Substituent Impact on Quinoxaline Performance and Degradation in Redox Flow Batteries.

Aqueous redox flow batteries (RFBs) are attractive candidates for low-cost, grid-scale storage of energy from renewable sources. Quinoxaline derivatives represent a promising but underexplored class of charge-storing materials on account of poor chemical stability in prior studies (with capacity fade rates >20%/day). Here, we establish that 2,3-dimethylquinoxaline-6-carboxylic acid (DMeQUIC) is vulnerable to tautomerization in its reduced form under alkaline conditions. We obtain kinetic rate constants for tautomerization by applying Bayesian inference to ultraviolet-visible spectroscopic data from operating flow cells and show that these rate constants quantitatively account for capacity fade measured in cycled cells. We use density functional theory (DFT) modeling to identify structural and chemical predictors of tautomerization resistance and demonstrate that they qualitatively explain stability trends for several commercially available and synthesized derivatives. Among these, quinoxaline-2-carboxylic acid shows a dramatic increase in stability over DMeQUIC and does not exhibit capacity fade in mixed symmetric cell cycling. The molecular design principles identified in this work set the stage for further development of quinoxalines in practical, aqueous organic RFBs.

Glass Transition Temperatures of Individual Submicrometer Atmospheric Particles: Direct Measurement via Heated Atomic Force Microscopy Probe.

The phase (solid, semisolid, or liquid) of atmospheric aerosols is central to their ability to take up water or undergo heterogeneous reactions. In recent years, the unexpected prevalence of viscous organic particles has been shown through field measurements and global atmospheric modeling. The aerosol phase has been predicted using glass transition temperatures (Tg), which were estimated based on molecular weight, oxygen:carbon ratio, and chemical formulae of organic species present in atmospheric particles via studies of bulk materials. However, at the most important sizes for cloud nucleation (∼50-500 nm), particles are complex mixtures of numerous organic species, inorganic salts, and water with substantial particle-to-particle variability. To date, direct measurements of Tg have not been feasible for individual atmospheric particles. Herein, nanothermal analysis (NanoTA), which uses a resistively heated atomic force microscopy (AFM) probe, is combined with AFM photothermal infrared (AFM-PTIR) spectroscopy to determine the Tg and composition of individual particles down to 76 nm in diameter at ambient temperature and pressure. Laboratory-generated proxies for organic aerosol (sucrose, ouabain, raffinose, and maltoheptaose) had similar Tg values to bulk Tg values measured with differential scanning calorimetry (DSC) and the Tg predictions used in atmospheric models. Laboratory-generated phase-separated particles and ambient particles were analyzed with NanoTA + AFM-PTIR showing intraparticle variation in composition and Tg. These results demonstrate the potential for NanoTA + AFM-PTIR to increase our understanding of viscosity within submicrometer atmospheric particles with complex phases, morphologies, and compositions, which will enable improved modeling of aerosol impacts on clouds and climate.

A Nonaqueous Redox-Matched Flow Battery with Charge Storage in Insoluble Polymer Beads.

We describe the nonaqueous redox-matched flow battery (RMFB), where charge is stored on redox-active moieties covalently tethered to non-circulating, insoluble polymer beads and charge is transferred between the electrodes and the beads via soluble mediators with redox potentials matched to the active moieties on the beads. The RMFB reported herein uses ferrocene and viologen derivatives bound to crosslinked polystyrene beads. Charge storage in the beads leads to a high (approximately 1.0-1.7 M) effective concentration of active material in the reservoirs while preventing crossover of that material. The relatively low concentration of soluble mediators (15 mM) eliminates the need for high-solubility molecules to create high energy density batteries. Nernstian redox exchange between the beads and redox-matched mediators was fast relative to the cycle time of the RMFB. This approach is generalizable to many different redox-active moieties via attachment to the versatile Merrifield resin.

Theoretical and Experimental Investigation of Functionalized Cyanopyridines Yield an Anolyte with an Extremely Low Reduction Potential for Nonaqueous Redox Flow Batteries.

