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Electrostatic Repulsion-Induced Desorption of Dendritic ViologenArranged Molecules Anchored on a Gold Surface through a Gold-Thiolate Bond Leading to a Tunable Molecular Template Takehiro Kawauchi, Takahiro Kojima, Hiroshi Sakaguchi, and Tomokazu Iyoda Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00858 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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Electrostatic Repulsion-Induced Desorption of Dendritic Viologen-Arranged Molecules Anchored on a Gold Surface through a Gold-Thiolate Bond Leading to a Tunable Molecular Template Takehiro Kawauchi,*, † Takahiro Kojima,‡ Hiroshi Sakaguchi,‡ and Tomokazu Iyoda§ †

Department of Materials Chemistry, Faculty of Science and Technology, Ryukoku

University, 1-5 Yokotani, Oe-cho, Seta, Otsu, Shiga 520-2194, Japan ‡

Institute of Advanced Energy, Kyoto University, Gokasyo, Uji, Kyoto 611-0011, Japan

§

Harris Science Research Institute, Doshisha University, 1-3 Tataramiyakodani, Kyotanabe,

Kyoto 610-0394, Japan

KEYWORDS: dendrimer / redox-active molecule / surface modification / molecular array / molecular template

ACS Paragon Plus Environment

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ABSTRACT: We investigated the adsorption and desorption behavior of self-assembled monolayers (SAMs) on gold derived from dendritic viologen-arranged molecules with an ωmercaptodecyl group (An, n (dendritic generation) = 0-3) at the apex of the dendritic structure in polar solvents. The adsorption of the dendritic molecules occurred quickly and saturated within a few minutes in an acetonitrile/ethanol (1/1, v/v) mixture at a concentration of 2 mM. Atomic force microscopy images of the SAMs showed flat surfaces regardless of the dendritic generation because the peripheral viologen units were closely packed at the surface of the molecular layer. Individual A3 molecules immobilized on the substrate were observed by scanning tunneling microscopy measurements of a mixed SAM with decanethiol. The desorption behaviors of dendritic molecules from the An-SAMs in several solvents such as water were also investigated. The spontaneous desorption of the An-SAM occurred more rapidly than that of a conventional n-alkanethiol SAM. However, the desorption was inhibited by adding electrolytes such as NaNO3 due to the shielding effect on the electrostatic repulsion between the dendritic molecules. These results indicate that the surface density of the dendritic molecules can be controlled through the desorption.


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INTRODUCTION Metal surfaces functionalized with well-organized redox-active molecular layers is of great interest because of their possible applications as biosensors,1,2 molecular electronics,3-14 photoinduced electron transfer systems,15-17 and molecular catalysts.18,19 To achieve surface modification, the self-assembly of adsorbate molecules in a solution phase is one of the most promising bottom-up approaches.20,21 Recently, we reported unique redox-active adsorbate molecules that consist of dendritically arranged viologen (1,1’-disubstituted-4,4’-bipyridilium salt) units and ω-mercaptodecyl groups, which form characteristic self-assembled monolayers (SAMs) on a gold substrate reflecting the molecular architecture.22 The dendritic molecules have a conical shape and are closely packed, forming cone arrays on the substrate. Introducing the anchor units at different positions in the dendritic structure results in apicaland basal-type cone arrays in which the spatial concentration of the viologen units can be precisely configured in the molecular cones, indicating three-dimensional arrays of redoxactive molecules with a precise molecular gradient on a metal electrode surface. Moreover, we demonstrated that the SAMs can accommodate various metal anionic complexes such as PtCl42- with an amount defined by the dendritic generation.24 In this paper, we describe the spontaneous adsorption and desorption behavior of the dendritic viologen-arranged molecules with an ω-mercaptodecyl group at the apex in polar solvents. The adsorption was investigated by electrochemical measurements, and the influence of dendritic generation on the formation of an SAM was also discussed. Scanning probe microscopic observations of the obtained SAM was then performed to confirm the morphological changes in solution and in air. We also investigated the desorption of the SAMs in various solvents. As a result, specific desorption phenomena of the viologenarranged dendritic molecules based on their multivalent ionic characteristics were found.


