Redox and Double-Layer Charging of Phenothiazine Functionalized

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Anal. Chem. 2001, 73, 921-929

Redox and Double-Layer Charging of Phenothiazine Functionalized Monolayer-Protected Clusters Deon T. Miles and Royce W. Murray*

Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290

Monolayer-protected Au clusters (MPCs) have been prepared with mixed monolayers of alkanethiolates and alkanethiolates terminally ω-functionalized with phenothiazine. The mixed monolayer MPCs can contain as many as 10 phenothiazines/MPC; these electron donors are electroactive in rapid, successive one-electron reactions. Surface adsorption of the functionalized MPCs is evident in cyclic voltammetry. Double-potential-step chronocoulometry with incremented potential steps was applied to unfunctionalized hexanethiolate-coated MPCs and to those functionalized with phenothiazine to analyze the coupling between the diffusion-controlled double-layer charging of the MPC cores and the oxidation of the phenothiazine centers. Apparent changes in ordering of the MPC alkanethiolate chains were observed with infrared spectroscopy in solutions of MPCs where alcohol, carboxylic acid, or phenothiazine moieties had been incorporated into the monolayer. Starting from the work of Schmid,1 nanometer-sized gold clusters stabilized by ligand monolayers have experienced substantial research attention, especially those stabilized by thiolate ligands.2 Thiolate-stabilized gold nanoparticles have been investigated3,4 to determine their chemical, electrochemical, structural, and physical properties,5,6 including size-dependent ones signaling intermediacy between bulk and molecule-like species.7 Other studies have pointed to possible uses as nanoscale electronic, catalyst, and sensor materials,8 in single electron device fabrication,9 in surface-enhanced Raman spectroscopy,10 and the colorimetric detection of polynucleotides.11 (1) Schmid, G. Chem. Rev. 1992, 92, 1709-1727. (2) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (3) (a) 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, 428-433. (b) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397-406. (4) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36. (5) Hostetler, M. J.; Wingate, J. E.; Zhong, C.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. 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. (6) Green, S. J.; Pietron, J. J.; Stokes, J. J.; Hostetler, M. J., Vu, H.; Wuelfing, W. P.; Murray, R. W. Langmuir 1998, 14, 5612-5619. (7) 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. 10.1021/ac0012647 CCC: $20.00 Published on Web 02/02/2001

© 2001 American Chemical Society

The nucleation/growth/passivation synthesis introduced by Brust et al.,2 is especially effective in producing nanoparticles that are quite small (1-3-nm diameter), air stable, and isolable. The stability is determined by the dense alkanethiolate ligand monolayer, which prevents core aggregation by inhibiting direct metalmetal contact. We refer4 to these nanoparticulate materials as monolayer protected clusters (MPCs). Alkanethiolate-MPCs have been synthesized with chain lengths from propanethiol (C3) to tetracosanethiol (C24).12 Functional groupings can be introduced into MPC monolayers by using functionalized thiols in the Brust reaction.2 On the other hand, changes in core size usually accompany this strategy. To maintain a constant average core size within an investigation, we prefer to introduce functional groupings in a subsequent step, by ligand place-exchange12 reactions between well-characterized alkanethiolate-MPCs and ω-functionalized alkanethiols. Using ω-carboxylic acid- or ω-alcohol-terminated alkanethiols allows subsequent further functionalization by ester and amide coupling to yet other reagents.13 Combinations of these three simple steps allow substantial flexibility in designing monolayers. The monolayer is readily characterized by thermogravimetry, nuclear magnetic resonance spectroscopy (NMR), and Fourier transform infrared spectroscopy (FT-IR).5,12,14 MPCs with monolayers containing multiple redox moieties are interesting because of their multiple electron-transfer properties. MPCs bearing ferrocenes,6,15 anthraquinones,16,17 and viologens17 have been described, the former two by place exchanges with redox-functionalized thiols. Following on a preliminary13 report, this paper describes the voltammetric properties of nanoparticles (8) Sampath, S.; Lev, O. Adv. Mater. 1997, 9, 410-413. (9) Sato, T.; Ahmed, H.; Brown, D.; Johnson, B. F. G. J. Appl. Phys. 1997, 82, 696-701. (10) (a) Chumanov, G.; Sokolov, K.; Gregory, B. W.; Cotton, T. M. J. Phys. Chem. 1995, 99, 9466-9471. (b) Freeman, R. G.; Grabar, K. C.; Alison, K. J.; Bright, R. M.; Davis, T. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 297, 1629-1632. (11) Elghanian, R.; Stohoff, J. J.; Mucic, R. C.; Letsinger, R. L.: Mirkin, C. A. Science 1997, 227, 1078-1080. (12) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 36043612. (13) Templeton, A. C.; Hostetler, M. J.; Warmoth, E. K.; Chen, S.; Hartshorn, C. M.; Krishnamurthy, V. M.; Forbes, M. D. E.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 4845-4849. (14) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 37823789. (15) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212-4213. (16) Ingram, R. S.; Murray, R. W. Langmuir 1998, 14, 4115-4121. (17) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 7081-7089.

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(abbreviated Ptz-MPCs) resulting from a sequence of placeexchange and ester coupling reactions that incorporate a phenothiazine carboxylic acid derivative (Ptz-COOH) into the MPC monolayer. Its structure and a cartoon of a Ptz-MPC nanoparticle are shown below.