Cyanopyridines and cyanophenylpyridines were investigated as anolytes for nonaqueous redox flow batteries (RFBs). The three isomers of cyanopyridine are reduced at potentials of -2.2 V or lower vs. ferrocene+/0 (Fc+/0 ), but the 3-CNPy⋅- radical anion forms a sigma-dimer that is re-oxidized at E≈-1.1 V, which would lead to poor voltaic efficiency in a RFB. Bulk electrochemical charge-discharge cycling of the cyanopyridines in acetonitrile and 0.50 M [NBu4 ][PF6 ] shows that 2-CNPy and 4-CNPy lose capacity quickly under these conditions, due to irreversible chemical reaction/decomposition of the radical anions. Density-functional theory (DFT) calculations indicated that adding a phenyl group to the cyanopyridines would, for some isomers, limit dimerization and improve the stability of the radical anions, while shifting their E1/2 only about +0.10 V relative to the parent cyanopyridines. Among the cyanophenylpyridines, 3-CN-6-PhPy and 3-CN-4-PhPy are the most promising as anolytes. They exhibit reversible reductions at E1/2 =-2.19 and -2.22 V vs. ferrocene+/0 , respectively, and retain about half of their capacity after 30 bulk charge-discharge cycles. An improved version of 3-CN-6-PhPy with three methyl groups (3-cyano-4-methyl-6-(3,5-dimethylphenyl)pyridine) has an extremely low reduction potential of -2.50 V vs. Fc+/0 (the lowest reported for a nonaqueous RFB anolyte) and loses only 0.21 % of capacity per cycle during charge-discharge cycling in acetonitrile.

Using JPP to Identify Ni Bidentate Phosphine Complexes In Situ.

Identifying intermediates of Ni-containing reactions can be challenging due to the high reactivity of Ni complexes and their sensitivity toward air and moisture. Many Ni bidentate phosphine complexes are diamagnetic and can be analyzed in situ via 31P NMR spectroscopy, but the oxidation state of Ni is difficult to determine using 31P chemical shift analysis alone. The J-coupling between P atoms, JPP, has been proposed to correlate with oxidation state, but few investigations have looked at how JPP is affected by parameters such as length of the linker or identity of the phosphine or other ligands. The present investigation into the JPP values of Ni bidentate phosphine complexes with two-carbon and three-carbon linkers shows that the JPP values observed in 31P NMR spectra, |JPP|, are competent indicators of the oxidation state at Ni. For complexes with two-carbon linkers, |JPP| > 40 Hz is typical of Ni0 while |JPP| < 30 Hz is typical of NiII; this trend is reversed for complexes with three-carbon linkers. Additionally, the Lewis acidity of the Ni and Lewis basicity of the phosphine ligand affect JPP predictably. For example, increased P-to-Ni donation arising from more-donating phosphines or more-withdrawing ligands trans to the P atoms causes a more negative JPP. These results should enable the oxidation state of Ni and properties of ligands in Ni bidentate phosphine complexes to be determined in situ during reactions containing these species.

Functionalized and Degradable Polyphthalaldehyde Derivatives.

Polymers that depolymerize back to monomers can be repeatedly chemically recycled, thereby reducing their environmental impact. Polyphthalaldehyde is a metastable polymer that can rapidly and quantitatively depolymerize due to its low ceiling temperature. However, the effect of substitution on the physical and chemical properties of polyphthalaldehyde derivatives has not been systematically studied. Herein, we investigate the cationic polymerization of seven o-phthalaldehyde derivatives and demonstrate that judicious choice of substituent results in materials with a wide range of ceiling temperatures (<-60 to 106 °C) and decomposition temperatures (109-196 °C). We anticipate that these new polymers and their derivatives will enable researchers to access degradable materials with tunable thermal, physical, and chemical properties.

The History of Palladium-Catalyzed Cross-Couplings Should Inspire the Future of Catalyst-Transfer Polymerization.