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EXPERIMENTAL SECTION Instruments. Electrochemical measurements were performed using a CompactStat electrochemical analyzer (Ivium, Florida, USA) in a standard three-electrode system equipped with an SAM-modified gold working electrode, a platinum counter electrode, and a saturated calomel reference electrode (SCE). The area of the working electrode was approximately 0.28 cm2. Water contact angle (CA) measurements were carried out using a DSA100S drop shape analyzer (KRÜSS, Hamburg, Germany) equipped with a monochromatic CCD camera. Atomic force microscopy (AFM) observations were performed at ambient temperature using standard silicon cantilevers (at a normal frequency of 300 kHz, Oxford Instruments, Oxfordshire, UK) and a Cypher S microscope (Oxford Instruments). Scanning tunneling microscopy (STM) measurements were performed under a currentconstant mode using a commercial instrument (PicoSPM; Agilent Technologies Inc., formerly Molecular Imaging) under Ar at room temperature. All STM images were acquired with a tip bias of 0.2 V with a constant current of 5-20 pA. An electrochemically etched Pt-Ir (80:20) wire was used as the tip. Molecular modeling was performed on a Windows PC using Materials Studio software (version 7.0, Accelrys, California, USA).

Materials. 1-Ethyl-1’-(10-mercaptodecyl)-4,4’-bipyridinium bishexafluorophosphate (Vio) and dendritic viologen-arranged molecules with an ω-mercaptodecyl group (A1, A2, and A3) were synthesized according to the previously reported method.22,23 Anhydrous ethanol and acetonitrile (>99.5%, water content < 0.001%), sodium nitrate (NaNO3, >99%), and potassium tetrachloroplatinate(II) (K2PtCl4, >98%) were purchased from Wako Pure Chemical Industries (Wako, Japan) and used as received. Water was purified by a Milli-Q


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system. Gold substrates were prepared by physical deposition onto chromium-deposited silicon wafers (Cr layer thickness: approximately 100 Å; Au layer thickness: approximately 500 Å) immediately before use. For the AFM observations, gold substrates deposited onto cleaved mica were purchased from Agilent Technologies and cleaned using a UV/ozone method immediately prior to use. Au (111) on mica substrates were prepared for STM using an e-beam vacuum-deposition system. Evaporated gold was deposited on the mica or glass substrate, which was heated at 350°C under a vacuum of 2 × 10-8 Torr. The deposition was performed at a rate of 0.5 Å s-1, up to a thickness of 30 nm.

Preparation of the SAMs. A typical experimental procedure is described below. A gold substrate was immersed into a solution of A3 in an acetonitrile/ethanol mixture (1/1, v/v) at a concentration of 2 mM under an argon atmosphere. After 48 h at room temperature, the resultant SAM was rinsed with acetonitrile for 10 s and dried under flowing pure nitrogen.

Preparation of a Mixed SAM comprised of Dodecanethiol and A3 for STM Observations. A gold substrate prepared on a mica was immersed into an ethanol solution of dodecanethiol (C10-SH, 2 mM) under an argon atmosphere for 10 min. After rinsing the dodecanethiol-SAM (C10-SAM) with ethanol and drying under flowing pure nitrogen, a drop of an acetonitrile/ethanol solution of A3 (2 mM) was placed on the C10-SAM. After drying in air at room temperature, the resultant mixed SAM (A3/C10-SAM) was rinsed with acetonitrile and dried under flowing pure nitrogen.


