Intramolecular Charge Transfer and Ion Pairing in N,N-Diaryl Dihydrophenazine Photoredox Catalysts for Efficient Organocatalyzed Atom Transfer Radical Polymerization

Photoexcited intramolecular charge transfer (CT) states in N,N-diaryl dihydrophenazine photoredox catalysts are accessed through catalyst design and investigated through combined experimental studies and density functional theory (DFT) calculations. These CT states are reminiscent of the metal to ligand charge transfer (MLCT) states of ruthenium and iridium polypyridyl complexes. For cases where the polar CT state is the lowest energy excited state, we observe its population through significant solvatochromic shifts in emission wavelength across the visible spectrum by varying solvent polarity. We propose the importance of accessing CT states for photoredox catalysis of atom transfer radical polymerization lies in their ability to minimize fluorescence while enhancing electron transfer rates between the photoexcited photoredox catalyst and the substrate. Additionally, solvent polarity influences the deactivation pathway, greatly affecting the strength of ion pairing between the oxidized photocatalyst and the bromide anion and thus the ability to realize a controlled radical polymerization. Greater understanding of these photoredox catalysts with respect to CT and ion pairing enables their application toward the polymerization of methyl methacrylate for the synthesis of polymers with precisely tunable molecular weights and dispersities typically lower than 1.10.