Controlling the Molecular Direction of Dinuclear Ruthenium

Sep 25, 2017 - We synthesized three types of binuclear Ru complexes (1–3) that contain pyrene anchors for the adsorption of 1–3 onto nanocarbon ma...
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Controlling the Molecular Direction of Dinuclear Ruthenium Complexes on HOPG Surface through Noncovalent Bonding Hiroaki Ozawa, Norihiko Katori, Tomomi Kita, Shota Oka, and Masa-aki Haga Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02194 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

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Controlling the Molecular Direction of Dinuclear Ruthenium Complexes on HOPG Surface through Noncovalent Bonding Hiroaki Ozawa,* Norihiko Katori, Tomomi Kita, Shota Oka, and Masa-aki Haga* †

Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan. KEYWORDS Ruthenium complexes, Pyrene anchor, Highly Oriented Pyrolytic Graphite (HOPG), Electrochemistry, Adsorption, Langmuir Isotherms ABSTRACT: We synthesized three types of binuclear Ru complexes (1-3) that contain pyrene anchors for the adsorption of 1-3 onto nanocarbon materials via non-covalent p-p interactions, in order to investigate their adsorption onto and their desorption from highly ordered pyrolytic graphite (HOPG). The adsorption saturation for 1 (6.22 pmol/cm2), 2 (2.83 pmol/cm2), and 3 (3.53 pmol/cm2) on HOPG was obtained from Langmuir isotherms. The desorption rate from HOPG electrodes decreased in the order 3 (2.4 ×10-5 s-1) > 2 (1.4 ×10-5 s-1) >> 1 (1.8×10-6 s-1). These results indicate that the number of pyrene anchors and their position of substitution in such complexes strongly affect the desorption behavior. However, neither the free energy of adsorption (DGads), nor the heterogeneous electron-transfer rate (kET) showed any significant differences among 1-3, albeit that the surface morphologies of the modified HOPG substrates showed domain structures that were characteristic for each Ru complex. In the case of 3, the average height changed from ~2 to ~4 nm upon increasing the concentration of the solution of 3 that was used for the surface modification. In contrast, the height for 1 and 2 remained constant (1.5-2 nm) upon increasing the concentration of the complexes in the corresponding solutions. While the molecular orientation of the Ru-Ru axis of 3 relative to the HOPG surface normal changed from parallel to perpendicular, the Ru-Ru axis in 1 and 2 remained parallel, which leads to an increased stability of 1 and 2.

INTRODUCTION Carbon nanomaterials, especially carbon nanotubes (CNTs) and graphene, are promising materials on account of their excellent thermal and electronic conductivity, high tensile strength, and large surface area.1-2 Accordingly, these materials have been widely investigated as electrodes in solar cells, supercapacitors, electronic devices, separators, and transistors.3-7 Functional composite materials based on carbon nanomaterials and functional molecules have also been investigated as potential materials for a large variety of applications in sensing devices, solar cells, supercapacitors, nanoelectronic devices, and field-effect transistors.8-10 Recently, we reported that the effect of the anchoring group chemistry on the charge transport properties of graphite-molecule contacts by means of the scanning tunneling microscopy break-junction technique and ab initio simulations.11 To endow the surface of nanocarbon materials with functionality, several types of molecules have been attached onto the carbon surface via covalent or noncovalent interactions.12-17 The covalent functionalization of nanocarbon materials can improve some properties such as solubility and stability, albeit under concomitant loss of other desirable electronic properties such as conductivity and electron or hole mobility. This is due to the fact that covalent functionalization usually disrupts the conjugation system by introducing chemical bonds that compromise the sp2-hybridization of the carbon atoms. On the other hand, noncovalent functionalization of nanocarbon mate-

rials may be able to maintain the intrinsic electronic structure of the p-conjugation systems and thus preserve the excellent physical and electronic properties of the nanocarbon materials.18 By modifying carbon materials with functional metal complexes, multifunctional properties such as magnetism and redox-activity can be introduced.19 In order to attach such functional metal complexes on the surface of the carbon materials via noncovalent interactions, p–conjugated molecules such as pyrenes,20-22 porphyrins,23-24 phthalocyanines,25 perylenetetracarboxylic dianhydrides,26-27 and others have been used.28-29 In particular, pyrene-anchor-substituted metal complexes have been used for the attachment of catalysts on the surface of carbon materials.30-32 Since conductive carbon surfaces are suitable for electrochemical measurements, many studies have focused on the electrochemical properties of metal complexes that are confined to the surface of nanocarbon materials via interactions with pyrene anchors.33 Dichtel has reported the adsorption behavior of cobalt complexes with multi- and monopodal pyrene anchors, in which the perpendicular orientation of the cobalt complex relative to the surface normal made it possible to monitor their dynamic movement on the HOPG surface.34 While the adsorption behavior and catalytic activity for the surface immobilization of molecules with pyrene anchors has been reported,35-36 only few studies have investigated how molecular vectors such as dipole moment or polarization might be controlled on the surface of nanocarbon materials.

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on single-walled carbon nanotubes (SWNTs), we designed dinuclear Ru complexes that contain two pyrene anchors that are aligned diagonally opposed with respect to the Ru-Ru axis, which allows an alignment of the molecular vector in parallel to the long axis of the SWNT (Figure 1c).38 Subsequently, we wanted to modify the position of the pyrene anchors on the dinuclear Ru complexes in order to understand the correlation between chemical structure and the adsorption stability of the molecular orientation of the complexes on the surface of nanocarbon materials (Figure 1b). For that purpose, we synthesized several ruthenium complexes that bear different numbers of pyrene anchors in different positions relative to the Ru-Ru axis in this study (1-3; Figure 2). 1-3 can be used as redox-active markers to quantitatively evaluate the adsorption on the HOPG surface. We also studied the adsorption/desorption behavior of 1-3 on HOPG substrates using electrochemical techniques in order to obtain information on how the substitution position of the pyrene anchors influences the adsorption of 1-3 on HOPG. Furthermore, the surface morphology of films of 1-3 on the HOPG substrates was investigated using AFM techniques.

