Influence of Solute Charge and Pyrrolidinium Ionic Liquid Alkyl Chain

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Influence of Solute Charge and Pyrrolidinium Ionic Liquid Alkyl Chain Length on Probe Rotational Reorientation Dynamics Jianchang Guo,† Shannon M. Mahurin,† Gary A. Baker,‡ Patrick C. Hillesheim,†,# Sheng Dai,† and Robert W. Shaw*,† †

Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Department of Chemistry, University of MissouriColumbia, Columbia, Missouri 65211, United States

‡

S Supporting Information *

ABSTRACT: In recent years, the effect of molecular charge on the rotational dynamics of probe solutes in room-temperature ionic liquids (RTILs) has been a subject of growing interest. For the purpose of extending our understanding of charged solute behavior within RTILs, we have studied the rotational dynamics of three illustrative xanthene fluorescent probes within a series of Nalkylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Cnmpyr][Tf2 N]) RTILs with different n-alkyl chain lengths (n = 3, 4, 6, 8, or 10) using timeresolved fluorescence anisotropy decay. The rotational dynamics of the neutral probe rhodamine B (RhB) dye lies between the stick and slip boundary conditions due to the influence of specific hydrogen bonding interactions. The rotation of the negatively charged sulforhodamine 640 (SR640) is slower than that of its positively charged counterpart rhodamine 6G (R6G). An analysis based upon Stokes−Einstein−Debye hydrodynamics indicates that SR640 adheres to stick boundary conditions due to specific interactions, whereas the faster rotation of R6G is attributed to weaker electrostatic interactions. No significant dependence of the rotational dynamics on the solvent alkyl chain length was observed for any of the three dyes, suggesting that the specific interactions between dyes and RTILs are relatively independent of this solvent parameter.

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INTRODUCTION

It is well-known that the length of the alkyl chain, either on the cation or the anion of RTILs, can affect both the physical and chemical properties of RTILs. A longer cation side chain is commonly accompanied by lower density, slower diffusion, and higher viscosity.14−16 Recent studies suggested that the effect of alkyl chain length on the rotational dynamics of dyes in RTILs is complicated.17−24 By studying the fluorescence anisotropy decay of a nonpolar solute perylene and the triple negatively charged solute, 8-methoxypyrene-1,3,6-sulfonate (MPTS) in 1alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Cnmim][Tf2N], n = 2, 4, 6, 8), Fruchey and Fayer have shown that the rotation of perylene follows slip hydrodynamics in [C2mim][Tf2N] and reaches subslip behavior in [C8mim][Tf2N].17 The observed results were rationalized as due to the alkane-like environment that the solute experienced for longerchain RTILs. The triply charged MPTS exhibited a more than 25% increase in the friction coefficient from [C2mim] to [C6mim], which was explained using the solventberg model. In a more recent study, Das and Sarkar18 observed a similar trend for coumarin 153 (C153) and 4-aminophthalimide (AP) in imidazolium alkylsulfate, in which they varied the alkyl chain length of the anion. The rotation of C153 and AP slowed with

Room-temperature ionic liquids (RTILs) have received intense interest in recent years due to their desirable properties such as negligible volatility, high electrochemical and thermal stability, and inherent ionic conductivity.1−11 Because a wide-ranging selection of cations and anions comprise RTILs, the specific physical properties of particular RTILs can be strongly affected by interionic interactions; these are complex due to numerous possible combinations of different cations and anions. A detailed understanding of the interionic interactions as well as solute−RTIL interactions is critical to not only offer new insight for physical properties but also to aid design of new materials. Despite broad interest in RTILs, how chemical structures and intermolecular interactions affect microscopic dynamics, that is, diffusional and rotational dynamics, remains relatively unexplored.12 A deeper understanding of solute− solvent interactions can provide information on intermolecular couplings, solvation modes, and related phenomena. Although a wealth of information is available on the rotational dynamics of many solutes in different conventional solvents, studies of rotational dynamics of solutes in RTILs are rather limited. The “dual nature” of RTILs13 and the interplay between different forces can complicate the understanding of intermolecular interactions, and a close integration of different experimental and computational techniques is therefore required. © 2014 American Chemical Society

Received: October 31, 2013 Revised: January 5, 2014 Published: January 8, 2014 1088

