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Spectroscopic Evidencefor Unusual Microviscosityin Imidazolium Ionic Liquid and Tetraethylene Glycol Dimethyl EtherCosolvent Mixtures.

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Title: Spectroscopic Evidencefor Unusual Microviscosityin Imidazolium Ionic Liquid and Tetraethylene Glycol Dimethyl EtherCosolvent Mixtures.
Authors: Li, Boxuan1, Wang, Yingying1, Wang, Xuefei1, Vdovic, Silvije1, Guo, Qianjin1, Xia, Andong1
Source: Journal of Physical Chemistry B. Nov2012, Vol. 116 Issue 44, p13272-13281. 10p.
Subjects: Viscosity, Imidazoles, Ionic liquids, Ethylene glycol, Methyl ether, Benzopyrans, Emission spectroscopy
Abstract: The rotational dynamics of coumarin 153 (C153) in imidazolium-basedionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and tetraethylene glycol dimethyl ether (TEGDME) mixturesacross all mole fractions have been investigated to determine thelocal viscosity of the microenvironment surrounding the probe molecules.The excimer-to-monomer fluorescence emission intensity ratio (IE/IM) of a well-knownmicroviscosity probe, 1,3-bis(1-pyrenyl)propane (BPP), is also employedto study the microviscosity of the mixtures as a complementary measurement.The rotational dynamics of C153 show that there are incompact andcompact domains within the heterogeneous structural [bmim][PF6], resulting in fast and slow components of C153 rotationaldynamics. The microviscosity in different structural domains of [bmim][PF6] before and after adding cosolvent TEGDME with differentmole fractions is further investigated by studying the fluorescenceanisotropy decay of probe molecules. The obtained average rotationtime constants show that the microviscosity of [bmim][PF6] is enhanced after mixing with a certain amount of TEGDME, althoughthe bulk viscosity of TEGDME itself is much lower than that of theionic liquid. This unusual behavior of microviscosity enhancementis further proven by the steady state fluorescence measurement withthe microviscosity probe of BPP. The microviscosity enhancement isreasonably demonstrated by the fast time constant of C153 rotationaldynamics and the departure between the experimentally observed andcalculated ratio of IE/IMof BPP, which shows that this effect is most pronouncedat intermediate mole fractions of the [bmim][PF6] and TEGDMEmixtures. The strengthening effects caused by the molecular interactionsbetween TEGDME and structural heterogeneous ionic liquid [bmim][PF6] are proposed to interpret the unusual microviscosity behaviors. [ABSTRACT FROM AUTHOR]
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Database: Engineering Source
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Abstract:The rotational dynamics of coumarin 153 (C153) in imidazolium-basedionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and tetraethylene glycol dimethyl ether (TEGDME) mixturesacross all mole fractions have been investigated to determine thelocal viscosity of the microenvironment surrounding the probe molecules.The excimer-to-monomer fluorescence emission intensity ratio (IE/IM) of a well-knownmicroviscosity probe, 1,3-bis(1-pyrenyl)propane (BPP), is also employedto study the microviscosity of the mixtures as a complementary measurement.The rotational dynamics of C153 show that there are incompact andcompact domains within the heterogeneous structural [bmim][PF6], resulting in fast and slow components of C153 rotationaldynamics. The microviscosity in different structural domains of [bmim][PF6] before and after adding cosolvent TEGDME with differentmole fractions is further investigated by studying the fluorescenceanisotropy decay of probe molecules. The obtained average rotationtime constants show that the microviscosity of [bmim][PF6] is enhanced after mixing with a certain amount of TEGDME, althoughthe bulk viscosity of TEGDME itself is much lower than that of theionic liquid. This unusual behavior of microviscosity enhancementis further proven by the steady state fluorescence measurement withthe microviscosity probe of BPP. The microviscosity enhancement isreasonably demonstrated by the fast time constant of C153 rotationaldynamics and the departure between the experimentally observed andcalculated ratio of IE/IMof BPP, which shows that this effect is most pronouncedat intermediate mole fractions of the [bmim][PF6] and TEGDMEmixtures. The strengthening effects caused by the molecular interactionsbetween TEGDME and structural heterogeneous ionic liquid [bmim][PF6] are proposed to interpret the unusual microviscosity behaviors. [ABSTRACT FROM AUTHOR]
ISSN:15206106
DOI:10.1021/jp304914e