J. Phys. Chem. B 2001, 105, 9663-9668
9663
The Cybotactic Region Surrounding Fluorescent Probes Dissolved in 1-Butyl-3-methylimidazolium Hexafluorophosphate: Effects of Temperature and Added Carbon Dioxide Sheila N. Baker, Gary A. Baker, Maureen A. Kane, and Frank V. Bright* Department of Chemistry, UniVersity at Buffalo, The State UniVersity of New York, Buffalo, New York 14260-3000 ReceiVed: January 26, 2001; In Final Form: July 20, 2001
We report on the local microenvironment that surrounds three fluorescent solutes (i.e., the cybotactic region) when they are dissolved in a 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) roomtemperature ionic liquid as a function of temperature and added CO2 (T ) 308 K; P ) 0-150 bar). In dry [bmim][PF6] at 293 K, the cybotactic region exhibits a dielectric constant and refractive index of 11.4 ( 1.0 and 1.523 ( 0.025, respectively. The activation energy that describes the [bmim][PF6] viscous flow is 38.4 ( 0.9 kJ mol-1. The activation energy for solute rotational reorientation in [bmim][PF6] is equivalent to the activation energy for [bmim][PF6] viscous flow, indicating that solute rotational dynamics are correlated entirely with the [bmim][PF6] dynamics. There is nanosecond dipolar relaxation surrounding a solute dissolved in dry [bmim][PF6] at 293 K. Even though CO2 is highly soluble in [bmim][PF6] (CO2 mole fraction ) 0.6 at 313 K and 68 bar), addition of up to 150 bar CO2 to [bmim][PF6] at 308 K causes the solute’s cybotactic region dipolarity to decrease by less than 15%. At a fixed temperature (308 K), we observe a 5-fold decrease in the apparent [bmim][PF6] bulk viscosity between 0 and 150 bar CO2.
Introduction Volatile organic compounds (VOCs) are common industrial solvents and worldwide VOC usage exceeds 5 billion dollars per year.1 Despite their widespread use, it is clear there are serious drawbacks associated with continued VOC use. Given this, there is substantial economic, environmental, political and social pressures to develop environmentally friendly alternatives to replace VOCs.2,3 Two approaches have emerged separately as replacement strategies: supercritical fluids (SFs)4 and room-temperature ionic liquids (RTILs).1,5 Blanchard et al.6 successfully combined these strategies by demonstrating the near quantitative extraction of naphthalene with CO2 from a 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) RTIL without extracting any of the RTIL. Most recently, Kazarian et al.7 used infrared spectroscopy to explore the intermolecular interactions between CO2 and the [bmim][PF6] RTIL. The results of this work were consistent with Lewis acid-base type interactions between CO2 and [PF6]- (a weak Lewis base). The results also showed that CO2 exhibits a solubility of 0.6 mole fraction in [bmim][PF6] at 313 K and 68 bar. In this paper we report on the local microenvironment that surrounds a solute (i.e., the cybotactic region) when it is dissolved in [bmim][PF6] as a function of temperature and added CO2. Toward these ends, we report on the fluorescence from pyrene, PRODAN (6-propionyl-2-(N,N-dimethylamino)naphthalene), and BTBP (N,N′-bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylenedicarboximide) dissolved separately at low concentration in [bmim][PF6]. In pyrene, solvent-induced perturbations of the π-electronic orbitals by Herzberg-Teller symmetry * To whom correspondence should be addressed. Voice: (716) 6456800 ext 2162. FAX: (716) 645-6963. E-mail:
[email protected].
distortions influence the B2u and B1u interstate coupling efficiency, resulting in a solvent-dependent I1/I3 emission band ratio.8 PRODAN is among a number of solvatochromic fluorescent probes,9 and its spectra shift in a predictable manner depending on the solvent. BTBP is a large fluorophore (molar volume ) 733 Å3/molecule), and its rotational reorientation dynamics are well described by a single rotational reorientation time that depends on the solvent viscosity.10 Theory The emission spectra of probes like pyrene and PRODAN “shift” on the basis of changes in the solvent dielectric constant () and refractive index (n). The pyrene I1/I3 band ratio is related to and n by11
I1/I3 ) A + Bf(,n2)
(1)
where f(,n2), the dielectric cross term, is given by
f(,n2) ) [( - 1)/(2 + 1)][(n2 - 1)/(2n2 + 1)]
(2)
The PRODAN Stokes shift (SS, difference between the absorbance and emission maxima in cm-1) depends on the solvent and n through the Lippert-Mataga expression:12
SS )
2∆f (µE - µG)2 + const hca3
(3)
In this expression, h is Planck’s constant, c is the speed of light, a is the cavity radius swept out by the PRODAN molecule, µE is the PRODAN’s excited-state dipole moment, µG is PRODAN’s ground-state dipole moment, and ∆f, the solvent’s orientational polarizability, is given by
10.1021/jp0103528 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/31/2001
9664 J. Phys. Chem. B, Vol. 105, No. 39, 2001
∆f ) [( - 1)/(2 + 1)] - [(n2 - 1)/(2n2 + 1)]
Baker et al.
