Electroactive Self-Assembled Monolayers on Gold via Bipodal

Jul 16, 2008 - Departments of Biochemistry and Molecular and Cell Biology, and Neurobiology, Physiology, and Radiology, Northwestern University...
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Langmuir 2008, 24, 9096-9101

Electroactive Self-Assembled Monolayers on Gold via Bipodal Dithiazepane Anchoring Groups Paul A. Bertin,*,† Dimitra Georganopoulou,† Taiyang Liang,† Amanda L. Eckermann,‡ Markus Wunder,† Michael J. Ahrens,† Gary F. Blackburn,† and Thomas J. Meade*,‡,§ Ohmx Corporation, 1801 Maple AVenue, Suite 6143, EVanston, Illinois 60201, and Departments of Chemistry, Biochemistry and Molecular and Cell Biology, Neurobiology and Physiology, and Radiology, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208-3113 ReceiVed April 14, 2008. ReVised Manuscript ReceiVed May 30, 2008 Novel dithiazepane-functionalized ferrocenyl-phenylethynyl oligomers 1 and 2 have been synthesized. Self-assembled monolayers (SAMs) of these ferrocene derivatives have been studied by X-ray photoelectron spectroscopy, ellipsometry, and cyclic voltammetry. It has been shown by XPS that monolayers of the dithiazepane-anchored molecules on gold electrodes contain gold-thiolate species. Cyclic voltammetry of the SAMs were characteristic of stable electroactive monolayers even for single-component SAMs of 1 and 2, with the more ideal responses recorded for the twocomponent SAMs diluted with undecanethiol. The small variation in peak splittings at progressively higher scan rates in these SAMs makes dithiazepane-bridged redox species promising candidates for further studies on molecular wires with bipodal anchoring.

Introduction Nanoscale charge transfer processes represent a rapidly advancing frontier of fundamental science,1–3 with applications in the development of molecular electronic devices4–9 and biosensor platforms.10–12 In particular, self-assembled monolayers (SAMs) of electroactive molecules adsorbed on noble metal electrodes have been intensely investigated as model systems for interfacial electron transfer events.13–17 The majority of electroactive SAMs studied to date comprise molecules with common design features, namely thiol-terminated organic bridges anchored to gold electrodes through gold-thiolate bonds with

ω-functionalized redox-active head groups. Since the pioneering work of Chidsey and co-workers13 in 1990, cyclic voltammetry (CV) studies of ferrocene-terminated SAMs have been extensively reported in the literature. The influence of bridge architecture,14,18–27 coadsorbed diluent molecules,18,28,29 and supporting electrolytes30–32 on the redox potential and shape of the voltammetric waves recorded for these SAMs has been well established.

* To whom correspondence should be addressed. E-mail: paul@ohmx. com (P.A.B.); [email protected] (T.J.M.). † Ohmx Corporation. ‡ Department of Chemistry, Northwestern University. § Departments of Biochemistry and Molecular and Cell Biology, and Neurobiology, Physiology, and Radiology, Northwestern University.

The most common type of bridge examined in this category has been the ferrocene-capped n-alkylthiolate system on gold (FcCnS-Au). This is in part due to synthetic feasibility, but primarily because these molecules are known to self-assemble into well-ordered two-component SAMs when diluted with nonelectroactive n-alkylthiols.16 Ideal reversible faradaic responses for these two-component FcCnS-Au SAMs, denoted by a symmetric waveshape with a full-width at half-maximum

(1) Gray, H. B.; Winkler, J. R. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 3534– 3539. (2) McCreery, R. L. Chem. Mater. 2004, 16, 4477–4496. (3) Adams, D. M.; Brus, L.; Chidsey, C. E. D.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat, P. V.; Lieberman, M.; Lindsay, S.; Marcus, R. A.; Metzger, R. M.; Michel-Beyerle, M. E.; Miller, J. R.; Newton, M. D.; Rolison, D. R.; Sankey, O.; Schanze, K. S.; Yardley, J.; Zhu, X. J. Phys. Chem. B 2003, 107, 6668–6697, and references therein. (4) Joachim, C.; Gimzewski, J. K.; Avarim, A. Nature 2000, 408, 541–548. (5) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384–1389. (6) Low, P. J. Dalton Trans. 2005, 2821–2824. (7) Flood, A. H.; Stoddart, J. F.; Steuerman, D. W.; Heath, J. R. Science 2004, 306, 2055–2056. (8) Tour, J. M. Molecular Electronics; World Scientific: Singapore, 2003. (9) Yasutomi, S.; Morita, T.; Imanishi, Y.; Kimura, S. Science 2004, 304, 1944–1947. (10) Yu, C. J.; Wan, Y.; Yowanto, H.; Li, J.; Tao, C.; James, M. D.; Tan, C. L.; Blackburn, G. F.; Meade, T. J. J. Am. Chem. Soc. 2001, 123, 11155–11161. (11) Willner, I.; Katz, E. Angew. Chem., Int. Ed. 2000, 39, 1180–1218. (12) (a) Kerman, K.; Mahmoud, K. A.; Kraatz, H.-B. Chem. Commun. 2007, 3829–3831. (b) Mahmoud, K. A.; Kraatz, H.-B. Chem. Eur. J. 2007, 13, 5885– 5895. (13) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301–4306. (14) Chidsey, C. E. D. Science 1991, 251, 919–922. (15) Tender, L.; Carter, M. T.; Murray, R. W. Anal. Chem. 1994, 66, 3173– 3181. (16) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103–1170. (17) Finklea, H. O. In Electroanalytical Chemistry: A Series of AdVances; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19.

