Mechanochromic Luminescence Based on Crystal-to-Crystal Transformation Mediated by a Transient Amorphous State

Photoluminescent materials that exhibit tunable emission properties when subjected to mechanical stimuli have numerous potential applications. Although many organic/inorganic and organometallic compounds display this property, called mechanochromic luminescence, most of these materials undergo a crystalline-to-amorphous (C -> A) phase transition; examples of crystalline-to-crystalline (C-1 -> C-2) transformation are rare. Single-crystal X-ray diffraction may allow direct analysis of the molecular packing of mechanochromic luminescence materials before and after C-1 -> C-2 transformation, which may help to understand the underlying mechanism of this transformation. Reported herein is a mechanochromic luminescence material that displays an unprecedented type of C-1 -> C-2 transformation mediated by a transient amorphous phase (C-1 -> [A] -> C-2). This mechanochromic luminescence material was developed by introducing soft triethylene glycol side chains in a crystalline gold(I) complex that exhibits mechanochromic luminescence based on a C A phase transition. When this new gold(I) complex bearing triethylene glycol chains was subjected to a mechanical or thermal stimulus, dynamic phase changes were observed with irreversible luminescence color changes from blue to yellow to green in both the cases. The crystallinity of the mechanically generated C-2 phase was lower than that of the thermally generated C-2 phase. This is because the mechanically induced C-1 -> [A] -> C-2 process was finished within seconds, whereas the thermal C-1 -> [A] -> C-2 process occurred over a few minutes. To control the C-1 -> [A] -> C-2 transformation, we doped the complex with an inactive soft component. This successfully made the transformation reversible (from green to blue) upon thermal annealing of the mechanically obtained C-2 phase. This approach allowed the development of an imaging process involving invisible information storage even under UV illumination.