Electrochemical Dimerization of 2-(2 '-Thienyl) pyridine Adsorbed on

Aug 19, 2004 - Emily Chung,Jeff L. Shepherd,Dan Bizzotto,* andMichael O. Wolf*. Department of Chemistry, University of British Columbia, Vancouver, Br...
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Electrochemical Dimerization of 2-(2′-Thienyl)pyridine Adsorbed on Au(111) Observed by in Situ Fluorescence Emily Chung, Jeff L. Shepherd, Dan Bizzotto,* and Michael O. Wolf* Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada Received June 17, 2004 The study of heterodentate molecules adsorbed on metal electrodes provides an opportunity to expand the functionality of modified surfaces while offering insights into the surface and intramolecular electronic interactions of organic adsorbates. The adsorption of 2-(2′-thienyl)pyridine, a molecule containing both pyridine and thiophene moieties, on a Au(111) electrode is reported. Adsorption was characterized by electrochemistry in neutral and basic aqueous electrolyte and was compared to that of pyridine. The aqueous electrochemistry of thiophene on Au(111) was also characterized for comparison purposes. At negative potentials, in the presence of 2-(2′-thienyl)pyridine, a diffuse, π-bonded monolayer was formed, and a phase transition to a close-packed N- and/or S-bonded configuration was observed near -0.4 V in a 1 mM solution of adsorbate, similar to that seen in pyridine on Au(111). The thiophene-like oxidative dimerization of the molecule was confirmed at positive potentials using in situ fluorescence microscopy by comparison with the spectrum of the chemically synthesized dimer.

* Authors to whom correspondence should be addressed. E-mail: [email protected] (D.B.), [email protected] (M.O.W.).

applications in electronic devices, such as transistors and batteries, as well as optoelectronic devices, such as organic light emitting diodes (OLEDs).8,11,12 Recently, polymerbased OLEDs have been incorporated into some commercial cellular phone and digital camera displays.13 Polythiophene is difficult to characterize due to its insolubility; thus, thiophene oligomers have been used as model systems in order to gain a better understanding of polymer properties, such as charge storage.8,14 In addition, derivatives of thiophene have been studied, as novel properties can be introduced by functionalizing the monomer precursor.15 Pyridine has been incorporated both into the backbone and as a side-chain of polythiophene in order to induce third-order nonlinear optical properties due to intramolecular charge transfer16 in order to stabilize the reduced form of the polymer15 or to allow binding of transition-metal ions.17 Pyridine has been a popular model compound for investigating the adsorption of neutral organic molecules to metal surfaces, as it can take on multiple surface orientations, interacting with the metal via either the aromatic π orbitals or through the lone pair of the nitrogen heteroatom.18,19 In contrast to the Au-thiol systems, pyridine on Au(111) is a system where the ligand-metal interaction is described as “moderate”, or weak chemisorption.20

(1) Mandler, D.; Turyan, I. Electroanalysis 1996, 8, 207-213. (2) Lavrich, D. J.; Wetterer, S. M.; Bernasek, S. L.; Scoles, G. J. Phys. Chem. B 1998, 102, 3456-3465. (3) Jin, Q.; Rodriguez, J. A.; Li, C. Z.; Darici, Y.; Tao, N. J. Surf. Sci. 1999, 425, 101-111. (4) Zharnikov, M.; Grunze, M. J. Phys.: Condens. Matter 2001, 13, 11333-11365. (5) Dishner, M. H.; Taborek, P.; Hemminger, J. C.; Feher, F. J. Langmuir 1998, 14, 6676-6680. (6) Dishner, M. H.; Hemminger, J. C.; Feher, F. J. Langmuir 1996, 12, 6176-6178. (7) Noh, J.; Ito, E.; Araki, T.; Hara, M. Surf. Sci. 2003, 532-535; 1116-1120. (8) Matsuura, T.; Nakajima, M.; Shimoyama, Y. Jpn. J. Appl. Phys. 2001, 40, 6945-6950. (9) Liu, G.; Rodriguez, J. A.; Dvorak, J.; Hrbek, J.; Jirsak, T. Surf. Sci. 2002, 505, 295-307. (10) Ishida, T.; Choi, N.; Mizutani, W.; Tokumoto, H.; Kojima, I.; Azehara, H.; Hokari, H.; Akiba, U.; Fujihira, M. Langmuir 1999, 15, 6799-6806.

(11) Casado, J.; Pappenfus, T. M.; Miller, L. L.; Mann, K. R.; Orti, E.; Viruela, P. M.; Pou-Amerigo, R.; Hernandez, V.; Lopez Navarrete, J. T. J. Am. Chem. Soc. 2003, 125, 2524-2534. (12) Noh, J.; Kobayashi, K.; Lee, H.; Hara, M. Chem. Lett. 2000, 630-631. (13) Souza, C. Electronic Buyer’s News 2003, 3. (14) Guay, J.; Kasai, P.; Diaz, A.; Wu, R.; Tour, J. M.; Dao, L. H. Chem. Mater. 1992, 4, 1097-1105. (15) Wang, J. X.; Keene, F. R. J. Electroanal. Chem. 1996, 405, 5970. (16) Lu, H.-F.; Chan, H. S. O.; Ng, S.-C. Macromolecules 2003, 36, 1543-1552. (17) Jenkins, I. H.; Salzner, U.; Pickup, P. G. Chem. Mater. 1996, 8, 2444-2450. (18) Stolberg, L.; Lipkowski, J.; Irish, D. E. J. Electroanal. Chem. 1987, 238, 333-353. (19) Yang, D. F.; Stolberg, L.; Lipkowski, J.; Irish, D. E. J. Electroanal. Chem. 1992, 329, 259-278.

