Mechanochromically luminescent cryst. org. materials that can reversibly change the color of their solid-state emissions upon exposure to mech. stimuli have been studied intensely in recent years owing to their wide range of potential applications. Although the mechanism of the mech. induced change of emission color is in most cases attributed to transitions between the cryst. and amorphous phases, the understanding of the relationship between the individual morphol. of a crystal (crystal habit) and its specific response to mech. stimuli still remains elusive. Herein, we demonstrate that differently substituted triphenylimidazolylbenzothiadiazole derivs. 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 derivs. form plate-like MCL crystals, whereas needle-like non-MCL microcrystals are obtained for the other derivs. with Me, methoxy, ethoxy, or trifluoromethyl substituents. We have confirmed that the slight modification of the steric bulk of a peripheral substituent dramatically changes both the morphol. and the MCL properties of these derivs. The mol. structures of the constituent compds. of both types of crystals have been detd. by single-crystal and powder X-ray diffraction analyses. A comparison of these structures revealed that a twisted mol. conformation is the origin of the mech.-stimuli-responsive emission color change in the plate-like crystals. In other words, mol. conformations fixed in the assembled structures det. the MCL properties of morphol. different crystals. The knowledge acquired in the present study can be expected to accelerate the development of cryst. MCL materials with controlled nano- or microstructures, with possible applications in microscale mechanosensors. Plate-like or needle-like crystals have been obtained for triphenylimidazolylbenzothiadiazole derivs. depending on the substituents on the Ph group attached to the benzothiadiazole ring. Nonsubstituted or fluoro-substituted derivs. form plate-like crystals and exhibit mechanochromic luminescence. In contrast, the emission color of the other derivs. that form needle-like microcrystals remains virtually unchanged after grinding. [on SciFinder(R)]
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