Spectroscopic Evidence for Unusual Microviscosity in Imidazolium Ionic Liquid and Tetraethylene Glycol Dimethyl Ether Cosolvent Mixtures

Abstract

The rotational dynamics of coumarin 153 (C153) in imidazolium-based ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim] [PF 6 ]) and tetraethylene glycol dimethyl ether (TEGDME) mixtures across all mole fractions have been investigated to determine the local viscosity of the microenvironment surrounding the probe molecules. The excimer-to-monomer fluorescence emission intensity ratio (I E /I M ) of a well-known microviscosity probe, 1,3-bis(1-pyrenyl)propane (BPP), is also employed to study the microviscosity of the mixtures as a complementary measurement. The rotational dynamics of C153 show that there are incompact and compact domains within the heterogeneous structural [bmim] [PF 6 ], resulting in fast and slow components of C153 rotational dynamics. The microviscosity in different structural domains of [bmim] [PF 6 ] before and after adding cosolvent TEGDME with different mole fractions is further investigated by studying the fluorescence anisotropy decay of probe molecules. The obtained average rotation time constants show that the microviscosity of [bmim] [PF 6 ] is enhanced after mixing with a certain amount of TEGDME, although the bulk viscosity of TEGDME itself is much lower than that of the ionic liquid. This unusual behavior of microviscosity enhancement is further proven by the steady state fluorescence measurement with the microviscosity probe of BPP. The microviscosity enhancement is reasonably demonstrated by the fast time constant of C153 rotational dynamics and the departure between the experimentally observed and calculated ratio of I E /I M of BPP, which shows that this effect is most pronounced at intermediate mole fractions of the [bmim] [PF 6 ] and TEGDME mixtures. The strengthening effects caused by the molecular interactions between TEGDME and structural heterogeneous ionic liquid [bmim] [PF 6 ] are proposed to interpret the unusual microviscosity behaviors.

Effect of ionic liquids on the conformation of a porphyrin-based viscometer

“…12,13 Possibilities for using neat ILs to control intermolecular interactions have been suggested primarily for cyanine dye self-assemblies. 14 Studies on viscosity of ILs using fluorescent dyes has been a subject of interest, [15][16][17] and the use of fluorescent probes for understanding the structure of ILs, in general, has been recently reviewed. 18 In addition, it was shown that the viscosity of molecular organic media could control the conformation of a conjugated porphyrin dimer (PD), resulting in an array of conformations with two distinct extremes: a planar and a twisted conformation (Fig.…”

Probing Laser‐Induced Heterogeneous Microenvironment Changes in Room‐Temperature Ionic Liquids

“…1‐Alkyl‐3‐methylimidazolium hexafluorophosphate ([C n mim][PF 6 ]) is one of the most commonly used ionic liquids; the material has a structural heterogeneity from the interconnected and permeated polar and nonpolar domains when the alkyl chain is butyl or longer ( n ≥4). Polar domains are formed by charge ordering of the imidazolium rings of the cations and anions through Coulomb electrostatic interactions, and nonpolar domains arise from aggregation of alkyl chains through van der Waals interactions . For ionic liquid [C 8 mim][PF 6 ], as shown in Scheme , the long octyl chains can either come close to each other in an ordered interdigitated way or individually coil themselves retaining a relatively large free volume, as reported in our previous study .…”

“…Modulation of the physical chemistry processes in various heterogeneous systems triggered by external optical or electrical control methods is currently a subject of intense research . Many probe molecules have been optimized to improve their responsive properties towards changes in their surrounding microenvironment . A heterogeneous solvent could provide a more versatile media in which simultaneous modulation of the microenvironment of solvent and the associated physical chemistry properties (such as spectral behavior) of the solute may enable many promising applications.…”

“…Especially, one of the most unique characteristics of ionic liquids is their structural heterogeneity, which provides a microenvironment with potentially designable heterogeneous microstructures composed entirely of cations and anions for photoelectric control . Both experimental and computational evidence indicate that some organized structures with different microstructural domains could form within ionic liquids because of their polar and nonpolar components . The distinct heterogeneous microenvironment of RTILs can be revealed by examining the dynamics of molecular probes through many spectroscopic methods; this approach has been studied by many research groups .…”

“…Moreover, because the dynamics of their triplet excited states are very sensitive to the microenvironment with respect to oxygen, porphyrin molecules are widely used as a probe to study the microstructure changes associated with oxygen‐related chemical reactions in many heterogeneous systems . Meanwhile, coumarin 153 (C153) has also become a well‐established probe for investigating the structural heterogeneity of RTIL systems …”