(18) Lee, L. Y. S.; Sutherland, T. C.; Rucareanu, S.; Lennox, R. B. Langmuir 2006, 22, 4438–4444. (19) Sumner, J. J.; Weber, K. S.; Hockett, L. A.; Creager, S. E. J. Phys. Chem. B 2000, 104, 7449–7454. (20) Collman, J. P.; Devaraj, N. K.; Chidsey, C. E. D. Langmuir 2004, 20, 1051–1053. (21) Seo, K.; Jeon, I. C.; Yoo, D. J. Langmuir 2004, 20, 4147–4154. (22) Sabapathy, R. C.; Bhattacharyya, S.; Leavy, M. C.; Cleland, W. E.; Hussey, C. L. Langmuir 1998, 14, 124–136. (23) Dudek, S. P.; Sikes, H. D.; Chidsey, C. E. D. J. Am. Chem. Soc. 2001, 123, 8033–8038. (24) Creager, S.; Yu, C. J.; Bamdad, C.; O’Connor, S.; MacLean, T.; Lam, E.; Chong, Y.; Olsen, G. T.; Luo, J.; Gozin, M.; Kayyem, J. F. J. Am. Chem. Soc. 1999, 121, 1059–1064. (25) Smalley, J. F.; Sachs, S. B.; Chidsey, C. E. D.; Dudek, S. P.; Sikes, H. D.; Creager, S. E.; Yu, C. J.; Feldberg, S. W.; Newton, M. D. J. Am. Chem. Soc. 2004, 126, 14620–14630. (26) Chambers, R. C.; Inman, C. E.; Hutchison, J. E. Langmuir 2005, 21, 4615–4621. (27) Weber, K.; Hockett, L.; Creager, S. E. J. Phys. Chem. B 1997, 101, 8286–8291. (28) Sumner, J. J.; Creager, S. E. J. Phys. Chem. B 2001, 105, 8739–8745. (29) Smalley, J. F.; Finklea, H. O.; Chidsey, C. E. D.; Linford, M. R.; Creager, S. E.; Ferraris, J. P.; Chalfant, K.; Zawodzinsk, T.; Feldberg, S. W.; Newton, M. D. J. Am. Chem. Soc. 2003, 125, 2004–2013. (30) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307–2312. (31) Rowe, G. K.; Creager, S. E. J. Phys. Chem. 1994, 98, 5500–5507. (32) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. J. Elecroanal. Chem. 1994, 372, 117–124.

10.1021/la801165b CCC: $40.75  2008 American Chemical Society Published on Web 07/16/2008

