Evaluation of Ligands Effect on the Photophysical Properties of Copper Iodide Clusters

Luminescent materials
based on copper complexes are currently receiving increasing attention
because of their rich photophysical properties, opening a wide field
of applications. The copper iodide clusters formulated [Cu4I4L4] (L = ligand), are particularly relevant
for the development of multifunctional materials based on their luminescence
stimuli-responsive properties. In this context, controlling and modulating
their photophysical properties is crucial and this can only be achieved
by thorough understanding of the origin of the optical properties.
We thus report here, the comparative study of a series of cubane copper
iodide clusters coordinated by different phosphine ligands, with the
goal of analyzing the effect of the ligands nature on the photoluminescence
properties. The synthesis, structural, and photophysical characterizations
along with theoretical investigations of copper iodide clusters with
ligands presenting different electronic properties, are described.
A method to simplify the analysis of the 31P solid-state
NMR spectra is also reported. While clusters with electron-donating
groups present classical luminescence properties, the cluster bearing
strong electron-withdrawing substituents exhibits original behavior
demonstrating a clear influence of the ligands properties. In particular,
the electron-withdrawing character induces a decrease in energy of
the unoccupied molecular orbitals, that consequently impacts the emission
properties. The modification of the luminescence thermochromic properties
of the clusters are supported by density functional theory (DFT) calculations.
This study demonstrates that the control of the luminescence properties
of these compounds can be achieved through modification of the coordinated
ligands, nevertheless the role of the crystal packing should not be
underestimated.