Probing the electronic structure of ether functionalised ionic liquids using X-ray photoelectron spectroscopy.

The charge distribution associated with individual components in functionalised ionic liquids (ILs) can be tuned by careful manipulation of the substituent groups incorporated into the ions. Here we use X-ray photoelectron spectroscopy to investigate the impact of substituent atoms on the electronic structure of similar imidazolium-based systems each paired with a common anion, [Tf2N]-. The experimental measurements revealed an unexpected variation in the charge density distribution within the IL cation when the oxygen atom in a poly-ether containing side chain is moved by just one atomic position. This surprising observation is supported by density functional theory calculations.


Introduction
X-ray photoelectron spectroscopy (XPS) is an established and commonly applied surface analysis technique, 1-3 which in the recent past, has been employed extensively in the study of ionic liquid-based systems. [4][5][6][7][8] As a general statement, XPS can provide detailed information about the electronic environment, and charge distribution associated with individual components at, or near to, the surface of a material. High-level atomistic data can be used to inform the design of more efficient processes, by delivering insight into local surface composition and orientation, 4,5,9 and complex interactions that can exist between charge carriers and solutes alike. [10][11][12][13] Continued structural modification and the inclusion of targeted functionalities and motifs that enable hydrogen bonding or recognition can deliver highly functionalised performance molecules or so-called "designer solvents". 14,[15][16][17] The polyethylene (PEG) moiety was initially grafted into the cationic and anionic components of ILs to yield ion-conducting polymers for electrochemical applications. 18,19 Since then, the relevance of ether-functionalised ILs has grown, with the materials demonstrating great potential for sustainable chemical applications. 20 For instance, PEG can be naturally derived from biorefinery and plant waste via solvolysis of soft wood biomass, 20 or the catalytic conversion of cellulose into ethylene glycol and ethylene glycol monoethers. 21 Improved catalytic performance and reduced viscosities, when compared to alkyl substituted analogues, is also noted. 14,[22][23][24][25][26][27] Ether and PEG substituted systems now find application as alternative solvents or performance molecules across multiple aspects of science, technology and engineering, [28][29][30][31][32][33] including, for example, as catalyst for the synthesis of butyl acetate and subsequent esterification of carboxylic acid, 22,23 for selective CO 2 capture, 34 and as anti-wear additives to improve tribological performance. 32,33 Early XPS studies of PEG-functionalized systems have largely focused upon the orientation of surface structure of imidazolium-based systems, 7,10,35 and were completed out using nonmonochromatic X-ray sources which sometimes suffer from reduced resolution of primary XP photoemission envelopes. A recent study, of surface enrichment in equimolar mixtures of nonfunctionalised and ether-functionalised imidazolium-based ILs, by Heller et al., 36 showed that the electronic structure of the aromatic region of the imidazolium cation is shifted to a significantly higher electron binding energies (BEs) when compared to [C 8 C 1 Im] + , when the ether oxygen is directly bound to the imidazolium charge carrier. Here we demonstrate how the use of a higher resolution, monochromated, X-ray source can reveal subtle new insights which can inform the design of next generation reaction systems and alternative solvents. In this contribution we utilize XPS and density functional theory (DFT) based BE calculations to evaluate intra-molecular interaction between the polyether oxygen and the imidazolium charge carrier group. We show that by manipulation of the position of the oxygen atom, nearest to the cationic charge carrier, we can tune the electronic properties of the resulting ionic liquid in a very delicate way. The relative positioning of the oxygen atom can have a marked impact upon on the immediate side chain chemistry of the IL, while exhibiting minimal impact on the imidazolium ring itself. This observation, although subtle in isolation, manifests as a distinct change in the overall profile of the C1s photoemission envelope which is easily recognized by inspection. Responding to this observation, we also propose a modified deconstruction model for ether containing systems, the detail of which is underpinned by the DFT calculations and modelling.
To assist in the development and description of these data, we propose a relatively simple nomenclature system which is designed to minimize ambiguities when visualising structures.
The system is defined by short hand notations ascribed to each N-based substituent in turn.
Structures studied herein include functionalised substituents, so our descriptor for the complex substituent must reflect this. If we apply the same logic, we can develop a descriptor for each substituent in turn, see Figure 1 and Table 1 for IL structures and linked nomenclature. Figure 1: Nomenclature system adopted for functionalised ionic liquids studied in this work    Table 2). Ion chromatography measurement confirmed that residue halide and/or metal impurities derived from salt exchange chemistries were below detection limits.

Density Functional Theory (DFT):
The structure was optimized using density functional theory with the B97-1 exchange-correlation functional, 38 and the pcSseg-1 basis set. 39 The structural model utilized for the calculations consist of the cation with two counterions placed near the imidazole ring, these structures are shown in the ESI. The electron BEs were computed using a ∆self-consistent field (∆SCF) approach, 40 with the PBE functional, 41 and larger pcSseg-2 basis set. These basis sets have been shown to be accurate for the calculation of coreionisation energies. 42   10.0 (10) 3.9 (4) 2.9 (3) 6.1 (6)

Comparison of the Photoemission Envelopes for β and γ -substituted Ether-functionalized Ionic liquids.
The high resolution C1s photoemission envelopes for two imidazolium-based ionic liquids, with an oxygen atom positioned at either the β or γ position of the longest substituent side chain are shown in Figure 2.  Similarly, a review of the O 1s region for β-IL revealed an asymmetry at ~533.5 eV, Figure   3b. This binding energy is much higher than those observed for the γ-IL and alkyl substituted ILs, i.e 532.6 eV for both ILs, with a binding energy difference, (ΔBE) equal to 0.9 eV. This is significantly higher than the experimental error and similar to those observed in a range of polymers. 49 Since O is both present in the cation and the anion, the higher BE shift is more

