Os Dinuclear Complexes: Probing

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9242

Langmuir 2004, 20, 9242-9248

Self-Assembled Monolayers of Ru/Os Dinuclear Complexes: Probing Monolayer Structure and Interaction Energies by Electrochemical Means Egbert Figgemeier,*,† Edwin C. Constable,*,‡ Catherine E. Housecroft,‡ and Yves C. Zimmermann‡ University of Uppsala, Department of Physical Chemistry, P. O. Box 579, 75123 Uppsala, Sweden, and University of Basel, Department of Chemistry, Spitalstrasse 51, 4056 Basel, Switzerland Received May 18, 2004. In Final Form: August 6, 2004 Monolayers of [Ru(bpy)2(µ-1)M(2)][PF6]4 salts (M ) Os, Ru; bpy ) 2,2′-bipyridine, 1 ) 4′-(2,2′-bipyridin4-yl)-2,2′:6′,2′′-terpyridine, tpy ) 2,2′:6′,2′′-terpyridine, and 2 ) 4′-(4-pyridyl)-2,2′:6′,2′′-terpyridine) were self-assembled on platinum and investigated by fast-scan electrochemistry. The electrochemistry of the complexes in solution and confined to the surface in self-assembled monolayers (SAMs) exhibited an almost ideal behavior. Scan-rate-dependent measurements of the peak current density (jp) were used to determine interaction energies within the monolayer. It is shown that the tpy coordination sites of the dinuclear complexes interact more strongly within the SAM than the bipyridine-coordinated fragments. This result was supported by peak potential shifts, which are due to interaction forces in SAMs. The alignment of the rodlike complexes relative to the surface is discussed, and the results of molecular mechanics calculations indicate that the species adopt a tilted orientation.

Introduction Metal polypyridine complexes have been investigated for a number of years because of their intriguing electrochemical and photophysical properties.1-3 This interest has been amplified by the prospect of high-impact practical applications in light-to-energy conversion devices such as dye-sensitized photoelectrochemical solar cells.4 Selfassembled monolayers (SAMs) of redox- and photoactive species are an essential part in such integrated chemical systems.5 Therefore, a detailed understanding of structure, adsorption, and electron-transfer properties of SAMs is a necessity for the development and improvement of such systems. The interaction forces, leading to self-organization of adsorbed species as they have been observed by scanning probe techniques, are also poorly understood. In this context, we recently reported the properties of SAMs consisting of the mononuclear complexes [M(tpy)(2)][PF6]2 (M ) Os, Ru; 2 is 4′-(4-pyridyl)-2,2′:6′,2′′-terpyridine and tpy is 2,2′:6′,2′′-terpyridine).6 The basic electrochemistry of the metal complexes on surfaces was discussed, and by calculating interaction energies between adsorbed species we were able to demonstrate that tpy ligands are less able to shield the charge of the metal ion than bpy ligands. * To whom correspondence should be addressed. E-mail: [email protected]; [email protected]. † University of Uppsala. ‡ University of Basel. (1) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85. (2) Sauvage, J.-P.; Collin, J.-P.; Chambron, J. C.; Guillerez, S.; Coudret, C.; Balzani, V.; Barigelletti, F.; Decola, L.; Flamigni, L. Chem. Rev. 1994, 94, 993. (3) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Chem. Rev. 1996, 96, 759. (4) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269. (5) Bard, A. J. Integrated Chemical Systems: A Chemical Approach to Nanotechnology; John Wiley & Sons: New York, 1994. (6) Figgemeier, E.; Merz, L.; Hermann, B. A.; Zimmermann, Y. C.; Housecroft, C. E.; Gu¨ntherodt, H.-J.; Constable, E. C. J. Phys. Chem. B 2003, 107, 1157.

Bearing in mind that multinuclear complexes are of great interest in terms of their fundamental properties (intramolecular charge-transfer and electronic interactions) and their light harvesting properties in for example photoelectrochemical cells, we consequently present here SAMs on Pt built by the dinuclear species [Ru(bpy)2(µ-1)M(2)][PF6]4 (M ) Os, Ru; bpy is 2,2′bipyridine and 1 is 4′-(2,2′-bipyridin-4-yl)-2,2′:6′,2′′-terpyridine).7,8 These complexes are anchored by the [M(tpy)(2)]2+ unit to platinum surfaces and expanded by a Rutris-bipyridine unit attached in the 4-position of the tpy ligand (Figure 1).9 The step-by-step extension of the complex nuclearity enables us to relate our results to the earlier experiments and to gain a deeper understanding of the properties of SAMs of these dinuclear complexes. The well-defined rodlike geometry of the complexes is also an advantage for the interpretation of the experimental data. Only recently have dinuclear complexes adsorbed on metal surfaces been investigated.10 On the other hand, there are a significant number of investigations dealing with mononuclear compounds on platinum or gold surfaces.11-19 (7) Barigelletti, F.; Flamigni, L. Chem. Soc. Rev. 2000, 29, 1. (8) Bignozzi, C. A.; Argazzi, R.; Kleverlaan, C. J. Chem. Soc. Rev. 2000, 29, 87. (9) The electrochemical properties of the multicenter complexes of the bridging ligand 1 in solution were investigated in parallel. The results together with the synthetic strategy are summarized in: Constable, E. C.; Figgemeier, E.; Housecroft, C. E.; Olsson, J.; Zimmermann, Y. C. J. Chem. Soc., Dalton Trans. 2004, 1918-1927. (10) Forster, R. J.; Keyes, T. E. J. Phys. Chem. B 2001, 105, 8829. (11) Forster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5444. (12) Forster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5453. (13) Acevedo, D.; Bretz, R. L.; Tirado, J. D.; Abrun˜a, H. D. Langmuir 1994, 10, 1300. (14) Forster, R. J.; Faulkner, L. R. Anal. Chem. 1995, 67, 1232. (15) Bretz, R. L.; Abrun˜a, H. D. J. Electroanal. Chem. 1995, 388, 123. (16) Hudson, J. E.; Abrun˜a, H. D. J. Phys. Chem. 1996, 100, 1036. (17) Forster, R. J.; Figgemeier, E.; Loughman, P.; Lees, A.; Hjelm, J.; Vos, J. G. Langmuir 2000, 16, 7871.

