Browsing by Author "Rauber, Daniel"
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- ItemAnion and ether group influence in protic guanidinium ionic liquids(Cambridge : RSC Publ., 2023) Rauber, Daniel; Philippi, Frederik; Becker, Julian; Zapp, Josef; Morgenstern, Bernd; Kuttich, Björn; Kraus, Tobias; Hempelmann, Rolf; Hunt, Patricia; Welton, Tom; Kay, Christopher W. M.Ionic liquids are attractive liquid materials for many advanced applications. For targeted design, in-depth knowledge about their structure-property-relations is urgently needed. We prepared a set of novel protic ionic liquids (PILs) with a guanidinium cation with either an ether or alkyl side chain and different anions. While being a promising cation class, the available data is insufficient to guide design. We measured thermal and transport properties, nuclear magnetic resonance (NMR) spectra as well as liquid and crystalline structures supported by ab initio computations and were able to obtain a detailed insight into the influence of the anion and the ether substitution on the physical and spectroscopic properties. For the PILs, hydrogen bonding is the main interaction between cation and anion and the H-bond strength is inversely related to the proton affinity of the constituting acid and correlated to the increase of 1H and 15N chemical shifts. Using anions from acids with lower proton affinity leads to proton localization on the cation as evident from NMR spectra and self-diffusion coefficients. In contrast, proton exchange was evident in ionic liquids with triflate and trifluoroacetate anions. Using imide-type anions and ether side groups decreases glass transitions as well as fragility, and accelerated dynamics significantly. In case of the ether guanidinium ionic liquids, the conformation of the side chain adopts a curled structure as the result of dispersion interactions, while the alkyl chains prefer a linear arrangement.
- ItemCurled cation structures accelerate the dynamics of ionic liquids(Cambridge : RSC Publ., 2021) Rauber, Daniel; Philippi, Frederik; Kuttich, Björn; Becker, Julian; Kraus, Tobias; Hunt, Patricia; Welton, Tom; Hempelmann, Rolf; Kay, Christopher W.M.Ionic liquids are modern liquid materials with potential and actual implementation in many advanced technologies. They combine many favourable and modifiable properties but have a major inherent drawback compared to molecular liquids – slower dynamics. In previous studies we found that the dynamics of ionic liquids are significantly accelerated by the introduction of multiple ether side chains into the cations. However, the origin of the improved transport properties, whether as a result of the altered cation conformation or due to the absence of nanostructuring within the liquid as a result of the higher polarity of the ether chains, remained to be clarified. Therefore, we prepared two novel sets of methylammonium based ionic liquids; one set with three ether substituents and another set with three butyl side chains, in order to compare their dynamic properties and liquid structures. Using a range of anions, we show that the dynamics of the ether-substituted cations are systematically and distinctly accelerated. Liquefaction temperatures are lowered and fragilities increased, while at the same time cation–anion distances are slightly larger for the alkylated samples. Furthermore, pronounced liquid nanostructures were not observed. Molecular dynamics simulations demonstrate that the origin of the altered properties of the ether substituted ionic liquids is primarily due to a curled ether chain conformation, in contrast to the alkylated cations where the alkyl chains retain a linear conformation. Thus, the observed structure–property relations can be explained by changes in the geometric shape of the cations, rather than by the absence of a liquid nanostructure. Application of quantum chemical calculations to a simplified model system revealed that intramolecular hydrogen-bonding is responsible for approximately half of the stabilisation of the curled ether-cations, whereas the other half stems from non-specific long-range interactions. These findings give more detailed insights into the structure–property relations of ionic liquids and will guide the development of ionic liquids that do not suffer from slow dynamics.
- ItemDynamics, cation conformation and rotamers in guanidinium ionic liquids with ether groups(Amsterdam : Elsevier, 2023) Rauber, Daniel; Philippi, Frederik; Morgenstern, Bernd; Zapp, Josef; Kuttich, Björn; Kraus, Tobias; Welton, Tom; Hempelmann, Rolf; Kay, Christopher W.M.Ionic liquids are modern materials with a broad range of applications, including electrochemical devices, the exploitation of sustainable resources and chemical processing. Expanding the chemical space to include novel ion classes allows for the elucidation of novel structure-property relationships and fine tuning for specific applications. We prepared a set of ionic liquids based on the sparsely investigated pentamethyl guanidinium cation with a 2-ethoxy-ethyl side chain in combination with a series of frequently used anions. The resulting properties are compared to a cation with a pentyl side chain lacking ether functionalization. We measured the thermal transitions and transport properties to estimate the performance and trends of this cation class. The samples with imide-type anions form liquids at ambient temperature, and show good transport properties, comparable to imidazolium or ammonium ionic liquids. Despite the dynamics being significantly accelerated, ether functionalization of the cation favors the formation of crystalline solids. Single crystal structure analysis, ab initio calculations and variable temperature nuclear magnetic resonance measurements (VT-NMR) revealed that cation conformations for the ether- and alkyl-chain-substituted are different in both the solid and liquid states. While ether containing cations adopt compact, curled structures, those with pentyl side chains are linear. The Eyring plot revealed that the curled conformation is accompanied by a higher activation energy for rotation around the carbon-nitrogen bonds, due to the coordination of the ether chain as observed by VT-NMR.