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RESULTS AND DISCUSSION Self-assembled monolayer formation of dendritic viologen-arranged molecules. A series of dendritic viologen-arranged molecules with ω-mercaptodecyl groups at the apex (Vio, A1, A2, and A3, as shown in Figure 1) were synthesized by a microwave-heating method.23 Previous studies have reported that dendritic molecules form unique SAMs reflecting the molecular architecture. Figure 2a shows the plausible “cone array” model for an A3-SAM constructed from electrochemical measurements and AFM observation, in which the ωmercaptodecyl units are sparsely anchored to the substrate with peripheral viologen units closely packed at the surface.22

Vio (1 V2+)

A1 (3 V2+)

A2 (7 V2+)

A3 (15 V2+)

Figure 1. Chemical structures of the dendritic viologen-arranged molecules with ω2+ mercaptodecyl groups. The number of viologen units (V ) is indicated in parentheses.


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Figure 2. (a) Space-filling model of a cone array for an A3-SAM on a gold substrate. (b) Cyclic voltammograms of Vio- and An-SAMs measured in 100 mM NaNO3 at a scan rate of -1 22

200 mV s . The SAMs were prepared with an immersion time of 48 h. (c) Changes in the surface coverage of the adsorbate molecule (Γ) with the immersion time.

First, we investigated the adsorption behavior of dendritic molecules onto the gold surface during SAM formation. The SAMs were prepared by immersing gold substrates into acetonitrile/ethanol (1/1, v/v) solutions of the dendritic molecules (2 mM) at room temperature for a predetermined immersion time and then subjected to electrochemical measurements. The SAMs prepared with an immersion time of 48 h showed a characteristic redox wave at approximately −400 mV vs. SCE corresponding to the V2+/V+• process in their cyclic voltammograms measured in 100 mM NaNO3 using SAM-modified gold substrates as


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electrodes (Figure 2b). The redox current increased with increasing dendritic generation. The surface density (Γ) of the dendritic molecule was determined by the amount of the transported charge resulting from the V2+/V+• process in chronocoulometric measurement.22 Figure 2c displays changes in the Γ values with immersion time. In the case of Vio, the Γ increased rapidly within first few minutes, but more time (several hours) was required to reach saturation. This behavior is similar to the SAM formation of a typical alkanethiol.24-26 The initial step corresponds to diffusion-controlled Langmuir adsorption, and the second step is known as a surface crystallization process, where the alkyl chains form a two-dimensional crystal. Thus, the adsorption behavior implied that the Vio molecules organized into a closepacked structure. Indeed, the saturated Γ of the Vio-SAM was estimated to be 4.1 × 10−10 mol·cm−2, which agrees with the value reported for the SAMs of n-alkanethiols bearing one viologen unit as the terminal functional group in which the viologen units are perpendicularly oriented to organize close-packed structures.27-30 In contrast, the Γ values of the dendritic molecules (A1, A2, A3) saturated within few minutes, suggesting that the crystallization step based on the van der Waals interaction of the decyl groups did not occur due to the bulky dendritic viologen-arranged terminal group. This adsorption behavior is consistent with the cone array model that expects weak interaction of each decyl unit (Figure 2a). Since the viologen units are stacked three-dimensionally on the substrate, the saturated Γ value increased with the dendritic generation; the Γ values of A1-, A2-, and A3-SAMs were estimated to be 1.8 × 10−10, 1.1 × 10−10, and 6.7 × 10−11 mol·cm−2, respectively

Direct observation by scanning prove microscopy. AFM observation of the saturated SAMs were performed to investigate the surface morphology in an aqueous solution of NaNO3 (100 mM) at room temperature, which are the same conditions as the electrochemical


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measurements (Figure 3). The tapping-mode AFM height image of the Vio-SAM revealed a flat and smooth surface with 0.11 nm of RMS roughness owing to the close-packed structure, as shown in Figure 3a.22 Some pit-like defects with ca. 0.25 nm depths were also observed as indicated by white arrows, which corresponds to gold vacancy islands.31 The RMS roughness of the A1-SAM slightly increased to 0.17 nm (b). The AFM images of the A2- and A3-SAMs also revealed relatively flat surfaces with a similar roughness (c, d). Although the molecular size increased significantly with the dendritic generation (Table S1 in the Supporting Information), the SAMs of the dendritic molecules displayed flat surfaces in 100 mM NaNO3 because the peripheral viologen units were closely packed at the surface.


