Thermoluminescent Antimony-Supported Copper-Iodo Cuboids: Approaching NIR Emission via High Crystallographic Symmetry

We report the syntheses, structures, and luminescence properties of a series of copper-iodo cuboids supported by L-type antimony ligands. The cuboids are of general formula [(SbR₃)₄Cu₄(I)₄] (1-4, 8), where SbR₃ is a series of homoleptic and heteroleptic stibines containing both phenyl and a variety of alkyl substituents (R = Cy, Pr, Bu, Ph); triphenyl, Pr₂Ph, and Me₂Ph stibines resulted in the formation of dimers of type [(SbR₃)₄(Cu)₂(I)₂] (5-7). While similar luminescent copper-halide cubes have been studied, the corresponding "heavy-atom" congeners have not been studied, and ligation of such heavy-atom moieties is often associated with long-lived triplet states and low-energy absorption and emission profiles. Overall, two obligate parameters are found to imbue NIR emission: (i) short Cu-Cu bonds and (ii) high crystallographic symmetry; both of these properties are found only in [(SbPr₃)₄Cu₄(I)₄] (1, in I23; λ_em = 711 nm). The correlation between NIR emission and high crystallographic symmetry (which intrinsically includes high molecular symmetry)-versus only molecular symmetry-is confirmed by the counterexample of the molecularly symmetric Bu-substituted cuboid [(SbBu₃)₄Cu₄(I)₄] (3, λ_em = 588 nm, in R-3), which crystallizes in the lower symmetry trigonal space group. Despite the indication that the stronger donor strength of the SbBu₃ ligand should red-shift emission beyond that of the SbPr₃-supported cuboid, the emission of 3 is limited to the visible region. To further demonstrate the connection between structural parameters and emission intensity, X-ray structures for 1 and 3 were collected between 100 and 300 K. Lastly, DFT calculations for 1 on its singlet (S₀) and excited triplet state (T₁) demonstrate two key factors necessary for low-energy NIR emission: (i) a significant contraction of the interconnected Cu₄ intermetallic contacts [∼2.45 → 2.35 Å] and (ii) highly delocalized (and therefore low-energy) A₁ symmetry HOMO/LUMO orbitals from which the emission occurs. Thus, any molecular or crystallographic distortion of the Cu₄ core precludes the formation of highly symmetric (and low-energy) HOMO/LUMO orbitals in T₁, thereby inhibiting low-energy NIR emission.