EXPERIMENTAL SECTION

Figure 1. Schematic illustration of the adsorption of (a) mono- and (b) dinuclear metal complexes bearing pyrene anchors on twodimensional carbon materials; color code: green circles = mononuclear Ru complexes; green rectangles = dinuclear Ru complexes; orange circles = pyrene anchors. The different directions of the metal-metal axis in such dinuclear metal complexes can be considered via varying the substitution position and the number of pyrene anchors as shown here. Red arrows indicate the molecular vector generated by mixed-valent metal-metal complexes. (c) Schematic illustration of the adsorption of dinuclear metal complexes that are aligned in parallel to the long axis of a CNT.38

Recently, we have systematically investigated the effect of the number of pyrene anchors in Ru complexes on their adsorption behavior on HOPG surfaces via noncovalent p-p interactions.37 In order to control the direction of the molecular axis

Instruments and Methods. NMR spectra were recorded on a JEOL JNM-ECA 599 spectrometer. ESI-TOF-MS and MALDI-TOF-MS spectra were recorded on a Micromass ESITOF mass spectrometer or a Shimadzu-Kratos AXIMA-CFR spectrometer. AFM measurements were conducted on an Agilent 5500. SEM images were recorded using a field-emission type SEM (S-5500, Hitachi High-Technologies). X-ray photoelectron spectra were recorded on a Kratos, Shimazu AXIS His-165 spectrometer. Electrochemical measurements were carried out on an ALS/CHI Electrochemical Analyzer Model 660A using a three-electrode cell system. The Spartan software package (Spartan 08, Wavefunction Inc.) was used to obtain CPK models of 1-3 adsorbed on benzene sheets. Materials. All reagents and solvents used for synthetic purposes were purchased from commercial suppliers and used without further purification. MeCN used in the electrochemical measurements was purified by twofold distillation from P2O5, while [n-Bu4N][PF6] (TBAPF6) (TCI) was purified by recrystallization from EtOH, followed by drying in vacuum.

Figure 2. Chemical structures of the dinuclear ruthenium complexes bearing pyrene anchors used in this study (1-3).

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Hydrazine hydrate was used as the reducing agent in the 1H NMR measurements. Synthesis. 2,6-(1-Hydroxylundecanyl-1’-octylbenzimidazol2-yl)pyridine (L2), 2,6-bis(1-octadecylbenzimidazol-2yl)pyridine (L3), and tetra(pyridyl)benzene (tpb) were synthesized according to previously reported procedures.38-40 2,6-Bis(1-hydroxylundecanylbenzimidazol-2-yl)pyridine (L1). 2,6-Bis(benzimidazolyl)pyridine (2.04 g, 6.56 mmol), 11bromo-1-undecannol (4.83 g, 19.2 mmol), and K2CO3 (2.98 g, 21.6 mmol) were dissolved in DMF (32 mL). The mixture was stirred for 16 h at 80 °C. After cooling to room temperature, the solution was concentrated under reduced pressure, before the residue was dissolved in CH2Cl2 and filtered over Celite®. The filtrate was washed three times with water, before the organic phase was dried over anhydrous Na2SO4 and concentrated under reduced pressure. The crude product was purified by column chromatography on silica gel (eluent: EtOAc) to give 4 as a pale yellow oil (2.68 g, 62%). 1H NMR (DMSO-d6, 500 MHz) d 8.33 (d, J = 8 Hz, 2H), 8.06 (t, J = 8 Hz, 1H), 7.88 (d, J = 9 Hz, 2H), 7.47 (dd, J = 6 and 2 Hz, 2H), 7.36 (m, 4H), 4.72 (t, J = 8 Hz, 4H), 3.63 (t, J = 6 Hz, 4H), 1.73 (t, J = 7 Hz, 4H), 1.55 (m, 4H), 1.30 (m, 4H), 1.17 (m, 4H), 1.02 (m, 20H). MALDI-TOF-MASS (m/z): [M+H]+ 652.71 (Calcd for C41H58N5O2, 651.45). Ru(L1)Cl3. RuCl3×3H2O (1.51 g, 5.75 mmol) and L1 (1.68 g, 2.56 mmol) were dissolved in EtOH (150 mL) under an atmosphere of N2. The solution was stirred under reflux for 10 h. After cooling to room temperature, the resulting brown precipitate was collected by filtration (1.97 g, 90%) and used in the next step without further purification. [Ru(L1)(tpb)Ru(L1)][PF6]3. Complex 5 (810 mg, 932 µmol) and tpb (122 mg, 311 µmol) were dissolved in ethylene glycol (20 mL). The mixture was heated using microwave irradiation (650 W) in an on-and-off manner (5 x 1 min). After cooling to room temperature, water (10 mL) and an excess of K[PF6] were added. The resulting precipitate was collected by filtration and purified by column chromatography on silica gel (eluent: CH3CN/KNO3aq = 9/1). After removing the solvent, CH3CN (5 mL), water (10 mL), and an excess of K[PF6] were added, and the resulting brown precipitate was collected by filtration (676 mg, 77%). 1H NMR (CDCl3, 500 MHz) δ 8.96 (d, J = 8 Hz, 4H), 8.47 (t, J = 8 Hz, 2H), 8.16 (d, J = 8 Hz, 4H), 7.79 (d, J = 8 Hz, 4H), 7.40 (t, J = 8 Hz, 4H), 7.25 (d, J = 6 Hz, 4H), 7.21 (t, J = 8 Hz, 4H), 6.91 (d, J = 8 Hz, 4H), 6.69 (t, J = 6 Hz, 4H), 6.45 (t, J = 8 Hz, 4H), 5.04 (t, J = 7 Hz, 8H), 3.35 (t, J = 7 Hz, 8H), 1.41 (m, 8H), δ 1.36 (m, 8H), 1.29 (m, 8H), 1.18 (m, 48H). ESI-TOF-MASS (m/z): [M]3+ 630.41 (calcd for C108H130N14O4Ru2, 630.28). Complex 1. A mixture of [Ru(L1)(tpb)Ru(L1)][PF6]3 (153 mg, 0.079 mmol), 1-pyrenebutyric acid (136 mg, 0.48 mmol), dicyclohexylcarbodiimide (DCC; 90 mg, 0.44 mmol), and dimethylaminopyridine (DMAP; 63 mg, 0.52 mmol) in CH2Cl2 (100 mL) was stirred for 24 h. The reaction mixture was filtered over Celite®, and the filtrate was washed with 1% HClaq and water. The organic layer was dried over anhydrous Na2SO4, and the solvent was evaporated under reduced pressure. Purification of the residue was carried out by column chromatography on silica gel (eluent: CH3CN ® CH3CN/MeOH = 95/5) to give 1 as a brown solid (140 mg, 59%). 1H NMR (DMSO-d6, 500 MHz) δ 8.92 (d, J = 7 Hz, 4H), 8.44 (t, J = 7 Hz, 2H), 8.29 (d, J = 11 Hz, 4H), 8.13 (m,