Conjugated polymers are the workhorse materials in organic electronics, a field that is rapidly growing to encompass energy storage devices such as supercapacitors and batteries. The highest-performing materials today have incredibly diverse structures and are accessed via step-growth polymerizations. This method results in limited control over the polymer's molecular weight, sequence, and dispersity, all of which can significantly impact device performance. The discovery of catalyst-transfer polymerization (CTP) in 2004 was predicted to change this landscape. Instead, nearly 14 years later, the CTP scope remains mostly limited to polymerizing small, electron-rich monomers. There is a pronounced gap between the rich array of structures utilized in organic electronics and what can be polymerized in a living, chain-growth fashion via CTP. Here, we suggest that palladium precatalysts could bridge this gap based on their huge versatility in the small-molecule cross-coupling literature. We highlight specific ancillary ligands from the small-molecule literature that we anticipate are candidates for enabling diverse conjugated polymer syntheses based on nearly a decade of research into the CTP mechanism. In addition, we describe several recent promising examples of CTP mediated by Pd precatalysts that serve as inspiration for the future. We present this Perspective as a call-to-action to advance organic electronics with CTP.

Ring-Walking in Catalyst-Transfer Polymerization.

Catalyst-transfer polymerization (CTP) has emerged as a useful method for synthesizing conjugated polymers with control over their length, sequence, and end-groups. However, the extent to which the polymerizations are living and chain-growth (or not) is highly catalyst and monomer dependent. Few studies have elucidated the impact of these identities on the stability and reactivity of the key intermediate, especially under polymerization-relevant conditions. We developed herein a simple experiment to identify catalyst stability and ring-walking ability using in situ-generated polymers. The combined results show that the ancillary ligand, metal, and polymer identity all play a crucial role. While each catalyst studied walks efficiently over large distances in poly(thiophene), the trends observed for poly(phenylene) highlight the differing roles of transition metal and ancillary ligand identities. The insights gained herein should be useful for extending CTP to other monomer and copolymer scaffolds.

Matchmaking in Catalyst-Transfer Polycondensation: Optimizing Catalysts based on Mechanistic Insight.

Catalyst-transfer polycondensation (CTP) has emerged as a useful living, chain-growth polymerization method for synthesizing conjugated (hetero)arene-based polymers with targetable molecular weights, narrow dispersities, and controllable copolymer sequences-all properties that significantly influence their performance in devices. Over the past decade, several phosphine- and carbene-ligated Ni- and Pd-based precatalysts have been shown to be effective in CTP. One current limitation is that these traditional CTP catalysts lead to nonliving, non-chain-growth behavior when complex monomer scaffolds are utilized. Because these monomers are often found in the highest-performing materials, there is a significant need to identify alternative CTP catalysts. Recent mechanistic insight into CTP has laid the foundation for designing new catalysts to expand the CTP monomer scope. Building off this insight, we have designed and implemented model systems to identify effective catalysts by understanding their underlying mechanistic behaviors and systematically modifying catalyst structures to improve their chain-growth behavior. In this Account, we describe how each catalyst parameter-the ancillary ligand(s), reactive ligand(s), and transition metal-influences CTP. As an example, ancillary ligands often dictate the turnover-limiting step of the catalytic cycle, and perhaps more importantly, they can be used to promote the formation of the key intermediate (a metal-arene associative complex) and its subsequent reactivity. The fidelity of this intermediate is central to the mechanism for the living, chain-growth polymerization. Reactive ligands, on the other hand, can be used to improve catalyst solubility and accelerate initiation. Additional advantages of the reactive ligand include providing access points for postpolymerization modification and synthesizing polymers directly off surfaces. While the most frequently used CTP catalysts contain nickel, palladium-based catalysts exhibit a higher functional group tolerance and broader substrate scope (e.g., monomers with boron, magnesium, tin, and gold transmetalating agents). Overall, we anticipate that applying the tools and lessons detailed in this Account to other monomers should facilitate a better "matchmaking" process that will lead to new catalyst-transfer polycondensations.

