Synthesis and photochemical reaction of novel, visible-wavelength oxidizable polymerization sensitizer based on the 12H-quinoxalino[2,3-b][1,4]benzothiazine skeleton

Novel dyes based on the 12H-quinoxalino[2,3-b] [1,4]benzothiazine skeleton were synthesized and subsequently characterized using 1H NMR. Their electrochemical and spectral properties, such as absorption and emission spectra, quantum yield of fluorescence, and quantum yield of singlet oxygen formation, were measured. These compounds were evaluated as sensitizers for alkoxypyridinium salt photodecomposition, and the results are discussed on the basis of the free energy change for electron transfer from benzothiazine dyes to alkoxypyridinium compounds. Benzothiazine dyes are useful oxidizable sensitizers for N-alkoxypyridinium photoinitiators. The mechanism of the dye photobleaching is supported by time-dependent density functional theory (TD-DFT) calculations and the quantum yields of sensitized proton formation. Photoredox pairs consisting of benzothiazine dyes and alkoxypyridinium salt were found to be effective initiation systems for free radical polymerization of methyl acrylate and trimethylolpropane triacrylate (TMPTA) using visible light.