20H), 8.01 (d, J = 3 Hz, 8H), 7.92 (t, J = 7 Hz, 4H), 7.83 (d, J = 7 Hz, 4H), δ 7.70 (d, J = 9 Hz, 4H), 7.31 (t, J = 6 Hz, 4H), 7.21 (d, J = 6 Hz, 4H), 7.16 (t, J = 6 Hz, 4H), 6.89 (d, J = 7 Hz, 4H), 6.61 (t, J = 6 Hz, 4H), 6.43 (t, J = 8 Hz, 4H), 4.96 (m, 8H,) 3.90 (t, J = 7 Hz, 8H), 2.35 (t, J = 5 Hz, 8H), 1.41 (t, J = 7 Hz, 8H), 1.34 (m, 8H), 1.27 (m, 8H), 1.07 (m, 64H). ESITOF-MASS (m/z): [M]3+ 990.95 (calcd for C188H186N14O8Ru2, 990.42). Ru(L2)Cl3. RuCl3×3H2O (1.20 g, 4.62 mmol) and L2 (1.61 g, 3.00 mmol) were dissolved in EtOH (150 mL) under an atmosphere of N2. The solution was stirred under reflux for 10 h. After cooling to room temperature, the obtained brown precipitate was collected by filtration and used for the next step without further purification (1.58 g, 71%). [Ru(L2)(tpb)Ru(L2)][PF6]3. Ethylene glycol (10 mL) was added to Ru(L2)Cl3 (279 mg, 350 µmol) and tpb (122 mg, 117 µmol). The mixture was heated using microwave irradiation (650 W) in an on-and-off manner (5 x 1 min). After cooling to room temperature, water (10 mL) and an excess of K[PF6] were added. The resulting precipitate was collected by filtration and the obtained solid was purified by column chromatography on silica gel (eluent: CH3CN/KNO3aq = 9/1). The solvent was evaporated and CH3CN (5 mL), water (10 mL), and an excess of K[PF6] (41 mg) were added, and the resulting brown precipitate was collected by filtration (153.2 mg, 74%). 1 H NMR (CDCl3, 500 MHz) δ 8.97 (d, J = 8 Hz, 4H), 8.48 (t, J = 8 Hz, 2H), 8.17 (d, J = 8 Hz, 4H), 7.81 (d, J = 8 Hz, 4H), 7.43 (t, J = 7 Hz, 4H), 7.24 (m, 8H), 6.92 (d, J = 8 Hz, 4H), 6.71 (t, J = 6 Hz, 4H), 6.47 (t, J = 8 Hz, 4H), 5.06 (t, J = 8 Hz, 8H), 3.36 (t, J = 7 Hz, 8H), 1.43 (m, 16H), 1.20 (m, 54H), 0.78 (t, J = 8 Hz, 6H). ESI-TOF-MASS (m/z): [M]3+ 591.72 (calcd for C102H118N14O2Ru2, 591.59). Complex 2. [Ru(L2)(tpb)Ru(L2)][PF6]3 (160.0 mg, 0.090 mmol), 1-pyrenebutyric acid (120.0 mg, 0.42 mmol), DCC (40.0 mg, 0.19 mmol), and DMAP (50.0 mg, 0.41 mmol) were dissolved in dry CH2Cl2 (100 mL) and stirred for 24 h. The reaction mixture was filtered over Celite®, and the filtrate was washed with 1% HClaq and water. The organic layer was dried over anhydrous Na2SO4, and the solvent was evaporated under reduced pressure. Purification of the thus obtained residue was carried out by column chromatography on silica gel (eluent: CH3CN/MeOH = 9/1) and Sephadex LH-20 (eluent: CH3CN/MeOH = 1/1) to give 2 as a brown solid (120.1 mg, 52%). 1H NMR (DMSO-d6, 500 MHz) δ 8.96 (dd, J = 13.2 and 6.5 Hz, 4H), 8.47 (t, J = 8 Hz, 2H), 8.34 (dd, J = 9 and 2 Hz, 4H), 8.17 (m, 12H), 8.05 (t, J = 3 Hz, 2H), 7.96 (td, J = 8 and 3 Hz, 2H), 7.88 (dd, J = 8 and 3 Hz, 2H), 7.77 (m, 4H,), 7.38 (t, J = 7 Hz, 4H), 7.23 (m, 8H), 6.92 (d, J = 7 Hz, 4H), 6.67 (t, J = 7 Hz, 4H), 6.46 (t, J = 8 Hz, 4H), 5.03 (s, 8H), 3.96 (m, 4H), 2.40 (t, J = 7 Hz, 6H), 1.48 (m, 4H), 1.36 (m, 8H), 1.13 (m, 56H), 0.75 (m, 6H). ESI-TOF-MASS (m/z): [M]3+ 771.81 (calcd for C142H148N14O4Ru2, 772.33). [Ru(L3)(tpb)][PF6]2. Ru(L3)Cl3 (74.3 mg, 0.10 mmol) and tpb (50.6 mg, 0.133 mmol) were dissolved in ethylene glycol (10 mL). The mixture was heated using microwave irradiation (650 W) in an on-and-off manner (5 x 1 min). After cooling to room temperature, MeOH (5 mL), water (10 mL), and an excess of K[PF6] were added. The resulting precipitate was collected by filtration and purified by column chromatography on silica gel (eluent: CH3CN/KNO3aq = 9/1). The solvent was evaporated and the resulting product was collected as a red purple solid (94.5 mg, 92%). 1H NMR (DMSO-d6, 500 MHz)

