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.