Ordered Molecular Assemblies of Substituted Bis(phthalocyaninato

Nov 17, 2005 - Department of Applied Chemistry, Graduate School of Engineering, Tohoku UniVersity, Aoba-yama 04,. Sendai 980-8579, Japan. ReceiVed ...
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Langmuir 2006, 22, 2105-2111

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Ordered Molecular Assemblies of Substituted Bis(phthalocyaninato) Rare Earth Complexes on Au(111): In Situ Scanning Tunneling Microscopy and Electrochemical Studies Houyi Ma,*,†,‡ Liang-Yueh Ou Yang,§ Na Pan,† Shueh-Lin Yau,*,‡,§ Jianzhuang Jiang,† and Kingo Itaya‡,§ Department of Chemistry, Shandong UniVersity, Jinan 250100, China, CREST, Japan Science and Technology, Kawaguchi Center Building, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan, and Department of Applied Chemistry, Graduate School of Engineering, Tohoku UniVersity, Aoba-yama 04, Sendai 980-8579, Japan ReceiVed September 20, 2005. In Final Form: NoVember 17, 2005 Substituted bis(phthalocyaninato) rare earth complexes ML2 (M ) Y and Ce; L ) [Pc(OC8H17)8]2, where Pc ) phthalocyaninato) were adsorbed onto single crystalline Au(111) electrodes from benzene saturated with either YL2 or CeL2 complex at room temperature. In situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV) were used to examine the structures and the redox reactions of these admolecules on Au(111) electrodes in 0.1 mol dm-3 HClO4. The CVs obtained with YL2- and CeL2-coated Au(111) electrodes respectively contained two and three pairs of redox peaks between 0 and 1.0 V (versus reversible hydrogen electrode). STM molecular resolution revealed that YL2 and CeL2 admolecules were imaged as spherical protrusions separated by 2.3 nm, which suggests that they were oriented with their molecular planes parallel to the unreconstructed Au(111)-(1 × 1). Both molecules when adsorbing from approximately micromolar benzene dosing solutions produced mainly ordered arrays characterized as (8 × 5x3)rect (θ ) 0.0125). The redox reactions occurring between 0.2 and 1.0 V caused no change in the adlayer, but they were desorbed or oxidized at the negative and positive potential limits. The processes of adsorption and desorption at the negative potentials were reversible to the modulation of potential. Electrochemical impedance spectroscopy (EIS) and CV measurements showed that YL2 and CeL2 adlayers could block the adsorption of perchlorate anions and mediating electron transfer at the Au(111) electrode, leading to the enhancement of charge transfer for the ferro/ferricyanide redox couple.

Introduction Sandwich-type phthalocyaninato and porphyrinato rare earth complexes (double-decker and triple-decker) with two or three macrocycles held in close proximity by rare earth metal ions represent extended conjugated systems, possessing interesting electronic, optical, and other properties.1 These materials have found applications as electrochromic materials,1b,2 molecular semiconductors,3 liquid crystals,4 and gas sensors.5 Rare earth double- and triple-deckers have been extensively studied since the first bis(phthalocyaninato) complex M(Pc)2 was synthesized in 1965.6 * To whom correspondence should be addressed. Fax: +81-22-7955869. E-mail: [email protected]. † Shandong University. ‡ JST. § Tohoku University. (1) (a) Jiang, J.; et al. J. Am. Chem. Soc. 2003, 11, 12257-12267. (b) Jiang, J.; Kasuga, K.; Arnold, D. P. In Supramolecular Photo-sensitiVe and ElectroactiVe Materials; Nalwa, H. S., Ed.; Academic Press: New York, 2001; pp 113210. (c) Ng, D. K. P.; Jiang, J. Chem. Soc. ReV. 1997, 26, 433. (2) (a) Capobianchi, A.; Ercolani, C.; Paoletti, A. M.; Pennesi, G.; Rossi, G.; Chiesi-Villa, A.; Rizzoli, C. Inorg. Chem. 1993, 32, 4605. (b) Nicholson, M. M.; Weismuller, T. P. J. Electrochem. Soc. 1984, 131, 2311. (c) Komatsu, T.; Ohta, K. Fujimoto, T.; Yamamoto, I. J. Mater. Chem. 1994, 4, 533. (d) Silver, J.; Lukes, P. J.; Hey, P. K.; O’Connor, J. M. Polyhedron 1989, 8, 1631. (3) (a) Robinet, S.; Clarisse, C. Thin Solid Films 1989, 170, L51. (b) Pzturk, Z. Z.; Musluoglu, E.; Ahsen, V.; Gul, A.; Bekaroglu, O. J. Mater. Sci. 1992, 27, 6183. (c) Madru, R.; Guillaud, G.; Al Sadoun, M.; Maitrot, M. Chem. Phys. Lett. 1990, 168, 41. (4) (a) Belarbi, Z.; Sirlin, C.; Simon, J.; Andre, J.-J. J. Phys. Chem. 1989, 93, 8105. (b) Toupance, T.; Bassoul, P.; Mineau, L.; Simon, J. J. Phys. Chem. 1996, 100, 11704. (5) (a) Passard, M.; Pauly, A.; Germain, J. P.; Maleysson, C. Synth. Met. 1996, 80, 25. (b) Aroca, R.; Bolourchi, H.; Battisti, D.; Najafi, K. Langmuir 1993, 9, 3138. (c) Capobianchi, A.; Paoletti, A. M.; Pennesi, G.; Rossi, G. Sens. Actuators, B 1998, 48, 333.

