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

Abstract

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. © 2021 Society of Industrial Chemistry.