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Integrated computational and experimental studies of concentrated redoxmer/electrolyte solutions reveal that charging leads to softening of the solution.

Energy density in nonaqueous redox flow batteries (RFBs) is often limited by the modest solubility of the redox-active organic molecules (ROMs). In addition, the lack of a separator that prevents ROMs from crossing between anolyte and catholyte solutions necessitates the use of 1:1 mixtures of two ROMs in both the anolyte and catholyte solutions in symmetric RFBs, further limiting concentrations. We show that permanently cationic oligomers of viologen, tris(dialkylamino)cyclopropenium, and phenothiazine molecules have high solubility in acetonitrile and cross over an anion exchange membrane at slow to undetectable rates, enabling the creation of asymmetric RFBs with low crossover. No added supporting electrolyte is necessary, with only the PF

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.

Metal-free redox flow batteries with TEMPO-based polypeptide catholytes and viologen-based polypeptide anolytes were demonstrated. Post-cycling analysis indicated the main source of capacity fade was degradation of the redox-active pendant groups.

Non-aqueous redox flow batteries which can be operated under subzero temperature are significant for applications in cold regions.

The advance non-aqueous redox flow batteries require redox-active organic molecules (ROM) with large solubilities in all states of charge (NMR-spectrometer icon by DBCLS and screen and workstation icons by Simon Duerr licensed under creative commons).

An operationally simple, reliable synthesis of ethynylated acenes suitable for an upper-division undergraduate organic chemistry laboratory course has been developed. This experiment requires students to synthesize two acene derivatives and can be completed in 6–8 laboratory sessions (3 h each); a shorter experiment could be realized in the synthesis of only one product. Synthesis is carried out by first treating a terminal alkyne with n-butyllithium in an oven-dried flask under nitrogen at 0 °C. An acene quinone is added, which produces a doubly ethynylated diol. Dehydration with SnCl2/HCl yields a bis(ethynylated) acene, which is purified via a short pad or column of silica gel. The intense colors of the products allow for great ease in determining reaction completion and following the products in chromatography. UV–vis and fluorescence spectroscopy may be used to better understand the effect of molecular structure on electronic properties.

Organic radical cations are important intermediates in a wide variety of chemical processes. To date, significant progress has been made to improve the stability of these charged materials for use ...

N-Ethyl-3,7-bis(trifluoromethyl)phenothiazine is a highly soluble redox shuttle for overcharge protection in lithium-ion batteries with an oxidation potential of ca. 3.8 V vs. Li+/0 in carbonate solvents. This compound has enabled extensive overcharge protection of LiFePO4/graphite cells and does so at high charging rates at high concentrations. Our initial synthesis of this compound suffered from low yields and difficult purifications. Here we report a cleaner, higher-yielding synthesis and additional characterization of the product and its stable radical cation salt.

In this issue of Chem, Michael Aziz, Roy Gordon, and collaborators report soluble, functionalized anthraquinones that serve to deactivate and recover from chemical reactions that lead to capacity fade in organic redox flow battery electrolytes. The result is an unprecedented low rate of capacity fade of

This study aims to advance our understanding of the physical and electrochemical behavior of nonaqueous redox electrolytes at elevated concentrations and to develop experimentally informed structur...

Redox flow batteries based on quinone-bearing aqueous electrolytes have emerged as promising systems for energy storage from intermittent renewable sources. The lifetime of these batteries is limited by quinone stability. Here, we confirm that 2,6-dihydroxyanthrahydroquinone tends to form an anthrone intermediate that is vulnerable to subsequent irreversible dimerization. We demonstrate quantitatively that this decomposition pathway is responsible for the loss of battery capacity. Computational studies indicate that the driving force for anthrone formation is greater for anthraquinones with lower reduction potentials. We show that the decomposition can be substantially mitigated. We demonstrate that conditions minimizing anthrone formation and avoiding anthrone dimerization slow the capacity loss rate by over an order of magnitude. We anticipate that this mitigation strategy readily extends to other anthraquinone-based flow batteries and is thus an important step toward realizing renewable electricity storage through long-lived organic flow batteries.

