Mechanochromic Luminescence and Liquid Crystallinity of Molecular Copper Clusters

Molecular copper iodide clusters with the [Cu4I4] 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 degrees C. Temperature-dependent 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 [Cu4I4] 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 [Cu4I4] inorganic cluster core geometry and in particular of the strength of the cuprophilic interactions.