Donor–Acceptor 1,2,4,5-Tetrazines Prepared by the Buchwald–Hartwig Cross-Coupling Reaction and Their Photoluminescence Turn-On Property by Inverse Electron Demand Diels–Alder Reaction

A facile efficient synthetic tool, Buchwald–Hartwig cross-coupling reaction, for the functionalization of 1,2,4,5-tetrazines is presented. Important factors affecting the Buchwald–Hartwig cross-coupling reaction have been optimized. Seven new donor–acceptor tetrazine molecules (TA1–TA7) were conveniently prepared in good to high yields (61–72%). They have been subsequently engaged in the inverse electron demand Diels–Alder (iEDDA) reaction with cyclooctyne. The photophysical and electrochemical properties of the new pyridazines have been studied. Some are fluorescent acting as turn-on probes. More importantly, two pyridazines (DA3 and DA6) exhibit room-temperature phosphorescence (RTP) properties.