Langmuir 1998, 14, 4115-4121
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Electroactive Three-Dimensional Monolayers: Anthraquinone ω-Functionalized Alkanethiolate-Stabilized Gold Clusters Roychelle S. Ingram† and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received January 5, 1998. In Final Form: May 18, 1998 This paper describes the synthesis and electrochemical reactivity of octanethiolate monolayer-protected Au cluster compounds (MPCs) in which some of the original octanethiolate monolayer has been substituted by place-exchange reactions with 1-(1,3-dithiapropyl)anthracene-9,10-dione. The mixed monolayer MPC can contain as many as 25 anthraquinone sites/cluster, which are all electroactive in rapid, successive one-electron reactions. The voltammetric behavior includes mass-transport-controlled charging of the electrical double layers of the Au cores, which are, in effect, diffusing nanoelectrodes. Mediated electrocatalytic reduction of a gem-dinitrocyclohexane by clusters bearing reduced anthraquinone sites yields electrocatalytic currents larger than those generated using freely diffusing anthraquinone radical anion monomer as mediator.
This paper describes the synthesis, spectroscopic characterization, and electrochemical behavior of alkanethiolate monolayer-protected gold cluster (MPC) molecules whose monolayers are mixtures of octanethiolate and an ω-anthraquinone-propanethiolate. We label them threedimensional self-assembled monolayers (3D-SAMs)1 because the monolayer shells are arrayed three-dimensionally around an approximately 2 nm diameter Au nanoparticle in solution, as opposed to the conventional twodimensionality on a large, flat surface (2D-SAMs).2 The cluster molecules are unusual in that each bears multiple anthraquinone (AQ) sites and also contains a Au core that undergoes double-layer charging when the diffusing molecule comes into contact with the working electrode surface. These properties are presented and quantified. The polyelectron voltammetry of the anthraquinone clusters has the characteristics of sequential, one-electron reactions as opposed to a concerted multielectron transfer, as established before for analogous ferrocenated MPCs.3 The cluster surface area-normalized double-layer capacitances resemble those of analogous 2D-SAMs on Au(111) surfaces.4 Synthesis of alkanethiolate monolayer-protected gold clusters was initially reported in ref 5, a report that touched off a vigorous further exploration of Au clusterforming syntheses and of the physical, spectroscopic, † Present address: Corning, Inc., Science & Technology, Chemical Analysis Dept., SP-FR-06, Corning, New York 14831.
(1) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.-H.; Poon, C.-D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S., Jr.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12357-12548. (2) (a) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (b) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989, 28, 506-512. (3) (a) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212-4213. (b) Green, S. J.; Stokes, J. J.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W. J. Phys. Chem. B 1997, 101, 2663-2668. (c) Green, S. J.; Pietron, J. J.; Stokes, J. J.; Hostetler, M. J.; Vu, H.; Wuelfing, P.; Murray, R. W. Submitted. (4) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (5) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802.
thermal, and electronic properties of gold clusters.1,3,6 These MPCs have the attractive characteristics of excellent air and good thermal stability as solids and in solutions, and adequate solubility in nonpolar solvents for characterization by NMR. Significant synthetic steps were taken3a,b,7 in accessing ω-alkanethiolate-functionalized versions of the clusters, among which place exchange reactions have proven highly versatile at producing clusters both poly-homofunctionalized3a,b and polyheterofunctionalized7b with multiple chemical groupings including electroactive ones. The clusters described in this paper (abbreviation AQSC3S:C8 MPCs) were prepared by place exchange of 1-(1,3-dithiapropyl)anthracene-9,10-dione (abbreviation (6) (a) Hostetler, M. J.; Wingate, J. E.; Zhang, C.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17-30. (b) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Reven, L. Langmuir 1996, 12, 1262-1269. (c) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604-3612. (d) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Reven, L. Langmuir 1996, 12, 1262. (e) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B. Chem. Eur. J. 1996, 2, 359. (f) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. Mater. 1996, 8, 428. (g) Ohara, P. C.; Leff, D. V.; Heath, J. R.; Gelbart, W. M. Phys. Rev. Lett. 1995, 75, 3466. (h) Luedtke, W. D.; Landman, U. J. Phys. Chem. 1996, 100, 13323. (i) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (j) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (k) Cleveland, C. L.; Landman, U.; Shafigullin, M.; Stephens, P. W.; Whetten, R. L. Z. Phys. D 1997, 40, 503. (l) Vezmar, I.; Alvarez, M. M.; Khoury, J. T.; Salisbury, B. E.; Whetten, R. L. Z. Phys. D 1997, 40, 147. (m) Johnson, S. R.; Evans, S. D.; Mahon, S. W.; Ulman, A. Langmuir 1997, 13, 51. (n) Badia, A.; Cuccia, L.; Demers, L.; Morin, F.; Lennox, R. B. J. Am. Chem. Soc. 1997, 119, 2682. (o) Yonezawa, T.; Tominaga, T.; Richard, D. J. Chem. Soc., Dalton Trans. 1996, 783. (p) Alvarez, M. M.; Khoury, J. T.; Schaaf, T. G.; Shafigullen, M.; Vezmar, I.; Whetten, R. L. Chem. Phys. Lett. 1997, 266, 91. (q) Yonezawa, T.; Sutoh, M.; Kunitake, T. Chem. Lett. 1997, 619. (r) Buining, P. A.; Humbel, B. M.; Philipse, A. P.; Verkleij, A. J. Langmuir 1997, 13, 3921. (s) Andrews, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (t) Andrews, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323. (7) (a) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Adv. Mater. 1995, 7, 795-797. (b) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175.
