Mechanochromism of a dumbbell D–π–A–π–D phenothiazine derivative
Phenothiazine has been regarded as a building block for the construction of materials that are responsive to mechanical force. These materials are typically D–π–A molecules that consist of one phenothiazine moiety. Mechanochromic (MC) molecules with two phenothiazine units are rare. In this study, a dumbbell D–π–A–π–D molecule (i.e., PBPAN) with two electron-withdrawing cyano groups and two electron-donating phenothiazine moieties was designed and synthesized. Then, the mechanochromism of PBPAN was investigated. The orange pristine PBPAN solids emitted orange fluorescence with the maximum at 567 nm. The fluorescence color changed to red with the maximal peak at 635 nm. Such a change indicated a bathochromic shift of 68 nm. The pristine solids were ground to form red solids. The fluorescence could be recovered through solvent fuming. X-ray diffraction patterns suggested that the MC process was accompanied by a reversible phase transition between the crystalline and amorphous states. Results illustrated that the conversion of the π-stacking type from H-aggregate to J-aggregate is responsible for the mechanochromism.