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Refereed/Peer-reviewed 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.

Nonaqueous redox flow batteries are a promising technology for grid-scale energy storage, however, their success relies on identifying redox-active materials that exhibit extreme potentials, high solubilities , and long cycling stabilities.

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

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.

Polymerizing electron-deficient arenes in a controlled, chain-growth fashion remains a significant challenge despite a decade of research on catalyst-transfer polycondensation. The prevailing hypothesis is that the chain-growth mechanism stalls at a strongly associated metal-polymer π-complex, preventing catalyst turnover. To evaluate this hypothesis, we performed mechanistic studies using thiazole derivatives and identified approaches to improve their chain-growth polymerization. These studies revealed a surprisingly high barrier for chain-walking toward the reactive C-X bond. In addition, a competitive pathway involving chain-transfer to monomer was identified. This pathway is facilitated by ancillary ligand dissociation and N-coordination to the incoming monomer. We found that this chain-transfer pathway can be attenuated by using a rigid ancillary ligand, leading to an improved polymerization. Combined, these studies provide mechanistic insight into the challenges associated with electron-deficient monomers as well as ways to improve their living, chain-growth polymerization. Our mechanistic studies also revealed an unexpected radical anion-mediated oligomerization in the absence of catalyst, as well as a surprising oxidative addition into the thiazole C-S bond in a model system.

Although Staudinger realized makromolekules had enormous potential, he likely did not anticipate the consequences of their universal adoption. With 6.3 billion metric tons of plastic waste now cont...

A two-week summer camp for high school students focused on sustainable polymers was developed and implemented. Students took part in experiments and interactive lessons that demonstrate polymer che...

Biomimetic systems that undergo macroscopic phase transformations by transducing and amplifying external cues are highly desirable for applications such as self-healing. Here, we report self-assembly of polyelectrolyte complexes into a vesicular structure that can accommodate hydrophilic guest molecules, including enzymes. Triggered depolymerization of one of the polyelectrolyte molecules in the complex causes the vesicle to disassemble and release its contents. Such a triggered release of enzymes causes molecular-scale events to be amplified due to the enzyme's catalytic properties. This feature has been utilized to demonstrate construction of hydrogels from the destruction of nanoscopic polymeric vesicles. The design principles developed here are broadly adaptable to other triggerable depolymerization motifs reported in the literature.

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.

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.

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 (31)P NMR spectroscopy, but the oxidation state of Ni is difficult to determine using (31)P chemical shift analysis alone. The J-coupling between P atoms, J(PP), has been proposed to correlate with oxidation state, but few investigations have looked at how J(PP) is affected by parameters such as length of the linker or identity of the phosphine or other ligands. The present investigation into the J(PP) values of Ni bidentate phosphine complexes with two-carbon and three-carbon linkers shows that the J(PP) values observed in (31)P NMR spectra, ∣J(PP)∣, are competent indicators of the oxidation state at Ni. For complexes with two-carbon linkers, ∣J(PP)∣ > 40 Hz is typical of Ni(0) while ∣J(PP)∣ < 30 Hz is typical of Ni(II); 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 J(PP) 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 J(PP). 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.

An estimated 6.3 billion metric tons of post-consumer polymer waste has been produced, with the majority (79%) in landfills or the environment. Recycling methods that utilize these waste polymers could attenuate their environmental impact. For many polymers, recycling via mechanical processes is not feasible and these materials are destined for landfills or incineration. One salient example is the superabsorbent material used in diapers and feminine hygiene products, which contain crosslinked sodium polyacrylates. Here we report an open-loop recycling method for these materials that involves (i) decrosslinking via hydrolysis, (ii) an optional chain-shortening via sonication, and (iii) functionalizing via Fischer esterification. The resulting materials exhibit low-to-medium storage and loss moduli, and as such, are applicable as general-purpose adhesives. A life cycle assessment demonstrates that the adhesives synthesized via this approach outcompete the same materials derived from petroleum feedstocks on nearly every metric, including carbon dioxide emissions and cumulative energy demand., For many polymers, recycling via mechanical processes is not feasible and these materials are destined for landfills or incineration. Here the authors show an open-loop recycling method for superabsorbent polymers that involves decrosslinking via hydrolysis, an optional chain-shortening step via sonication, and functionalizing via Fischer esterification.

