Anion Templated Formation of Pseudorotaxane and Rotaxane

Feb 10, 2009 - Liyun Zhao, Jason J. Davis,* Kathleen M. Mullen, Michał J. Chmielewski,. Robert M. J. Jacobs, Asha Brown, and Paul D. Beer*. Chemistry...
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Langmuir 2009, 25, 2935-2940

2935

Anion Templated Formation of Pseudorotaxane and Rotaxane Monolayers on Gold from Neutral Components Liyun Zhao, Jason J. Davis,* Kathleen M. Mullen, Michał J. Chmielewski, Robert M. J. Jacobs, Asha Brown, and Paul D. Beer* Chemistry Research Laboratory, UniVersity of Oxford, Mansfield Road, Oxford, UK OX1 3TA ReceiVed December 1, 2008. ReVised Manuscript ReceiVed January 10, 2009 The surface covalent attachment of indolocarbazole axles enables anion templation to be exploited in the formation of pseudorotaxane assemblies via the threading of neutral isophthalamide macrocycles from solution. The anion selectivity of this templating process can be monitored by a number of surface spectroscopic methods and shows subtle differences compared to the same process in solution. Though the fluxional and disordered nature of ethylene glycol extended axle adlayers prohibits detectable threading on the surface, rotaxane monolayers can be generated by a preassociation of the components and templating anion in solution. The threaded macrocycles therein can subsequently be released and detected by mass spectrometry by reductive stripping of the axle.

Introduction From a molecular design, molecular machine, and sensory perspective, supramolecular interlocked structures have attracted an enormous amount of interest, and their synthesis has experienced considerable advances during the last two decades.1-7 The controlled interfacing of these molecules with electroactive or optically transparent solid surfaces is key to proposed applications in triggered motion, data storage, or sensing. In addition to the possibility of generating a robust, renewable, and repeatedly “read” surface capable of operation in a variety of fluid media, there exists considerable evidence that the thermodynamics of host-guest association is favored at such interfaces.8,9 Despite this, only a comparatively low number of reports of surface confined interlocked molecules exist.10-18 Though the use of host-guest binding in the effective synthesis * Corresponding author. E-mail: [email protected]; [email protected]; Tel: 44 01865 275914; Tel: 44 01865 285142. (1) Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew. Chem., Int. Ed. Engl. 2007, 46, 72–191. (2) Dietrich-Buchecker, C.; Sauvage, J. P. Catenanes, Rotaxanes and Knots. A Journey Through the World of Molecular Topology; Wiley-VCH: Germany, 1999. (3) Sauvage, J.-P. Acc. Chem. Res. 1990, 23, 319–327. (4) Pease, A. R.; Jeppesen, J. O.; Stoddart, J. F.; Luo, Y.; Collier, C. P.; Heath, J. R. Acc. Chem. Res. 2001, 34, 433–444. (5) Balzani, V.; Credi, A.; Silvi, S.; Venturi, M. Chem. Soc. ReV. 2006, 35, 1135–1149. (6) Tian, H.; Wang, Q.-C. Chem. Soc. ReV. 2006, 35, 361–374. (7) Saha, S.; Stoddart, J. F. Chem. Soc. ReV. 2007, 36, 77–92. (8) Beer, P. D.; Davis, J. J.; Drillsma-Milgrom, D. A.; Szemes, F. Chem. Commun. 2002, 1716–1717. (9) Davis, J. J.; Beer, P. D. Encyclopedia of Nanoscience and Nanotechnology; Marcel Dekker: New York, 2004; pp 2477-2492. (10) Chia, S.; Cao, J.; Stoddart, J. F.; Zink, J. I. Angew. Chem., Int. Ed. Engl. 2001, 40, 2447–2451. (11) Cecchet, F.; Rudolf, P.; Rapino, S.; Margotti, M.; Paolucci, F.; Baggerman, J.; Brouwer, A. M.; Kay, E. R.; Wong, J. K. Y.; Leigh, D. A. J. Phys. Chem. B 2004, 108, 15192–15199. (12) Raehm, L.; Kern, J.-M.; Sauvage, J.-P.; Hamann, C.; Palacin, S.; Bourgoin, J.-P. Chem. Eur. J. 2002, 8, 2153–2162. (13) Weber, N.; Hamann, C.; Kern, J.; Sauvage, J. Inorg. Chem. 2003, 42, 6780–6792. (14) Kim, K.; Jeon, W. S.; Kang, J.-K.; Lee, J. W.; Jon, S. Y.; Kim, T.; Kim, K. Angew. Chem., Int. Ed. 2003, 42, 2293–2296. (15) Katz, E.; Lioubashevsky, O.; Willner, I. J. Am. Chem. Soc. 2004, 126, 15520–15532. (16) Berna, J.; Leigh, D. A.; Lubomska, M.; Mendoza, S. M.; Perez, E. M.; Rudolf, P.; Teobaldi, G.; Zerbetto, F. Nat. Mater. 2005, 4, 704. (17) Braunschweig, A. B.; Northrop, B. H.; Stoddart, J. F. J. Mater. Chem. 2006, 16, 32–44. (18) Willner, I.; Basnar, B.; Willner, B. AdV. Funct. Mater. 2007, 17, 702–717.

