Optical Kerr effect spectroscopy of CS2 in monocationic and dicationic ionic liquids: insights into the intermolecular interactions in ionic liquids.

A comparative study of the intermolecular dynamics of CS2 in monocationic and dicationic ionic liquids (ILs) was performed using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The reduced spectral densities (RSDs) of mixtures of CS2 in 1-alkyl-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([CnC1im][NTf2] for n = 3-5) and 1,2n-bis(3-methylimidazolium-1-yl) alkane bis[(trifluoromethane)sulfonyl]amide ([(C1im)2C2n][NTf2]2 for n = 3-5) were investigated as a function of concentration at 295 K. An additivity model was used to obtain the CS2 contribution to the RSD of a mixture in the 0-200 cm-1 region. One of the aims of this study is to show how CS2 can be used as a probe of intermolecular/interionic interactions in ILs. The concentrations were chosen such that the CS2-to-imidazolium ring mole fraction of a mixture with [(C1im)2C2n][NTf2]2 (DIL(2n)) is the same as that of a mixture with [CnC1im][NTf2] (MIL(n)). As found previously for CS2 in monocationic ILs, the intermolecular spectrum of CS2 in dicationic ILs is lower in frequency and narrower than that of neat CS2. The new result is that the intermolecular spectrum of CS2 is higher in frequency in DIL(2n) than in the corresponding MIL(n), indicating that CS2 molecules experience a stiffer potential in dicationic ILs than in monocationic ILs. The intermolecular dynamics of CS2 being higher in frequency in DIL(2n) than in MIL(n) is consistent with recent molecular dynamics simulations (Lynden-Bell and Quitevis, J. Chem. Phys., 2018, 148, 193844) that show the stiffer potential is the result of greater confinement of CS2 in DIL(2n) than in MIL(n). We also show in this study how effects due to dilution and the intermolecular potential seen by a solute molecule in solution are unraveled.

Comparative study of the intermolecular dynamics of imidazolium-based ionic liquids with linear and branched alkyl chains: OHD-RIKES measurements.

This article describes a comparative study of the low-frequency (0-450 cm-1) Kerr spectra of the branched 1-(iso-alkyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([(N - 2)mCN-1C1im][NTf2] with N = 3-7) ionic liquids (ILs) and that of the linear 1-(n-alkyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([CNC1im][NTf2] with N = 2-7) ILs. The spectra were obtained by use of femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The intermolecular spectrum of a branched IL is similar to that of a linear IL that is of the same alkyl chain length rather than of the same number of carbon atoms in the alkyl chain. This similarity and the lack of a correlation of the first spectral moments and widths of the intermolecular spectra with chain length is mainly attributed to the increase in the dispersion contribution to the total molar cohesive energy being compensated by stretching of the ionic network due to the increasing size of the nonpolar domains, which is dependent only on the length of the alkyl chain.

Solubility of n-butane and 2-methylpropane (isobutane) in 1-alkyl-3-methylimidazolium-based ionic liquids with linear and branched alkyl side-chains.

The solubility of n-butane and 2-methylpropane (isobutane) in three ionic liquids - 1-(2-methylpropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [(2mC3)C1im][Ntf2], 1-(3-methylbutyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [(3mC4)C1im][Ntf2] and 1-methyl-3-pentylimidazolium bis(trifluoromethylsulfonyl)imide [C5C1im][Ntf2] - has been measured at atmospheric pressure from 303 to 343 K. Isobutane is less soluble than n-butane in all the ionic liquids. Henry's constant values range from 13.8 × 10(5) Pa for n-butane in [C5C1im][Ntf2] at 303 K to 64.5 × 10(5) Pa for isobutane in [(2mC3)C1im][Ntf2] at 343 K. The difference in solubility between the two gases can be explained by a more negative enthalpy of solvation for n-butane. A structural analysis of the pure solvents and of the solutions of the gases, probed by molecular dynamics simulations, could explain the differences found in the systems: (i) the nonpolar domains of the ionic liquids accommodate better the long and more flexible n-butane solute; (ii) the small differences in solubility of each gas in the ionic liquids with the same number of carbon atoms in the alkyl side-chains are explained by the absence of large structural differences in the pure solvents. In all cases, the structural analysis of the four ionic liquids confirms that the studied gases can act as probes of the molecular structure of the ionic liquids, the simulations being always compatible with the experimental solubility data.

An OHD-RIKES and simulation study comparing a benzylmethylimidazolium ionic liquid with an equimolar mixture of dimethylimidazolium and benzene.

