s-Tetrazine derivative exhibiting unprecedented polymorphism-dependent emission properties

The first example of a tetrazine derivative exhibiting emission color-tunable crystals upon external stimuli such as heat and an organic solvent is described. Bis(4-methylphenoxy)tetrazine (1) forms several crystalline phases by heating and cooling processes accompanying unprecedented photoluminescence color transitions including green, yellow, red, and traditional orange colors. These thermoresponsive polymorphisms and colors could be also reversibly recovered to their original state by solvent vapor annealing. The formation of mild excimer-like species and energy state transitions (much-stabilized ground state than the S1 state) presumably contributed to the unique bathochromic and hypsochromic shifts, respectively. In addition, a novel mechanical feature from 1 -the formation of flexible crystals- was achieved, which plausibly originated from the unique T-shape arrangement of the tetrazine rings and the phenyl rings observed in the molecular packing.