Introduction The modification of surfaces through the adsorption of organic molecules has been extensively studied as a way of chemically tuning surfaces to engineer specific characteristics and functionalities.1 Thus far, there has been little research focused on the heterodentate binding of adsorbates capable of interacting with a metal surface through more than one functional group simultaneously. Such a system would expand the functionality of the modified surface and provide new insights into the interplay between intramolecular interactions of organic adsorbates and electronic effects in adsorbate-metal binding. The strong and stable gold-sulfur interaction enjoyed by thiols has made thiol-on-gold systems a popular choice for the chemical modification of surfaces.2-4 Thiophenes, in which the interaction with gold is weaker, have recently been shown to produce stable monolayers also.5-10 Thiophene polymerizes to polythiophene, a π-conjugated organic polymer that is unusually stable in both neutral and oxidized (conductive) forms.8 Polythiophenes and other conductive polymers also have interesting optical properties and have been widely studied for possible

10.1021/la0485024 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/19/2004

Electrochemical Dimerization of 2-(2′-Thienyl)pyridine

Thiophene can be displaced from a Au surface by methanethiol, and thiophene monolayers can be annealed at 40 °C. Both facts suggest that the Au-thiophene interaction is weaker than the Au-thiol interaction,5 although the Gibbs free energy of adsorption for thiophene has been calculated as being comparable to that of thiols.8 The kinetics of self-assembly of the layer are also reported to be very slow.8 Thiophene monolayers deposited on gold from ethanolic solution have been characterized by STM,5-7 IR,8 and XPS.7,9,10 In addition, there are a number of studies of thiophene deposited from the gas phase.2,8 Recently, a slow potential-dependent phase transition from a diffuse, flat-lying configuration to a close-packed, vertical transition was observed by STM in thiophene monolayers adsorbed from acidic aqueous solution, although no corresponding signals were observed electrochemically.21 Thiophene has been observed to form both diffuse and more closely packed monolayers, the latter of which appear to be coordinated through the sulfur with the ring oriented normal to the surface. There have been a few adsorption studies of molecules theoretically capable of adsorbing to Au through more than one type of functional group simultaneously, such as aminothiophenol,22 mercaptobenzimidazole,23 2-mercapto3-n-octylthiophene,24 and 2-mercaptobenzthiazole.25 A study of 2-mercaptopyridine suggested that it interacts with Au via both the nitrogen and the sulfur.26 However, prior to the work described herein, there have been no studies detailing the electrochemical behavior of the individual functional groups of a multifunctional adsorbate in order to determine how each moiety might influence another’s response to changes in potential. Preliminary results of this work have appeared in a communication.27 2-(2′-Thienyl)pyridine (TP) is an interesting model for examining how linking two surface-active functional groups affects their very different individual interactions with the electrodes. This study bridges those of weakly chemisorbed species, such as pyridine, and more strongly adsorbed surfactants and is an interesting starting point for the investigation of heterodentate adsorbates on surfaces.

Experimental Section Reagents and Materials. All chemicals used in the synthesis of TP and PTTP (5,5′-bis(2-pyridyl)-2,2′-bithiophene) were reagent grade (Aldrich) and were used without further purification, with the exception of NiCl2(dppp) (dppp ) Ph2P(CH2)3PPh2), which was synthesized in-house using a literature procedure.28 Et2O and THF were dried over Na and benzophenone before use. (20) Bizzotto, D.; Zamlynny, V.; Burgess, I.; Jeffrey, C.; Li, H.-Q.; Rubinstein, J.; Galus, Z.; Nelson, A.; Pettinger, B.; Merril, R.; Lipkowski, J. In Interfacial Electrochemistry; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; pp 405-425. (21) Su, G.-J.; Zhang, H.-M.; Wan, L.-J.; Bai, C.-L. Surf. Sci. 2003, 531, L363-L368. (22) Batz, V.; Schneeweiss, M. A.; Kramer, D.; Hagenstrom, H.; Kolb, D. M.; Mandler, D. J. Electroanal. Chem. 2000, 491, 55-68. (23) Whelan, C. M.; Smyth, M. R.; Barnes, C. J.; Brown, N. M. D.; Anderson, C. A. Appl. Surf. Sci. 1998, 134, 144-158. (24) Peng, Z.; Dong, S. Langmuir 2001, 17, 4904-4909. (25) Sandhyarani, N.; Skanth, G.; Berchmans, S.; Yegnaraman, V.; Pradeep, T. J. Colloid Interface Sci. 1999, 209, 154-161. (26) Taniguchi, I.; Yoshimoto, S.; Sunatsuki, Y.; Nishiyama, K. Electrochemistry 1999, 67, 1197-1199. (27) Chung, E.; Bizzotto, D.; Wolf, M. O. Chem. Commun. 2002, 30263027.