Although teaching laboratories offer students the opportunity to act and think like chemists, in many cases students simply follow written procedures to generate predetermined outcomes. In recent years, there has been a movement toward inquiry-, problem-, and discovery-based learning. In a similar vein, the first-semester introductory organic chemistry laboratory curriculum at the University of Michigan was recently overhauled, informed by the strategies of meaningful learning. The new course consists of three interconnected modules, and we present herein the second module, which focuses on liquid–liquid extraction (LLE). In the first week, students learn how to perform LLE by designing their own separation of a dye mixture, gaining an understanding of how the solution pH impacts molecular structure and solubility. In the second week, they practice using LLE by designing a separation method to remove a pollutant from water, and then, in the third week, they apply LLE to isolate a reaction product. We assessed student learning through a writing assignment at the end of the three-week module. We also assessed how the overall course affects student understanding of LLE concepts and their confidence in the lab. Our findings suggest that this learn, practice, apply approach for teaching LLE leads to modest learning gains and increased confidence.

Infographics are increasingly used to communicate complex topics to the public. By coupling visually appealing graphics with compelling narratives, infographics can both engage and inform the audie...

A 3-week module incorporating the principles of green chemistry was developed for a large-enrollment, introductory organic chemistry laboratory course. An emphasis was placed on students planning their own experiments with the goal of obtaining a greener reaction (week 1). Students executed their designed experiments in week 2 and were given an opportunity to repeat the experiment or further optimize the reaction conditions to improve the yields during week 3. A postlab writing assignment as well as pre- and postsemester surveys were used to evaluate student learning and confidence. These data revealed an increase in students’ understanding of green chemistry concepts as well as their confidence in their ability to modify a reaction to improve the results. We anticipate that this overall framework can be adapted to other large- and small-enrollment laboratory courses.

Polymers that depolymerize back to monomers can be repeatedly chemically recycled, thereby reducing their environmental impact. Polyphthalaldehyde is a metastable polymer that is rapidly and quantitatively depolymerized 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 distinct o‑phthalaldehyde derivatives and demonstrate that judicious choice of substituents results in materials with a wide range of ceiling temperatures (from < –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.

Introductory-level laboratory courses provide students with hands-on experience using the discipline’s tools and theories. These courses often rely on recipe-based experiments due to the constraint...

Catalyst-transfer polymerization has enabled synthesizing conjugated polymers with unmatched control over both polymer sequence and end-group identity. These attributes provide access to complex materials, including conjugated–conjugated and conjugated–nonconjugated block copolymers, gradient copolymers, nonlinear copolymers, and hybrid materials. This review discusses these materials, highlighting their syntheses as well as their applications.

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.

Transmetalation is a ubiquitous transformation used for synthesizing organic molecules. In catalyst-transfer polymerization (CTP), conjugated monomers are polymerized using transmetalation of Grignard reagents to make versatile organic semiconductors such as poly(3-hexylthiophene). This study presents the complete mechanistic viewpoint for this transmetalation reaction, taking into consideration the catalyst electronic states, steric environment, and realistic models of each reagent. These quantum chemical results reveal that singlet–triplet crossing is routine in these transmetalation reactions, and switching between low-spin square-planar and high-spin tetrahedral geometries naturally occurs during the catalytic cycle. In this cycle transmetalation preferentially occurs from a triplet state but forces the metal center back into a singlet state after monomer addition. Furthermore, the relative preference of singlet vs triplet state can be modulated by the ancillary ligand. This model therefore captures r...

Catalyst-transfer polymerization (CTP) is a living, chain-growth method for accessing conjugated polymers with control over their length and sequence. Typical catalysts utilized in CTP are either Pd or Ni complexes with bisphosphine or N-heterocyclic carbene ancillary ligands. More recently, diimine-ligated Ni complexes have been employed; however, in most cases nonliving pathways become dominant at high monomer conversions and/or low catalyst loading. We report herein an alternative Ni diimine catalyst that polymerizes 3-hexylthiophene in a chain-growth manner at low catalyst loading and high monomer conversion. In addition, we elucidate the chain-growth mechanism as well as one chain-transfer pathway. Overall, these studies provide insight into the mechanism of conjugated polymer synthesis mediated by Ni diimine catalysts.