Redox-functionalized MPCs are unusual in that each nanoparticle undergoes two kinds of electron transfers when it equilibrates its potential with that of a working electrode surface with which it comes into contact. Namely, electron transfers of any redox groupings present are accompanied by those accomplishing double-layer charging of the MPC by electron transfers with its metal-like core.18 We present here a detailed method to measure these two electron-transfer process for the Ptz-MPCs. Their existence, for other redox/MPC examples, has been previously noted,6,7,13 but they have not been analyzed in detail. Additionally, a monolayer chain-ordering sensitivity to the nature of terminal alkanethiolate functionalities was uncovered in FT-IR measurements. Phenothiazine, besides being a well-behaved electron donor molecule, has been used in insecticides, in pharmaceutical manufacturing, and as an anthelmintic substance that can destroy intestinal parasitic worms. Phenothiazine is light-sensitive and has been hypothesized to form a sulfoxide by photolysis.19 Decomposition through this light sensitivity can lead to anomalous voltammetry, and the Ptz-MPC syntheses were accordingly conducted with a minimum of light exposure. EXPERIMENTAL SECTION Chemicals. HAuCl4‚xH2O and 10H-(phenothiazine-10) propionic acid were synthesized according to literature procedures.20,21 Tetraoctylammonium bromide, carbon tetrachloride, 11-mercapto1-undecanol, 6-mercapto-1-hexanol, 3-mercapto-1-propanol, 3-mercaptopropionic acid, and 11-mercaptoundecanoic acid (Aldrich), sodium borohydride (Johnson Matthey), dichloromethane and acetonitrile (Fisher), absolute ethanol (AAPER), tetrabutylammonium hexafluorophosphate and tetrabutylammonium perchlorate (18) Double-layer charging of the gold core at the electrode surface occurs via electron tunneling through the alkanethiolate monolayer shell. The tunneling reaction is expected to be fast when the alkanethiolate chain is short, as in the present C6 case. (19) Felmeister, A.; Discher, C. A. J. Pharm. Sci. 1964, 53, 756-762. (20) (a) Handbook of Preparative Inorganic Chemistry; Brauer, G., Ed.; Academic Press: New York, 1965; p 1054. (b) Block, B. P. Inorg. Synth. 1953, 4, 14. (21) Godefroi, E. F.; Wittle, E. L. J. Org. Chem. 1956, 21, 1163-1168.

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(Fluka), and toluene, acetone, and tetrahydrofuran (Mallinckrodt) were used as received, as were the coupling reagents13 HOBt, BOP, DMAP, and NMM (Aldrich) and DCC (Sigma).22 Water was purified with a Barnstead NANOpure system. Synthesis. Monolayer protected clusters with butanethiolate, hexanethiolate, and dodecanethiolate monolayers were prepared using a modification of the Brust reaction,2,5,23 in which the reactant thiol/AuCl4- mole ratio was 3:1 and the reducing agent (NaBH4) was added at 0 °C. After stirring at room temperature for 24 h, the solvent was removed under vacuum and the crude MPC product collected on a medium-porosity glass frit, washed with copious amounts of ethanol and acetone, dried, and weighed. From the hexanethiolate-MPC (C6 MPC) preparation, samples were separated into ethanol-soluble and non-ethanol-soluble fractions as previously described.24 These samples are labeled later as C6 MPC-A and -B, respectively. Studies4,5of dodecanethiolateMPCs by thermogravimetry, small-angle X-ray scattering, and high-resolution transmission electron microscopy (HRTEM) have determined that MPCs produced under these reaction conditions are slightly polydisperse in core sizes. The average core radius is 0.8 nm,5 and modeling based on a truncated octahedral core shape indicates that the average MPC has ∼140 core Au atoms and ∼53 alkanethiolate ligands. Ligand Place-Exchange and Subsequent Ester Coupling Reactions. Ligand place exchange15,16,25,26 was performed by stirring THF solution mixtures of the desired ω-functionalized alkanethiol and alkanethiolate-MPC (40-80 µM) for 72 h at room temperature. Solvent was removed under vacuum, and the exchanged MPC product was collected on a frit where unreacted materials were removed by copious washing with acetonitrile. An example is the place-exchange reaction between the functionalized thiol 11-mercaptoundecanol (C11OH) and the nanoparticle dodecanethiolate-MPC (C12-MPC), whose product is designated as C11OH:C12-MPC. The mole ratio of C11OH and C12 in the mixed monolayer of the MPC product depends on the mole ratio used in the place-exchange reactionswhich was either 1:4 or 1:3sand was determined by NMR as described below. MPCs prepared for coupling with Ptz-COOH were place-exchanged with ω-hydroxyalkanethiols. Acid-terminated alkanethiols were used to prepare mixed-monolayer MPCs for monolayer structural studies as described in the final section of this report. Ester coupling of the phenothiazine carboxylic acid (PtzCOOH, structure above) to MPCs with mixed monolayers of ω-alcohol alkanethiolates and alkanethiolates was performed using BOP as described previously,13 where 20 equiv (relative to moles of MPC alcohol) of the acid and BOP reagents (BOP, HOBt, NMM, DMAP) were reacted in low-water THF. Following a short activation period (10 min), the MPC (to a concentration ∼2 mg (22) HOBt ) 1-hydroxybenzotriazole hydrate; BOP ) benzotriazolyl-N-oxytris(dimethylamino)phosphonium hexafluorophosphate; DMAP ) 4-(dimethylamino)pyridine; NMM ) 4-methylmorpholine; DCC ) N,N′-dicyclohexylcarbodiimide. (23) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 36043612. (24) Hicks, J. F.; Templeton, A. C.; Chen, S.; Sheran, K. M.; Jasti, R.; Murray, R. W.; Debord, J.; Schaaff, T. G.; Whetten, R. L. Anal. Chem. 1999, 71, 37033711. (25) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175-9178. (26) Green, S. J.; Stokes, J. J.; Hostetler, M. J.; Pietron, J.; Murray, R. W. J. Phys. Chem. B 1997, 101, 2663-2668.