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(a) Vio-SAM (RMS = 0.10 nm)

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(b) A1-SAM (RMS = 0.17 nm)

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Figure 3. Tapping-mode AFM height images of the Vio- (a), A1- (b), A2- (c), and A3-SAMs (d) observed in a 100 mM NaNO3 aqueous solution. The RMS roughness values were calculated using the area indicated by the white-dotted squares. The height profiles measured along the red lines in the images are also shown.

Next, we carried out AFM measurements of the SAMs in air. The saturated SAMs were prepared using the same procedure, then dried under vacuum at room temperature for


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12 h and subjected to observation in air. The AFM images are shown in Figure 4. The height image of the Vio-SAM showed a smooth surface (a), except for dust, supporting that the Vio molecules remained a close-packed structure even under these conditions. In contrast, the surface morphology of the SAMs derived from the dendritic molecules was drastically changed. The AFM image of the A1-SAM revealed a rather rough surface with an RMS roughness of 0.51 nm (b). The RMS roughness increased with increasing dendritic generation. The AFM image of the A3-SAM showed a reticulate surface, suggesting that the partial aggregation of A3 occurred on the substrate during the drying process. The depth of the holes resulting from the reticular structure was roughly estimated to be 3-5 nm in cross section, which is in fair agreement with the size of the A3 molecule (Table S1 in the Supporting Information). These morphological changes of the An-SAMs suggest that the flat surfaces of the An-SAMs observed in 100 mM NaNO3 were due to well-organized peripheral viologen units.


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Figure 4. Tapping-mode AFM height images of Vio- (a), A1- (b), A2- (c), and A3-SAMs (d) formed on Au substrates. The measurements were performed in air at ambient temperature. The RMS roughness values were calculated using the area indicated by the white-dotted squares. The height profiles measured along the red lines in the images are also shown.

The dendritic molecules in the SAMs could not be visualized individually. The individual A3 molecules anchored on a substrate were successfully observed in a mixed SAM with an alkyl mercaptan. A droplet of the acetonitrile solution of A3 (2 mM) was placed on


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an SAM of n-decyl mercaptan (C10-SAM) prepared on a gold substrate. After drying in air, the A3/C10-SAM substrate was rinsed with acetonitrile, dried under flowing nitrogen, and then subjected to scanning tunneling microscopic (STM) analysis. Figure 5 shows the constant-current STM images of the mixed SAM. Well-ordered domains with (√3 × √3)R30° unit cell were separated by domain boundaries, which correspond to the C10-SAM. Despite treatment with a concentrated A3 solution, the C10-SAM structure remained. Note that some bright spots are observed at the domain boundaries in the images. The spot diameter is 3-4 nm, which is consistent with the expected diameter of A3. Therefore, the bright spots were attributed to A3 molecules embedded in the boundary defects, as illustrated in Figure 5d. Since the A3 molecules were separated individually, the conductivity of the isolated A3 molecule might be measured by conductive AFM of the mixed SAM.32,33 The specific electron transport ability along the branching framework can be anticipated via a throughbond hopping mechanism among the redox-active viologen units.34 The conductive AFM study will be published elsewhere.


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Figure 5. (a-c) Constant-current STM images of the A3/C10-SAM formed on gold. The height profile measured along the red lines in the image (c) is also shown. The observation was performed under Ar. (d) A schematic illustration of the A3/C10-SAM.

Desorption behavior of the dendritic viologen-arranged molecules. Because of the multivalent ionic character of the dendritic viologen-arranged molecules, Coulomb repulsion among the molecules in the cone array structure is expected. Additionally, the van der Waals interaction among the alkyl chains is weak due to the bulky dendritic terminal groups. We therefore expected the desorption of the molecules assembled on the substrate by immersing the SAM into a solvent such as water.


