Mechanochromic hydrogen-bonded cocrystals with a salient effect upon heating†

Abstract

Herein, we report the characterization of two hydrogen-bonded cocrystals, termed AC-TFTA and AC-PFBA, with mechanofluorochromism upon grinding. The heating of single crystals of AC-TFTA promotes microscopic displacements due to a decarboxylation process, which was studied through calorimetry-based techniques, gas chromatography/mass spectrometry, and hot-stage microscopy. Using solid-state nuclear magnetic resonance (ssNMR), powder X-ray diffraction (PXRD), and UV-Vis/fluorescence spectroscopy, we demonstrated that AC-TFTA displays sharper photophysical changes than its analog AC-PFBA, attributable to differences in the energies of their non-covalent interactions. Furthermore, the reversibility of the amorphous phase in both cocrystals was explored. However, fatigue tests led us to conclude that AC-TFTA displays potential for application as an anticounterfeiting agent, in contrast with the more robust crystalline AC-PFBA. This work emphasizes the importance of cocrystallization in generating accessible and functional mechanofluorochromic materials.