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 [CuIL] (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 P 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.

Molecular copper iodide clusters with the [Cu 4 I 4 ] cubane core have been functionalized by phosphine ligands carrying protomesogenic gallate-based derivatives bearing either long alkyl chains (C8, C12, and C16) or cyanobiphenyl (CBP) fragments. The mesomorphic properties of the functionalized clusters were studied by combining differential scanning calorimetry (DSC), polarized optical microscopy (POM), and small-angle X-ray scattering (SAXS) experiments. Whereas clusters functionalized solely with long alkyl chains present amorphous or crystalline states, the cluster carrying CBP fragments displays liquid crystal properties with the formation of a smectic A mesophase from room temperature up to 100 °C. Temperaturedependent photoluminescence measurements show that the CBP derivative displays an unusual luminescence thermochromism which is possibly due to a resonance energy transfer mechanism between the emissive [Cu 4 I 4 ] inorganic and CBP moieties. The emission properties of this original cluster are also sensitive to variation of local order of the molecular assembly. Moreover, the liquid crystalline properties imported on the inorganic core allow for a facile deformation of its local environment, leading to mechanochromic properties related to modulation of intramolecular interactions. Indeed, mechanical constraints on the molecularly self-assembled structure induce changes at the molecular level by modification of the [Cu 4 I 4 ] inorganic cluster core geometry and in particular of the strength of the cuprophilic interactions.