelectron spectroscopy (XPS) spectrum. The results of the XPS and X-ray crystallography revealed that there are two independent CuIIand CuIcenters in nanocluster 1with the relative abundances of 66.6 and 33.3% for CuIIand CuI, respectively. The nanocluster contains three four-coordinated CuIions with a square-planar geometry and six five-coordinated CuIIions with a square pyramid geometry. The nanocluster shows strong near-infrared optical absorption in the solid state and excellent photothermal conversion ability (the equilibrium temperature ∼78.2 °C) with the light absorption centers in 286–917 nm over previous reported pentanucleus CuI4CuIIclusters and CuIIcompounds.