@article{StrehmelSarkerLahtietal.2005, author = {Strehmel, Veronika and Sarker, A. M. and Lahti, P. M. and Karasz, F. E. and Heydenreich, Matthias and Wetzel, Hendrik and Haebel, Sophie and Strehmel, Bernd}, title = {One- and two-photon photochemistry and photophysics of poly(arylenevinylene)s containing a biphenyl moiety}, issn = {1439-4235}, year = {2005}, abstract = {Photochemical and photophysical properties were investigated for poly(arylenevinylene)s containing a flexible biphenyl "hinge" unit by applying one-photon (OP) and two-photon (TP) excitation to explore excited-state properties. The poly(arylenevinylene)s were poly[(2,5-dihexyloxy-p-phenylenevinylene)-alt-(4,4'-dihexyloxy-3,3'-biph enylenevinylene)] (1), poly[(2,5-dihexyloxy-p-phenylenevinylene)-alt-(2,2'-dihexyloxy-3,3'-biph enylenevinylene)] (2), and poly[(2,5-dihexyloxy-p-phenylenevinylene)-alt-(2,2'-biphenylene-vinylene )] (3). Effective emission quantum yields and related photonic properties were evaluated on a realistic per-chromophore basis using effective conjugation lengths based on the Strickler-Berg relationship. intramolecular photocyclization was deduced to occur in the one case where the biphenyl molecular connectivity permitted the reaction, based on matrix- assisted loser desorption/ionization time-of-flight (MALDI-TOF), heteronuclear multiple-quantum coherence (HMQC)-NMR, and gel-permeation chromatography (GPC) results. The various photoprocesses could be induced by either OP or TP excitation, though the first excited singlet state is the photoactive state. The higher excitation energy 1 of the TP excited state favors indirect population of the S, state by electronic coupling between the TP and OP excited states [lambda(max)(TPE) (nm): 726; delta (GM)([9]): 1 = 229, 2 = 215, 3 = 109). Photochemical processes occurring from the lowest OP excited state (S-1) could therefore also be indirectly induced by TP excitation}, language = {en} } @article{StrehmelSarkerLahtietal.2004, author = {Strehmel, Veronika and Sarker, A. M. and Lahti, P. M. and Karasz, F. E. and Strehmel, Bernd}, title = {One and two-photon optical properties of ionic block copolymers with phenylenediethylenebispyridinium- chromophores}, issn = {0065-7727}, year = {2004}, language = {en} } @article{StrehmelWishartPolyanskyetal.2009, author = {Strehmel, Veronika and Wishart, James F. and Polyansky, Dmitry E. and Strehmel, Bernd}, title = {Recombination of photogenerated lophyl radicals in imidazolium-based ionic liquids}, issn = {1439-4235}, doi = {10.1002/cphc.200900594}, year = {2009}, abstract = {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.}, language = {en} } @article{StrehmelBerdzinskiStrauchetal.2014, author = {Strehmel, Veronika and Berdzinski, Stefan and Strauch, Peter and Hoffmann-Jacobsen, Kerstin and Strehmel, Bernd}, title = {Investigation of molecular solvents and ionic liquids with a dual probe}, series = {Zeitschrift f{\"u}r physikalische Chemie : international journal of research in physical chemistry and chemical physics}, volume = {228}, journal = {Zeitschrift f{\"u}r physikalische Chemie : international journal of research in physical chemistry and chemical physics}, number = {2-3}, publisher = {De Gruyter}, address = {Berlin}, issn = {0942-9352}, doi = {10.1515/zpch-2014-0453}, pages = {155 -- 169}, year = {2014}, abstract = {A dual probe was investigated by UV-Vis, fluorescence, and ESR spectroscopy. It comprises the pyrene chromophore and the paramagnetic 2,2,6,6-tetramethylpiperidinyl-N-oxyl radical that are covalently linked together via an ester bridge. The dual probe was used to investigate molecular solvents of different polarity as well as ionic liquids bearing either imidazolium or pyrrolidinium cations and various anions, such as bis(trifluoromethylsulfonyl)imide, tetrafluoroborate, tris(pentafluoroethyl)trifluorophosphate, or dicyanamide. The dual probe does not show solvatochromism that is typical for some pyrenes. Furthermore, the dual probe is considerable less mobile compared to 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO) without additional substituent as detected by ESR spectroscopy. This is caused by the bulky pyrenyl substituent bound at the dual probe resulting in a reduced mobility of the dual probe.