Employing Singlet-Singlet Energy Transfer for Boosting the Reactivity of Type I Photoinitiators in Radical Photopolymerization.

A remarkable and unexpected increase in the photopolymerization efficiency of an acrylic resin by a bisacylphosphine oxide photoinitiator was observed when an optical brightener was present in the medium. High values for the maximal rates of photopolymerization were obtained by RT-FTIR at 365 nm under a very low irradiance of 1 mW/cm. Fluorescence studies revealed that the quenching process occurs through singlet-singlet energy transfer between the first singlet excited state of the optical brightener and the ground state photoinitiator. This mechanism acts as an additional pathway for the excitation of the photoinitiator, thereby increasing the total amount of initiating radicals. Using the Förster resonance energy transfer model, we calculated the relative efficiency of the photosensitization process with respect to the direct excitation efficiency of the photoinitiator. The results demonstrate that the photosensitization process can be predicted, paving the way for further improvements in photoinitiating systems.