Reducing Organic Photoredox Catalysts for Visible Light-Driven Polymer and Small Molecule Synthesis

Introduction

Photoredox catalysis has emerged as a powerful synthetic methodology to form challenging covalent bonds under mild reaction conditions using light irradiation. Recently, phenoxazine (901111) and dihydrophenazine (901112) organic photoredox catalysts were computationally designed and developed to possess photophysical and electrochemical properties pertinent to photoredox catalysis. Specifically, they absorb visible light, have high molar absorptivity, redox reversibility, charge transfer excited states, long triplet excited state lifetimes, and high triplet quantum yields.

Phenoxazine- and dihydrophenazine-derived compounds were demonstrated in the application of photoredox catalyzed atom transfer radical polymerization (ATRP) for controlled polymer synthesis and small molecule transformations such as trifluoromethylation and dual photoredox/nickel catalyzed C-N and C-S cross-couplings. In collaboration with the Garret Miyake group, we now offersphenoxazine and dihydrophenazine organic photoredox catalysts.

Figure 1. Phenoxazine (901111) and dihydrophenazine (901112) organic photoredox catalysts.

E0 (2PC•+/3PC*) = -1.80 V vs. SCE	E0 (2PC•+/3PC*) = -1.80 V vs. SCE
E0 (2PC•+/1PC) = 0.65 V vs. SCE	E0 (2PC•+/1PC) = 0.21 V vs. SCE
Etriplet = 2.45 V	Etriplet = 1.90 V
λmax,abs = 388nm (ε = 26600 M-1cm-1)	λmax,abs = 343nm (ε = 5950 M-1cm-1)
𝜏triplet = 480 µs (Φtriplet = 90 %)	𝜏triplet = 480 µs (Φtriplet = 2 %)

Advantages

• Cost-effective and sustainable alternatives to precious metal complexes
• Highly reducing excited states
• Exhibiting properties important for photoredox catalysis: visible light absorption, high molar absorptivity, redox reversibility, charge transfer excited states, long triplet excited state lifetimes, and high triplet quantum yield

Applications

References

1. Du Y, Pearson RM, Lim C, Sartor SM, Ryan MD, Yang H, Damrauer NH, Miyake GM. 2017. Strongly Reducing, Visible-Light Organic Photoredox Catalysts as Sustainable Alternatives to Precious Metals. Chem. Eur. J.. 23(46):10962-10968. https://doi.org/10.1002/chem.201702926

2. Theriot JC, Lim C, Yang H, Ryan MD, Musgrave CB, Miyake GM. 2016. Organocatalyzed atom transfer radical polymerization driven by visible light. Science. 352(6289):1082-1086. https://doi.org/10.1126/science.aaf3935

3. Pearson RM, Lim C, McCarthy BG, Musgrave CB, Miyake GM. 2016. Organocatalyzed Atom Transfer Radical Polymerization Using N-Aryl Phenoxazines as Photoredox Catalysts. J. Am. Chem. Soc.. 138(35):11399-11407. https://doi.org/10.1021/jacs.6b08068

4. Lim C, Ryan MD, McCarthy BG, Theriot JC, Sartor SM, Damrauer NH, Musgrave CB, Miyake GM. 2017. Intramolecular Charge Transfer and Ion Pairing in N,N-Diaryl Dihydrophenazine Photoredox Catalysts for Efficient Organocatalyzed Atom Transfer Radical Polymerization. J. Am. Chem. Soc.. 139(1):348-355. https://doi.org/10.1021/jacs.6b11022