Synthetic chemists customarily tune the redox characteristics of π-conjugated molecules by introducing electron-donating or electron-withdrawing substituents onto the molecular core, or by modifying the length of the π-conjugated pathway. Any steric effects of such efforts on molecular geometry typically affect both the neutral and charged (oxidized or reduced) states indiscriminately. However, in electroactive systems that undergo significant conformational changes upon oxidation or reduction, we can leverage the steric and inductive effects of substitution to attain considerable control over individual redox potentials. Here, we make use of density functional theory to elucidate the interplay between electronic and geometric effects of peripheral substitution on the model system of phenothiazine. For instance, we introduce substituents at positions ortho to the nitrogen atom (positions 1 and 9) to induce steric strain in the radical-cation state without significant effect on the neutral molecule, thereby augmenting the overall ionization potential. Notably, this steric effect persists for electron-donating substituents; the resulting ionization potentials therefore deviate from outcomes foretold by Hammett constants. Moreover, the same procedure has limited effect on electron affinities because of differences in phenothiazines' relaxation process upon reduction compared to oxidation. Our results promote molecular design guidelines for manipulating redox potentials in classes of electroactive compounds that experience dramatic changes in geometry upon ionization.

We report viscous flow properties of a redox-active organic molecule, N-(2-(2-methoxyethoxy)ethyl)phenothiazine (MEEPT), a candidate for non-aqueous redox flow batteries, and two of its radical cation salts. A microfluidic viscometer enabled the use of small sample volumes in determining viscosity as a function of shear rate and concentration in the non-aqueous solvent, acetonitrile, both with and without supporting salts. All solutions tested show Newtonian behavior over shear rates of up to 30,000 1/s, which is rationalized by scaling arguments for the diffusion-based relaxation time of a single MEEPT molecule without aggregation. Neat MEEPT is flowable but with a large viscosity (412 mPa s) at room temperature), which is approximately 1,000 times larger than acetonitrile. When dissolved in acetonitrile, MEEPT solutions have low viscosities; at concentrations up to 0.5 M, the viscosity increases by less than a factor of two. From concentration-dependent viscosity measurements, molecular information is inferred from intrinsic viscosity (hydrodynamic diameter) and the Huggins coefficient (interactions). Model fit credibility is assessed using the Bayesian Information Criterion (BIC). It is found that the MEEPT and its charged cation are "flowable" and do not flocculate at concentrations up to 0.5 M. MEEPT has a hydrodynamic diameter of around 0.85 nm, which is largely insensitive to supporting salt and state of charge. This size is comparable to molecular dimensions of single molecules obtained from optimized structures using density function theory calculations. The results suggest that MEEPT is a promising candidate for redox flow batteries in terms of its viscous flow properties., 22 pages, 11 figures in manuscript; 8 pages, 5 figures in supporting information

Nonaqueous redox flow batteries use liquid electrolytes containing redox-active organic molecules (redoxmers) as their energy storage medium. To maximize energy density, the redoxmer concentration needs to be maximized while maintaining low viscosity and high ionic conductivity. During charge, a redoxmer molecule pairs with an ion in the electrolyte while another ion migrates across the membrane to maintain electric neutrality. In a crowded electrolyte, this reconstitution changes physical and chemical properties of the solution. To explore these behaviors, a phenothiazine redoxmer fully miscible with acetonitrile was used, and electrochemical charge was mimicked by chemical oxidation. The solutions were examined using small-angle X-ray scattering, nuclear magnetic resonance, and conductometry and modeled using classical molecular dynamics. Our study indicates that physical and structural properties of redoxmer solutions in both states of charge make it exceedingly difficult to increase the redoxmer concentrations over 2 M at any temperature without compromising dynamic properties of such solutions. The cause for this limitation is proximity to a gel-like regime in which fluidity, diffusivity, and ionic conductivity exponentially decrease with increasing concentration. This tendency is compounded by non-Arrhenius behavior of the electrolyte: a small increase in the concentration outruns gains in fluidity and conductivity at a higher temperature. Thus the properties of crowded electrolytes generally make it impossible to operate when gel-like behavior sets in. Pushing the redoxmer concentration to 2.5–3 M might be possible for small redoxmer molecules, but it would require the use of ionic liquid electrolytes at 340–360 K.