Langmuir, Vol. 20, No. 19, 2004 8271 The aqueous electrochemical experiments were performed in 0.05 M KClO4 (Fluka, puriss. grade, 99%+, recrystallized) with NaOH (1 mM, Fluka, puriss. grade, calcined overnight at 290 °C) made up in purified water (Millipore >18.2 MΩ‚cm). The solutions were deaerated with Ar (Praxair, filtered through a Supelco charcoal purifier). Pyridine (HPLC grade, 99.9%+) and thiophene (99+%) were both used as received from Aldrich. Organic electrochemistry was performed under N2 (Praxair) in 0.1 M tetrabutylammonium hexafluorophosphate (recrystallized) and acetonitrile (Fisher, HPLC grade, dried over 3 Å molecular sieves (Fisher, grade 562)). Bis(pentamethylcyclopentadienyl)iron (Strem, 99%) was used as received. Synthesis of 2-(2′-Thienyl)pyridine. TP was synthesized by Kumada coupling of 2-bromopyridine and 2-bromothiophene, using NiCl2(dppp) as the catalyst, based on a general procedure in the literature.29 The organic layer was washed with dilute HCl and saturated NaHCO3 solution. The crude solid was then purified by column chromatography using a 2:1 hexanes/ethyl acetate eluant to give a pale yellow solid. The melting point and 1H NMR spectrum of the product agreed with literature values.29,30 To ensure high purity, the TP was further purified prior to use by recrystallization from petroleum ether and double sublimation to give colorless crystals, which were >99% pure, as determined by GC. Synthesis of 5,5′-Bis(2-pyridyl)-2,2′-bithiophene (PTTP). This compound was synthesized on the basis of a literature procedure.30,31 TP was dissolved in THF and lithiated by dropwise addition of tert-butyllithium at -50 °C. After being stirred for 1 h, the solution was allowed to warm to room temperature. CuCl was added, and O2 was bubbled through the solution, which turned black. It was heated to reflux under O2 for 3 h, at which point the red solution containing a pale precipitate was removed from heat. HCl (10%) was added to quench any remaining metalated TP, and NaHCO3 was added until the solution was neutralized and a precipitate formed. The crude product was purified by column chromatography using 2:1 hexanes/EtOAc as the eluent and then recrystallized from CH2Cl2 to give yellow crystals. To ensure high purity, these crystals were recrystallized from acetone. The melting point and 1H NMR spectrum of the product agreed with literature values.30,31 Electrochemical Measurements. All aqueous measurements were performed in a three-electrode cell with a Au(111) single-crystal working electrode, a Au coil counter electrode, and a saturated calomel electrode (SCE) reference. The electrolyte was 0.05 M aqueous KClO4, and the pH was altered by addition of NaOH solution, primarily to suppress H2 evolution at the negative potentials required for adsorbate desorption in the chronocoulometry and fluorescence experiments. The solution was purged with Ar before the start of each experiment, and a blanket of Ar was maintained over the solution to exclude O2. TP was added predissolved as a 1 mM or 0.1 mM aqueous solution. PTTP was spread at the air-solution interface by introducing a 1 mg/mL chloroform solution via syringe and allowing the chloroform to evaporate, following an established procedure.32 The working electrode was flame annealed and cooled before each use and was in contact with the solution via a hanging meniscus. The electrochemical results were obtained using a potentiostat (FHI ELAB) connected via a data acquisition board (National Instruments PCI-MIO-16E-1, 12-bit, 1.25 MHz) to a computer. Cyclic voltammograms were performed at 20 mV/s. The differential capacitance measurements were performed at 5 mV/s, using a 25 Hz, 5 mV rms ac perturbation, and the capacitance was calculated assuming a series RC circuit. Chronocoulometry was performed under computer control and the transients integrated as detailed in the literature.18 The absolute charge densities were calculated using the potential of (28) Van Hecke, G. R.; Horrocks, W. D., Jr. Inorg. Chem. 1966, 5, 1968-1974. (29) Tamao, K.; Kodama, S.; Nakajima, I.; Kumada, M. Tetrahedron 1982, 38, 3347-3354. (30) Constable, E. C.; Sousa, L. R. J. Organomet. Chem. 1992, 427, 125-139. (31) Kauffmann, T.; Wienhoefer, E.; Woltermann, A. Angew. Chem., Int. Ed. Engl. 1971, 10, 741-743. (32) Shepherd, J.; Yang, Y.; Bizzotto, D. J. Electroanal. Chem. 2002, 524-525, 54-61.

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Chung et al. (pentamethylcyclopentadienyl)iron was added as an internal reference, and values of potential are reported vs SCE. Fluorescence Measurements. Solution-phase fluorescence spectra were obtained with a Varian Cary Eclipse spectrometer. In situ measurements were performed in an electrochemical cell setup similar to that used by Shepherd et al.,32,33 as shown in Figure 1. Light from a Xenon arc lamp was passed through a 11000v2 filter set from Chroma Technology Corp. (excitation 340-380 nm, dichroic 400 nm, emission >430 nm), and the selected excitation wavelengths were focused on the electrode from below by the 10× objective (NA ) 0.3, wd ) 10 mm) of an inverted microscope (Olympus IX70). The emitted and reflected light were collected through the same microscope objective and then passed through the dichroic and emission filters. A fiber optic spectrometer (Ocean Optics S2000) was used to measure the spectrum of the emitted light.

Results and Discussion

Figure 1. The instrumental setup used for in situ fluorescence measurements. zero charge (pzc) of the Au(111) electrode (measured separately). The pH was measured after each experiment using an Accumet Model 810 pH meter. Organic electrochemistry was performed with a polycrystalline Au bead electrode, a Pt grid counter electrode, and a Ag wire quasi-reference using a potentiostat (Pine model AFCBP1). Bis-

Cyclic Voltammetry and Differential Capacitance of TP. The steady-state cyclic voltammograms (CVs) and differential capacitance plots (as measured by ac voltammetry) of 1 mM TP in basic and neutral solution are shown in Figure 2. Three main features are visible in the positive scan of both the CV and capacitance curves (labeled in the CVs). In the latter plots, a pair of sharp peaks, A, appears at -0.42 and -0.46 V. A broader peak, B, appears at +0.35 V at pH 7 and at +0.13 V at pH 11. On the negative scan, a large peak, C, occurs at -0.50 V at pH 7 and at -0.67 V at pH 11. In the steady-state CVs in basic pH, a small peak also appears near -0.9 V. Peak A is pH independent,

Figure 2. Cyclic voltammograms (a and b) and differential capacitance curves (c and d) at pH 7 (a and c) and pH 11 (b and d) in the absence (dotted line) and presence (solid line) of 1 mM TP on Au(111) in 0.05 M KClO4. The blank differential capacitance plots include only the forward scan for clarity. For cyclic voltammograms, the scan rate was 20 mV/s; differential capacitance was recorded at 5 mV/s, 25 Hz, and 5 mV rms. Uppercase letters refer to different peaks and Roman numerals refer to different adsorbed states.

