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Organocatalyzed atom transfer radical polymerization (O-ATRP) is a controlled radical polymerization method mediated by organic photoredox catalysts (PCs) for producing polymers with well-defined structures. While N,N-diaryl dihydrophenazine PCs have successfully produced polymers with low dispersity (Đ < 1.3) in O-ATRP, low initiator efficiencies (I* ∼ 60–80%) indicate an inability to achieve targeted molecular weights and have been attributed to the addition of radicals to the PC core. In this work, we measure the rates of alkyl core substitution (AkCS) to gain insight into why PCs differing in N-aryl group connectivity exhibit differences in polymerization control. Additionally, we evaluate how PC properties evolve during O-ATRP when a non-core-substituted PC is used. PC 1 with 1-naphthyl groups in the N-aryl position resulted in faster AkCS (k1 = 1.21 ± 0.16 × 10–3 s–1, k2 = 2.04 ± 0.11 × 10–3 s–1) and better polymerization control at early reaction times as indicated by plots of molecular weight (number average molecular weight = Mn) vs conversion compared to PC 2 with 2-naphthyl groups (k1 = 6.28 ± 0.38 × 10–4 s–1, k2 = 1.15 ± 0.07 × 10–3 s–1). The differences in rates indicate that N-aryl connectivity can influence polymerization control by changing the rate of AkCS PC formation. The rate of AkCS increased from the initial to the second substitution, suggesting that PC properties are modified by AkCS. Increased PC radical cation (PC•+) oxidation potentials (E1/2 = 0.26–0.27 V vs SCE) or longer triplet excited-state lifetimes (τT1 = 1.4–33 μs) for AkCS PCs 1b and 2b compared to parent PCs 1 and 2 (E1/2 = 0.21–0.22 V vs SCE, τT1 = 0.61–3.3 μs) were observed and may explain changes to PC performance with AkCS. Insight from evaluation of the formation, properties, and performance of AkCS PCs will facilitate their use in O-ATRP and in other PC-driven organic transformations.
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Impact of Oxygen and Sulfur Heteroatom Core Substitution on Catalyst Properties of Phenoxazines and Their Performance in Organocatalyzed Atom Transfer Radical Polymerization (O‐ATRP)
Abstract Phenoxazines are a successful class of organic photoredox catalysts (PCs) with tunable redox and photophysical properties. Originally, we aimed to realize more reducing phenoxazine PCs through heteroatom core substituted (HetCS) derivatives, while maintaining an efficiently oxidizing PC·+. However, core modification with thioether or ether functionality to a PC that exhibits photoinduced intramolecular charge transfer (CT) negligibly alters the singlet excited state reduction potential (ES1°*), while yielding a less oxidizing PC·+(E1/2) (E1/2 = 0.50–0.64 V vs. SCE) compared to the noncore modified PC1(0.68 V vs. SCE). Photophysical characterization of HetCS PCs revealed that increasing electron density on the core of a CT exhibiting PC stabilizes the emissive state and PC·+, resulting in a relatively unchangedES1°* compared to PC1. In contrast, modifying the core of a PC that does not exhibit CT yields a highly reducingES1°* (PC3= −2.48 V vs. SCE) compared to its CT equivalent (PC1d= −1.68 V vs. SCE). The impact of PC property on photocatalytic ability was evaluated through organocatalyzed atom transfer radical polymerization (O‐ATRP). HetCS PCs were able to yield poly(methyl methacrylate) with low dispersity and moderate targeted molecular weight as evaluated by initiator efficiency (I*) in DMAc (Ð= 1.20–1.26;I*= 47–57%). Ultimately, this work provides insight into how phenoxazine PC properties are altered through structural modification, which can inform future PC design.
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- PAR ID:
- 10614132
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Chemistry – A European Journal
- Volume:
- 31
- Issue:
- 50
- ISSN:
- 0947-6539
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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