Mechanofluorochromism of a Series of Benzofuro[2,3-c]oxazolo[4,5-a]-carbazole-Type Fluorescent Dyes

Mechanofluorochromism is found for a series of benzofuro[2,3-c]oxazolo[4,5-a]-carbazole-type fluorophores (1a-1d) having cyano groups as acceptor: grinding of 1a-1d induces a fluorescent color change with an enhanced quantum yield, and the fluorescent color is recovered by heating or exposure to solvent vapor. Interestingly, both absorption and emission peaks are redshifted by grinding, in conflict with the common view. On the basis of time-resolved fluorescence spectroscopy, X-ray powder diffraction (XRD), differential scanning calorimetry (DSC), and single-crystal X-ray structuralanalysis, the mechanofluorochromism observed with this new class of fluorescent dyes is found to accompany a reversible switching between crystalline and amorphous states with changes of molecular arrangement and intermolecular pi-pi interactions. An essential role of a strong donor-pi-acceptor character of the dyes is pointed out for the manifestation of the observed mechanofluorochromism. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)