Heteropolycyclic donor–acceptor π-conjugated (D–π–A) fluorescent dyes with strong electron-withdrawing substituents show mechanofluorochromism (MFC): grinding of as-recrystallized dyes induces a fluorescent color change accompanying an enhanced fluorescence quantum yield, followed by a reversion to the original fluorescent color by heating or exposure to solvent vapor. Interestingly, both absorption and emission peaks are red-shifted by grinding, and the degrees of the red-shift are dependent on the electron-accepting ability of acceptor, steric sizes of the substituents, and D–π–A system. In order to clarify the mechanism of MFC observed with this new class of D–π–A fluorescent dyes, time-resolved fluorescence spectroscopy, X-ray powder diffractometry, differential scanning calorimetry and density measurements were performed before and after grinding of the solids. On the basis of experimental results and semi-empirical molecular orbital calculations (AM1 and INDO/S), we have revealed that the MFC is attributed to a reversible switching between crystalline and amorphous states with changes of dipole–dipole interaction and intermolecular π–π interaction by changes of the densities of the solids before and after grinding. This study demonstrates that the most important point for developing mechanofluorochromic dyes is to design fluorescent dye molecules of strong D–π–A characters with large dipole moments.
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