(4)
The BTBP rotation dynamics are described by an isotropic rotor model10 and a single rotational reorientation time (φ). The Debye-Stokes Einstein expression:13
φ)
ηV RT
(5)
provides a link between φ, the solvent viscosity η, and the volume of the reorienting unit V. T and R are the Kelvin temperature and gas constant, respectively. Experimental Section Materials and Reagents. BTBP, pyrene, 1-methylimidazole, 1-chlorobutane, ethyl acetate, and hexafluorophosphoric acid were purchased from Aldrich and used a received. PRODAN was from Molecular Probes. Supercritical fluid chromatographic grade CO2 was a product of Scott Specialty Gases. [bmim][PF6] Synthesis and Characterization. Preliminary experiments were performed with [bmim][PF6] synthesized following the protocol given by Rogers and co-workers.14 The product purity was confirmed by 1H NMR. Subsequent experiments used electrochemical grade, 99+ % purity ( 515 nm). At high frequencies, the phase
Figure 2. Effects of temperature on the PRODAN emission spectra dissolved in [bmim][PF6]. (A) Emission maxima. (B) Emission full width at half-maximum.
Figure 3. Multifrequency phase-modulation data for PRODAN dissolved in [bmim][PF6] at 293 K when we monitor the entire emission profile (λem > 400 nm) and the emission red edge (λem > 515 nm). The solid trace represent the best fit of the data to a triple exponential decay law. The recovered kinetic terms are collected in Table 2.
TABLE 2: Recovered Intensity Decay Kinetics for PRODAN Dissolved in [Bmim][PF6] at 293 Ka wavelength (nm)
τ1 (ns)
τ2 (ns)
τ3 (ns)
R1
R2
χ2
>400 nm >515 nm
>1000 >1000
3.83 3.67
0.288 0.847
-0.001 0.001
1.50 3.52
1.113 1.184
a
ΣRi ) 1.00
angle for the red edge trace clearly exceeds 90° (- - ), indicating an excited-state reaction. The parameters that described the PRODAN/[bmim][PF6] fluorescence intensity decay kinetics are given in Table 2. These results are in line with nanosecond dipolar relaxation of the [bmim][PF6] solvent surrounding PRODAN at 293 K. The details of this process are under further investigation in our laboratories. Pyrene Dissolved in [bmim][PF6]. Figure 4 presents the I1/I3 vs f (,n2) plot for pyrene dissolved in a number of pure liquids (O) and [bmim][PF6] (0) at 293 K. In contrast to the PRODAN results, these data suggest that the cybotactic re-
9666 J. Phys. Chem. B, Vol. 105, No. 39, 2001
Baker et al.
Figure 4. Pyrene I1/I3 vs dielectric cross term in a number of normal liquids (O) and bmim][PF6] (0) at 293 K.
Figure 6. Effects of temperature on the rotational reorientation dynamics of BTBP dissolved in [bmim][PF6]. (A) Effects of temperature on the BTBP rotational reorientation time (φ). (A, inset) Arrhenius plot. ( B) Effects of temperature on the BTBP apparent molar volume. Figure 5. Effects of added CO2 on the pyrene I1/I3 in [bmim][PF6] at 308 K. The arrows on the right edge of Figure denote the pyrene I1/I3 values recorded in liquid DMSO, D2O, and H2O at 308 K.