S0743-7463(98)00045-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/30/1998
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AQSC3SH) with octanethiolate sites on C8 monolayer-
protected Au clusters (abbreviation C8 MPCs). The extent of place exchange was determined by 1H NMR, with further characterization by UV/vis and FTIR spectroscopies. MPC diffusion coefficients determined from microelectrode voltammetry produced estimates of their hydrodynamic radii, which are helpful in understanding the nature of the dissolved forms of the cluster molecules. Of special interest regarding the polyelectron transfer properties of the anthraquinone clusters are preliminary data on their mediated electrocatalytic reduction of 1,1-dinitrocyclohexane. Experimental Section Chemicals. HAuCl4,8 1-(1,3-dithiapropyl)anthracene-9,10dione,9 and 1,1-dinitrocyclohexane10 were synthesized on the basis of literature methods. (HAuCl4 is of course commercially available, but it is more economical to recycle the precious metal locally.) Tetraoctylammonium bromide (Aldrich), sodium borohydride (Johnson Matthey), dichloromethane (Fisher), toluene (Mallinckrodt), absolute ethanol (AAPER), acetone (Mallinckrodt), and octanethiol (Aldrich) were used as received, while tetrabutylammonium perchlorate (Bu4NClO4, Fluka) and tetrahexylammonium perchlorate (Hx4NClO4, Alfa) were dried under vacuum at 70 °C. Acetonitrile (Fisher) was distilled over calcium hydride. Water was purified with a Barnstead NANOpure system. Synthesis. Gold cluster compounds were synthesized as described before,6a in a modification of the original Brust et al.5 method. Briefly, tetrachloroaurate (aqueous) was phase transferred into toluene using tetraoctylammonium bromide as the phase-transfer reagent. After the organic was removed, a twofold molar excess of octanethiol (relative to Au) was added to it and allowed to react, whereupon an aqueous solution containing a ninefold molar excess of sodium borohydride was added rapidly at room temperature to complete the gold reduction. The solution, which immediately turns dark, was stirred for 3 h; the organic phase was isolated and the toluene removed by rotary evaporation at low heat. The black solid was filtered and washed with 100 mL each of absolute ethanol and acetone. Other studies6a based on thermogravimetric, small angle X-ray scattering and highresolution transmission electron microscopy (HRTEM) of clusters prepared with dodecanethiolate ligands have determined that these reaction conditions produce clusters with a somewhat polydisperse mixture of core sizes and an average core diameter of 2.0 nm. HRTEM and modeling based on a truncated octahedral core shape indicate the mixture is mainly 225 and 314 Au atom cores. We will assume the latter core size for the C8 MPCs, for which each cluster core bears about 91 alkanethiolate chains. Values of cluster solution concentrations are slightly dependent on the preceding assumption but evaluated diffusion coefficients are not.3b Some clusters were prepared as above but carrying out the reduction at 0 °C, in which case (again from previous characterization data6a) the cluster cores produced are smaller and are estimated as about 140 Au atoms with 53 alkanethiolate ligands. These smaller clusters were employed in the electrocatalysis experiments. (8) (a) Handbook of Preparative Inorganic Chemistry; Brauer, G., Ed.; Academic Press: New York, 1965; pp 1054-1059. (b) Block, B. P. Inorg. Synth. 1953, 4, 14-17. (9) Zhang, L.; Lu, T.; Gokel, G. W.; Kaifer, A. E. Langmuir 1993, 9, 786-791. (10) Kaplan, R. R.; Schechter, H. J. J. Am. Chem. Soc. 1961, 83, 3535-3536.