ElectroactiVe SAMs Via Bipodal Anchoring on Gold

(∆Efwhm) of 3.53RT/nF (90.6 mV at 25 °C)33 and zero peak splitting (∆Ep ) 0) at low scan rates, are rarely observed and have only been reported at low ferrocene surface coverages.13,14 Ideal CV responses are predicted for electroactive SAMs composed of identical, noninteracting tethered redox groups.17 Conversely, several studies have reported abnormal asymmetric peak broadening, peak splitting, or the appearance of additional peaks for SAMs with higher surface coverages of ferrocene.13,18,21,22,26,31,35–39 The irregular electrochemical behavior has been attributed to thermodynamic and/or kinetic heterogeneity of redox centers. Such behavior could be due in part to steric or electrostatic effects, domain boundaries, isolated microenvironments within the monolayers, and double layer capacitance differences due to spatial dispersions.18,40 In addition, studies have emerged involving electroactive SAMs prepared from ferrocene derivatives with conjugated “molecular wire” bridges (i.e., oligophenylethynylene (OPE)24,25 or oligophenylvinylene (OPV)23,41) of variable length and structure for the purpose of characterizing the influence of bridge length and composition on interfacial electron-transfer kinetics. The strong electronic coupling reported in these systems suggests that unsaturated, fully conjugated organic bridges may be useful components for wiring molecular electronic elements. Our interest in the development of electrochemical biosensor platforms from complex molecular redox subunits wired to gold electrode arrays has prompted our investigation into alternate OPE-bridged electroactive monolayers with bipodal anchoring groups. Multivalent anchoring in SAMs is often desirable because molecules are less dependent on intermolecular self-assembly interactions to impart rigidity and may exhibit enhanced stabilities toward exchange and desorption.42–46 Recently, SAMs of dithiolated scaffolds on gold have been studied as supports for the immobilization of antibodies in a biosensor application.47 In addition, SAMs of redox-active molecules bearing multipodal anchors have demonstrated superior stability over those with single-site anchors without sacrificing electrochemical performance in studies on molecular information storage.48 Here, we report our strategy for preparing bipodally anchored electroactive SAMs utilizing novel dithiazepane-functionalized ferrocenyl-phenylethynyl oligomers 1 and 2 as examples (Figure 1A). The molecules vary by one p-phenylethynyl unit to probe the influence of length on the CV response. Importantly, the air-stable dithiazepane moiety in the bridge precludes the need for sulfur protecting groups during synthetic transformations. (33) Laviron, E. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1982; Vol. 12. (34) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510–1514. (35) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687–2693. (36) Voicu, R.; Ellis, T. H.; Ju, H.; Leech, D. Langmuir 1999, 15, 8170–8177. (37) Rowe, G. K.; Creager, S. E. Langmuir 1994, 10, 1186–1192. (38) Quist, F.; Tabard-Cossa, V.; Badia, A. J. Phys. Chem. B 2003, 107, 10691– 10695. (39) Yao, X.; Wang, J.; Zhou, F.; Wang, J.; Tao, N. J. Phys. Chem. B 2004, 108, 7206–7212. (40) Calvente, J. J.; Andreu, R.; Molero, M.; Lo´pez-Pe´rez, G.; Domı´nguez, M. J. Phys. Chem. B 2001, 105, 9557–9568. (41) Sikes, H. D.; Smalley, J. F.; Dudek, S. P.; Cook, A. R.; Newton, M. D.; Chidsey, C. E. D.; Feldberg, S. W. Science 2001, 291, 1519–1523. (42) Wooster, T. T.; Gamm, P. R.; Geiger, W. E. Langmuir 1996, 12, 6616– 6626. (43) Liu, H.; Liu, S.; Echegoyen, L. Chem. Commun. 1999, 1493–1494. (44) Dong, Y.; Abaci, S.; Shannon, C.; Bozack, M. J. Langmuir 2003, 19, 8922–8926. (45) Garg, N.; Lee, T. R. Langmuir 1998, 14, 3815–3819. (46) Park, J.-S.; Vo, A. N.; Barriet, D.; Shon, Y.-S.; Lee, T. R. Langmuir 2005, 21, 2902–2911. (47) Fragoso, A.; Laboria, N.; Latta, D.; O’Sullivan, C. K. Anal. Chem. 2008, 80, 2556–2563. (48) Wei, L.; Padmaja, K.; Youngblood, W. J.; Lysenko, A. B.; Lindsey, J. S.; Bocian, D. F. J. Org. Chem. 2004, 69, 1461–1469.

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Figure 1. (A) Dithiazepane-functionalized ferrocenyl-phenylethynyl oligomers 1 and 2. (B) Schematic representation of single-component SAMs of 1 and 2 (SAM-1a, SAM-2a) and two-component SAMs (SAM1b, SAM-2b) deposited with undecanethiol (20 equiv).

Additionally, we anticipated 1 and 2 would adsorb on gold via cleavage of the disulfide bond in the dithiazepane ring to form stable alkanethiolate-gold bonds.49 SAMs of 1 and 2 were prepared in the absence (single-component SAMs 1a and 2a) and presence (two-component SAMs 1b and 2b) of undecanethiol diluents (Figure 1B). The monolayers were characterized by X-ray photoelectron spectroscopy (XPS), ellipsometry, and cyclic voltammetry (CV).

Results and Discussion Design and Synthesis of Ferrocene Derivatives. The synthesis of 1 and 2 is summarized in Scheme 1. A modified literature procedure was developed for 3 to avoid using ethylene oxide as a reagent en route to the key intermediate 5-(4-iodophenyl)[1,2,5] dithiazepane (5).50 Bisalkylation of 4-iodoaniline with 2-chloroethanol in the presence of K2CO3 afforded 3. Subsequent chlorination with POCl3 gave 4 and further treatment with KSCN yielded the dithiazepane subunit 5. This compound was coupled with the appropriate terminal alkynyl-functionalized ferrocene derivative via Sonogashira methodology to yield 1 and 2. Crystals of 2 were characterized by X-ray diffraction to reveal the structure of the dithiazepane moiety (Figure 2). The rod-like ferrocene-derivative is slightly bowed with a molecular length (S1 to Fe1) of 18.26 Å. The dithiazepane moiety adopts a boatlike conformation with torsional angles of 41.0° (N1-C27C28-S1) and 62.5° (N1-C30-C29-S2). Dihedral angles between the phenyl ring planes and between the first ring (C13-C18) and the ferrocene Cp ring (C6-C10) are 148.6° and 79.6°, respectively. This significant deviation from coplanarity is in contrast to the nearly coplanar arrangement reported for a similar thioether-terminated OPE-bridged ferrocene.51 Monolayer Preparation and Characterization. SAM-1a and SAM-2a were prepared by incubating polycrystalline Au(111) electrodes in THF solutions of 1 or 2 (0.1 mM) for 24 h. For two-component mixed monolayers (SAM-1b and SAM-2b), undecanethiol (2 mM, 20 equiv) was included in the deposition solutions. Following deposition, the electrodes were washed extensively and dried over a stream of Ar as described in the Experimental Section. The elemental compositions of SAM-1a and SAM-2a were analyzed by XPS and representative spectra are given in Figure 3. In addition to the characteristic signals from bulk gold, the spectra exhibit multiple peaks consistent with the molecular (49) Ishida, T.; Yamamoto, S.; Mizutani, W.; Motomatsu, M.; Tokumoto, H.; Hokari, H.; Azehara, H.; Fujihira, M. Langmuir 1997, 13, 3261–3265. (50) Li, Q.; Rukavishnikov, A. V.; Petukhov, P. A.; Zaikova, T. O.; Keana, J. F. W. Org. Lett. 2002, 4, 3631–3634. (51) Hsung, R. P.; Chidsey, C. E. D.; Sita, L. R. Organometallics 1995, 14, 4808–4815.

