Intramolecular Charge Transfer and Ion Pairing inN,N-Diaryl Dihydrophenazine Photoredox Catalysts for Efficient Organocatalyzed Atom Transfer Radical Polymerization

Abstract

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.

Photoinduced Organocatalyzed Atom Transfer Radical Polymerization Using Continuous Flow

“…17 In the case of PC 4, in typical batch reactions Đ of 1.25–1.17 and I * of ∼100% were reported. 24 A summary of key photophysical and redox properties of these PCs can be found in Table 1.…”

How the catalyst circulates and works in organocatalyzed atom transfer radical polymerization

“…[1][2][3] This technique provides the capability to synthesize well-defined functional polymers with a wide range of monomer types under mild conditions. [8][9][10][11][12][13][14][15][16][17][18] In O-ATRP systems, the organic photoredox catalysts (PCs) could completely replace transition metal catalysts. The high cost of post purification makes this technique losing competitiveness in terms of industrial application.…”

Strongly Reducing, Visible‐Light Organic Photoredox Catalysts as Sustainable Alternatives to Precious Metals

“…To address this challenge, we have introduced visible light organic PCs including perylene, [19a] N,N -diaryl dihydrophenazines, [19b, 23] and N -aryl phenoxazines [24] as organic PCs to mediate O-ATRP via an oxidative quenching pathway. [25] Dihydrophenazine and phenoxazine contain electron rich chromophore motifs that form rather stable radical cations upon oxidation and enable them to be strong excited state reductants.…”

“…[8b, 9a] Recently, we reported that the lowest energy excited state of dihydrophenazine 3 is also CT in nature. [23] Specifically, intramolecular CT occurs from the electron rich phenazine core (donor) to one of the 2-naphthyl N -substituents (acceptor). Here we show that phenoxazine PC 4 similarly undergoes photoinduced intramolecular CT, as evidenced by a significant solvatochromic effect in the emission (Figure 2A).…”