Technical Notes Anal. Chem. 1996, 68, 918-920
Dielectric Properties of a Binary Cryoelectrochemical Solvent According to a Solvatochromic Probe Michael T. Carter,† Robert A. Osteryoung,‡ and Royce W. Murray*
Kenan Laboratories of Chemistry, CB #3290, University of North Carolina, Chapel Hill, North Carolina 27599
The application of a solvatochromic dye to determination of the dielectric properties of organic solvent mixtures is described. The dye 2,6-diphenyl-4-(2,4,6-triphenylpyridino)phenolate (Reichardt’s dye) features an intramolecular charge transfer band in its visible spectrum with a large associated change in dipole moment between the ground and excited states. The position of the charge transfer band is sensitive to local dielectric environment and so is an indicator of solvation of the dye. Room-temperature spectra of Reichardt’s dye in the cryoelectrochemical solvent system 2:1 (v/v) ethyl chloride/butyronitrile and in the neat components are reported and related to the dielectric properties of the mixture. The effective dielectric properties of the mixture scale with the value of the Pekar factor, 1/EOP - 1/ES, that is predicted from the mole fraction-weighted (or volume fraction-weighted) sum of the dielectric constants for the pure components, suggesting that strongly selective solvation of the dye by one component of the solvent mixture does not occur. The results are pertinent to a recent study of electrode kinetics in the solvent mixture. reports1-7
In recent we described the electrochemical behavior of self-assembled monolayers (SAMs) composed of mixed ω-ferrocenylcarboxy- and alkanethiols on bulk gold and on metal-coated superconducting electrodes at ultralow temperatures. The cryo† Present Address: Oak Ridge National Laboratory, Chemical and Analytical Sciences Division, Bldg. 4500S, Oak Ridge, TN 37831-6142. ‡ Department of Chemistry, Box 8204, North Carolina State University, Raleigh, NC 27695-8204. (1) Rowe, G. K.; Carter, M. T.; Richardson, J. N.; Murray, R. W. Langmuir 1995, 11, 1797-1806. (2) Carter, M. T. ; Rowe, G. K.; Richardson, J. N.; Tender, L. M.; Terrill, R. H.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 2896-2899. (3) Richardson, J. N.; Rowe, G. K.; Carter, M. T.; Tender, L. M.; Curtin, L. S.; Peck, S. R.; Murray, R. W. Electrochim. Acta 1995, 40, 1331-1338. (4) Peck, S. R.; Curtin, L. S.; Tender, L. M.; Carter, M. T. ; Terrill, R. H.; Murray, R. W.; Collman, J. P.; Little, W. P.; Duan, H. M.; Dong, C.; Hermann, A. M. J. Am. Chem. Soc. 1995, 117, 1121-1126. (5) Richardson, J. N.; Peck, S. R.; Curtin, L. S.; Tender, L. M.; Terrill, R. H.; Carter, M. T.; Murray, R. W.; Rowe, G. K.; Creager, S. E. J. Phys. Chem. 1995, 99, 766-772. (6) Tender, L. M.; Carter, M. T. ; Murray, R. W. Anal. Chem. 1994, 66, 31733181. (7) Curtin, L. S.; Peck, S. R.; Tender, L. M.; Murray, R. W.; Rowe, G. K.; Creager, S. E. Anal. Chem. 1993, 65, 386-392.
918
Analytical Chemistry, Vol. 68, No. 5, March 1, 1996
solvent employed8 was a 2:1 (v/v) mixture of ethyl chloride (EtCl) and butyronitrile (PrCN), containing tetra-n-butylammonium hexafluorophosphate (Bu4NPF6) as supporting electrolyte, that is liquid over a wide range of subambient temperatures. SAMs offer an attractive avenue to study electron transfer dynamics, by eliminating diffusional contributions to the measured current, by decreasing nonfaradaic currents by lowering the interfacial capacitance, and by allowing control of the architecture of the electrochemical interface through choice of redox head group, alkane spacer length, and surface coverage dilution with alkanethiol (Figure 1). An issue of importance, in comparisons of experimental kinetic results1-7 for reorganizational barrier energies2,11,12 to predictions of Marcus dielectric continuum theory13-15 for an “outer-sphere” barrier energy, is the values used in the predictions for the dielectric constants (OP, optical, and S, static) of the solvent medium surrounding the ferrocene moiety16
λOS ) (Ne2/8πO)(1/rO - 1/2d)(1/OP - 1/S)
(1)
