Effect of the Steric Molecular Structure of Azobenzene on the

Dec 19, 2012 - Effect of the Steric Molecular Structure of Azobenzene on the Formation of Self-Assembled Monolayers with a Photoswitchable Surface Mor...
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Effect of the Steric Molecular Structure of Azobenzene on the Formation of Self-Assembled Monolayers with a Photoswitchable Surface Morphology Daisuke Ishikawa,† Eisuke Ito,‡ Mina Han,*,§ and Masahiko Hara*,†,‡ †

Department of Electronic Chemistry, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan Flucto-Order Functions Research Team, RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan § Department of Chemistry and Department of Electronic Chemistry, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan ‡

S Supporting Information *

ABSTRACT: The growth processes of self-assembled monolayers (SAMs) of two azobenzene disulfides formed on flat gold surfaces were studied to confirm the effect of the intermolecular interactions between azobenzene molecules on the light-triggered surface morphologies of the SAMs. Scanning tunneling microscopy (STM), atomic force microscopy (AFM), thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), and ultraviolet−visible (UV−vis) absorption spectroscopy were employed to study the SAMs and their growth processes. The SAMs studied were of bulky-substituted azobenzene disulfide (Et-2S), and nonsubstituted azobenzene disulfide (Me-2S), formed on a goldcovered substrate, and had a twisted and a planar structure, respectively. STM-based imaging of the initial stage of the selfassembly of the Et-2S molecules revealed that cleavage of the disulfide bond occurred on the gold surface, and phase-separated domains composed of azobenzenethiolate and dodecanethiolate were formed. Time-dependent AFM-based imaging illustrated the mechanism through which the Et-2S SAM grewit was through the formation of dendritic aggregates and islands eventually resulting in phase-separated domains with a wormlike structure. This wormlike structure showed noticeable changes in its surface morphology upon irradiation with UV and visible light. On the other hand, while the growth process for the Me-2S SAM was similar to that of the Et-2S SAM, the final Me-2S SAM had smooth domains whose morphology did not exhibit photoswitchability. The TD and XP spectra of the SAMs showed that the number of adsorbed Et-2S molecules reached a point of saturation after a 24 h long immersion while the number of Me-2S molecules increased even after a 336 h long immersion. Furthermore, the area occupied by the azobenzene moiety in the Et-2S SAM was constant regardless of the immersion time, whereas that in the Me-2S SAM decreased with the immersion time. These results indicated that the strength of the interactions between the azobenzene molecules significantly influenced the aggregate-forming ability in SAMs.

1. INTRODUCTION Smart surfaces that react to external stimuli such as light,1−3 temperature,4,5 and electrical potential6,7 show promise for use not only in molecular switches and molecular recognition devices8,9 but also in biomimetic systems.10 As an external stimulus, light is an appropriate trigger for inducing changes in molecular conformations because its wavelength, intensity, and polarization can be adjusted. Azobenzene and its derivatives have been extensively studied as they exhibit simple photoinduced isomerization. It is well-known that azobenzene shows a transition from the thermodynamically stable trans form to the cis form upon irradiation with UV light, and this transformation is reversed upon irradiation with visible light or on heating. Because the photoisomerization of azobenzene is accompanied by a change in its dipole moment, drastic changes in its absorption spectra, and collective subnanometer-scale © XXXX American Chemical Society

changes in its structure, azobenzenes have been investigated for use as storage materials11,12 as well as for the fabrication of mechanical working devices13,14 and photoswitchable sensors.15 Self-assembled monolayers (SAMs) have been used to prepare functional surfaces on metallic and nonmetallic substrates.16,17 When attempting to control the surface properties of SAMs at the nanoscale, the terminal functional group of the constituent molecules can be of critical importance. Furthermore, the spatial distribution (both normal and lateral) and orientation of and molecular interactions between the components of the SAMs can also be very important. In fact, the density and orientation of their Received: June 24, 2012 Revised: November 18, 2012