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The as-prepared A3-SAM was subject to CV in 100 mM NaNO3 to confirm the Γ value before desorption. The area was fixed at 0.28 cm2. The counter PF6- of the viologen units were exchanged with NO3- by the measurement. After rinsing with water, the substrate was immersed in water (4 mL) at room temperature. Changes in the CV of the A3-SAM with immersion time are shown in Figure 6a, and the results are summarized in Table 1. The redox current apparently decreased with the immersion time, indicating that the desorption of the A3 molecules occurred in water. The half-wave potential for the V2+/V+• process (E1/2) shifted slightly to a negative potential by the desorption, probably due to the relaxation of the Coulomb repulsion between the viologen units. The desorption ratio was determined by the decreasing ratio of the amount of the transported charge obtained by chronocoulometric measurement (Figure 6b). Figure 6c(i) shows plots of the desorption ratio of the A3-SAM in water versus the immersion time. The desorption ratio of an SAM of octadecanethiol has been reported to be 20% in water for 1 day,35 whereas the desorption ratio of the A3-SAM reached approximately 80% in water even after 2 h. To compare the relative rates of desorption, the rate constants (k) were estimated on the basis of first-order kinetics (Table 2). The k of the A3-SAM in water was estimated to be 5.9 x 10-4 s-1, which is approximately one order of magnitude larger than that of the SAM of octadecanethiol on gold in water.35


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Figure 6. (a) CVs of the A3-SAMs after immersion in H2O several times measured in 100 -1

mM NaNO3 at a 200 mV s scan rate. (b) Charge-time curves of a bare Au substrate and the A3-SAM after immersing in H2O for several times obtained by chronocoulometry with the potential step from -650 mV to -200 mV vs. SCE. (c) Changes in the desorption ratios of the A3-SAMs after immersion into H2O (i), CH3CN (ii), and NaNO3 aqueous solutions at 1 (iii), 10 (iv), and 100 mM (v) versus the immersion time. (d) Water contact angles on a bare gold substrate (vi), and an A3-SAM with NO3 anions (vii). The sample vii was immersed into CH3CN for 30 min and then subjected to a repeated measurement (viii).


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Table 1. Desorption of the A3-SAM in water at room temperature. Immersion [V2+] b Desorption Rc E1/2 a Γc c time / min ratio / % mV vs. unit mol nm mol cm-2 -2 SCE cm 0 -392 1.0 x 10-9 6.7 x 10-11 0 0.9 5 -398 6.8 x 10-10 4.5 x 10-11 32 1.1 30 -409 3.8 x 10-10 2.5 x 10-11 62 1.5 120 -416 2.3 x 10-10 1.5 x 10-11 77 1.9 a Half-wave potential for the first redox process (E1/2) was estimated by CV measurements in 100 mM NaNO3 aq. See Figure 6a. b Concentration of the electrically active viologen unit ([V2+]) was determined by potentialstep chronoamperometry.22 See Figure 6b. c Surface coverage (Γ), desorption ratio, and average radius of the area occupied by one dendritic molecule (R) were evaluated on the basis of [V2+]. Table 2. First-order rate constants for the desorption of A3 on gold in several solvents at room temperature.a kb Solvent 10-4 s H2 O 5.9 1 mM NaNO3 aq. 0.71 10 mM NaNO3 aq. 0.29 100 mM NaNO3 aq. 0.15 100 mM K2PtCl4 aq. 0.50 200 mM glucose aq. 5.4 acetonitrile 13 a Area of SAM 0.28 cm2, solvent 4 mL. b Estimated by fitting the desorption data to first-order kinetics according to Γt / Γ0 = exp(-kt), where Γ0 is the initial surface density of A3 and Γt is the surface density of A3 at time t.

