X-ray photoelectron spectroscopy of piperidinium ionic liquids: a comparison to the charge delocalised pyridinium analogues

In this study, nine piperidinium-based ionic liquids are analysed by X-ray photoelectron spectroscopy. The effect of alkyl substituent length and the nature of the anion on the electronic environment of the cation are investigated. The electronic environment of the hetero carbon and the cationic nitrogen is compared between two structurally similar cations, 1-octyl-1-methylpiperidinium ([C8C1Pip]+) versus 1-octylpyridinium ([C8Py]+). Due to the charge delocalisation, the hetero carbon component within [C8Py]+ is more positively charged, which exhibits much higher binding energy; whilst the cationic nitrogen component is in the similar electronic environment. The impact of the charge delocalisation on the electronic environment of the anion is also compared between [C8C1Pip]+ and [C8Py]+. It is found that for the more basic anion, the cation can significantly affect the electronic environment of the anion; for the less basic anion, such an effect concentrates on the component bearing more negative point charges.


Introduction
Ionic liquids (ILs) have generated considerable excitement, ascribed to their liquid nature and negligible volatility. Being composed of cations and anions, they have shown many characteristic properties, i.e. low melting point, high thermal stability and high conductivity, and therefore been attractive for many practical applications. 1-7 It has been concluded that by simply changing the cationic and/or anionic constituent, the physicochemical properties of ILs can be effectively tuned.
Piperdinium has been the most versatile family in applications next to imidazolium and pyridinium. Due to the lack of more acidic protons, piperidinium ILs often show higher thermal stability than imidazolium or pyridinium ILs. 8,9 When compared to their structurally similar charge delocalised pyridinium analogues, piperdinium ILs often show higher melting points. For example, the melting point for [C4C1Pip]Br is 514 K, 8 which is 136 K higher than that of [C4Py]Br. 10 However, for a specific case, i.e. switching the anion of the above two ILs to the larger delocalised bis  11 Apart from those, piperidinium ILs also show higher viscosity 8,12,13 and wider electrochemical windows, 14,15 compared to imidazolium or pyridinium ILs. All the above properties have made piperdinium ILs potentially useful particularly in electrochemical applications.
To date, X-ray photoelectron spectroscopy (XPS) has been accepted as an effective tool to investigate ionic liquid-based systems. 16,17 The focus of research effort in the area of ultrahigh vacuum (UHV) characterisation has been upon a host of families of ILs, including imidazolium, 18,19 pyridinium, 20 pyrrolidinium, 21,22 ammonium, 23 3 guanidinium 24 and phosphonium. 23,25 The analysis initially includes the calculation of surface composition of ILs, the identifying of a certain component present in an ionic liquid, and the distinguishing of subtle change in electronic environment for either a cation-or an anion-based component. The scope of such a research topic is later extended to the investigation of the cation-anion interactions, which have also been probed by NMR 26 and Kamlet-Taft parameters. 27 The study of the cation-anion interactions of ILs aids proper understanding of their physicochemical properties.
In this work, XPS is used to analyse nine piperidinium-based ILs. The effect of the alkyl chain length on the electronic environment of the cation is investigated by varying n from 2 to 12. It is found that the charge-transfer effect from the anion to the cation and the inductive effect from the alkyl substituent can both affect the electronic environment of the cation-based components, i.e. Ncation 1s.

Materials
All chemicals were purchased from Sigma Aldrich and were used as received. Ionic liquids investigated in this study were prepared in our laboratory using established synthetic protocols, and were characterised by NMR recorded on a JEOL 400YH spectrometer as solutions in DMSO-d6. The procedures of synthesis of ionic liquids, NMR data and XP spectra of ionic liquids are demonstrated in detail in Electronic Supplementary Information.

XPS Data Collection
All XP spectra were recorded using a Thermo Scientific Kα spectrometer employing a focused, monochromated Al Kα source (h = 1486.6 eV), hemispherical analyser, charge neutraliser and a 128-channel detector. The instrument employs an oval X-ray spot. The largest spot size (long axis) is 400 microns.
All ionic liquid (IL) samples prepared in this work were purified under high vacuum at 60 o C for at least 12 h prior to use. The IL sample was firstly transferred into a load-lock of the XPS instrument as a sample droplet on a stainless sample holder.
Pumping of samples was conducted to achieve ~10 -4 mbar. After achieving the base pressure in the load-lock, samples were maintained pumping overnight, before transferring to the main analytical chamber.
The base pressure in the main analytical chamber is below 1  10 -9 mbar without any samples being analysed. When analysing liquid samples, the charge neutraliser is switched off which helps the maintenance of the pressure to be below ~10 -8 mbar. When solid samples are being analysed, the charge neutraliser is turned on. The pressure in this case is usually below ~10 -7 mbar. It suggests that all volatile impurities under those vacuum conditions, such as water and organic solvents, can be completely removed, leading to high purity samples. 28 Consequently, the comparison of binding energies derived from XP spectra is reliable. 5

