Radical Cations of Phenoxazine and Dihydrophenazine Photoredox Catalysts and Their Role as Deactivators in Organocatalyzed Atom Transfer Radical Polymerization.

Radical cations of photoredox catalysts used in organocatalyzed atom transfer radical polymerization (O-ATRP) have been synthesized and investigated to gain insight into deactivation in O-ATRP. The stability and reactivity of these compounds were studied in two solvents, ,-dimethylacetamide and ethyl acetate, to identify possible side reactions in O-ATRP and to investigate the ability of these radical cations to deactivate alkyl radicals. A number of other factors that could influence deactivation in O-ATRP were also probed, such as ion pairing with the radical cations, radical cation oxidation potential, and halide oxidation potential.

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

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 (* = / × 100%) near unity is achieved, and the synthesized polymers possess a low dispersity (). ,-Diaryl dihydrophenazine catalysts typically produce polymers with low dispersity ( < 1.

Interrogation of O-ATRP Activation Conducted by Singlet and Triplet Excited States of Phenoxazine Photocatalysts.

Organocatalyzed ATRP (O-ATRP) is a growing field exploiting organic chromophores as photoredox catalysts (PCs) that engage in dissociative electron-transfer (DET) activation of alkyl-halide initiators following absorption of light. Characterizing DET rate coefficients () and photochemical yields across various reaction conditions and PC photophysical properties will inform catalyst design and efficient use during polymerization. The studies described herein consider a class of phenoxazine PCs, where synthetic handles of core substitution and -aryl substitution enable tunability of the electronic and spin characters of the catalyst excited state as well as DET reaction driving force (Δ).

Impacts of Performing Electrolysis During Organocatalyzed Atom Transfer Radical Polymerization.

An electrochemical variant of organocatalyzed atom transfer radical polymerization (O-ATRP) is developed and investigated. Inspired by electrochemically mediated atom transfer radical polymerization (eATRP), potentiostatic electrolysis is used to manipulate the catalyst's redox states in O-ATRP to understand whether deactivation in O-ATRP can be enhanced to improve polymerization control. During the course of this work, several possible side reactions are investigated, and the electrochemical apparatus is optimized to reduce side reactions at the counter electrode.

Controlling Polymer Composition in Organocatalyzed Photoredox Radical Ring-Opening Polymerization of Vinylcyclopropanes.

Although radical polymerizations are among the most prevalent methodologies for the synthesis of polymers with diverse compositions and properties, the intrinsic reactivity and selectivity of radical addition challenge the ability to impart control over the polymerization propagation and produce polymers with defined microstructure. Vinylcyclopropanes (VCPs) can be polymerized through radical ring-opening polymerization to produce polymers possessing linear () or cyclic () repeat units, providing the opportunity to control polymer structure and modify the polymer properties. Herein, we report the first organocatalyzed photoredox radical ring-opening polymerization of a variety of functionalized VCP monomers, where high monomer conversions and spatial and temporal control were achieved to produce poly(VCPs) with predictable molecular weight and low dispersity.

Effects of Naphthyl Connectivity on the Photophysics of Compact Organic Charge-Transfer Photoredox Catalysts.

Modular chromophoric systems with minimal electronic coupling between donor and acceptor moieties are well suited for establishing predictive relationships between molecular structure and excited-state properties. Here, we investigate the impact of naphthyl-based connectivity on the photophysics of phenoxazine-derived orthogonal donor-acceptor complexes. While compounds in this class are themselves interesting as potent organic photocatalysts useful for visible-light-driven organocatalyzed atom-transfer radical polymerization and small-molecule synthesis, many other systems (e.

What happens in the dark? Assessing the temporal control of photo-mediated controlled radical polymerizations.

A signature of photo-mediated controlled polymerizations is the ability to modulate the rate of polymerization by turning the light source 'on' and 'off.' However, in many reported systems, growth can be reproducibly observed during dark periods. In this study, emerging photo-mediated controlled radical polymerizations are evaluated with H NMR monitoring to assess their behavior in the dark.

Exploiting Charge-Transfer States for Maximizing Intersystem Crossing Yields in Organic Photoredox Catalysts.

A key feature of prominent transition-metal-containing photoredox catalysts (PCs) is high quantum yield access to long-lived excited states characterized by a change in spin multiplicity. For organic PCs, challenges emerge for promoting excited-state intersystem crossing (ISC), particularly when potent excited-state reductants are desired. Herein, we report a design exploiting orthogonal π-systems and an intermediate-energy charge-transfer excited state to maximize ISC yields (Φ) in a highly reducing ( E* = -1.

Structure-Property Relationships for Tailoring Phenoxazines as Reducing Photoredox Catalysts.

Through the study of structure-property relationships using a combination of experimental and computational analyses, a number of phenoxazine derivatives have been developed as visible light absorbing, organic photoredox catalysts (PCs) with excited state reduction potentials rivaling those of highly reducing transition metal PCs. Time-dependent density functional theory (TD-DFT) computational modeling of the photoexcitation of N-aryl and core modified phenoxazines guided the design of PCs with absorption profiles in the visible regime. In accordance with our previous work with N, N-diaryl dihydrophenazines, characterization of noncore modified N-aryl phenoxazines in the excited state demonstrated that the nature of the N-aryl substituent dictates the ability of the PC to access a charge transfer excited state.

Organocatalyzed Atom Transfer Radical Polymerization: Perspectives on Catalyst Design and Performance.

The recent development of organocatalyzed atom transfer radical polymerization (O-ATRP) represents a significant advancement in the field of controlled radical polymerizations. A number of classes of photoredox catalysts have been employed thus far in O-ATRP. Analysis of the proposed mechanism gives insight into the relevant photophysical and chemical properties that determine catalyst performance.

Structural Color for Additive Manufacturing: 3D-Printed Photonic Crystals from Block Copolymers.

The incorporation of structural color into 3D printed parts is reported, presenting an alternative to the need for pigments or dyes for colored parts produced through additive manufacturing. Thermoplastic build materials composed of dendritic block copolymers were designed, synthesized, and used to additively manufacture plastic parts exhibiting structural color. The reflection properties of the photonic crystals arise from the periodic nanostructure formed through block copolymer self-assembly during polymer processing.

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

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.

Organocatalyzed Atom Transfer Radical Polymerization Using N-Aryl Phenoxazines as Photoredox Catalysts.

N-Aryl phenoxazines have been synthesized and introduced as strongly reducing metal-free photoredox catalysts in organocatalyzed atom transfer radical polymerization for the synthesis of well-defined polymers. Experiments confirmed quantum chemical predictions that, like their dihydrophenazine analogs, the photoexcited states of phenoxazine photoredox catalysts are strongly reducing and achieve superior performance when they possess charge transfer character. We compare phenoxazines to previously reported dihydrophenazines and phenothiazines as photoredox catalysts to gain insight into the performance of these catalysts and establish principles for catalyst design.