of cluster/mL) was added and stirred at room temperature for 15 h. Coupling was alternatively performed using DCC reagents, where 20 equiv (relative to moles of MPC alcohol) of Ptz-COOH and DCC reagents (DCC and DMAP) were reacted with MPC in low-water THF, stirring for 15 h at room temperature. In both procedures, solvent was removed under vacuum, and the coupled MPC product was collected on a frit where unreacted reagents were removed by copious washing with acetonitrile (in which the MPCs are insoluble). MPC Monolayer Analysis. The composition of mixed-monolayer MPCs (both place-exchanged and ester-coupled) was determined using proton NMR. I2-induced decomposition of the place-exchanged MPC monolayer into disulfides was performed as previously described. About 10 drops of an I2 solution (2 mg/2 mL of CD2Cl2) are added to an NMR tube containing 20 mg of MPC/1 mL of CD2Cl2. Disulfide formation is signaled by a change in solution color from dark brown to orange for MPCs containing ω-alcohol thiolates and to pink for ω-acid thiolates. The decomposition to soluble disulfides is quantitative;13 the Au MPC core precipitates as a brown aggregated mass. 1H NMR spectra were obtained on a Bruker AMX 200-MHz spectrometer, using a linebroadening factor of 1 Hz to improve signal-to-noise (S/N) resolution and a relaxation delay of 5 s. FT-IR spectra of mixed monolayer MPCs were obtained with a BioRad 6000 spectrometer. Thin films of solid MPC were prepared by drop-casting out of toluene on either NaCl or KBr plates (SpectraTech). N2-purged MPC solutions were studied in a Spectra-Tech heated-cooled demountable liquid IR cell, with either KBr or NaCl plates, two 32-mm-diameter Viton gaskets, and one 0.5-mm Teflon spacer. Background spectra of the CCl4 solvent were obtained before each spectral acquisition. Electrochemical Measurements. Cyclic voltammetry, Osteryoung square wave voltammetry, and chronocoulometry were performed using a BAS 100B electrochemical analyzer. The Pt working macroelectrode (0.0010 cm2) was polished with 0.25-µm diamond paste (Buehler), rinsed with NANOpure water, sonicated in absolute ethanol and acetone, and potential-cycled between -0.5 and 1.4 V vs a Ag wire quasi-reference electrode in 0.5 M H2SO4 for 15 min to further clean the electrode surface. The Pt working microelectrode (0.0030-cm diameter) was polished in a manner similar that for the macroelectrode, but it was not potential-cycled. In electrochemical measurements, a Pt coil counter electrode was isolated with a frit and a Ag/Ag+ reference electrode or Ag wire quasi-reference electrode resided in the same cell compartment as the working electrode. Ptz-MPC solutions in 2:1 (v/v) toluene/ CH3CN, containing 0.050 M Bu4NPF6 electrolyte, were degassed and then blanketed with solvent-saturated N2. Voltammetry or chronocoulometry of the electrolyte solutions was taken prior to each experiment to check for any characteristics that might be attributed to insufficient cleaning of the working electrode. RESULTS AND DISCUSSION Monolayer Analysis. The extent of place exchange of ω-functionalized alkanethiols onto the corresponding MPCs, and of the subsequent ester-coupling reactions, was determined with proton NMR. Spectra of intact MPCs are characteristically broadened, by a variety of mechanisms,27,28 and for thiolate monolayer analysis, it is advantageous to decompose13 the monolayer into free disulfides by reaction of MPCs with I2; hence the sharpened

Table 1. Number of ω-Alcohols Exchanged and Phenothiazines Coupled per MPCa chain length

no. of alcohols/MPCb

no. of Ptz/MPCc

C12 C12 C12 C12 C12 C12 C12 C6 C6 C4

11 11 11 11 14 14 14 13 19d 23d

6 10 2e 2 4 3 10 7 5f 15

a Average number of species (based on 53 ligands/MPC model) due to sample polydispersity. Uncertainty of NMR for determining number of alcohols or phenothiazines per MPC is (1. b A 1:4 mole feed ratio of ω-alcohol alkanethiolate chains to n-alkanethiolate chains on MPC obtained from place-exchange reaction, unless otherwise noted. c Twenty equivalents (relative to the mole of MPC alcohol groups) of the Ptz-COOH and corresponding BOP reagents used for coupling reaction, unless otherwise noted. d A 1:3 mole feed ratio of ω-alcohol alkanethiolate chains to n-alkanethiolate chains on MPC obtained for place-exchange reaction. e Five equivalents of Ptz-COOH and BOP reagents used for coupling reaction. f Coupling performed using DCC reagents.

resonance line integrals are more accurately determined. Examples of NMR spectra of Ptz-MPC and I2-decomposed C11OH: C12-MPC are given in Figures S-1 and S-2 (Supporting Information), respectively. The relative number of ω-alcohol alkanethiolates and alkanethiolates on a place-exchanged MPC is typically measured using the area ratio of R-CH2 and CH3 peaks. In the case of Ptz-MPCs, the relative numbers of ester-coupled phenothiazine groupings, unreacted ω-alcohol alkanethiolates, and alkanethiolates are measured from the peak area ratios of phenothiazine ring, R-CH2 and CH3 resonances. Ptz-MPCs were prepared starting from place exchanges of three combinations of alkanethiolate-MPC with ω-alcohol alkanethiols: C12-MPC with C11OH, C6-MPC with C6OH, and C4MPC with C3OH. The results of NMR analysis of the placeexchange and subsequent phenothiazine-coupled mixed monolayer products are given in Table 1. The numbers of each ligand are based on an average of 53/MPC (from previous5 alkanethiolateMPC studies). The results show that 20-40% of the original alkanethiolate ligands were place-exchanged by ω-alcohol alkanethiolates and that the ester coupling reaction efficiency was a rather variable 20->90%. As few as 2 and as many as 15 phenothiazine units per MPC were obtained. It should be emphasized that the numbers of phenothiazines per MPC are averages and there is very likely some (statistical) polydispersity in both the number of ligands/MPC that are place-exchanged and the number of ester bonds/MPC that are formed. We have no analysis at this time that can determine this nanoparticle-tonanoparticle ligand dispersity. (27) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.; Poon, C.;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, 12537. (28) (a) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B. Chem. Eur. J. 1996, 2, 359-363. (b) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Reven, L. Langmuir 1996, 12, 1262-1269. (c) Badia, A.; Cuccia, L.; Demers, L.; Morin, F.; Lennox, R. B. J. Am. Chem. Soc. 1997, 119, 2682-2692. (d) Badia, A.; Demers, L.; Dickinson, L.; Morin, F. G.; Lennox, R. B.; Reven, L. J. Am. Chem. Soc. 1997, 119, 11104-11105.

