Achieving dual-state emission and high-contrast mechanochromic luminescence based on donor-acceptor type phenothiazine methanone derivatives

The development of fluorescent materials that can achieve dual-state emission (DSE) and high-contrast mechanochromic luminescence has attracted more and more attention. However, such materials are very rare, and there are no clear molecular design principles to follow. In current work, two novel DSE emitters are designed and synthesized by modifying phenothiazine and benzene or naphthalene rings on both sides of the methanone. Their photophysical properties in both dilute solution and the solid state are studied and compared. Optical researches have shown that D-A type phenothiazine methanone derivatives exhibit well-defined ICT (intramolecular charge transfer) characteristics. At the same time, they show intense luminescent efficiencies in both solution (ΦPTZ-CO-B = 0.645; ΦPTZ-CO-NA = 0.624) and solid states (ΦPTZ-CO-B = 0.386; ΦPTZ-CO-NA = 0.357), indicating their DSE properties. It is noteworthy that compounds PTZ-CO-B and PTZ-CO-NA exhibit significant fluorescence response to mechanical force, and the emitting wavelengths are red-shifted by 33 nm and 46 nm, respectively. XRD patterns and DSC curves confirm that the mechanochromism results from reversible changes in stacking pattern. The current work develops materials that can achieve DSE emission and high-contrast mechanochromism, laying the foundation for the development of such materials.