The principal difference between 1-benzyl-3-methyl-imidazolium triflimide [BzC1im][NTf2] and an equimolar mixture of benzene and dimethylimidazolium triflimide [C1C1im][NTf2] is that in the former the benzene moieties are tied to the imidazolium ring, while in the latter they move independently. We use femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) and molecular simulations to explore some properties of these two systems. The Kerr spectra show small differences in the spectral densities; the simulations also show very similar environments for both the imidazolium rings and the phenyl or benzene parts of the molecules. The low frequency vibrational densities of states are also similar in the model systems. In order to perform the simulations we developed a model for the [BzC1im](+) cation and found that the barriers to rotation of the two parts of the molecule are low.

Probing the interplay between electrostatic and dispersion interactions in the solvation of nonpolar nonaromatic solute molecules in ionic liquids: an OKE spectroscopic study of CS2/[C(n)C(1)im][NTf(2)] mixtures (n = 1-4).

The intermolecular dynamics of dilute solutions of CS2 in 1-alkyl-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([CnC1im][NTf2] for n = 1-4) were studied at 295 K using femtosecond optical Kerr effect (OKE) spectroscopy. The OKE spectra of the CS2/ionic liquid (IL) mixtures were analyzed using an additivity model to obtain the CS2 contribution to the OKE spectrum from which information about the intermolecular modes of CS2 in these mixtures was gleaned. The intermolecular spectrum of CS2 in these mixtures is lower in frequency and narrower than that of neat CS2, as found previously for CS2 in [C5C1im][NTf2]. Moreover, a dependence of the spectra on alkyl chain length is observed that is attributed to the interplay between electrostatic and dispersion interactions. The surprising result in this study is the solubility of CS2 in [C1C1im][NTf2], which involves the interaction of a nonpolar nonaromatic molecular solute and only the charged groups of the IL. We propose that the solubility of CS2 in [C1C1im][NTf2] is determined by three favorable factors - (1) large polarizability of the solute molecule; (2) small size of the solute molecule; and (3) low cohesive energy in the high-charge density regions of the IL.

Local structure and intermolecular dynamics of an equimolar benzene and 1,3-dimethylimidazolium bis[(trifluoromethane)sulfonyl]amide mixture: Molecular dynamics simulations and OKE spectroscopic measurements.

The local structure and intermolecular dynamics of an equimolar mixture of benzene and 1,3-dimethylimidazolium bis[(trifluoromethane)sulfonyl]amide ([dmim][NTf2]) were studied using molecular dynamics (MD) simulations and femtosecond optical Kerr effect (OKE) spectroscopy. The OKE spectrum of the benzene/[dmim][NTf2] mixture at 295 K was analyzed by comparing it to an ideal mixture spectrum obtained by taking the volume-fraction weighted sum of the OKE spectra of the pure liquids. The experimental mixture spectrum is higher in frequency and broader than that of the ideal mixture spectrum. These spectral differences are rationalized in terms of the local structure around benzene molecules in the mixture and the intermolecular dynamics as reflected in the density of states from the MD simulations. Specifically, we attribute the deviation of the OKE spectrum of the mixture from ideal behavior to benzene molecules seeing a stiffer intermolecular potential due to their being trapped in cages comprised of ions in the first solvation shell.

Pyrenylpyridines: Sky-Blue Emitters for Organic Light-Emitting Diodes.

A novel sky-blue-emitting tripyrenylpyridine derivative, 2,4,6-tri(1-pyrenyl)pyridine (2,4,6-TPP), has been synthesized using a Suzuki coupling reaction and compared with three previously reported isomeric dipyrenylpyridine (DPP) analogues (2,4-di(1-pyrenyl)pyridine (2,4-DPP), 2,6-di(1-pyrenyl)pyridine (2,6-DPP), and 3,5-di(1-pyrenyl)pyridine (3,5-DPP)). As revealed by single-crystal X-ray analysis and computational simulations, all compounds possess highly twisted conformations in the solid state with interpyrene torsional angles of 42.3°-57.2°. These solid-state conformations and packing variations of pyrenylpyridines could be correlated to observed variations in physical characteristics such as photo/thermal stability and spectral properties, but showed only marginal influence on electrochemical properties. The novel derivative, 2,4,6-TPP, exhibited the lowest degree of crystallinity as revealed by powder X-ray diffraction analysis and formed amorphous thin films as verified using grazing-incidence wide-angle X-ray scattering. This compound also showed high thermal/photo stability relative to its disubstituted analogues (DPPs). Thus, a nondoped organic light-emitting diode (OLED) prototype was fabricated using 2,4,6-TPP as the emissive layer, which displayed a sky-blue electroluminescence with Commission Internationale de L'Eclairage (CIE) coordinates of (0.18, 0.34). This OLED prototype achieved a maximum external quantum efficiency of 6.0 ± 1.2% at 5 V. The relatively high efficiency for this simple-architecture device reflects a good balance of electron and hole transporting ability of 2,4,6-TPP along with efficient exciton formation in this material and indicates its promise as an emitting material for design of blue OLED devices.