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Figure 1. Microelectrode voltammetry of 86 µM Ptz10C11OH1C1242MPC (0.84 mM in Ptz) (solid line) and of 1.4 mM phenothiazine carboxylic acid (dotted line) in 2:1 (v/v) toluene/acetonitrile/0.050 M Bu4NPF6 at a 0.0030-cm-diameter Pt electrode. Current scale of PtzMPC voltammogram is scaled up by 6-fold for comparison purposes. iLIM (MPC) ) 1.0 nA, iLIM (phenothiazine carboxylic acid) ) 10.1 nA. MPC: E1/2 ) 560 mV (vs Ag/Ag+ reference) and υ ) 5 mV/s. phenothiazine carboxylic acid: E1/2 ) 540 mV and υ ) 10 mV/s. Inset is log[(iLIM - i)]/i] vs E plot of the voltammetric current-potential wave shape of 86 µM Ptz-MPC. Average anodic slope, 62 mV.

The abbreviation for the MPCs to be used in the following electrochemical studies, for example, for the second entry in Table 1, is (Ptz)10(C11OH)1(C12)42-MPC, where the subscripts represent the average relative numbers of each ligand. This formula indicates that 10 of the original 11 C11OH ligands became estercoupled to the phenothiazine carboxylic acid. Microelectrode Voltammetry. Figure 1 shows Pt microelectrode voltammetry of the phenothiazine carboxylic acid derivative (- - -) and of Ptz10C11OH1C1242-MPC (s). The latter voltammogram is of a much more dilute solution than the former. Microelectrode limiting currents for redox MPCs are given by29

iLIM ) 4nFRDCLUCθSITES

(1)

where n is electrons transferred per phenothiazine, F is the Faraday, r is microelectrode radius, DCLU is the MPC diffusion coefficient, and C its bulk solution concentration (mol/cm3), and θSITES is, for the case of Ptz-MPCs, the average number of reacting phenothiazine sites per MPC. Phenothiazine is a well-known30 one-electron donor (n ) 1). Application of eq 1 to the Figure 1 phenothiazine carboxylic acid monomer solution gives D ) 1.2 × 10-5 cm2/s. D equals 2.2 × 10-6 cm2/s for the Figure 1 Ptz10C11OH1C1242-MPC voltammetry. An analogous determination for a shorter-chain Ptz-MPC, Ptz7C6OH6C640-MPC, gave D ) 2.6 × 10-6 cm2/s. Taylor dispersion measurements of alkanethiolate-MPCs have produced31 similar results, such as D ) 2.9 × 10-6 cm2/s in toluene for a nonfunc(29) Wightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15, pp 271-291. (30) (a) Opallo, M.; Kapturkiewicz, A. Electrochim. Acta 1985, 30, 1301-1306. (b) Paduszek, B.; Kalinowski, M. K. Electrochim. Acta 1983, 28, 639-642. (c) Tinker, L. A.; Bard, A. J. J. Am. Chem. Soc. 1979, 101, 2316-2319. (31) Wuelfing, W. P.; Templeton, A. C.; Hicks, J. F.; Murray, R. W. Anal. Chem. 1999, 71, 4069-4074.

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tionalized C12-MPC. The ∼5:1 ratio of the diffusion coefficients of the phenothiazine carboxylic acid monomer versus the PtzMPCs indicates that their effective hydrodynamic diameters lie in the inverse ratio (i.e., 1:5). This seems reasonable and consistent with our earlier discussion31 of the potentially freedraining character of the outer portions of MPC monolayers. The preceding calculation of D was based on all of the phenothiazine sites attached to the MPC being electroactive; i.e., θSITES equals the average number of phenothiazines per MPC. This is not an assumption since thin-layer electrode coulometry in the preliminary report13 demonstrated exhaustive reaction. The capacity to engage in polyelectron transfers is an interesting aspect of redox-derivatized nanoparticles such as the PtzMPCs. In the absence of strong electronic coupling between the electron donor/acceptor site and the Au core, such polyelectron transfers are expected6 to occur by serial one-electron reactions that involve fast rotational diffusion or site-site electron hopping. That the reactions are indeed serial one-electron transfers can be shown by determining the incremental free energy per electrontransfer barrier-crossing by a log[(iLIM - i)/i] versus E plot of the voltammetric current-potential wave shape (Figure 1 inset). The plot encompasses a (35-mV range around the half-wave potential (0.56 vs Ag/Ag+ or vs SCE). The slope, for fast electron transfers, is ideally 0.059/n; that in the Figure 1 inset, 62 mV, is nearly the ideal value. Thus, the 10 electrons/Ptz-MPC donor are delivered in 10 serial, one-electron transfers. Analogous oneelectron results were obtained in previous6,16,17 electrochemical studies of MPCs derivatized with ferrocene, anthraquinone, and viologen groupings. At least for MPCs where long linker chains couple the groupings to the MPC core (poor intrasite electronic coupling), serial one-electron transfer seems now an established generality. The result bears analogy to older studies of solutions of redox polymers.32 An additional feature of Figure 1(s) is the sloping current baseline preceding and following the phenothiazine oxidation wave. Rotated disk electrode studies26 of MPCs with attached ferrocenes have shown that such sloping currents arise from the transport-controlled (∆i/∆E varies with the square root of electrode rotation rate) double-layer charging of the metal-like MPC cores as they come into contact with the working electrode. The MPC cores act as soluble, diffusing, nanoelectrodes that require electronic charge to be passed when their potential is changed, just as the case for any ordinary macroscopic electrode/ solution interface.16 (The MPC double-layer charging can be observed7,24 in incremental, one-electron waves when the MPC sample is sufficiently core-monodisperse; that is not the case with the Ptz-MPCs in the present work, which are insufficiently monodisperse.) The relation between ∆i/∆E in microelectrode voltammetry and the double-layer capacity per cluster, CCLU (F/MPC), is16

∆i/∆E ) 4rDCLUCNACCLU

(2)

where NA is Avogadro’s number and CCLU ) ACLUCDL, where CDL is the MPC surface-area-normalized double-layer capacitance (32) Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson, F. C. J. Am. Chem. Soc. 1978, 100, 4248-4253.

Figure 2. Osteryoung square wave voltammetry of 4.2 µM solution of Ptz7C6OH6C640-MPC (30 µM in Ptz) in 2:1 (v/v) toluene/acetonitrile/ 0.05 M Bu4NPF6 at Pt electrode. For oxidation scan (lower wave), Ep ) 592 mV (vs Ag/Ag+ reference) and ip ) 35.4 nA; for reduction scan (upper wave), Ep ) 584 mV and ip ) 32.3 nA. Arrows indicate direction of scan for experiment.