Electrochemical Dimerization of 2-(2′-Thienyl)pyridine

Figure 3. Steady-state cyclic voltammograms in the presence of 1 mM TP (solid line) and (a) 1 mM pyridine at pH 7 and pH 11 and (b) 1 mM thiophene (dashed lines) at pH 11 on Au(111) in 0.05 M KClO4; scan rate, 20 mV/s.

while peak B shifts close to 60 mV/decade toward negative potentials as the pH increases in the CV, suggesting it is due to a faradaic process involving 1 equiv of protons. The apparent pH shift of peak C in this figure is slightly less than 60 mV/decade and was found to be scan-rate dependent. If the positive limit of the first potential scan is restricted to below the onset of B, A appears as a single sharp peak at -0.42 V rather than two peaks and is reversible, giving a similar peak at almost the same potential on the negative scan (Figure 3a). However, after scanning positive of B and reversing the scan direction, significant hysteresis is observed (Figure 2). The reverse peak of A disappears, and a single broad peak, C, appears at -0.6 V. On subsequent positive scans, a second, broader peak appears at -0.46 V in addition to the peak at -0.42 V, and two peaks are seen at A. Generally, starting at -1.0 V vs SCE, where TP is desorbed, and scanning the potential initially in a positive direction, several potential regions in which TP is adsorbed to the electrode surface can be distinguished, corresponding to three states. These are labeled with Roman numerals in Figure 2c and d. At potentials negative of peak A, the adsorbed TP molecules are in state I, where the capacitance is similar to that of the bare electrode. At potentials between peak A and peak B, the adsorbed TP results in a low-capacitance region corresponding to state II, which can be converted reversibly back to state I by reversing the scan direction at any potential negative of peak B (Figure 3a). On scanning the potential positive of B, the adsorbed TP undergoes a transition to state III, which is characterized by an even lower capacitance than state II, and cannot be reversibly converted back to state II. When the potential is reversed, state III is stable until the onset of peak C, a process that results in state IV, along with significant desorption. On subsequent positive (33) Shepherd, J. L.; Bizzotto, D. J. Phys. Chem. B 2003, 107, 85248531.

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scans, peak A is split into two peaks. It is interesting to note that, for lower concentrations of TP, peak A is shifted to more positive potentials and peak B is shifted to more negative potentials; hence, at high TP concentrations, the potential region of stability for state II increases. The small, unlabeled peak in the steady-state CV for the full potential range near -0.9 V is likely due to a desorption process. The molecules are completely desorbed at -1.0 V, as demonstrated by the equivalence of the capacitance with and without the TP molecule at this potential. Comparison of Electrochemistry of TP to Thiophene and Pyridine. To gain some insight into the origin of the peaks, electrochemical characterizations of thiophene and pyridine were performed under similar conditions. CVs of these molecules at pH 11 compared to TP are shown in parts a and b of Figure 3, respectively. In the presence of 1 mM pyridine at pH 11, a broad peak at -0.55 V and a sharp peak at +0.10 V are observed. Both peaks are reversible, with little separation between the anodic and cathodic peaks and appear at the same potential in neutral solution, consistent with what has been observed in a detailed study of pyridine on Au(111) by Stolberg et al.34 In that study, the peak at -0.55 V is attributed to adsorption of pyridine, bonded through the aromatic π orbitals, with the plane of the ring parallel to the electrode. The peak at +0.10 V (near the pzc of the covered electrode) is attributed to a two-dimensional phase transition, such that positive of the peak, the plane of the ring is normal to the surface and bonding to the surface occurs through the lone pair of the nitrogen. The peak associated with this orientational change in pyridine closely resembles peak A in TP with regard to shape and reversibility, as shown in Figure 3a. Typical of peaks due to reorientational phase changes and changes in dielectric constant, they are sharp and reversible, with minimal charge beneath the peak, and broaden dramatically as the concentration of surfactant decreases. Both peak A and the pyridine peak at +0.10 V result in a similar drop in the capacitance positive of the peak. In the case of pyridine, this lower capacitance is due to the lower dielectric constant (and higher surface concentration) of the perpendicular pyridine monolayer relative to the parallel-oriented pyridine monolayer. At a more basic pH, this drop in capacitance is not observed due to overlap with another process. On the basis of studies of OHadsorption on Au (111), the latter is likely the adsorption of OH- and the lifting of the (23 × x3) gold reconstruction near the positive-potential limit.35 A CV of a 1 mM thiophene solution shows two major broad peaks (Figure 3b). At pH 11, the first peak occurs at +0.13 V on the positive scan in the cyclic voltammogram. The second peak occurs on the negative scan at -0.61 V. These peaks are pH dependent and occur at +0.25 V and +0.35 V, respectively, in neutral solution. They are similar in shape and in the large separation between them to features typically observed during the polymerization of thiophenes in organic and aqueous solvents on Pt and Au electrodes.15,36,37 Typically, the oxidation peak encompasses both a step involving the formation and coupling of radical cations and a charging, or “doping”, step. The reduction peak releases the stored charge. The large separation between the cathodic and anodic peaks has been attributed to slow kinetics caused by phase transi(34) Stolberg, L.; Morin, S.; Lipkowski, J.; Irish, D. E. J. Electroanal. Chem. 1991, 307, 241-262. (35) Chen, A.; Lipkowski, J. J. Phys. Chem. B 1999, 103, 682-691. (36) Heinze, J. In Electrochemistry IV; Steckhan, E., Ed.; SpringerVerlag: Berlin, 1990; Vol. 152, pp 1-47. (37) Mu, S.; Park, S.-M. Synth. Met. 1995, 69, 309-312.

