Donor-acceptor π-conjugated quinoxaline derivatives exhibiting multi-stimuli-responsive behaviors and polymorphism-dependent multicolor solid-state emission

Two phenothiazine-based quinoxaline derivatives QPT1 and QPT2 have been rationally designed and synthesized. Optical analysis and theoretical chemical calculations illustrated that they showed typical intramolecular charge transfer (ICT) process from electron-rich phenothiazine unit to electron-withdrawing quinoxaline moiety. Interestingly, QPT1 exhibited the polymorphism feature and high-contrast mechanochromic luminescence. Two types of crystals were prepared by solvent evaporation method. A green needle-like crystal (G-form) and a yellow ribbon-like crystal (Y-form) were yielded from dichloromethane/petroleum ether and THE solution, respectively. The emission peaks of G-form and Y-form in pristine powders appeared at 501 nm and 549 nm, and red-shifted to 570 nm and 594 nm in ground powders, respectively, giving the emission spectral shifts of 69 nm for G-form and 45 nm for Y-form. Besides, QPT2 exhibited moreover autonomously self-recovering mechanochromic luminescence. X-ray diffraction measurements confirmed that the reversible phase transition between the crystalline and amorphous states has been proposed as the MFC mechanism. Moreover, QPT1 and QPT2 also showed stable acidofluorochromic characteristics. The solution and film of QPT1 and QPT2 achieved a fast response to TFA with high sensitivity and low detection limit. The multiple stimuli responsive performances of QPT1 and QPT2 make them potential candidates for smart materials.