Preparation of ordered molecular assemblies of sandwichtype rare earth complexes on appropriate solid substrates can be important in fabricating functional materials of these chemical species. Highly ordered thin films of phthalocyanine (Pc) derivatives and monolayer or multilayer bis(phthalocyaninato) (or bis(naphthalocyaninato)) rare earth complexes have been successfully prepared by using Langmuir-Blodgett (LB) technique.7-9 Meanwhile, it has been shown that Pc or porphyrin (Por) derivatives and their metal compounds can be adsorbed spontaneously from solution onto a substrate in an ordered form.10 For example, we have illustrated the spontaneous adsorption of a number of metallophthalocyanine (MPc) and metalloporphyrin (MPor) molecules, producing well-ordered adlattices on Au(111) and Au(100) surfaces.11 Lackinger et al. showed that the electronic properties of Pc molecules, such as the gap between HOMO and LUMO and the redox potential, may vary with different metal centers and ring (6) Kirin, I. S.; Moskalev, P. N.; Makashev, Y. A. Russ. J. Inorg. Chem. 1965, 10, 1065. (7) Van Nostrum, C. F.; Nolte, R. J. M. J. Chem. Soc., Chem. Commun. 1996, 2385. (8) (a) Liu, Y.; Shigehara, K.; Hara, M.; Yamada, A. J. Am. Chem. Soc. 1991, 113, 440. (b) Aroca, R.; Johnson, E. Langmuir 1992, 8, 3137. (c) RodrignezMendez, M. L.; Aroca, R.; DeSaja, J. A. Chem. Mater. 1992, 4, 1017. (9) Chen, Y.; Liu, H.; Pan, N.; Jiang, J. Thin Solid Films 2004, 460, 279-285. (10) Kaifer, A. E.; Gomez-Kaifer, M. Supramolecular Electrochemistry; WileyVCH: Weinheim, 1999; pp 191-206. (11) (a) Yoshimoto, S.; Tsutsumi, E.; Honda, Y.; Murata, Y.; Murata, M.; Komatsu, K.; Ito, O.; Itaya, K. Angew. Chem., Int. Ed. 2004, 43, 3044-3047. (b) Suto, K.; Yoshimoto, S.; Itaya, K. J. Am. Chem. Soc. 2003, 125, 14976-14977. (c) Yoshimoto, S.; Suto, K.; Tada, A.; Kobayashi, N.; Itaya, K. J. Am. Chem. Soc. 2004, 126, 8020-8027. (d) Yoshimoto, S.; Higa, N.; Itaya, K. J. Am. Chem. Soc. 2004, 126, 8540-8545. (e) Yoshimoto, S.; Tada, T.; Suto, K.; Itaya, K. J. Phys. Chem. B 2003, 107, 5836.

10.1021/la052553z CCC: $33.50 © 2006 American Chemical Society Published on Web 01/27/2006

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Figure 2. Cyclic voltammograms at 50 mV s-1 obtained with a bare (a), YL2-modified (b) and CeL2-modified (c) Au(111) electrodes in 0.1 mol dm-3 HClO4.

on the nature of the tetrapyrrole ligands. High-resolution STM imaging yielded ordered surface structures of these molecules. In addition, it was found that the adlayers of YL2 and CeL2 enhanced the rate of electron-transfer reaction at the electrified interface of Au(111). Experimental Section

Figure 1. (a) Schematic molecular structures for double-decker Y[Pc(OC8H17)8]2 and Ce[Pc(OC8H17)8]2 sandwich complexes; (b) chemical structure of substituted phthalocyanine ligands.

derivatizations.12a This idea can be applied to adjust molecular characteristics of sandwich-type rare earth complexes. Meanwhile, the Hipps group has examined the electronic structures of CoPC, CoTPP, and NiOEP using ultraviolet photoelectron spectroscopy and scanning tunneling spectroscopy (STM).12b-d It is of both theoretical and practical importance to characterize ordered adlayers of rare earth sandwich complexes at the electrode/solution interface. Although a large number of molecular assemblies of unsubstituted and substituted Pc’s and MPc’s have been studied by STM in ultrahigh vacuum (UHV),12,13 in air,14 and in aqueous solutions,11b-d,15 there has been no report on the adsorption of sandwich-type rare earth complexes at single crystal metal surfaces. Here we employed in situ STM to examine two rare earth double-deckers, YL2 and CeL2, on Au(111) electrode. Figure 1 shows their molecular structures. Y is not a lanthanide element, and Y3+ ion has a relatively small radius (∼104 pm) compared to an ionic radius of 115 pm for Ce3+. The electronic configuration of cerium is [Xe]4f15d16s2, whose valence electrons can be completely removed to reach a +4 oxidation state.1a Accordingly, the cerium center may adopt a valence of +3 or +4, depending (12) (a) Lackinger, M.; Muller, T.; Gopakumar, T. G.; Muller, F.; Hietschold, M.; Flynn, G. W. J. Phys. Chem. B 2004, 108, 2279-2284. (b) Barlow, D. E.; Schdiero, L.; Hipps, K. W. Langmuir 2004, 20, 4413. (c) Scudiero, L.; Barlow, D. E.; Hipps, K. W. J. Phys. Chem. B 2002, 106, 996. (d) Scudiero, L.; Barlow, D. E.; Mazur, U.; Hipps, K. W. J. Am. Chem. Soc. 2002, 123, 4073. (13) (a) Chizhov, I.; Scoles, G.; Kahn, A. Langmuir 2000, 16, 4358-4361. (b) Lu, X.; Hipps, K. W.; Wang, X. D.; Mazur, U. J. Am. Chem. Soc. 1996, 118, 7197-7202. (c) Barlow, D. E.; Hipps, K. W. J. Phys. Chem. B 2000, 104, 59936000. (d) Lackinger, M.; Hietschold, M. Surf. Sci. 2002, 520, L619-L624. (14) (a) Lei, S.-B.; Wang, C.; Yin, S.-X.; Wang, H.-N.; Xi, F.; Liu, H.-W.; Xu, B.; Wan, L.-J.; Bai, C.-L. J. Phys. Chem. B 2001, 105, 10838-10841. (b) Qiu, X.; Wang, C.; Yin, S.; Zeng, Q.; Xu, B.; Bai, C.-L. J. Phys. Chem. B 2000, 104, 3570-3574. (15) (a) Yoshimoto, S.; Tada, A.; Suto, K.; Itaya, K. J. Phys. Chem. B 2003, 107, 5836-5843. (b) Yoshimoto, S.; Tada, A.; Suto, K.; Yau, S.-L.; Itaya, K. Langmuir 2004, 20, 3159-3165.