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Bertin et al. Scheme 1. Synthesis of 1 and 2a

a Reaction conditions: (a) 2-chloroethanol, K2CO3; (b) POCl3, pyridine; (c) KSCN, EtOH/H2O, ∆; (d) ethynyl ferrocene, Pd(PPh3)2Cl2, CuI, DIPEA/THF; (e) (4-iodophenylethynyl)-trimethylsilane, Pd(PPh3)2Cl2, CuI, THF/TEA; (f) TBAF, THF; (g) 5, Pd(PPh3)2Cl2, CuI, THF/TEA.

Figure 2. X-ray structure of crystalline 2 from two perspectives. Hydrogen atoms omitted for clarity.

structures of 1 and 2. The C 1s region of the monolayers shows identical peaks at 284.5 eV for both SAM-1a and SAM-2a. The symmetry of the C 1s binding energies (BEs) is expected considering the C atoms in 1 and 2 are nearly identical. For SAM-1a, two peaks exist in the Fe 2p region at 707.5 eV (Fe 2p3/2) and 721.0 eV (Fe 2p1/2). These same Fe 2p peaks are shifted ∼0.5 eV in SAM-2a to 708.0 eV (Fe 2p3/2) and 721.5 eV (Fe 2p1/2). Both sets of values are within the experimental range of those reported previously for ferrocene-terminated SAMs on Au.52 The character of the sulfur-gold bonding in SAMs is indicated by the high-resolution core level data in the S 2p region. For alkyl thiols or dialkyl disulfides bound to gold, the spectra are defined by the S 2p3/2, S 2p1/2 doublet at BEs of 162 and 163.2 eV with an intensity ratio of 2:1.53,54 Although the sulfur region exhibited peaks with low signal-to-noise ratios due to significant attenuation by the overlying atoms in the monolayers, the BEs of the S 2p3/2 peak and the S 2p1/2 peak for SAM-1a were observed at 162.1 and 163.4 eV, respectively, with an intensity ratio near 1.5:1 (Figure 3, insets). These peaks are slightly shifted for SAM2a to 162.4 and 163.7 eV with an intensity ratio closer to 1:1. The observed S 2p3/2 BEs of ca. 162 eV for both SAMs are consistent with sulfur atoms bound to the gold electrodes as thiolate species.53 Deviations from the expected 2:1 peak intensity of the S 2p doublets for SAMs 1a and 2a are most likely explained by incomplete binding of some S atoms to gold. It has been shown previously by XPS that SAMs prepared with sulfurcontaining multipodal tethers may exhibit incomplete binding of (52) Zhang, X. G.; Shi, Y. L.; Li, H. L. J. Colloid Interface Sci. 2002, 246, 296–301. (53) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083– 5086. (54) Ishida, T.; Choi, N.; Mizutani, W.; Tokumoto, H.; Kojima, I.; Azehara, H.; Hokari, H.; Akiba, U.; Fujihara, M. Langmuir 1999, 15, 6799–6806.

Figure 3. XPS spectra of (A) SAM-1a and (B) SAM-2a. The highresolution spectra of the S 2p regions are shown (inset) and spectra were referenced by setting the Au 4f7/2 binding energy to 84.0 eV.

the sulfur atoms to gold resulting in higher peak intensities at ca. 164 eV.46,55 Alternatively, previous studies of polymer monolayers anchored to gold through dialkyl disulfide side chains have reported S 2p3/2 BEs of ca. 164 eV for unbound disulfides.56 However, the existence of unbound disulfides in SAMs 1a and 2a is less likely since the monolayers were rigorously washed prior to analysis. Average monolayer thickness values were estimated from multiple ellipsometry measurements of each SAM at different spots (Table 1). As a control, bare gold electrodes handled in a similar manner gave average thickness measurements of 0.06 ((0.01) nm. For SAMs 1a and 1b, the thickness values were (55) Wei, L.; Tiznado, H.; Liu, G.; Padmaja, K.; Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Phys. Chem. B 2005, 109, 23963–23971. (56) Sun, F.; Castner, D. G.; Grainger, D. W. Langmuir 1993, 9, 3200–3207.

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Table 1. SAM Thicknesses and Electrochemical Properties entry SAM-1a SAM-1b SAM-2a SAM-2b a

thicknessa (nm) 1.04((0.10) 1.12((0.05) 1.84((0.08) 1.60((0.09)

E0′ (mV vs Ag/AgCl) 386((2) 430((8) 436((8) 468((18)

∆Ep (mV) 31((4) 10((3) 25((6) 8((3)

∆Efwhm (mV)

Γ (mol cm-2)

no. (cm-2)

160 125 140 105

1.1 × 10 7.0 × 10-12 2.2 × 10-10 9.0 × 10-12

6.5 × 1013 4.0 × 1012 1.3 × 1014 5.4 × 1012

-10

Determined by ellipsometry and compared to the theoretical length of 1 (1.11 nm)57 and molecular length of 2 (1.83 nm).