(F/cm2). From experiments like Figure 1, CCLU ≈ 1.0 aF/MPC from current-potential slopes at either potentials negative or positive of the phenothiazine oxidation wave. The corresponding value of CDL for Ptz-MPCs is ∼12 µF/cm2, respectively, using an estimated ACLU ) 8.0 × 10-14 cm2 for the average Au140 core. The result for CDL is an upper limit, since no correction is made for background currents not associated with MPC double-layer charging. The CCLU results are generally consistent with previous values for anthraquinone-MPCs (1.2 aF/MPC) and are larger than those24 measured by quantized double-layer charging for underivatized, monodisperse alkanethiolate-MPCs (0.6 and 0.4 for C6-MPCs and C12-MPCs, respectively). A further analysis of the MPC double-layer charging, and an explanation of the difference just noted, is given later. Square Wave Voltammetry. Ptz-MPC solutions were briefly inspected (Figure 2) by Osteryoung square wave voltammetry33 (OSWV), for which the voltammetric peak-width width at halfheight, W1/2, for an electrode reaction with fast electron-transfer kinetics is equal to 3.52RT/nF. Deviations from this parameter (ideally 90 mV at 25 °C when n ) 1) can be attributed to slow electron transfers, uncompensated solution resistance, or, in the present case, a dispersity in the formal potentials of the multiple phenothiazines on each MPC. W1/2 ) 112 mV from the Figure 2 voltammogram; this value is consistent with n ) 1 and a mild nonideality. Dispersity in formal potentials is the most likely effect; it has been postulated34 to account for similar results in chemically modified electrode voltammetry. When redox sites on a surface are not widely dispersed, they can interact in such a way that the overall formal potential varies with the fraction that has been oxidized or reduced. (33) Osteryoung, J. G.; Osteryoung, R. A. Anal. Chem. 1985, 57, 101A-110A. (34) (a) Murray, R. W. Introduction to Molecularly Designed Electrode Surfaces. In Molecular Design of Electrode Surfaces; Techniques of Chemistry Series; Murray, R. W., Ed.; John Wiley and Sons: New York, 1992; p 9. (b) Murray, R. W. Annu. Rev. Mater. Sci. 1984, 14, 145-169. (c) Albery, W. J.; Boutelle, M. G.; Colby, P. J.; Hillman, A. R. J. Electroanal. Chem. 1982, 133, 135145.

Figure 3. Macroelectrode voltammetry of (A) 19.5 µM solution of Ptz7C6OH6C640-MPC (0.14 mM in Ptz) and (B) 15.5 µM solution of Ptz10C11OH1C1242-MPC (0.15 mM in Ptz) in 2:1 (v/v) toluene/ acetonitrile/0.05 M Bu4NPF6 at Pt electrode. Ptz-MPC (C6): Epa ) 580 mV, Epc ) 552 mV, and ∆Ep ) 28 mV. Ptz-MPC (C12): Epa ) 588 mV, Epc ) 555 mV, and ∆Ep ) 33 mV.

The OSWV peak current can be estimated from33

i)

nθSITESFADCLU1/2C

xπtp

ψ(∆Es,Esw)

(3)

where A is electrode area, tP the square wave pulse width, and ψ(∆Es,Esw) a dimensionless peak current factor which equals 0.93 for a kinetically fast reaction.33 Using the diffusion coefficient determined from microelectrode voltammetry, experimental peak currents for the phenothiazine carboxylic acid derivative and currents calculated from eq 3 agreed within several percent. In contrast, the theoretical estimate of a peak current, ∼10 nA, for a 4.2 µM (Ptz)7(C6OH)6(C6)40-MPC solution (30 µM in Ptz) is much smaller than the Figure 2 result (35 nA) and implies adsorption of Ptz-MPCs on the Pt electrode. MPC adsorption is common, but weak, being usually reversed when the electrode is moved to MPC-free solvent.6,16,35 Ferrocenated MPCs have been established to physisorb.15,26 Viologen-MPC adsorption on Au has been measured using an electrochemical quartz crystal microbalance (EQCM) at a coverage Γο ) 1.3 × 10-11 mol/cm2, which is roughly a monolayer of cluster.35 Adsorption of Ptz-MPCs is additionally indicated by macroelectrode cyclic voltammetry (Figure 3) of dilute Ptz-MPC solutions, which has the oxidation-reduction wave symmetry and low ∆Ep (30 mV) typical of currents where adsorbed reactants are present as well as diffusing ones. (35) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 7081-7089.

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Double-Potential-Step Chronocoulometry. This experiment allows a more detailed examination of MPC adsorption and doublelayer charging and of coupling of the latter with phenothiazine oxidation. We simplify the analysis by first considering hexanethiolate-coated MPCs that are unfunctionalized. The charges QF (an anodic charge) and QR (cathodic charge) for double-layer charging of an unfunctionalized MPC in a diffusion-controlled process in which the potential is stepped by ∆E first in a positive-going direction, from an initial value EINIT to a final one EFINAL, and then after time τ returned to EINIT, are

QF )

-2(∆EF)CCLUFADCLU1/2Ct1/2 eπ1/2

-

[

(∆EF) AQDL + QR )

2(∆ER)CCLUFADCLU1/2Cθ eπ1/2

[

]

CCLUFAΓINIT (4) e

+

(∆ER) AQDL +

]