Ingram and Murray Functionalization of both C8 MPC sizes with AQSC3S ligands was performed via place exchange reactions3a,b,7b in which CH2Cl2 solution mixtures of AQSC3SH and the C8 MPC containing the desired mole ratio of AQSC3SH to C8 MPC alkanethiolate chains were stirred for 24 h at room temperature. The AQSC3S: C8 MPC product contains a mixed monolayer of octanethiolate and AQSC3S ligands and was collected by filtration and washed with acetonitrile, absolute EtOH, and acetone, respectively, and analyzed for relative AQSC3S and C8 content by proton NMR. Measurements. The viscosity of 0.1 M Bu4NClO4 in 2:1 (v/v) toluene/CH3CN is 0.51 cP at 297 K, as measured with a Cannon Ubbelohde (size 50) viscometer, having determined the density of this solution by weighing a known volume. Cyclic voltammetry was performed using a locally built potentiostat interfaced with an IBM compatible PC with a 16 bit DAS-HRES A/D, D/A board and locally written software. Platinum working electrodes of variously 1 mm, 28.8 µm, 27.8 µm, and 25.6 µm diameter were polished with 0.5 µm alumina (Buehler) followed by a NANOpure water and EtOH rinse before each experiment. A Pt wire coil and Ag wire served as counter and quasi-reference electrodes, respectively. The cell was sealed to minimize oxygen contamination and solvent evaporation, and solutions were purged with N2 for 20 min and blanketed during experiments with solvent-saturated N2. Electrochemical studies of AQSC3S:C8 MPCs containing 314 core Au atoms were in 0.1 M Bu4NClO4 in 2:1 toluene/CH3CN, and those of AQSC3S:C8 MPCs containing 140 core Au atoms (electrocatalysis experiments) were in 0.05 M Hx4NClO4 in 2:1 (v:v) toluene/DMF. UV/vis spectra in CH2Cl2 solutions were obtained with a ATI Unicam spectrometer, 1H NMR spectra in C6D6 or CD2Cl2 solutions with a Bruker AMX 200 Hz spectrometer, and FTIR spectra of drop cast thin films on a KBr plate with a BioRad 6000 spectrometer.
Results and Discussion Spectroscopic Characterization of AQSC3S:C8 Clusters. Following earlier procedures,3a,b,7b the extent of place exchange of octanethiolate on C8 MPCs with AQSC3S ligands was determined with proton NMR. For example, in one experiment a 1:3 mole feed ratio of AQSC3SH to alkanethiolate chains on the C8 MPC in the exchange solution resulted in a 1:2.6 mole ratio of AQSC3S to octanethiolates on the product cluster, as determined from the ratio of peak integrals of the anthraquinone and methyl proton NMR resonances (Supporting Information, Figure S1). On the basis of an average 314 Au atom core bearing approximately 91 ligands, this corresponds to an average of approximately 25 anthraquinones per cluster. In addition to the mild polydispersity of Au core sizes present,6a some dispersity undoubtedly also occurs in the number of octanethiolate ligands replaced on individual clusters by AQSC3S ligands; there is no analysis yet available of the extent of this compositional dispersity. We have previously established7b that the extent of place exchange of ω-functionalized thiols onto alkanethiolateprotected Au clusters depends on the steric bulk of the ω-functional group as well as on the relative chain lengths of the protecting and incoming ligands. Comparing the present results to analogous exchanges3 with ω-ferrocenyloctanethiol, the AQSC3S ligand exchanges more readily onto C8 MPCs than does the FcC8S ligand, even though the linker chain of the AQSC3S ligand is shorter than the surrounding octanethiolate units. Apparently the planar shape of the AQSC3S ligand, relative to the more spherically bulky ferrocene moiety, is a significant steric factor in this case. NMR resonance peaks of the monolayers on Au clusters are broadened by a variety of mechanisms, including dipolar spin relaxation in the densely packed regions nearest the thiolate/Au interface, chemical shift differences of different Au surface binding sites, and spin lattice relaxation.1,6b Species that are not bound to the clusters
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Figure 1. UV/vis spectrum of (a) 28 µM AQSC3SH, (b) 3.6 µM C8 MPC (Au314), and (c) 3.0 µM 1:2.6 AQSC3S:C8 MPC (75 µM AQ sites, Au314) in CH2Cl2.
