Novel photoinitiator systems working under visible radiation were studied. The photoredox pair constructed with dye derivatives of 12H-quinoxalino-[2,3-<i>b</i>][1,4]-benzothiazine (<b>1–3</b>) and 2,4,6-tris(trichloromethyl)-1,3,5-triazine (<b>Tz</b>) were found to be effective initiators for free radical polymerization of trimethylolpropane triacrylate (TMPTA) using VIS light. Photosensitization occurred through electron transfer, which was confirmed by the observation of a radical cation of the studied dyes. The <b>1<sup>•+</sup> </b> was also characterized in cryogenic matrices (mixture of CH<sub>2</sub>Cl<sub>2</sub> and ionic liquid: 1-butyl-3-methylimidazolium hexafluorophosphate (<svg style="vertical-align:-3.3907pt;width:78.625px;" id="M1" height="18.3375" version="1.1" viewBox="0 0 78.625 18.3375" width="78.625" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(.017,-0,0,-.017,.062,14.05)"> <path id="x42" d="M372 352v-2q176 -27 176 -159q0 -99 -95 -151q-73 -40 -196 -40h-221v28q65 5 78.5 19t13.5 76v404q0 63 -12 77t-72 18v28h257q102 0 150 -34q26 -18 41 -48.5t15 -65.5q0 -111 -135 -150zM213 362h47q80 0 117.5 32t37.5 96q0 56 -33 92t-100 36q-41 0 -56 -10
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t96 -62.5q53 0 78 45.5t25 105.5q0 68 -35 116t-99 48z" /> </g> </svg> <sup>−</sup>)) and its reactivity was investigated by means of pulse radiolysis in solution at room temperature. In a halogenated solvent and in a mixture of CH<sub>2</sub>Cl<sub>2</sub> and <svg style="vertical-align:-3.3907pt;width:78.625px;" id="M2" height="18.3375" version="1.1" viewBox="0 0 78.625 18.3375" width="78.625" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"> <g transform="matrix(.017,-0,0,-.017,.062,14.05)"> <use xlink:href="#x42" /> </g> <g transform="matrix(.017,-0,0,-.017,10.058,14.05)"> <use xlink:href="#x4D" /> </g> <g transform="matrix(.017,-0,0,-.017,25.204,14.05)"> <use xlink:href="#x49" /> </g> <g transform="matrix(.017,-0,0,-.017,31,14.05)"> <use xlink:href="#x4D" /> </g> <g transform="matrix(.012,-0,0,-.012,46.15,5.887)"> <use xlink:href="#x2B" /> </g> <g transform="matrix(.017,-0,0,-.017,53.75,14.05)"> <use xlink:href="#x50" /> </g> <g transform="matrix(.017,-0,0,-.017,63.252,14.05)"> <use xlink:href="#x46" /> </g> <g transform="matrix(.012,-0,0,-.012,72.225,18.137)"> <use xlink:href="#x36" /> </g> </svg> <sup>−</sup>, the radical cation <b>1<sup>•+</sup> </b> underwent deprotonation to form a neutral radical <b>1<sup>•</sup> </b>, which was stable in the second time scale. During photolysis of the <b>1/Tz</b> photoredox pair in 1-methyl-2-pyrrolidone and monomer, the formation of a neutral radical <b>1<sup>•</sup> </b> was not observed.