10.1021/la048762l CCC: $27.50 © 2004 American Chemical Society Published on Web 09/17/2004

SAMs of Ru/Os Dinuclear Complexes

Figure 1. Structural diagram of [Ru(bpy)2(µ-1)M(2)]4+ (M ) Os, Ru) and ring numbering used in the Experimental Section.

In the current study, the basic electrochemistry of the metal complexes [Ru(bpy)2(µ-1)M(2)][PF6]4 (M ) Os, Ru) in solution and confined on surfaces is presented and interpreted. Moreover, assuming an ordered array of the species on platinum, the interaction energies for the different fragments of the dinuclear complexes are calculated and compared. Finally, the alignment of the adsorbed rodlike ions relative to the underlying surface is discussed. Experimental Section General. 1H NMR spectra were recorded on a Bruker AM 250 spectrometer; δ is relative to TMS. IR spectra were recorded on a Mattson Genesis FT spectrophotometer with samples in compressed KBr disks. UV/vis measurements were performed using a Perkin-Elmer Lambda 19 spectrophotometer. Fluorescence experiments used a Perkin-Elmer luminescence spectrometer LS 50. Electrospray mass spectra (ES-MS) were recorded using a Bruker FTMS 4.7T BioAPEX II or a Finnigan MAT LCQ workstation. Reagents. For the electrochemical experiments, acetonitrile (Fluka) with a water content lower than 0.001% stored over 4 Å molecular sieves was used. Dichloromethane (Aldrich) was distilled and also stored over 4 Å molecular sieves. The supporting electrolyte for electrochemical experiments in acetonitrile and dichloromethane was tetra-n-butylammonium hexafluorophosphate ([nBu4N][PF6], Aldrich) and this was dried in a vacuum oven at 120 °C for at least 24 h before use. LiClO4 (99.9% Aldrich) was used for the electrochemistry in aqueous solutions. For the synthetic procedures, commercially available chemicals were of reagent grade and used without further purification. The compounds [Ru(tpy)Cl3)],20 [Os(tpy)Cl3)],21 4′-(4-pyridyl)-2,2′: 6′,2′′-terpyridine,22 4′-(2,2′-bipyridin-4-yl)-2,2′:6′,2′′-terpyridine (1),9 and [(bpy)2Ru(1)][PF6]29 were prepared according to literature methods. Synthesis of [Ru(bpy)2(µ-1)Ru(2)][PF6]4. A solution of [(bpy)2Ru(1)][PF6]2 (0.015 g, 0.013 mmol) and RuCl3‚3H2O (0.004 g, 0.013 mmol) in EtOH (15 mL) was heated to reflux for 3 h. The dark precipitate of [(bpy)2Ru(µ-1)RuCl3][PF6]2 was collected by filtration and washed with EtOH but was not purified further. This solid (0.018 g, 0.014 mmol), ligand 2 (0.005 g, 0.016 mmol), and 2 drops of N-ethylmorpholine were added to ethane-1,2-diol (5 mL), and the mixture was heated to reflux in a microwave oven (800 W) for 5 min. Saturated aqueous NH4PF6 (25 mL) was added to the dark-red solution. The precipitate so formed was collected on Celite and then dissolved in MeCN. The solvent was removed in vacuo, and the residue was purified by column chromatography (SiO2 eluting with MeCN/saturated aqueous KNO3/H2O 7:1.5:0.5). An excess of NH4PF6 was added to the major red fraction, and the solution was reduced in volume. The precipitate was collected on Celite, washed with MeCN, and dried. (18) Finklea, H. O.; Yoon, K.; Chamberlain, E.; Allen, J.; Haddox, R. J. Phys. Chem. B 2001, 105, 3088. (19) Forster, R. J.; Figgemeier, E.; Lees, A.; Hjelm, J.; Vos, J. G. Langmuir 2000, 16, 7867. (20) Constable, E. C.; Cargill Thompson, A. M. W.; Tocher, D. A.; Daniels, M. A. M. New J. Chem. 1992, 16, 855. (21) Buckingham, D. A.; Dwyer, F. P.; Sargeson, A. M. Aust. J. Chem. 1964, 17, 622. (22) Constable, E. C.; Cargill Thompson, A. M. W. J. Chem. Soc., Dalton Trans. 1992, 2947.

