S-Tetrazine Derivative Exhibiting Unprecedented Polymorphism-Dependent Emission Properties

•Bis(4-methylphenoxy)tetrazine exhibited external stimuli-responsive photoluminescence colors in the condensed state.•Thermoresponsive polymorphism was observed showing emission colors of green, yellow, and red in addition to the traditional orange color.•The original packing structure and photoluminescence color could almost restore by solvent vapor exposure.•Flexible crystals were formed and showed mechanical bending motions.