Stable electron-donating organic compounds are of interest for numerous applications that require reversible electron-transfer reactions. Although many organic compounds are stable one-electron donors, removing a second electron from a small molecule to form its dication usually leads to rapid decomposition. For cost-effective electrochemical energy storage utilizing organic charge-storage species, the creation of high-capacity materials requires stabilizing more charge whilst keeping molecular weights low. Here we report the simple modification of N-ethylphenothiazine, which is only stable as a radical cation (not as a dication), and demonstrate that introducing electron-donating methoxy groups para to nitrogen leads to dramatically improved stability of the doubly oxidized (dication) state. Our results reveal that this derivative is more stable than an analogous compound with substituents that do not allow for further charge delocalization, rendering it a promising scaffold for developing atom-efficient, two-electron donors.

Non-aqueous redox flow batteries (NAqRFBs) employing redox-active organic molecules show promise to meet requirements for grid energy storage. Here, we combine the rational design of organic molecules with flow cell engineering to boost NAqRFB performance. We synthesize two highly soluble phenothiazine derivatives, N-(2-methoxyethyl)phenothiazine (MEPT) and N-[2-(2-methoxyethoxy)ethyl]phenothiazine (MEEPT), via a one-step synthesis from inexpensive precursors. Synthesis and isolation of the radical-cation salts permit UV-vis decay studies that illustrate the high stability of these open-shell species. Cyclic voltammetry and bulk electrolysis experiments reveal the promising electrochemical properties of MEPT and MEEPT under dilute conditions. A high performance non-aqueous flow cell, employing interdigitated flow fields and carbon paper electrodes, is engineered and demonstrated; polarization and impedance studies quantify the cell's low area-specific resistance (3.2–3.3 Ω cm2). We combine the most soluble derivative, MEEPT, and its tetrafluoroborate radical-cation salt in the flow cell for symmetric cycling, evincing a current density of 100 mA cm−2 with undetectable capacity fade over 100 cycles. This coincident high current density and capacity retention is unprecedented in NAqRFB literature.

Electron-withdrawing substituents are introduced onto the phenothiazine core to raise its oxidation potential for use as a redox shuttle in high-voltage lithium-ion batteries. A perfluorinated derivative oxidizes at 4.3 V vs. Li+/0, and functions for ca. 500 h of 100% overcharge in LiNi0.8Co0.15Al0.05O2/graphite coin cells at a charging rate of C/10.

Two new small-molecule enzyme mimics of carbonic anhydrase were prepared and characterized. These complexes contain the salen-like ligand bis(hydroxyphenyl)phenanthroline. This ligand is similar to the salen-type ligands previously incorporated into carbonic anhydrase mimics but contains no hydrolyzable imine groups and therefore serves as a promising ligand scaffold for the synthesis of a more robust CO2 hydration catalyst. These homogeneous catalysts were investigated for CO2 hydration in concentrated primary amine solutions through which a dilute CO2 (14%) fluid stream was flowed and showed exceptional activity for increased CO2 absorption rates.

A phenothiazine derivative with high solubility in carbonate solvents containing lithium salts showed extensive overcharge protection and, as a result, has been evaluated as a catholyte for non-aqueous redox flow batteries. We report the testing of 3,7-bis(trifluoromethyl)-N-ethylphenothiazine as a catholyte and 2,3,6-trimethylquinoxaline as the anolyte in redox flow batteries containing 0.05, 0.15, and 0.35 M active material and found the longest capacity retention over about 60 cycles at 0.15 M. To our knowledge, this is the most soluble catholyte candidate with a robust radical cation.