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δ 8.86 (d, J = 8 Hz, 2H), 8.79 (d, J = 5 Hz, 2H), 8.41 (t, J = 8 Hz, 1H), 8.00 (t, J = 8 Hz, 2H), 7.75 (d, J = 8 Hz, 2H), 7.70 (d, J = 8 Hz, 2H), 7.56 (d, J = 5 Hz, 1H), 7.55 (d, J = 4 Hz, 1H), 7.28 (t, J = 8 Hz, 2H), 7.17 (t, J = 7 Hz, 2H), 7.02 (m, 4H), 6.72 (d, J = 8 Hz, 2H), 6.58 (t, J = 7 Hz, 2H), 6.26 (t, J = 9 Hz, 2H), 4.97 (t, J = 7 Hz, 4H), 1.30 (m, 4H), 1.21 (m, 4H), 1.14 (m, 16H), 0.77 (t, J = 7 Hz, 6H). ESI-TOF-MASS (m/z): [M]2+ 511.53 (calcd for C61H61N9Ru, 511.14) [Ru(L3)(tpb)Ru(L1)][PF6]3. [Ru(L3)(tpb)][PF6] (80.7 mg, 78.3 µmol) and L1 (68.3 mg, 79.1 µmol) were dissolved in ethylene glycol (10 mL). The mixture was heated using microwave irradiation (650 W) in an on-and-off manner (5 x 1 min). After cooling to room temperature, CH3CN (5 mL), water (10 mL), and an excess of K[PF6] (34 mg) were added. The resulting precipitate was collected by filtration and purified in the same manner as [Ru(L2)(tpb)Ru(L2)][PF6]3. The product was obtained as a brown solid (113.2 mg, 81%). 1H NMR (DMSO-d6, 500 MHz) δ 8.97 (dd, J = 8 and 3 Hz, 4H), 8.48 (t, J = 8 Hz, 2H), 8.17 (d, J = 8 Hz, 4H), 7.81 (d, J = 7 Hz, 4H), 7.43 (t, J = 8 Hz, 4H), 7.23 (m, 8H), 6.91 (d, J = 8 Hz, 4H), 6.71 (t, J = 7 Hz, 4H), 6.46 (t, J = 8 Hz, 4H), 5.06 (m, 8H), 1.42 (m, 16H), 1.19 (m, 54H), 0.78 (t, J = 7 Hz, 6H). ESITOF-MASS (m/z): [M]3+ 591.71 (calcd for C102H118N14O2Ru2, 591.59) Complex 3. A mixture of [Ru(L3)(tpb)Ru(L1)][PF6]3 (100.8 mg, 56.2 µmol), 1-pyrenebutyric acid (40.1 mg, 141 µmol), N'-diisopropylcarbodiimide (DIC; 13.9 mg, 110 mmol), and DMAP (17.8 mg, 146 mmol) were dissolved in dry CH2Cl2 (10 mL) and stirred for 16 h. The reaction mixture was filtered over Celite®, and the filtrate was washed with 1% HClaq, NaHCO3aq, and water. The organic layer was dried over anhydrous Na2SO4, filtered, and the solvent of the filtrate was evaporated under reduced pressure. The purification was carried out by column chromatography on silica gel (eluent: CH3CN/MeOH = 9/1) and Sephadex LH-20 (eluent: CH3CN/MeOH = 1/1). After evaporating the eluent, the residue was dissolved in CH3CN (5 mL), and an excess of K[PF6] in water (10 mL) was added, which resulted in the precipitation of a brown solid that was collected by filtration (34 mg, 25%). 1H NMR (DMSO-d6, 500 MHz) δ 8.94 (dd, J = 14 and 8 Hz, 4H), 8.46 (t, J = 8 Hz, 2H), 8.31 (d, J = 8 Hz, 2H), 8.24 (t, J = 6 Hz, 2H), 8.14 (m, 12H), 8.03 (m, 4H), 7.95 (m, 2H), 7.86 (t, J = 8 Hz, 2H), 7.78 (m, 2H), 7.73 (m, 2H), 7.38 (m, 4H), 7.22 (m, 8H), 6.90 (d, J = 9 Hz, 4H), 6.66 (m, 4H), 6.45 (t, J = 7 Hz, 4H), 5.02 (m, 8H), 3.96 (m, 4H), 2.37 (t, J = 7 Hz, 4H), 1.38 (m, 12H), 1.14 (m, 56H), 0.74 (m, 6H). ESI-TOFMASS (m/z): [M]3+ 771.81 (calcd for C142H148N14O4Ru2, 772.33). Electrochemical measurements. All cyclic voltammetry (CV) measurements were carried out using a CHI 620D electrochemical workstation with a one-compartment electrochemical cell under an atmosphere of N2. Fresh HOPG surfaces was used as working electrodes. The areas of the electrodes that were exposed to the electrolyte solutions were confined by an O-ring (diameter: 6 mm), and the electrodes were mounted at the bottom of a cone-shaped solution cell. All potentials were measured using a saturated Ag/AgNO3 (0.01 M AgNO3 and 0.1 M TBAPF6 in CH3CN) reference electrode, and converted to the ferrocenium/ferrocene (Fc+/Fc) couple without regard for the liquid junction potential. All measurements were carried out in MeCN using 0.1 M TBAPF6 as the supporting electrolyte.