Figure 4. Representative cyclic voltammograms (vs Ag/AgCl) of SAM1a (solid line) and SAM-2a (dashed line) in PBS buffer measured at 100 mV/s.

1.04 ((0.10) and 1.12 ((0.05) nm, respectively. These values are in close agreement with the calculated molecular length of 1 and undecanethiol.57 The thicknesses of SAMs 2a and 2b were 1.84 ((0.08) and 1.60 ((0.09) nm, in general agreement with the molecular length of 2 measured in the crystal structure. The decreased thickness for SAM-2b is reasonable considering the shorter length of the diluent. Given the near zero value for the bare gold surface, we regard the thickness values for both sets of SAMs as close approximations of the actual physical thickness of densely packed monolayers of 1 and 2 in the presence and absence of undecanethiol diluents. Both sets of SAMs containing 1 or 2 were tested electrochemically by CV in an aqueous phosphate buffer and the apparent formal potentials, E0′, (defined as the mean of the oxidation and reduction peak potentials), the peak splittings, ∆Ep, the ∆Efwhm values, the surface coverages (Γ), and the corresponding surface excess (no.) of redox-active molecules are reported in Table 1. Representative CVs for SAMs 1a and 2a at 100 mV/s are shown in Figure 4. The E0′ of SAM-2a is 50 mV more positive than SAM-1a, a value consistent with the potential difference observed between 1 and 2 in solution (see Experimental Section). The dependence of the ferrocene formal potential on the bridge length is interesting and suggests strong electronic communication. The higher positive E0′ for SAM-2a compared to 1a is in general agreement with previous reports on SAMs of OPE-bridged ferrocene derivatives where E0′ values trended more positive at increasing bridge lengths.25,58 Both CVs are broad Gaussian waves with the ∆Efwhm of SAM 1a (160 mV) slightly wider than SAM2a (140 mV) with similar surface coverages. The peak splittings of 31((4) mV for SAM-1a and 25((6) mV for SAM-2a suggest a slight structural hysteresis in the monolayers.23 Although the deviation from ideal reversible faradaic responses for both SAMs 1a and 2a is not surprising, it is noteworthy that neither CV exhibits significant asymmetry nor contains additional peaks as (57) Molecular lengths calculated using Mm+ in HyperChem Lite (version 2.0) (Hypercube, Inc., Gainesville, FL 32601). (58) Smalley, J. F.; Newton, M. D.; Feldberg, S. W. Electrochem. Commun. 2000, 2, 832–838.

Figure 5. Cyclic voltammograms (vs Ag/AgCl) for SAMs 1a, 1b, 2a, and 2b overlaid at the following scan rates: 0.1, 0.2, 0.5, 0.7, 1, 2, 5, 7, and 10 V/s.

has been observed previously for single-component alkyl-bridged FcCnS-Au SAMs or SAMs at high ferrocene mole fractions.13,18 Interestingly, electrochemical studies of single-component SAMs prepared from conjugated “molecular wire” functionalized ferrocene derivatives on gold have been limited.48 The pronounced peak broadening for SAMs 1a and 2a suggest either interaction between ferrocene sites, structural inhomogeneity, or ferrocene interaction with the interfacial charge of the double layer.59,60 Additional spectroscopic studies aimed at elucidating the molecular order of SAMs 1a and 2a may be needed to further explain the observed electrochemical responses. The peak splittings and ∆Efwhm values in the CVs for twocomponent SAMs 1b and 2b are decreased relative to the singlecomponent SAMs and approach more ideal behavior. While both SAMs 1b and 2b have positive potential increases with respect to SAMs 1a and 2a, the statistically more significant potential increase for SAM-1b may be explained by a slight burying of the shorter ferrocene derivative in neighboring undecanethiol molecules.19 The high surface excess for SAMs 1b and 2b suggest adequate SAM formation and packing of the dithiazepanefunctionalized ferrocene derivatives with undecanethiol. Cyclic voltammograms at increasing scan rates (from 0.1 to 10 V/s) for each SAM are overlaid in Figure 5. Interestingly, repeated scans did not influence the CV peak height and shape suggesting SAM stability. Plots of anodic peak currents versus scan rates for each SAM (data not shown) were fit linearly confirming surface confinement of ferrocene.17 The slopes of the fitted lines were used to derive the surface excess values presented in Table 1. The higher currents in SAMs 1a and 2a compared to 1b and 2b can be attributed to the higher ferrocene coverage in the absence of diluents. The positive potential window edges (59) Smith, C. P.; White, H. S. Anal. Chem. 1992, 64, 2398–2405. (60) Ohtani, M.; Kuwabata, S.; Yoneyama, H. Anal. Chem. 1997, 69, 1045– 1053.