CCLUFAΓFINAL (5) e

where ∆EF is EFINAL - EINIT (a positive number), ∆ER is EINIT EFINAL (negative), θ ) [τ1/2 + (t - τ)1/2 - t1/2], t is time, ΓINIT and ΓFINAL are the quantities of adsorbed MPC at EINIT and EFINAL, respectively, e is the electronic charge, and (∆E)CCLU/e ) nDL is the average number of electrons passed per MPC to charge its double layer. The negative signing of eq 4 reflects QF being an anodic charge. Equations 4 and 5 are new expressions.36 They parallel those known37 for chronocoulometry of an adsorbed redox moiety, but differ in that the number of electrons passed depends linearly on the values of ∆E. Inspection of the equations indicates that (a) the slopes SF and SR of plots of QF and QR against t1/2 and θ, respectively, should vary linearly with ∆E (assuming that CCLU does not vary appreciably with potential) and that plots of the slopes SF and SR against ∆EF and ∆ER, respectively, should yield (b) zero intercepts and (c) equal slopes SSF,∆E ) SSR,∆E ) 2CCLUFADCLU1/2C/eπ1/2. Further inspection of eqs 4 and 5 reveals that (a) the intercepts IF and IR of plots of QF and QR against t1/2 and θ, respectively, should also vary linearly with ∆E (because the nDL delivered to charge the double layers of adsorbed MPCs varies with ∆E) and that plots of the intercepts IF and IR against ∆EF and ∆ER, respectively, should yield (b) zero intercepts and (c) slopes SIF,∆E and SIF,∆E, the difference between which is determined by the difference in adsorption between EINIT and EFINAL or CCLUFA[ΓINIT - ΓFINAL]/e. CCLU of adsorbed MPCs is assumed to be the same as that of diffusing MPCs. The above relations were examined with four sets of potentialstep experiments on hexanethiolate-coated MPCs, in which ∆E was varied from +0.30 to +1.00 V. In one experiment, EINIT was held constant at -0.20 V and EFINAL was varied out to +0.80 V, in another, EINIT was varied and EFINAL was held constant at +0.80 V. Varied-EFINAL and varied-EINIT experiments were performed on each of two hexanethiolate MPC samples. Sample C6 MPC-A is (36) This new methodology was derived from chronocoulometry equations in: Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980. (37) Anson, F. C.; Osteryoung, R. A. J. Chem. Educ. 1983, 60, 293-296.

926 Analytical Chemistry, Vol. 73, No. 5, March 1, 2001

Figure 4. Chronocoulometry plots of concentration-normalized QF and QR against t1/2 (upper) and θ (lower), respectively, for 48 µM solution of C6 MPC-A in methylene chloride/0.05 M Bu4NClO4 at Pt electrode. EFINAL ) +200 (closed circles), +400 (open circles), +600 (closed triangles), and +800 mV (open triangles); EINIT ) -200 mV.

the ethanol extract of a C6 MPC preparation and is more nearly monodisperse than C6 MPC-B, which is the nonethanol soluble fraction. Figure 4 shows examples of plots of QF and QR against t1/2 and θ, for the C6 MPC-A sample, for the case of varied-EFINAL potential steps. Analogous plots for the other three sets of experiments are given in Figures S-3-S-5 (Supporting Information). Figure 5 shows plots of the slopes SF and SR and intercepts IF and IR against ∆E for sample C6 MPC-B; Figure S-6 (Supporting Information) shows analogous plots for sample C6 MPC-A. Figures 5 and S-6 (upper) verify an essential aspect of eqs 4 and 5, namely that the diffusion-controlled slopes of plots like those in Figure 4 reflect varying numbers of electrons removed from the MPC cores in the double-layer charging process. Values of the slopes (SSF,∆E, SSR,∆E, SIF,∆E, and SIR,∆E) resulting from the Figures 5 and S-6 ∆E plots are presented in Table 2 for the four sets of potential-step experiments. The intercepts of such plots (ideally zero), also included in the table, are discussed later. Table 2 gives values of the average double-layer capacitance (CCLU) per MPC, which is calculated from the slopes SSF,∆E and SSR,∆E (vide infra) using DCLU ) 2.5 × 10-6 cm2/s (a value consistent with that determined above for Ptz-MPCs and for unsubstituted ones31). The results for CCLU give an average of 1.2 ( 0.2 aF/ MPC. This average result is close to that measured from the Figure 1 microelectrode voltammetry of Ptz-MPCs and other substituted MPCs. The Table 2 average CCLU is, on the other hand, larger than that obtained24 (0.6 aF) from quantized double-layer charging data on C6 MPC samples, including ones like the C6 MPC-A sample used here. This is a real and anticipated difference that arises from the C6 MPC sample not being perfectly monodisperse. The C6 MPC samples have a dominant population of MPCs with Au140

Figure 5. Plots of slopes SF and SR (upper panel) and intercepts IF and IR (lower panel) of chronocoulometry plots (as in Figures S-4 and S-5) against ∆E, for 54 µM solution of C6 MPC-B in methylene chloride/0.05 M Bu4NClO4 at Pt electrode. EINIT ) constant ) -200 mV (closed square) and EFINAL ) constant ) +800 mV (closed circle).

coresswhich gives rise to the quantized double-layer charging phenomenon established in earlier DPV and CV studies.24 The samples also however contain a substantial portion of larger core MPCs. The chronocoulometric double-layer charging experiment (and earlier rotated disk electrode experiments26) responds to charging of all MPC cores present, including the larger ones. The present results indicate that the average core area of all MPCs present in these samples (and thus the average CCLU) is ∼2-fold larger than that of MPCs with solely Au140 cores. Examination of MPC populations in TEM-derived core-size histograms in the manner previously described24 gives an average core size consistent with this result. As noted above, the difference between the slopes SIF,∆E and SIR,∆E is CCLUFA[ΓINIT - ΓFINAL]/e; the resulting values of ΓINIT ΓFINAL are given in Table 2 for the four experiments. The ΓINIT ΓFINAL results indicate a minor difference between the amount of C6 MPC adsorption at negative and positive potentials with most data indicating slightly more adsorption at EFINAL. The results correspond to fractions of a monolayer, which for the C6 MPC is ∼2.2 × 10-11 mol/cm2. Given the degree of scatter (∼(20%) in the slope-derived CCLU results, and that the ΓINIT - ΓFINAL result is a difference of the corresponding intercepts, the small ΓINIT ΓFINAL values may not be experimentally meaningful. Other experiments35 based on electrochemical quartz crystal microbalance results indicating little potential dependency of the amount of adsorbed MPCs would support this view.