Block copolymers containing both insulating and conducting segments have been shown to exhibit improved charge transport properties and air stability. Nevertheless, their syntheses are challenging, relying on multiple post-polymerization functionalization reactions and purifications. A simpler approach would be to synthesize the block copolymer in one pot using the same catalyst to enchain both monomers via distinct mechanisms. Such multitasking polymerization catalysts are rare, however, due to the challenges of finding a single catalyst that can mediate living, chain-growth polymerizations for each monomer under similar conditions. Herein, a diimine-ligated Ni catalyst is evaluated and optimized to produce block copolymer containing both 1-pentene and 3-hexylthiophene. The reaction mixture also contains both homopolymers, suggesting catalyst dissociation during and/or after the switch in mechanisms. Experimental and theoretical studies reveal a high energy switching step coupled with infrequent catalyst dissociation as the culprits for the low yield of copolymer. Combined, these studies highlight the challenges of identifying multitasking catalysts, and suggest that further tuning the reaction conditions (e.g., ancillary ligand structure and/or metal) is warranted for this specific copolymerization. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017

Synthesizing conjugated polymers via catalyst-transfer polymerization (CTP) has led to unprecedented control over polymer sequence and molecular weight. Yet many challenges remain, including broadening the monomer scope and narrowing the molecular weight dispersities. Broad polymer dispersities can arise from nonliving pathways as well as slow initiation. Previously, slow initiation was observed in Ni-mediated CTP of phenylene monomers. Although precatalysts with faster initiation rates have been reported, the rates still do not exceed propagation. Herein a second- and third-generation of reactive ligands are described, along with a simple method for measuring initiation rates. A precatalyst with an initiation rate that exceeds propagation is now reported, however, the resulting polymer samples still exhibit broad dispersities, suggesting that slow initiation is not the most significant contributing factor in Ni-mediated phenylene polymerizations. In addition, initiation rates measured under authentic polymerization conditions revealed that both exogenous triphenylphosphine and an ortho-trifluoroethoxy substituent on the reactive ligand have a strong influence. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 1530–1535

Feedback-driven online homework systems provide students with a comprehensive set of practice questions that can accompany and enhance other instructional resources. However, the available e-homework systems do not contain content that aligns well with our course objectives, provide too few questions in key areas, and use assessment format(s) that do not match the ones used on our exams. Motivated to create our own questions, we used this gap as an opportunity to engage students in constructing and reviewing course-aligned content within a commercial e-homework platform. The students successfully generated approximately 1,000 largely open-ended organic chemistry questions, some with mechanistic and structural drawing capabilities, by modifying old exam questions. The students’ questions spanned a variety of cognitive levels that skewed, as intended, toward skill-building. According to our assessment scheme, 75% of the questions were evaluated to be of the highest quality. As a consequence, we advocate tha...

The living, chain-growth polymerization of (hetero)aryl monomers by carbene-ligated palladium precatalysts gives conjugated polymers with narrow dispersities as well as specified molecular weights and sequences. Despite this success, the mechanism for Pd precatalyst initiation and subsequent polymer growth is unknown. A quantum chemical study is presented herein to provide insight into thiophene homopolymerization initiated by (IPr)PdCl2(3-chloropyridine) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) using novel reaction discovery tools for generating and evaluating mechanistic hypotheses. This study reveals the role for the “throw-away” 3-chloropyridine ligand during initiation via a stepwise transmetalation pathway that involves pyridine dissociation, followed by pyridine reassociation to liberate the Grignard byproduct (MgCl2) and stabilize the resulting Pd intermediate. In contrast, 3-chloropyridine association to Pd during propagation hinders catalyst turnover by producing a low-energy, of...

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.

Catalyst-transfer polycondensation (CTP) is a relatively new method for synthesizing conjugated polymers in a chain-growth fashion using transition metal catalysis. Recent research has focused on screening catalysts to broaden the monomer scope. In this effort, small molecule reactions have played an important role. Specifically, when selective difunctionalization occurs, even with limiting quantities of reaction partner, it suggests an associative intermediate similar to CTP. Several new chain-growth polymerizations have been discovered using this approach. We report herein an attempt to use this method to develop chain-growth conditions for synthesizing poly(2,5-bis(hexyloxy)phenylene ethynylene) via Sonogashira cross-coupling. Hundreds of small molecule experiments were performed and selective difunctionalization was observed with a Buchwald-type precatalyst. Unexpectedly, these same reaction conditions led to a step-growth polymerization. Further investigation revealed that the product ratios in the small molecule reactions were dictated by reactivity differences rather than an associative intermediate. The lessons learned from these studies have broad implications on other small molecule reactions being used to identify new catalysts for CTP.

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.

