The electronic structure of ionic liquids based on the TFSI anion: A gas phase UPS and DFT study

Abstract

The valence bands of [EMIM][TFSI], [DEME][TFSI] and [PYR1,4][TFSI] gas-phase ion pairs have been investigated using ultraviolet photoelectron spectroscopy (UPS). The photoelectron spectra are interpreted by using several density functional and ab initio calculation methods. Although the experimental vapor phase spectra are similar, the different calculation methods make different predictions about the HOMO molecular state of the ion-pairs of the ionic liquids. The HOMO state of the [DEME][TFSI] ion-pair is due to the TFSI anion, while in [EMIM][TFSI] it is due to the EMIM cation. However, it is difficult to make conclusive assignments for the [PYR1,4][TFSI] ionic liquid. All calculation methods predict the LUMO to be of cationic origin in all the studied ion-pairs.

Electronic and Optical Properties of Polythiophene Molecules and Derivatives

“…Nayak and Periasamy [13] used DFT functional B3LYP basis set 6-31g(d) to investigate the electron affinity, ionization potential, transport gap, optical band gap (exciton energy), and exciton binding energy of organic molecules in the solid state. Kuusik et al [14] used hybrid DFT functional M06 and ωB97X-D to investigate the electronic structure of ionic liquids based on the trifluoromethyl sulfonyl imide anion.…”

Frontier orbitals and quasiparticle energy levels in ionic liquids

“…In recent years, a number of experimental and theoretical investigations of the electronic structure of ILs have been reported. For example, photoelectron spectroscopy has been used to study the valence band electronic structure of liquid ILs and IL vapors consisting of neutral cation-anion pairs [13][14][15][16][17][18][19] .…”

“…To gain a detailed understanding of the properties of ILs, many groups have carried out calculations based on density-functional theory (DFT) [20][21][22][23][24] . To model photoemission experiments, several studies compared the Kohn-Sham (KS) eigenvalues obtained from DFT calculations to the measured photoelectron spectra 16,17,19,25 . This practice is based on the observation that in many materials KS eigenvalues can be useful approximations to quasiparticle energies that are measured in photoemission spectroscopy.…”

Experimental measurement and prediction of ionic liquid ionisation energies

“…E i (IL) = 9.4 eV from this work for [C 8 C 1 Im][BF 4 ], an excellent match to the E i (IL) value derived from data in reference107 . Im][BF 4 ] from reference49 was significantly lower than E th (IL) = 7.8 eV for [C 4 C 1 Im][BF 4 ] from reference 50 and E th (IL) = 8.3 eV for [C 8 C 1 Im][BF 4 ] from our work, strongly suggesting a problem with the charge referencing for the data in reference49 .Using gas phase UPS of neutral ion pairs, E th (ion pair) values were measured by two groups, Leone and co-workers[15][16][17][18] and Kuusik and co-workers[19][20][21][22][23][24][25] . Most of these E th (ion pair) values were for a combination of imidazolium cations and an imide anion, e.g.…”

“…14 Almost all E i values for ILs, E i (IL), have been measured on vaporised gas phase neutral ion pairs. [15][16][17][18][19][20][21][22][23][24][25][26] Whilst measuring E i is relatively facile in the gas phase, a major problem is that most ILs are very tricky to vaporise without significant thermal decomposition occurring/dominating, meaning many IL ion pairs cannot be easily studied in the vapour phase; [27][28][29][30][31][32] furthermore, vapour phase ion pairs do not have the complete solvation environment of the liquid phase. A major hurdle for measuring reliable, reproducible, and comparable binding energies (E B ) 14 and E i for liquid phase ILs is dealing with sample charging during XPS measurements, which is not understood.…”