Development of a multi-step screening procedure for redox active molecules in organic radical polymer anodes and as redox flow anolytes.

Benzo[d]-X-zolyl-pyridinyl (XO, S, NH) radicals represent a promising class of redox-active molecules for organic batteries. We present a multistep screening procedure to identify the most promising radical candidates. Experimental investigations and highly correlated wave function-based calculations are performed to determine benchmark redox potentials. Based on these, the accuracies of different methods (semi-empirical, density functional theory, wave function-based), solvent models, dispersion corrections, and basis sets are evaluated. The developed screening procedure consists of three steps: First, a conformer search is performed with CREST. The molecules are selected based on the redox potentials calculated using GFN2-xTB. Second, HOMO energies calculated with reparametrized B3LYP-D3(BJ) and the def2-SVP basis set are used as selection criteria. The final molecules are selected based on the redox potentials calculated from Gibbs energies using BP86-D3(BJ)/def2-TZVP. With this multistep screening approach, promising molecules can be suggested for synthesis, and structure-property relationships can be derived.

Development of Novel Redox-Active Organic Materials Based on Benzimidazole, Benzoxazole, and Benzothiazole: A Combined Theoretical and Experimental Screening Approach.

Sustainability is one of the hot topics of today's research, in particular when it comes to energy-storage systems such as batteries. Redox-active molecules implemented in organic batteries represent a promising alternative to lithium-ion batteries, which partially rely on non-sustainable heavy metal salts. As an alternative, we propose benzothiazole, -oxazole and -imidazole derivatives as redox-active moieties for polymers in organic (radical) batteries. The target molecules were identified by a combination of theoretical and experimental approaches for the investigation of new organic active materials. Herein, we present the synthesis, electrochemical characterization and theoretical investigation of the proposed molecules, which can later be introduced into a polymer backbone and used in organic polymer batteries.

Oxymethylene Ether (OME) Fuel Catalyst Screening Using In Situ NMR Spectroscopy.

Online NMR measurements are introduced in the current study as a new analytical setup for investigation of the oxymethylene dimethyl ether (OME) synthesis. For the validation of the setup, the newly established method is compared with state-of-the-art gas chromatographic analysis. Afterwards, the influence of different parameters, such as temperature, catalyst concentration and catalyst type on the OME fuel formation based on trioxane and dimethoxymethane is investigated. As catalysts, AmberlystTM 15 (A15) and trifluoromethanesulfonic acid (TfOH) are utilized. A kinetic model is applied to describe the reaction in more detail. Based on these results, the activation energy (A15: 48.0 kJ mol-1 and TfOH: 72.3 kJ mol-1 ) and the order in catalyst (A15: 1.1 and TfOH: 1.3) are calculated and discussed.

Novel Biobased Self-Healing Ionomers Derived from Itaconic Acid Derivates.

This article presents novel biobased ionomers featuring self-healing abilities. These smart materials are synthesized from itaconic acid derivates. Large quantities of itaconic acid can be produced from diverse biomass like corn, rice, and others. This study presents a comprehensive investigation of their thermal and mechanical properties via differential scanning calorimetry (DSC), thermo gravimetric analysis (TGA), and FT-Raman and FT-IR measurements as well as dynamic mechanic analysis. Within all these measurements, different kinds of structure-property relationships could be derived from these measurements. For example, the proportion of ionic groups enormously influences the self-healing efficiency. The investigation of the self-healing abilities reveals healing efficiencies up to 99% in 2 h at 90 °C for the itaconic acid based ionomer with the lowest ionic content.

Red-light sensitized hole-conducting polymer for energy conversion.

We report a novel hole conductive polymer with photoactive Os(ii) complexes in the side chains. This PPV derivative can be activated upon absorption of red visible light and delivers notable photocurrents when used as photocathode material. Thus, the polymer presents as a stepping stone towards developing soft matter alternatives to NiO photocathodes, which function under visible light irradiation. To show the concept we combine electrical impedance spectroscopy with steady state spectroscopy. As light-driven hole injection from Os complex to the PPV polymer is thermodynamically feasible both based on reductive quenching of photoexcited PPV and based on oxidative quenching of the photoexcited Os chromophores we investigate the impact of illumination wavelengths on the photocathode behavior and photochemical stability of the material. While both blue and red light excitation, i.e., excitation of the chromophoric units PPV and excitation of the metal-to-ligand charge transfer transitions in the side-chain pendant Os chromophores yield cathodic photocurrents, the photochemical stability is drastically enhanced upon red-light excitation. Hence, the results of the investigations discussed show the validity of the concept developing red-light sensitized hole-conducting polymers for energy conversion.

An aqueous, polymer-based redox-flow battery using non-corrosive, safe, and low-cost materials.