Two homoleptic bis(phthalocyaninato) rare earth double-decker compounds, Y[Pc(OC8H17)8]2 and Ce[Pc(OC8H17)8]2, were synthesized according to the procedure described previously.16 Benzene (spectroscopically pure, Cica-reagent, Darmstadt, Germany) was used to prepare the dosing solutions of YL2 and CeL2, whose saturated concentrations are ca. 5.7 × 10-5 and 6.2 × 10-5 mol dm-3, respectively. Ultrapure HClO4 and K4[Fe(CN)6] were purchased from Cica-Merck (Darmstadt, Germany) and Kanto Chemical Co. (Tokyo, Japan). A 0.1 mol dm-3 HClO4 solution, prepared by dilution with Millipore triply distilled water (resistivity g18.2 MΩ cm, TOC < 5 ppb), was used in all electrochemical and STM measurements. The Au single crystal bead was prepared from gold wire by melting the end of a wire.17 The as-prepared Au electrode was flame-annealed and water-quenched. It was blown dry with ultrapure nitrogen before it was immersed for 20 min in benzene dosing solutions containing CeL2 or YL2. The gold electrode was rinsed with Millipore water before it was transferred to an electrochemical cell or STM cell. The electrochemical cell had a three-electrode configuration. A bright Pt plate and a reversible hydrogen electrode (RHE) acted as the counter and reference electrodes, respectively. Cyclic voltammetry was performed using a CH 600 voltammetric analyzer (Austin, TX). The measurements of electrochemical impedance spectroscopy (EIS) were performed by using an electrochemical workstation composed of a PAR 273A potentiostat and a 5210 lock-in amplifier. CV and EIS measurements were conducted at room temperature (∼22 °C) with the hanging meniscus method under a nitrogen atmosphere. The microscope used for in situ STM was a Nanoscope IV (Digital Instruments, Santa Barbara, CA). Two Pt wires served as the quasireference and counter electrodes in the STM cell, respectively. All electrode potentials reported here were referred to an RHE scale. The tunneling tips were prepared by electrochemically etching a tungsten wire (diameter ) 0.25 mm) in 1.0 mol dm-3 KOH solution by applying a 15 V alternating current. The tungsten tips were coated with transparent nail polish to minimize residual faradic current.

Results and Discussion Voltammetric Measurements in HClO4. Figure 2a shows the cyclic voltammogram (CV) at 50 mV s-1 of a bare Au(111) electrode in 0.1 mol dm-3 HClO4 solution. This CV profile contains typical characteristics of an ordered Au(111) electrode, including an extended double-layer charging region between 0 and 0.6 V and a pair of broad peaks at 0.75 V due to lifting of the Au(111) reconstruction.11c,18 All neutral rare earth doubledeckers except CePc2 can be expressed as [M3+(ring-12-)(ring(16) (a) Liu, W.; Jiang, J.; Du, D.; Arnold, D. P. Aust. J. Chem. 2000, 53, 131-136. (b) Jiang, J.; Liu, R. C. W.; Mak, T. C. W.; Chan, T. W. D.; Ng, D. K. P. Polyhedron 1997, 16, 515-520. (c) Jiang, J.; Xie, J.; Choi, M. T. M.; Ng, D. K. P. J. Porphyrins Phthalocyanines 1999, 3, 322-328. (17) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205.