and conformational control of interlocked structures has been established,19,20 the use of anions in templating surface assembly has only recently been reported,21,22 where, specifically, the anion templated interpenetration of pyridinium cationic threads through the annulus of an isophthalamide macrocycle has been noted. We have recently reported an extension of this work to the anion templated threading of a disulfide-appended and surface-assembled neutral indolocarbazole thread and neutral isophthalamide macrocycle, where the underlying surface effectively acts as a stopper.22 This work utilizes the preorganized hydrogen bond donating and conjugated planar properties of the indolocarbazole moiety, characteristics that make it an attractive building block for the construction of interpenetrated assemblies. Within this, the provision of only two hydrogen bonds (leaving associated anions coordinatively unsaturated) is a satisfied prerequisite. We report here the synthesis and surface assembly of two disulfide-appended indolocarbazole axles (Scheme 1). Once assembled, specific anions are able to promote the threading of neutral isophthalamide macrocycles from solution in the creation of surface-confined pseudorotaxanes (Scheme 2). This threading is reversible and can be followed by surface plasmon resonance (SPR). The surface assembly of neutral components prethreaded in solution by fluoride or sulfate to form surface confined rotaxanes was confirmed by mass spectroscopic detection of macrocycles released by reductively cleaving the axle-surface bond.

Results and Discussion Self-assembled monolayers of the indolocarbazole axle 1 on gold were reliably prepared by the immersion of clean substrates in low-concentration DMF solutions. Reflectance FTIR analyses (Figure 1) resolve the associated methylene (asymmetric str 2930 cm-1, symmetric str 2865 cm-1), ester (1749 cm-1), methyl (1375 cm-1), and aromatic (1400-1600 cm-1, 700-800 cm-1) stretches. Ellipsometric assessments (assuming a refractive index of 1.45 (19) Hiratani, K.; Kaneyama, M.; Nagawa, Y.; Koyama, E.; Kanesato, M. J. Am. Chem. Soc. 2004, 126, 13568–13569. (20) Kaiser, G.; Jarrosson, T.; Otto, S.; Ng, Y. F.; Bond, A. D.; Sanders, J. K. M. Angew. Chem., Int. Ed. Engl. 2004, 43, 1959–1962. (21) Bayley, S. R.; Gray, T. M.; Chmielewski, M. J.; Beer, P. D.; Davis, J. J. Chem. Commun. 2007, 2234–2236. (22) Chmielewski, M. J.; Zhao, L.; Brown, A.; Curiel, D.; Sambrook, M. R.; Thompson, A. L.; Santos, S. M.; Felix, V.; Davis, J. J.; Beer, P. D. Chem. Commun. 2008, 3154–3156.

