Influence of DihydrophenazinePhotoredox CatalystExcited State Character and Reduction Potentials on Control in OrganocatalyzedAtom Transfer Radical Polymerization

The development of <i>N</i>,<i>N</i>-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 (<i>Đ</i> < 1.30) and near-unity initiator efficiency (<i>I</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 (<i>E</i>_S1°*). We observe that a highly reducing <i>E</i>_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 (<i>E</i>_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 <i>E</i>_S1°* (−2.07 V vs SCE), more LE character (Stokes shift = 124 nm), and has an oxidizing PC radical cation (<i>E</i>_1/2 = 0.30 V vs SCE). The CS PCs with improved properties, including more negative <i>E</i>_S1°*, perform best in O-ATRP of MMA with the HetCS PC showing the best control in both DMAc (<i>Đ</i> = 1.08, <i>I*</i> = 89%) and EtOAc (<i>Đ</i> = 1.06, <i>I*</i> = 97%). Additionally, the HetCS PC was found to mediate the controlled polymerization of <i>n</i>-butyl acrylate (<i>n</i>-BA) (<i>Đ</i> = 1.24, <i>I*</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 (<i>Đ</i> = 1.33, <i>I*</i> = 69%). The relationship between excited state character, <i>E</i>_S1°*, and polymerization control observed in this work provides a foundation for increasing the utility of phenazine PCs across photoredox catalysis.