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Modification of HOPG surfaces. HOPG surface modifications with 1-3 were carried out by immersing the carbon electrode surfaces (HOPG) for 15 h in a CH2Cl2 solution of 1-3 (100 µM). Subsequently, the modified carbon substrates were washed with CH2Cl2 and dried under a flow of N2.

RESULTS AND DISCUSSION Synthesis of Ru dinuclear complexes bearing pyrene an-

Figure 3. (a) Cyclic voltammogram of 1 in CHCN 3 (supporting electrolyte: 0.1 M TBAPF6) and (b) scan rate dependency of Vs1 .

chors (1-3). The synthesis of 1-3 is summarized in Schemes S1-3. The auxiliary ligand L1 was obtained from the reaction between 2,6-bis(benzimidazol-2-yl)pyridine and 11-bromo-1undecanol, and used for the preparation of Ru(L1)Cl3. The reaction of Ru(L1)Cl3 with stoichiometric amounts of the bridging ligand tpb in ethylene glycol afforded the dinuclear complex [Ru(L1)(tpb)Ru(L1)][PF6]3. Subsequently, the side arms of the alkyl alcohol on [Ru(L1)(tpb)Ru(L1)][PF6]3 were treated with 1-pyrenebutyric acid to afford 1 as a brown solid. Complex 2 was synthesized in a similar manner using asymmetric ligand L2. Complex 3 was prepared by a stepwise introduction of auxiliary ligands into [Ru(L3)Cl3]: initially, the reaction between [Ru(L3)Cl3] and equal amounts of the bridging ligand tpb in ethylene glycol afforded [Ru(L3)(tpb)][PF6], which was used in an ensuing reaction with Ru(L1)Cl3 to yield the non-symmetric dinuclear Ru complex [Ru(L3)(tpb)Ru(L1)][PF6]3. Subsequently, the esterification of the non-symmetric dinuclear Ru complex with 1pyrenebutyric acid generated 3. Complexes 1-3 were purified by column chromatography on silica gel (eluent: CH3CN/MeOH = 9/1) and Sephadex LH-20 gel-filtration chromatography (eluent: CH3CN/MeOH = 1/1). The resulting

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Figure 4. (a) Cyclic voltammograms of HOPG-immobilized 1 in CH3CN (0.1 M TBAPF6) at a scan rate of 100 mV/s. (b) Plots of the peak current as a function of the scan rate for HOPG-immobilized 1. (c) Laviron plots for HOPGimmobilized 1-3 in CH 3CN (0.1 M TBAPF 6): 1 (red), 2 (green), and 3 (blue).

solids were characterized by 1H NMR and FT-IR spectroscopy, as well as MALDI-TOF and ESI-TOF mass spectrometry. Cyclic voltammograms of 1-3 in solution. The cyclic voltammogram of 1 in CH3CN (supporting electrolyte: 0.1 M TBAPF6) using a platinum disk electrode revealed welldefined one-electron reduction and one-electron oxidation waves at -0.36 and 0.11 V vs Fc+/Fc, which were assigned to Ru(III)-Ru(II)/Ru(II)-Ru(II) and Ru(II)-Ru(III)/Ru(III)-Ru(III) processes, respectively (Figure 3a).41 Since the open circuit potential showed -0.12 V, the complex 1 holds a mixed-valent Ru(II)-Ru(III) state. Each peak current in the cyclic voltam-