IL ions. (DFT data is provided in the ESI, Table SI 2 and 3). This observation can be correlated
to the density (ρ) of the two ILs. Although ρ is a bulk measurement parameter, it strongly correlates to intra and intermolecular interaction as well as packing efficiency of ILs. 50 Hence the previously measured ρ for both β and γ-ILs at 1.5015 g cm -3 and 1.4577 g cm -3 respectively, 48 61 mN m -1 ). 48 While melting point (T m ) for γ-IL was observed to be -7.6 °C, but was not observed for β-IL at the same temperature. 48 Other studies where slight variation in methylene spencer unit in ether functionalised ILs led to significant changes in electrochemistry, 14,53,54 density, 55 viscosity, 53 and T d, 29, 55 have also been published.
To further investigate the incidence of intramolecular interactions a series of DFT calculations was executed, all of which suggest that no significant interaction was observed, thus supporting the XPS data (Figure 4a and Figure 4 shows the three highest occupied molecular orbitals (HOMOs) for the β and γ-ILs.
These orbitals represent the π molecular orbitals of the imidazolium ring and can provide some insight into the weak interaction between the oxygen and imidazolium ring that is observed for both of these ILs. For both of these ILs there is little overlap between the p-type orbitals localised on the oxygen and the p orbitals of the ring owing to both the distance of the oxygen atom from the ring and also its alignment with respect to the imidazolium ring. Crucially for the β-IL where the oxygen is closer to the ring, the oxygen does not lie in the plane of the imidazolium which means that it cannot form part of a delocalised π molecular orbital spanning the imidazolium ring and the ether oxygen, and this is the case for both the β and γ-ILs. As a consequence, there is only a small interaction between the oxygen and the imidazolium ring for both of these ILs.
In order to firmly establish that the C 7 1s component is mostly impacted within the β-IL, the C 2 position was methylated, hence turning off hydrogen bonding interaction within the site, and distorting charge transfer between the C 2 and other potential interaction sites. 56-58

Figure 5: Overlaid C 1s high resolution XP spectra for non-methylated (red) and methylated (green) β-IL, [(C 1 OC 2 )C 1 Im][Tf 2 N] and [(C 1 OC 2 )C 1 C 1 Im][Tf 2 N] respectively. Intensity normalized to the area of the C CF 3 1s for non-methylated β-IL, [(C 1 OC 2 )C 1 Im][Tf 2 N]. Charge correction is achieved indirectly by setting the value of the F 1s photoemission equal to that observed in a standard reference material [C 8 C 1 Im][Tf 2 N], (F = 688.8 eV). 47
Inspection of the overlaid spectra for both ILs, (Figure 5), reveals there is an apparent broadening of the region with the highest intensity for the methylated β-IL (green colour) at ~286.0 eV. This is indicative of the presence of the methyl group directly bonded to the C 2 carbon. While the region with the highest binding energy (~288.5 eV) exhibited good alignment between the two XP spectra. This suggests that the C 2 1s component in both ILs are in the same electronic environment, and independent of the methylation at the C 2 position. This data has therefore established the fact that the electronic structure of the C 2 1s component is not impacted by substituting polyether oxygen at β position to the imidazolium ring.

Towards a Refined C1s Deconstruction Model for Ether-functionalized Imidazolium-based Ionic liquids
The systematic analysis of both β-IL and γ-IL demonstrates that their electronic structure is distinctively different, as a result, a new fitting model is required to correctly deconstruct the C 1s environment for β-IL. Consequently, the C 1s environment for γ-IL was deconstructed based upon the fitting model established for imidazolium-based alkyl substituted ILs. 6,8,47,57,59 ( Figure 6a). The C 1s component for both β-IL and γ-ILs were fitted based upon data obtained from XPS and DFT calculations. Parameter constraints was employed in both cases to achieve a reliable fit. This include constraining the FWHM to between 0.8 -1.2 eV, 47 while area constraints was carried out in line with the stoichiometry of the ILs. Detailed parameter description used to fit the ILs is provided in the ESI (Table SI 1.1). After accounting for shake up/off phenomena which affects ~ 20 % of all photoemissions originating from the aromatic region of the imidazolium cation, the relative peak ratio for the four components in β-IL and γ-ILs is set at 1.8 : 1.6 : 2 : 1 for components C 2+7 : C 4+5 : C hetero : C aliphatic and 0.8 : 1.6 : 4 : 1 for components C 2 : C 4+5 : C hetero : C aliphatic respectively. In the model, for γ-IL, the component from the CF 3 i.e C hetero 1s atoms, comprising C 6 , C 7 , C 8 and C 10, was fitted to 286.5 eV. While the terminal carbon which is involved in a C-C aliphatic type bond is fitted to 285.1 eV. The model for β-IL was fitted in line with the description above for the γ-IL (Figure 6b).

Conclusions
The designer aspect of ionic liquids was exploited to graft polyether groups into IL structural framework, thus creating an opportunity to investigate fundamental interactions within the IL, The above XPS data was supported by DFT core-electron binding energy calculations which showed excellent agreement with the experimental observations. The C 1s environment for β-IL was consequently fitted based upon the data obtained from both XPS experiment and DFT calculation.
This data has provided critical insight that can aid the design of new materials with specific properties that can potentially impact hydrogen bonding, both intermolecularly and intramolecularly. This study has added to the depth of information already established for ether-