Radical Addition to N,N-Diaryl Dihydrophenazine Photoredox Catalysts and Implications in Photoinduced Organocatalyzed Atom Transfer Radical Polymerization

Abstract

Photoinduced organocatalyzed atom transfer radical polymerization (O-ATRP) is a controlled radical polymerization methodology catalyzed by organic photoredox catalysts (PCs). In an efficient O-ATRP system, good control over molecular weight with an initiator efficiency (I* = M n,theo /M n,exp × 100%) near unity is achieved, and the synthesized polymers possess a low dispersity (Đ). N,N-Diaryl dihydrophenazine catalysts typically produce polymers with low dispersity (Đ < 1.3) but with less than unity molecular weight control (I* ∼ 60−80%). This work explores the termination reactions that lead to decreased control over polymer molecular weight and identifies a reaction leading to radical addition to the phenazine core. This reaction can occur with radicals generated through reduction of the ATRP initiator or the polymer chain end. In addition to causing a decrease in I*, this reactivity modifies the properties of the PC, ultimately impacting polymerization control in O-ATRP. With this insight in mind, a new family of core-substituted N,N-diaryl dihydrophenazines is synthesized from commercially available ATRP initiators and employed in O-ATRP. These new core-substituted PCs improve both I* and Đ in the O-ATRP of MMA, while minimizing undesired side reactions during the polymerization. Further, the ability of one core-substituted PC to operate at low catalyst loadings is demonstrated, with minimal loss of polymerization control down to 100 ppm (weight average molecular weight [M w ] = 10.8 kDa, Đ = 1.17, I* = 104% vs M w = 8.26, Đ = 1.10, I* = 107% at 1000 ppm) and signs of a controlled polymerization down to 10 ppm of the catalyst (M w = 12.1 kDa, Đ = 1.36, I* = 107%).

Phenanthrene-Extended Phenazine Dication: An Electrochromic Conformational Switch Presenting Dual Reactivity

“…These derivatives are known to be easily oxidized and have been synthesized by various groups, 27 − 31 leading to their use as emitters, redox-active materials, or organocatalysts. 32 − 37 Interestingly, while the radical cations of such derivatives have been extensively explored, 30 , 34 , 38 , 39 the aromatic dicationic diamagnetic states have been widely neglected and studied almost exclusively in solution, 39 , 40 with some notable exceptions successfully isolating N-alkyl derivatives of the parent phenazinium dication, which, however, were reported as highly reactive ( Figure 1 ). 41 , 42 More recently, an elegant work from the group of Shinokubo reported the synthesis of an anthracene-extended dihydrophenazine ( Figure 1 ), which was oxidized in solution to the radical and dicationic states, resulting in a marked electrochromism.…”

Removal of photoredox catalysts from polymers synthesized by organocatalyzed atom transfer radical polymerization

“…To investigate the removal of PCs from polymers synthesized by O‐ATRP, PC 1 was used to synthesize PMMA under previously reported conditions 8,17 . Large scale O‐ATRP polymerizations often suffer from non‐uniform irradiation and produce poorly defined polymers, 18 so multiple small‐scale polymerizations were performed to maintain optimal polymerization control.…”

“…Reported oxidation potentials 7–10,17,21,22 measured by cyclic voltammetry for photoredox catalysts (PCs) used to synthesize poly(methyl methacrylate) via organocatalyzed atom transfer radical polymerization. Solvent used in cyclic voltammetry is indicated under E 1/2 values.…”

Design strategies and applications of novel functionalized phenazine derivatives: a review

“…Surprisingly, N , N ′-dihydrophenazine and its derivatives demonstrate a distinctive vibration-induced emission (VIE) behavior that facilitates multicolor emission owing to excited-state configuration transformations from bending to planar. 67–73 Most notably, the photophysical properties of dihydrophenazine derivatives might generate extraordinary stimuli responsiveness to external environmental factors, such as temperature, humidity, pH, solvent polarity and viscosity. 74–80 As a consequence, through altering the environmental factors, such special VIE-active molecules might achieve a significant change in fluorescence color from blue to orange-red and even white.…”