When acetonitrile, whose dielectric constant (ε) is less than water (acetonitrile: ε = 37.5, water: ε = 80.2), was used as the immersion solvent of desorption, the desorption occurred more quickly (k = 1.3 x 10-3 s-1) than that in water (Figure 6c(ii)). The desorption of A3 molecules was also confirmed by contact angle (CA) measurements, as shown in Figure 6d. While the CA of bare gold was 48° (vi), the surface became hydrophilic after the formation of the A3-SAM with NO3- counter anions (vii). After immersing the substrate in CH3CN for 30 min, the surface changed from hydrophilic to


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hydrophobic, similar to the bare gold, owing to the desorption of the hydrophilic A3 molecules from the substrate (viii). The solubility of A3 with NO3- in acetonitrile is quite low. However, the desorption of A3 proceeded spontaneously in the solvent. This result implies that the solution-phase desorption of A3 seems to be dominated by not the solubility of A3 in the immersion solvent but by Coulomb repulsion between the multivalent ionic molecules. We therefore investigated influence of electrolytes such as NaNO3 on the desorption in water (iii-V in Figure 6c). Interestingly, the desorption of A3 could be inhibited by adding NaNO3. The desorption ratios decreased drastically with increasing concentration of NaNO3, and the desorption ratio in a 100 mM NaNO3 solution for 2 h was estimated to be 10% (k = 1.5 x 10-5 s-1), which means that the A3-SAM in 100 mM NaNO3 is as stable as that of the octadecanethiol SAM in water. Moreover, the surface density was maintained even after further immersion in the solution for 24 h. The Coulomb repulsion among the A3 molecules might be shielded by the electrolyte, which prevents desorption. The divalent electrolyte K2PtCl4 also acted as an inhibitor for the desorption (Figure S1 in the Supporting Information). On the other hand, no effect was observed in the case of 200 mM glucose as the additive (Figure S1 in the Supporting Information). These results suggest that the solutionphase desorption of the dendritic viologen-arranged molecules was accelerated by Coulomb repulsion among the molecules, which could be controlled by adding electrolytes. In addition, the influence of steric hindrance of the dendritic molecules on the desorption is discussed as follows. To evaluate the molecular size of A3 in several solvents, the diffusion coefficients (Ds) in various solutions were determined using the acetyl-capped A3 molecule (AcS-A3, the thiol group was protected by the acetyl group) by diffusion NMR experiments. The hydrodynamic radius (RH) of AcS-A3(30PF6-) in deuterated acetonitrile was


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calculated to be 1.8 nm based on the D value according to the Stokes equation (Table 3). In the deuterated acetonitrile/ethanol mixture corresponding to the immersion solvent for the preparation of the SAM, the RH changed to 1.3 nm, possibly because ethanol is a poor solvent for AcS-A3. The shrunken molecules might easily form the close-packed structure in the SAM. In contrast, the RH of AcS-A3 after anion exchange for NO3- measured in deuterated water was slightly increased (2.0), which indicates that the A3 molecules tend to adopt an expanded conformation in water. However, the RH measured in a 100 mM NaNO3 aqueous solution was the same as that in water (1.9), confirming that the influence of the bulkiness of the dendritic molecules on the desorption was small. These results support that the inhibition of desorption of the dendritic viologen-arranged molecules by the addition of electrolytes was due to the shielding effect of the Coulomb repulsion.

Table 3. Diffusion coefficients (D) and hydrodynamic radii (RH) of acetyl-capped A3 (AcSA3) in several solvents.a D RH Solvent m2 s-1 nm AcS-A3(30PF6-) CD3CN 3.2 x 10-10 1.8 CD3CN/EtOD (1/1, v/v) 2.5 x 10-10 1.3 AcS-A3(30NO3-)b D2O 1.2 x 10-10 2.0 -10 D2O + 100 mM NaNO3 1.3 x 10 1.9 a The D was determined by NMR measurements at 25°C, and the RH was calculated using the D value according to the Stokes equation. b The counter anion was exchanged for NO3- to dissolve in water.

