Polymorphic Copper Iodide Clusters: Insights into the Mechanochromic Luminescence Properties

An in-depth study of mechanochromic
and thermochromic luminescent
copper iodide clusters exhibiting structural polymorphism is reported
and gives new insights into the origin of the mechanochromic luminescence
properties. The two different crystalline polymorphs exhibit distinct
luminescence properties with one being green emissive and the other
one being yellow emissive. Upon mechanical grinding, only one of the
polymorphs exhibits great modification of its emission from green
to yellow. Interestingly, the photophysical properties of the resulting
partially amorphous crushed compound are closed to those of the other
yellow polymorph. Comparative structural and optical analyses of the
different phases including a solution of clusters permit us to establish
a correlation between the Cu–Cu bond distances and the luminescence
properties. In addition, the local structure of the [Cu4I4P4] cluster cores has been probed by 31P and 65Cu solid-state NMR analysis, which readily
indicates that the grinding process modifies the phosphorus and copper
atoms environments. The mechanochromic phenomenon is thus explained
by the disruption of the crystal packing within intermolecular interactions
inducing shortening of the Cu–Cu bond distances in the [Cu4I4] cluster core and eventually modification of
the emissive state. These results definitely establish the role of
cuprophilic interactions in the mechanochromism of copper iodide clusters.
More generally, this study constitutes a step further into the understanding
of the mechanism involved in the mechanochromic luminescent properties
of metal-based compounds.