Structure–Property–Performance Relationships of Tetra-Alkylated Phenazine Photoredox Catalysts in Organocatalyzed Atom Transfer Radical Polymerization

Organocatalyzed atom transfer radical polymerization (O-ATRP) is a light-driven controlled radical polymerization method employing organic photoredox catalysts (PCs) for the synthesis of polymers with targeted molecular weights (initiator efficiency [I*] ∼ 100%) and low dispersity (Đ < 1.3). Understanding the impact of structural modification on PC properties and control in O-ATRP requires extensive and systematic photoredox characterization and implementation to address current limitations like increasing monomer scope and lowering catalyst loading. In this work, the structure–property relationships of tetra-alkylated N,N-diaryl dihydrophenazines and their performance in O-ATRP are explored. Ethyl α-bromophenylacetate (EBP), itself a common alkyl-bromide initiator in O-ATRP, is used in a photochemical radical coupling to generate the novel core substituted PCs. These tetra-alkylated PCs (1a and 2a) have greatly improved triplet yields (ϕISC ∼ 17% for PC 1a and 12% for PC 2a) and triplet lifetimes that are an order-of-magnitude longer than their parent PCs (τ = 11.3 μs for PC 1a and 220 μs for PC 2a). They also exhibit increased locally excited character in their triplet excited state while retaining charge-transfer character in their singlet excited state. 1a and 2a show improved control in O-ATRP over their forebears and can be employed at lower catalyst loadings. We attribute this improved performance to their higher molar absorptivity, longer lifetimes, and higher triplet yields. The design, characterization, and application of the PCs herein contribute to structure-property-performance relationships that will guide future structural modifications for producing high-performing PCs for O-ATRP as well as other macromolecular and small molecule transformations.