Veronika Strehmel

The dipolarity/polarisability of 1-alkyl-3-methylimidazolium ionic liquids as function of anion structure and the alkyl chain length

Based on the developed tool to measure Kamlet–Taft polarity parameters α (hydrogen bond donating ability), β (hydrogen bond accepting ability), and π* (dipolarity/polarisability) for ionic liquids (ILs), it is now possible to precisely determine the influence of both cation and anion structure on π*. α, β, and π* values of 38 1-alkyl-3-methylimidazolium ILs ([Rmim]X) are presented in this work to give a solid background for the discussion. The dipolarity/polarisability of [Rmim]X determined by means of the established π*-sensitive solvatochromic probe 4-tert-butyl-2-(dicyanomethylene)-5-[4-(diethylamino)benzylidene]-Δ3-thiazoline is a blend of dipolarity and polarisability in equal parts. The large π* parameters of ionic liquids are attributed to two cumulative effects: the ion pairing strength and the individual polarisabilities of both the cation and anion. This is shown by the influence of anion and cation structures (alkyl chain length of R) on the dipolarity/polarisability. For all studied ionic liquids we observed a general trend. The stronger the ion pairing effect, the greater is the determined π* value.

The curing of epoxy resins as studied by various methods

Abstract The sol-gel-glass transformations were examined in the thermosets of diglycidyl ether of bisphenol A (DGEBA) and diglycidylaniline (DGA) cured with 4,4′-diaminodiphenylmethane (DDM) at 70°C by using time-resolved fluorescence, Fourier-transform ( FT ) Raman spectroscopy, an ultrasonic technique and torque measurements. The rotational correlation times of substituted styryl and cyanine dyes showing a twisted intramolecular charge-transfer increase as the isothermal cure reaction proceeds and are sensitive to gelation. The chromophores detect the local viscosity of the surroundings. In the case of DGEBA/DDM, the extent of the epoxide ring reaction, obtained from FT -Raman studies over the whole curing process, was used to determine the variation of the glass transition temperature and the reduced free volume during crosslinking. Thus, it was possible to relate quantitatively the increase of the rotational correlation time of the dyes to the decrease of the reduced free volume. The variation of the ultrasonic velocity and absorption in the course of curing indicates vitrification (dynamic glass transition), but shows no characteristics with respect to gelation. Differences in the curing behaviour of DGEBA/DDM and DGA/DDM were clearly evident.

Fluorescence probes for investigation of epoxy systems and monitoring of crosslinking processes

The fluorescence behavior of 1,1′-dimethyl-2,2′-carbocyanine and p-N,N-dimethylamino-styryl-2-ethylpyridinium was investigated in several epoxy systems. Time-correlated single photon counting was used for all fluorescence measurements to obtain the rate constant for viscosity or mobility-dependent nonradiative processes of the probe. Microviscosity effects were discussed on the basis of a model describing the microfriction between matrix and probe molecules. The probes investigated are able to detect the glass-transition temperature of the materials investigated. These compounds also show a dependence on the mobility in the glassy state. The probes applied in this work also can be used to monitor the crosslinking process of several epoxy systems containing 4,4′-diaminodiphenylmethane (DDM) as curing agent. The epoxides used for the crosslinking process were 2,2′-[(1-methylethylidene)bis(4,1-phenyleneoxymethylene)bis-oxiranemethaneamine] [common name, diglycidyl ether of bisphenol A (DGEBA)], N-oxiranylmethyl-N-phenyl-oxiranylmethane [common name, diglycidyl aniline (DGA)], and epoxy novolacs of different functionality. The networks obtained have a different morphology, which was studied by the fluorescence probe technology. The structure of the epoxy compound has an important influence on the probe behavior because both network density and size of the free volume influence the photochemical behavior of the probe.