The outer sphere reorganizational energy, λOS, controls, in the energetically simplest case, the overpotential of the electron transfer reaction. In the studies in EtCl/PrCN mixed solvent, we assumed1-7 an ideal behavior, namely, that the effective dielectric constants OP and S of the solvent mixture are the mole fractionweighted sums of the corresponding values for the two pure components. In this paper, we seek to verify the assumption of a mole fraction-weighted dielectric continuum, against the eventuality that the EtCl/PrCN solvent exhibits a strong differential solvation of (8) Ching, S.; McDevitt, J. T.; Peck, S. R. ; Murray, R. W. J. Electrochem. Soc. 1991, 138, 2308-2315. (9) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (10) Ridick, J. A.; Bunder, J. B. In Organic Solvents: Physical Properties and Methods of Purification; Weissberger, A., Ed.; Techniques of Chemistry Series 2; Wiley: New York, 1970. (11) Weaver, M. J. Chem. Rev. 1992, 92, 463-480. (12) Liu, Y. P. ; Newton, M. D. J. Phys. Chem. 1994, 98, 7162-7169. (13) Marcus, R. A. J. Phys. Chem. 1963, 67, 853-857. (14) Marcus, R. A. J. Chem. Phys. 1965, 43, 679-701. (15) Chidsey, C. E. D. Science 1991, 251, 919-922. (16) In eq 1, N is Avogadro’s number, e the charge of the electron, O the permitivity of vacuum, rO the radius of the redox center, and d the distance of the redox center from the electrode surface. 0003-2700/96/0368-0918$12.00/0
© 1996 American Chemical Society
Figure 1. Schematic diagram of SAM-coated gold electrode in contact with 2:1 (v/v) EtCl/PrCN. The monolayer is shown tilted 30o off the normal to the gold surface as described previously.9 The alltrans methylene chains of the diluent alkanethiolates are shown as straight lines. (Electroactive SAMs are typically diluted with nonelectroactive alkane thiol to minimize lateral interactions between the redox sites.15) Optical and static dielectric constants are from published compilations;10 those shown for the alkane spacer are for n-dodecane.
ionic species (such as ferricenium), which would invalidate the assumption. Hupp et al.17 have given an example of how selective solvation can influence electron transfer in a mixed solvent. We describe the use of a solvatochromic dye to determine the dielectric properties of the mixed EtCl/PrCN solvent. The dye, 2,6-diphenyl-4-(2,4,6-triphenylpyridino)phenolate (Reichardt’s dye),18,19 features an intramolecular charge transfer band in its visible spectrum with a large associated change in dipole moment between the ground and excited states. The position of the charge transfer band is extremely sensitive to local dielectric environment and thus is an indicator of solvation of the dye. EXPERIMENTAL SECTION Reichardt’s dye (RD) and butyronitrile (both from Aldrich Chemical Co.) were used as received. Ethyl chloride (Matheson) was vapor phase transferred on a vacuum line to the optical cuvette. Standard Pyrex glass 1-cm-square cuvettes were fitted with air-tight, in-line NMR tube valves (Brunfedlt Co.) for vacuum line manipulations. RD was added to the cuvette as an acetone solution; the acetone was removed under vacuum prior to transfer of the solvents. UV/visible spectra were recorded on an HewlettPackard Model 8452A photodiode array spectrometer. RESULTS AND DISCUSSION Room-temperature spectra of RD18,19 are shown in Figure 2 for the cryosolvent mixture (without supporting electrolyte) and for the individual solvent components. The position of the intramolecular charge transfer band (CT) lies at 668 nm (1.856 eV) in the more polar solvent butyronitrile (OP ) 1.91, S ) 24.83) (17) Roberts, J. A.; Bebel, J. C.; Absi, M. L.; Hupp, J. T. J. Am. Chem. Soc. 1992, 114, 7957-7958. (18) (a) Reichardt, C. Angew. Chem., Int. Ed. Engl. 1965, 4, 29-40. (b) Reichardt, C. Angew. Chem., Int. Ed. Engl. 1979, 18, 98-110. (c) Reichart, C. Chem. Rev. 1994, 94, 2319-2358. (19) Zachariasse, K. H.; Phuc, N. V.; Kozankiewicz, B. J. Phys. Chem. 1981, 85, 2676-2683.
Figure 2. Room-temperature visible spectra of RD in the mixed solvent and in its neat components. Dye concentrations: 0.1 mM for PrCN and 2:1 EtCl/PrCN; 0.2 mM for EtCl.