Langmuir, Vol. 20, No. 21, 2004 9243 [(bpy)2Ru(µ-1)Ru(2)][PF6]4 was isolated as a red powder (0.020 g, 81%). 1H NMR (250 Hz, CD3CN): δ 9.27 (1H, m, F3), 9.17 (2H, s, G3), 9.07 (2H, s, K3), 9.02 (1H, d, J ) 8.1 Hz, E3), 8.96 (2H, m, L2), 8.74 (2H, d, J ) 7.7 Hz, H3), 8.67 (2H, d, J ) 8.1 Hz, J3), 8.59-8.53 (4H, m, A3, B3, C3, D3), 8.19 (1H, dt, J ) 2.2, 8.0 Hz, E4), 8.16-8.04 (7H, m, A,4 B,4 C,4 D,4 F,5 L3), 7.99-7.95 (6H, m, E,6 F,6 H,4 J4), 7.83-7.77 (4H, m, A,6 B,6 C,6 D6), 7.50-7.39 (9H, m, A,5 B,5 C,5 D,5 H,6 J6), 7.23-7.18 (4H, m, H,5 J5). IR (cm-1): 3426m, 1601m, 1466m, 1446w, 1409m, 1384m, 1358m, 1028w, 840s, 788m, 761w, 732w, 558s. UV/vis (CH3CN): λmax/nm (/dm3 mol-1 cm-1) 241.7 (44 800), 285.0 (89 200), 444.0 (17 600), 502.4 (36 100). Fluorescence (CH3CN): λmax/nm (λirr/nm) 685.5 (503). ES-MS: m/z 750.5 ([M - 2PF6]2+), 452.0 ([M - 3PF6]3+), 302.8 ([M - 4PF6]4+). Anal. Calcd for C65H47N13Ru2P4F24‚6H2O (MW 1900.2): C, 41.0; H, 3.1; N, 9.6. Found: C, 41.0; H, 3.3; N, 9.7%. Synthesis of [(bpy)2Ru(µ-1)Os(2)][PF6]4. A solution of [(bpy)2Ru(1)][PF6]2 (0.015 g, 0.013 mmol) and [NH4]2[OsCl6] (0.010 g, 0.022 mmol) in EtOH (15 mL) was heated to reflux overnight. The solution was cooled to room temperature, and H2O (15 mL) was added. The black precipitate of [(bpy)2Ru(µ-1)OsCl3][PF6]2 that formed was collected by filtration and washed with EtOH. This solid (0.030 g, 0.021 mmol), ligand 2 (0.015 g, 0.048 mmol), and 2 drops of N-ethylmorpholine were added to ethane-1,2-diol (5 mL), and the mixture was heated to reflux in a microwave oven (800 W) for 5 min. Saturated aqueous NH4PF6 (25 mL) was added to the brown solution. The precipitate that formed was collected on Celite and dissolved in MeCN. The solvent was removed in vacuo, and the residue was purified by column chromatography (SiO2 eluting with MeCN/saturated aqueous KNO3/H2O 7:1.5:0.5). An excess of NH4PF6 was added to the second fraction (orange-brown), and the solution was reduced in volume. The precipitate was collected on Celite, washed with MeCN, and dried. [(bpy)2Ru(µ-1)Os(2)][PF6]4 was isolated as a red powder (0.010 g, 25%). 1H NMR (250 Hz, CD3CN): δ 9.21 (1H, m, F3), 9.15 (2H, s, G3), 9.08 (2H, s, K3), 9.00 (1H, d, J ) 8.1 Hz, E3), 8.95 (2H, m, L2), 8.71 (2H, d, J ) 7.7 Hz, H3), 8.64 (2H, d, J ) 8.1 Hz, J3), 8.59-8.54 (4H, m, A3, B3, C3, D3), 8.19 (1H, dt, J ) 2.2, 8.0 Hz, E4), 8.14-8.02 (8H, m, A,4 B,4 C,4 D,4 F,5 F,6 L3), 7.92 (1H, m, E6), 7.87-7.76 (8H, m, A,6 B,6 C,6 D,6 H,4 J4), 7.49-7.44 (5H, m, A,5 B,5 C,5 D,5 E5), 7.33 (2H, m, J6), 7.26 (2H, m, H6), 7.17-7.08 (4H, m, H,5 J5). IR (cm-1): 3449m, 1638m, 1602m, 1475m, 1447w, 1397w, 1354w, 1089w, 1029w, 840s, 786m, 755w. UV/vis (CH3CN): λmax/nm (/dm3 mol-1 cm-1) 235.0 (67 400), 285.6 (113 900), 500.5 (41 300), 677.8 (8900). Fluorescence (CH3CN): λmax/nm (λirr/nm) 776.1 (501), 773.4 (680). ESMS: m/z 1736.3 ([M - PF6]+), 795.6 ([M - 2PF6]2+). Anal. Calcd for C65H47N13[Ru(bpy)2(µ-1)Os(2)]2+ P4F24‚6H2O (MW 1989.3): C, 39.2; H, 3.0; N, 9.1. Found: C, 39.0; H, 3.1; N, 9.0%. Electrochemistry. Cyclic voltammetry (CV) was performed using an EcoChem PGSTAT30 and a conventional three-electrode cell with a BAS Ag/AgCl electrode as reference. Microelectrodes were fabricated from platinum wires (Goodfellow Metals Ltd.) of radii between 5 and 25 µm by sealing them into soft glass using a procedure described previously by Faulkner et al.12 Microdisk electrodes were exposed by removing excess glass using 600 grit emery paper followed by successive polishing with 12.5, 5, 1, 0.3, and 0.05 µm alumina. The polishing material was removed between changes of particle size by sonicating the electrodes in deionized water for at least 5 min. The polished electrodes were electrochemically cleaned by cycling in 0.5 M H2SO4 between potential limits chosen to first oxidize and then to reduce the surface of the platinum electrode. Excessive cycling was avoided in order to minimize the extent of surface roughening. The real, or microscopic, surface area of microelectrodes and the macroelectrodes was found by calculating the charge under the oxide or hydrogen adsorption-desorption peaks. Before the electrode was removed from the cell, the potential was then held in the double-layer region at a sufficiently negative value to ensure complete reduction of any surface oxide. Finally the electrode was cycled between -0.300 and 0.700 V in 0.1 M LiClO4 solution until hydrogen desorption was complete. Typically the surface roughness factor was between 1.5 and 2.0. RC cell time constants were between 0.03 and 3 µs, depending on the electrode radius and the supporting electrolyte concentration.