gion surrounding pyrene is more dipolar than even water (I1/I3 RTIL ∼ 2.08 ( 0.01; water ∼ 1.91) and is instead similar to that of DMSO (2.07). This result is also in contrast to reports from Gra¨tzel and co-workers18 on pyrene dissolved in [emim]+ [Tf2N]- (I1/I3 ) 1.18 ( 0.07). To reconcile the apparent contradiction between the PRODAN and pyrene results, we used the apparent ∆f (0.203) and f (,n2) (0.102) and simultaneously solved eqs 2 and 4 for and n. The results of this exercise yield ) 11.4 ( 1.0 and n ) 1.523 ( 0.025 which agree reasonably well with previous independent results which suggest an upper limit near 10 for and reported values of n between 1.40 and 1.45 for a variety [bmim] cation-based RTILs.18 Figure 5 summarizes the effects of added CO2 (T ) 308 K) on the pyrene I1/I3 in a [bmim][PF6] RTIL. For comparison, the pyrene I1/I3 in neat supercritical CO2 at 313 K over this same pressure range changes from ∼0.65 at low CO2 densities to ∼0.95 at higher CO2 densities. The results in Figure 5 show that the pyrene I1/I3, as expected, decreases with added CO2; however, the decrease is relatively small (2.02 to 1.92) even though the [bmim][PF6] RTIL is taking up a significant amount of CO2. (Note: At 68 bar and 313 K, Kazarian et al. estimated the solubility of CO2 in [bmim][PF6] to be 0.6 mole fraction.)7 These results argue that the intermolecular interactions between pyrene and the RTIL are “disrupted” by the addition of CO2, but not significantly so. These results also reveal that the largest change on adding CO2 is observed between 0 and 70 bar. Rotational Mobility of BTBP Dissolved in [bmim][PF6]. We questioned how the [bmim][PF6] RTIL influenced the rotational dynamics of a simple isotropic rotor. Noe¨l et al.20 used EPR spectroscopy to investigate the rotational dynamics of 2,2,6,6-tetramethylpiperidine-1-oxyl (tempo) and 4-amino2,2,6,6-tetramethylpiperidine-1-oxyl (tempoamine) dissolved in RTILs composed of [emim]+ Cl- and AlCl3. In a basic melt, the tempoamine rotational reorientation time was about 27-fold
greater than tempo in the same RTIL. The authors explained this result in terms of a solvated cation, forming an adduct with the solvent (i.e., [tempamine-emim]+), which “drags” along solvent ions and reorients more slowly. Figure 6 summarizes the effects of temperature on the BTBP rotational reorientation dynamics (φ) in [bmim][PF6]. Figure 6A shows that the BTBP mobility increases with increasing temperature, the dynamics follow a simple Arrhenius model (inset), and the activation energy for rotational reorientation Er (38.8 ( 1.9 kJ/mol) is statistically equivalent at the 95% confidence level to the [bmim][PF6] Eη. These results argue that the BTBP rotational dynamics in neat [bmim][PF6] arise completely from the [bmim][PF6] viscous flow. Using the data from Figure 6A and the viscosities from Table 1, we estimated the apparent V of the BTBP species reorienting within the [bmim][PF6] RTIL as a function of temperature. The results of this exercise are presented in Figure 6B. Two aspects of these data merit further discussion. First, over the temperature range studied, the apparent V is significantly larger than the reported10a,b BTBP molar volume (733 Å3/molecule). This result is consistent with a solvent/ion attachment scenario as proposed by Noe¨l et al.;20 however, unlike tempo and tempoamine, BTBP is nonpolar and it is uncharged. Second, as temperature increases, the apparent V increases and levels off between 320 and 335 K to a value near 2100 Å (ca. 3 times the BTBP molar volume). This result suggests that there is greater ion association/attachment with/to the BTBP at elevated temperatures. (A single layer of [bmim]+ or [PF6]- associating with BTBP could readily account for such an elevated V.) If there were a simple ion attachment model operating alone, one would anticipate that the BTBP-ion(s) interactions would decrease with increasing temperature and one would expect to see the apparent V (actually a weighted average of all forms of BTBP in various solvation states) decrease with increasing temperature. We suggest that the results in Figure 6B arise from at least two competing equilibria: (1) intermolecular interactions between the RTIL components proper ([bmim]+ and [PF6]-)
Dissolved Fluorescent Probes
J. Phys. Chem. B, Vol. 105, No. 39, 2001 9667 [bmim][PF6] RTIL viscous flow is 38.4 ( 0.9 kJ mol-1 which is about 9 kJ mol-1 less than another [bmim] cation-based RTIL with a triflate anion,18 illustrating that the anion plays a significant role. The activation energy for BTBP rotational reorientation in a [bmim][PF6] RTIL is equivalent to the activation energy for [bmim][PF6] viscous flow, indicating that solute dynamics are correlated entirely with the [bmim][PF6] dynamics. The addition of CO2 to [bmim][PF6] leads to a decrease in the local dipolarity surrounding pyrene, but the decrease is only about 10-15% relative to the value seen for pyrene dissolved in neat CO2. Finally, the addition of CO2 to [bmim][PF6] leads to a 5-fold increase in the BTBP rotational mobility. Acknowledgment. This work was supported by the U.S. Department of Energy. We also thank Erik Indra at Bausch & Lomb (Rochester, NY) for graciously providing access to their viscometer. References and Notes
Figure 7. Effects of added CO2 on the BTBP dynamics in [bmim][PF6] at 308 K. (A) BTBP rotational reorientation times φ. (B) Estimated [bmim][PF6] viscosity as a function of added CO2 for the case where the BTBP molar volume is 733 or 1850 Å3/molecule.