M[Pc(OC8H17)8]2 Rare Earth Complexes on Au(111)

2•-)], in which a trivalent metal center (M3+) is sandwiched by a dianionic macrocycle and a radical anionic ligand.1a The redox reactions seen in Figure 2b,c are likely associated with successive removal or addition of electrons from or to the ligand-based orbitals.1a (Benzene is shown to be nonadsorbed on Au(111) in the present experimental conditions.)19 The redox potentials of bis(phthalocyaninato) rare earth complex in nonaqueous solvents are known, but those of molecular monolayers in aqueous solutions have not been reported.20 The CV characteristics seen in Figure 2 are assigned as follows. For YL2, the oxidation state of Y was fixed at +3. The redox couples at 0.12 and 0.63 V, unseen for metal phthalocyanine complexes, are attributed to the YL2/YL2- and YL2+/YL2 redox couples, respectively.20b For CeL2, not only the ligands but also the central cerium ions could be involved in the redox reactions. It is proposed that the redox couple at 0.1 V should be due to the [CeXLn-2]/[CeIIIL2-2]couple (3 < X < 4), whereas the ones at about 0.6 and 0.8 V are ascribed to CeXL2/CeXL2+ and CeXL2+/CeXL22+ couple, respectively.1a Note that the oxidation peak (A) seen for YL2-modified Au(111) is composed of a prepeak (A1) at 0.13 V and a main peak (A2) at 0.22 V. The corresponding cathodic peak (A′) also exhibits a similar morphology. These features could arise from coupled processes of adsorption/desorption of YL2 and the redox reaction of YL2/YL2- on Au(111). This is consistent with dynamic STM results (vide infra) showing reversible adsorption-desorption process of YL2 molecules, as the electrode potential was modulated near 0.2 V. This potential of molecular desorption is 100 mV more positive than that reported for metal phthalocyanine, for example, Co(II) phthalocyanine.15 This difference could mean weaker adsorption strength of YL2 and CeL2 than that of Co(II) phthalocyanine on Au(111). In Situ Scanning Tunneling Microscopy. The as-prepared adlayers of YL2 and CeL2 on Au(111) electrodes were examined by in situ STM in 0.1 mol dm-3 HClO4. Extensive STM imaging experiments showed that the STM resolution varied greatly with the potentials of Au(111) and tip electrodes. Holding the potential of Au(111) and tip electrodes at 0.4 and 0.05 V, respectively, yielded the optimal resolution. Meanwhile, it was necessary to set the tunneling resistance higher than 300 MΩ; otherwise, the tip could interact with the Au(111) electrode so strongly that the molecular adlayer was interrupted. Even the Au(111) substrate could be damaged if the tunneling resistance was lowered to 1 MΩ or less. Y[Pc(OC8H17)8]2 Adlayers. Figure 3a is a typical large-scale (100 × 100 nm) STM scan obtained with the YL2 adlayer at 0.4 V, which shows that terraces were decorated with symmetryequivalent squarelike arrays marked “I”, “II”, and “III”. These local molecular arrays are rotational and translational domains of an identical structure defined by a rectangular unit cell (vide infra). The population of a particular rotational domain was random and varied with the imaged areas, which suggests a nucleation-and-growth process. However, ordered arrays near step ledges tended to arrange themselves with the shorter edges of the rectangular cell along the neighboring step ledge, possibly because this allowed the most compact molecular array at step sites and the lowest surface energy. (18) (a) Hamelin, A. J. Electroanal. Chem. 1996, 407, 1-11. (b) Yoshimoto, S.; Tada, A.; Suto, K.; Narita, R.; Itaya, K. Langmuir 2003, 19, 672-677. (19) (a) Yoshimoto, S.; Narita, R.; Itaya, K. Chem. Lett. 2002, 356. (b) Yoshimoto, S.; Narita, R.; Wakisaka, M.; Itaya, K. J. Electroanal. Chem. 2002, 532, 331. (20) (a) Zhu, P.; Pan, N.; Ma, C.; Sun, X.; Arnold, D. P.; Jiang, J. Eur. J. Inorg. Chem. 2004, 518-523. (b) Zhu, P.; Lu, F.; Pan, N.; Arnold, D. P.; Zhang, S.; Jiang, J. Eur. J. Inorg. Chem. 2004, 510-517. (c) Nyokong, T.; Furuya, F.; Kobayashi, N.; Du, D.; Liu, W.; Jiang, J. Inorg. Chem. 2000, 39, 128-135. (d) He, Y.; Ye, T.; Borquet, E. J. Am. Chem. Soc. 2002, 124, 11964.

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Figure 3. Typical large-scale (100 × 100 nm) (a) and high-resolution (20 × 20 nm) (b) and (15 × 15 nm) (d) STM images of the ordered YL2 adlayer on Au(111) acquired in 0.1 mol dm-3 HClO4 solution at 0.4 V. The tip potential and the tunneling current were -0.18 V and 0.3 nA, respectively. “I”, “II”, and “III” in (a) denote three rotational domains of the (5x3 × 8) structure marked in (b). Three types of protrusions A, B, and C are indicated in the molecular resolution image of (b). The dimmest features of C ascribed to the uncoordinated ligands were imaged as a four-spot indicated by the white dots in (d).