The substitution of sterically bulky groups at precise locations along the periphery of fused-ring aromatic systems is demonstrated to increase electrochemical oxidation potentials by preventing relaxation events in the oxidized state. Phenothiazines, which undergo significant geometric relaxation upon oxidation, are used as fused-ring models to showcase that electron-donating methyl groups, which would generally be expected to lower oxidation potential, can lead to increased oxidation potentials when used as the steric drivers. Reduction events remain inaccessible through this molecular design route, a critical characteristic for electrochemical systems where high oxidation potentials are required and in which reductive decomposition must be prevented, as in high-voltage lithium-ion batteries. This study reveals a new avenue to alter the redox characteristics of fused-ring systems that find wide use as electroactive elements across a number of developing technologies.

The energy required to run carbon capture systems (CCS) constitutes a huge fraction (30% or more) of that which is produced by coal-fired power plants (1). This parasitic consumption is a major impediment to CCS implementation and retrofitting. The most energy-intensive element of CCS is the reboiler which provides thermal energy for the regeneration of amine-based CO2capture solvents. Our efficiency-boosting method stores excess electrical energy (produced during off-peak hours at baseload power plants) as chemical free energy. This device, resembling a secondary battery, is then optimized to preferentially produce thermal energy in its discharge phase for on-demand, solvent-regenerating heat when required during peak electricity use. We have developed several embodiments of the technology utilizing low cost electrode materials (Zn, Al, Sn, Bi and/or Pb) and molten salt eutectic electrolytes (ZnCl2:KCl, SnCl2:KCl, AlCl3:NaCl:KCl) (2). Figure 1(a) shows heat-generation for a Zn-Zn(Sn) system where a Zn anode is reversibly plated and stripped during charge and discharge, respectively (0 V discharge shown). This uses an air-stable ZnCl2:KCl eutectic electrolyte and molten Sn cathode where Zn reversibly forms an alloy. A ΔT max = 6 °C, steady current density of 10 mA cm-2, and 64% capacity retention after the first cycle demonstrates the proof-of-concept of this novel system which utilizes only ~50 g of earth-abundant active materials in this embodiment. The simple design – one active charge carrying species, Zn2+, in a one-compartment, separator-free cell – has shown promise as a battery technology with other materials, albeit at higher temperatures (3, 4). In an alternate, traditional galvanic cell design, Figure 1(b) demonstrates the temperature dependency of the open circuit voltage (OCV) during cooling (at t=0, cell T= 377 °C and heater is switched off) of a fritted H-cell containing Zn(s)|ZnCl2:KCl(l, eut.)||SnCl2:KCl(l, eut.)|Sn(l) (m.p. = 230, 176, 232 °C). As the molten materials freeze (around 2 h), there is an optimal operating temperature for this binary compartment cell. Interestingly, the OCV is poorer at higher temperatures (an opportunity for energy/cost reduction) and is still high and fluctuating significantly well after all materials are frozen (>3 hours). Ongoing experiments are exploring the mechanism behind this behavior and investigating lower temperature charge/discharge capacities. The thermal energy dissipated upon discharge of these cells can be subsequently captured via heat-transfer fluids (e.g. steam) for injection into the CCS stripper tower. Our studies are fine-tuning the chemistry and engineering necessary to control the balance of heat/electricity generation in these secondary cells; we ultimately envision a molten salt-based hybrid thermal/electrical energy storage medium. It is anticipated from Figure 1(b) that the electrochemical cell can self-heat to desired operating temperatures (achieving molten phase transition) with the excess electricity available during the charging phase. Molten salt electrolytes for electric-to-thermal energy transduction from this study may lead to further progress in niche battery applications since few commercial products of this type operate in the 250-400 °C range. References 1. E.S. Rubin, et al., Progr. Energy Combust. Sci., 38(5), 630 (2012). 2. J.K. Neathery, et al., A Method for Energy Storage to Utilize Intermittent Renewable Energy and Low-Value Electricity for CO2 Capture and Utilization. 2014. 3. H. Kim, et al., J. Power Sources, 241, 239 (2013). 4. D.J. Bradwell, et al., J. Am. Chem. Soc., 134(4), 1895 (2012). Figure 1. (a) Current density (black curve, primary axis) and heat generated (red curve, secondary axis) during 1 h of 0 V discharge of the Zn-Zn(Sn) cell with ZnCl2:KCl eutectic electrolyte. (b) OCV sampled during cooling of the second Zn-Sn cell type with frit-separated ZnCl2:KCl and SnCl2:KCl eutectics; OCV was steady at 0.22 V and T=377 °C when heater switched off at t=0.