This work introduces fully π-conjugated gradient copolymers as promising materials to control and stabilize the nanoscale morphology of polymer:fullerene solar cells. Gradient and block sequence copolymers of 3-hexylselenophene (3HS) and 3-hexylthiophene (3HT) are utilized as the donors (D) in bulk-heterojunction (BHJ) solar cells with phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor (A). We show that for the same overall copolymer composition, the ordering of molecular constituents along the copolymer chain (copolymer sequence) significantly influences the nanoscale morphology and phase separation behavior of π-conjugated copolymer:fullerene devices. In addition, energy-filtered transmission electron microscopy (EFTEM) of the blends revealed that relative to the block copolymer:PCBM, the gradient copolymer:PCBM sample formed a more uniform, continuous and interconnected network of polymer fibrils within the acceptor-rich phase, associated with a large D/A interface. Charge extraction of photogenerated carriers by linearly increasing voltage (photo-CELIV) shows that the gradient copolymer:PCBM device possesses the highest initial carrier density, n(0) = (3.92 ± 0.3) × 1018 cm−3, consistent with a larger D/A interfacial area suggested by the observed morphology, albeit at the expense of increased carrier recombination rate. Accelerated degradation studies show that the gradient copolymer:PCBM system maintains the highest efficiency over prolonged heat treatment.

Bulk heterojunction (BHJ) solar cells are fabricated using active material blends of poly(3-hexylthiophene) (P3HT) donor, indene-C60 bisadduct (ICBA) acceptor, and an all-conjugated random copolymer (RCP) additive. By optimizing RCP loading, power conversion efficiencies (PCEs) up to 20% higher than those of a binary P3HT:ICBA mixture are achieved. The improved device characteristics are rationalized in terms of the differences between the photoactive thin film morphologies. Energy-filtered transmission electron micro­scopy reveals that incorporation of the RCP improves the degree of structural order of the BHJ fibrillar network and increases the extent of microphase separation between P3HT and ICBA. Additionally, a combination of atomic force microscopy and X-ray photoelectron spectroscopy analysis indicates segregation of the RCP at the free interface, leading to a shift in the surface potentials measured by Kelvin probe force microscopy. These changes, both in the bulk morphology and in the interfacial composition/energetics, are correlated to improved carrier collection efficiency due to a reduction of non-geminate recombination, which is measured by charge extraction of photo­generated carriers by linearly increasing voltage.

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.

The process of selecting and modifying a known gelator scaffold to develop a new nitrite-based sensor is described. Five new azo-sulfonate gelators were discovered and characterized. The most promising scaffold exhibits a stable diazonium intermediate, proceeds in a high yield, and gels nitrite-spiked tap, river, and pond water.

The recent discovery of a living, controlled chain-growth method for synthesizing π-conjugated polymers has ignited the field and led to the development of many new materials. This Perspective focuses on the mechanistic underpinnings of the synthetic transformation, highlighting the controversial hypotheses and supporting data. A critical analysis of the literature revealed that the monomer scope remains largely limited to electron-rich monomers at this time. Last, a brief overview of some exciting new materials accessed via this method is provided.

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).

Nickel-catalyzed chain-growth copolymerizations of thiophene and selenophene derivatives afforded well-defined π-conjugated copolymers with narrow molecular weight distributions, defined end-groups, and specific sequences. In particular, a π-conjugated copolymer with a novel linear gradient sequence was prepared. Compared to the analogous block and random copolymers, the gradient copolymer displayed unique optical and thermal properties as well as thin-film morphology. Moreover, the gradient copolymer exhibited an intermediate extent of phase separation into thiophene-rich and selenophene-rich domains compared to the block and random copolymers. Because this gradient sequence provides access to new solid-state properties, these materials should be further explored in applications where phase-separated morphology is important (e.g., solar cells, transistors, and light-emitting diodes).

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.

The effect of polymeric additives on molecular gelation was explored using poly(acrylic acid) and pyridine-based gelators. A significant reduction in the critical gel concentration (cgc) and an increase in gel strength were observed when the polymer was present during gel formation. Detailed studies revealed that the polymer is adsorbing onto the growing fibers, reducing the growth rates, and leading to thinner fibers. These and other morphological changes lead to improved gel properties by increasing the number of fiber–fiber entanglements. Several other polymers were briefly examined and these studies revealed that polymer structure is important. The polymer containing a complementary functional group relative to the gelator (e.g., H-bond donor/acceptor) provided the lowest cgc.

The role of ligand-based steric effects was investigated in the polymerization of 4-bromo-2,5-bis(hexyloxy)phenylmagnesium chloride. Three different Ni(L-L)Cl2 catalysts were synthesized using comm...