XPS Data Analysis
CasaXPS software was used for data interpretation. A spline linear background subtraction was used. Peaks were fitted using GL (30) lineshapes: a combination of a Gaussian (70%) and Lorentzian (30%). 29

Sample purity
The sample purity of ILs is confirmed by semi-quantitative analysis from XPS. The experimental surface composition for each IL is calculated from high resolution XP spectra, according to the relative sensitivity factor for each element taken from literature. 29 Table 2 demonstrates the surface composition for each of the IL studied in this paper. In order to give a visual comparison, the nominal stoichiometry is also included. Taking into account the error of semi-quantitative analysis from XPS, it suggests that the experimental surface composition is the same with the nominal one calculated from the empirical formulae, for each of the IL in this study.  (1.0)

Electronic environment of the carbon regions: Fitting model
Initially, the C 1s spectrum was fitted according to an established model developed for 1-alkyl-1-methylpyrrolidinium ILs reported in literature. 21 Figure S1 shows the (C 2 and C 6 to C 8 ) and Caliphatic 1s (C 3 to C 5 and C 9 onwards).
Therefore, a two-component model (apart from the signal originated from -CF3 7 group) is developed. Figure 1 shows [CnC1Pip]Br, where n = 2-12, and [C8C1Pip][PF6] contain no -CF3 group in the structure. Therefore, only the unresolved cation-based doublet C 1s peak can be observed. However, the same fitting procedure is also applicable for these ILs.

Effect of alkyl chain length on the aliphatic C 1s binding energies
In order to investigate the effect of alkyl chain length on Caliphatic 1s binding energies, two families of ionic liquids are employed: one of the least basic anions, i.e.
[Tf2N] -, and Br -, which is a typical more basic anion. Figure 2 shows the C 1s XP spectra for these two IL families.
As demonstrated in Figure 2a, for [Tf2N] -, there is an apparent shift of Caliphatic 1s binding energy towards higher value, along with the decreasing of the alkyl chain length, i.e. from dodecyl to ethyl. When n = 4, the Caliphatic 1s binding energy is found at 285.2 eV, which is 0.2 eV higher than that for n = 8. This is because the alkyl carbons present in [C4C1Pip] + locates more closed to the positively charged nitrogen. Therefore, the inductive effect from alkyl chain to the positive headgroup leads to the decrease of the electron density on the alkyl carbon component, which subsequently shows higher binding energy. As the alkyl chain length decreases from butyl to ethyl, the Caliphatic 1s binding energy is further increased to 285.3 eV. It must be noted that when n = 12, the Caliphatic 1s binding energy is 284.9 eV, which is 0.1 eV lower than that for n = 8. Since the experimental error associated with XPS is of the order ± 0.1 eV, a noticeable change in binding energy should be no less than 0.2 eV. Therefore, it suggests that when increasing n value from 8 to 12, the electronic environment of Caliphatic 1s component is not changed.
This observation is in good agreement with the conclusion that has been made for pyridinium ionic liquids. 20 As a result, it is concluded that the electronic environment of Caliphatic 1s component is always identical when n ≥ 8.   Figure 3  This phenomenon can be interpreted as below. For the more basic anion, the charge-transfer effect from the anion to the cation is significant, which leads to the increase of the electron density on nitrogen centre. Consequently, the N 1s binding energy is lower. The opposite is also true for the less basic anion. 9 The same trend can also be found for Chetero 1s binding energy, which can be found in detail in Table 3.    A same binding energy shift can also be observed for Br -ILs (see Table 3 for details).

Electronic environment of the nitrogen regions: Impact of the anion
It suggests that by switching the cation from [C8C1Pip] + to [C8Py] + , it is feasible to tune the electronic environment of the cationic hetero carbons, with only subtle impact from the anion basicity.
However, such a shift is not measurable for Ncation component. As shown in Table 3,    Figures 4a, 4c-4e). On the other hand, a noticeable change in binding energy is found for O 1s, which is more than 0.2 eV (see Figure 4b). This observation further confirms that for the less basic anion, the charge shielding effect is more concentrated on the component bearing more point charges, i.e. oxygen within [Tf2N] -. [33][34][35] Binding energies for all elements can be found in Table 3   For the more basic anion, i.e. Br -, the measured binding energy shift of Br 3d5/2 is more than 0.2 eV, which can reflect the change in electronic environment of the anion. 12 Bras one of the most basic anions can transfer much more point charges to the caiton.
Consequently, when switching the cation from [C8Py] + to [C8C1Pip] + , the charge shielding effect is much stronger, enabling a measurable Br 3d5/2 binding energy shift. Figure 6 summaries in detail the shift in binding energy for each anion-based component for the two anions.