Influence of Dihydrophenazine Photoredox Catalyst Excited State Character and Reduction Potentials on Control in Organocatalyzed Atom Transfer Radical Polymerization

The development of N, N -diaryl dihydrophenazine organic photoredox catalysts (PCs) has enabled numerous examples of organocatalyzed atom transfer radical polymerization (O-ATRP) of methyl methacrylate (MMA) monomer to polymers with low dispersity ( Đ < 1.30) and near-unity initiator efficiency ( I * ∼ 100%), as well as small molecule synthesis. In this work, we investigate the influence of core substitution (CS) by alkyl, aryl, and heteroatom groups on singlet excited state reduction potential ( E S1 °*). We observe that a highly reducing E S1 °* is in part a result of a locally excited (LE)-dominated hybridized local and charge transfer (HLCT) excited state in CS PCs, which is influenced by the identity of the core substituent. Additionally, the PCs that possess a LE-dominated HLCT character maintain a relatively oxidizing PC radical cation oxidation potential ( E 1/2 ) for deactivation in O-ATRP compared to fully LE PCs reported in prior work. For example, a thiophenol core substituted (heteroatom CS, HetCS) PC shows the most negative E S1 °* (−2.07 V vs SCE), more LE character (Stokes shift = 124 nm), and has an oxidizing PC radical cation ( E 1/2 = 0.30 V vs SCE). The CS PCs with improved properties, including more negative E S1 °*, perform best in O-ATRP of MMA with the HetCS PC showing the best control in both DMAc ( Đ = 1.08, I* = 89%) and EtOAc ( Đ = 1.06, I* = 97%). Additionally, the HetCS PC was found to mediate the controlled polymerization of n -butyl acrylate ( n -BA) ( Đ = 1.24, I* = 97%), which has remained challenging in O-ATRP without supplemental deactivation strategies. An aryl CS PC was found to have moderate control as low as 1 ppm PC, indicating facilitation of low PC loadings ( Đ = 1.33, I* = 69%). The relationship between excited state character, E S1 °*, and polymerization control observed in this work provides a foundation for increasing the utility of phenazine PCs across photoredox catalysis.