Geometry Flexibility of Copper Iodide Clusters: Variability in Luminescence Thermochromism

Abstract

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.

Subtle Modulation of Cu4X4L2 Phosphine Cluster Cores Leads to Changes in Luminescence

“…22 The 3 CC transition occurs through distortions of the Cu 4 X 4 core in the excited state, an effect enhanced by strong cuprophilic interactions. 39 As noted by Kitagawa et al, the flexibility in the Cu 4 X 4 core is the key factor that influences the photoemissive properties of these complexes. 9 Octahedral Cu 4 X 4 L 2 motifs also possess interesting photophysical properties but are less-studied relative to the cubane Cu 4 X 4 L 4 compounds.…”

“…22 The 3 CC transition occurs through distortions of the Cu 4 X 4 core in the excited state, an effect enhanced by strong cuprophilic interactions. 39 As noted by Kitagawa et al, the flexibility in the Cu 4 X 4 core is the key factor that influences the photoemissive properties of these complexes. 9…”

Thermochromic Luminescent Materials and Multi-Emission Bands in d10 Clusters

“…Among the thermochromic luminescent materials, the ones built with transition-metal complexes are of particular interest89101112. The analysis of the ordered molecular assemblies allows understanding and rationalizing the mechanisms that are involved in these phenomena.…”

A Bis(Diphosphanyl N‐Heterocyclic Carbene) Gold Complex: A Synthon for Luminescent Rigid AuAg2 Arrays and Au5 and Cu6 Double Arrays

“…This arrangement results in two types of Cu–Cu separations, that is, between opposing copper atoms of different arrays and between adjacent copper atoms of the same array, all falling within the range 2.669(2)–2.872(2) Å, and consistent with d 10 –d 10 interactions. Five iodides are bridging six copper atoms in an μ 2 fashion with Cu–I distances in the range of 2.533(2)–2.680(2) Å, which is comparable to typical bond lengths in copper clusters having doubly‐bridging iodides (see the Supporting Information) . To the best of our knowledge, the Cu 6 arrangement in Cu6 is unprecedented and presumably originates from the unique donor characteristics and rigidity of the PC NHC P ligand.…”