Developing a Gel-Based Sensor Using Crystal Morphology Prediction.

The stimuli-responsive nature of molecular gels makes them appealing platforms for sensing. The biggest challenge is in identifying an appropriate gelator for each specific chemical or biological target. Due to the similarities between crystallization and gel formation, we hypothesized that the tools used to predict crystal morphologies could be useful for identifying gelators. Herein, we demonstrate that new gelators can be discovered by focusing on scaffolds with predicted high aspect ratio crystals. Using this morphology prediction method, we identified two promising molecular scaffolds containing lead atoms. Because solvent is largely ignored in morphology prediction but can play a major role in gelation, each scaffold needed to be structurally modified before six new Pb-containing gelators were discovered. One of these new gelators was developed into a robust sensor capable of detecting lead at the U.S. Environmental Protection Agency limit for paint (5000 ppm).

My maize and blue brick road to physical organic chemistry in materials.

Similar to Dorothy's journey along the yellow brick road in The Wizard of Oz, this perspective carves out the path I took from my early childhood fascinations with science through my independent career at the University of Michigan (maize and blue). The influential research projects and mentors are highlighted, including some fortuitous experimental results that drew me into the field of supramolecular chemistry, specifically, and organic materials, broadly. My research group's efforts toward designing new sensors based on small molecule gelators are described. In particular, I highlight how our design strategy has evolved as we learn more about molecular gelators. This perspective concludes with some predictions about where molecular gels, as well as my personal and professional life, are headed.

Effect of Solvent on Surface Ordering of Poly(3-hexylthiophene) Thin Films.

Enhancement of charge transport in organic polymer semiconductors is a crucial step in developing optimized devices. A variety of sample preparation conditions, such as film fabrication method, solvent species, and annealing, were found to influence the hole mobility of organic polymers. Despite the fact that many factors can influence their performance, it is believed that polymer surface ordering plays a key role in determining organic polymer function. Here, sum frequency generation (SFG) vibrational spectroscopy was used to nondestructively map the surface/interfacial ordering of poly(3-hexylthiophene) (P3HT) films prepared using different solvents; we believe that solvent interactions determine the degree of surface/interfacial ordering. Both X-ray diffraction (XRD) spectroscopy and scanning electron microscopy (SEM) were used to supplement SFG to systematically study bulk crystallinity and surface morphology. We conclude that SFG is a powerful tool to elucidate the surface/interfacial structural information on polymer semiconducting films. We demonstrate that the solvent composition used to prepare P3HT thin films influences the resulting film surface morphology, surface/interfacial ordering, and bulk crystallinity.

Tools for identifying gelator scaffolds and solvents.

Small molecule gelators are serendipitously discovered more often than they are designed. As a consequence, it has been challenging to develop applications based on the limited set of known materials. This synopsis highlights recent strategies to streamline the process of gelator discovery, with a focus on the role of unidirectional intermolecular interactions and solvation. We present these strategies as a series of tools that can be employed to help identify gelator scaffolds and solvents for gel formation. Overall, we suggest that this guided approach is more efficient than random derivatization and screening.

Improving Hg-triggered gelation via structural modifications.

The relationship between chemical structure and gelation ability was examined for a series of nine Hg-containing compounds. Both solid-state properties (dissolution enthalpies/entropies and packing structure) and gel properties (strength, morphology, cation selectivity, and anion tolerance) were examined. Overall, the results reveal a complex relationship between chemical structure and properties. The remediation potential of these Hg-triggered gelations was also investigated, revealing that >98% of the Hg(2+) in water can be removed through gel formation.

Characterization of the bridged hyponitrite complex {[Fe(OEP)](2)(µ-N(2)O(2))}: reactivity of hyponitrite complexes and biological relevance.

