Valence electronic structure of [EMIM][BF4] ionic liquid: photoemission and DFT+D study

Electronic and Optical Properties of Polythiophene Molecules and Derivatives

The electronic and optical properties of polythiophene (PT) for polymer light-emitting diodes (PLEDs) were calculated using density functional theory (DFT) and time-dependent DFT. We calculated the electronic and optical properties of thiophene and PT polymers with degrees of polymerization (DP) from 2 to 30 monomers (T1–T30) and their derivatives. The associated highest occupied molecular orbital (HOMO) energy, lowest unoccupied molecular orbital (LUMO) energy, band gaps, electron orbitals, and molecular structures were determined. As the DP increased, the LUMO energy gradually decreased, and the HOMO energy gradually increased. The band gap of PT approached 2 eV as the DP of the PT polymer increased from 1 to 30. The calculations and exchange–correlation functional were verified against values in the literature and experimental data from cyclic voltammetry (redox potential) and ultraviolet-visible, photoluminescence, and ultraviolet photoelectron spectra. The color of PT PLEDs can be adjusted by controlling the DP of the polymer and the substituents.

Experimental measurement and prediction of ionic liquid ionisation energies

Ionic liquid (IL) valence electronic structure provides key descriptors for understanding and predicting IL properties. The ionisation energies of 60 ILs are measured and the most readily ionised valence state of each IL (the highest occupied molecular orbital, HOMO) is identified using a combination of X-ray photoelectron spectroscopy (XPS) and synchrotron resonant XPS. A structurally diverse range of cations and anions were studied. The cation gave rise to the HOMO for nine of the 60 ILs presented here, meaning it is energetically more favourable to remove an electron from the cation than the anion. The influence of the cation on the anion electronic structure (and vice versa) were established; the electrostatic effects are well understood and demonstrated to be consistently predictable. We used this knowledge to make predictions of both ionisation energy and HOMO identity for a further 516 ILs, providing a very valuable dataset for benchmarking electronic structure calculations and enabling the development of models linking experimental valence electronic structure descriptors to other IL properties, e.g. electrochemical stability. Furthermore, we provide design rules for the prediction of the electronic structure of ILs.

Imidazolium-fulleride ionic liquids - a DFT prediction

Ionic liquids (ILs) exhibit tunable physicochemical properties due to the flexibility of being able to select their cation-anion combination from a large pool of ions. The size of the ions controls the properties of the ILs in the range from ionic to molecular, and thus large ions play an important role in regulating the melting temperature and viscosity. Here, we show that the exohedral addition of anionic X^- moieties to C₆₀ (X = H, F, OH, CN, NH₂, and NO₂) is a thermodynamically viable process for creating large X-fulleride anions (C₆₀X)^-. The addition of X^- to C₆₀ is modelled by locating the transition state for the reaction between C₆₀ and 1,3-dimethyl-2X-imidazole (IMX) at the M06L/6-311++G(d,p)//M06L/6-31G(d,p) level. The reaction yields the ion-pair complex IM⁺⋯(C₆₀X)^- for X = H, F, OH, CN, NH₂, and NO₂ and the ordered pair of (activation free energy, reaction free energy) is found to be (14.5, 1.1), (6.1, 3.1), (16.7, 2.3), (14.7, -7.9), (27.9, 0.5) and (11.9, 12.4), respectively. The low barrier of the reactions suggests their feasibility. The reaction is slightly endergonic for X = H, F, OH, and NH₂, while X = CN shows a significant exergonic character. The X-fulleride formation is not observed when X = Cl and Br. The ion-pair interactions (E_ion-pair) observed for IM⁺⋯(C₆₀X)^- range from -64.0 to -73.0 kcal mol^-1, which is substantially lower (∼10%) than the typically reported values for imidazolium-based ionic liquids such as [EMIm]⁺[trz]^-, [EMIm]⁺[dc]^-, [EMIm]⁺[dtrz]^-, and [EMIm]⁺[NH₂tz]^-. The quantum theory of atoms in molecules (QTAIM) analysis showed that the C-X bonding in (C₆₀X)^- is covalent, while that in (IM⁺⋯X^-)⋯C₆₀ (for X = Cl and Br) is non-covalent. Furthermore, molecular electrostatic potential (MESP) analysis showed that the X-fulleride could behave as a large spherical anion due to the delocalization of the excess electron in the system over the entire carbon framework. The large anionic character of the X-fulleride is also revealed by the identification of several close lying local energy minima for the IM⁺⋯(C₆₀X)^- ion-pair. The low E_ion-pair value, the significant contribution of dispersion to the E_ion-pair and the spherical nature of the anion predict low-melting point and highly viscous IL formation from X-fullerides and the imidazolium cation.