For renewable energy sources such as solar, wind, and hydroelectric to be effectively used in the grid of the future, flexible and scalable energy-storage solutions are necessary to mitigate output fluctuations. Redox-flow batteries (RFBs) were first built in the 1940s and are considered a promising large-scale energy-storage technology. A limited number of redox-active materials--mainly metal salts, corrosive halogens, and low-molar-mass organic compounds--have been investigated as active materials, and only a few membrane materials, such as Nafion, have been considered for RFBs. However, for systems that are intended for both domestic and large-scale use, safety and cost must be taken into account as well as energy density and capacity, particularly regarding long-term access to metal resources, which places limits on the lithium-ion-based and vanadium-based RFB development. Here we describe an affordable, safe, and scalable battery system, which uses organic polymers as the charge-storage material in combination with inexpensive dialysis membranes, which separate the anode and the cathode by the retention of the non-metallic, active (macro-molecular) species, and an aqueous sodium chloride solution as the electrolyte. This water- and polymer-based RFB has an energy density of 10 watt hours per litre, current densities of up to 100 milliamperes per square centimetre, and stable long-term cycling capability. The polymer-based RFB we present uses an environmentally benign sodium chloride solution and cheap, commercially available filter membranes instead of highly corrosive acid electrolytes and expensive membrane materials.

Novel, Stable Catholyte for Aqueous Organic Redox Flow Batteries: Symmetric Cell Study of Hydroquinones with High Accessible Capacity.

Owing to their broad range of redox potential, quinones/hydroquinones can be utilized for energy storage in redox flow batteries. In terms of stability, organic catholytes are more challenging than anolytes. The two-electron transfer feature adds value when building all-quinone flow battery systems. However, the dimerization of quinones/hydroquinones usually makes it difficult to achieve a full two-electron transfer in practical redox flow battery applications. In this work, we designed and synthesized four new hydroquinone derivatives bearing morpholinomethylene and/or methyl groups in different positions on the benzene ring to probe molecular stability upon battery cycling. The redox potential of the four molecules were investigated, followed by long-term stability tests using different supporting electrolytes and cell cycling methods in a symmetric flow cell. The derivative with two unoccupied ortho positions was found highly unstable, the cell of which exhibited a capacity decay rate of ~50% per day. Fully substituted hydroquinones turned out to be more stable. In particular, 2,6-dimethyl-3,5-bis(morpholinomethylene)benzene-1,4-diol (asym-O-5) displayed a capacity decay of only 0.45%/day with four-week potentiostatic cycling at 0.1 M in 1 M H3PO4. In addition, the three fully substituted hydroquinones displayed good accessible capacity of over 82%, much higher than those of conventional quinone derivatives.

DNA Origami Meets Polymers: A Powerful Tool for the Design of Defined Nanostructures.

The combination of DNA origami nanostructures and polymers provides a new possibility to access defined structures in the 100 nm range. In general, DNA origami serves as a versatile template for the highly specific arrangement of polymer chains. Polymer-DNA hybrid nanostructures can either be created by growing the polymer from the DNA template or by attaching preformed polymers to the DNA scaffold. These conjugations can be of a covalent nature or be based on base-pair hybridization between respectively modified polymers and DNA origami. Furthermore, the negatively charged DNA backbone permits interaction with positively charged polyelectrolytes to form stable complexes. The combination of polymers with tuneable characteristics and DNA origami allows the creation of a new class of hybrid materials, which could offer exciting applications for controlled energy transfer, nanoscale organic circuits, or the templated synthesis of nanopatterned polymeric structures.

In-depth characterization of self-healing polymers based on π-π interactions.

The self-healing behavior of two supramolecular polymers based on π-π-interactions featuring different polymer backbones is presented. For this purpose, these polymers were synthesized utilizing a polycondensation of a perylene tetracarboxylic dianhydride with polyether-based diamines and the resulting materials were investigated using various analytical techniques. Thus, the molecular structure of the polymers could be correlated with the ability for self-healing. Moreover, the mechanical behavior was studied using rheology. The activation of the supramolecular interactions results in a breaking of these noncovalent bonds, which was investigated using IR spectroscopy, leading to a sufficient increase in mobility and, finally, a healing of the mechanical damage. This scratch-healing behavior was also quantified in detail using an indenter.

Fluorescence upconversion by triplet-triplet annihilation in all-organic poly(methacrylate)-terpolymers.

Fluorescence upconversion by triplet-triplet annihilation is demonstrated for a fully polymer-integrated material, i.e. in the limit of restricted diffusion. Organic sensitizer and acceptor are covalently attached to a poly(methacrylate) backbone, yielding a metal-free macromolecular all-in-one system for fluorescence upconversion. Due to the spatial confinement of the optically active molecular components, i.e. annihilator and sensitizer, UC by TTA in the constrained polymer system in solution is achieved at exceptionally low averaged annihilator concentrations. However, the UC quantum yield in the investigated systems is found to be low, highlighting that only chromophores in specific local surroundings yield upconversion in the limit of restricted diffusion. A photophysical model is proposed taking the heterogeneous local environment within the polymers into account.