10.1021/la803960z CCC: $40.75  2009 American Chemical Society Published on Web 02/10/2009

2936 Langmuir, Vol. 25, No. 5, 2009

Zhao et al.

Scheme 1. Structures of the Non-Stoppered Indolo[2,3-a]carbazole Axle 1, Isophthalamide Macrocycle 2, and Stoppered Indolo[2,3-a]carbazole Axle 3

Scheme 2. Schematic Representation of Macrocycle 2 Threading over a Surface Confined Non-Stoppered Indolocarbazole Axle 1 on Gold in the Presence of a Templating Anion (Fluoride or Sulfate)

for the monolayer over a gold layer with pseudo optical constants of n + ik ) 0.003039 + i × 3.570),23 gave an average layer thickness of 2.18 ( 0.04 nm, a figure broadly consistent with Chem3D modeling and suggestive of good layer homogeneity. Adlayer formation was also independently confirmed by X-ray reflectometry, though the contributions of adsorbed water to electron density profile precluded an accurate thickness determination. Reductive stripping analyses of molecular coverage ((1.70 ( 0.26) × 10-10 mol cm-2, translating to a footprint of

Figure 1. Comparative FTIR spectra of the indolo[2,3-a]carbazole axle 1 in SAM form (solid curve) and in the solid state (dotted curve). Small differences in peak position and intensity for the monolayer and the solid phases arise from differences in both the recording modes and chain orientation/conformation.32

1.02 nm2) were fully consistent with the relative steric bulk of the indolocarbazole moiety and supportive of potential macrocycle threading (potentially prohibited at high axle surface densities). Redox probe assessments of this surface (see Supporting Information, Figure S1) additionally confirmed the exposure of an appreciable amount of underlying bare gold surface to solution. Molecular modeling was performed by applying an MM2 minimized energy calculation on molecule 1 with minimum rms gradient set at 0.0100 within Chem3D Ultra 10.0 software (CambridgeSoft, 2006). This calculation estimates the distance between the terminal methyl and disulfide groups to be 2.03-2.13 nm. An orthogonal surface molecular orientation would, accordingly, generate an adlayer of this thickness. The effects of significant molecular tilt or low surface coverage/homogeneity would be to reduce observed ellipsometric thickness; since observations are 2.18 ( 0.04 nm, we conclude that adlayers of 1 are homogeneous with molecular axis aligned approximately with the surface normal. The potential association between surface confined axles and solution-phase macrocycles can be reliably probed by surface plasmon resonance.14,21,22 Solution-phase NMR experiments have demonstrated that association of indolocarbazole with macrocycle 2 can be templated by specific anions.22 Figure 2 shows an example of an SPR sensorgram showing the reversible threading, dethreading, and rethreading of macrocycle 2 over a SAM of axle 1 using a fluoride anion template. Control experiments confirm (see Supporting Information Figure S2) the requirement of both indolocarbazole axle and specific (see below) anion in mediating the adsorption of macrocycle 2 from solution. (23) Basabe-Desmonts, L.; Beld, J.; Zimmerman, R. S.; Hernando, J.; Mela, P.; Parajo, M. F. G.; Hulst, N. F. v.; Berg, A. v. d.; Reinhoudt, D. N.; CregoCalama, M. J. Am. Chem. Soc. 2004, 126, 7293–7299.

Pseudorotaxane and Rotaxane MLs on Gold

Figure 2. SPR sensorgram showing reversible threading, dethreading, and rethreading of macrocyle 2 (0.5 mM) over a surface confined SAM of axle 1 in the presence of fluoride ion (0.5 mM TBAF in acetonitrile): (a) the threading of macrocycle 2 over 1 on the addition of a 1:1 mol equiv of the macrocycle 2 and F-; (b) the dethreading of macrocyle 2 after washing the surface with water; and (c) the F- template induced rethreading of macrocycle 2 over surface confined 1. The upper x axis represents regions of flow in the indicated solvent.