in their solutions readily show up as much sharper resonance peaks. Comparisons of proton NMR of AQSC3SH and AQSC3S:C8 MPC solutions (Supporting Information, Figures S2 and S1, respectively) show that the latter exhibits only solvent impurity sharp peaks and none corresponding to unbound thiol. The methylene and AQ resonances of the cluster-bound AQSC3S ligand appear as highly broadened peaks at approximately 3.5 and 7.6 ppm, respectively. The peak shape of the latter, broadened AQ ring resonance, relative to those in unbound AQSC3SH (Figure S2), suggests that the protons on the part of the AQ ring system most remote from the Au surface are least broadened, implying that dipolar spin relaxation caused by the dense chain packing is the principal broadening mechanism. FTIR spectra of drop cast (solid state) films of AQSC3S: C8 MPCs (Supporting Information, Figure S3) show the carbonyl stretch ν(CdO) at 1730 cm-1; this position is shifted from that (1720 cm-1) of the free thiol AQSC3SH. The absence of ν(SsH) vibrations between 2600 and 2800 cm-1 confirms that the cluster samples contain no free AQSC3SH. The CsH methylene stretching vibrations provide a signature11 for the degree of chain ordering; crystalline polyethylene (2850, 2920 cm-1) and liquid heptane (2855, 2924 cm-1) reflect the extremes of order/ disorder. These bands for AQSC3S:C8 MPCs lie at 2850 and 2920 cm-1, which shows that the previously observed6c high level of chain ordering (trans conformation) in solidstate C8 MPCs is not seriously disturbed by the presence of the AQSC3S ligands. On the other hand, recent FTIR measurements12 on MPC solutions indicate that dissolution causes the cluster chains to become a much more fluid-like monolayer. Figure 1 shows UV/vis spectra of solutions of AQSC3SH thiol, C8 MPC, and AQSC3S:C8 MPC. The 446 nm λmax of AQSC3SH is, as a AQSC3S:C8 ligand, slightly redshifted (to 464 nm). The areas under the free and ligated anthraquinone absorbance peaks are 0.74:1, which, considering the uncertainty in the spectral baseline for the latter, implies that little change occurs in the oscillator strength of the anthraquinone transitions. The C8 MPC solution exhibits a surface plasmon band at 514 nm (11) Maroncelli, M.; Qi, S. P.; Strauss, H. L.; Snyder, R. G. J. Am. Chem. Soc. 1982, 104, 6237-6247. (12) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906-1911.
Figure 2. Cyclic voltammetry of 62 µM 1:2.6 AQSC3S:C8 MPC (Au314) in 2:1 (v/v) toluene/MeCN/0.05 M Bu4NClO4 at (A) a 1 mm diameter Pt disk, 100 mV/s, ∆EPEAK ) 140 mV, and (B) a 27.8 µm diameter Pt disk, 20 mV/s, ilim ) 1.9 nA.
(characteristically weak,13 for a small Au core size) that, on the basis of a second derivative of the absorbance spectrum, may have become somewhat red-shifted in the mixed monolayer AQSC3S:C8 MPC, but the unfortunate overlap with the anthraquinone absorbance makes this effect difficult to quantitate. Surface plasmon peaks of metal clusters are known to be sensitive13c to the local dielectric medium. AnthraquinoneVoltammetry in AQSC3S:C8 Cluster Solutions. The electrochemical behavior of cluster molecules and of redox-substituted cluster molecules is of interest with respect to both the double-layer charging of the cluster metal core and the electrode reactions of polyelectron donors and acceptors. This section considers anthraquinone polyelectron transfers of the AQSC3S:C8 MPCs. Anthraquinone voltammetry is known to be solventdependent, occurring by successive one-electron reductions in aprotic solvents to the radical anion and dianion in waves typically separated by 0.4 to 0.5 V depending on solvent and electrolyte.14 In aqueous acid, protonation of the radical anion collapses the reaction to a single twoelectron (ECE) process which lies at -0.1 V versus SSCE.9,14c The AQSC3S:C8 MPCs are not soluble in polar solvents and were investigated in an aprotic mixed solvent system (2:1 (v/v) toluene/MeCN with 0.05 M Bu4NClO4 electrolyte), which is a compromise between adequate MPC and electrolyte solubilities. In this solvent the reduction waves for free AQSC3SH thiol lie at -0.8 and -1.3 V versus the Ag quasi-reference. Figure 2A shows cyclic voltammetry of a 62 µM AQSC3S:C8 MPC solution (concentration expressed as MPC concentration) at a 1 mm diameter Pt electrode over potentials accessing the first stage of the (13) (a) Kreibig, U., Vollmer, M., Eds. Optical Properties of Metal Clusters; Springer-Verlag: New York, 1995. (b) Vezmar, I.; Alvarez, M. M.; Khoury, J. T.; Salisbury, B. E.; Shafigullin, M. N.; Whetten, R. L. Z. Phys. D 1997, 40, 147-151. (c) Mulvaney, P. Langmuir 1996, 12, 788-800. (14) (a) Robertson, P. K. J.; Eggins, B. R. Analyst 1994, 119, 827832. (b) Stewart, R. F.; Miller, L. L. J. Am. Chem. Soc. 1980, 102, 49995004. (c) Chambers, J. Q. In The Chemistry of Quinoid Compounds; Patai, S., Ed.; Wiley: New York, 1974; p 737.
