Investigation of 1, 10-Phenanthroline based Ionic Liquids using X-ray photoelectron spectroscopy

Ionic liquids—low temperature molten salts composed entirely of mobile ions—are a fascinating class of materials that have experienced an incredible growth in research over recent years. This is because of their immense applications in not only academic but also industrial processes. Ultra high vacuum technique, i.e. X-ray photoelectron spectroscopy is now accepted as a reliable method for the study of ionic liquid-based systems. To date, the main focus of research effort employing X-ray photoelectron spectroscopy has been upon imidazoliumbased ionic liquids, quite simply because these are the materials most often employed by synthetic chemists. However, X-ray photoelectron spectroscopy data (fitting models) for other than 1, 3-dialkyimidazolium based ionic liquids are required and need to be developed. 1, 10-Phenanthrolinium based ionic liquids are among the new compounds reported recently. However, no attempt is observed to investigate the electronic environments of its component atoms. In this report, therefore, the electronic environments of six N-alkyl-1, 10-Phenathrolinium based ionic liquids, [CnPhen][Tf2N] (n=1, 2, 4, 6, 8, 10), were investigated using the X-ray photoelectron spectroscopy and a good fitting is developed for C 1s, and N 1s that applies to each of the compounds studied. This model allows accurate charge correction and the determination of reliable and reproducible binding energies of all the atoms for each compound studied. Hence, this model could be taken as benchmark in investigating any compound containing 1, 10-phenanthroline derivatives.


INTRODUCTION
Ionic liquids (ILs) are low temperature (below 100 °C) melting salts composed solely of mobile ions. ILs are a fascinating class of materials which have magnetized the research direction of scientists (Plechkova et al., 2008;Wasserscheid and Welton, 2008;Weingartner, 2008;). Unlike molecular solvents, the properties of ILs can be tuned by the structurally diversified cation/anion combination (Coasne et al., 2011). Features such as negligible vapor pressure, non-flammability, widespread liquidus temperature range, high ionic conductivity, excellent dissolving power for many materials, and a wide potential window make ILs the subject of research as alternative *Corresponding author: atakiltabebe1@gmail.com phosphate, [PF 6 ] -, or bis (trifluoromethylsulfonyl) imide, [Tf 2 N] - (Schaltin et al., 2011). To increase the solubility, either employing new cations and/ or anions or modifications of the traditional ones can be used (Nockemann et al., 2006,;Nockemann et al., 2010). In view of this, a new set of 1, 10-phenanthrolinium cation based ILs were synthesized and their potential applications were reported elsewhere (Villar-Garcia et al., 2012).
However, there is no evidence that attempted to investigate the latter class of compounds using ultra-high vacuum (UHV) characterization techniques.
The electronic environment of each element present in the compounds is discussed. For ionic liquids, the most common, complex and often relevant element is carbon. Therefore, the development of a C 1s fitting model which deconstructs these different electronic
All X-ray photoelectron spectra were recorded using a Kratos Axis Ultra spectrometer employing a focused, monochromated Al Kα source (hν = 1486.6 eV), hybrid (magnetic/electrostatic) optics, hemispherical analyzer and a multi-channel plate and delay line detector (DLD) with an X-ray incident angle of 0° (relative to the surface normal).

XPS data analysis
Relative Sensitivity Factors (RSF) were taken from the Kratos Library (RSF of F 1s = 1) and were used to determine atomic percentages (Wagner et al., 1981;Smith et al, 2006).
This line shape has been used consistently in the fitting of XP spectra, and has been found to mach experimental line shapes in ionic liquid systems (Smith et al., 2006;Villar-Garcia et al., 2011).
All X-ray photoelectron spectra were charge corrected by setting the binding energy of the aliphatic C 1s photoemission peak (C aliphatic 1s) of

Binding energies
In order to obtain absolute binding energies for components within 1, 10-Phenanthrolinium based compounds, it is necessary to charge correct the X-ray photoelectron spectra using an appropriate internal reference. To investigate whether such an internal reference exists for photoelectrons. In this regard the peak due to C 1 , (C 4 +C 5 ) and C 3 is reduced by 20%. As the peak area of aliphatic carbons is not affected by the shakeup phenomenon, the relative peak area ratios for the four cationic components would be 1: 2.6: 1.6: 6.4. While the Full Width at Half Maximum, FWHM, for (C 1 +C 2 ), (C 3 ), and (C 4 +C 5 ) were set to be equal and set to be 1.1, for CF 3 was set from 0.9 to 0.95. In general, for all compounds studied here, similar procedures were followed and the fit shows an excellent agreement to the experimentally acquired signal.
In this regard, the binding energy separation be-  surrounded with large electronegative fluorine atoms which offsets the negative charge that leads it to be nearly neutral and to be signaled nearly at the binding energy of the neutral nitrogen. This binding energy is comparable with that observed for a range of imidazolium based ionic liquids (Smith et al., 2006;Lovelock et al., 2009;Villar-Garcia et al., 2011).
The N 1s fitting model (Figure 3) in [C 1 phen] [Tf 2 N] is a little different from the rest in the same reason as the C 1s of CF 3 , which is the proximity of the electron donating CH 3 group created a little discrimination between N Anion and unalkylated nitrogen of 1, 10-phenanthroline, N ' Cation .
Fluorine, oxygen and sulfur of these Compounds each show a single electronic environment (Smith et al., 2006;Lovelock et al., 2009;Villar-Garcia et al., 2011). This is because of the obvious reason that the six fluorine atoms are chemically indistinguishable. This fact works the same for the four oxygen atoms and the two sulfur atoms. The doublet due to the region of sulfur does not signify the existence of two electronically different regions. This doublet peak is created because this element acquires two electronic states with respect to its 2p orbitals due to spin-orbit splitting into the S 2p 1/2 and 2p 3/2 levels with area ratio of 1:2 when bombarded and excited by the X-ray photons (Figure 4).  eV. This is due to the obvious reason that they are bonded to the positively charged alkylated nitrogen and also they are part of the electron deficient ring.
On the other hand, regardless of the alkyl chain length, little or no effect is observed on the binding

CONCLUSIONS
We have successfully measured X-ray photoelec- were obtained for C Alkyl 1s, charge corrected binding energies (absolute binding energies) for all components could be obtained. Variation of n was shown to have little or no effect on the electronic