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chromophore/luminophore contamination. Inert atmospheres, careful control of temperature, and protection from light were implemented for all syntheses. To prepare a given sample of the dye (R6G, RhB, or SR640) in [Cnmpyr][Tf2N] for TRFAD experiments, a small volume of 5 × 10−4 M ethanolic dye stock solution was micropipetd into an appropriate volume of [Cnmpyr][Tf2N] to obtain a 2 × 10−6 M dye concentration. The resulting solution was vacuum pumped overnight and was then transferred into a 1 cm path length sample cuvette. Time-Resolved Fluorescence Anisotropy Decay Kinetics. The TRFAD apparatus consisted of a visible collinear optical parametric amplifier (OPA 9400, Coherent Lasers, Santa Clara, CA) pumped by a Ti:sapphire regenerative amplifier laser (RegA 9000, Coherent Lasers) at a 250 kHz repetition rate, along with a single-photon counting fluorescence detection setup. The output beam power from the amplifier was about 1.2 W, which was used to produce a wavelength-tunable OPA output in the 490−700 nm spectral range. Excitation was at 532 nm with a 150 fs fwhm typical pulse duration and 10, an alkyl chain length dependence was noted.24 For the case of N-alkyl-N-methylmorpholinium bis(trifluoromethylsulfonyl)imides, an alkyl chain length dependence was observed even though the alkyl chain length was varied only from ethyl to octyl.22,23 In the current study, we cannot exclude the effects of organized structures in Nalkylpyrrolidinium bis(trifluoromethylsulfonyl)imide RTILs. However, the weakly associating character of the [Tf2N]− anion and the limited chain length range used in this study may have contributed to relatively disorganized solvent structures.

making them slightly weaker acids even than water. Given the lack of strongly acidic sites available on the ions comprising the RTILs, the sulfonate groups will remain fully dissociated. Therefore, although the sulfonates can certainly participate in hydrogen bonding by acting as good hydrogen bond acceptors, they cannot act in the capacity of hydrogen bond donors. In light of the discussion of the hydrogen bonding capability of [Cnmpyr][Tf2N] above, it is no surprise that some degree of hydrogen bond formation between the charged R6G and SR640 with [Cnmpyr][Tf2N] exists. However, the mixture of electrostatic interactions and hydrogen bond formation is a delicate balance. Besides a Coulombic interaction between the R6G cation and the [Tf2N]− anion, a hydrogen bond can also occur between the N−H of R6G and the [Tf2N]− anion. However, a lower boundary condition parameter observed relative to SR640 and the ionic liquids indicates that the Coulombic interaction between R6G and the [Tf2N]− anion is not as strong as that between SR640 and the [Cnmpyr]+ cation. Although hydrogen bonding also is present between R6G and SR640 and ionic liquids, it may not be as strong as the Coulombic interaction. In the event that there is no Coulombic interaction as for RhB and ionic liquids, the hydrogen bond interaction will dominate the rotational dynamics. RTIL Alkyl Chain Length Effect. To help understand the effect of alkyl chain length on the observed boundary condition parameters, plots of Cobs versus the number of carbon atoms in the RTIL alkyl chain for all three dyes are shown in Figure 5.

Figure 5. Plot of Cobs for R6G (empty squares), RhB (empty circles), and SR640 (empty triangles) versus the number of carbon atoms in the alkyl chain attached to the solvent pyrrolidinium ring, as shown in Figure 1. The horizontal lines in the figure are drawn to illustrate trends.

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CONCLUSIONS In this study, we have carefully examined the rotational dynamics of three representative rhodamine dyes with different formal charges in a series of pyrrolidinium bis(trifluoromethylsulfonyl)imide ([Cnmpyr][Tf2N]) RTILs with paraffinic side chain lengths of n = 3, 4, 6, 8, and 10. The rotation of neutral RhB lies between the slip and stick boundary conditions, which can be rationalized by the existence of hydrogen bonding between RhB and the [Cnmpyr]+ cation. The rotational dynamics of the negatively charged SR640 follow stick boundary conditions, while the rotation of the positively charged R6G is proximal to the stick boundary limit but still falls between the slip and stick limits. This is probably due to weaker Coulombic interaction between the R6G cation and [Tf2N]− relative to the interaction between the SR640 anion and the [Cnmpyr]+ cation. Within our measurement

Cobs for the same dye is almost constant with increasing alkyl chain length for all three of the differently charged dyes. Each symbol in Figure 5 represents an average of five measurements made at the five experimental temperatures. The variation of the points within each dye/carbon number set can be used to approximate error bars for each plotted point. There is no particular Cobs trend with changing sample temperature for each cluster of points, and the range of Cobs values for each cluster is approximately ±15% of the mean. This value represents a lower bound for rigorous error bars. Given this uncertainty, all that can be stated regarding the dependence on the cation alkyl chain length is that there is no significant change in Cobs. The effect of alkyl chain length on rotational dynamics has been a subject of extensive discussion in recent years, with the 1094

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precision, there is no significant dependence of Cobs for any of the three rhodamine dyes on the alkyl chain length of the [Cnmpyr]+ cation, consistent with recent findings by the Dutt group. An examination of Kamlet−Taft acidity and basicity for [Cnmpyr][Tf2N] suggests that the hydrogen bond donating and accepting abilities remain almost the same for ionic liquids with different alkyl chain lengths, yielding fairly similar rotational hydrodynamics spanning the five ionic liquid homologues studied herein.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

The power law fitting parameters for the traces shown in Figure 4 are tabulated. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Phone: 865-574-4920. Present Address #

P.C.H.: Department of Chemistry, Mississippi State University, Starkville, MS 39762. E-mail: phillesheim@chemistry. msstate.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported as part of the Fluid Interface Reactions, Structures, and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

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REFERENCES

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