Tetragonal Diiodotetrapyridinedicopper(I): Structure, Luminescence, and Computational Modeling

Abstract

We report on a new crystal modification of (CuIPy₂) _n (Py = pyridine), a compound first reported by White et al. In contrast to White’s orthorhombic structure, which is comprised of rhomboid iodide-bridged dimers, Cu₂I₂Py₄, our new tetragonal crystal structure in space group P4₁2₁2 is disordered and can be interpreted as either iodide-bridged dimers or helical chains. To determine the structure type, variable temperature X-ray diffraction and luminescence measurements were carried out. The photoluminescence spectrum shows a distinct cluster-centered transition at high excitation energies which is consistent with the dimer structure. DFT and TD-DFT calculations were performed to explain the difference between the emission spectrum at high energies compared to low energies. Furthermore, correlation of the luminescence spectrum with the X-ray results as temperature is varied demonstrates that the cluster-centered luminescence band in Cu₂I₂Py₄ arises from close Cu⋯Cu distances which vary with temperature. A low temperature X-ray crystallographic redetermination of the cubane tetrameric Cu₄I₄Py₄ is also presented. Both Cu₂I₂Py₄ and Cu₄I₄Py₄ structures show distortion of the Cu _n I _n core cluster at low temperature resulting in reduced Cu⋯Cu distances, but with I⋯I distances roughly unchanged.