Microviscosity and Micropolarity Effects of Imidazolium Based Ionic Liquids Investigated by Spin Probes Their Diffusion and Spin Exchange

Different polar common spin probes (TEMPO, TEMPOL, and CAT-1) as well as 15N spin probes (15N-TEMPO and 15N-TEMPOL-D17) were investigated to get information about microviscosity and micropolarity of ionic liquids. Rotational correlation times and hyperfine coupling constants of the spin probes were obtained by complete simulation of the ESR spectra. Microviscosity effects as shown by the Gierer–Wirtz theory may explain the spin probe behavior. Investigation of spin exchange of TEMPO, TEMPOL, and CAT-1 dissolved in ionic liquids shows an increased tendency of aggregation in the case of the nonpolar spin probe TEMPO. Two different kinds of species (isolated and aggregated species) were observed in the case of the more polar spin probes TEMPOL and CAT-1. ESR tomographic investigation of lateral diffusion of selected spin probes in an ionic liquid corresponds to the results obtained in rotational diffusion experiments. Furthermore, the strongly decreased mobility of radicals in ionic liquids makes detection of a polymer radical possible that was observed during thermal induced free radical polymerization of a methacrylate substituted by a sulfobetaine structure.

Diffusion in ionic liquids: the interplay between molecular structure and dynamics

Diffusion in a series of ionic liquids is investigated by a combination of Broadband Dielectric Spectroscopy (BDS) and Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR). It is demonstrated that the mean jump lengths increase with the molecular volumes determined from quantum-chemical calculations. This provides a direct means—via Einstein–Smoluchowski relation—to determine the diffusion coefficient by BDS over more than 8 decades unambiguously and in quantitative agreement with PFG NMR measurements. New possibilities in the study of charge transport and dynamic glass transition in ionic liquids are thus opened.

Temperature dependence of interactions between stable piperidine-1-yloxyl derivatives and an ionic liquid.

2,2,6,6-Tetramethylpiperidine-1-yloxyl derivatives substituted with either hydrogen bonding [-OH, -OSO(3)H] or ionic [-OSO(3) (-)Na(+), -OSO(3) (-)K(+), N(+)(CH(3))(3)I(-), N(+)(CH(3))(3) N(-)(SO(2)-CF(3))(2)] substituents are investigated in 1-butyl-3-methylimidazolium tetrafluoroborate over a wide temperature range covering both glassy and viscous states. The Vogel-Fulcher-Tammann equation describes the temperature dependence of the ionic liquid viscosity. Quantum chemical calculations of the spin probes at the UB3LYP/6-311(2d,p++) level are done to describe the dependence of the spin density on nitrogen on the substitution pattern of the 4-position of the probe. The results of these calculations are also used to understand the experimental results obtained by applying the Spernol-Gierer-Wirtz theory to analyze the viscosity dependence of the rotational correlation time of the spin probes. Significant differences are found between 2,2,6,6-tetramethylpiperidine-1-yloxyl and its derivatives containing substituents that are able to form hydrogen bonds with the ionic liquid. Moreover, derivatives substituted with ionic groups at the 4-position have a large effect on temperature-induced solvent viscosity, as this is particularly dependent on the nature of the substituent at the 4-position. These dependencies include the temperature region that can be used to describe interactions between the spin probes and the ionic liquid, diffusion into the free volume during non-activated (neutral spin probes) and activated (charged spin probes) processes. Additional parameters are the radii of the ionic liquid and the spin probes, which are calculated and measured approximately. In addition, the temperature dependence of the isotropic hyperfine coupling constants of the spin probes results in information about the micropolarity of the ionic liquid. At room temperature, this is comparable to that of the solvent dimethylsulfoxide.

Radicals in Ionic Liquids

Stable radicals and recombination of photogenerated lophyl radicals are investigated in ionic liquids. The 2,2,6,6-tetramethylpiperidine-1-yloxyl derivatives contain various substituents at the 4-position to the nitroxyl group, including hydrogen-bond-forming or ionic substituents that undergo additional interactions with the individual ions of the ionic liquids. Some of these spin probes contain similar ions to ionic liquids to avoid counter-ion exchange with the ionic liquid. Depending on the ionic liquid anion, the Stokes-Einstein theory or the Spernol-Gierer-Wirtz theory can be applied to describe the temperature dependence of the average rotational correlation time of the spin probe in the ionic liquids. Furthermore, the spin probes give information about the micropolarity of the ionic liquids. In this context the substituent at the 4-position to the nitroxyl group plays a significant role. Covalent bonding of a spin probe to the imidazolium ion results in bulky spin probes that are strongly immobilized in the ionic liquid. Furthermore, lophyl radical recombination in the dark, which is chosen to understand the dynamics of bimolecular reactions in ionic liquids, shows a slow process at longer timescale and a rise time at a shorter timescale. Although various reactions may contribute to the slower process during lophyl radical recombination, it follows a second-order kinetics that does not clearly show solvent viscosity dependence. However, the rise time, which may be attributed to radical pair formation, increases with increasing solvent viscosity.

