Donor-acceptor 1,2,4,5-Tetrazines Prepared by 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, in the synthesis of 1,2,4,5-tetrazines is presented. The important factors of Buchwald-Hartwig cross-coupling reaction for tetrazine functionalization are discussed, which could be of practical importance for researchers performing Buchwald-Hartwig coupling reaction on difficult substrates. Donor-acceptor tetrazine molecules (TA1-TA7) are conveniently prepared in high yields (61%-72%) which could act as new tetrazine turn-on probes by inverse Electron Demand Diels-Alder (iEDDA) reaction. The fluorescence turn-on property is presented by exemplifying the reaction of tetrazine TA2 with cyclooctyne. The photophysics and electrochemistry of the resulting pyridazines are also presented, of which two pyridazines DA3 and DA6 exhibit promising room temperature phosphorescence (RTP) properties.