Fluorogenic heptazines functionalized with tetrazine rings for conjugation through inverse Electron Demand Diels-Alder reaction

Fluorogenic probes that can selectively target molecules or materials thanks to click chemistry are promising tools for labelling or functionalization. Heptazines are heterocycles that gather appealing electrochemical and photophysical properties, combining high photooxidation potentials with long fluorescence lifetimes. However, selective grafting of molecular heptazines on a substrate is still underdeveloped. In this work, we report the synthesis, photophysical and electrochemical properties of two heptazine derivatives conjugated with one and three tetrazine rings respectively. These heptazines conjugated to tetrazines are very weakly fluorescent. We demonstrate that they can react in a fast manner with a strained alkyne, through an Inverse Electron Demand Diels-Alder reaction. This reaction has been followed thanks to a home-built photokinetics setup: in a timecale of a few seconds blue emitting heptazine pyridazine conjugates are generated with an 8.6 to 13.6 fluorescence turnon.