Strategies for Red-Shifting Type I Photoinitiators: Internal Electric Fields versus Lewis Acids versus Increasing Conjugation

Time-dependent density functional theory calculations were performed on derivatives of Irgacure 2959, a water-soluble, acetophenone-Type photoinitiator, in order to assess the relative merits and drawbacks of three distinct ways of modifying its photochemistry: Lewis acid complexation, changing the amount of conjugation in the molecule, and application of an internal electric field through inclusion of a remote charged functional group. The effectiveness of each of the three methods was evaluated against the magnitude of the change in energy of the excited states. Internal electric fields were shown to provide the best method for targeting specific excited states in a controlled and rational manner. The other strategies also had significant effects but it was more difficult to independently target different transitions. Nonetheless, for the specific case of Irgacure 2959, we predict that its complexation with Mg²⁺ ions in a range of solvents will both red-shift the initiator's absorbance while improving its efficiency and it is thus a promising candidate for testing as a visible light photoinitiator.