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Table 1. Redox Potentials of [Ru(bpy)2(µ-1)Os(2)]4+ and [Ru(bpy)2(µ-1)Ru(2)]4+ in Acetonitrile Solution (E0sol) and Adsorbed on Platinum (E0surf)a E0sol/V [Ru(bpy)2(µ-1)Ru(2)]2+ [Ru(bpy)2(µ-1)Os(2)]2+ a

0.97 0.94

0.97 0.62

-1.32 -1.37

-1.62 -1.65

0 Esurf /V

-1.78 -1.81

-1.88 -1.93

-2.21 -2.25

0.96 0.94

1.03 0.66

All potentials refer to Fc-Fc+.

Figure 2. Cyclic (A,C) and differential pulse (B,D) voltammograms of [Ru(bpy)2(µ-1)Os(2)]4+ (vs ferrocene-ferrocenium). The voltammograms were recorded in acetonitrile and [nBu4N][PF6] (0.1 M) as the supporting electrolyte. The electrochemical behavior of all self-assembled monolayers was measured in acetonitrile with tetrabutylammonium hexafluorophosphate (0.1 M) as the supporting electrolyte, if not stated otherwise. All potentials given are quoted versus the ferrocene-ferrocenium redox couple. Preparation of Monolayers. The freshly prepared platinum electrodes were immersed into aqueous acetone solutions of the complexes for at least 1 h. In this time, the complexes spontaneously formed adsorbed monolayers. The surfaces were thoroughly rinsed with the solvent to remove nonbound material before performing electrochemistry. Molecular Mechanics Calculations. The conformation analysis of the complexes was done by means of molecular dynamics methods included in the software package HyperChem. Using MM+ as the force field, a simulation temperature of 1000 K, a simulation time of 1 ps, and a time step of 1 fs were applied to all systems. The conformations obtained by this method were optimized by standard methods (steepest descent, conjugate gradient, and quasi-Newton methods). The Pt surface (111 crystal face) was created according to known crystal parameters and was held constant during the calculations. Its size was 10 × 10 × 2 unit cells.

Results Synthesis and Characterization of Complexes. The complexes [Ru(bpy)2(µ-1)Ru(2)]4+ and [Ru(bpy)2(µ-1)Os(2)]4+ were prepared in a stepwise manner. The pendant tpy-binding domain in the precursor [(bpy)2Ru(1)]2+ 9 reacted with RuCl3‚3H2O or [OsCl6]2- to yield [(bpy)2Ru(µ-1)MCl3]2+ with M ) Ru or Os, respectively. Under reducing conditions, treatment of [(bpy)2Ru(µ-1)MCl3]2+ with ligand 222 gave [Ru(bpy)2(µ-1)Ru(2)]4+ and [Ru(bpy)2(µ-1)Os(2)]4+ in 81% and 25% yield after chromatographic workup. The complexes were isolated as hexafluorophosphate salts. In the electrospray (ES) mass spectrum of the diruthenium complex, peaks at m/z 750.5, 452.0, and 302.8 were assigned to [M - 2PF6]2+, [M - 3PF6]3+, and [M - 4PF6]4+, respectively. The ES mass spectrum of the heterometallic complex exhibited peaks at m/z 1736.3 and

795.6 assigned to [M - PF6]+ and [M - 2PF6]2+, respectively. For both spectra, observed isotope distributions matched those calculated for each peak. The solution 1H NMR spectra of [Ru(bpy)2(µ-1)Ru(2)][PF6]4 and [Ru(bpy)2(µ-1)Os(2)][PF6]4 resembledthoseof[Ru(bpy)2(µ-1)Os(tpy)][PF6]4 and [Ru(bpy)2(µ-1)Ru(tpy)][PF6]4,9 with the signal for the 4′-proton of the terminal tpy ligand being replaced by signals for the 4′-pyridyl protons (L2 and L3, see Figure 1). The spectra were assigned with the aid of COSY techniques and are fully consistent with the structure shown in Figure 1. Electrochemistry in Solution. The redox behavior of the dinuclear complexes [Ru(bpy)2(µ-1)Os(2)][PF6]4 and [Ru(bpy)2(µ-1)Ru(2)][PF6]4 was investigated in acetonitrile with [nBu4N][PF6] as supporting electrolyte. The potentials for all redox processes detected are summarized in Table 1. As shown in Figure 2A, two reversible processes could be detected for the compound [Ru(bpy)2(µ-1)Os(2)]4+ within the oxidative region of the potential window. The differential pulse voltammogram (DPV) also shows two signals, which have identical integrated areas indicating two one-electron-transfer processes. We assign the process at less positive potential to the OsII/III (0.62 V) redox couple and the one at more positive values to RuII/III (0.94 V). For [Ru(bpy)2(µ-1)Ru(2)]4+, only one peak within the potential window was detected (Figure 3). In comparison to [Ru(bpy)2(µ-1)Os(2)]4+, the signal in the cyclic voltammogram is slightly asymmetric as can be seen in Figure 3, indicating that two processes with only slightly different potentials are covered by the signal. This is a sign that there might be a small interaction between the metal centers mediated by the bridging ligand. Nevertheless we were not able to resolve the two peaks. We recently presented extensive studies for a series of trinuclear and dinuclear complexes involving bridging ligand 1.9 These investigations led to the conclusion that ligand 1 does not effectively promote electronic interactions between the two metal centers. This

SAMs of Ru/Os Dinuclear Complexes

Figure 3. Cyclic (A) and differential pulse (B) voltammograms of [Ru(bpy)2(µ-1)Ru(2)]4+ (vs ferrocene-ferrocenium). The voltammograms were recorded in acetonitrile and [nBu4N][PF6] (0.1 M) as the supporting electrolyte.