In addition, the STM identified patches of a hexagonal phase denoted “P” in Figure 3a, which appeared sparsely on the surface. It amounted to only ca. 1% of the whole adlayer. It is likely that the coverage and spatial structure of the molecular adlayer were determined by [YL2] in the dosing solution. Unfortunately, the limited solubility (approximately micromoles) of YL2 and CeL2 in benzene averted examination of the relationship between concentration and the packing of the adlayer. To gain more insight into this issue, it is desirable to employ other organic solvents that allow more concentrated YL2 and CeL2 solutions. A further closeup view (20 × 20 nm) of the molecular adlayer presented in Figure 3b was acquired at a domain boundary with a typical imaging condition of 0.2 GΩ (or 300 pA in setpoint current and 60 mV in bias voltage). Two nearest-neighboring protrusions are separated by ca. 2.3 nm, a value corresponding to the dimension of a YL2 molecule, suggesting horizontal molecular orientation with their phthalocyanine planes parallel to the Au(111) surface. YL2 admolecules appear as corrugated protrusions labeled “A”, “B”, and “C” in descending order. The features of A and C distributing randomly on the surface account for about 5% of all admolecules. The dimmest features of C were 0.15 ( 0.05 and 0.082 ( 0.01 nm lower than A and B, respectively. The features C are 0.8 nm higher than the Au(111) substrate. The relative corrugation heights of these features can be seen in the section plot shown in Figure 3c. Features C were imaged as four spots arranged in squares in a further high-resolution scan in Figure 3d. This is reminiscent of the results reported for phthalocyanine on Au(111),15 implying that C could be metalfree Pc(OC8H17)8 ligands. The features of both A and B cannot be ascribed to YL2 admolecules, because of the 0.1 nm difference in corrugation height. One plausible account for these STM results is that the most popular features of B were YL2 admolecules, whereas spots A were impurities. The fact that A and B were

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Figure 4. Structural model for the double-decker YL2 adlayers on Au(111) surface.

adsorbed homogeneously on Au(111) implies they have similar molecular structure, and possibly similar intermolecular interactions. Overall, a monolayer of YL2 arranged in an ordered array was produced on Au(111) via spontaneous adsorption from benzene saturated with micromoles of YL2 complexes. No multilayer adsorption was ever observed. However, compared to the well-defined four-leaf-clover (or propeller) molecular resolution observed for the monomeric metalcoordinated phthalocyaninato molecules,11c,12 the internal molecular structures of the double-deckers could not be discerned. This lack of STM resolution was not due to the blunt tip or inappropriate imaging conditions. Rather, it is likely to be associated with the molecular and electronic structures of these sandwich-type complexes. It is thought that the two parallel phthalocyanine macrocyclic rings staggered with a nearly 45° skew angle could limit STM resolution.1b,9 In addition, the HOMO-LUMO gaps and energies of molecular orbitals vary with the ring-to-ring distance in the double-deckers, the ionic size and identities of the sandwiched metal centers, and ring variability.20a,b X-ray crystallographic studies of bis(phthalocyaninato) rare earth compounds show that the two isoindole N4 planes are very close to one another, which results in overlaps in electronic states and poor STM resolution.1b In Figure 3b, the unit cell of the squarelike array of YL2 is defined by two unit vectors pointing in the [1h10] and [1h1h2] directions. They are 2.31 ( 0.07 and 2.50 ( 0.07 nm in length, which correspond to 8 and 5x3 times the interatomic spacing of Au(111) substrate, respectively. Thus, the ordered structure is determined to be (8 × 5x3)rect with one molecule per unit cell (θ ) 0.0125). The STM measurements were calibrated against the Au(111)-(1 × 1) atomic lattice, obtained with the same tip but using different imaging conditions of the STM (vide infra). This surface coverage corresponds to 2.9 × 10-11 mol‚cm-2, assuming a 1.5 × 1015 atoms‚cm2 atomic density of the Au(111) surface. Figure 4 shows a real-space model of the YL2 adlayer on Au(111). The eight side chains of OC8H17 are omitted for simplicity. Because the STM results do not inform the spatial alignment of admolecules with respect to the Au(111) substrate, we adopt the model of monomeric MPc molecules on Au(111), where the longer C2 axis of YL2 molecules was aligned with atomic rows of the Au(111) substrate.11e The side chains that were not seen by the STM could occupy the intermolecular space and interact weakly with the Au(111) surface. The aforementioned STM results were collected at 0.2 V in 0.l mol dm-3 HClO4. Although these conditions should have favored the reconstructed Au(111), the adsorbed YL2 admolecules appeared to cause lifting of reconstruction, as evidenced by the

Figure 5. Time-sequenced (100 × 100 nm) STM images of YL2 adlayer on Au(111) in 0.1 mol dm-3 HClO4. The potential was at 0.2 V initially (a), followed by stepping to 0 V. STM images in (b), (c), and (d) were collected at 6, 10, and 15 min after the potential step. The tip potential was 0 V, and the tunneling current was 1 nA.

STM results. By adjusting the STM imaging conditions, we could observe the atomic structure of the Au(111) substrate. We noted that using 3 nA feedback current and 5 mV bias voltage discerned the typical herringbone and atomic features of Au(111)-(22 × x3), rather than the (1 × 1) surface. This result is reconciled by the removal of those YL2 admolecules in the scan area to expose a local bare gold area. Switching the imaging conditions to those normally used to see admolecules rendered redeposition of double-deckers on an unreconstructed Au(111). Lowering the tunneling resistance further could result in physical contact between the tip and the Au(111) substrate, producing a depression on the substrate and damaging the tip at the same time. Real-time STM imaging was used to examine how the electrochemical potential could affect the real-space structures of YL2 within the potential region between 1.0 and 0 V. Shown in Figure 5 is a sequence of STM images obtained after the potential was stepped from 0.2 to 0 V. Figure 5a shows an STM image acquired at 0.2 V, in which ordered arrays of YL2 were seen as that in Figure 3. The STM images in Figure 5b-d were acquired at the time marks of 6, 10, and 15 min after the potential was switched to 0 V. It appears that those YL2 admolecules near the upper step ledge were desorbed first, while those at the lower terrain were unaffected and were seen to leave the Au(111) surface last. Protracted potential holding at 0 V for 15 min resulted in detachment of nearly all YL2 molecules. However, switching the potential back to 0.2 V or more positive values caused readsorption of YL2 molecules arranged similarly to that seen in Figure 3a, but the degree of ordering was not as good as before. Rather than being dissolved into solution, YL2 molecules could be displaced from the surface but they stayed close to the Au(111) electrode at E < 0.2 V. These results are in line with those reported for porphyrin molecules adsorbed on gold electrode.20d It is shown that the electrochemical potential greatly influences the mobility of porphyrin molecules on gold electrode, as admolecules drift too fast to be imaged at negative potentials and bind too strongly to enable the formation of ordered adlattices at positive potentials.20d Ordered structures of porphyrin were observed only within the middle range of potential (0.2-1.0 V). We speculate that electrochemical potential dominates the binding

