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 reductive cleavage of the N−O bond to give an alkoxy radical that can be used to initiate polymerization. Bond-dissociation energies obtained from calculations based on density-functional theory are in agreement with predictions 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 and indicate that the fragmentation of the radical is highly exothermic. The energetic requirements for the photochemical electron transfer are discussed in terms of a simplified model that shows that the initiation efficiency of the radical polymerization can be correlated with a single parameter, the reduction potential of the sensitizing dye. Dyes from many classes and with absorption bands spanning the entire visible region were found to be effective in initiating photopolymerization of acrylate monomers in this system. Doubling of the photoresponse can be achieved through coupling of the reductive cleavage of the N-alkoxypyridinium with an oxidative cleavage of a C−B bond of an alkyltriarylborate, a process that utilizes the chemical potential stored in the oxidized dye following electron transfer to the pyridinium salt.