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ratio of FcC8 to C8 chains) solution in CH2Cl2. The latter consistency supports the assumption of exhaustive reaction of the 25 anthraquinone sites per cluster (i.e., θsites ) 25) on the AQSC3S:C8 MPC used in Figures 2 and 3. On typical voltammetric time scales, it should be emphasized that exhaustive reaction of polyelectron donors and acceptors at electrode surfaces is generally to be expected (and indeed has been previously observed18 for solutions of redox polymers). The following calculation is instructive concerning this point. Rotational diffusion time constants (τrot.) are normally much shorter than those for translational diffusion, as can be seen by simple comparison of the relations Figure 3. Plot of ∆i/∆E and ilim versus microdisk electrode radius, showing that limiting current (2), prewave ∆i/∆E slope (9), and postwave ∆i/∆E slope (b) are proportional to the electrode radius. Measurements of ilim and prewave and postwave ∆i/∆E slopes are illustrated in Figure S4.
anthraquinone reduction (to radical anion). The second reduction step was consistently chemically irreversible and is not discussed further. The peak potential separation in Figure 2A (∆Ep ) 140 mV) is larger than the 59 mV reversible value and increases slowly with potential scan rate, v, effects which may be due either to slow electron transfer kinetics or to the uncompensated resistance (iRunc) of the solvent. Others9,15 have observed deviations from reversibility for the AQ0/- couple with ω-substituted anthraquinone alkanethiolate 2D-SAMs. We have previously observed3 mild physisorption of ferrocenated MPCs on electrodes. The somewhat pointy oxidation peak in Figure 2A suggests that some adsorption of the MPC AQ radical anion may occur. Plots of reduction peak currents against [v]1/2 (Randles-Sevcik analysis16) for AQSC3S:C8 MPC solutions were, however, nicely linear (and against [v] decidedly curved), indicating the reduction currents are dominated by diffusing AQSC3S: C8 MPCs. The linearity of plots of voltammetric limiting currents at observed at microdisk electrodes (Figure 2B) against electrode radius r (Figure 3, 2, albeit over a narrow range), as anticipated by the microelectrode equation17
Ilim ) 4nFrDCθsites
(1)
further confirms diffusion control of reduction of the anthraquinone moieties on the AQSC3S:C8 MPC clusters. Such minor physisorption as may occur for these clusters does not appear to affect the behavior of the anthraquinone reduction voltammetry. In eq 1, n is the number of electrons transferred per anthraquinone (one), F is the Faraday constant, D and C are the MPC diffusion coefficient and concentration, respectively, and θsites is the average number of reacting anthraquinone sites per cluster. Assuming exhaustive reaction of the anthraquinone sites, cluster diffusion coefficients were calculated from the currents in Figure 3 and are given in Table 1. The D values agree within experimental uncertainty for the different electrode radii and are also close to diffusion coefficients measured for ferrocenated C8 MPCs3 having similar core sizes; for example, D ) 2.8 × 10-6 cm2/s for a FcC8:C8 MPC (1:5.5 (15) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. E.; Wrighton, M. S. Science 1991, 252, 688. (16) Bard, A. J.; Faulkner, L. F. Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons: New York, 1980. (17) (a) Wightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15, pp 271-291. (b) θsites was added to the equation to account for the multiple AQ sites.