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tions within the polymer38 or to a nonfaradaic process following charge-transfer involving the movement of solvent and anions in which conformational changes may occur.39 The potentials at which the oxidative process for thiophene is observed are remarkably low. Thiophene polymerization is typically observed in organic solvents at a potential of +1.6 to +2.0 V vs SCE at a Pt electrode.40 There are a few reports of thiophene polymerization in acidic aqueous media.37,41,42 In all cases, the onset of polymerization was observed at considerably less positive potentials than required in organic solvents. Bazzaoui et al.41 and Jin and Xue42 attributed the low oxidation potential to the formation of π complexes between the monomer and the HClO4, reducing the aromaticity of the rings, and to stabilization of the cation radical by the acid medium. Hu et al. observed that the oxidation potential of bithiophene was lowered in acetonitrile solution upon addition of perchloric acid but attributed this to enhanced solution conductivity in the aqueous medium.43 Thiophene oligomerization on gold at a potential of 0 to +0.6 V vs SCE in neutral aqueous electrolyte has also been reported,44 with polymerization occurring at +0.8 to +1.0 V. This is consistent with our results, and combined with the observation that the oxidation potential of thiophene is even lower in basic solution, it suggests that perhaps the low oxidation potential is due primarily to the use of an aqueous solvent rather than acidity per se. Indeed, we found that, in acetonitrile solution, a potential of +1.2 V vs SCE was required to oxidize 15 mM TP. It is also worth noting that a steady state in the CV of thiophene is reached quickly and after the second scan very little change in the size, shape, or potential of the thiophene peaks was observed, giving no evidence of polymerization. This too is consistent with the work of Fujita et al., which shows that more positive potentials are needed to produce a polymer.44 In general, the oxidation and reduction peaks observed for thiophene are quite similar to TP peaks B and C. They occur at similar potentials, have a similar peak separation, and show a similar pH dependence. In addition, after oxidation of thiophene, there is a drop in the capacitance similar to that seen in TP.27 The similarities between TP peaks B and C and features in the CV of thiophene suggest these peaks are due to the thiophene portion of the molecule and are caused by similar processes, namely an oxidative dimerization and charging process, followed by reductive discharging on the subsequent negative scan. This hypothesis is consistent with studies of 2-(2′-thienyl)pyridine-based polymers synthesized by chemical polymerization that show TP-based polymers can store charge and also show a large separation between oxidation and reduction peaks.45,46 Chronocoulometric Characterization of States I and II of TP. Plots of charge density, σM, vs potential (38) Heinze, J. In Organic Electrochemistry, 4th ed.; Lund, H., Hammerich, O., Eds.; Marcel Dekker: New York, 2001, pp 1309-1339. (39) Visy, C.; Kankare, J. Electrochim. Acta 2000, 45, 1811-1820. (40) Tourillon, G. In Handbook of Conducting Polymers Vol. 1; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1986; pp 301-350. (41) Bazzaoui, E. A.; Aeiyach, S.; Lacaze, P. C. J. Electroanal. Chem. 1994, 364, 63-69. (42) Jin, S.; Xue, G. Macromolecules 1997, 30, 5753-5757. (43) Hu, X.; Wang, G.; Wong, T. K. S. Synth. Met. 1999, 106, 145150. (44) Fujita, W.; Teramae, N.; Haraguchi, H. Chem. Lett. 1994, 511514. (45) Yamamoto, T.; Zhou, Z. H.; Maruyama, T.; Kanbara, T. Synth. Met. 1993, 55, 1209-1213. (46) Zhou, Z. H.; Maruyama, T.; Kanbara, T.; Ikeda, T.; Ichimura, K.; Yamamoto, T.; Tokuda, K. J. Chem. Soc., Chem. Commun. 1991, 1210-1212.

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Figure 4. Charge density, σM, as a function of potential for various bulk concentrations of TP at pH 11, from chronocoulometry measurements. σM ) 0 is marked with a gray dashed line.

were derived from chronocoulometric measurements in basic solution performed from -1.10 to -0.25 V at +0.025 V intervals in order to determine the surface excess of the TP monolayer in the potential range where the changes in the layer are reversible and where it appears no faradaic reactions occur. The charge-potential curves for six different concentrations of TP, as well as two curves for Au(111) in 50 mM KClO4 (pH ) 6, and pH ) 11) in the absence of TP are shown in Figure 4. The two curves for bare gold diverge from one another at around -0.20 V due to adsorption of OH- in the presence of basic solution.35 The slopes of both curves increase suddenly near the positive limit due to the onset of gold oxidation. In the presence of TP, all curves merge with the bare Au curves around -0.95 V, indicating that TP is completely desorbed at this potential despite the absence of any distinct features in the CV and capacitance curves and a negligible change in the capacitance of the interface on adsorption. The lack of peaks in the CV and capacitance data suggests that adsorption is a kinetically slow process, which is consistent with the observation that extremely long waiting times (up to 400 s) were required to reach equilibrium for each potential step in the chronocoulometry experiments. The absence of significant features in the capacitance upon adsorption is similar to that observed for pyridine adsorbed in its initial flat-lying configuration. For all curves, there are two regions of sharply changing slope. The first occurs between -0.95 and -0.80 V, followed by a region of constant slope (constant capacitance). When [TP] g 1 × 10-4 M, there is a change in slope between -0.50 and -0.20 V that grows steeper and shifts to more negative potentials with an increase in concentration. This corresponds to and is consistent with peak A in the CV and capacitance plots (Figure 2). At very high concentrations, this is followed by a second region of constant slope. At lower concentrations, peak B is shifted to more negative potentials and its onset appears to prevent the occurrence of this second constant capacitance region. Regions of constant slope represent stable phases of constant capacitance. An extrapolation of these slopes

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gives a hypothetical case where these phases are stable throughout the potential range in which TP is adsorbed. Following these extrapolations to σM ) 0 for 1 mM TP (shown with green dotted lines) can give an estimate of the orientation of the adsorbed molecules based on their dipole moments, using eq 1.18

EN ) (4πµ j /)Γmax

(1)