mogram of 1 was proportional to the square root of the scan rate, indicating that both electron-transfer processes are diffusion-controlled (Figure 3b). Similarly, two clear redox peaks were observed for 2 (-0.37/0.10 V) and 3 (-0.37/0.10 V) on a Pt working electrode in CH3CN (0.1 M TBAPF6), and the peak currents were also proportional to the square root of the scan rate (Figure S1). Complexes 1-3 showed almost identical potentials for both processes, indicating that the redox potentials of the complexes are not significantly influenced by the number and position of the pyrene side arms. Surface electrochemistry of 1-3 adsorbed onto HOPG carbon electrodes. HOPG surfaces were modified with 1-3 by exposing HOPG surfaces for 15 h to CH2Cl2 solutions of 1-3 (100 µM), followed by washing with CH2Cl2 and drying under a flow of N2. Figure 4a shows the cyclic voltammogram of 1 Two well-defined waves were observed at -0.29 and 0.16 V vs Fc+/Fc on the HOPG electrode, which were similar to those of 1 in CH3CN solution (Figure 3a). Since the open circuit potential was -0.08 V vs Fc+/Fc, the two waves correspond to the reduction process for Ru(III)-Ru(II)/Ru(II)-Ru(II) couple and the oxidation for Ru(II)-Ru(III)/Ru(III)-Ru(III) ones, respectivelyadsorbed on HOPG in CH3CN (0.1 M TBAPF6).. The electrochemical properties of 1 are thus preserved upon adsorption on the HOPG surface. The experimental surface coverage of HOPG with 1 (5.95 pmol/cm2), which was calculated from the integration of the oxidation peak, is ~2/3 lower than the theoretical value obtained from molecular modeling calculations using the Spartan software (9.16 pmol/cm2). This discrepancy may be due to the high motional degree of freedom of the long alkyl chains with the pyrene anchors, which could hamper ideal adsorption. To further examine the adsorption of 1 on the HOPG surface, the relationship between the peak currents and the scan rate was examined. The linear correlation between the anodic and cathodic peak currents for the two redox processes of 1 and the scan rate indicated that the redox processes arise from the adsorbed species (Figure 4b). Subsequently, we examined how the differences with regard to structural positioning of the pyrene side arms in HOPGimmobilized 1-3 affect the heterogeneous electron-transfer rates. From the Laviron plots of 1-3 adsorbed on HOPG (Figure 4c), the two kET values, which correspond to the first and second waves, were obtained for 1 (120 and 130 s−1; α = 0.52 and 0.49), 2 (136 and 138 s−1; α = 0.54 and 0.49), and 3 (128 and 139; α = 0.48 and 0.55). These electron-transfer rates are almost identical, and thus indicate that the structural positioning of the pyrene side arms should not influence the electrontransfer events. Langmuir isotherms of 1-3 on HOPG. Then, we investigated the adsorption tendency and surface coverage of 1-3, which contain pyrene anchors in different structural positions, onto the HOPG surfaces through non-covalent p-p interactions by Langmuir adsorption isotherms. By changing the concentration of 1-3, Langmuir isotherm plots of the surface coverage as a function of the concentration of 1-3 were obtained (Figure 5).34,37 The surface coverage gradually increases with increasing concentration of 1-3. The plots of 2 follow the typical Langmuir isotherm model, indicating that 2 was adsorbed on the HOPG surface, and that subsequent interactions between the adsorbed complexes do not occur.42 However, the plots of 1 and 3 in high concentration condition (over 80 µM) were deviated from typical Langmuir isotherm model. The observed

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deviation from the ideal Langmuir adsorption model may have to be ascribed to the formation of multilayer or π-π interactions between molecules adsorbed on HOPG according to previously reports.34,37 The isotherm plots can be analyzed using equation (1): 0

-DGabs Gi = exp CB RT Gs - Gi

(1)

Equation 1 delivers the free energy of adsorption for the Ru complexes, ΔGads (1: -29.7 kJ/mol; 2: -27.7 kJ/mol; 3: -29.4 kJ/mol). The adsorption energies of 1-3 are almost identical, despite the differences with respect to structural positioning and number of pyrene anchors. Recently, we reported the free energy of adsorption for Ru complexes with two (-27.9 kJ/mol) or four pyrene groups (-30.4 kJ/mol), whereby the number of pyrene anchors showed little influence on the adsorption equilibrium. Similar results on the free energy of adsorption of pyrene groups were also reported by Dichtel et al., who speculated that noncovalent intermolecular interactions between the pyrene anchors that do not participate in the immobilization might interfere with the binding to graphene, resulting in similar free energy values.34 From the isotherm plots, the surface density values for saturated adsorption of 1 (6.22 pmol/cm2), 2 (2.83 pmol/cm2), and 3 (3.53 pmol/cm2) were obtained. The value for 1 is higher than those for 2 and 3, which is consistent with larger amounts of Ru complexes being adsorbed upon increasing the number of pyrene anchors in the complexes. Therefore, the adsorption saturation depends on the strength of the p-p interactions between HOPG and the Ru complexes. However, the footprint, i.e., the area that one molecule of 1 occupies on HOPG, should be higher than those of 2 or 3. Since the increase of the number of pyrene anchors in these complexes might lead to an increased footprint, the number of molecules of 1 to reach full surface coverage should be lower than that of 2 and 3. However, the obtained value for the saturated surface coverage of 1 is larger than those of 2 and 3. Recently, we reported a similar dependency of the adsorption saturation for complexes bearing pyrene anchors with longer alkyl chains.37 Moreover, Dichtel has reported that the adsorption saturation values for cobalt complexes with pyrene anchors decreases with increasing number of pyrene anchors.34 Considering this distinct contrast

Figure 5. Langmuir isotherm plots of the surface coverage as a function of the concentration of 1 (red), 2 (blue), and 3 (green) in CH3CN (0.1 M TBAPF6) at 25 °C.

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regarding the surface coverage, we anticipated that the molecular structure around the pyrene anchors should lead to a significantly different structure on the carbon surface upon adsorption of the complex. X-ray Photoelectron Spectroscopy (XPS) measurements. The elemental composition of 1 on HOPG was examined by XPS measurements (0-800 eV), which showed the expected element signals for P (2p), Ru (3d5/2), C (1s), N (1s), O (1s), and F (1s) at 136.5, 282.3, 284.6, 402.4, 532.5, and 686.5 eV, respectively (Figure 6a). The presence of signals for Ru, together with the P and F signals from the [PF6]− counter anion, suggests that 1 is present as an ion pair of [dinuclear Ru cation][PF6]3 on the HOPG surface. Further, the O1s signal appeared as one broad peak when 1 was adsorbed from the lower Ru concentration, but the O1s signal exhibited two broad peaks at 530.2 and 532.7 eV as shown in Figure S4. For HOPG-immobilized 2 and 3, similar XPS spectra were observed (Figure S4). Upon the higher concentration of the Ru complex, the O1s signal was split into two separated peaks, which might reflect on the presence of two different arrangements of the C (=O)O groups in the Ru complex such as interactions between ester group and HOPG, and adsorption of water on the HOPG surface. Especially, the split peak appeared remarkably for 1 and 3, which have two pyrene anchors in the same ancillary ligand. However, the attribution of the split peak is under consideration. AFM measurements on HOPG surfaces modified with 1-3. In order to obtain information on the surface morphology of HOPG-adsorbed 1-3, AFM measurements were performed on HOPG substrates that were exposed to solutions that contained

Figure 6. XPS spectra of 1 adsorbed on HOPG from 100 µM Ru solution: (a) wide scan spectrum and narrow scan spectra of the (b) P 2s, (c) C 1s, (d) N 1s, (e) O1s, and (f) F 1s regions.

