Antimony-Supported Cu4I4 Cuboid with Short Cu-Cu Bonds: Structural Premise for Near-Infrared Thermoluminescence

Herein we report the synthesis, temperature-dependent X-ray structures (100, 150, 200, 250, 273, and 300 K), and solid-state emissive properties of the antimony-copper(I)-iodo cluster [Cu4(I)4(Sb(i)Pr3)4] (1), supported by the trialkylantimony donor Sb(i)Pr3. Overall, 1 exhibits a distorted cuboidal structure, wherein a twisting of the "cube" generates an interconnected Cu4 tetrahedron, as well as long Cu-I and Cu-Sb bonds [e.g., 2.707(2) and 2.571(2) Å]; in the 100 K X-ray structure, the Cu-Cu bonds are quite short [2.761(3) Å]. Solid-state emission spectra of 1 were obtained over a range of temperatures (163-298 K), wherein 1 exhibits only a low-energy emission feature centered near 700 nm (λEm = 711 nm, fwhm = 150 nm, and λEx = 390 nm). Because 1 contains no aryl units, the emission spectrum (and absorption) can be unambiguously attributed to the Cu4I4 core [no L (pnictogen ligand) component]. The luminescence is sharply attenuated upon warming from 150 to 200 K. An overlaid plot of the emissive properties of 1 and its Cu-Cu bond distances [2.761(3)-2.836(4) Å] reveal that ∼2.80 Å represents a critical crossing point for low-energy thermoluminescence in Cu4-based clusters.