N-substituted quinoxalinobenzothiazine/iodonium salt systems as visible photoinitiators for hybrid polymerization

Sensitizers based on the 12-substituted 12H-quinoxalino-[2,3-b][1,4]-benzothiazine skeleton were synthesized and characterized using 1H NMR spectroscopy, mass spectrometry and elemental analysis. Their electrochemical and spectral properties, including absorption and emission spectra, fluorescence quantum yield and singlet lifetime were also measured. These benzothiazines combine with commercially available diphenyliodonium hexafluorophosphate to create novel visible initiator systems of free radical/cationic hybrid polymerization of glycidyl methacrylate. The efficiency of these initiator systems is discussed on the basis of the free energy change for electron transfer from the benzothiazines to the iodonium compound. During the photopolymerization photobleaching of the benzothiazines was observed; a mechanism to account for this bleaching is proposed.

► A two component initiator system for visible-light initiated hybrid polymerization is described. ► N-substituted benzothiazines are employed as oxidizable sensitizers. ► A diphenyliodonium salt as the co-initiator. ► A mechanism for the bleaching of benzothiazines was proposed.