Detailed Analysis of the Influencing Parameters on the Self-Healing Behavior of Dynamic Urea-Crosslinked Poly(methacrylate)s.

For this paper, the self-healing ability of poly(methacrylate)s crosslinked via reversible urea bonds was studied in detail. In this context, the effects of healing time and temperature on the healing process were investigated. Furthermore, the impact of the size of the damage (i.e., area of the scratch) was monitored. Aging processes, counteracting the self-healing process, result in a decrease in the mechanical performance. This effect diminishes the healing ability. Consequently, the current study is a first approach towards a detailed analysis of self-healing polymers regarding the influencing parameters of the healing process, considering also possible aging processes for thermo-reversible polymer networks.

Thermally Switchable Fluorescence Resonance Energy Transfer via Reversible Diels-Alder Reaction of π-Conjugated Oligo-(Phenylene Ethynylene)s.

The combination of reversible, dynamic covalent bonds and π-conjugated oligo-(phenylene ethynylene)s is utilized for exchange reactions between two acceptors and one donor containing copolymer in the solid state upon thermal treatment. The specific molecular design of the polymers allows upon thermally triggering the reshuffling of the π-conjugated donor and acceptor moieties. Depending on the nature of the acceptor, an increased and a decreased fluorescence resonance energy transfer (FRET), respectively, can be observed for the mixing of the copolymers or the demixing of a donor-acceptor copolymer.

Influence of Aspartate Moieties on the Self-Healing Behavior of Histidine-Rich Supramolecular Polymers.

Aspartate incorporated into the protein structure of mussel byssal threads is believed to play an important role, besides the reversible histidine-zinc interactions, in the self-healing behavior of mussel byssal threads. Therefore, copolymers containing both aspartate and histidine moieties are synthesized in order to investigate the influence of aspartate on the complexation of zinc(II) as well as on the self-healing behavior and the mechanical properties of the resulting supramolecular networks. For this purpose, isothermal titration calorimetry measurements of a model aspartate compound as well as of these copolymers are performed and the thermodynamic parameters are utilized for the design of self-healing copolymers. For this purpose, n-lauryl methacrylate-based copolymers containing histidine and aspartate are synthesized and crosslinked with zinc(II) acetate. The self-healing behavior of the supramolecular networks is investigated using scratch healing tests and the mechanical properties by nanoindentation.

Palladium-SCS Pincer Complexes as Cross-Linking Moieties in Self-Healing Metallopolymers.

Metallopolymers combine properties of metal complexes and polymers and are thus interesting materials for a wide field of different applications. One current major field is the utilization as self-healing polymers, and for this purpose, palladium-SCS pincer complexes are synthesized. Well-defined copolymers featuring those metal complexes in the side chain are obtained using the reversible addition-fragmentation chain-transfer polymerization technique. By the addition of a tetravalent pyridine cross-linkers, reversible cross-linked supramolecular networks are prepared, enabling self-healing properties. By utilizing density functional theory calculations, IR, and Raman spectroscopy, as well as isothermal titration calorimetry, the complex formation, reversibility, and stability are studied. The information of these experiments also enables further optimizations for the design of similar reversible systems in the future.

Hydrogel-Embedded Model Photocatalytic System Investigated by Raman and IR Spectroscopy Assisted by Density Functional Theory Calculations and Two-Dimensional Correlation Analysis.

The presented study reports the synthesis and the vibrational spectroscopic characterization of different matrix-embedded model photocatalysts. The goal of the study is to investigate the interaction of a polymer matrix with photosensitizing dyes and metal complexes for potential future photocatalytic applications. The synthesis focuses on a new rhodamine B derivate and a Pt(II) terpyridine complex, which both contain a polymerizable methacrylate moiety and an acid labile acylhydrazone group. The methacrylate moieties are afterward utilized to synthesize functional model hydrogels mainly consisting of poly(ethylene glycol) methacrylate units. The pH-dependent and temperature-dependent behavior of the hydrogels is investigated by means of Raman and IR spectroscopy assisted by density functional theory calculations and two-dimensional correlation spectroscopy. The spectroscopic results reveal that the Pt(II) terpyridine complex can be released from the polymer matrix by cleaving the C═N bond in an acid environment. The same behavior could not be observed in the case of the rhodamine B dye although it features a comparable C═N bond. The temperature-dependent study shows that the water evaporation has a significant influence neither on the molecular structure of the hydrogel nor on the model photocatalytic moieties.