Returning to the intercepts (IIF,∆E and IIR,∆E, respectively, ideally zero) of the plots (Figures 5 and S-3-5) of IF and IR against ∆E, Table 2 shows that these are rather small and we attribute them simply to experimental errors. The intercepts (ISF,∆E and ISR,∆E, respectively, again ideally zero) of plots of SF and SR against ∆E are on the other hand, larger. These intercepts arise from an effect not represented in eqs 4 and 5 but which can be qualitatively rationalized. The intercepts ISF,∆E and ISR,∆E of ∆E plots against SF and SR correspond to hypothetical potential steps of ∆E ) 0. For the varied-EINIT and varied-EFINAL experiments, ∆E ) 0 corresponds simply to poising the electrode at potentials of +0.8 and -0.2 V, respectively, and measuring the charge-time response. These potentials will typically differ (especially +0.8 V) from that of the solution MPC population, and accordingly, even after lengthy equilibration at t ) 0, there will be a diffusive flux of charge for a ∆E ) 0 potential “step”. This flux should be larger when the ∆E ) 0 potential is +0.8 V, and the intercepts ISF,∆E and ISR,∆E in Table 2 are in fact larger for the +0.8-V case, i.e., EFINAL being the potential held constant. The effect just described is analogous to a chronocoulometry experiment in a solution of electron donors and acceptors, but starting from a potential differing from the solution rest potential. Consider next analogous double-potential-step chronocoulometric measurements on solutions of MPCs with attached phenothiazines (Ptz-MPCs). Equations 4 and 5 are modified to account concurrently for the MPC core charging and for phenothiazine oxidation and subsequent reduction when ∆E amounts to a step completely across the phenothiazine wave (in this case to an EFINAL ) +0.8 V):

QF )

-2[nDL + θSITES]FADCLU1/2Ct1/2 π1/2

-

[

(∆EF) AQDL + QR )

2[nDL + θSITES]FADCLU1/2Cθ π1/2

[

]

CCLUFAΓINIT (6) e

+

(∆ER) AQDL +

]

CCLUFAΓFINAL (7) e

The interesting part of these (new) relations is the presence of both MPC core charging (represented by nDL ) (∆E)CCLU/e and thus ∆E-dependent) and redox charging of the phenothiazine sites (represented by θSITES ) 7.4 for the Ptz-MPC material employed). Figure 6 shows plots of QF and QR against t1/2 and θ, respectively, for the Ptz-MPC sample. Equations 6 and 7 indicate that (a) plots of the slopes SF and SR of QF and QR against t1/2 and θ, respectively, should vary linearly with ∆E (assuming that CCLU does not vary appreciably with potential) and should display (b) slopes SSF,∆E ) SSR,∆E ) 2CCLUFADCLU1/2CCLU°/eπ1/2 and equal to those for C6 MPCs lacking attached phenothiazines and intercepts ISF,∆E and ISR,∆E that are each equal to 2θSITESFADCLU1/2CCLU°t1/2/ π1/2. The ratios SSF,∆E/ISF,∆E and SSR,∆E/ISR,∆E ) CCLU/eθSITES, which affords a straightforward way to calculate CCLU knowing θSITES, or vv. Figure 7A shows plots of SF and SR against ∆E and the expected linearity is confirmed. The average ratio (of the forward Analytical Chemistry, Vol. 73, No. 5, March 1, 2001

927

Table 2. Chronocoulometric Data for C6-MPC and Ptz-MPC.a,b MPC C6-A C6-A C6-B C6-B Ptz

E held constant EFINAL EINIT EFINAL EINIT EFINAL

CCLU (aF)c 1.3 ( 0.5 1.2 ( 0.1 1.0 ( 0.4 1.2 ( 0.1 1.5

ΓINIT - ΓFINAL (mol/cm2) 10-11

-1.13 × -2.07 × 10-12 -5.52 × 10-12 2.90 × 10-12 -2.21 × 10-11

SSF,∆E

ISF,∆E

SSR,∆E

ISR,∆E

SIF,∆E

IIF,∆E

SIR,∆E

IIR,∆E

-4.79 -7.27 -3.82 -7.69 -4.27

-2.30 0.61 -2.46 1.13 -8.99

-8.80 -6.34 -7.85 -6.49 -10.2

3.86 0.85 2.46 0.45 -2.49

-1.74 -1.54 -1.45 -1.03 -3.99

0.38 -0.31 0.38 0.11 -0.03

-0.92 -1.39 -1.05 -1.24 -1.99

-0.41 0.18 -0.10 0.22 -1.19

aSlope and intercept values multiplied by a factor of 108. b Definition of headings: Q vs t1/2 plot has a slope of S and an intercept of I . Q vs F F F R θ plot has a slope of SR and an intercept of IR. All headings are either the slope (S) or intercept (I) of the subscripted values. For example, SSF,∆E is the slope from the plot of SF (slope of QF vs t1/2 plot) vs ∆E, and IIR,∆E is the intercept from the plot of IR (intercept of QR vs θ plot) vs ∆E. c Average of C CLU obtained from forward and reverse potential steps.

Figure 6. Chronocoulometry plots of QF and QR against t1/2 (upper) and θ (lower) for 19.5 µM solution of Ptz7C6OH6C640-MPC (0.14 mM in Ptz) in 2:1 (v/v) toluene/acetonitrile/0.05 M Bu4NPF6 at Pt electrode. EINIT ) -200 (dark circle), 0 (open circle), 200 (dark triangle), and 400 mV (open triangle); EFINAL ) +800 mV. Chronocoulometry of C6 MPC-B solution at EINIT ) 0 mV (dark square) and EFINAL ) +800 mV shown for comparison.