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1-3 in two different concentrations. Figure 7 shows the representative AFM images and cross-section analyses of HOPG surfaces modified with lower (µM) (Figures 7a-c) or higher concentrations (100 µM) of 1-3 (Figures 7d-f). For lower concentrations, the HOPG surfaces were homogeneously covered with films consisting of a small-domain morphology (average height: 1.5-2 nm). For high concentrations of 1 and 2, a homogeneous film morphology was observed (average height: ~2 nm). The average height obtained from high concentrations was thus identical to that obtained from lower concentrations. On the other hand, films obtained from higher concentrations of 3 exhibited an average height of ~4 nm, which is twice the height of the molecular cross-section. Based on a molecular model, the observed differences of the molecular height can be rationalized in terms of a vertical or horizontal arrangement of the Ru-Ru axis of 3 relative to the HOPG surface normal. Consequently, at low concentrations, 3 “lies down” on the HOPG surface (Figure 8a), while at high surface coverage, 3 “stands upright” on the HOPG surface (Figure 8b). Similar phenomena have been observed during the formation of selfassembly monolayers (SAMs) of alkyl thiols, which stand up or lie down on gold surfaces, depending on the concentration, temperature, and the immersion time.43-45 Considering the Langmuir isotherm of 3 in response to the result of AFM observation (Fig. 5), we expected that the deviation from Langmuir isotherm model might be caused by the difference of arrangement of 3 on HOPG surface. Photoelectron yield spectroscopy (PYS) for different molecular orientations of 1-3 on HOPG. Based on the AFM measurements, we speculated that the orientation of adsorbed 3 might change according to the concentration of the immersion solution. To further investigate the effects of the molecular orientation of 3 on HOPG, we compared the photoelectron

yield (PYS) spectra of samples with different molecular orientations on HOPG (Figure 9). The work functions of the surfaces were obtained from the threshold of the photoelectron emission, while the photoemission yield in the PYS spectra is related to the polarization state and film thickness on the substrate surface. For 1, the photoemission yield increased with increasing concentration of 1 on HOPG (Figure 9a). The PYS measurements delivered work functions of 5.35 eV and 5.41 eV for low and high concentrations, respectively. The values also increased with increasing amounts of adsorbed 1. Complex 1 adopts an orientation on HOPG surface, where the Ru-Ru axis is aligned in parallel to the surface normal for all concentrations. Therefore, the electronic states of HOPG are significantly influenced with increasing amounts of adsorbed 1, which changes the work function. On the other hand, for 3, identical work functions (5.35 eV) were observed for low and high concentrations. The PYS spectra and work functions did not change upon increasing the amount of adsorbed complex on HOPG (Figure 9b). Since 3 exhibits an “upright” orientation on the HOPG surface at high concentrations, the distance between the metal centers in 3 and the HOPG surface is increased, which should reduce the electronic contribution from 3 to the electronic states of the HOPG surface. Therefore, the changes of the PYS spectra and the work function decrease, which stands in sharp contrast to those of 1 with a parallel orientation relative to the surface normal. These results suggest that the orientation of the complexes might influence the electronic states of the HOPG surface. Desorption kinetics. We evaluated the kinetic stability of monolayers of 1-3 on HOPG electrodes in electrochemical cells containing a CH3CN solution of 0.1 M TBAPF6 in accordance with a previously reported method.34 The desorption

Figure 7. AFM images and line profiles of modified HOPG surfaces obtained from the immersion of HOPG substrates in solutions of 1-3 at (a-c) low concentrations (10 µM) and (d-f) high concentrations (100 µM). White lines correspond to the line-scan position.

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kinetics were monitored by CV measurements at regular time intervals. This method provides a good measure of the adsorption stability of the redox-active molecules adsorbed via the pyrene groups through non-covalent p-p interactions. Figure 10 shows the plots of the fractional coverage percentage (G-G0) /G0)×100 (%) as a function of time for 1-3, wherein G0 and G represent the initial surface coverage and the coverage after a particular time interval, respectively. The coverage of 1 slowly decreases over time, with a first-order rate constant (k = 1.8 ´ 10-6 s-1). As 1 contains four pyrene anchors, the number of pyrene anchors seems to affect the adsorption

Figure 8. Illustrating the orientation of 3 on the HOPG surface for (a) low concentrations and (b) high concentrations. Color code: green rectangles = dinuclear metal cores; orange circles = pyrene anchors.

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stability. The first-order rate constants of 2 (14 ´ 10-6 s-1) and 3 (24 ´ 10-6 s-1) for the desorption are larger than that of 1(1.8 ´ 10-6 s-1). Thus, the desorption of 2 and 3 is easier compared to 1. Even though 2 and 3 contain the same number of pyrene anchors, their desorption rates differ significantly. This discrepancy should be attributed to the difference of the molecular orientation of the complexes upon adsorption on the surface. The molecular Ru-Ru axis of 3 on the HOGP surface exhibits a relatively high degree of freedom, given that 3 contains only two pyrene anchors on the same side relative to the Ru-Ru axis. On the other hand, 2 contains also only two pyrene side arms, but those are located on diagonally opposing sides relative to the Ru-Ru axis. To understand the difference in stability caused by the molecular structure, we also carried out molecular mechanics simulations using the Spartan software in order to model the interaction between 2 and 3, graphene sheets, and 2 and 3 absorbed on graphene sheets in a lying arrangement (Figure S5 and Table S1), and thus roughly estimate the binding energy.46 The potential energy of 2 (1274.11 kJ/mol) is consistent with the value of 3 (1272.11 kJ/mol). The total potential energy of 2 and graphene sheet (41709.08 kJ/mol) was similar to that of 3 and graphene sheet (41707.08 kJ/mol). However, the potential energy of the composite of graphene sheet with 2 and 3, respectively are markedly different, i.e., the binding for 2 with graphene sheet (245.14 kJ/mol) is more stabilized than that for 3 with graphene sheet (-216.34 kJ/mol), indicating that 2 should be adsorbed strongly on the HOPG surface. The surface arrangement of 3 might be interchanged between “lying” state and “stand-up” one in good solvent, which lead to the easier desorption of 3 from the HOPG surface.