Bi-diketopyrrolopyrrole (Bi-DPP) as a novel electron accepting compound in low band gap π-conjugated donor-acceptor copolymers/oligomers.

The synthesis and characterization of a novel 2,5-diketopyrrolo[3,4-c]pyrrole(DPP)-based accepting building block with the scheme DPP-neutral small linker-DPP (Bi-DPP) is presented, which was utilized as electron accepting moiety for low band gap π-conjugated donor-acceptor copolymers as well as for a donor-acceptor small molecule. The electron accepting moiety Bi-DPP was prepared via a novel synthetic pathway by building up two DPP moieties step by step simultaneously starting from a neutral phenyl core unit. Characterization of the synthesized oligomeric and polymeric materials via cyclic voltammetry afford LUMO energy levels from -3.49 to -3.59 eV as well as HOMO energy levels from -5.07 to -5.34 eV resulting in low energy band gaps from 1.52 to 1.81 eV. Spin coating of the prepared donor-acceptor oligomers/polymers resulted in well-defined films. Moreover, UV-vis measurements of the investigated donor-acceptor systems showed a broad absorption over the whole visible region. It is demonstrated that Bi-DPP as an electron accepting moiety in donor-acceptor systems offer potential properties for organic solar cell devices.

Redox-Flow Batteries: From Metals to Organic Redox-Active Materials.

Research on redox-flow batteries (RFBs) is currently experiencing a significant upturn, stimulated by the growing need to store increasing quantities of sustainably generated electrical energy. RFBs are promising candidates for the creation of smart grids, particularly when combined with photovoltaics and wind farms. To achieve the goal of "green", safe, and cost-efficient energy storage, research has shifted from metal-based materials to organic active materials in recent years. This Review presents an overview of various flow-battery systems. Relevant studies concerning their history are discussed as well as their development over the last few years from the classical inorganic, to organic/inorganic, to RFBs with organic redox-active cathode and anode materials. Available technologies are analyzed in terms of their technical, economic, and environmental aspects; the advantages and limitations of these systems are also discussed. Further technological challenges and prospective research possibilities are highlighted.

Polymeric Halogen-Bond-Based Donor Systems Showing Self-Healing Behavior in Thin Films.

The synthesis and comprehensive characterization of a systematic series of cleft-type anion receptors imbedded into a polymeric architecture is presented. For the first time, isothermal calorimetric titrations on polymeric halogen-bond-based donors were exploited to evaluate the dependence of the anion affinity on different key parameters (i.e. monomeric versus polymeric receptor, halogen versus hydrogen bonding, charge assistance). The combination of these donor systems with a copolymer bearing accepting carboxylate groups led to supramolecular cross-linked polymer networks showing excellent intrinsic self-healing behavior. FT-Raman spectroscopy and nano-indentation measurements were utilized to clarify the thermally induced self-healing mechanism based on the formation of halogen bonds. These first self-healing materials based on halogen bonds pave the way for new applications of halogen-bond donors in polymer and material science.

Assorted Phenoxyl-Radical Polymers and Their Application in Lithium-Organic Batteries.

The synthesis and electrochemical characterization of novel polymers bearing phenoxyl-radicals as redox-active side chains is described. The monomers are synthesized from the corresponding phenols and quinones, respectively. These compounds are subsequently poly-merized via ring-opening metathesis polymerization. The electrochemical properties of the phenoxyl-radical polymers are characterized using cyclic voltammetry and the most promising polymer is investigated as active material in a lithium coin-cell, creating the first phenoxyl-lithium battery. These phenoxyl-containing polymers represent interesting anode materials for organic radical and lithium batteries due to their suitable redox-potentials and possibility to create batteries with higher potentials as well as straightforward synthesis procedures.

An Aqueous Redox-Flow Battery with High Capacity and Power: The TEMPTMA/MV System.

Redox-flow batteries (RFB) can easily store large amounts of electric energy and thereby mitigate the fluctuating output of renewable power plants. They are widely discussed as energy-storage solutions for wind and solar farms to improve the stability of the electrical grid. Most common RFB concepts are based on strongly acidic metal-salt solutions or poorly performing organics. Herein we present a battery which employs the highly soluble N,N,N-2,2,6,6-heptamethylpiperidinyl oxy-4-ammonium chloride (TEMPTMA) and the viologen derivative N,N'-dimethyl-4,4-bipyridinium dichloride (MV) in a simple and safe aqueous solution as redox-active materials. The resulting battery using these electrolyte solutions has capacities of 54 Ah L-1 , giving a total energy density of 38 Wh L-1 at a cell voltage of 1.4 V. With peak current densities of up to 200 mA cm-2 the TEMPTMA/MV system is a suitable candidate for compact high-capacity and high-power applications.