4π(RH)3η r2 τdiff ) versus τrot. ) D 3kBT
(2)
where τdiff is the time constant for a cluster molecule to pass through the radial diffusion layer around a microelectrode of radius r, η is viscosity, and RH is the hydrodynamic radius of the cluster, which can be calculated from the Stokes-Einstein equation:19
RH )
kBT 6πηD
(3)
where kB is the Boltzmann constant and T is temperature. Table 1 contains results for RH calculated from the diffusion coefficients there, which leads to a value of τrot. ) 3 ns, that is far smaller than the τdiff ) 0.8 s calculated for a cluster to traverse the diffusional depletion layer around a 14 µm radius microdisk electrode. In other words, cluster molecules arriving diffusionally at the electrode/solution interface rotate sufficiently rapidly to amply expose all AQ sites on the cluster molecule surface to the electrode for electron transfer. When we examine the Table 1 hydrodynamic radii, RH ) 1.8-2.0 nm, it is worth noting that these values are smaller than that based on the sum of the 1.0 nm Au core radius and the 1.5 length of a fully extended octanethiolate chain (1.5 nm). This difference has been noticed before, for ferrocenated clusters,3a and is presumed to reflect the small radius of curvature of the Au core surface, whereby chain tilt or folding and/or fully solvated, free draining chain ends depress the hydrodynamic radius value. It is implicit in the preceding discussion (use of n ) 1 in eq 1 and consideration of rotational diffusion) that the 25 anthraquinone sites on the MPC undergo one-electron reductions serially (i.e., one at a time, albeit in rapid succession), as opposed to a concerted reaction wherein n ) 25. This interpretation of the voltammetry is further supported by analysis of the microelectrode voltammetric wave shape on the basis of the following equation:16
E ) E1/2 +
[
]
ilim - i 0.059 log n i
(4)
Figure 4 shows a plot of log[(ilim - i)/i] versus E, taking a (35 mV range around E1/2 (-0.85 V versus AgQRE) of the AQSC3S:C8 MPC voltammogram in Figure 2B. The average slopes of the plots for negative- and positive-going potential scans, 67 and 70 mV, respectively, are close to the 59 mV ideal for a one-electron (n ) 1) reaction, which (18) (a) Jehoulet, C.; Bard, A. J. J. Am. Chem. Soc. 1991, 113, 54565457. (b) Flanagan, J. B.; Margel, S.; Bard, A. J., Anson, F. C. J. Am. Chem. Soc. 1978, 100, 248. (19) Brett, C. M. A.; Oliveira Brett, A. M. Electrochemistry; Oxford University Press: New York, 1993.
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Table 1. Diffusion Coefficients, Hydrodynamic Radii, and Cluster Double-Layer Capacitances for 62 µM (1:2.6) AQSC3S:C8 MPC electrode radius (µm) 12.8 13.9 14.4
106Da
(cm2/s)
2.1 2.3 2.3
RH (nm) 2.0 1.8 1.8
CDLb (µF/cm2) postwave postwave 5.9 6.3 6.3
9.9 9.1 9.1
postwave CDL/prewave CDL 1.7 1.5 1.4
1023(CDLAclus)/MWclusc (F mol/g) prewave postwave 1.5 1.6 1.6
2.5 2.3 2.3
a Calculated from i b lim, where θsites ) 25. Double-layer capacitance of cluster compound calculated from ∆i/∆E slopes based on the 314 atom model. c No assumptions regarding the gold core shape required. Aclus (ca. 2.0 × 10-13 cm2) and MWclus (ca. 79 kDa) are the average Au core surface area and the cluster molecular weight, respectively.
expected for diffusion control of the double-layer charging process, as the above picture requires. The MPC double-layer charging phenomenon was first detected and described in detail in connection with rotated disk voltammetry of ferrocenated MPCs.3 The MPC double-layer capacitance CDL per unit Au core surface area Aclus is related to the ∆i/∆E slope between potentials EA and EB (which are both in either the “prewave” or “postwave” potential region; see also Supporting Information, Figure S4) by
iA - i B ∆i ) ∆E EA - EB
CDL )
σA - σB (EA - EB)NAclus
iA - iB σA - σB ) 4nrDC EA - E B EA - EB
Figure 4. Analysis of anthraquinone waveshape from the microelectrode voltammogram of 62 µM (1:2.6) AQSC3S:C8 MPC (Au314) with a 28 µm diameter Pt disk at 5 mV/s, where E1/2 is -0.85 V versus AgQRE. The average cathodic slope (b) is 0.067 V, and the anodic slope (0) is 0.070 V.
further shows that the AQSC3S:C8 MPC reacts by 25 rapid, but successive, one-electron reactions. That the plot in Figure 4 is somewhat curved is reasonably interpreted as a secondary effect from electrostatic (and perhaps other) interactions between the multiple anthraquinone and radical anion sites packed together in a small molecular space. As shown elsewhere for analogous plots for ferrocenated clusters,3c the curvature can be modeled as a Gaussian dispersion of the formal potentials of the anthraquinone sites. Such effects are known for 2D-SAMs.20 Double-Layer Charging of AQSC3S:C8 Clusters. Turning to the double-layer charging of the MPC Au core, examination of the microelectrode voltammogram of Figure 2B shows that both the current baseline at potentials more positive than the anthraquinone reduction wave (“prewave” region) and the limiting current region (“postwave” region) are more sloping than ideally expected. These effects are not experimental artifacts. The sloping currents (∆i/∆E) arise from the double-layer charging of the Au cores of the MPCs. In a negative-going voltammetric scan, as clusters arrive at the microelectrode surface, a larger double-layer charge (thus a larger current flow) must be delivered at each progressively more negative applied potential to equilibrate the MPC core potential with that of the microelectrode. Figure 3 (b, 9) shows that the slopes of the current-potential curves ∆i/ ∆E are proportional to the microelectrode radius, as (20) Rowe, G. K.; Carter, M. T.; Richardson, J. N.; Murray, R. W. Langmuir 1995, 11, 1797.