Overcharge, a condition in which cell voltage rises to undesirably high potentials, can be prevented in lithium-ion batteries by incorporating redox shuttles into the battery electrolyte. Although extensive overcharge protection has been demonstrated in batteries with LiFePO4 cathodes, the redox shuttles that work in these batteries are incompatible with higher voltage cathodes. Designing stable additives with higher oxidation potentials is necessary to protect high voltage batteries from overcharge. Toward that goal, we synthesized diarylamines with varied structures, including fused heteroaromatic ring systems and electron-withdrawing substituents. We found that trends in oxidation potentials correlated with those in calculated adiabatic ionization potentials. Some diarylamine derivatives protected batteries from overcharge with varying degrees of success.

3,7-Bis(trifluoromethyl)-N-ethylphenothiazine (BCF3EPT) was evaluated as a redox shuttle for overcharge protection in lithium-ion batteries. Constant-charging experiments were performed to compare the compound to 1,4-di-tert-butyl-2,5-dimethoxybenzene and N-ethylphenothiazine. BCF3EPT showed significantly longer overcharge protection when compared to either benchmark at the same concentrations in LiFePO4/graphite batteries.

We report homogeneous catalysts that are soluble and stable in primary amine-based CO2 capture solvents. The zinc(II) and cobalt(III) complexes, which contain electron-donating multi-dentate anionic ligands, perform catalytic CO2 hydration at unparalleled observed rates under conditions conducive to industrial post-combustion carbon capture processes.

A variety of mechanisms lead to the failure of lithium-ion batteries. One is overcharge, a condition in which a battery's voltage rises above its designed end-of-charge potential. Electrolyte additives called redox shuttles limit cell potential by preferentially oxidizing, and cycling between the cathode and anode in their radical cation and neutral forms. Currently, testing requires coin cell assembly and repeated cycling, which can be an expensive and time consuming process. It is commonly accepted that degradation of the radical cation form of a redox shuttle leads to overcharge protection failure. We thus studied the stability of the radical cation forms of a series of redox shuttle additives to determine if there is a correlation between radical cation stability and the number of cycles of overcharge protection. While the reversibility of oxidations in cyclic voltammetry did not correlate to trends in overcharge performance, results from both UV-vis and electron paramagnetic resonance spectroscopy showed a correlation between stability and overcharge protection. Our results reveal trends within a few hours for what otherwise takes months of battery cycling to determine, providing a fast and relatively inexpensive method for predicting redox shuttle performance.

In this issue of Joule, Michael Aziz, Roy Gordon, Alan Aspuru-Guzik, and colleagues report a new derivative of anthraquinone that exhibits greater solubility and stability in basic solutions. Cells operated at ambient temperatures at pH 12 that contain the new quinone and potassium ferri-/ferrocyanide show remarkably improved stability.

