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Relationship between Crystal Structure, Crystal Morphology, and Mechanochromic Luminescence of Triphenylimidazolylbenzothiadiazole Derivatives
journal contribution
posted on 2020-06-01, 12:05 authored by Suguru Ito, Sayaka Nagai, Takashi Ubukata, Takuya Ueno, Hidehiro UekusaMechanochromically
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.