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Figure 7. Cyclic voltammograms at 50 mV s-1 obtained with Au(111) (solid trace) and polycrystalline Au electrode (dotted trace) in 0.1 mol dm-3 HClO4 + 1 mmol dm-3 K4[Fe(CN)6].

Figure 6. Representative large-scale (100 × 100 nm) (a) and highresolution (25 × 25 nm) (b) STM images for double-decker CeL2 adlayers on Au(111) obtained in 0.1 mol dm-3 HClO4 solution at 0.2 V. The tip potential was -0.07 V, and the tunneling current was 323 pA. The arrows indicate the close-packed directions of the Au(111) substrate.

strength and thus the mobility of porphyrin molecules on gold electrodes. The redox processes seen in the CV of YL2 introduced no change in STM imaging. Ce[Pc(OC8H17)8]2 Adlayers. In situ STM imaging of CeL2 adlayer on Au(111) was also performed to understand whether the chemical identity of the central metal ion would affect the real-space structures of rare earth double-deckers. Figure 6a shows a large-scale STM image obtained with Au(111) coated with a monolayer of CeL2 admolecules at 0.2 V. The adlayer was partially ordered with an average domain size of 5 × 6 nm. Areas marked I, II, and III in Figure 6a are rotational domains of a squarelike structure, whereas domain IV denotes a hexagonal phase. Figure 6b shows a high-resolution (25 × 25 nm) STM image at 0.2 V. Two molecular rows were aligned in the [11h0] and [1h1h2] directions of the Au(111) substrate. Two neighboring molecules are separated by 2.23 ( 0.07 nm in the [1h10] direction and 2.46 ( 0.07 nm in the [1h1h2] direction. These results lead to an assignment of (8 × 5x3), as that of YL2 adlayer. A unit cell of the CeL2 adlayer is indicated by the dotted lines in Figure 6b. Similarly to the results of YL2, where not all protrusions exhibit identical intensity, our previous interpretation of the STM results of YL2 should be valid also here. Effect of Double-Deckers on Electron-Transfer Reactions. Cyclic Voltammetry. Figure 7 shows cyclic voltammograms (CVs)

Figure 8. Cyclic voltammograms obtained with YL2-modified (a) and CeL2-modified (b) Au(111) electrode in 0.1 mol dm-3 HClO4 + 1 mmol dm-3 K4[Fe(CN)6]. The redox couple near 0.7 V is associated with the ferro/ferricyanide couple. The scan rate was 5 mV s-1. (c) and (d) reveal the linear relationship between ip and (scan rate)1/2 for these systems.

at 50 mV/s obtained with Au(111) (solid trace) and polycrystalline Au (dotted trace) electrodes in 0.1 mol dm-3 HClO4 + 1 mmol dm-3 K4[Fe(CN)6] solution. Both profiles contain a pair of peaks near 0.7 V, attributable to the ferro/ferricyanide couple redox. At Au(111), the anodic and cathodic peaks were separated by 0.28 V, classifying it as a quasi-reversible redox process. The small hump at 0.67 V seen in the positive scan of the solid trace, but not of the dotted line, is probably associated with the lifting of Au(111) reconstruction. This phenomenon can be associated with the adsorption of anions, ClO4- in this case.21a The adsorption of perchlorate on gold electrodes has been scrutinized by using vibrational spectroscopy, showing that the adsorption of perchlorate anions concurs the lifting of reconstructed Au(111).21b However, the surface-bound perchlorate anions are likely in hydrated states with unknown numbers of water molecules. This hydrated adsorption state is thought to be the reason for the weak interaction between perchlorate anion and transition metal (21) (a) Dakkouri, A. S.; Kolb, D. M. In Interfacial Electrochemistry; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999. (b) Ataka, K.; Yotsuyanagi, T.; Osawa, M. J. Phys. Chem. 1996, 100, 10664. (c) Borkowska, Z.; Stimming, U. In Interfacial Electrochemistry; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999. (d) Angerstein-Kozlowska, H.; Conway, B. E.; Hemelin, A.; Stoicoviciu, L. Electrochim. Acta 1986, 31, 1051.

2110 Langmuir, Vol. 22, No. 5, 2006

Ma et al.