Figure 4. Cyclic voltammograms of monolayers of [Ru(bpy)2(µ-1)Os(2)]4+ (A) and [Ru(bpy)2(µ-1)Ru(2)]4+ (B) on a platinum microelectrode (d ) 25 µm) at 200 (vs ferrocene-ferrocenium). Both voltammograms were recorded in acetonitrile and [nBu4N][PF6] (0.1 M) as the supporting electrolyte.

result was concluded from a comparison of the redox potentials of the dimeric and trimeric complexes in comparison to their monomeric counterparts.9 The electrochemistry at negative potentials is very similar for [Ru(bpy)2(µ-1)Os(2)]4+ and [Ru(bpy)2(µ-1)Ru(2)]4+ as typified in Figure 2 (panels C and D) for [Ru(bpy)2(µ-1)Ru(2)]4+: In total, five peaks were detected with similar potentials for both compounds. Assuming that in polypyridine complexes the reductions take place successively on the ligands and the number of peaks matches the number of ligands in the complex, we can conclude from our earlier study mentioned above that the first two peaks represent the reduction of the two tpy ligands followed by the successive reduction of the three bpy ligands.1,9 It is also shown that the bpy part of the bridging ligand is reduced at most negative potentials due to the increased charge on the attached tpy. Electrochemistry of Self-Assembled Monolayers. After applying a monolayer of [Ru(bpy)2(µ-1)Ru(2)]4+ or [Ru(bpy)2(µ-1)Os(2)]4+ in the way described in the Experimental Section, the electrochemistry was measured in acetonitrile with [nBu4N][PF6] without any free complex being present in the electrolyte solution. The E0surf values are listed in Table 1. The cyclic voltammograms of the self-assembled monolayers of [Ru(bpy)2(µ-1)Os(2)]4+ and [Ru(bpy)2(µ-1)Ru(2)]4+ are shown in Figure 4A,B, respectively. Like the solution redox behavior, [Ru(bpy)2(µ-1)Os(2)]4+ shows two peaks corresponding to the oxidation and reduction of OsII/III and RuII/III. The shape and behavior

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Figure 5. Example of the peak current density as a function of scan rate for a monolayer of [Ru(bpy)2(µ-1)Os(2)]4+ on a platinum microelectrode (d ) 25 µm) (OsII/III, open triangles; RuII/III, filled triangles). All voltammograms were recorded in acetonitrile and [nBu4N][PF6] (0.1 M) as the supporting electrolyte.

of the signals have the typical characteristics of surfacebound species: The peak-to-peak separation between the anodic and cathodic waves, ∆Ep, is for both redox processes of [Ru(bpy)2(µ-1)Os(2)]4+ smaller than 20 mV at scan rates below 1000 V/s when using a 25 µm platinum electrode. At higher scan rates, an increase of ∆Ep was observed, which is probably caused by the iR-drop and/or the influence of heterogeneous electron-transfer kinetics. As theoretically predicted, the peak currents of the monolayers show in all cases a linear increase with the scan rate (see Figure 5).23 The full width at half-maximum, ∆Ep,1/2, of the electrochemical responses of monolayers of [Ru(bpy)2(µ-1)Os(2)]4+ is between 100 and 110 mV for the ruthenium oxidation and between 130 and 140 mV for the osmium-based process. The saturation surface coverage (Γs) of the [Ru(bpy)2(µ-1)Os(2)]4+ complex is between 3.5 and 4.0 × 10-11 mol cm-2 as calculated from the charge under the oxidation and reduction peaks of the cyclic voltammograms. We also tried to measure the reduction waves of the species adsorbed on the surface, but such experiments led to complete desorption of the complex. The compound [Ru(bpy)2(µ-1)Ru(2)]4+ showed a somewhat different behavior when adsorbed onto the surface compared to its behavior in solution. As can be seen in Figure 4B, two processes under the anodic and cathodic waves become visible. We were able to fit the data by assuming that two redox one-electron processes are beneath the signal. An example for the fitting is shown in Figure 6. The background-corrected oxidation process (squares) of the CV experiment is represented together with a simulation (solid line), which represents the sum of two Gaussian curves (dotted lines). For the fitting procedure, it was assumed that both Gaussian curves have the same area; the other parameters (height, width, and position with respect to the potential) were varied in order to match the experimentally obtained function. We assign the two peaks to two RuII/RuIII redox processes of the complex [Ru(bpy)2(µ-1)Ru(2)]4+. From these fits, the E0surf (see Table 1) and ∆Ep,1/2 values for both Ru-based redox processes could be determined. Numbers for ∆Ep,1/2 were in the range of 100-110 mV for the signal at 0.96 V and 130-140 mV for the peak at 1.03 V. The surface coverage of [Ru(bpy)2(µ-1)Ru(2)]4+ was in all cases between 3.5 and 4 × 10-11 mol cm-2 and therefore similar to that of [Ru(bpy)2(µ-1)Os(2)]4+. (23) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications; John Wiley & Sons: New York, 1980.