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of the CV responses for SAMs 1b and 2b were restricted due to the initiation of an irreversible oxidative peak at higher positive potentials. This redox process was not observed in SAMs 1a and 2a, but reproducibly occurred in the presence of diluents. Further studies aimed at elucidating this process are ongoing. As expected, conventional electrochemical techniques (CV, chronoamperometry) were not sufficient to accurately measure the electron transfer (ET) rates for the SAMs studied herein. However, previous studies on SAMs of ferrocene-OPE derivatives with single thiol anchors have measured ET rates using an indirect laser induced temperature jump (ILIT) technique,9 or ACV techniques.24 Regardless, the small variation in peak splittings at progressively higher scan rates for SAMs of 1 and 2 phenomenologically indicates ET rates of at least 104 s-1, consistent with similarly behaving electroactive moieties in a monolayer.27

Conclusions Novel dithiazepane-functionalized ferrocenyl-phenylethynyl oligomers 1 and 2 have been synthesized, structurally characterized, and used to prepare stable, bipodally anchored electroactive SAMs in the presence and absence of undecanethiol diluents. XPS analyses of single-component SAMs 1a and 2a confirm the dithiazepane-anchored bridges contain gold-thiolate species. The thickness values for each SAM, estimated by ellipsometry, were consistent with molecular monolayers. The cyclic voltammetry of all SAMs described were characteristic of stable electroactive monolayers with the more ideal responses recorded for twocomponent SAMs 1b and 2b diluted with undecanethiol. Therefore, the preparation of alternate monolayers utilizing the dithiazepane group as a bipodal anchoring unit may be of broad utility in future molecular electronics applications. Hence, further studies will focus on elucidating the observed electrochemical behavior and gain insight into ET rates and the influence of diluents.

Experimental Section Unless otherwise noted, all synthetic manipulations were performed under a dry argon atmosphere using standard Schlenk techniques. For reaction media, solvents were dried over neutral alumina via the Dow-Grubbs solvent system61 acquired from Glass Contours (Laguna Beach, CA). These solvents were deoxygenated with argon prior to use. Flash chromatography was carried out using silica gel 60 (particle size: 40-63 µm) (Sorbent Technologies, Atlanta, GA) under a positive pressure of laboratory air. 1H and 13C NMR spectra were recorded on a Varian INOVA 500 FT-NMR spectrometer (500 MHz for 1H NMR, 125 MHz for 13C NMR). 1H NMR data are reported as follows: chemical shift, multiplicity (b ) broad, s ) singlet, d ) doublet, t ) triplet, q ) quartet, pt ) pseudo triplet from a nonresolved doublet of doublets, and m ) multiplet), integration, and peak assignments. 1H and 13C chemical shifts are reported in ppm downfield from tetramethylsilane (TMS). Absorbance spectra were collected using an Ocean Optics S200 Dual Channel spectrometer equipped with a DH-2000-BAL light source. Matrix-assisted laser desorption ionization time-offlight (MALDI-TOF) mass spectrometry was obtained on a Perspective Biosystems Voyager DE-Pro mass spectrometer. Electrospray ionization (ESI) mass spectrometry was obtained on a Finnigan LCQ Advantage mass spectrometer. Elemental analyses were performed by Quantitative Technologies, Inc. (Whitehouse, NJ). Electrochemical experiments were carried out with a CHI model 660A electrochemical analyzer (CHI Instruments Inc.) in a threeelectrode system, with a Ag/AgCl reference wire, a platinum wire as counter electrode (Bioanalytical Systems) and evaporated gold (61) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518–1520.

