Solvatochromism and Click Reactivity of Yellow Tetrazines.

Hydroxypyridyl-substituted 1,2,4,5-tetrazines have recently been developed as advanced scissors for bioorthogonal click-to-release chemistry. In contrast to the otherwise characteristic pink color of 1,2,4,5-tetrazines, these compounds were observed to give yellow solutions in methanol and buffered aqueous media. This study shows that the solvatochromism of hydroxypyridyl-tetrazines has its origin in varying protonation states of the phenolic OH group and reveals the impact of deprotonation on bioorthogonal click reactivity. We demonstrate that the rate of the Diels-Alder cycloaddition is significantly affected by the pH of the aqueous solution, thereby providing critical insights for the development of molecular tools for bioorthogonal bond cleavage.