}, language = {en} } @article{StrehmelLaschewskyWetzel2006, author = {Strehmel, Veronika and Laschewsky, Andr{\´e} and Wetzel, Hendrik}, title = {Homopolymerization of a highly polar zwitterionic methacrylate in ionic liquids and its copolymerization with a non-polar methacrylate}, year = {2006}, abstract = {Free radical homo- and copolymerization of the highly polar 3-(N-[2-methacryloyloxyethyl]-N,N-dimethylammonio) propane sulfonate with the nonpolar n-butylmethacrylate was investigated in the ionic liquids 1-butyl-3-methyl imidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium hexafluoro phosphate, and compared to analogous polymerizations in standard solvents. Higher molar masses are obtained for the zwitterionic homopolymer when the polymerization is carried out in an ionic liquid compared to the classical reaction in water. Although homopolymerization of the sulfobetain monomer as well as of n-butylmethacrylate results in phase separation during the polymerization process, copolymerization of a stoichiometric ratio of the two monomers in the ionic liquids produced transparent gels indicating that no macrophase separation occurs. The use of ionic liquids as reaction medium improved the copolymerization behavior of the two methacrylates significantly. Whereas only minor amounts of n-butyl methacrylate were incorporated in the copolymer when synthesized in acetonitrile, the content of the non-polar monomer units in the zwitterionic copolymer approached increasingly its content in the polymerization mixture when ionic liquids were employed as solvents}, language = {en} } @article{TejedorOriolPinoletal.2005, author = {Tejedor, R. M. and Oriol, L. and Pinol, M. and Serrano, J. L. and Strehmel, Veronika and Stiller, Burkhard and Stumpe, Joachim}, title = {Photoreactive main-chain liquid-crystalline polyesters : Synthesis, characterization, and photochemistry}, issn = {0887-624X}, year = {2005}, abstract = {Three series of semiflexible and rigid main-chain polyesters containing photoreactive mesogenic units derived from p-phenylenediacrylic acid (PDA) and cinnamic acid have been synthesized by high-temperature polycondensation. The thermal and mesomorphic properties of the polymers have been determined. The photochemical behavior of polymer P-[1]-T, which contains a PDA unit, has been studied both in solution and in films. In solution, [2+2] photocycloaddition, E/Z photoisomerization, and photo-Fries rearrangement can take place. In contrast, the dominant process in spin-coated films is the [2+2] photocycloaddition reaction, which causes crosslinking of the polymer. In films, the photochemistry and induction of anisotropy are strongly influenced by the aggregation of the PDA phenylester unit. A dichroism of about 0.2 has been induced in films by irradiation with linearly polarized UV light, and thus the capability of these films to induce optical anisotropy and align liquid crystals has been demonstrated. Liquid-crystalline cells have been made with polarized irradiated films of P[1]-T as aligning layers. A commercial liquid-crystalline mixture has been used for this study, and a similar liquid-crystalline order determined by polarized Fourier transform infrared to a commercial cell with rubbed polyimide as an aligning layer has been detected. Because of crosslinking of the irradiated P[1]-T photoaligning layer, the photoinduced anisotropy is stable at high temperatures, and the liquid-crystalline molecules are insoluble in the irradiated polymer. (c) 2005 Wiley Periodicals, Inc}, language = {en} } @article{StrehmelKraudeltWetzeletal.2004, author = {Strehmel, Veronika and Kraudelt, Heide and Wetzel, Hendrik and Gornitz, Eckhard and Laschewsky, Andr{\´e}}, title = {Free radical polymerization of methacrylates in ionic liquids}, issn = {0065-7727}, year = {2004}, language = {en} } @article{JyotishkumarKoetzTierschetal.2009, author = {Jyotishkumar, P. and Koetz, Joachim and Tiersch, Brigitte and Strehmel, Veronika and oezdilek, Ceren and Moldenaers, Paula and H{\"a}ssler, R{\"u}diger and Thomas, Sabu}, title = {Complex phase separation in poly(acrylonitrile-butadiene-styrene)-modified epoxy/4,4 '-diaminodiphenyl sulfone blends : generation of new micro- and nanosubstructures}, issn = {1520-6106}, doi = {10.1021/Jp8094566}, year = {2009}, abstract = {The epoxy system containing diglycidyl ether of bisphenol A and 4,4'-diaminodiphenyl sulfone is modified with poly(acrylonitrile-butadiene-styrene) (ABS) to explore the effects of the ABS content on the phase morphology, mechanism of phase separation, and viscoelastic properties. The amount of ABS in the blends was 5, 10, 15, and 20 parts per hundred of epoxy resin (phr). Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were employed to investigate the final morphology of ABS-modified epoxy blends. Scanning electron microscopic studies of 15 phr ABS-modified epoxy blends reveal a bicontinuous structure in which both epoxy and ABS are continuous, with substructures of the ABS phase dispersed in the continuous epoxy phase and substructures of the epoxy phase dispersed in the continuous ABS phase. TEM micrographs of 15 phr ABS-modified epoxy blends confirm the results observed by SEM. TEM micrographs reveal the existence of nanosubstructures of ABS in 20 phr ABS-modified epoxy blends. To the best of our knowledge, to date, nanosubstructures have never been reported in any epoxy/thermoplastic blends. The influence of the concentration of the thermoplastic on the generated morphology as analyzed by SEM and TEM was explained in detail. The evolution and mechanism of phase separation was investigated in detail by optical microscopy (OM) and small-angle laser light scattering (SALLS). At concentrations lower than 10 phr the system phase separates through nucleation and growth (NG). However, at higher concentrations, 15 and 20 phr, the blends phase separate through both NG and spinodal decomposition mechanisms. On the basis of OM and SALLS, we conclude that the phenomenon of complex substructure formation in dynamic asymmetric blends is due to the combined effect of hydrodynamics and viscoelasticity. Additionally, dynamic mechanical analysis was carried out to evaluate the viscoelastic behavior of the cross-linked epoxy/ABS blends. Finally, apparent weight fractions of epoxy and ABS components in epoxy- and ABS-rich phases were evaluated from T-g analysis.}, language = {en} } @article{StrehmelLaschewskyWetzeletal.2006, author = {Strehmel, Veronika and Laschewsky, Andr{\´e} and Wetzel, Hendrik and Gornitz, Eckhard}, title = {Free radical polymerization of n-butyl methacrylate in ionic liquids}, doi = {10.1021/Ma0516945}, year = {2006}, abstract = {Ionic liquids based on imidazolium, pyridinium, and alkylammonium salts were investigated as solvents in free radical polymerization of the model monomer n-butyl methacrylate. The properties of the ionic liquids were systematically varied by changing the length of the alkyl substituents on the cations, and by employing different anions such as tetrafluoroborate, hexafluorophosphate, tosylate, triflate, alkyl sulfates and dimethyl phosphate. Results were compared to analogous polymerizations in toluene and in bulk. The solvents have no detectable influence on polymer tacticity. However, the molar masses obtained and the degree of polymerization, respectively, are very sensitive to the choice of the solvent. The degrees of polymerization are significantly higher when polymerizations were carried out in ionic liquids compared to polymerization in toluene, and can even exceed the values obtained by bulk polymerization. Imidazolium salts unsubstituted at C-2 result in an increase in the degree of polymerization of the poly(butyl methacrylate) with increasing viscosity of these ionic liquids. Methyl substitution at C-2 of the imidazolium ion results in an increase in the viscosity of the ionic liquid and in a viscosity independent degree of polymerization of the poly(butyl methacrylate). Ionic liquids based on imidazolium salts seem preferable over pyridinium and alkylammonium salts because of the higher degree of polymerization of the poly(butyl methacrylate)s obtained in the imidazolium salts. The glass transition temperatures and thermal stabilities are higher for poly(butyl methacrylate)s synthesized in the ionic liquids compared to the polymer made in toluene}, language = {en} } @article{StrehmelLungwitzRexhausenetal.2010, author = {Strehmel, Veronika and Lungwitz, Ralf and Rexhausen, Hans and Spange, Stefan}, title = {Relationship between hyperfine coupling constants of spin probes and empirical polarity parameters of some ionic liquids}, issn = {1144-0546}, doi = {10.1039/C0nj00253d}, year = {2010}, abstract = {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 alpha, beta, and pi* and the hyperfine coupling constants A(iso)(N-14) 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 (beta) 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.