UV laser spectroscopy of mass-selected ionic liquid building blocks in the gas-phase

doi:10.1016/j.cplett.2015.06.023

Chemical Physics Letters

Volume 634, 1 August 2015, Pages 216-220

https://doi.org/10.1016/j.cplett.2015.06.023 Get rights and content

Highlights

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First UV spectra of mass-selected gas-phase aggregates of ionic liquids.
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Anionic and cationic aggregates of [BMIM⁺][Tf₂N⁻] and [EMIM⁺][Tf₂N⁻] have been studied.
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Our aggregate photofragmentation spectra are very similar to previous spectra obtained for gas-phase IL ion pairs.
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Technique is a facile route to obtain gas-phase UV spectra of ILs.

Abstract

We report the first UV laser action spectra of mass-selected aggregates composed of the cationic and anionic constituents of two ionic liquids, i.e. [BMIM⁺]_n[Tf₂N⁻]_m and [EMIM⁺]_n[Tf₂N⁻]_m where n ≠ m and n, m = 1,2. Each aggregate cluster displays a maximum absorption at 220 nm, the high energy limit scanned. However, the anionic and cationic aggregate photofragmentation cross-sections are notably different, indicating distinctive decay mechanisms. We compare our spectra to other gas-phase spectroscopic methods and discuss the general utility of our technique as a facile route to obtaining gas-phase electronic spectra of isolated ionic liquids.

Graphical abstract

Introduction

Ionic liquids have attracted considerable attention in both the fundamental and industrial chemical community over the last decade as a result of their unique physiochemical properties such as low vapour pressure, high viscosity and good conductivity. Proposed applications range from green industrial solvents, CO₂ extraction media, through to rocket propellants [1], [2], [3], [4]. A very considerable number of spectroscopic studies have been conducted on condensed phase ionic liquids (ILs) with the aim of developing analytical methods for characterising ILs as well as to investigate their fundamental structure [5], [6]. Such work has been complemented by gas-phase studies of IL vapours over recent years, following the discovery that certain ILs were sufficiently volatile to allow transfer into the vapour phase within a high vacuum environment [7], [8]. These studies indicate that the IL vapours typically consist of ion-pairs for aprotic ILs [9], [10], [11], and spectroscopic characterisation of such systems therefore provides an excellent opportunity for testing the large number of theoretical calculations that have been conducted on IL ion pairs [12], [13], [14].

In this letter, we present UV laser photodissociation spectra of mass selected aggregate clusters of the [BMIM⁺][Tf₂N⁻] and [EMIM⁺][Tf₂N⁻] ILs, i.e. [BMIM⁺]_n[Tf₂N⁻]_m and [EMIM⁺]_n[Tf₂N⁻]_m where n ≠ m and n, m = 1, 2. Schematic structures of these ILs are displayed in Scheme 1. These spectra are the first such gas-phase UV spectra where the identity of the IL aggregate cluster can be definitively identified via mass selection prior to spectroscopic characterisation. Our general strategy mirrors that recently employed by Johnson and co-workers in their IR laser spectroscopy study of [EMIM⁺]_n[BF₄⁻]_m n ≠ m aggregate ions [15]. We have chosen to study the [BMIM⁺][Tf₂N⁻] and [EMIM⁺][Tf₂N⁻] systems for this work as both have recently been the subject of vapour-phase UV spectroscopy studies [16], [17], allowing us to address the key point of how closely the spectra of our mass-selected aggregates resemble those of the vapour-phase ILs. Wang et al. presented the first UV spectroscopic measurements of IL vapours (including the BMIM and EMIM systems) using high-temperature vaporisation of ILs into quartz cuvettes [16], while Ogura et al. subsequently acquired a UV absorption spectrum of vapour phase [EMIM⁺][Tf₂N⁻] using cavity ring down spectroscopy [17]. Cooper et al. [18] have introduced [EMIM⁺][Tf₂N⁻] into the gas-phase in a supersonic expansion and applied UV photofragmentation to the resulting (non-mass selected) jet-cooled neutral clusters. In our current study, electrospray ionisation is used to transfer the ILs from the condensed phase into the gas-phase, prior to mass selection in a laser-interfaced mass spectrometer. We present electronic spectra of the anionic and cationic aggregates for both ILs for comparison with the previous gas-phase studies.

Section snippets

Experimental

Experiments were conducted in Bruker Esquire 6000 and Bruker AmaZon Quadrupole Ion Trap mass spectrometers that have been custom-modified for performing laser spectroscopy [19], [20]. The ILs were purchased from Aldrich and used without any further purification. IL clusters were generated using positive and negative mode electrospray (100 °C dry gas temperature) ionisation of 10⁻⁴ mol dm⁻³ (Esquire) and 10⁻⁶ mol dm⁻³ (AmaZon) solutions of the respective IL in acetonitrile, and specific mass-selected

Results and discussion

Figure 1a displays the positive ion mode electrospray ionisation mass spectrum (ESI-MS) obtained by electrospraying the [BMIM⁺][Tf₂N⁻] IL in acetonitrile. The mass spectrum is dominated by the [BMIM⁺]₂[Tf₂N⁻] aggregate cluster peak, with the [BMIM⁺] cation also clearly visible. Negatively charged aggregate clusters were observed when the instrument was operated in negative ion mode (Figure 1b), as illustrated by the strong appearance of [BMIM⁺][Tf₂N⁻]₂. Similar spectra were obtained in positive

Further discussion

The similarity between the [EMIM⁺]_n[Tf₂N⁻]_m aggregate photofragment spectra observed in this work, and the [EMIM⁺][Tf₂N⁻] supersonic jet spectra measured by Cooper et al. suggest that similar photophysics may be present in photoexcitation of the aggregates as in the isolated ion-pairs that are thought to be present in the jet experiment. Cooper et al. have proposed a mechanism that involves initial photoexcitation to an electronic excited state of the [EMIM⁺][Tf₂N⁻] ion-pair [18]. They

Summary

Since the chemical and physical properties of ILs can be controlled by the combination of cationic and anionic components, ILs can be used for an increasingly wide range of technical applications [1], [2], [3], [4]. Given the wealth of possible cation-anion combinations, it is highly desirable to be able to rationally predict the properties of an IL based on the constituents [30]. Ab initio computational studies of ILs make a considerable contribution to this general effort, but such

Acknowledgements

This work was supported through the European Research Council grant 208589-BIOIONS. The Bruker Esquire 6000 was provided by Science City York and Yorkshire Forward using funds from the Northern Way Initiative. We also thank the STFC for the provision of equipment from the EPSRC Laser Loan Pool (Grant # 13250030).

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UV laser spectroscopy of mass-selected ionic liquid building blocks in the gas-phase

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental

Results and discussion

Further discussion

Summary

Acknowledgements

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J. Mol. Spectrosc.

Chem. Phys. Lett.

Chem. Phys. Lett.

Chem. Soc. Rev.

Chem. Rev.

Angew. Chem. Int. Ed.

Angew. Chem. Int. Ed.

Ionic Liquids (Topics in Current Chemistry)

The Structure of Ionic Liquids (Soft and Biological Matter)

J. Phys. Chem. B

Nature

Phys. Chem. Chem. Phys.

Phys. Chem. Chem. Phys.

J. Phys. Chem. A

Chem. Eur. J.