Intramolecular
Charge Transfer and Ion Pairing in N,N-Diaryl Dihydrophenazine
Photoredox Catalysts for Efficient
Organocatalyzed Atom Transfer Radical Polymerization
posted on 2016-12-14, 00:00authored byChern-Hooi Lim, Matthew D. Ryan, Blaine G. McCarthy, Jordan C. Theriot, Steven M. Sartor, Niels H. Damrauer, Charles B. Musgrave, Garret M. Miyake
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.