Enhanced activity as polymerization photoinitiators of Rose Bengal and Eosin esters with an O-benzoyl-α-oxooxime group: The role of the excited state reactivity

New Eosin and Rose Bengal ester with an O-benzoyl-α-oxooxime group were synthesized and evaluated as visible light photoinitiators for polymerization in the presence of an amine. The dyes EO-AO and RB-AO are clearly better photoinitiators, with higher Rp values, than the corresponding physical mixtures dye/acyloxime, or than the corresponding pure dyes. However, the relative efficiencies of the different dye/amine and dye/amine/AO mixtures show that if RB is better than EO in the binary mixture, the situation was reversed after addition of AO. The spectroscopic and electrochemical properties of the dyes were measured and the forecasts of thermodynamic calculations were confirmed by time resolved laser spectroscopy experiments which show that the species formed by reaction of the excited dyes eosin and RB with AO were not the same. One observes the dominating formation of the oxidized form of the acyloxime with eosin and its reduced form in the case of Rose Bengal which lead in the case of EO to a larger amount of radicals per photon absorbed and a better initiating efficiency.