Excited state reactions of acridinedione dyes with onium salts: mechanistic details

Diaryliodonium and triarylsulphonium salts are thermally stable UV photoini tiators for cationic polymerization. Diaryliodonium salts (DPI-1-7) and tri arylsulphonium (type-I, type-II and type-III) salts with different substitu ent have been prepared. Excited state reactions of these onium salts with d ecahydroacridinedione dyes (ADD-1-6) have been studied. ADD acts as an effi cient sensitizer for the decomposition of these onium salts. Photosensitiza tion occurs through electron transfer, which is confirmed by the observatio n of enol radical cation of ADD and diarylsulphinium radical cation in the laser flash photolysis. The mechanism of photosensitization and the mechani sm of photoacid release have been discussed. Both singlet and triplet state of the ADD are involved in the photosensitization of diaryliodonium salts, whereas singlet state of ADD is alone involved in the photosensitization o f triarylsulphonium salts. The quantum yield of photoacid generated in dire ct and the sensitized process is determined and this allows to gauge the pr actical efficiency of onium salts/ADD combinations as the photoinitiator fo r the cationic polymerization applications. (C) 2001 Elsevier Science B.V. All rights reserved.