}, language = {en} } @article{StrehmelRexhausenStrauch2010, author = {Strehmel, Veronika and Rexhausen, Hans and Strauch, Peter}, title = {2,2,6,6-Tetramethylpiperidine-1-yloxyl bound to the imidazolium ion by an acetamido group for investigation of ionic liquids}, issn = {0040-4039}, doi = {10.1016/j.tetlet.2009.11.124}, year = {2010}, abstract = {New spin probes bearing the 2,2,6,6-tetramethylpiperidine-1-yloxyl covalently bound to the imidazolium ion via a methylene spacer and an amide group are synthesized. If the anion is bis(trifluoromethylsulfonylimide) instead of iodide, the new spin probe has a similar structure as that of an ionic liquid. Nevertheless, the new spin probes are useful tools to investigate ionic liquids.}, language = {en} } @article{StrehmelRexhausenStrauch2010, author = {Strehmel, Veronika and Rexhausen, Hans and Strauch, Peter}, title = {Influence of imidazolium bis(trifluoromethylsulfonylimide)s on the rotation of spin probes comprising ionic and hydrogen bonding groups}, issn = {1463-9076}, doi = {10.1039/B920586a}, year = {2010}, abstract = {The influence of the alkyl chain length in 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonylimide)s is studied to explore the rotation of piperidine-1-yloxyl derivatives substituted with either hydrogen bonding hydroxy group or ionic substituents, such as the cationic trimethylammonium or the anionic sulfate group placed at the 4 position. Structural variation of the ionic liquids results in differences of their viscosity influencing the rotation of the spin probes. The size of the average rotational correlation times of the spin probes dissolved in the ionic liquids depends further on the additional substituent in 4-position at these spin probes. The rotational correlation time exhibits a linear dependence on the ionic liquid viscosity in the case of the spin probe forming hydrogen bonding with the ionic liquids. In contrast to this, a deviation from the Stokes-Einstein behavior is found in the case of rotation of the charged spin probes in the 1-alkyl-3-methylimidazolium bis( trifluoromethylsulfonylimide) s substituted with a longer alkyl chain. This effect may be explained by phase separation on a molecular level between the charged part of the ionic liquid and the longer alkyl chains bound at the imidazolium ion. Although the neutral and the cationic spin probes show only a slight dependence between ionic liquid structure variation and the hyperfine coupling constants, structural effects cause changes in the hyperfine coupling constants in the case of the anionic spin probes. These probes strongly interact with the imidazolium ion.}, language = {en} } @article{StrehmelRexhausenStrauchetal.2010, author = {Strehmel, Veronika and Rexhausen, Hans and Strauch, Peter and Strehmer, Bernd}, title = {Temperature dependence of interactions between stable piperidine-1-yloxyl derivatives and a semicrystalline ionic liquid}, issn = {1439-4235}, doi = {10.1002/cphc.200900977}, year = {2010}, abstract = {The stable 2,2,6,6-tetramethylpiperidine-1-yloxyl and its derivatives with hydrogen-bond-forming (-OH, -OSO3H), anionic (-OSO3- bearing K+ or [K(18-crown-6)](+) as counter ion), or cationic (-N+-(CH3)(3) bearing I-, BF4-, PF6- or N- (SO2CF3)(2) as counter ion) substituents are investigated in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide over a wide temperature range. The temperature dependence of the viscosity of the ionic liquid is well described by the Vogel-Fulcher-Tammann equation. Interestingly, the temperature dependence of the rotational correlation time of the spin probes substituted with either a hydrogen-bond-forming group or an ionic substituent can be described using the Stokes-Einstein equation. In contrast, the temperature dependence of the rotational correlation time of the spin probe without an additional substituent at the 4-position to the nitroxyl group does not