When considering organic redox couples as active charge storage materials in redox flow batteries, several characteristics are important, including redox potential, solubility, and stability. Although redox potentials can be reliably estimated using density functional theory calculations, predicting solubility and stability remains a significant challenge, especially when considering the charged states of a material. The majority of reports on the solubility of organic materials have been limited to the uncharged (often neutral) forms of redox couples. Stability is usually probed using electrochemical techniques (e.g. cyclic voltammetry and bulk electrolysis) and with flow cell cycling. Although some instances of in situ spectroscopic analysis of flow cell electrolytes has been reported, for the most part, experimental determination of the failure mechanisms of an organic redox couple has been attempted via post-mortem analysis of an electrolyte after cycling. In none of these experiments is the charged form analyzed separately from components beyond the electrolyte salt and solvent – such as electrode surfaces, membranes, and potential fields – that may lead to decomposition. My group is interested in characterizing the charged forms of organic molecules using methods that allow us to determine the sources and mechanisms of decomposition. Thus, in addition to the standard electrochemical methods and cycling studies, we have focused on the generation and isolation of charged species using chemical means. By isolating the charged species, we are able to measure solubility and analyze stability in controlled environments, choosing when to introduce any component that is not the active species itself. Here I will give an overview of the methods we have used to generate, isolate, and characterize charged organic molecules, as well as and what we have learned, both independent of and complementary to electrochemical methods for analysis.

Redox-active organic compounds are added to lithium-ion battery electrolytes (i) to serve as redox shuttles to mitigate excess current during overcharge and (ii) to passivate electrode surfaces to stabilize interfaces that would otherwise lead to electrolyte decomposition. Especially when expanding the voltage range in charging of lithium-ion cells, overcharge protection efficacy of reported redox shuttles has been limited, yet this may not be a challenge if redox shuttle failure results in cathode passivation, which becomes more critical to cell longevity in higher voltage cells. In this study, we explored a series of phenothiazine-based electrolyte additives with oxidation potentials above 4 V vs. Li in high-voltage (e.g. NMC cathode-containing) lithium-ion cells. Using electrochemical and spectroscopic techniques, we sought to determine if these additives act in shuttling (reversible) or passivating (irreversible) mechanisms.

Due to their wider electrochemical windows of solvents and higher cell voltages, non-aqueous electrolytes containing organic active materials are of interest for use in redox flow batteries (RFBs) for grid energy storage.1 The main challenges to achieving high performance in a nonaqueous RFBs include (i) improving the solubility and stability of the active organic molecules, (ii) preventing the negolyte and posolyte molecules from crossing over separators or membranes, and (iii) to do so without also limiting charging rates due to high cell resistance.2 Using a range of commercially available membranes and separators, we sought to improve the performance of full flow cells containing two materials developed in our laboratory: N-[2-(2-methoxyethoxy)ethyl]phenothiazine (MEEPT)3 and bis[2-(2-methoxyethoxy)ethyl]viologen bis(trifluoromethanesulfonyl)imide (B(MEE)Vol-TFSI2). We first screened electrolyte salts and solvents to optimize for a combination of electrochemical stability and solubility after which we measured the impedance of various membranes and separators with a subset of the electrolytes. Finally, we assembled flow cells with select electrolytes, comparing performance using a separator vs. with ion-selective membranes. Our results showed that the two active materials are stable in cycling and that cell performance was ultimately limited by membrane performance: Either the capacity was halved with a non-selective separator, or impedance rose upon fouling of ion-selective membranes. These results highlight the need for membrane development for nonaqueous RFBs containing small molecule active materials.