where EN is the shift in the pzc,  is the dielectric constant of the inner layer, Γmax is the maximum surface concentration of the adsorbate, and µ j is the component of the effective dipole moment perpendicular to the electrode surface, which accounts for the H2O molecules displaced by the adsorbate. Extrapolation of the constant capacitance region that encompasses -0.80 to -0.60 V shows that, for this phase, the pzc of the modified surface differs very little from that of the water-covered electrode (in the absence of high [OH-], since OH- would be excluded from the surface by the adsorbed layer), indicating that the dipole perpendicular to the surface of the electrode for the adsorbate is small. This evidence suggests that, initially, TP is adsorbed with the plane of the rings parallel to the surface. The steep change in charge density that appears positive of this first stable phase (near -0.45 V in the case of 1 × 10-3 M TP) is further evidence that the corresponding peak, A, in the CV and capacitance curves is due to a two-dimensional phase transition similar to that seen in pyridine. At high concentrations, this steep part of the plot culminates in a region of constant slope corresponding to another stable phase, where the shift in pzc, EN, is approximately -0.43 V, implying an average orientation with a larger component of the dipole moment perpendicular to the electrode, with the more electronegative end (the N and probably the S) interacting with the gold surface. Unlike pyridine34 and 2,2′-bipyridine,47 the steep rise in σM does not pass through σM ) 0. In addition, the shift in pzc is modest compared to that seen for pyridine adsorbed on Au(111) of -0.86 V. This is likely due in part to the much smaller dipole moment of the thiophene moiety relative to pyridine. In addition, it is possible that the TP molecules are in a tilted configuration, or one in which the rings are not coplanar. That the phase transition occurs at more negative potentials relative to pyridine is expected considering that the thiophene ring is π-electron rich compared to pyridine, and greater electrostatic repulsion between the aromatic π electrons and the negatively charged electrode would favor interaction with Au via the heteroatom(s) at less-positive potentials. In addition, the presence of two rings per molecule may enhance intermolecular π-π interactions and is consistent with the observation that, for 2,2′-bipyridine, the analogous phase transition also occurs at more negative potentials.47 On the basis of thermodynamic relationships, film pressure, π, as a function of E and surface pressure, φ, as a function of σ were calculated from the charge density curves.18 The surface coverage, Γ, was determined using the electrocapillary equation at various fixed potentials. The surface excess is reported as relative to H2O and assumes a monolayer model.48 From the film pressure, the maximum Γ observed at a bulk concentration of 1 × 10-3 M TP at the constant capacitance region near -0.70 V, corresponding to the flat-lying orientation, is 7 ((5) × (47) Yang, D.; Bizzotto, D.; Lipkowski, J.; Pettinger, B.; Mirwald, S. J. Phys. Chem. 1994, 98, 7083-7089. (48) Lipkowski, J.; Stolberg, L. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P., Eds.; VCH: New York, 1992; pp 171-238.

10-11 mol/cm2. At -0.25 V, the maximum coverage is approximately 5 ((1) × 10-10 mol/cm2 by this method. Using the surface pressure gives a similar value of of 6 ((1) × 10-10 mol/cm2. These values compare favorably to literature values of surface coverage for the similar-sized molecule 2,2′-bipyridine47 (2 × 10-10 and 5 × 10-10 mol/ cm2, respectively, for flat- lying, π-bonded and vertical N-bonded orientations). The surface coverage of TP at -0.25 V is similar to the surface coverage of 4.6 ((0.4) × 10-10 mol/cm2 calculated by assuming peak C in the CV is due to a one-electron process, integrating underneath it at equilibrium, and accounting for the capacitive charge.27 This consistency suggests that the coverage likely remains the same after the transition at peak B and during the reverse scan, where the capacitance remains constant (although the value assumes one electron per TP molecule, which may be an overestimate of charging and, therefore, an underestimate of surface coverage). The Gibbs energy of adsorption, ∆Gads, was calculated by two different methods, by extrapolating π vs ln c at Γmax to π ) 0,47 and by fitting the φ curves to a virial adsorption isotherm.49 In the latter case, the fit was optimized with the viral coefficient, B ) 1.8, corresponding to 92 Å2/molecule. B is a measure of intermolecular interactions. A positive value indicates repulsive electrostatic forces between molecules, and B is predicted to be twice the molecular area for a rigid, circular molecule. The small value of B relative to the size of TP (for comparison, B ) 120 Å2/molecule for thiourea, a much smaller molecule, adsorbed on Hg)49 indicates that repulsive interactions are weak. It should be noted that this method favors data at lower concentration and charge values and should give a value closer to ∆Gads at infinite dilution, where intermolecular interactions are insignificant. The two methods of calculating ∆Gads gave similar values of -26 (( 5) and -31 (( 5) kJ/mol. These values compare favorably to a ∆Gads of -37 kJ/mol for pyridine on Au(111), a ∆Gads of -30 and -50 kJ/mol for flat-lying and vertical orientations, respectively, of 2,2′-bipyridine on Au(111), and a ∆Gads of -22 kJ/mol for thiophene,8 suggesting that the interaction of TP with gold is comparable in magnitude to that of similar species; however, the magnitude of the lateral π-π interactions between TP molecules may not be as strong as that between 2,2′-bipyridine molecules. Electrochemical and in Situ Fluorescence Characterization of PTTP and State III of Adsorbed TP. To test the hypothesis that peaks B and C were due to a dimerization and charging process, the dimer, 5,5′-bis(2-pyridyl)-2,2′-bithiophene (PTTP) was chemically synthesized in order to compare its properties with those of TP in state III. Cyclic voltammetry and differential capacitance measurements of adsorbed PTTP (Figure 5) showed a slight decrease in the capacitance indicative of adsorption near -0.80 V vs SCE. A single pair of peaks at -0.48 V showed no pH dependence, and the anodic and cathodic peaks were separated by +0.08 V. These peaks could be due to adsorption, reorientation, or charging. At potentials positive of these peaks, there was a slightly larger decrease in the capacitance. At pH 10, Au reconstruction and OHpeaks appear at potentials positive of 0 V vs SCE. The minimum capacitance does not fall below 12 µF/cm2, in contrast to a minimum capacitance of 7 µF/cm2 for a similar differential capacitance experiment in 1 mM TP solution, suggesting that the adsorbed PTTP layer formed by this method is relatively diffuse and/or disorganized. (49) Parsons, R. Proc. R. Soc. (London) 1961, A261, 79-90.

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Figure 5. Capacitance plots (a and b) and cyclic voltammograms (c and d) of chemically synthesized PTTP at neutral (a and c) and basic (b and d) pH.

Figure 6. (a) A comparison of the fluorescence spectrum of chemically synthesized PTTP in CHCl3; the background-subtracted fluorescence, ∆I, observed at the desorption potential near a Au (111) electrode in contact with chemically synthesized PTTP at the air/solution interface after holding at a given potential of interest, Ei, for 15 min; and ∆I near a Au(111) electrode in contact with 1 mM TP after holding at Ei for 15 or 30 min. (b) The background-subtracted fluorescence, ∆I, observed at the desorption potential near Au(111) in contact with 1 mM TP as a function of Ei and time spent at Ei. Lines represent smoothed/filtered data, while points represent the raw data.

