Geometry flexibility of copper iodide clusters: Variability in luminescence thermochromism

An original copper(I) iodide cluster of novel geometry obtained by using a diphosphine ligand is reported and is formulated [Cu₆I₆(PPh₂(CH₂)₃PPh₂)₃] (1). Interestingly, this sort of "eared cubane" cluster based on the [Cu₆I₆] inorganic core can be viewed as a combination of the two known [Cu₄I₄] units, namely, the cubane and the open-chair isomeric geometries. The synthesis, structural and photophysical characterisations, as well as theoretical study of this copper iodide along with the derived cubane (3) and open-chair (2) [Cu₄I₄(PPh₃)₄] forms, were investigated. A new polymorph of the cubane [Cu₄I₄(PPh₃)₄] cluster is indeed presented (3). The structural differences of the clusters were analyzed by solid-state nuclear magnetic resonance spectroscopy. Luminescence properties of the three clusters were studied in detail as a function of the temperature showing reversible luminescence thermochromism for 1 with an intense orange emission at room temperature. This behavior presents different feature compared to the cubane cluster and completely contrasts with the open isomer, which is almost nonemissive at room temperature. Indeed, the thermochromism of 1 differs by a concomitant increase of the two emission bands by lowering the temperature, in contrast to an equilibrium phenomenon for 3. The luminescence properties of 2 are very different by exhibiting only one single band when cooled. To rationalize the different optical properties observed, density functional theory calculations were performed for the three clusters giving straightforward explanation for the different luminescence thermochromism observed, which is attributed to different contributions of the ligands to the molecular orbitals. Comparison of 3 with its [Cu₄I₄(PPh₃)₄] cubane polymorphs highlights the sensibility of the emission properties to the cuprophilic interactions.