Predicting the solubility and stability of redox-active organics across all states of charge is a challenging task. A trained chemist can often predict trends, but when it comes to numerical values, even the best computer software cannot be counted upon. In screening active materials for redox flow batteries (RFBs) – in which both properties are important – we are slave to trial and error, modifying organic molecules in attempt to optimize (increase) solubility and stability without compromising other important properties such as redox potential.1,2We hope to change our approach to materials development by better predicting properties in advance. Using quantitative structure-property relationship (QSPR), we have shown that properties of unknown organic molecules can be predicted when experimental properties of a training set of related compounds is provided.3Here we sought to expand this approach to a different redox core, phenothiazines, which we have evaluated as posolyte candidates for RFBs. We measured the solubility of about a dozen phenothiazine derivatives – in both relevant states of charge (neutral and radical cation) – in a nonaqueous electrolyte. The structure of these derivatives included variations in the number, type, and position of substituents to offer a diverse training set. We used our experimental values and the results of density functional theory calculations to develop a QSPR model for the phenothiazine core, following which we synthesized and measured the solubility of a new set of phenothiazines. Here we will report on the degree of success of our model for solubility prediction. References Kowalski, J. A.; Casselman, M. D.; Kaur, A. P.; Milshtein, J. D.; Elliott, C. F.; Modekrutti, S.; Attanayake, N. H.; Zhang, N.; Parkin, S. R.; Risko, C.; Brushett, F. R.; Odom, S. A., “A Stable Two-Electron-Donating Phenothiazine for Application in Nonaqueous Redox Flow Batteries.” J. Mater. Chem. A 2017, 5, 24371. Milshtein, J. D.; Kaur, A. P.; Casselman, M. D.; Kowalski, J. A.; Modekrutti, S.; Zhang, P. L.; Attanayake, N. H.; Elliott, C. F.; Parkin, S. R.; Risko, C.; Brushett, F. R.; Odom, S. A., “High Current Density, Long Duration Cycling of Soluble Organic Active Species for Non-Aqueous Redox Flow Batteries.” Energy Environ. Sci. 2016, 9, 3531. Sevov, C. S.; Hickey, D. P.; Cook, M. E.; Robinson, S. G.; Barnett, S.; Minteer, S. D.*; Sigman, M. S.; Sanford, M. S. "Physical Organic Approach to Persistent, Cyclable, Low-Potential Electrolytes for Flow Battery Applications." J. Am. Chem. Soc. 2017, 139, 2924.

Radical cations have been generated for 10 bis[4-(diarylamino)styryl]arenes and heteroarenes to investigate the effect of the electron-richness of the terminal groups and of the bridging (hetero)arene on delocalization. The intervalence charge-transfer bands of these radical cations vary from weak broad Gaussians, indicative of localized class-II mixed-valence species, to strong relatively narrow asymmetric bands, characteristic of delocalized class-III bis(diarylamino) species, to narrow symmetric bands in cases where the bridge contribution to the singly occupied molecular orbital is largest. Hush analysis of these bands yields estimates of the electronic coupling varying from 480 cm -1 (electron-poor bridge, most electron-rich terminal aryl groups) to 1000 cm -1 (electron-rich bridge, least electron-rich termini) if the diabatic electron-transfer distance, R ab, is equated to the N-N separation. Computational and electron spin resonance (ESR) evidence for displacement of the diabatic states into the bridge (reduced R ab) suggests that these values are underestimates and that even more variation is to be expected through the series. Several dications have also been studied. The vis-NIR absorption of the dication of (E,E)-1,4-bis{4-[bis(4-n-butoxyphenyl) amino]styryl}-2,5-dicyanobenzene is seen at an energy similar to that of the strongest band in the spectrum of the corresponding weakly coupled monocation, with approximately twice the absorptivity, and its ESR spectrum suggests essentially noninteracting radical centers. In contrast, the electronic spectra of class-III monocations show no clear relationship to those of the corresponding dications, which ESR reveals to be singlet species. © 2012 American Chemical Society.

Dr. S. A. Odom , O. Zhao , Prof. J. S. Moore Department of ChemistryBeckman Institute for Advanced Science & TechnologyUniversity of Illinois at Urbana-Champaign405 N. Mathews Ave. Urbana, IL 61801, USA E-mail: jsmoore@illinois.edu S. Chayanupatkul , B. J. Blaiszik , A. C. Jackson , Prof. P. Braun , . V Prof. N. R. Sottos Department of Materials Science & EngineeringBeckman Institute for Advanced Science & TechnologyUniversity of Illinois at Urbana-Champaign405 N. Mathews Ave. Urbana, IL 61801, USA Prof. S. R. White Department of Aerospace EngineeringBeckman Institute for Advanced Science & TechnologyUniversity of Illinois at Urbana-Champaign405 N. Mathews Ave. Urbana, IL 61801, USAE-mail: swhite@illinois.edu

