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 [(SbR 3 ) 4 Cu 4 (I) 4 ] ( 1 - 4 , 8 ), where SbR 3 is a series of homoleptic and heteroleptic stibines containing both phenyl and a variety of alkyl substituents (R = Cy, i Pr, Bu, Ph); triphenyl, i Pr 2 Ph, and Me 2 Ph stibines resulted in the formation of dimers of type [(SbR 3 ) 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 [(Sb i Pr 3 ) 4 Cu 4 (I) 4 ] ( 1 , in I 23; 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 [(Sb Bu 3 ) 4 Cu 4 (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 Sb Bu 3 ligand should red-shift emission beyond that of the Sb i Pr 3 -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 (S 0 ) and excited triplet state (T 1 ) demonstrate two key factors necessary for low-energy NIR emission: ( i ) a significant contraction of the interconnected Cu 4 intermetallic contacts [∼2.45 → 2.35 Å] and ( ii ) highly delocalized (and therefore low-energy) A 1 symmetry HOMO/LUMO orbitals from which the emission occurs. Thus, any molecular or crystallographic distortion of the Cu 4 core precludes the formation of highly symmetric (and low-energy) HOMO/LUMO orbitals in T 1 , thereby inhibiting low-energy NIR emission.