(5)
(6)
where σA and σB are the charge per mole of MPC at EA and EB, N is Avogadro’s number, and Aclus ≈ 2.0 × 10-13 cm2 on the basis of6a a 314 Au atom core model. Table 1 gives ∆i/∆E slope results from voltammetry like those of Figure 2B in terms of the product CDLAclus/MWclus, which requires no assumptions about Au core shape or area, and as CDL based on the above value for Aclus. Typical MPC “prewave” and “postwave” capacitances are approximately 6 and 9 µF/cm2, which are somewhat larger than but reasonable in light of analogous results for 2D-SAMs.4 Table 1 also shows that the “postwave” capacitance is approximately 1.5 times larger than the “prewave” value, which is not surprising, since the MPC monolayer in the former case has highly charged chain termini. Observation of the MPC core double-layer charging has significance in showing that the core is metal-like and in demonstrating that the MPCs can be viewed as soluble, diffusing, nanoelectrodes that can be charged to various potentials like an ordinary electrode/solution interface. Their double-layer charges could in principle be used to drive electrochemical reactions, just as ordinary electrodes are made to do in the “coulostatic” experiment.16 The double-layer capacitance values correspond to an average charge of about 1.2 aF/cluster, which, were the core to be charged by 1 V, would mean that the cluster could deliver six electrons for complete discharge to its PZC. The individual cluster capacitances are also small enough to, in principle, exhibit “ensemble Coulomb staircase” charging; they do not in the above experiments because the cluster samples used are somewhat polydisperse in core size (and thus capacitance). The multiple Coulomb staircase voltammetric peaks recently reported21 for MPC solutions require that the clusters be reasonably monodisperse as to core size; otherwise, there are multiple, overlapping Coulomb staircase peaks and the staircase detail is lost, as in the present voltammetry. (21) Ingram, R. S.; Hostetler, M. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Whetten, R. L.; Bigioni, T. P.; Guthrie, D. K.; First, P. N. J. Am. Chem. Soc. 1997, 119, 9279-9280.
4120 Langmuir, Vol. 14, No. 15, 1998
Ingram and Murray
Figure 5. ECE reduction scheme of 1,1-dinitrocylcohexane mediated by anthraquinone radical anion where the C step involves loss of nitrite.22
Mediated Electrocatalysis with AQSC3S:C8 MPCs. The use of redox-functionalized MPCs as electron-transfer catalysts has not previously been demonstrated. We include here preliminary experiments which show not only that electrocatalysis is possible with redox MPCs but also that it can be kinetically more facile than the analogous electrocatalysis using analogous but monomeric dissolved redox species. A reaction previously described by Evans et al.22 was chosen for the preliminary investigation. In that report, a series of quinones, including 9,10-anthraquinone, were used to mediate the reduction of 1,1-dinitrocyclohexane (DNC). The direct (unmediated) reduction of DNC occurs at -1.53 V in DMF, and production of the radical anion is followed by very rapid cleavage of a C-N bond, yielding nitrite and 1-nitrocyclohexyl radical, the latter being immediately further reduced. Figure 5 shows the mediated reduction scheme (which for brevity omits some further reaction details),22 in which the initial electrontransfer reaction (with the electrogenerated quinone radical anion) is the slowest step. Of the quinones investigated, anthraquinone produced the fastest reaction, calculated as 1 × 105 s-1 in dimethylformamide. Figure 6b shows the voltammetric wave for the mediated reduction of DNC by electrogenerated monomer anthraquinone radical (0.12 mM) in 2:1 (v:v) toluene/DMF23 with 0.05 M Hx4NClO4, and Figure 6a shows the voltammetry of the anthraquinone monomer alone, at the same concentration. As observed by the indistinct current plateau, the response appears to be under kinetic control; however, variations in anthraquinone concentration are expected to affect the rate of reaction and change the shape of the wave. In the presence of the DNC reaction substrate, the reduction current for anthraquinone is enhanced by 18-fold (Table 2, icat./iAQ ) 18), which reflects the occurrence of the mediated reaction of Figure 5. Figure 6c shows the voltammetric wave (which has a diffusion-controlled shape) for the mediated reduction of DNC by AQSC3S:C8 MPC (in this case a 140 Au core atom cluster bearing an average of ca. 11 AQSC3S ligands (22) Ru¨hl, J. C.; Evans, D. H.; Hapiot, P.; Neta, P. J. Am. Chem. Soc. 1991, 113, 5188-5194. (23) The solvent used differs from that in ref 22 due to dissolution requirements of the hydrophobic MPC.