Figure 9. Nyquist impedance spectra of a bare (a), YL2-modified (b), and CeL2-modified (c) Au(111) electrode in 0.1 mol dm-3 HClO4 + 1 mmol dm-3 K4[Fe(CN)6] at open-circuit potentials. Table 1. Values of Elements of Equivalent Circuit in Figure 10 To Fit Impedance Spectra for Au(111) Electrodes Uncovered and Covered by the Rare Earth Double-Decker Adlayers Shown in Figure 9, and Calculated Values of Cdl bare Au(111) Y[Pc(OC8H17)8]2-modified Au(111) Ce[Pc(OC8H17)8]2-modified Au(111)

Rs/Ω‚cm2

QdlY0/Ω-1 cm-2 sn

Cdl/F‚cm-2

Rt/Ω‚cm2

Zw/Ω‚cm2

4 2 2.2

1.23 × (n ) 0.93) 1.82 × 10-5 (n ) 0.93) 8.7 × 10-5 (n ) 0.91)

1.06 × 10 9.04 × 10-6 3.67 × 10-5

1.13 × 5.02 3.66 × 102

1.21 × 10-3 8.64 × 10-4 4.58 × 10-4

surfaces.21c In contrast, a peak separation of 0.16 V is observed at a polycrystalline gold electrode. This result could derive from differences in atomic and electronic structures of these two electrodes. The atomic arrangement of electrode surfaces is known to influence greatly the adsorption of perchlorate anions, as reported for the unlike interaction of perchlorate anions with single crystalline (111), (100), and (110) electrodes.21d It is possible that perchlorate anion interacted more strongly with Au(111), producing a more uniform adlayer than that at polycrystalline gold electrode. Still, perchlorate anion was held too weakly to uphold STM imaging on Au(111). The perchlorate adlayer could act as a barrier for charge transfer, manifesting in the delay of the oxidation of Fe(CN)64-. In contrast, the reduction of Fe(CN)63- occurring at potentials where no perchlorate anions were adsorbed was not affected by this double-layer effect. On the other hand, the results of Figure 7 could have an electronic origin. For example, single crystalline and polycrystalline gold electrodes are known to possess different points of zero charge (pzc’s). Specifically, a well-ordered Au(111) electrode is shown to have a pzc of 0.45 V in 0.1 mol dm-3 HClO4.21a The pzc of a polycrystalline Au electrode, although poorly characterized, likely lies a few hundred millivolts more negative than that of Au(111). For example, Au(210), a rougher surface than Au(111), has a pzc 170 mV more negative than that of Au(111).22a As Fe(CN)64- is oxidized at potentials positive of the pzc of both electrodes, these two electrodes are expected to be positively charged. The ferro/ferrihexacyano complexes with -4 and -3 charges would be pulled to the electrodes electrostatically. The polycrystalline Au, with a more negative pzc, should exert a greater electrostatic attraction on the Fe(CN)64- complex, shifting its oxidation negatively by 120 mV with respect to that of Au(111). On the other hand, Fe(CN)63- is reduced at potentials near the pzc of these electrodes; the electrostatic effect might not be pronounced enough to cause a shift in potential. This doublelayer effect was proposed to explain the reduction of Co(NH3)63+ and Fe(H2O)62+ at some single crystalline gold electrodes.22b In comparison, the ferro/ferricyanide redox became evidently more reversible (∆Ep ) 60 mV) at Au(111) electrode coated with a monolayer of YL2 or CeL2. (Figure 8). These results indicate that YL2 or CeL2 adlayers promoted rather than blocked the heterogeneous electron transfer of Fe(CN)64-/3-. This contrasts (22) (a) Hromadova, M.; Fawcett, W. R. J. Phys. Chem. A 2000, 104, 4356. (b) Fawcett, W. R. Electrochim. Acta 1997, 42, 833. (c) Kawiak, J.; Kulesza, P. J.; Galus, Z. J. Electroanal. Chem. 1987, 226, 305.

10-4

-4

103

markedly with the alkanethiols adsorbed on Au(111), which are shown to block charge transfer.10 This promoting effect of YL2 and CeL2 adlayers could be partly ascribed to the strong adsorption of these double-deckers, which inhibited the adsorption of perchlorate anions or species produced in the decomposition of Fe(CN)63-. In addition, these admolecules could act as mediators for charge transfer at the electrode because they could undergo redox reactions within the potential window of the ferri/ferrohexacyano complexes. This contention is supported by the CVs in Figure 2, where a pair of reversible features near 0.6 V is seen for YL2 and CeL2 adlayers. Parts c and d of Figure 8 show the plots of peak current (ip) vs (scan rate)1/2 at Au(111) electrodes modified with YL2 and CeL2, respectively. A linear relationship was observed in both cases, suggesting that the redox processes are diffusion-controlled. Our results resemble those obtained in the presence of an electron mediator, methyl viologen.23a It is difficult to determine if the redox species of Fe(CN)64-/3- interacted with YL2 and CeL2 admolecules strongly enough to produce surface confined species. Meanwhile, it is reported recently that multilayers of electrostatically assembled tetraruthenated porphyrin adlayer exhibit high electrochemical activity.23b Electrochemical Impedance Spectroscopy. The impedance spectra of the Au(111) electrodes modified by two rare earth double-decker SAMs were measured and compared with that of a bare Au(111) electrode. Figure 9 shows Nyquist impedance plots of a Au(111) electrodes without and with rare earth doubledeckers in 0.1 mol dm-3 HClO4 + 1 mmol dm-3 K4[Fe(CN)6] solution at respective open-circuit potentials. The Nyquist plot for a bare Au(111) electrode (Figure 9a) contains a capacitive loop at high frequency and a straight line with a slope of 45° (i.e., the Warburg impedance) at low frequency. The capacitive loop is usually related to the relaxation time constant of the charge-transfer resistance (Rt) and the doublelayer capacitance (Cdl) at the electrode/electrolyte interface,24 and the Warburg impedance is ascribed to mass transfer of soluble reactant and product species.25 In the high-frequency region, the redox process of the ferro/ferricyanide couple was under charge(23) (a) Lee, C.; Anson, F. J. Electroanal. Chem. 1992, 323, 381. (b) Rocha, J. R. C.; Demets, G. J.-F.; Bertotti, K.; Araki, K.; Toma, H. E. J. Electroanal. Chem. 2002, 526, 69. (24) (a) Ma, H.; Cheng, X.; Li, G.; Chen, S.; Quan, Z.; Zhao, S.; Niu, L. Corros. Sci. 2000, 42, 1669. (b) Barcia, O. E.; Matoos, O. R.; Pebere, N.; Tribollet, B. J. Electrochem. Soc. 1993, 140, 2825. (c) Barcia, O. E.; Matoos, O. R. Electrochim. Acta 1990, 35, 1601.