Figure 7. Slopes SF and SR (A) and intercepts IF and IR (B) against ∆E of 19.5 µM of Ptz7C6OH6C640-MPC (0.14 mM in Ptz) in 2:1 (v/v) toluene/acetonitrile/0.05 M Bu4NPF6 at Pt electrode.

and reverse potential steps) of the slope and intercept of Figure 7A affords a value of CCLU ) 1.5 aF. This MPC capacitance is in reasonable agreement with that of the unsubstituted C6 MPC described above. Equations 6 and 7 also show that the intercepts IF and IR of plots of QF and QR against t1/2 and θ, respectively, should also vary linearly with ∆E, with zero intercepts and slopes the difference between which is CCLUFA[ΓINIT - ΓFINAL]/e. The slope difference yields in this case a ΓINIT - ΓFINAL value of ∼2 × 10-11 mol/cm2, with again the uncertainties mentioned above. It is interesting to compare the number of electrons per MPC consumed for the conversion of the average Ptz-MPC in a -0.2 to +0.8 V potential-step experiment. Using the average value CCLU obtained above, nDL ≈ 9 electrons as compared to the ∼7 electrons required per MPC for phenothiazine oxidation. The average MPC

core double-layer charging clearly cannot be neglected in experiments in which the core potential is changed by a significant value. Over a smaller range of potentialssjust those centered around the actual Ptz-MPC wave, say, 0.2 VsnDL ≈ 2. This is a smaller core-charging increment but is still not negligible relative to the ∼7 electrons required to oxidize the phenothiazine sites per MPC. Whether the extra 2 electrons/MPC would affect the limiting current measured in a microelectrode voltammetry experiment like Figure 1 would depend on how well the baseline extrapolation used in measuring the current accounts for the core-charging. We have, in the past, proceeded on the assumption that it does;6,16,17 the present results do not refute that view but add to it a maximum potential error in the DCLU obtained from the limiting currentsin the present case a factor of (9/7).

928 Analytical Chemistry, Vol. 73, No. 5, March 1, 2001

Table 3. Infrared Spectroscopic Data for n-Alkanethiolate MPCs, ω-Functionalized MPCs, and 2-D SAMsa drop-cast filmc

solutionc,d

C4 Ptz-MPC (C4) C3OH:C4 C2COOH:C4

2928.9, 2856.7 (she) 2928.6 (sh), 2848.9 2923.7, 2856.2 (sh) 2920.8, 2855.4 (sh)

2929.7, 2858.8 2928.5, 2856.3 2927.0, 2856.5 2925.9, 2856.9

C12 Ptz-MPC (C12) C11OH:C12 C10COOH:C12

2922.0, 2851.9 2921.1, 2850.6 2920.9, 2850.9 2920.2, 2850.8

2925.3, 2853.3 2921.4, 2851.0 2925.1, 2853.7 2925.6, 2854.2

MPC

2-D SAMb

C10 CH3(CH2)9SH HOCH2(CH2)10SH HO2C(CH2)10SH

2922.2, 2852.2 2920.5, 2850.5 2920.0, 2850.5 2925.0, 2853.0

CH3(CH2)15SH HOCH2(CH2)15SH HO2C(CH2)15SH

2917.8, 2849.5 2919, 2851 2919, 2851 2918, 2852

C16

a

Error ) (0.5 cm-1. b See refs 38c and 39. c d-,d+; in cm-1. d In CCl4. e sh ) shoulder.

Vibrational Spectroscopy. Qualitative success of the ester coupling reaction is readily assessed by the appearance of the ester carbonyl stretch υ(CdO) at 1734 cm-1 in an FT-IR spectrum of the Ptz-MPC product (Figure S-7, Supporting Information). This band lies at 1705 cm-1 for the carboxylic acid group of the phenothiazine carboxylic acid. Infrared spectroscopy has been utilized for investigating the conformation of alkane chains on 2-D SAMS.38 For adsorbed alkanethiolates, the positions of the antisymmetric (d-) and symmetric (d+) C-H methylene stretching vibrations are sensitive indicators of MPC chain conformation.23 Table 3 shows positions of these bands for n-alkanethiolate MPCs and ω-functionalized MPCs as a function of chain length in the solid state as drop-cast films and in CCl4 solution. The vibrational energies of the corresponding alkanethiolates as a 2-D SAM on a flat gold surface are listed for comparison.39 A lower wavenumber indicates that there is more ordering in the cluster chains. For comparison, the d-, d+ values, which are valuable guidelines for understanding the range of order and disorder, are for crystalline polyethylene, 2920, 2850 cm-1, and for liquid heptane, 2924, 2855 cm-1.40 For C4 MPCs in the solid state, it can be seen that when the MPC is functionalized by a phenothiazine, alcohol, or carboxylic acid group, the vibrational energy is decreased, indicating that chain ordering is increased. This effect is more dramatic for the alcohol and carboxylic acid groups. This significant increase in ordering could be due to hydrogen bonding among the alcohol or carboxylic acid groups, either on the same cluster (intramono(38) (a) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Reven, L. Langmuir 1996, 12, 1262-1269. (b) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (c) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (d) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (39) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682-691. (40) (a) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (b) Snyder, R. G.; Maroncelli, M.; Strauss, H. L.; Hallmark, V. M. J. Phys. Chem. 1986, 90, 5623.

layer H-bonding) or in the solid state, on different clusters (intercluster H-bonding). This effect has been observed previously in MPCs with longer chains terminated with carboxylic acids.41 There is little effect seen when the substituent is phenothiazine; its sizessignificantly larger than either the alcohol or carboxylic acid groupsmay yield a steric constraint on the ability of the thiolate chains to become more ordered. In solutions, the ordering effects are slight in all cases. For C12 MPCs, there is a slight increase in ordering when the MPC is functionalized, but to a lesser degree as observed in C4 MPCs. The increase in ordering is less because of the difference between the C12 and C4 alkanethiolate chain lengths; the unsubstituted C12 chain is already substantially ordered. The C10COOH:C12 MPC and the C10 carboxylic acid 2-D SAM offer an interesting comparison; placing a carboxylic acid on the end of the MPC thiolate chain increases order, while in the 2-D SAM it induces disorder. The comparison for the C16 chain length shows a slight lowering of order in accord with previous observations. ACKNOWLEDGMENT This research is supported in part by grants from the National Science Foundation and the Office of Naval Research. SUPPORTING INFORMATION AVAILABLE 1H NMR spectra of Ptz-MPC and C11OH:C12 exchanged MPC, chronocoulometry plots of C6 MPC-A and C6 MPC-B, slope and intercept analysis of C6 MPC-A chronocoulometry plots, and FTIR spectrum of Ptz-MPC. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review December 27, 2000.

October

26,

2000.

Accepted

AC0012647 (41) Schmitt, H.; Badia, A.; Dickinson, L.; Reven, L.; Lennox, R. B. Adv. Mater. 1998, 10, 475-480.

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