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Figure 6. Oxidation of [Ru(bpy)2(µ-1)Ru(2)]4+ (CV, anodic wave) as a self-assembled monolayer on a platinum microelectrode (d ) 25 µm) in acetonitrile/[nBu4N][PF6] (squares, original data; dotted lines, simulation of two Gaussian curves; solid line, sum of the two Gaussians).

Discussion Interaction energies (W) between surface-bound species are responsible for the order within the monolayer and therefore an essential part of self-assembling systems. From a number of electrochemically determined parameters, it is possible to estimate the sign (repulsion vs attraction) as well as the magnitude of W. Interaction Energies. E0surf versus E0sol. According to the surface activity theory by Brown and Anson, the formal potentials of interacting species, like adjacent species within a monolayer E0surf, depend on the interaction parameters of the reduced (rR) and oxidized forms (rO):24

E0surf ) E0 +

RT (r - rO)ΓT nF R

(1)

where E0 is the potential in the case of no interaction forces, n is the number of electrons transferred in the reaction, F is the Faraday constant, R is the gas constant, T is the temperature in kelvin, and ΓT is the total surface coverage. According to eq 1, repulsive interactions lead to negative values of rR and rO.24 For [Ru(bpy)2(µ-1)Ru(2)]4+ and [Ru(bpy)2(µ-1)Os(2)]4+, rO assumes higher absolute values than rR, because of the higher charge of the oxidized forms. This gives us E0surf > E0. Assuming that the interaction forces in solution can be assumed to be much smaller than those within the self-assembled monolayer due to the much larger average distance, one can conclude that E0sol is equivalent to E0 in eq 1. As can be seen from Table 1, E0surf is larger than E0sol for both dinuclear complexes for the tpy-coordinated metal ions but no such effect is seen for the bpy-coordinated Ru. Therefore we can conclude that within monolayers of [Ru(bpy)2(µ-1)Ru(2)]4+ and [Ru(bpy)2(µ-1)Os(2)]4+ repulsive interaction forces are dominant for tpy-coordinated metal ions but not for their bpy counterparts. The mononuclear building blocks ([M(tpy)(2)]2+) (M ) Ru, Os) of the dinuclear complexes showed an identical shift of E0surf versus E0sol.6 Finally it should be mentioned that the difference in potential shift between the tpy- and bpy-coordination site of [Ru(bpy)2(µ-1)Ru(2)]4+ is the reason that it is possible to resolve the two Ru-based redox processes for the redoxactive SAMs (compare Figure 3A and Figure 4B). ∆Ep,1/2. Another measure for interaction forces within self-assembled monolayers is the full width at halfmaximum (∆Ep,1/2) of potential sweep experiments.23 The (24) Brown, A. P.; Anson, F. C. Anal. Chem. 1977, 49, 1589.

cyclic voltammograms of [Ru(bpy)2(µ-1)Os(2)]4+ as well as [Ru(bpy)2(µ-1)Ru(2)]4+ show a particular difference for ∆Ep,1/2 of the two peaks detected. Whereas for the tpycoordinated metal ions ∆Ep,1/2 is for both dinuclear complexes between 130 and 140 mV, only 100-110 mV was detected for the bpy-coordinated Ru(II). It is generally accepted that values larger than 90.6 mV indicate repulsive interactions within the monolayer, and this is concluded from the electrochemical application of the Frumkin isotherm, expanding the Langmuir isotherm by an interaction parameter.25,26 Therefore, on a qualitative level we conclude that repulsive interactions within the monolayers predominate and that the repulsion for the tpy-coordination site within [Ru(bpy)2(µ-1)Os(2)]4+ is stronger than for the bpy-coordinated Ru. For [Ru(bpy)2(µ-1)Ru(2)]4+, it is not obvious which Ru(II) is oxidized first, but comparing ∆Ep,1/2 values of [Ru(bpy)2(µ-1)Os(2)]4+ and [Ru(bpy)2(µ-1)Ru(2)]4+ indicates that the peak at more positive potential represents the tpy-coordinated Ru(II) of the complex. jp ) f(υ). So far the discussion has shown that repulsive interactions are dominant within the self-assembled monolayer and that they are strongest for the tpycoordinated metal ions. A quantitative approach to interaction energies (W) was proposed by Matsuda et al.27,28 and applied to the mononuclear building blocks of [Ru(bpy)2(µ-1)Os(2)]4+ and [Ru(bpy)2(µ-1)Ru(2)]4+ earlier.6 This method is based on a statistical mechanical treatment of redox-active adsorbed species. The authors derived an expression for the peak current density jp as a function of the scan rate of potential sweep experiments for a simple surface redox reaction (R h O + ne-):

(jp)a ) -(jp)c )

nFΓT(nFυ/RT) 4{1 - (z/2)(1 - 1/)}

(2)

for z ln{z/(z - 2)} g W/RT g z ln βr and

(jp)a ) -(jp)c )

nFΓT(nFυ/RT)(1 - βr2) 4{1 - (z/2)(1 - 1/βr)}(1 - 2)

(3)

for W/RT < z ln βr, where (jp)a and (jp)c represent the anodic and the cathodic peak current densities, υ is the potential sweep rate, and  is given by

 ) exp(W/zRT)

(4)

where z is the number of nearest neighbors and W is the interaction energy which is given by

W ) (WRR + WOO) - WRO

(5)

where WRR is the interaction energy of reduced particles, WOO is that of oxidized particles, and WOR is that of pairs of oxidized and reduced species. β depends on the mole fractions of the oxidized and reduced species as well as on . The index r indicates a solution for β in an algebraic equation of third order, appearing when deriving the above given equations.27,28 To be able to calculate W from eq 2 and eq 3 and by measuring jp as a function of υ in voltammetric experi(25) Frumkin, A. N.; Damaskin, B. B. Mod. Aspects Electrochem. 1964, 3, 149. (26) Delahay, P. Double Layer and Electrode Kinetics; Interscience: New York, 1965. (27) Matsuda, H.; Aoki, K.; Tokuda, K. J. Electroanal. Chem. 1987, 217, 1. (28) Matsuda, H.; Aoki, K.; Tokuda, K. J. Electroanal. Chem. 1987, 217, 15.