follow this trend. The activation energy for the mobility of the unsubstituted spin probe, determined from an Arrhenius plot of the spin-probe mobility in the ionic liquid above the melting temperature, is comparable with the activation energy for the viscous flow of the ionic liquid, but is higher for spin probes bearing an additional substituent at the 4-position. Quantum chemical calculations of the spin probes using the 6-31G+d method give information about the rotational volume of the spin probes and the spin density at the nitrogen atom of the radical structure as a function of the substituent at the spin probes in the presence and absence of a counter ion. The results of these calculations help in understanding the effect of the additional substituent on the experimentally determined isotropic hyperfine coupling constant.}, language = {en} } @article{StrehmelLaschewskyStoesseretal.2006, author = {Strehmel, Veronika and Laschewsky, Andr{\´e} and Stoesser, Reinhard and Zehl, Andrea and Herrmann, Werner}, title = {Mobility of spin probes in ionic liquids}, doi = {10.1002/poc.1072}, year = {2006}, abstract = {The spin probes TEMPO, TEMPOL, and CAT-1 were used to investigate microviscosity and micropolarity of imidazolium based ionic liquids bearing either tetrafluoroborate or hexafluorophosphate as anions and a variation of the substitution at the imidazolium ion. The average rotational correlation times (r) obtained by complete simulation of the X-band ESR spectra of TEMPO, TEMPOL, and CAT-1 increase with increasing viscosity of the ionic liquid although no Stokes Einstein behavior is observed. This is caused by microviscosity effects of the ionic liquids shown by application of the Gierer-Wirtz theory. Interestingly, the jump of the probe molecule into the free volume of the ionic liquids is a nonactivated process. The hyperfine coupling constants (A(iso) (N-14)) of TEMPO and TEMPOL dissolved in the ionic liquids do not depend on the structure of the ionic liquids. The A(iso) (N-14) values show a micropolarity of the ionic liquids that is comparable with methylenchloride in case of TEMPO and with dimethylsulfoxide in case of TEMPOL. Micropolarity monitored by CAT-1 strongly depends on structural variation of the ionic liquid. CAT-1 dissolved in imidazolium salts substituted with shorter alkyl chains at the nitrogen atom exhibits a micropolarity comparable with dimethylsulfoxide. A significant lower micropolarity is found for imidazolium. salts bearing a longer alkyl substituent at the nitrogen atom or a methyl substituent at C-2. Copyright (c) 2006 John Wiley \& Sons, Ltd}, language = {en} } @article{StoesserHerrmannZehletal.2006, author = {Stoesser, Reinhard and Herrmann, Werner and Zehl, Andreas and Strehmel, Veronika and Laschewsky, Andr{\´e}}, title = {ESR spin probes in ionic liquids}, doi = {10.1002/cphc.200500651}, year = {2006}, abstract = {The spin probes 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL), and 2,2,6,6-tetramethyl-4-trimethylammoniumpiperidine-1-oxyllodide (CAT-1) are examined in a number of ionic liquids based on substituted imidazolium cations and tetrafluoroborate and hexafluorophosphate anions, respectively. The reorientation correlation times tau(R) of the spin probes in these systems have been determined by complete spectra simulation and, for rapid reortientation, by analysis of the intensities of the hyperfine lines of the electron spin resonance (ESR) spectra. A comparison of the results with those from the model system glycerol/water and selected organic solvents is made. Additions of diamagnetic and paramagnetic ions allow the conclusion that salt effects and spin exchange are present, and that both are superimposed by motional effects. Specific interactions in the ionic liquids, as well as between the spin-probe molecules and the constituents of the ionic liquids are reflected in the spectra of the spin probes, depending on their molecular structure}, language = {en} } @article{StrehmelKraudeltWetzeletal.2004, author = {Strehmel, Veronika and Kraudelt, Heide and Wetzel, Hendrik and G{\"o}rnitz, Eckhard and Laschewsky, Andr{\´e}}, title = {Free radical polymerization of methacrylates in ionic liquids}, year = {2004}, language = {en} } @article{SangoroIacobAgapovetal.2014, author = {Sangoro, Joshia R. and Iacob, C. and Agapov, A. L. and Wang, Yangyang and Berdzinski, Stefan and Rexhausen, Hans and Strehmel, Veronika and Friedrich, C. and Sokolov, A. P. and Kremer, F.