We report a new core-shell microcapsule system for the visual detection of mechanical damage. The core material, 1,3,5,7-cyclooctatetraene, is a conjugated cyclic olefin and a precursor to intensely colored polyacetylene. A combination of poly(urea-formaldehyde) and polyurethane is required to effectively encapsulate the volatile core material. Increasing the outer shell wall thickness and including a core-side prepolymer improves the thermal stability and free-flowing nature of these capsules, which tend to leach and rupture with thinner shell walls. Capsules ruptured in the presence of the Grubbs-Love ruthenium catalyst show immediate color change from nearly colorless to red-orange and dark purple over time, and color change in thin films resulted from scratch damage.

Stimuli-responsive capsules are of interest in drug delivery, fragrance release, food preservation, and self-healing materials. Many methods are used to trigger the release of encapsulated contents. Here we highlight mechanisms for the controlled release of encapsulated cargo that utilize chemical reactions occurring in solid polymeric shell walls. Triggering mechanisms responsible for covalent bond cleavage that result in the release of capsule contents include chemical, biological, light, thermal, magnetic, and electrical stimuli. We present methods for encapsulation and release, triggering methods, and mechanisms and conclude with our opinions on interesting obstacles for chemically induced activation with relevance for controlled release.

There is an increasing demand for stationary energy storage systems to facilitate the integration of intermittent, renewable energy sources (e.g. wind, solar) and to improve the efficiency, reliability, and resiliency of the existing fossil fuel infrastructure. Redox flow batteries (RFBs) are electrochemical energy storage devices that are well suited for grid storage due to decoupled power and energy scaling, long operating lifetimes, and simplified manufacturing1. While state-of-the-art RFBs utilize transition metal salts to store energy, they are currently too expensive to meet the stringent targets set by the U.S. Department of Energy ($100/kWh)2, motivating research in novel organic charge storage materials which may enable a pathway to low cost through tunable molecular structure and inexpensive synthesis routes3. This work focuses on the use of phenothiazine and its derivatives as a robust learning platform to investigate the impact of substituent group addition on the molecular properties with an overarching goal of developing structure-function relations that enable deterministic multi-property optimization. Specifically, we seek to improve the equivalent charge concentration of redox electrolytes containing phenothiazines, by enhancing the solubility and intrinsic storage capacity through a combination of molecular engineering and electrochemical analysis. Building from N-ethylphenothiazine, we find judicious application of substituent groups can lead to significant performance enhancements, but care must be taken to avoid improving one property at the expense of others. References A. Z. Weber et al., J. Appl. Electrochem., 41, 1137–1164 (2011). A. A. Akhil et al., Ed Albuq. NM Sandia Natl. Lab. (2013) http://www.emnrd.state.nm.us/ECMD/RenewableEnergy/documents/SNL-ElectricityStorageHandbook2013.pdf. J. A. Kowalski, L. Su, J. D. Milshtein, and F. R. Brushett, Curr. Opin. Chem. Eng., 13, 45–52 (2016).

Redox-active organic compounds have been explored as electrolyte additives for lithium-ion batteries, for purposes ranging from mitigation of excess current in overcharging cells to passivation of electrode surfaces to stabilize surfaces that would otherwise lead to electrolyte decomposition. Especially when expanding the voltage range in charging of lithium-ion cells, passivation becomes more critical to cell longevity during operation. Electrolyte additives containing purposefully weak covalent bonds have been employed in high-voltage (e.g. NMC cathodes) lithium-ion cells, which are tailored to react with hydrofluoric acid – a product of decomposition of fluorine-containing anions present in the electrolyte. Here we report results from the investigation of organic molecules as electrolyte additives with functionality aimed to react with/at cathode surfaces to form passivating films. These organic molecules with varying oxidation potentials (3.4 V vs. Li+/0 and above) house main group ad-atoms unique from those contained in the electrode, binder, conductive fillers, and electrolyte solutions. This allows for surface imaging and electrolyte analysis to determine the location and reaction of such additives following formation cycling.