TP and PTTP are both fluorescent but have different characteristic emission spectra. TP has an excitation maximum at 303 nm and a single emission maximum at 360 nm in chloroform or dichloromethane solution. Its emission is shifted to 370 nm in basic, aqueous electrolyte. PTTP excitation and emission occur at longer wavelengths due to its longer conjugation length. In chloroform or

dichloromethane solution, its excitation maximum is at 390 nm, and it shows two emission maxima at 435 and 460 nm, with shoulders at 500 and 530 nm (Figure 6a). (Spectra were taken in CHCl3 because PTTP is poorly soluble in aqueous solution). Equivalent concentrations of TP and PTTP (accounting for the fact that it takes two TP molecules to produce a

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single molecule of PTTP) show similar fluorescence intensities at their excitation maxima in a polar environment. PTTP is negligibly affected by solvent polarity, while TP fluorescence intensity is reduced as the polarity of the solvent is decreased and shows a small red shift in emission at a concentration of 1 mM in aqueous electrolyte solution relative to a 0.015 mM solution in dichloromethane, which may indicate some π stacking. To compare TP adsorbed in state III with PTTP, fluorescence spectra of the adsorbed monolayer were measured in situ. The electrochemical cell was positioned above the microscope objective, which focused the excitation light (340 to 380 nm) onto the working electrode from below. Light reflected and emitted from the electrode and the solution passed back through the objective, and the dichroic and emission filters allowed wavelengths >430 nm to be detected by the spectrometer. The electrode was held at a potential of interest, Ei, typically for 15 min. Fluorescence from any species adsorbed on the electrode at Ei should be completely quenched since nonradiative energy transfer to the metal significantly decreases the lifetime of an excited state within close proximity to a metal surface.50,51 Thus, during this time, a background spectrum, I(Ei), was taken of the fluorescently quenched adsorbed monolayer. This spectrum was used to account for leakage through filters, solubilized fluorophores (e.g., TP) in the optical path between the microscope objective and the electrode, and any other stray light. This background should be independent of the applied potential. After waiting at Ei long enough to reach a stable adsorbed state and taking a background spectrum, the potential was stepped to the desorption potential, Edes (which was -1.00 V), where another spectrum, I(Edes), was collected immediately. The change in E will not affect the fluorescence of the species in solution. However, organic molecules adsorbed on the electrode at Ei are desorbed into solution at Edes. Upon desorption, the molecules separate from the metal surface, significantly decreasing the efficiency of metal-mediated quenching, resulting in a large increase in the fluorescence intensity, since the rate of energy transfer to the metal surface is inversely related to the distance from the electrode. Therefore, ∆I, the spectral changes due to the potential perturbation (with the background subtracted), would result only from the species that had been adsorbed and nonfluorescent at Ei, which become desorbed and fluorescent at Edes:

∆I ) I(Edes) - I(Ei)

(2)

The resulting spectra are shown in Figure 6. For chemically synthesized PTTP adsorbed from the air/ solution interface, fluorescence is observed even after holding the electrode at the very negative potential of -0.80 V, and ∆I increases for more positive values of Ei until it reaches a maximum at -0.60 V. For values of Ei more positive than -0.60 V, ∆I remains constant; suggesting that the peaks observed in the CV of PTTP are not associated with a sudden increase in adsorbed material. For TP, at adsorption potentials e -0.60 V, ∆I was negligible. At an adsorption potential of -0.40 V, fluorescence from the monolayer was characteristic of TP (i.e., (50) Axelrod, D.; Hellen, E. H.; Fulbright, R. M. In Topics in Fluorescence Spectroscopy; Lakowicz, J. R., Ed.; Plenum Press: New York, 1992; Vol. 3, pp 289-343. (51) Chance, R. R.; Prock, A.; Silbey, R. In Advances in Chemical Physics; Prigogine, I., Ed.; Interscience: New York, 1978; Vol. 37, pp 1-65.

identical to the background from TP in solution), and ∆I increased for more positive adsorption potentials, reaching a maximum at -0.25 V (Figure 6b). This is consistent with electrochemical data showing a maximum in the surface coverage and a minimum in the capacitance are reached at this potential. At potentials g -0.25 V, ∆I decreases, suggesting a loss of TP at the onset of dimerization. After the potential was held for 2 min at +0.20 V, a fluorescence spectrum characteristic of PTTP was observed, and after being held for 30 min at this potential, the individual peaks of an emission spectrum could be clearly resolved. It should be noted that this spectrum is not actually the spectrum of the species that had been adsorbed at +0.20 V but of the desorbed species, which has undergone a reduction near -0.70 V (peak C, Figure 2) prior to desorption at -1.0 V (vide infra). It is believed that this species is electrochemically generated PTTP. The spectrum of the desorbed species created after adsorbing TP at +0.20 V and the spectrum of chemically synthesized PTTP deposited on the electrode from the air/solution interface are both very similar to that of chemically synthesized PTTP in chloroform solution. However, the species desorbed from the electrode show spectra that are either broadened or more intense at longer wavelengths than the solution spectrum. There is no red shift in the peak maxima that might suggest π stacking of the molecules, despite the fact that small aromatic molecules, such as pyridine,52 2,2′-bipyridine,53,54 and thiophene,55 appear to form organized, close-packed rows in STM images. This may be because the dimer does not π stack efficiently or because the desorption (and prior events such as reduction and possibly reorientation) process disturbs stacking that may occur on the surface. That TP fluorescence is observed at all is somewhat surprising (given that equivalent amounts of TP and PTTP show similar emission intensities), as TP generally shows little absorption between 340 and 380 nm and very little emission at wavelengths longer than 430 nm. It is possible that excitation and emission of TP may be red-shifted or broadened in our in situ experiments. Red-shifting and broadening of spectra have previously been observed for fluorophores attached to metal surfaces,56,57 and it is possible that ordering similar to that on the surface and/ or proximity to the electrode are sufficient to produce these effects. However, with the available fluorescence filter set, only a very small tail of the emission spectrum could be seen; thus, it is impossible to tell for certain whether a red shift or broadening occurs in the case of TP, although broadening is likely since it is observed for both the chemically synthesized and electrochemically synthesized PTTP. The fluorescence intensity of TP adsorbed at -0.25 V at 430 nm suggests that fluorescence at the emission maximum would likely be significantly higher than that observed for the species formed after holding at +0.20 V (Figure 6b). Since no TP emission is observed at +0.20 V, it is unlikely that the low intensity of dimer emission is (52) Andreasen, G.; Vela, M. E.; Salvarezza, R. C.; Arvia, A. J. J. Electroanal. Chem. 1999, 467, 230-237. (53) Cunha, F.; Tao, N. J.; Wang, X. W.; Jin, Q.; Duong, B.; D’Agnese, J. Langmuir 1996, 12, 6410-6418. (54) Dretschkow, T.; Lampner D.; Wandlowski, T. J. Electroanal. Chem. 1998, 458, 121-138. (55) Noh, J.; Ito, E.; Nakajima, K.; Kim, J.; Lee, H.; Hara, M. J. Phys. Chem. B 2002, 106, 7139-7141. (56) Kittredge, K. W.; Fox, M. A.; Whitesell, J. K. J. Phys. Chem. B 2001, 105, 10594-10599. (57) Pope, J. M.; Buttry, D. A. J. Electroanal. Chem. 2001, 498, 7586.