}, title = {Decoupling of ionic conductivity from structural dynamics in polymerized ionic liquids}, series = {Soft matter}, volume = {10}, journal = {Soft matter}, number = {20}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1744-683X}, doi = {10.1039/c3sm53202j}, pages = {3536 -- 3540}, year = {2014}, abstract = {Charge transport and structural dynamics in low molecular weight and polymerized 1-vinyl-3-pentylimidazolium bis(trifluoromethylsulfonyl) imide ionic liquids (ILs) are investigated by a combination of broadband dielectric spectroscopy, dynamic mechanical spectroscopy and differential scanning calorimetry. While the dc conductivity and fluidity exhibit practically identical temperature dependence for the non-polymerized IL, a significant decoupling of ionic conduction from structural dynamics is observed for the polymerized IL. In addition, the dc conductivity of the polymerized IL exceeds that of its molecular counterpart by four orders of magnitude at their respective calorimetric glass transition temperatures. This is attributed to the unusually high mobility of the anions especially at lower temperatures when the structural dynamics is significantly slowed down. A simple physical explanation of the possible origin of the remarkable decoupling of ionic conductivity from structural dynamics is proposed.}, language = {en} } @article{StrehmelRexhausenStrauch2012, author = {Strehmel, Veronika and Rexhausen, Hans and Strauch, Peter}, title = {New spin probes starting from 4-amino-2,2,6,6-tetramethylpiperidine-1-yloxyl}, series = {Tetrahedron letters}, volume = {53}, journal = {Tetrahedron letters}, number = {13}, publisher = {Elsevier}, address = {Oxford}, issn = {0040-4039}, doi = {10.1016/j.tetlet.2012.01.063}, pages = {1587 -- 1591}, year = {2012}, abstract = {This Letter describes four new 4-trimethylammonio-2,2,6,6-tetramethylpiperidine-1-yloxyls bearing camphorsulfonate, triflate, tosylate, or lactate as counter ions. These spin probes were made by anion metathesis of 4-trimethylammonio-2,2,6,6-tetramethylpiperidine-1-yloxyl iodide using the corresponding silver salts. The latter is made by the alkylation of 4-amino-2,2,6,6-tetramethylpiperidine-1-yloxyl. Furthermore, the Letter gives an improved synthetic way to 4-sulfonamido-2,2,6,6-tetramethylpiperidine-1-yloxyl using chlorosulfuric acid trimethylsilylester and 4-amino-2,2,6,6-tetramethylpiperidine-1-yloxyl. All the spin probes are highly interesting for the investigation of ionic liquids.}, language = {en} } @article{SangoroIacobNaumovetal.2011, author = {Sangoro, J. R. and Iacob, C. and Naumov, S. and Valiullin, R. and Rexhausen, Hans and Hunger, J. and Buchner, R. and Strehmel, Veronika and Kaerger, J. and Kremer, F.}, title = {Diffusion in ionic liquids the interplay between molecular structure and dynamics}, series = {Soft matter}, volume = {7}, journal = {Soft matter}, number = {5}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1744-683X}, doi = {10.1039/c0sm01404d}, pages = {1678 -- 1681}, year = {2011}, abstract = {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.}, language = {en} } @article{StrehmelBerdzinskiRexhausen2014, author = {Strehmel, Veronika and Berdzinski, Stefan and Rexhausen, Hans}, title = {Interactions between ionic liquids and radicals}, series = {Journal of molecular liquids}, volume = {192}, journal = {Journal of molecular liquids}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0167-7322}, doi = {10.1016/j.molliq.2013.12.007}, pages = {153 -- 170}, year = {2014}, abstract = {Ionic liquids were investigated with both stable radicals on the basis of 2,2,6,6-tetramethylpiperidine-1-yloxyl (TEMPO) and photogenerated lophyl radicals. The ionic liquids are composed either of bis(trifluoromethylsulfonyl)imide (NTf2) as anion and various cations or they contain an imidazolium ion in combination with various anions. The cations include imidazolium, pyrrolidinium, piperidinium, polymethine or ammonium ions. Furthermore, BF4-, PF6-, triflate, camphorsulfonate, lactate, tosylate or tris(pentafluoroethyl) trifluorophosphate (FAP) are