Figure 6. Cyclic voltammograms of (a) 0.12 mM AQSC3SH thiol, (b) 0.12 mM AQSC3SH thiol + 4.6 mM DNC, and (c) 1:4 AQSC3S:C8 MPC (Au140, contains overall 0.37 mM AQ) + 4.6 mM DNC in 2:1 toluene/DMF/0.05 M Hx4ClO4 at 100 mV/s using a 1 mm diameter Pt electrode. Table 2. Mediated Electrocatalytic Reduction of 1,1-Dinitrocyclohexane Using Anthraquinones as Catalystsa catalyst AQSC3S:C8 MPCb monomer AQ
CAQc CDNCd iAQe iCATf (mM) (mM) (µA) (µA) icat.CAQ/iAQCDNCg 0.37 0.12
4.6 4.6
0.31 10.8 0.31 5.7
2.8 0.48
a From cyclic voltammetry in 2:1 (v/v) toluene/DMF with 0.05 M Hx4ClO4 at 100 mV/s at a 1 mm diameter Pt disk. b Approximately 11 AQSC3S ligands per cluster and a 140 Au atom core; microelectrode voltammetry gives D ) 2.7 × 10-6 cm2/s and RH ) 15.8 Å for this smaller cluster. c Concentration of anthraquinone sites on clusters in solution. d Concentration of 1,1-dinitrocyclohexane. e Cathodic peak currents for AQ reduction alone. f Catalytic peak currents for DNC reduction catalyzed by AQ monomer and by AQ sites on the cluster. g Ratio of currents in the presence (catalytic) and absence of DNC substrate times the ratio of catalyst to substrate concentration; this is a measure of the relative size of the current enhancement due to catalysis.
per cluster) in a solution in which the concentration of MPC was adjusted so that the reduction of the AQSC3S ligands on the MPC gave the same current (catalyst flux)
Electroactive Three-Dimensional Monolayers
as that for the 0.12 mM anthraquinone solution. (This required a larger MPC concentration because of the slower MPC diffusion rate.) The results show that (a) reduction of the anthraquinone moieties on a AQSC3S:C8 MPC yields a larger electrocatalytic current for DNC reduction (Table 2, icat./iAQ ) 35) than does an equal flux of anthraquinone monomer and (b) when the currents are normalized for relative catalyst and substrate concentrations so as to reflect catalytic efficiencies (icat.CAQ/iAQCDNC), the AQSC3S:C8 MPC is nearly 6-fold more efficient (Table 2). The detailed origin of this intriguing result is unclear at the present time. It is possible that, owing to their hydrophobic character, the MPC and DNC weakly associate in the solution, in which case the rate enhancement is of a “precursor complex” kind. Alternatively, the rate enhancement may arise from the availability of multiple electron donor sites on the MPC, in the collision complex leading to the rapid delivery of a second electron to the once-reduced DNC or its immediate decay product. Further studies underway aim at resolving these questions. In any event, the mere presence of an effective electrocatalysis indicates that further study of redox MPCs as catalysts is clearly worthwhile.
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In conclusion, stable, ω-functionalized redox clusters can be synthesized and characterized using spectroscopy and electrochemistry and provide interesting polyelectron voltammetry and double-layer charging currents. The electrocatalysis results represent only a preliminary report of a highly functionalized cluster’s reactivity; nonetheless, these experiments provide proof-of-concept evidence of design of potentially useful donor/acceptor reactions on cluster molecules. Acknowledgment. We thank Dr. Michael J. Hostetler for synthesis of some of the room-temperature-prepared C8 cluster material and Jeremy Pietron for useful comments. This research was supported in part by grants from the National Science Foundation and the Office of Naval Research. Supporting Information Available: Description of NMR spectroscopy and FTIR measurements, 1H NMR spectra of a AQSC3S:C8MPC and AQSC3SH thiol, FTIR spectrum of a AQSC3S:C8 MPC on evaporated Au, and cyclic voltammogram (6 pages). See any current masthead page for ordering and internet access information. LA9800452
