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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 (E_S1°*), while yielding a less oxidizing PC^·+(E_1/2) (E_1/2 = 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 unchangedE_S1°* compared to PC1. In contrast, modifying the core of a PC that does not exhibit CT yields a highly reducingE_S1°* (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. 

Photoredox catalysis driven by visible light has improved chemical synthesis by enabling milder reaction conditions and unlocking distinct reaction mechanisms. Despite the transformative impact, visible-light photoredox catalysis remains constrained by the thermodynamic limits of photon energy and inefficiencies arising from unproductive back electron transfer, both of which become particularly pronounced in thermodynamically demanding reactions. In this work, we introduce an organic photoredox catalyst system that overcomes these obstacles to drive chemical transformations that require super-reducing capabilities. This advancement is accomplished by coupling the energy of two photons into a single chemical reduction, whereas inefficiencies from back electron transfer are mitigated through a distinct proton-coupled electron transfer mechanism embedded in the catalyst design. The super-reducing capabilities of this organic catalyst system are demonstrated through efficient application in a broad scope of challenging arene reductions. 

Abstract Herein, we describe a new strategy for the carbonylation of alkyl halides with different nucleophiles to generate valuable carbonyl derivatives under visible light irradiation. This method is mild, robust, highly selective, and proceeds under metal‐free conditions to prepare a range of structurally diverse esters and amides in good to excellent yields. In addition, we highlight the application of this activation strategy for¹³C isotopic incorporation. We propose that the reaction proceeds by a photoinduced reduction to afford carbon‐centered radicals from alkyl halides, which undergo subsequent single electron‐oxidation to form a carbocationic intermediate. Carbon monoxide is trapped by the carbocation to generate an acylium cation, which can be attacked by a series of nucleophiles to give a range of carbonyl products. 

Abstract Herein, we report a selective photooxidation of commodity postconsumer polyolefins to produce polymers with in‐chain ketones. The reaction does not involve the use of catalyst, metals, or expensive oxidants, and selectively introduces ketone functional groups. Under mild and operationally simple conditions, yields up to 1.23 mol % of in‐chain ketones were achieved. Installation of in‐chain ketones resulted in materials with improved adhesion of the materials and miscibility of mixed plastics relative to the unfunctionalized plastics. The introduction of ketone groups into the polymer backbone allows these materials to react with diamines, forming dynamic covalent polyolefin networks. This strategy allows for the upcycling of mixed plastic waste into reprocessable materials with enhanced performance properties compared to polyolefin blends. Mechanistic studies support the involvement of photoexcited nitroaromatics in consecutive hydrogen and oxygen atom transfer reactions. 

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. 

Benzo[ghi]perylene monoimides (BPIs) have recently been employed as organic photocatalysts for challenging reductions. In probing their function, we identify a thermal degradation product involving imide ring opening, and this in turn motivates the development and synthesis of a high-symmetry model systema benzo[ghi]perylene diester (BPDE)whose structural simplicity is useful for mechanistic exploration relevant to the broader photocatalyst class. Using electrochemical and spectroscopic tools, we probe both the singly and doubly reduced states of BPDE and report the generation of [BP-H]−, a twoelectron, one-proton activated closed-shell super-reductant. This catalytically active species, after visible photon absorption, operates from its singlet excited state, where the motions of the added proton are coupled to an electron transfer event, which enables direct reduction of inert substrates like benzene and fluorobenzene. Traditional Birch chemistry on benzene has been previously realized only by solvated electrons or electrochemistry. The function of this model system uncovered in these mechanistic explorations suggests modes of operation for this photocatalyst class that will enable future optimizations.