Highly conjugated isoindigo and quinoxaline dyes as sunlight photosensitizers for onium salt-photoinitiated cationic polymerization of epoxy resins

In this study, photoinitiated cationic polymerization of epoxy resins by photoinduced electron transfer reactions using four new chromophoric dyes, namely (3E)-1,1 '-bis(2-ethylhexyl)-6,6 '-dipyren-1-yl-3,3 '-biindole-2,2 '(1H,1 ' H)-dione (ISOIV), (3 ' E)-1,1 '''-diethyl-1 ',1 ''-bis(2-ethylhexyl)-1H,1 ''' H-5,6 ':3 ',3 '':6 '',5 '''-quaterindole-2 ',2 ''(1 ' H,1 '' H)-dione (ISOV), (3E)-6,6 '-bis(9-ethyl-9H-carbazole-3-yl)-1-[(2R)-2-ethylhexyl]-1 '-[(2S)-2-ethylhexyl]-3,3 '-biindole-2,2 '(1H,1 ' H)-dione (ISOVIII) and 3,3 '-(6-bromokinoksalin-2,3-diyl)bis(9-ethyl-9H-carbazole) (TPDC6), and diphenyliodonium hexafluorophosphate (Ph2I+PF6-; DPI) under sunlight is reported. Upon irradiation, photoexcited dye forms an exciplex with DPI. Electron transfer reactions within the exciplex result in the formation of ionic species capable of initiating ring-opening polymerization of an epoxy resin. It is shown that among the investigated dyes, ISOVIII and TPDC6 exhibited higher initiation efficiency in accordance with their stronger electron-donating nature due to the presence of carbazole groups. (c) 2021 Society of Industrial Chemistry.