Phenothiazine-based benzoxazole derivates exhibiting mechanochromic luminescence: the effect of a bromine atom

Two phenothiazine-based benzoxazole derivates (PVB and BPVB) were designed and synthesized, and their photophysical properties were studied and compared. The results showed that both compounds emitted strong fluorescence in solution and in the solid state. Their emission wavelengths were also strongly affected by solvent polarity, indicating intramolecular charge transfer (ICT) transitions. Interestingly, the two compounds exhibited different mechanochromism. The ground film of PVB without bromine atoms can self-cure and change into the original film within 15 min at room temperature. The fluorescence of the ground film for BPVB showed no changes for at least 2 weeks. Meanwhile, a higher contrast fluorescence change was observed for the solid film of BPVB with bromine atoms under grinding, although PVB and BPVB exhibited similar spectral red shifts after grinding. Based on the spectral results and single-crystal structures, a mechanochromic mechanism is given.