Figure 10. Time-dependence of the CV-derived fractional coverage percentage (G-G0)/(Γ0)´100 for 1 (red), 2 (blue), and 3 (green) at room temperature.

CONCLUSION

Figure 9. Spectra for the photoelectron yield of HOPGimmobilized (a) 1 and (b) 3 under atmospheric conditions; color code: black = HOPG, red = low concentration of the complex on HOPG, blue = high concentration of the complex on HOPG.

We synthesized rod-shaped dinuclear Ru complexes with two or four pyrene groups at different substitution positions (1-3), which allows a systematic comparison of their adsorption and desorption behavior on HOPG surfaces. In order to control the molecular orientation of the Ru-Ru axis in 1-3 upon adsorption, the number of pyrene groups and the positioning of the anchors are dominant factors. The adsorption and desorption behavior of 1-3 on HOPG surfaces were examined by CV and AFM techniques. In particular, the influence of the substitu-

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tion position and the number of pyrene anchors in 1-3 on the adsorption behavior on HOPG surfaces were investigated by the adsorption isotherms and adsorption stability studies. While the desorption rates increase in the order 1 < 2 < 3, the kinetic stability of the adsorption on HOPG decreases in the order 1 >> 2 > 3, indicating that the substitution positions and the number of pyrene anchors in 1-3 are of crucial importance. 1-3 exhibit similar heterogeneous electron-transfer rates (kET) and free energy (ΔGads) of adsorption values, which suggests that the number of pyrene side arms and the molecular structures have little influence on the electron transfer between HOPG and the complexes. The morphologies of the HOPG surfaces that were immersed in solutions of 1 and 2 were homogeneously covered with a film of the complexes (height: ~ 2 nm). For 3, the average height changed from ~ 2 to ~ 4 nm upon changing the concentration of the immersion solution from 10 to 100 µM. In the case of two pyrene anchors that are located on diagonally opposing sides relative to the Ru-Ru axis, the alignment changed from parallel to perpendicular relative to the surface normal upon increasing the concentration of the complex in the immersion solution. In order to maintain a molecular orientation of the Ru-Ru axis parallel to the HOPG and graphene surface upon adsorption of [(tridentate)Ru(BL)Ru(tridentate)]-type complexes, two terminal tridentate auxiliary ligands with pyrene anchors are required. Potentially, such complex modifications via p-p interactions of p-conjugated anchors may provide useful guidelines for the design of new molecular catalysts and devices including nanocarbon materials such as CNTs and graphene.

ASSOCIATED CONTENT Supporting Information. Synthetic routes to 1-3; cyclic voltammograms of 2 and 3 in solution as well as of 2 and 3 adsorbed on HOPG; XPS spectra of 2 and 3 on HOPG; molecular models of 2 and 3 on benzene sheets; this material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author *Email: [email protected], [email protected].

ACKNOWLEDGMENT This work was supported by JSPS KAKENHI grant 16K05732 and by the Foundation of the Kondo Scholarship Association for H. O., by JSPS KAKENHI grant JP17H05383(Coordination Asymmetry), by the Science Research Promotion Fund from the Promotion and Mutual Aid Corporation for Private Schools of Japan, and by the Institute of Science and Engineering of Chuo University for M. H. We are grateful to Prof. Daisuke Tanaka and Ms.Nao Shiojiri at Kwansei Gakuin University for the help with preliminary STM measurements.

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41 Nagashima, T.; Nakabayashi, T.; Suzuki, T.; Kanaizuka, K.; Ozawa, H.; Zhong, Y.-W.; Masaoka, S.; Sakai. K.; Haga, M. Tuning of Metal-Metal Interactions in Mixed-Valence States of Cyclometalated Dinuclear Ruthenium and Osmium Complexes Bearing Tetrapyridylpyrazine or –benzene. Organometallics 2014, 33, 4893-4904.

42. Xu, J.; Wang, L.; Zhu, Y. F. Decontamination of Bisphenol A from Aqueous Solution by Graphene Adsorption. Langmuir 2012, 28, 8418-8425. 43. Yamada, R.; Uosaki, K. In Situ, Real-Time Monitoring of the Self-Assembly Process of Decanethiol on Au(111) in Liquid Phase. A Scanning Tunneling Microscopy Investigation. Langmuir 1997, 13, 5218-5221. 44. Yamada, R.; Uosaki, K. In Situ Scanning Tunneling Microscopy Observation of the Self-Assembly Process of Alkanethiols on Gold(111) in Solution. Langmuir 1998, 14, 855-861. 45. Yamada, R.; Wano, H.; Uosaki, K. Effect of Temperature on Structure of the Self-Assembled Monolayer of Decanethiol on Au(111) Surface. Langmuir 2000, 16, 5523-5525. 46. Ozawa, H.; Fujigaya, T.; Niidome, Y.; Hotta, N.; Fujiki, M.; Nakashima, N. Rational Concept To Recognize/Extract SingleWalled Carbon Nanotubes with a Specific Chirality. J. Am. Chem. Soc. 2011, 133, 2651-2657.

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