Figure 3. SPR traces showing the association of solution-phase macrocycle 2 with a SAM of axle 1 in the presence of F- and SO42-. A quantitative analysis shows surface interlocking is considerably more effective with the former (35% threading for F- and 21% for SO42-). Association constants, determined by a linearization method,33,34 are (2.78 ( 0.33) × 104 M-1 and (6.77 ( 0.59) × 103 M-1 for F- and SO42-, respectively. Negligible axle threading (300 °C. 1H NMR (500 MHz, DMSO-d6) δ: 12.61 (br s, 2H), 11.51 (s, 2H), 8.83 (d, 3J ) 1.3 Hz, 2H), 8.09 (s, 2H), 8.03 (dd, 3J ) 8.5 Hz, 4J ) 1.6 Hz, 2H), 7.78 (d, 3J ) 8.5 Hz, 2H). 13C NMR (125 (30) Sambrook, M. R.; Beer, P. D.; Wisner, J. A.; Paul, R. L.; Cowley, A. R.; Szemes, F.; Drew, M. G. B. J. Am. Chem. Soc. 2005, 127, 2292–2302.

Langmuir, Vol. 25, No. 5, 2009 2939 MHz, DMSO-d6) δ: 168.1, 141.7, 126.2, 126.1, 123.4, 122.2, 121.5, 120.8, 112.7, 111.4; m/z (EI) 344.0808 (M+ C20H12N2O4 344.0797). Bis(2-(2-(2-hydroxyethoxy)ethoxy)ethyl) 11,12-Dihydroindolo[2,3a]carbazole-3,8-dicarboxylate 6. 11,12-Dihydroindolo[2,3-a]carbazole-3,8-dicarboxylic acid 5 (0.10 g, 0.29 mmol) was dissolved in dry, degassed DMF (4 mL), and the mixture was purged with N2 for 20 min. Dry Et3N (1 mL, 7.2 mmol) and 2-[2-(2-chloroethoxy)ethoxy]ethanol (0.21 mL, 1.4 mmol) were added via syringe. The mixture was heated to 140 °C using microwave irradiation for 3 h. After cooling to room temperature, the solvent was removed in vacuo to leave a pale yellow oil. This was dry-loaded onto silica and purified by column chromatography (SiO2; 5% MeOH in CH2Cl2) to afford the product as a waxy white solid (0.12 g, 69%); mp 158-160 °C. 1H NMR (300 MHz, DMSO-d6) δ: 11.63 (2H, s), 8.85 (2H, d, 4J ) 1.5 Hz), 8.12 (2H, s), 8.05 (2H, dd, 3J ) 8.4 Hz, 4J ) 1.5 Hz), 7.81 (2H, d, 3J ) 8.4 Hz), 4.59 (2H, br t), 4.47-4.43 (4H, m), 3.83-3.80 (4H, m), 3.67-3.64 (4H, m), 3.59-3.56 (4H, m), 3.51-3.40 (8 H, m). 13C NMR (125 MHz, DMSO-d6) δ: 166.5, 142.0, 126.3, 126.0, 123.4, 121.1, 120.81 120.5, 112.8, 111.7, 72.4, 70.0, 69.8, 68.6, 63.7, 60.2; m/z (ES) 631.2262 (M + Na+, C32H36N2NaO10 631.2268). 4,4′-((4-Isocyanatophenyl)(phenyl)methylene)bis(tert-butylbenzene) 7. 