Luminescence Mechanochromism of an Anionic Dinuclear Copper Iodide Complex

This article reports the mechanochromic luminescence properties of a copper iodide complex formulated (PrPh3P)2[Cu2I4] (1), which displays a contrasted reversible emission color switch from green (λmax = 535 nm) to yellow (λmax = 565 nm), upon mechanical solicitations. In addition to being a rare example of an ionic dinuclear copper-based mechanochromic compound, this complex is capable of congruently melting at a relatively low temperature (159 °C) to form a yellow emissive glassy phase (λmax = 590 nm). This amorphous state is considered as the reference state of the partial crystalline-to-amorphous transformation that occurs upon grinding. Comparison of these two amorphous phases helps mechanism investigations. In particular, solid-state NMR (nuclear magnetic resonance) analysis allowed quantification of the amorphous fraction, constituting 23% of the mechanically altered phase. Mechanical grinding disrupts long-range order and induces small changes in the molecular structure of [Cu2I4]2- leading to different emissive properties. The thermochromic luminescence properties also displayed originate from the two independent emissive centers: (PrPh3P)+ and [Cu2I4]2-. Thanks to the combined luminescent mechanochromic properties and congruent melting of 1, luminescent and transparent glass films and mechanochromic crystalline films were prepared, demonstrating its potential for developing low-cost and environmentally friendly materials for a large panel of applications.