Bertin et al. substrates as the working electrodes. For SAMs, experiments were run at room temperature in phosphate buffer (0.05 M K3PO4, 0.05 M KCl, and 0.05 M KPF6, pH 7.4) using cyclic voltammetry at various scan rates. Phosphate buffer was selected as a supporting electrolyte in association with our parallel studies of utilizing similar SAMs as probes for ligand-receptor interactions in a biosensor platform.62 Electrochemical measurements in solution were carried out in THF using a freshly cleaned platinum microdisc electrode (d ) 2 mm, CHI Instruments) with a three-electrode system as above, and 0.1 M tetrabutylammonium hexafluorophospate (TBAPF6) as the electrolyte. Solution measurements were performed with a Agquasireference electrode that was calibrated with ferrocene as a potential standard. The electrochemical data for each SAM in Table 1 is reported as an average of four separate experiments with standard deviations. Materials. Compounds 4, 5, 6, and 7 were synthesized as described previously.50,51 Cold storage under an inert atmosphere is recommend for aryl-iodide compounds 3, 4, and 5 as slight decomposition was observed for each sample after prolonged exposure to ambient atmosphere at room temperature. Chloroform-d1 was purchased from Cambridge Isotope Laboratories. All other reagents were purchased from commercial sources and used without further purification unless otherwise noted. Reactions were monitored by TLC (aluminum backed silica gel sheets 60 F254; EMD Chemicals, Inc., Gibbstown, NJ) and spots were visualized by fluorescence quenching upon exposure to UV light. For the electrochemical measurements, deionized water was used after it was passed through an Aqua Solutions system equipped with a combined reverse osmosis deionized system and a UV sterilization lamp, for a final product that has a resistivity of 18.0 MΩ cm. Evaporated gold substrates (glass slides, 2.54 cm × 7.62 cm, with an underlayer of chromium (5 nm) followed by 99.9% gold (100 nm)) were obtained from EMF Corporation (Ithaca, NY). Monolayer Preparation. Gold-evaporated electrodes were cleaned with plasma ionization (19.99% oxygen:argon, Harrick Plasma) for 5 min, washed with ethanol and THF, and immersed in THF deposition solutions for 24 h at room temperature in the dark. For single-component SAMs, THF solutions of 1 or 2 (0.1 mM) with NH4PF6 (0.1 M) were used whereas for two-component mixed monolayers, undecanethiol (2 mM) was added to the deposition solution as diluent. The deposition solutions included NH4PF6 for protocol consistency with alternate electroactive SAMs in our laboratory. The electrodes were washed extensively with THF and ethanol and dried with a stream of argon. X-ray Photoelectron Spectroscopy. XPS measurements were performed using an Omicron ESCA Probe with monochromated Al KR radiation located at Northwestern University Keck Interdisciplinary Surface Science Center. The sample was oriented with a 45° photoelectron takeoff angle from the sample surface to the hemispherical analyzer. An analyzer pass energy of 70 eV with 500 meV steps was used for single-sweep survey scans. High-resolution spectra were averaged over six sweeps using an analyzer pass energy of 26 eV with 20 meV steps. XPS spectra were processed using Omicron EIS v2.4 software and peaks were fit with a GaussianLorentzian sum function using Spectral Data Processor v4.3. All spectra were referenced to the Au 4f7/2 binding energy at 84.00 eV. The XPS substrates were SAM-modified EMF evaporated gold substrates (1.27 cm2), washed extensively with THF and ethanol and dried over a stream of argon prior to XPS analysis. Ellipsometry. SAM thicknesses were estimated using a MOSS ES4G variable angle spectroscopic ellipsometer. Measurements were carried out immediately after monolayer adsorption. Data was collected at 76° from the surface normal over a wavelength range of 200 to 2500 nm. In this study, a refractive index (nf) of 1.60 for all the SAMs was used to estimate monolayer thickness values. Literature reports of oligophenylethynylene (OPE) or oligophenylvinylene (OPV) SAMs have used refractive indices ranging from (62) Eckermann, A. L.; Barker, K. D.; Hartings, M. R.; Ratner, M. A.; Meade, T. J. J. Am. Chem. Soc. 2005, 127, 11880–11881.

ElectroactiVe SAMs Via Bipodal Anchoring on Gold 1.45 to 1.8 to estimate monolayer thickness.63 Optical constants of the underlying gold were used as supplied by the software manufacturer (Winelli). N,N-Bis(2-hydroxyethyl)-4-iodoaniline (3). This procedure was developed as an alternative to the previously reported method for 3.50 To a 250-mL Schlenk flask was added K2CO3 (12.0 g, 90.0 mmol). The flask was placed under vacuum (20 min) with mild heating. 4-Iodoaniline (3.90 g, 18.0 mmol) was added and the flask was placed under an Ar atmosphere. 2-Chloroethanol (30 mL, ∼450 mmol) was added and the flask was fitted with a reflux condenser and heated to 55 °C under Ar for 16 h. The reaction mixture was concentrated in vacuo and the crude residue was transferred to a 500-mL separatory funnel with CH2Cl2 (200 mL). The organic phase was washed with brine (3 × 100 mL), dried over Na2SO4, filtered, and concentrated to a yellow oil. The residue was purified by chromatography on silica gel (1%MeOH:CH2Cl2 f 5%MeOH: CH2Cl2 gradient). All fractions containing the product (5%MeOH: CH2Cl2, Rf ) 0.5) were combined to yield a viscous clear oil. Recrystallization from ethyl acetate/hexanes yielded the desired product as a white, microcrystalline solid (2.95 g, 9.61 mmol, 53%). 1H NMR (CDCl ): δ 3.50 (t, J 3 H-H ) 4.8 Hz, 4H, 2 HOCH2CH2N); 3.77 (t, JH-H ) 4.8 Hz, 4H, 2 HOCH2CH2N); 4.01 (bs, 2H, HOCH2); 6.44 (d, JH-H ) 9.0 Hz, 2H, aromatic-H); 7.45 (d, JH-H ) 9.0 Hz, 2H, aromatic-H). 13C NMR (CDCl3): δ 55.39, 60.64, 77.85, 114.92, 137.96, 147.48. MS (ESI) m/z: 308.02 (M+H)+. 5-(4-Ferrocenylethynylphenyl)-[1,2,5]dithiazepane (1). To a 50-mL Schlenk flask was added 5 (0.100 g, 2.97 mmol). The flask was placed under Ar and THF (5 mL) was added followed by Pd(PPh3)2Cl2 (0.010 g, 0.014 mmol), copper(I) iodide (0.006 g, 0.032 mmol), and ethynyl ferrocene (0.067 g, 3.19 mmol) as solids under Ar. N,N-Diisopropylethylamine (0.26 mL, 1.5 mmol) was added and the flask was fitted with a reflux condenser and heated to 60 °C for 16 h. The volatiles were removed in vacuo and the crude brown residue was purified by chromatography on silica gel (40% CH2Cl2: hexanes) to yield the pure product as an orange solid (0.050 g, 0.12 mmol, 40%). 1H NMR (CDCl3): δ 3.11 (t, JH-H ) 5.5 Hz, 4H, 2 SCH2CH2N), 4.00 (t, JH-H ) 5.5 Hz, 4H, 2 SCH2CH2N), 4.22 (pt, 2H, ferrocene-H), 4.25 (bs, 5H, ferrocene-H), 4.81 (pt, 2H, ferroceneH), 6.59 (d, JH-H ) 9.0 Hz, 2H, aromatic-H), 7.38 (d, JH-H ) 9.0 Hz, 2H, aromatic-H). 13C NMR (CDCl3): δ 37.0, 52.6, 66.6, 68.7, 70.1, 71.4, 85.9, 86.4, 111.3, 111.5, 133.2, 146.1. MS (MALDITOF) m/z: 420.30 (M+H)+. λabs (THF) 315, 280 (sh), 445 (wk) nm. Solution electrochemistry in THF, 0.1 M TBAPF6: Eo) 482 mV vs Ag/AgCl, D)1.78 x10-8 cm2s-1. Elemental analysis for C22H21FeNS2: calcd. C 63.01, H 5.05, N 3.34; found C 62.96, H 5.09, N 3.24. (63) Richter, L. J.; Yang, C. S.-C.; Wilson, P. T.; Hacker, C. A.; van Zee, R. D.; Stapleton, J. J.; Allara, D. L.; Yao, Y.; Tour, J. M. J. Phys. Chem. B 2004, 108, 12547–12559.