A

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assumed that the strength of the intermolecular interaction between the azobenzene moieties and the presence of dodecanethiolate strongly affected these morphological changes. If the influence of the intermolecular interactions on the photoreactive surface morphology of SAMs can be elucidated, it may be possible to control the surface properties of SAMs. Here we report the steric effect that azobenzene molecules have on the photoreactive surface morphologies supposedly formed via the separation of azobenzenethiolate and alkanethiolate into separate phases. This was determined by studying the growth processes of SAMs formed using Et-2S and Me-2S solutions. The topographies of two distinct azobenzene disulfide SAMs, formed by the immersion of gold-covered substrates in dilute 1 μM Et-2S and Me-2S solutions, respectively, were investigated in detail. The number of molecules of azobenzene adsorbed in the case of each SAM was also determined. Our study reveals the importance of the molecular structure of SAMs in controlling their surface properties and in amplifying the changes in their light-triggered nanoscopic characteristics such that they are reflected in their macroscopic properties as well.

components have a significant effect on the reactivity of SAMs. For example, Vaidya et al.18 showed that the interfacial reaction rate of the hydrolysis of ester groups immobilized on gold surfaces depends on the packing density and orientation of the components of the corresponding SAMs. Additionally, the surface morphology of a SAM can affect the reactivity of the monolayer at the air/solution interface. Mrksich et al.19 studied biomolecule−substrate interactions by combining patterned SAMs and electrochemical techniques. Thus, it is important for the construction of functional surfaces to control not only the terminal groups but also the orientation of the components and the morphology of SAMs. We have recently reported on photoreactive SAMs consisting of ortho-diethylated azobenzene disulfide (Et-2S) and thiol (EtSH) on gold surfaces, in which the molecular interactions between the azobenzene molecules were relaxed by the steric hindrance of the substituents and the existence of dodecanethiolate as a spacer (Figure 1).20,21 These SAMs exhibited

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. The syntheses of the azobenzene disulfides (Et-2S and Me-2S) and thiols (Et-SH and MeSH) were performed using procedures described previously in the literature.21,23 Thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM)- and atomic force microscopy (AFM)-based imaging were used to characterize the SAMs. Au(111) substrates were prepared by the thermal deposition of gold on mica in a vacuum chamber. Singlecrystal gold was thermally deposited onto a freshly cleaved mica surface at 350 °C after prebaking the mica film at 400 °C in a vacuum at 10−6 Pa. The deposition rate for the layer of gold was less than 1 Å/ s. After deposition, the gold-coated substrates were annealed at 420 °C for 2 h to produce atomically flat single-crystal surfaces. For ultraviolet−visible (UV−vis) absorption spectroscopy, polycrystalline gold films with a thickness of 20 nm were deposited onto clean quartz surfaces in vacuum. The 1 μM Et-2S and Me-2S solutions were prepared in dichloromethane. The azobenzene SAMs were prepared by the immersion of the Au(111)/mica or polycrystalline gold substrates into each azobenzene solution for specified durations. After immersion, the samples were carefully rinsed with pure dichloromethane to remove physisorbed molecules from their surfaces and blown dry with nitrogen gas. 2.2. Light Irradiation. The azobenzene SAMs were exposed at room temperature in a dark room to UV light (365 nm) using a highpressure mercury arc lamp (Supercure-204S, Tokina) and a combination of Toshiba color filters (UV-35+UV-D36A) to induce the trans-to-cis isomerization or visible light (436 nm), also using a combination of Toshiba color filters (Y-43+V-44), to induce the cis-totrans isomerization. 2.3. Atomic Force Microscopy (AFM)- and Scanning Tunneling Microscopy (STM)-Based Imaging. AFM images of the azobenzene SAMs formed on the Au(111)/mica substrate were obtained using a NanoScope IV (Veeco Instruments Inc.) with a silicon cantilever (NCH Pointprobe, NanoWorld, Switzerland) in the tapping mode in order to characterize the surface topography of the samples. The molecular structure of the gold layer was observed by STM (NanoScope IV, Veeco instruments Inc.) using a commercially available Pt/Ir tip (80:20). The STM-based imaging was performed in air using the constant current mode. The bias voltage and tunneling current were set at 550 mV and 100 pA, respectively. All AFM- and

Figure 1. Molecular structures of asymmetrical azobenzene disulfides Et-2S and Me-2S and azobenzene thiols Et-SH and Me-SH as reference compounds.