Ionic Liquid Vapors in Vacuum: Possibility to Derive Anodic Stabilities from DFT and UPS

Ultraviolet photoelectron spectroscopy (UPS) investigations of several gas-phase ionic liquid (IL) ion pairs have been conducted. [EMIM][OTF], [PYR14][OTF], [EMIM][DCA], [PYR14][DCA], [PYR14][TCM], [PYR14][FSI], [PYR14][PF6], [S222][TFSI], [P4441][TFSI], and [EMMIM][TFSI] vapor UPS spectra are presented for the first time. The experimental low-binding-energy cutoff value (highest occupied molecular orbital, HOMO energy) of the ionic liquid ion pairs, which is of great interest, has been measured. Many studies use calculated gas-phase electronic properties to estimate the liquid-phase electrochemical stability. Hybrid density functional theory (DFT) calculations have been used to interpret the experimental data. The gas-phase photoelectron spectra in conjunction with the theoretical calculations are able to verify most HOMO energies and assign them to the cation or anion. The hybrid M06 functional is shown to offer a very good description of the ionic liquid electronic structure. In some cases, the excellent agreement between the UPS spectra and the M06 calculation validates the conformer found and constitutes as a first indirect experimental determination of ionic liquid ion-pair structure. Comparisons with recent theoretical studies are made, and implications for electrochemical applications are discussed. The new data provide a much-needed reference for future ab initio calculations and support the argument that modeling of IL cations and anions separately is incorrect.

Screening nature of the van der Waals density functional method: a review and analysis of the many-body physics foundation

We review the screening nature and many-body physics foundation of the van der Waals density functional (vdW-DF) method [Berland Ket al2015Rep. Prog. Phys.78066501], a systematic approach to construct truly nonlocal exchange-correlation energy density functionals. To that end we define and focus on a class of consistent vdW-DF versions that adhere to the Lindhard screening logic of the full method formulation. The consistent-exchange vdW-DF-cx version [Berland K and Hyldgaard P 2014Phys. Rev. B89035412] and its spin extension [Thonhauser Tet al2015Phys. Rev. Lett.115136402] represent the first examples of this class; in general, consistent vdW-DFs reflect a concerted expansion of a formal recast of the adiabatic-connection formula [Hyldgaard Pet al2014Phys. Rev. B90075148], an exponential summation of contributions to the local-field response, and the Dyson equation. We argue that the screening emphasis is essential because the exchange-correlation energy reflects an effective electrodynamics set by a long-range interaction. Two consequences are that (1) there are, in principle, no wiggle room in how one balances exchange and correlation, for example, in vdW-DF-cx, and that (2) consistent vdW-DFs have a formal structure that allows them to incorporate vertex-correction effects, at least in the case of levels that experience recoil-less interactions (for example, near the Fermi surface). We explore the extent to which the strictly nonempirical vdW-DF-cx formulation can serve as a systematic extension of the constraint-based semilocal functionals. For validation, we provide a complete survey of vdW-DF-cx performance for broad molecular processes, for the full set of 55 benchmarks in GMTKN55 [Goerigk Let al2017Phys. Chem. Chem. Phys.1932184] and comparing to the quantum-chemistry calculations that are summarized in that paper. We also provide new vdW-DF-cx results for metal surface energies and work functions that we compare to experiment. Finally, we use the screening insight to separate the vdW-DF nonlocal-correlation term into pure-vdW-interaction and local-field-susceptibility effects and present tools to compute and map the binding signatures.

Valence electronic structure of [EMIM][B(CN)4]: ion-pair vs. bulk description

The ultraviolet photoelectron spectrum of the [EMIM][B(CN)₄] ionic liquid was recorded and analyzed. Together with different ab initio calculation methods, detailed insight into the electronic structure of this simple room temperature ionic liquid is possible. The ion-pair approximation to the liquid electronic structure was not sufficient. Therefore bulk ab initio calculations were performed on a proposed crystal structure. The modelling of bulk electronic spectra is able to explain the experimental electronic structure of the ionic liquid. Most notably, the dispersion corrected PBE calculation (PBE-D3BJ) showed good agreement with the experimental UPS spectrum. The spectra simulated by the B97-D and the BLYP-D3(BJ) functionals were also in agreement with the experimental data. The LDA approximation only provided qualitative agreement while the optB88-vdW and CX-vdW functionals were not good. However, it will be shown that many requirements have to be met in order to accurately describe the electronic structure of this ionic liquid.

The ultraviolet photoelectron spectrum of the [EMIM][B(CN)₄] ionic liquid was recorded and simulated using different ab initio ion-pair and bulk calculation methods.