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Microscopic aspects in dicationic ionic liquids through the low-frequency spectra by femtosecond Raman-induced Kerr
Hideaki Shirota1, Tateki Ishida
1Department of Nanomaterial Science, Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi, Inage-ku, Chiba 263-8522, Japan. shirota@faculty.chiba-u.jp
The Journal of Physical Chemistry. B
|August 13, 2011
Summary
Femtosecond spectroscopy reveals distinct interionic vibrations in dicationic ionic liquids (ILs) compared to monocationic ILs. Differences in spectral shapes indicate varying microstructures influenced by cation length and linker groups.
Area of Science:
- Physical Chemistry
- Materials Science
- Spectroscopy
Background:
- Ionic liquids (ILs) are salts that are liquid at room temperature, with tunable properties based on their cation and anion structure.
- Imidazolium-based ILs are widely studied due to their versatile applications, but their microscopic structure and dynamics remain areas of active research.
- Understanding the relationship between molecular structure and vibrational dynamics is crucial for designing ILs with specific functionalities.
Purpose of the Study:
- To investigate the interionic vibrational dynamics in imidazolium-based dicationic ionic liquids (ILs) with bis(trifluoromethylsulfonyl)amide ([NTf(2)](-)) counteranions.
- To compare the microscopic structure of dicationic ILs with their corresponding monocationic ILs using femtosecond Raman-induced Kerr effect spectroscopy.
- To elucidate how cation structure, specifically the length of the alkylene linker or alkyl groups, influences the vibrational spectra and microstructure of ILs.
Main Methods:
- Employed femtosecond optical-heterodyne-detected Raman-induced Kerr effect (FS-OHD-RIKES) spectroscopy to probe low-frequency interionic vibrations.
- Studied dicationic ILs ([C(n)(MIm)(2)][NTf(2)](2), n=6, 10, 12) and compared them with monocationic ILs ([C(n)MIm][NTf(2)], n=3, 5, 6).
- Analyzed spectral profiles and line-shapes in the 0-200 cm(-1) frequency range to infer structural characteristics.
Main Results:
- Both dicationic and monocationic ILs exhibited a bimodal spectral profile in the low-frequency range.
- Spectral shape differences between dicationic and monocationic ILs were more pronounced for shorter linker/alkyl groups ([C(6)(MIm)(2)][NTf(2)](2) vs. [C(3)MIm][NTf(2)]).
- The monocationic IL [C(3)MIm][NTf(2)] showed a distinct line-shape, suggesting a homogeneous microstructure, unlike other ILs indicating microsegregation due to longer nonpolar groups.
Conclusions:
- The study reveals subtle yet significant differences in interionic vibrations and microstructures between dicationic and monocationic ILs.
- The length of the nonpolar segments in the cations plays a critical role in dictating the degree of microsegregation within the ILs.
- FS-OHD-RIKES spectroscopy is a powerful tool for differentiating subtle structural variations in ionic liquids based on their vibrational dynamics.
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