Desorption behavior similar to the A3-SAM was also observed for the A1-, and A2SAMs in water (Figure 7a). Since the van der Waals interaction of the decyl groups was expected in the Vio-SAM, the desorption ratios of the Vio-SAM at each immersion time were less than those of the SAMs of the dendritic molecules (Vio-SAM: k = 1.9 x 10-4 s-1). On the basis of the fraction of remaining SAMs, the average radius of the area occupied by one adsorbate molecule (R) was estimated at each immersion time (Figure 7b). The changes in R


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became large with increasing dendritic generation. The R of the A3-SAM drastically increased from 0.89 to 1.86 nm, and the diameter reached 3.7 nm after desorption, which is in good agreement with the diameter of an individual A3 molecule of the A3/C10-SAM, as observed by STM (Figure 5). As described above, the solution-phase desorption of the SAMs derived from the dendritic viologen-arranged molecules was inhibited by adding electrolytes such as NaNO3. Therefore, the orientation of the dendritic molecules on gold can be controlled by utilizing the specific desorption induced by Coulomb repulsion.

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A1 1 Vio

0.5 0

0

50 100 Immersion time / min

Figure 7. Changes in the desorption ratio (a) and R (b) of the Vio-, A1-, A2-, and A3-SAMs by immersion into H2O with various immersion times. R: the average radius of the area occupied by one dendritic molecule on the substrate.


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CONCLUSIONS We have reported that the solution-phase adsorption and desorption of dendritic viologen-arranged molecules with ω-mercaptodecyl groups at the apex of dendritic structure on gold. The adsorption of the dendritic molecules onto a gold substrate occurred quickly and saturated within a few minutes in a mixture of acetonitrile/ethanol at a concentration of 2 mM. AFM measurements of the SAMs in a 100 mM NaNO3 aqueous solution revealed flat surfaces regardless of the dendritic generation because the peripheral viologen units were closely packed at the surface. Although the dendritic molecules in the SAMs could not be detected individually, the individual 3rd-generation dendritic molecules embedded in the boundary defects of the C10-SAM were successfully visualized as a 3-4 mm diameter bright spots by STM observation, which might enable a conductive measurement of a single dendritic molecule. The desorption of the SAMs on gold derived from the dendritic molecules was observed in various solvents such as water and acetonitrile and occurred more rapidly than that of a conventional n-alkanethiol SAM. However, the desorption was inhibited by adding electrolytes such as NaNO3, indicating that the surface density of the dendritic molecules can be controlled through the desorption. Several dendronized adsorbates were developed to control the surface density based on the steric bulkiness of the dendritic terminal groups.36,37 We showed a different approach that utilizes the dendritic structure to control the surface density of the functional molecules on a substrate. Thanks to the defined charge and specific molecular size of the dendritic An adsorbates, the surface densities of the An-SAMs can be tailored through the solution-phase desorption controlled by electrolytes. Moreover, the counter anions of the viologen units in the SAM can be readily exchanged for various metal anionic complexes such as PtCl42-, CuCl42-, and PdCl42- with the amount accommodated


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defined by dendritic generation.24 Therefore, the An-SAMs may act as surface-densitytunable molecular templates to fabricate few-atom metal dot arrays with a nanometer-scalecontrolled distance on a substrate. This work is now in progress.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. Table S1 showing calculated-radii of gyration and hydrodynamic radii of the dendritic adsorbates, and Figure S1 showing changes in the desorption ratio of A3-SAM with time by immersing into 100 mM K2PtCl4, and 200 mM glucose aqueous solutions. .

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (T.K.). Notes The authors declare no competing financial interest.

ACKNWLEDGMENTS


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We are deeply grateful to Ms. Y. Oguchi (Tokyo Institute of Technology) for AFM observation. This work was supported by JST ERATO Grant Number JPMJER1001 and JSPS KAKENHI Grant Number 15K04590.

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Desorption control 80

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In H2O

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