with one another and (2) intermolecular interactions between the RTIL components and the BTBP. We suggest the following equilibria (stoichiometry not implied):
[bmim]+ + [PF6]- a [bmim][PF6] BTBP + [bmim]+ a [bmim]+-BTBP BTBP + [PF6]- a [PF6]--BTBP
KbP
(6a)
KbB (6b) KPB
(6c)
To explain the data in Figure 6B requires that increasing temperature decreases KbP more rapidly than the corresponding decrease in KbP and/or KPB. Figure 7 summarizes the effects of added CO2 (T ) 308 K) on the BTBP rotational dynamics in a [bmim]+[PF6]- RTIL. Figure 7A shows that φ decreases by about 5-fold between 0 and 150 bar CO2. The drop in φ can be assigned to a decrease in V and/or a decrease in the bulk liquid η due to CO2 uptake. Unfortunately, we do not have information on the [bmim]+[PF6]viscosity as a function of added CO2. Given these limitations, we can estimate the [bmim]+[PF6]- viscosity as a function of added CO2 using two limiting cases: (1) V is that of an unsolvated BTBP molecule (733 Å3/molecule)10a,b and (2) V is that of BTBP dissolved in [bmim]+[PF6]- at 308 K (1850 Å3/ molecule) without CO2. We further assume that the V does not change with added CO2. The small change in the pyrene I1/I3 with added CO2 (Figure 5) supports the latter assumption. The results of this exercise are presented in Figure 7B as the limiting ranges over which the [bmim]+[PF6]- viscosity might change upon adding CO2 at 308 K. Conclusions The cybotactic region surrounding three solutes dissolved within a [bmim]+[PF6]- RTIL have been measured. At 293 K the local dielectric constant and refractive index are 11.4 ( 1.0 and 1.523 ( 0.025, respectively. The activation energy for the
(1) (a) Seddon, K. R.; J. Chem. Technol. Biotechnol. 1997, 68, 351. (b) Freemantle, M. Chem. Eng. News 2001, 79, 21. (2) (a) Noble, D. Anal. Chem. 1993, 65, 693A. (b) Via, J.; Taylor, L. T. CHEMTECH 1993, p 38. (c) A presidential directive published in the Federal Register (S8 FR: 65018. Dec 10, 1993) implemented January 1, 1996. Federal Register; U.S. Government Printing Office: Washington, DC, 1993. (3) (a) Hester, R. E., Harrison, R. M., Eds. Issues in EnVironmental Science and Technology. Volatile Organic Compounds in the Atmosphere; Royal Society of Chemistry, London, UK 1995; Issue 4. (b) Shen, T. T.; Schmidt, C. E.; Card, T. R. Assessment and Control of VOC Emission; Wiley & Sons: New York, 1997. (c) Rafson, H. J. Odor and VOC Control Handbook; McGraw-Hill: New York, 1998. (d) Placet, M.; Mann, C. O.; Gilbert, R. O.; Niefer, M. J. Atmos. EnViron. 2000, 34, 2183. (4) (a) Johnston, K. P., Penninger, J. M. L., Eds. Supercritical Fluid Science and Technology; ACS Symposium Series 406; American Chemical Society: Washington, DC, 1989. (b) Bruno, T. J.; Ely, J. F., Eds. Supercritical Fluid TechnologysReViews in Modern Theory and Applications; CRC Press: Boca Raton, FL, 1991. (c) Bright, F. V., McNally, M. E. P., Eds Supercritical Fluid TechnologysTheoretical and Applied Approaches in Analytical Chemistry; ACS Symposium Series 488; American Chemical Society: Washington, DC, 1992. (d) Kiran, E., Brennecke, J. F., Eds. Supercritical Fluid Engineering SciencesFundamentals and Applications; ACS Symposium Series 514; American