M[Pc(OC8H17)8]2 Rare Earth Complexes on Au(111)

Langmuir, Vol. 22, No. 5, 2006 2111

Figure 10. Equivalent circuit to fit impedance spectra with a highfrequency capacitive loop and low-frequency Warburg impedance.

transfer control. In the low-frequency region, the appearance of the Warburg impedance indicates that the system was under diffusion control. Nyquist plots for the Au(111) electrodes coated with YL2 or CeL2 monolayers (see Figure 9b,c) appear to be linear, which resembles that in Figure 9a, except the high-frequency capacitive loops were insignificant. This is more evident for the YL2modified Au(111) electrode (Figure 9b). The aforementioned three impedance spectra can be fitted well by the equivalent circuit in Figure 10. In this circuit, Rs is the solution resistance between the working electrode and the reference electrode, Rt is the charge-transfer resistance reflecting the electrochemical reaction of the ferro/ferricyanide couple at the electrode/solution interface, Zw is the Warburg impedance, and Qdl represents the constant phase elements (CPEs) modeling the double-layer capacitance (Cdl), which is substituted for the capacitors to fit more exactly the high-frequency capacitive loop. Admittance and impedance of a CPE are, respectively, defined as

YQ ) Y0(jω)n

(1)

1 (jω)-n Y0

(2)

and

ZQ )

where subscript Q denotes a CPE, Y0 is the modulus, ω is the angular frequency, and n is the phase. 25a,26 The value of n is an important index to evaluate whether the double-layer capacitance behaves like a capacitor. The double-layer capacitance will act as an ideal capacitor when the value of n approaches +1.27 The (25) Ma, H.; Chen, S.; Niu, L.; Shang, S.; Zhao, S.; Li, S.; Quan, Z. J. Electrochem. Soc. 2001, 148, B208. (26) Wu, X.; Ma, H.; Chen, S.; Xu, Z.; Sui, A. J. Electrochem. Soc. 1999, 146, 1847. (27) Ma, H.; Yang, C.; Chen, S.; Jiao, Y.; Huang, S.; Li, D.; Luo, J. Electrochim. Acta 2003, 48, 4277-4289.

values of elements of the circuit obtained by fitting are given in Table 1. The values of the double-layer capacitance were calculated by means of the method proposed by us26 and are also listed in this table. It is interesting that the value of Rt markedly decreased after the Au(111) electrode was covered by YL2 or CeL2 adlayers. The smaller the charge-transfer resistance, the faster the redox reaction rate becomes. As a result, we can conclude that these adlayers accelerated greatly the rate of charge transfer at the electrode/solution interface, making the ferro/ferricyanide redox reaction diffusion-controlled. The EIS results explain why reversible peaks due to ferro/ferricyanide were observed in the CVs of YL2- and CeL2-modified Au(111) electrodes. In addition, the values of n for the Qdl element were over 0.9 with or without the modification by the rare earth sandwich complexes. It is fair to state that the electrical double layers at bare and modified Au(111) electrodes behave as ideal capacitors. The decrease in the double-layer capacitance (Cdl) after modifications could arise from lower permittivity values for YL2 and CeL2 admolecules than that for water.

Conclusion Bis(phthalocyaninato) rare earth complexes M[Pc(OC8H17)8]2 (M ) Y and Ce) are adsorbed in ordered (8 × 5x3)rect structure on Au(111) electrodes, as characterized by high-resolution STM in 0.1 M HClO4. They are oriented horizontally with the planes of their ligands aligned parallel to the Au(111) substrate. These molecules undergo a series of redox processes involving the ligand-based and metal-related orbitals, leading to two and three pairs of redox peaks in the current-potential profiles of YL2 and CeL2, respectively. Using the ferro/ferricyanide couple as the redox probe, we found that YL2 and CeL2 adlayers accelerate substantially the rate of charge transfer at electrode/solution interfaces. This accelerating effect arises partly from their blocking effect on the adsorption of perchlorate anions and unknown species produced in the redox process of ferro/ferricyanide, and partly from the redox property of these complexes, which enables shuttling electrons across the electrified interface. Acknowledgment. This work was supported by Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology Agency (JST). LA052553Z