Langmuir, Vol. 24, No. 16, 2008 9101 5-[4-(4-Ferrocenylethynylphenylethynyl)phenyl]-[1,2,5]dithiazepane (2). To a 100-mL Schlenk flask was added 5 (0.100 g, 0.31 mmol). The flask was placed under Ar and deoxygenated triethylamine (4 mL) was added followed by Pd(PPh3)2Cl2 (0.011 g, 0.015 mmol), copper(I) iodide (0.006 g, 0.030 mmol), and 7 (0.200 g, 0.95 mmol) as solids under Ar. THF (2 mL) was added and the mixture stirred at room temperature for 16 h. The volatiles were removed in vacuo and the crude residue was suspended in CH2Cl2/hexanes (1:1 v/v) and filtered through Celite. The filtrate was concentrated in vacuo and the crude residue was purified by chromatography on silica gel (40% CH2Cl2:hexanes) to yield the pure product (Rf ) 0.5) as an orange solid (0.118 g, 0.23 mmol, 73%). 1H NMR (CDCl3): δ 3.11 (t, JH-H ) 5.5 Hz, 4H, 2 SCH2CH2N), 4.00 (t, JH-H ) 5.5 Hz, 4H, 2 SCH2CH2N), 4.27 (m, 7H, ferrocene-H), 4.52 (pt, 2H, ferrocene-H), 6.62 (d, JH-H ) 9.0 Hz, 2H, aromatic-H), 7.42 (d, JH-H ) 9.0 Hz, 2H, aromatic-H), 7.45 (m, 4H, aromatic-H). 13C NMR (CDCl3): δ 37.0, 52.7, 65.3, 69.1, 70.2, 71.6, 85.9, 87.7, 90.3, 92.3, 110.6, 111.4, 123.2, 123.5, 131.3, 131.4, 133.4, 146.7. MS (MALDI-TOF) m/z: 520.64 (M+H)+. λabs (THF) 367, 281 (sh), 445 (wk) nm. Solution electrochemistry in THF, 0.1 M TBAPF6: Eo ) 532 mV vs Ag/AgCl, D ) 1.98 × 10-8 cm2 s-1. Elemental analysis for C30H25FeNS2: calcd. C 69.36, H 4.85, N 2.70; found C 68.42, H 4.98, N 2.47. Crystals suitable for X-ray analysis were obtained from CH2Cl2/ pentane at 5 °C. Single-crystal X-ray diffraction data were collected on a Bruker SMART-1000 diffractometer equipped with Mo KR radiation (λ ) 0.71073 Å). Reflections were integrated with the SAINT-Plus program. The structure was solved in the space group P21/c by direct methods and refined against F2 using full-matrix least-squares techniques. The crystal is a merohedral twin with the two components related by the twin law (100/010/001). One component is rotated from first domain by 180 degrees about the reciprocal axis 100 and real axis 100. Absorption correction on one twinned data set was applied using Twinabs, a part of the SAINT integration program. That set of diffraction points was used to solve the structure including anisotropic displacement parameters before a final refinement with all reflections. The twin fraction refined to 50.6%. The Cambridge Crystallographic Data Centre contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from CCDC 658504 via www.ccdc.cam.ac.uk/data_request/cif.

Acknowledgment. We acknowledge financial support from the Illinois DCEO. We thank Charlotte L. Stern in the Integrated Molecular Structure Education and Research Center at Northwestern University for X-ray data collection. LA801165B