Relationship between hyperfine coupling constants of spin probes and empirical polarity parameters of some ionic liquids

The polarity of 1-alkyl-3-methylimidazolium-based ionic liquids containing hexafluorophosphate, tetrafluoroborate, dicyanoimide, or bis(trifluoromethanesulfonyl)imide as anions and a variation of the alkyl-chain length of the cation are investigated by both solvatochromic dyes and spin probes. Two different polarity scales are used for discussion of the polarity of these ionic liquids. These polarity scales are the empirical Kamlet–Taft parameters α, β, and π* and the hyperfine coupling constants Aiso(14N) obtained for spin probes substituted either with an ammonio or a sulfate group at 4-position. The results show that both polarity scales are valid for description of the ionic liquid polarity although differences are found between the two polarity scales. The most clear trend is found in all ionic liquids investigated for the hydrogen-bond accepting ability (β) and the hyperfine-coupling constant of the anionic spin probe, where both parameters increase for all ionic liquids investigated until an alkyl chain length of eight carbon atoms and keep constant at longer alkyl chains.

Recombination of Photogenerated Lophyl Radicals in Imidazolium‐Based Ionic Liquids

Laser flash photolysis is applied to study the recombination reaction of lophyl radicals in ionic liquids in comparison with dimethylsulfoxide as an example of a traditional organic solvent. The latter exhibits a similar micropolarity as the ionic liquids. The ionic liquids investigated are 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (1), 1-hexyl-3-methylimidazolium hexafluorophosphate (2), and 1-butyl-3-methylimidazolium tetafluoroborate (3). The recombination of the photolytic generated lophyl radicals occur significantly faster in the ionic liquids than expected from their macroscopic viscosities and is a specific effect of these ionic liquids. On the other hand, this reaction can be compared with the macroscopic viscosity in the case of dimethylsulfoxide. Activation parameters obtained for lophyl radical recombination suggest different, anion-dependent mechanistic effects. Quantum chemical calculations based on density functional theory provide a deeper insight of the molecular properties of the lophyl radical and its precursor. Thus, excitation energies, spin densities, molar volumes, and partial charges are calculated. Calculations show a spread of spin density over the three carbon atoms of the imidazolyl moiety, while only low spin density is calculated for the nitrogens.

Recombination of Lophyl Radicals in Pyrrolidinium-Based Ionic Liquids

The recombination of photolytically generated lophyl radicals has been investigated by UV/Vis spectroscopy in 1-alkyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imides (NTf2) in comparison with 1-butyl-3-methylimidazolium NTf2 , dimethyl sulfoxide, and triacetin. The 1-alkyl-1-methylpyrrolidinium-based ionic liquids contain an alkyl substituent varying between butyl and decyl groups. Optically pure ionic liquids are used in these studies. Temperature-dependent investigation of lophyl radical recombination shows an increase in the radical recombination rate with increasing temperature in each solvent, which is caused by decreasing viscosity with increasing temperature. Furthermore, the viscosity of the 1-alkyl-1-methylpyrrolidinium NTf2 increases nearly linearly within the row of these ionic liquids. In contrast, the recombination of the photolytically generated lophyl radicals is significantly faster in the ionic liquids than in the traditional organic solvents under investigation. Moreover, the recombination rate increases with the length of the alkyl chain bound at the cation of the ionic liquid at a given temperature. This may be caused by an increase in the extent of lophyl radical recombination within the solvent cage. Solvent cage effects dominate in the case of lophyl radical recombination in ionic liquids bearing a long alkyl chain or if the temperature is near the melting temperature of the ionic liquid. The positive value of the activation entropy supports this hypothesis. The results obtained are important for discussion of bimolecular radical reactions in ionic liquids.