- ItemEther functionalisation, ion conformation and the optimisation of macroscopic properties in ionic liquids(Cambridge : RSC Publ., 2020) Philippi, Frederik; Rauber, Daniel; Kuttich, Björn; Kraus, Tobias; Kay, Christopher W.M.; Hempelmann, Rolf; Hunt, Patricia A.; Welton, TomIonic liquids are an attractive material class due to their wide liquid range, intrinsic ionic conductivity, and high chemical as well as electrochemical stability. However, the widespread use of ionic liquids is hindered by significantly higher viscosities compared to conventional molecular solvents. In this work, we show how the transport properties of ionic liquids can be altered significantly, even for isostructural ions that have the same backbone. To this end, structure–property relationships have been determined for a set of 16 systematically varied representative ionic liquids. Variations in molecular structure include ammonium vs. phosphonium, ether vs. alkyl side chains, and rigid vs. flexible anions. Ab initio calculations are used to relate molecular structures to the thermal, structural and transport properties of the ionic liquids. We find that the differences in properties of ether and alkyl functionalised ionic liquids are primarily dependent on minimum energy geometries, with the conformational flexibility of ether side chains appearing to be of secondary importance. We also show unprecedented correlations between anion conformational flexibility and transport properties. Critically, increasing fluidity upon consecutive introduction of ether side chains and phosphonium centres into the cation is found to be dependent on whether the anion is flexible or rigid. We demonstrate that targeted design of functional groups based on structure–property relationships can yield ionic liquids of exceptionally high fluidity.
- ItemPentacene in 1,3,5-Tri(1-naphtyl)benzene: A Novel Standard for Transient EPR Spectroscopy at Room Temperature(Wien [u.a.] : Springer, 2021) Schröder, Mirjam; Rauber, Daniel; Matt, Clemens; Kay, Christopher W. M.Testing and calibrating an experimental setup with standard samples is an essential aspect of scientific research. Single crystals of pentacene in p-terphenyl are widely used for this purpose in transient electron paramagnetic resonance (EPR) spectroscopy. However, this sample is not without downsides: the crystals need to be grown and the EPR transitions only appear at particular orientations of the crystal with respect to the external magnetic field. An alternative host for pentacene is the glass-forming 1,3,5-tri(1-naphtyl)benzene (TNB). Due to the high glass transition point of TNB, an amorphous glass containing randomly oriented pentacene molecules is obtained at room temperature. Here we demonstrate that pentacene dissolved in TNB gives a typical “powder-like” transient EPR spectrum of the triplet state following pulsed laser excitation. From the two-dimensional data set, it is straightforward to obtain the zero-field splitting parameters and relative populations by spectral simulation as well as the B1 field in the microwave resonator. Due to the simplicity of preparation, handling and stability, this system is ideal for adjusting the laser beam with respect to the microwave resonator and for introducing students to transient EPR spectroscopy. © 2021, The Author(s).
- ItemPolyacrylonitrile-containing amphiphilic block copolymers: self-assembly and porous membrane formation(Cambridge : RSC Publ., 2023) Gemmer, Lea; Niebuur, Bart-Jan; Dietz, Christian; Rauber, Daniel; Plank, Martina; Frieß, Florian V.; Presser, Volker; Stark, Robert W.; Kraus, Tobias; Gallei, MarkusThe development of hierarchically porous block copolymer (BCP) membranes via the application of the self-assembly and non-solvent induced phase separation (SNIPS) process is one important achievement in BCP science in the last decades. In this work, we present the synthesis of polyacrylonitrile-containing amphiphilic BCPs and their unique microphase separation capability, as well as their applicability for the SNIPS process leading to isoporous integral asymmetric membranes. Poly(styrene-co-acrylonitrile)-b-poly(2-hydroxyethyl methacrylate)s (PSAN-b-PHEMA) are synthesized via a two-step atom transfer radical polymerization (ATRP) procedure rendering PSAN copolymers and BCPs with overall molar masses of up to 82 kDa while maintaining low dispersity index values in the range of Đ = 1.13-1.25. The polymers are characterized using size-exclusion chromatography (SEC) and NMR spectroscopy. Self-assembly capabilities in the bulk state are examined using transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) measurements. The fabrication of isoporous integral asymmetric membranes is investigated, and membranes are examined by scanning electron microscopy (SEM). The introduction of acrylonitrile moieties within the membrane matrix could improve the membranes’ mechanical properties, which was confirmed by nanomechanical analysis using atomic force microscopy (AFM).
- ItemRoom temperature ionic liquids with two symmetric ions(Cambridge : RSC, 2023) Rauber, Daniel; Philippi, Frederik; Schroeder, Daniel; Morgenstern, Bernd; White, Andrew J. P.; Jochum, Marlon; Welton, Tom; Kay, Christopher W. M.Room temperature ionic liquids typically contain asymmetric organic cations. The asymmetry is thought to enhance disorder, thereby providing an entropic counter-balance to the strong, enthalpic, ionic interactions, and leading, therefore, to lower melting points. Unfortunately, the synthesis and purification of such asymmetric cations is typically more demanding. Here we introduce novel room temperature ionic liquids in which both cation and anion are formally symmetric. The chemical basis for this unprecedented behaviour is the incorporation of ether-containing side chains - which increase the configurational entropy - in the cation. Molecular dynamics simulations indicate that the ether-containing side chains transiently sample curled configurations. Our results contradict the long-standing paradigm that at least one asymmetric ion is required for ionic liquids to be molten at room temperature, and hence open up new and simpler design pathways for these remarkable materials.