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due to incomplete dimerization. One possible explanation for the difference in emission intensities between TP and the dimer is their distance from the electrode upon desorption. As previously mentioned, the effect of electrodemediated quenching is related to the distance between the surfactant and the electrode. With this type of relationship, even small differences in distance from the electrode can lead to large differences in fluorescence intensity. As PTTP is larger and less soluble than TP, it might be expected that the dimer would not move as far away from the electrode as TP at the same desorption potential and would therefore be quenched to a greater extent. This would be consistent with fluorescence microscopy studies of insoluble surfactants showing that such adsorbates remain close to the electrode for long periods of time upon desorption (>60 min), without drifting or diffusing.32 Another possible explanation for the lower observed fluorescence of the dimer relative to the monomer is that either the monomer radical cation or some of the dimer has reacted with other species in solution and is no longer fluorescent. However, when the electrode is held at +0.20 V for up to 60 min, the fluorescence intensity on desorption remains relatively constant with time spent at the oxidizing potential, suggesting that both the TP radical cation and the dimer are chemically very stable on the electrode (and that long time scales are required for reaction completion) and may be stabilized by intermolecular interactions due to the organization of the monomer prior to dimerization. In addition, the lack of charge mobility within the oxidized dimer evidenced by the low capacitance suggests anions in solution may be involved in stabilizing the adsorbed cationic species. In particular, the presence of OH- near the electrode is favored at these positive potentials.35 Also, it has been proposed that some anions, such as ClO4-, are able to shield charged polythiophene from nucleophilic attack.58 The evidence presented here is strong that, at positive potentials, TP does indeed dimerize to form PTTP and that, at negative potentials, the neutral dimer is desorbed from the surface. In the intervening potentials, the electrochemistry is consistent with the occurrence of a charging process similar to that seen in thiophene, although such a process cannot be observed using this fluorescence technique. Summary and Conclusions The proposed model for the adsorption and dimerization as a function of potential for 2-(2′-thienyl)pyridine (TP) is shown in Figure 7. TP shows adsorption behavior similar to that of pyridine and 2,2′-bipyridine. At potentials near -0.80 V vs SCE, evidence suggests that TP adsorbs as a monolayer with the plane of the rings parallel to the electrode surface. Near -0.45 V, a two-dimensional phase transition to a close-packed, N-bonded configuration with the rings more normal to the surface occurs. At positive potentials, an oxidative dimerization similar to the oligomerization observed in thiophene occurs, where the dimer is immediately charged and likely undergoes a conformational change; when scanned or pulsed to a negative potential, stored charge is discharged and the neutral dimer is (58) Pud, A. A. Synth. Met. 1994, 66, 1-18.

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Figure 7. Proposed model of TP-electrode interactions as a function of E.

desorbed, as observed by fluorescence spectroscopy. On scanning the potential again in a positive direction, some dimer is re-adsorbed along with TP from solution, as shown by changes in the CV on subsequent scans. A new, sharp peak, slightly negative of peak A, which may be attributed to the dimer, appears in addition to peak A itself, which has grown smaller (Figure 2). Interestingly, the pyridine half of the molecule appears to dominate the adsorption behavior of TP (although the thiophene part of the molecule does have an influence as a π-electron rich substituent) despite the fact that the thiophene S might be expected to interact more strongly with gold. The thiophene half of the molecule confers electrochemical reactivity to the molecule that is influenced by the pyridine-like potentially driven organization of the layer. The preorganization of a close-packed TP layer allows for the electrochemical generation of an adsorbed layer of the dimer that is highly organized and closely packed. These properties would be difficult to achieve by direct adsorption of chemically synthesized PTTP. We show here that the in situ fluorescence technique is a versatile method allowing characterization of the layer as it is formed at different potentials, as well as the identification of the product of a surface-confined electrochemical reaction on a gold electrode via its characteristic fluorescence spectrum. Overall, this system shows that multifunctional adsorbates can create chemically tuned surfaces capable of switching between multiple states and allow subtle comparisons of the influence of surface-adsorbate interactions between different functional groups. Acknowledgment. We thank NSERC and UBC (Laird Fellowship for E.C.) for funding and Dr. Cerrie Rogers for the synthesis of the catalyst. Supporting Information Available: Absorption, excitation, and emission spectra of TP and PTTP; backgroundsubtracted fluorescence, ∆I, as a function of Ei for adsorbed PTTP; and ∆I as a function of time spent at +0.20 V in the presence of 1 mM TP. This material is available free of charge via the Internet at http://pubs.acs.org. LA0485024