Energetics of electron-transfer reactions of photoinitiated polymerization: Dye-sensitized fragmentation of N-alkoxypyridinium salts

Electron transfer from excited dyes to N-alkoxypyridinium salts leads to re ductive cleavage of the N-O bond to give an alkoxy radical that can be used to initiate polymerization. Bond-dissociation energies obtained from calcu lations based on density-functional theory are in agreement with prediction s from a thermochemical cycle. These data show a difference of ca. 290-315 kJ/mol between the BDE of the pyridinium and that of the pyridyl radical an d indicate that the fragmentation of the radical is highly exothermic. The energetic requirements for the photochemical electron transfer are discusse d in terms of a simplified model that shows that the initiation efficiency of the radical polymerization can be correlated with a single parameter, th e reduction potential of the sensitizing dyed Dyes from many classes and wi th absorption bands spanning the entire visible region were found to be eff ective in initiating photopolymerization of acrylate monomers in this syste m. Doubling of the photoresponse can he achieved through coupling of the re ductive cleavage of the N-alkoxypyridinium with an oxidative cleavage of a C-B bond of an alkyltriarylborate, a process that utilizes the chemical pot ential stored in the oxidized dye following electron transfer to the pyridi nium salt.