higher trans-to-cis photoisomerization yields than those exhibited by nonsubstituted azobenzene molecules (Me-2S and Me-SH) that form H-aggregates of azobenzene moieties. This was because of the strong intermolecular interactions such as π−π stacking between the phenyl rings of the azobenzene molecules. Moreover, we reported noticing changes in the surface morphology of the Et-2S SAM: its structure changed from being wormlike, consisting of phase-separated domains ∼10 nm in size, to a smoother one, on being irradiated with UV light.22 This smoother structure reverted to the wormlike structure within ∼2 min of the SAM being irradiated with visible light. These morphological changes in the Et-2S SAM were accompanied by molecular conformation changes, i.e., transformations between the trans and cis forms of the azobenzene molecules. However, no pronounced morphological changes were observed in the other three azobenzene SAMs after they had been irradiated with UV or visible light (Figures S1−S3 in the Supporting Information). It has been B

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Figure 2. Topographic images and cross-sectional profiles of the Et-2S SAMs on Au(111) surfaces, monitored by AFM, after immersion in the 1 μM Et-2S solution for various durations. The area of each frame is 400 nm × 400 nm.

Figure 3. Topographic images and cross-sectional profiles of the Me-2S SAMs on Au(111) surfaces, monitored by AFM, after the immersion in the 1 μM Me-2S solution for various durations. The area of each frame is 400 nm × 400 nm. STM-based imaging procedures were performed at 19−25 °C and 25− 40% relative humidity. 2.4. Thermal Desorption Spectroscopy (TDS). The TDS measurements were performed with an EMD-WA1000S (ESCO Ltd., Japan) TDS system that had an ultrahigh vacuum chamber (2 × 10−7 Pa) and a quadrupole mass spectrometer (QMG-421, Blazers, Liechtenstein). The heat rate was 1 K/s. The surface temperatures were monitored with a W−Re thermocouple tightly attached to the surfaces of the samples. 2.5. X-ray Photoelectron Spectroscopy (XPS). The XPS measurements were performed using a Theta Probe (Thermo Fischer Scientific, U.K.). Monochromatic Al Kα (photon energy of 1486.6 eV and spot size of 400 μm) was used for excitation. The base pressure within the measurement chamber was ∼4 × 10−7 Pa. The pass energy level for the measurements was set at 100 eV. The obtained spectra were analyzed using a peak-fitting software (Thermo Avantage ver. 3.25). 2.6. Ultraviolet−Visible (UV−Vis) Absorption Spectroscopy. The UV−vis absorption spectra of the samples were measured with a Shimadzu UV-3150 UV−vis−near-infrared (NIR) spectrophotometer in the absorbance mode with air in the reference path. To obtain the spectra of the azobenzene SAMs, the spectra of bare gold substrates, measured before the preparation of the samples, were subtracted from the spectra of the samples after the preparation of the SAMs.

SAM was monitored during various stages of the growth process using AFM. Figure 2 displays topographic images of the SAM after the immersion of the gold-covered substrate in the 1 μM Et-2S solution for predetermined periods. In the beginning of a cleavage of disulfide bond of Et-2S molecule followed by the phase separation into azobenzene thiolate and dedecanethiolate domains (Figure S4), two-dimensional aggregates such as dendrites emerged, which had a width of 22 ± 4 nm and were randomly distributed over the surface, as shown in Figure 2a. These aggregates were produced by the assembling of small protrusions with a diameter of ∼22 nm. After a 2 h long immersion, the aggregates grew into islands and small aggregates appeared. After a 24 h long immersion, much larger islands were formed such that they were in contact with other islands. During the coalescence stage, which lasted from the 24 h period to the 168 h period, i.e., for 168 h, slow growth, which was characterized by the filling of the voids between the islands, finally resulted in the formation of a wormlike structure with a width of about 10−15 nm, as shown in Figure 2e. This wormlike structure closely resembled that of a fully assembled Et-2S SAM prepared using a 0.1 mM solution. In addition, it was found that this wormlike structure was the terminal morphology of the Et-2S SAM because even after the immersion of the substrate in the solution for over 168 h, the morphology of the formed SAM remained the same. The surface morphology after 168 h was light-sensitive, and its structure changed from a wormlike one to a smoother one and

3. RESULTS AND DISCUSSION 3.1. AFM Image of the Azobenzene SAMs during Various Stages of Growth and the Photoswitchability of Their Surface Morphologies. The topography of the Et-2S C

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Figure 4. Topographic images and cross-sectional profiles of the Et-2S SAM after immersion for 24 h, monitored by AFM (400 nm × 400 nm). The shown surface morphologies are of (a) the freshly as-prepared SAM, exposed to (b) UV light, and subsequently exposed to (c) visible light.