4-(Bis(4-tert-butylphenyl)(phenyl)methyl)aniline (0.12 g, 0.27 mmol) (synthesized as previously reported)31 and triphosgene (0.0039 g, 0.13 mmol) were dissolved in dry toluene (35 mL), and distilled triethylamine (0.040 mL, 0.29 mmol) was added. The solution was heated to 70 °C under a nitrogen atmosphere for 4 h, filtered to remove triethylamine hydrochloride, and concentrated in vacuo to give the product as a pale yellow oil, which was dried under high vacuum for 60 min before being used immediately in the next step without further purification or characterization. 3-(2-(2-(2-(4-(Bis(4-tert-butylphenyl)(phenyl)methyl)phenylcarbamoyloxy)ethoxy)ethoxy)ethyl) 8-(2-(2-(2-hydroxyethoxy)ethoxy)ethyl) 11,12-Dihydroindolo[2,3-a]carbazole-3,8-dicarboxylate 8.Bis(2-(2-(2-hydroxyethoxy)ethoxy)ethyl)11,12-dihydroindolo[2,3-a]carbazole-3,8-dicarboxylate 6 (0.16 g, 0.26 mmol) was suspended in dry CH3CN (100 mL) under N2, and the mixture was heated to reflux until it had formed a homogeneous solution. After cooling to room temperature, di-n-butyltindilaurate (2 drops) was added. A solution of 4,4′-((4-isocyanatophenyl)(phenyl)methylene)bis(tert-butylbenzene) 7 (0.12 g, 0.26 mmol) in dry CH3CN (50 mL) was added dropwise over a 3 h period via an addition funnel. After addition was complete, the reaction mixture was allowed to stir at room temperature under nitrogen for a further 48 h. The solvent was removed in vacuo, and the residual solid was purified by column chromatography (SiO2; 2% MeOH in CH2Cl2) to yield the product as a white solid (0.12 g, 44%); mp 182-184 °C. 1H NMR (500 MHz, DMSO-d6) δ: 11.69 (s, 2H), 9.77 (s, 1H), 8.88 (s, 2H), 8.13 (s, 2H), 8.09-8.06 (m, 2H), 7.85-7.83 (1H, d, 3J ) 8.8 Hz), 7.84-7.82 (1H, d, 3J ) 8.8 Hz), 4.62-4.60 (t, 1H, 3J ) 5.4 Hz), 4.46-4.43 (m, 4H), 4.20-4.18 (m, 2H), 3.83-3.81 (t, 4H, 3J ) 4.9 Hz), 3.68-3.64 (m, 6H), 3.62-3.61 (m, 2H), 3.59-3.57 (m, 2H) 3.51-3.43 (m, 4H). 13C NMR (125 MHz, DMSO-d6) δ:166.5, 153.5, 147.8, 146.9, 143.7, 141.8, 140.7, 136.7, 130.7, 130.3, 130.0, 127.8, 126.4, 126.0, 125.7, 124.4, 123.3, 122.1, 120.7, 120.4, 120.3, 117.6, 115.3, 112.7, 111.5, 72.4, 69.9, 69.8, 68.8, 68.6, 63.7, 63.5, 63.1, 60.2, 34.0, 31.1. m/z (ES) 1104.47 (M + Na+, C66H71N3NaO11 1104.50). (31) Wisner, J. A.; Beer, P. D.; Drew, M. G. B.; Sambrook, M. R. J. Am. Chem. Soc. 2002, 124, 12469–12476. (32) Tolstoy, V. P.; Chernyshova, I. V.; Skryshevsky, V. A. Handbook of infrared spectroscopy of ultrathin films; John Wiley & Sons, Inc.: Hoboken, NJ, 2003. (33) Morton, T. A.; Myszka, D. G.; Chaiken, I. M. Anal. Biochem. 1995, 227, 176–185. (34) AutoLab ESPRIT Surface Plasmon Resonance Kinetic EValuation Manual; Eco Chemie B. V., The Netherlands, 2006.