the counter ions in the imidazolium salts. The structural variation of the ionic liquids results in differences in glass formation, semiaystallinity, or crystallinity, as well as in viscosity differences. Furthermore, a vinyl substituent at the imidazolium ion and a methacryloyloxyethyl substituent at the ammonium ion result in polymerizable ionic liquids that were converted via a radical mechanism in amorphous polymerized ionic liquids with a glass transition temperature, which is significantly higher compared to the ionic liquids. An additional substituent at TEMPO causes additional hydrogen bond formation or additional Coulomb interactions with the individual ions of the ionic liquids compared to TEMPO. This influences the mobility of these radicals in the ionic liquid expressed by differences in the average rotational correlation time (T-rot). The mobility of the radicals in the ionic liquids as function of the temperature describes ionic liquids either as continuum in analogy to molecular solvents using the Stokes-Einstein model, that is the case for 1-butyl-3-methylimidazolium NTf2, or as medium where free volume effects are important for the mobility of a solute in the ionic liquid using the model of Spernol, Gierer, and Wirtz. The 1-butyl-3-methylimidazolium BF4- fits well into the latter. Furthermore, the isotropic hyperfine coupling constant (A(iso)(N-14)) of the stable radicals gives information about micropolarity of the ionic liquids only if the mobility of the radical is high enough in the ionic liquid. In addition to the rotational mobility of the stable radicals, the photogenerated lophyl radicals give information about translational diffusion of radicals and solvent cage effects in the ionic liquids. The application of the Eyring equation results mostly in the expected negative values of the activation entropy for the transition state that is typical for bimolecular reactions. Only few examples show a less negative or positive activation entropy for the bimolecular reaction, which may be attributed to radical recombination within the solvent cage to a high extent. The results obtained during investigation of radicals in ionic liquids are important to understand the radical processes in ionic liquids that may occur for example in dye sensitized solar cells, photo or thermally induced reactions or radical polymerizations in ionic liquids.}, language = {en} } @article{MondalMuellerJungingeretal.2014, author = {Mondal, Suvendu Sekhar and Mueller, Holger and Junginger, Matthias and Kelling, Alexandra and Schilde, Uwe and Strehmel, Veronika and Holdt, Hans-J{\"u}rgen}, title = {Imidazolium 2-substituted 4,5-dicyanoimidazolate ionic liquids: synthesis, crystal structures and structure-thermal property relationships}, series = {Chemistry - a European journal}, volume = {20}, journal = {Chemistry - a European journal}, number = {26}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {0947-6539}, doi = {10.1002/chem.201304934}, pages = {8170 -- 8181}, year = {2014}, abstract = {Thirty six novel ionic liquids (ILs) with 1-butyl-3-methylimidazolium and 3-methyl-1-octylimidazolium cations paired with 2-substitited 4,5-dicyanoimidazolate anions (substituent at C2=chloro, bromo, methoxy, vinyl, amino, methyl, ethyl, propyl, isopropyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl and phenyl) have been synthesized and characterized by using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and single-crystal X-ray crystallography. The effects of cation and anion type and structure on the thermal properties of the resulting ionic liquids, including several room temperature ionic liquids (RTILs) are examined and discussed. ILs exhibited large liquid and crystallization ranges and formed glasses on cooling with glass transition temperatures in the range of -22 to -68 degrees C. The effects of alkyl substituents of the imidazolate anion reflected the crystallization, melting points and thermal decomposition of the ILs. The Coulombic packing force, van der Waals forces and size of the anions can be considered for altering the thermal transitions. Three crystal structures of the ILs were determined and the effects of changes to the cations and anions on the packing of the structure were investigated.}, language = {en} }