and shifts to 720 nm (1.722 eV) in the less polar component, ethyl chloride (OP ) 1.86, S ) 9.45). The CT band of RD represents an electronic transition which decreases the dipole moment of the molecule in the excited state vs the ground state.18 This shift reflects destabilization of the ground state dipole vs the excited state one in a less polar as compared to a more polar solvent; the energy difference between the two peak positions therefore is a measure of difference in solvation of RD between the more polar PrCN and less polar EtCl. In the 2:1 mixed solvent, the CT band peak lies at an intermediate energy (696 nm, 1.781 eV); i.e., the solvent mixture has dielectric properties intermediate between those of its two components. The room-temperature dielectric constants calculated5 for the mixed solvent based on the ideal mole fraction weighting assumption are OP ) 1.875 and S ) 13.91. The calculated values of the Pekar factors, 1/OP - 1/S (see eq 1), that relate the surrounding dielectric to the energetics of a charge displacement are 0.432, 0.461, and 0.483 for EtCl, the mixed solvent, and PrCN, respectively. If we assume that the energy shift of the CT band scales linearly with the solvent Pekar factor, then from the above values we would predict that the CT band would lie at 689 nm (1.798 eV) in the mixed solvent. The tick on the spectrum in Figure 2 shows that this energy is in fact quite close to the experimental band maximum (1.781 eV). The justification for using the Pekar factor is the parallel between its representation of the energetics of reactant/product electron transfer charge displacement in eq 1 and the charge displacement in RD giving rise to the solvatochromic effect. If the shift in the CT band energy were presumed to scale linearly with the mole fraction of the two solvent components, then given that the mole fraction of ethyl chloride in the 2:1 (v/ v) mixture is 0.71, one predicts that the CT band in the mixed solvent should lie at 682 nm (1.817 eV), which value is indicated Analytical Chemistry, Vol. 68, No. 5, March 1, 1996
919
by the heavy bar on the left-hand side of the spectral peak in Figure 2. The above comparisons show that the energy of the CT transition in the mixed solvent scales much more closely with the Pekar factor predicted from mole fraction-weighting of dielectric parameters than it does with the mole fractions of the mixture’s components.20 Clearly the magnitudes of the dielectric constants of the solvent components must be considered in addition to the simple mole fractions. The consistency of the mole fraction-weighting of dielectric terms with the behavior of the RD solvatochromic dye serves as a diagnostic of selective solvation and is evidence that strongly selective solvation of RD by one of the two solvent components can be ruled out. It remains an assumption, but now a better grounded one, that strongly selective solvation of electrode reaction participants (such as ferrocene or ferricenium in a SAM) also does not occur and that the dielectric properties of the EtCl/ PrCN solvent mixture are adequately described by mole fractionweighted dielectric constants OP and S. Of course the use of (20) (a) An alternative and perhaps more accurate way of modeling dielectric constants for a binary mixture is to employ weighted volume rather than mole fractions, which follows from the use of dipole moment density in treatment of the dielectric parameters. In the present case, it makes little difference. For example, by volume fractions, S ) 14.58, OP ) 1.879, and the Pekar factor is 0.464, which values are minimally changed from those evaluated with mole fraction. The wavelength maximum predicted from volume fractions is 686 nm vs that predicted using mole fractions (689 nm). (b) Hill, N. E. Dielectric Properties and Molecular Behavior; Van Nostrand Reinhold: London, New York, 1969; p 41. (21) Johnson, B. P.; Khaledi, M . G.; Dorsey, J. G. Anal. Chem. 1986, 58, 2354-2365. (22) Fannin, A. A.; Floreani, D. A.; King, L. A.; Sanders, J. J.; Piersma, B. J.; Stech, D. J.; Vaughn, R. L.; Wilkes, J. S.; Williams, J. L. J. Phys. Chem. 1984, 88, 2614-2621. (23) Reichart, C. Solvents and Solvent Effects in Organic Chemistry, 2nd ed.; VCH: New York, 1988; pp 119-120.
920
Analytical Chemistry, Vol. 68, No. 5, March 1, 1996
dielectric constants to describe the solvation properties of mixed solvent systems is always an approximation; a complete description of solute/solvent interactions would include specific chemical interactions (e.g., hydrogen bonding, donor/acceptor pair complexes, etc.) as well as nonspecific (electrical) ones.18 It is also worth adding that using the experimental CT band energy in the solvent mixture to estimate an improved Pekar factor gives a value (0.454) that is little different from the calculated one and would result in lowering the value of λOS calculated from eq 1 by only about 2%. In summary, the experiment reported here provides a simple way to assess the solvation characteristics of electrochemical solvent mixtures. RD has been used previously in the prediction of reaction rates and equilibrium position in mixtures of varying composition,18 determination of solute partitioning in micellar solution,19 and evaluation of polarity and retention characteristics of mixed mobile phases in chromatography.21 The simplicity of this approach should be of interest to electrochemists in work with solvent systems of unknown dielectric properties, e.g., cryoelectrochemical solvents, mixed solvents at ambient temperature, or ambient temperature molten salts.22,23 ACKNOWLEDGMENT This research has been supported by grants from the Office of Naval Research and the National Science Foundation.
Received for review June 19, 1995. Accepted November 27, 1995.X AC950617C X
Abstract published in Advance ACS Abstracts, January 1, 1996.
