Mechanofluorochromism of heteropolycyclic donor–π-acceptor type fluorescent dyes

This paper reports that mechanofluorochromism is found for a series of benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorophores (1a–5a) with different p-substituted phenyl groups as acceptor and dibutylamino groups as donor. Grinding of as-recrystallized dyes 1a–3a with strong electron-accepting group induces a fluorescent color change with an enhanced quantum yield and the fluorescent color is recovered by heating or exposure to solvent vapor. On the basis of experimental results and semi-empirical molecular orbital calculations (AM1 and INDO/S), we clarified that the mechanofluorochromism 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.