2940 Langmuir, Vol. 25, No. 5, 2009 3-(2-(2-(2-(5-(1,2-Dithiolan-3-yl)pentanoyloxy)ethoxy)ethoxy)ethyl) 8-(2-(2-(2-(4-(Bis(4-tert-butylphenyl)(phenyl)methyl)phenylcarbamoyloxy)ethoxy)ethoxy)ethyl) 11,12-Dihydroindolo[2,3-a]carbazole-3,8-dicarboxylate 3 3-(2-(2-(2-(4-(Bis(4-tert-butylphenyl)(phenyl)methyl) phenylcarbamoyloxy)ethoxy)ethoxy)ethyl) 8-(2(2-(2-hydroxyethoxy)ethoxy)ethyl) 11,12-dihydroindolo[2,3-a]carbazole-3,8-dicarboxylate 8 (0.10 g, 0.092 mmol), D,L-6,8-thioctic acid (0.047 g, 0.23 mmol), N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (0.027 g, 0.14 mmol), and 4-(dimethylamino)pyridine (0.010 g, 0.082 mmol were dissolved in dry CH2Cl2 (5 mL) under nitrogen. The reaction mixture was stirred at room temperature for 18 h. The solvent was removed in vacuo, and the residual solid was purified by column chromatography (SiO2; CH2Cl2/ MeOH 99:1) to yield the title compound as a white solid (0.11 g, 91%); mp 154-156 °C. 1H NMR (500 MHz, DMSO-d6) δ: 11.68 (2H, s), 9.77 (1H, s), 8.88 (2H, s), 8.11 (2H, s), 8.09-8.06 (2H, m), 7.84-7.82 (2H, d, 3J ) 8.8 Hz), 7.37-7.35 (2H, d, 3J ) 8.8 Hz), 7.29-7.28 (4H, d, 3J ) 8.3 Hz), 7.19-7.14 (3H, m), 7.08-7.06 (4H, d, 3J ) 8.3 Hz), 7.04-7.03 (2H, d, 3J ) 8.8 Hz), 4.45-4.44 (4H, m), 4.20-4.18 (2H, br t),. 4.11-4.09 (2H, br t), 3.82 (4H, m), 3.66-3.59 (6H, m), 3.62-3.59 (6H, m), 3.49-3.44 (1H, m), 3.12-3.07 (1H, m), 3.04-2.99 (1H, m), 2.31-2.25 (1H, m), 2.21-2.18 (2H, t, J ) 7.3 Hz), 1.77-1.70 (1H, m), 1.55-1.47 (2H, m), 1.45-1.39 (4H, m). 13C NMR (125 MHz, DMSO-d6) δ: 172.7, 166.5, 153.5, 147.8, 146.9, 143.7, 141.9, 140.7, 136.7, 130.6, 130.3, 130.0, 127.6, 126.2, 125.9, 125.7, 124.4, 123.3, 122.0, 120.8, 120.4, 117.5, 112.7, 111.6, 69.9, 69.8, 68.7, 68.6, 68.3, 63.7, 63.5, 63.1, 56.1, 56.0, 38.0, 34.0, 33.2, 31.1, 28.0, 24.1. m/z (MALDI-TOF) 1269.62.(M+ C74H83N3O12S2 requires 1269.94), 1292.69 (M + Na+, C74H83N3NaO12S2 1292.53). Formation of Non-Stppered Idolo[2,3]carbazole Axle or Stoppered Axle SAMs on Gold. Gold electrodes for FTIR, ellipsometric, and SPR analyses were treated with piranha solution (concentrated H2SO4 and 33% aqueous H2O2 in a 3:1 ratio), washed copiously with deionized water, blown dry, then exposed to a 1 mM solution of 1 in DMF or 3 in DCM for 24 h. (Caution: Piranha solution should be handled with great care: it has been reported to detonate unexpectedly). The surface was then sonicated in DMF (or DCM) for 1 min, rinsed with DMF (or DCM) and ethanol, and then blown dry in a stream of high purity nitrogen. For electrochemical analyses, polycrystalline electrodes were utilized. Prior to adlayer formation on these surfaces, an additional electrochemical polishing step was implemented. Formation of SAMs from Mixture Solution of Stoppered Indolo[2,3]carbazole Axle 3 and Macrocycle 2 in the Presence of Different Anions on Gold. Mechanically cleaned gold electrodes were immersed in a 1 mM/10 mM/10 mM mixture solution of 3/2/ anion (TBAF, (TBA)2SO4, TBACl, or TBAHSO4) in DCM. The electrodes were then sonicated in DCM for 1 min, washed copiously with DCM and ethanol, and blown dry with nitrogen. Characterization. FTIR spectra were recorded using a Varian Digilab FTS 7000 series FTIR spectrometer, with a liquid nitrogen cooled MCT detector. For reflectance measurements on gold surface, a Pike Veemax Specular reflectance accessory was used, with the sample inverted on top of this accessory, supported on a mask with an aperture diameter of 16 mm. An angle of incidence of 75° was used for all reflectance measurements in order to give as large a signal/noise ratio as possible within the constraints of the IR beam and sample size. Prior to measurements, the instrument and the reflectance accessory were purged with CO2 and H2O free air from

