OD1–OD4 bearing various counteranions (X−=Cl−, Br−, I−, or BPh4−) have been designed and developed to gain greater insight into the influences of the electronic structures of D–π-A type pyridinium dye on the specific solvatochromism in halogenated solvents: the intramolecular charge transfer absorption bands of the dyes OD1–OD4 in halogenated solvents show a large bathochromicshift compared with those
We have newly designed and synthesized unsymmetrical carbazole-type D-pi-A fluorescent dyes. The dyes show a bathochromic shift-type mechanofluorochromism (MFC): grinding of as-recrystallized dyes induces a bathochromic shift of fluorescent color and the fluorescent color is recovered by heating or exposure to solvent vapor. In order to clarify the MFC mechanism for the carbazole-type D-pi-A fluorescent dyes, time-resolved fluorescence spectroscopy, X-ray powder diffractometry, single-crystal X-ray structural analysis, IR spectroscopy, and differential scanning calorimetry are 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 intermolecular hydrogen bonding and pi-pi interaction. (C) 2011 Elsevier Ltd. All rights reserved.