SAMs of Ru/Os Dinuclear Complexes

Langmuir, Vol. 20, No. 21, 2004 9247

ments, one has to determine the number of nearest neighbors (z) within the monolayer. For the mononuclear complex [Ru(tpy)(1)]2+, a scanning probe picture revealed z ) 6.6 We have not been able so far to image the dinuclear complexes [Ru(bpy)2(µ-1)Ru(2)]4+ and [Ru(bpy)2(µ-1)Os(2)]4+ on Pt.29 However, assuming non-neglectable interaction forces and the similarity in structure it seems likely that z is either 4 or 6. A random distribution does not seem likely. Therefore we have calculated W for z ) 4 and z ) 6 with eqs 2 and 3. z ) 4. According to Matsuda et al., for z ) 4, βr has a value of 0.6527, and therefore we obtain from eqs 2 and 3

(jp)a ) -(jp)c )

nFΓT(nFυ/RT) 2(4 - 2)

(6)

Table 2. Values for W of [Ru(bpy)2(µ-1)Os(2)]4+, [Ru(bpy)2(µ-1)Ru(2)]4+, and Monomeric Model Complexes Calculated by Determining the Slope of jp ) f(υ) for z ) 4 and z ) 4a W (kJ/mol) bpy-coord complex

z)4

z)6

z)4

z)6

[Ru(bpy)2(µ-1)Os(2)]4+ [Os(tpy)(2)]2+ [Ru(bpy)2(µ-1)Ru(2)]4+ [Ru(tpy)(2)]2+ [Os(bpy)2(bpe)Cl]+

-3.6

-3.8

-4.2

-3.2

-3.7

-4.8

-4.5 -4.4 -5.2 -5.4

(jp)a ) -(jp)c )

4(1 - 2)

The measurements were recorded in acetonitrile with 0.1 M [nBu4N][PF6] as the supporting electrolyte (bpe ) 1,2-bis(4pyridyl)ethene).

for 2.43 g W/RT g -2.19 and

m2(z ) 6) ) (7)

for W/RT < -1.71. Equations 6 and 7 represent linear relations between jp and the scan rate υ with the slopes m1(z ) 4) and m2(z ) 4) given by

m1(z ) 4) )

n2F2ΓT  RT [2(4 - 2)]

(8)

for 2.77 g W/RT g -1.71 and

n2F2ΓT 0.168 m2(z ) 4) ) RT [4(1 - 2)]

(9)

for W/RT < -1.71, respectively. After determining the slope of jp ) f(υ),  can be calculated from eqs 8 and 9, which gives us W(z ) 4) according to eq 4. z ) 6. For z ) 6, βr equals 0.6943 and therefore we obtain from eqs 2 and 3

(jp)a ) -(jp)c )

nFΓT(nFυ/RT) 4(3 - 2)

(10)

for 2.43 g W/RT g -2.19 and

(jp)a ) -(jp)c )

nFΓT(nFυ/RT)0.223 4(1 - 2)

(11)

for W/RT < -2.19. Equations 10 and 11 represent linear relations between jp and the scan rate υ with the slopes m1(z ) 6) and m2(z ) 6) given by

m1(z ) 6) )

n2F2ΓT  RT [4(3 - 2)]

(12)

(29) Images of monolayers of the dinuclear complexes by scanning tunneling microscopy (STM) could not be recorded so far. The mobility of pyridine complexes on platinum seems to be rather high, leading to distortions of the image when the STM tip approaches the surface. The metal complexes also have a rather low conductivity in comparison to purely aromatic systems usually imaged with STM, so that high bias voltages need to be applied.

-3.2

a

for 2.77 g W/RT g -1.71 and

nFΓT(nFυ/RT)0.168

W (kJ/mol) tpy-coord

n2F2ΓT 0.223 RT [4(1 - 2)]

(13)