Chemical Society: Washington, DC, 1993. (e) Noyori, R., Ed. Chem. ReV. 1999, 99, 353-634. (f) Jessop, P. G.; Leitner, W., Eds. Chemical Synthesis Using Supercritical Fluids; Wiley-VCH: New York, 1999. (5) (a) Hussey, C. L. Pure Appl. Chem. 1988, 60, 1763. (b) Chauvin, Y.; Olivier,-Bourbigou, H. CHEMTECH 1995, 25, 26. (c) Welton, T. Chem. ReV. 1999, 99, 2071. (d) Rooney, D. W.; Seddon, K. R. In Handbook of SolVents; Wypych, G., Ed.; William Andrew: Toronto, 2000; pp 14591484. (6) Blanchard, L. A.; Hancu, D.; Beckman, E. J.; Brennecke, J. F. Nature 1999, 399, 28. (7) Kazarian, S. G.; Briscoe, B. J.; Welton, T. Chem. Commun. 2000, 100, 2047. (8) (a) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: New York, 1970. (b) Lochmuller, C. H.; Marshall, D. B.; Wilder, D. R. Anal. Chim. Acta 1981, 130, 31. (c) Lochmuller, C. H.; Marshall, D. B.; Harris, J. M. Anal. Chim. Acta 1981, 131, 263. (d) Dong, D. C.; Winnik, M. Photochem. Photobiol. 1982, 35, 17. (e) Kaufman, V. R.; Avnir, D. Langmuir 1986, 2, 717. (f) Wong, A. L.; Hunnicutt, M. L.; Harris, J. M. Anal. Chem. 1991, 63, 1076. (g) Karpovich, D. S.; Blanchard, G. J. J. Phys. Chem. 1995, 99, 3951. (h) Hautala, R. R.; Schore, N. E.; Turro, N. J. J. Am. Chem. Soc. 1973, 95, 5508. (9) Sun, S.; Heitz, M. P.; Bruckenstein, S.; Perez, S. A.; Colo´n, L. A.; Bright, F. V. Appl. Spectrosc. 1997, 51, 1316 and references therein. (10) (a) Ben-Amotz, D.; Drake, J. M. J. Chem. Phys. 1988, 89, 2. (b) Williams, A. M.; Ben-Amotz, D. Anal. Chem. 1992, 64, 700. (c) Niemeyer, E. D.; Bright, F. V. Macromolecules 1998, 31, 77. (11) (a) Dong, D. C.; Winnik, M. A. Can. J. Chem. 1984, 62, 2560. (b) Waris, R.; Acree, W. E., Jr.; Street, K. W., Jr. Analyst 1988, 113, 1465. (c) Rice, J. K.; Niemeyer, E. D.; Dunbar, R. A.; Bright, F. V. J. Am. Chem. Soc. 1995, 117, 5832.
9668 J. Phys. Chem. B, Vol. 105, No. 39, 2001 (12) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.; Plenum Press: New York, 1999; Chapters 6 and 7. (13) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.; Plenum Press: New York, 1999; Chapters 10-12. (14) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; Rogers, R. D. Chem. Commun. 1998, 98, 1765. (15) (a) Gratton, E.; Jameson, D. M.; Hall, R. D. Annu. ReV. Biophys. Bioeng. 1984, 13, 105. (b) Jameson, D. M.; Gratton, E.; Hall, R. D. Appl. Spectrosc. ReV. 1984, 20, 55. (c) Bright, F. V.; Betts, T. B.; Litwiler, K. S. CRC Crit. ReV. Anal. Chem. 1990, 21, 389. (d) Bright, F. V. Appl. Spectrosc. 1995, 49, 14A. (16) Gordon, C. M.; McLean, A. J. Chem. Commun. 2000, 100, 1395.
Baker et al. (17) (a) Seddon, K. R.; Stark, A.; Torres, M. J. ACS AdV. Chem. Ser. 2001. In press. (b) Seddon, K. R.; Stark, A.; Torres, M. J. Pure Appl. Chem. 2000, 72, 2275. (18) Bonhoˆte, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Gra¨tzel, M. Inorg. Chem. 1996, 35, 1168. (19) (a) Badea, M. G.; De Toma, R. P.; Brand, L. Biophys. J. 1978, 24, 197. (b) Lakowicz, J. R.; Bevan, D. R.; Maliwal, B. P.; Cherek, H.; Balter, A. Biochemistry 1983, 22, 5714. (c) Lakowicz, J. R.; Cherek, H.; Lazcko, G.; Gratton, E. Biochim. Biophys. Acta 1984, 777, 183. (20) Noe¨l, M. A. M.; Allendoerfer, R. D.; Osteryoung, R. A. J. Phys. Chem. 1992, 96, 2391.
