Relationship between Crystal Structure, Crystal Morphology, and Mechanochromic Luminescence of Triphenylimidazolylbenzothiadiazole Derivatives

Mechanochromically luminescent crystalline organic materials that can reversibly change the color of their solid-state emissions upon exposure to mechanical stimuli have been studied intensely in recent years owing to their wide range of potential applications. Although the mechanism of the mechanically induced change of emission color is in most cases attributed to transitions between the crystalline and amorphous phases, the understanding of the relationship between the individual morphology of a crystal (crystal habit) and its specific response to mechanical stimuli still remains elusive. Herein, we demonstrate that differently substituted triphenylimidazolylbenzothiadiazole derivatives exhibit distinct mechanochromic luminescence (MCL) if they crystallize as plate-like crystals, but those that crystallize as needles do not. Specifically, nonsubstituted and fluoro-substituted derivatives form plate-like MCL crystals, whereas needle-like non-MCL microcrystals are obtained for the other derivatives with methyl, methoxy, ethoxy, or trifluoromethyl substituents. We have confirmed that the slight modification of the steric bulk of a peripheral substituent dramatically changes both the morphology and the MCL properties of these derivatives. The molecular structures of the constituent compounds of both types of crystals have been determined by single-crystal and powder X-ray diffraction analyses. A comparison of these structures revealed that a twisted molecular conformation is the origin of the mechanical-stimuli-responsive emission color change in the plate-like crystals. In other words, molecular conformations fixed in the assembled structures determine the MCL properties of morphologically different crystals. The knowledge acquired in the present study can be expected to accelerate the development of crystalline MCL materials with controlled nano- or microstructures, with possible applications in microscale mechanosensors.