for W/RT < -2.19, respectively. After determining the slope of jp ) f(υ),  can be calculated from eqs 8 and 9 for z ) 6, which enables us to determine W(z ) 6) according to eq 4. The values for W(z ) 4) and W(z ) 6) for [Ru(bpy)2(µ1)Ru(2)]4+ and [Ru(bpy)2(µ-1)Os(2)]4+ together with numbers for model complexes are listed in Table 2. All numbers calculated for W are negative, indicating repulsive interactions, which is in qualitative agreement with the more commonly applied predictions from the difference between E0surf and E0sol and from the Frumkin isotherm (∆Ep,1/2) as discussed earlier in this contribution. The comparison of W(z ) 4) and W(z ) 6) does not show a strong difference. The comparison of interaction energies for [Ru(bpy)2(µ-1)Os(2)]4+ and [Ru(bpy)2(µ-1)Ru(2)]4+ with their monomeric building blocks shows a remarkable similarity and is only affected to a small extent by the choice for z. It was previously suggested that bpy ligands screen the positive charges of the metal ions better than tpy ligands.6 This result is fully supported by the current results as well as the fact that the interaction energies of Ru moieties are higher than those for the Os analogues. It was speculated that this is due to the additional electron sphere of Os in comparison to Ru.6 Packing and Molecular Orientation. The rodlike geometry of the dinuclear complexes raises the question about the tilt angle between the surface and the long axis (the axis going through both metal centers) of the complexes. A first approach to answer this question is to examine the surface coverage; this was determined to be between 3.5 and 4.0 × 10-11 mol cm-2 for both complexes. From these values, a contact area per molecule between 4.15 × 10-14 cm2 (415 Å2) and 4.74 × 10-14 cm2 (474 Å2) can be calculated, which is significantly larger than the projected maximum van der Waals area of approximately 300 Å2 gained from crystallographic data of related complexes in combination with molecular mechanics simulations. These numbers would allow tilt angles of between 0° and 90°. Though no direct experimental evidence is available, it is assumed that the pyridine functionality is responsible for binding each complex to platinum surfaces, since a binding of complexes without pyridine functionality is not observed. Therefore, a geometry in which the long axis of the dinuclear complex and the surface are parallel is highly unlikely, since, for geometric reasons, the nitrogen of the pyridine could not interact with the surface.

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Figgemeier et al.

Figure 7. Image of the molecular mechanics calculation of [Ru(bpy)2(µ-1)Ru(2)]4+ adsorbed on Pt (crystal face 111).

To gain additional information about the geometry of the complexes on the surface, molecular mechanics calculations were performed: [Ru(bpy)2(µ-1)Ru(2)]4+ was placed above a Pt surface (crystal face 111) at a variety of distances, and a molecular dynamics simulation was run in order to explore the conformational space. Following this simulation, the energy was minimized according to the procedures mentioned in the Experimental Section. This procedure was repeatedly applied, and a representative picture of the resulting geometry of the [Ru(bpy)2(µ-1)Ru(2)]4+ dinuclear complex on a Pt(III) surface is given in Figure 7. The calculations point out that besides the nitrogen-platinum interactions also an interaction between the aromatic rings of the tpy moiety and the surface plays a role and this leads to a tilted structure being favored as depicted in Figure 7. The calculation performed is somewhat limited since it is performed on a single species on a limited platinum surface and without counterions. Nevertheless it gives an impression about the forces between the surface and the polypyridine complex. The adsorption enthalpy (∆Hads) as calculated from the difference between the sum of the free energies of the Pt surface (∆HPt) and the complex (∆Hcomp) and the minimized complex/surface combined system (∆Hagg) is -13 kJ/mol. This value is significantly lower than experimentally determined values (-50 to -60 kJ/mol) for similar complexes.6 The difference reflects the limitations of the calculation. However, the negative value of ∆Hads shows that the simulation is able to recognize the forces leading to the adsorption and orientation of the dinuclear complexes on Pt. This gives confidence in the validity of the predicted tilted orientation of [Ru(bpy)2(µ-1)Ru(2)]4+ when adsorbed on Pt. Summary We have shown that SAMs of the dinuclear complexes [Ru(bpy)2(µ-1)Ru(2)]4+ and [Ru(bpy)2(µ-1)Os(2)]4+ bound to platinum by a pyridine functionality and by additional interactions between the aromatic systems of the tpy ligands exhibit an ideal electrochemical response. From the evaluation of the peak current density as a function of the scan rate, the interaction energies within the monolayers could be determined, confirming that bpy ligands are more capable of screening repulsive interac-

tions than tpy groups. This was confirmed by different peak shifts for the bpy and tpy moieties of the complexes due to repulsive interactions according to Brown and Anson.24 Finally, the implications of our findings for the applications of self-assembled monolayers of such complexes in dye-sensitized solar cells should be pointed out: Terpyridine-based dyes (e.g., [Ru(L1)(SCN)3]- with L1 ) 4,4′,4′′-triscarboxy-2,2′:6′,2′′-terpyridine) show a lower surface coverage than bpy analogues (e.g., [Ru(L2)2(SCN)2] with L2 ) 4,4′-dicarboxy-2,2′-bipyridine), which leads to lower light-to-energy conversion efficiencies.30 For our complexes, we also find that the surface coverage of the terpyridine complexes is reduced by a factor of 2 in comparison to that of the bipyridine analogues. The result that terpyridine ligands are screening charges less efficiently than bipyridine ones might explain the decreased surface coverage on metals and on semiconductors in dyesensitized nanostructured solar cells. Another consequence of our investigation refers to electron-transfer processes between rodlike complexes such as [Ru(bpy)2(µ-1)Ru(2)]4+ and [Ru(bpy)2(µ-1)Os(2)]4+ and the surface, since the interpretation of electrontransfer rates needs to take into account that most likely an upstanding geometry does not apply. Therefore the electron-transfer distance must be corrected accordingly. Vice versa, the electron-transfer rates could give a clue about the tilt angle between the complexes and the surface since the distance dependence of the transfer rate is reasonably well understood.11 This issue will be addressed in future investigations. Acknowledgment. This work was supported by the University of Basel, Switzerland, the University of Uppsala, Sweden, and the Swiss National Science Foundation. The financial contribution of the Portlandcementfabrik is gratefully acknowledged. E.F. thanks the European Community for financial support (Marie-Curie Fellowship Contract No. MCFI-2001-01264). LA048762L (30) Nazeeruddin, Md. K.; Gra¨zel, M. In Comprehensive Coordination Chemistry II; McCleverty, J. A., Meyer, T. J., Eds.; Elsevier: Amsterdam, 2004; Vol. 9, p 719.