Zhao et al. a FTIR Purge Gas Generator (Whatman). Background scans were recorded using a clean, bare gold substrate. Data were averaged over 2000 scans using a resolution of 4 cm-1 for each measurement. Spectra of solid samples were recorded using a DurasamplIR II diamond attenuated total reflectance (ATR) accessory. Ellipsometry measurements of the SAM on gold were performed on a Beaglehole Instruments picometer ellipsometer assuming a refractive index of 1.45 for the monolayer over the gold layer (with pseudo optical constants of n + ik ) 0.003039 + i × 3.570 determined from a fit to a bare gold sample identical to those used for the deposition). Raster scans were taken of 5 points per gold substrate and their values averaged. Electrochemical characterization (reductive desorption and redox probe analyses) was performed using a potentiostat Autolab type II (Windsor Scientific UK Ltd.). Gold working electrodes and a saturated calomel (SCE) reference electrode, together with a platinum counter electrode, were used for reductive desorption. For redox probe experiments in acetonitrile solution, a self-assembled Ag/Ag+ reference electrode (MF-2062, Bioanalytical Systems, Inc.) was used. In all cases, gold electrodes were pretreated by reductive desorption in 0.5 M NaOH aqueous solution by cyclic voltammetry from -0.5 to -1.5 V at a sweep rate of 100 mV s-1 (40 scans). They were then rinsed with purified water, treated with hot Piranha solution for 20-25 min, rinsed again with water, polished on 0.05 µm alumina slurry (Buehler MicroPolish II, 0.05 Micron), and sonicated in water for 3 min. Further electrochemical polishing and reduction of the gold electrodes were performed in 0.1 M H2SO4 aqueous solution from -0.2 to 1.5 V at 100 mV s-1 (25 scans) and from 0.75 to 0.2 V at the same speed for 10 scans. Reductive stripping measurment were performed in 0.1 M NaOH solution from 0 to -1.6 V at 100 mV s-1. SPR measurements were performed on an AutoLab ESPRIT surface plasmon resonance instrument. In all cases, a double-channel SPR reader was used (Eco Chemie) and the gold sensor disk (diameter 17 mm) mounted to the optical lens through index-matching oil. An auto sampler was used to inject or remove solutions. Stripping mass spectrometry: Mechanically and electrochemically cleaned gold electrodes were immersed in a 1 mM/10 mM/10 mM mixture solution of 3/2/anion (TBAF, (TBA)2SO4, TBACl or TBAHSO4) in DCM for rotaxane assembly on gold. The electrodes were then sonicated in DCM for 1 min, rinsed copiously with DCM and ethanol, and then blown dry. Electrochemical stripping was performed in a three-electrode system in 0.1 M NaOH aqueous solution (0 to -1.6 V at rate of 100 mV s-1). Collected aqueous samples were dispersed with a 5-10-fold order of dilution in acetonitrile for mass spectroscopy assessments on a Bruker FTICR-MS Apex Qe(9.4T) system.

Acknowledgment. We acknowledge financial support from the EPSRC (postdoctoral fellowships for L.Z., K.M. and M.C.; DTA studentship for A.B.), and Dr. Robin Proctor for assistance with mass spectrometry. Supporting Information Available: Synthesis scheme, SPR control experiments, redox probe analysis of nonstoppered axle 1 SAM on gold, FTIR and electrochemical analysis of stoppered axle 3 SAM on gold, comparison of solution-phase 1H NMR titrations of 2 and indolocarbazole in the presence of different anions. This material is available free of charge via the Internet at http://pubs.acs.org. LA803960Z