Mechanofluorochromism of D-π-A Fluorescent Dyes

Mechanofluorochromic dyes can exhibit a change in fluorescent colour by external mechanical stimuli to the solids, being accompanied by a reversion to the original fluorescent color by heating or exposure to solvent vapor, which have attracted much attention from chemists, physicists, and engineers because of enormous scientific interest in the fundamental research fields of photochemistry and photophysics and interesting smart materials for next generation optoelectronic devices. We have found that newly developed heteropolycyclic donor-acceptor π-conjugated (D-π-A) fluorescent dyes with electron-withdrawing substituents as acceptor show a bathochromic shift-type mechanofluorochromism (MFC). Our work revealed that the MFC of D-π-A fluorescent dyes is attributed to a reversible switching between crystalline and amorphous states with changes of dipole-dipole interaction and intermolecular π-π interaction. We propose that the most important point for developing MFC dyes is to design D-π-A fluorescent dye molecules with moderate dipole moments (ca. 5 Debye) which are controlled by tuning the electron-donating ability of D, electron-accepting ability of A, steric size of a substituent, and D-π-A system. On the other hand, we demonstrated that the bathochromic shift-type MFC of unsymmetrical carbazole-type D-π-A fluorescent dyes is attributed to a reversible switching between crystalline and amorphous states with changes of intermolecular hydrogen bonding and π-π interaction. We also propose that the use of various counter anions for D-π-A pyridinium dyes can be one of the most effective strategies, not only to express a pronounced MFC, but also to control changes of fluorescence properties by the grinding and heating, resulting in fine-tuning of fluorescent color change by the MFC.