The detoxification of nitric oxide (NO) by bacterial NO reductase (NorBC) represents a paradigm of how NO can be detoxified anaerobically in cells. In order to elucidate the mechanism of this enzyme, model complexes provide a convenient means to assess potential reaction intermediates. In particular, there have been many proposed mechanisms that invoke the formation of a hyponitrite bridge between the heme b3 and nonheme iron (FeB) centers within the NorBC active site. However, the reactivity of bridged iron hyponitrite complexes has not been investigated much in the literature. The model complex {[Fe(OEP)]2(µ-N2O2)} offers a unique opportunity to study the electronic structure and reactivity of such a hyponitrite-bridged complex. Here we report the detailed characterization of {[Fe(OEP)]2(µ-N2O2)} using a combination of IR, nuclear resonance vibrational spectroscopy, electron paramagnetic resonance, and magnetic circular dichroism spectroscopy along with SQUID magnetometry. These results show that the ground-state electronic structure of this complex is best described as having two intermediate-spin (S = (3)/2) iron centers that are weakly antiferromagnetically coupled across the N2O2(2-) bridge. The analogous complex {[Fe(PPDME)]2(µ-N2O2)} shows overall similar properties. Finally, we report the unexpected reaction of {[Fe(OEP)]2(µ-N2O2)} in the presence and absence of 1-methylimidizole to yield [Fe(OEP)(NO)]. Density functional theory calculations are used to rationalize why {[Fe(OEP)]2(µ-N2O2)} cannot be formed directly by dimerization of [Fe(OEP)(NO)] and why only the reverse reaction is observed experimentally. These results thus provide insight into the general reactivity of hyponitrite-bridged iron complexes with general relevance for the N-N bond-forming step in NorBC.

Revisiting poly(vinyl chloride) reactivity in the context of chemical recycling.

Poly(vinyl chloride) (PVC) is one of the highest production volume polymers due to its many applications, and it is one of the least recycled due to its chemical structure and frequent formulation with additives. Developing efficient PVC recycling techniques would enable PVC waste to be reused or repurposed in other processes. Within this context, the literature on PVC modification offers considerable insight into versatile reaction pathways, potentially inspiring new approaches for repurposing PVC waste into value-added products. This perspective provides an overview of PVC functionalization through a lens of chemical recycling, discussing various PVC reactivity trends and their applications with a critical assessment and future outlook of their recycling implications.

Using waste poly(vinyl chloride) to synthesize chloroarenes by plasticizer-mediated electro(de)chlorination.

New approaches are needed to both reduce and reuse plastic waste. In this context, poly(vinyl chloride) (PVC) is an appealing target as it is the least recycled high-production-volume polymer due to its facile release of plasticizers and corrosive HCl gas. Herein, these limitations become advantageous in a paired-electrolysis reaction in which HCl is intentionally generated from PVC to chlorinate arenes in an air- and moisture-tolerant process that is mediated by the plasticizer. The reaction proceeds efficiently with other plastic waste present and a commercial plasticized PVC product (laboratory tubing) can be used directly. A simplified life-cycle assessment reveals that using PVC waste as the chlorine source in the paired-electrolysis reaction has a lower global warming potential than HCl. Overall, this method should inspire other strategies for repurposing waste PVC and related polymers using electrosynthetic reactions, including those that take advantage of existing polymer additives.

Chain-growth polymerization of aryl Grignards initiated by a stabilized NHC-Pd precatalyst.

An N-heterocyclic carbene-ligated palladium catalyst was discovered to mediate living, chain-growth polymerizations of both phenylene- and thiophene-based monomers. Polymerization of a fluorene-based monomer, on the other hand, did not proceed through a living, chain-growth pathway. Excitingly, block copolymerizations of phenylene and thiophene proceeded via a chain-growth pathway, regardless of the order of monomer addition. Although some chain termination was observed during these copolymerizations, this pathway could be minimized when the second monomer was added shortly after consumption of the first monomer. These results suggest that the catalyst resting-state at the end of polymerization is unstable. As a result, modifications to the NHC-scaffold or the 3-chloropyridine ligand will be necessary to generate an improved catalyst.