The growth of the islands in the Et-2S and Me-2S SAMs may be explained on the basis of the diffusion-limited aggregation (DLA) model, which is related to the dendritic growth observed after 1 h (Figures 2a,b and 3a).25,26 In the DLA model, molecules in solutions randomly adsorb onto surfaces and then diffuse by moving among atomic sites owing to the attractive interactions among them, leading to the growth of small clusters. Diffusing molecules are more probable to attach to the tip of the cluster than the fjord, and then grow to islands, characterized by dendrites. The growing islands coalesce with each other, whereas the fjords between the branches of dendrites and voids within the aggregates are slowly filled, which results in a fully covered surface. It can be assumed that strong intermolecular interactions such as π−π stacking between the phenyl groups of Et-2S and Me-2S may be partly responsible for the growth process, characterized by the formation of dendritic aggregates through the DLA model. In fact, thiols and symmetric or asymmetric disulfides without an aromatic ring can form dendrites. For example, molecules of n-butanethiol,27 n-dioctadecyl sulfide,28 and symmetrical and asymmetrical disulfides with an ester group midway in the alkyl chains29 can assemble into islands that have a few twiglike dendrites. In addition, poly(ethylene glycol) thiol also assembles on Au surfaces during the early stages of SAM growth to form islands that have multiple branches along the terrace steps.30 Thus, the growth process of both the Et-2S and Me-2S SAMs through dendrites can be affected by the interactions between the alkyl chains that are bonded to gold atoms instead of the aromatic rings near the air−molecule interface. As can be seen from Figures 2e and 3f, the phases of the completely formed Et-2S and Me-2S SAMs are quite dissimilar from each other. It appears that the interactions between the phenyl groups affect the condition of the aggregates at the final stage of SAM growth. As mentioned above, whereas the azobenzene moieties of Me-2S can be influenced by these interactions, resulting in H-aggregates along the long axis of the azobenzene structure in the Me-2S SAM, Et-2S is not affected owing to the steric hindrance of the two ethyl groups at ortho positions. The change in the molecular conformation of azobenzene in the Me-2S SAM was not observed as a change in its surface morphology because the densely aggregated

vice versa on being irradiated by UV and visible light, respectively (Figure S5). The growth process for the Me-2S SAM (images of the Me-2S SAM at various stages of the process are shown in Figure 3) was similar to that of Et-2S; however, the final stage was fairly different. Unlike in the case of the Et-2S SAM, whose terminal structure was wormlike structure, consisting of small phase-separated domains, the growth of the islands in the case of the Me-2S SAM led to flat domains that were separated by valleys with a depth of 1 nm (Figure 3f). After 336 h, the flat surface of the Me-2S SAM did not show any morphological changes on being irradiated with UV or visible light. In order to determine the mechanism of the formation of the islands that were brighter in color, changes in the surface morphology of the Et-2S SAM were observed midway through the growth process. Figure 4 shows AFM images of the photoinduced changes in the morphology of the Et-2S SAM after 24 h. The changes in the height of the brighter colored islands can also be seen. On being irradiated with UV light, the height of the islands decreased by ∼0.5 nm, and the small aggregates connected with each other. Subsequent exposure to visible light increased the height of the islands, and the separated small domains reappeared. These results indicate that bright-colored islands mainly consisted of azobenzenethiolate and that there was enough space for the azobenzene moieties to isomerize into aggregates, irrespective of the islands’ sizes. Given the discrepancy between the molecular length of azobenzenethiolate and the difference in the heights of the bright and dark regions, it is assumed that the dark regions consisted of dodecanethiolates and azobenzenethiolates in conformations similar to a “lying-down” structure. Because the azobenzenethiolate molecules of Et-2S have flexible ether bonds connecting the phenyl ring to alkyl chains, the azobenzene moiety can have conformations that result in heights similar to that of “standing-up” dodecanethiolate. Unlike an all-trans alkanethiolate molecules that do not have any ether bonds, alkanethiolate molecules with flexible ether bonds could adopt various conformations owing to the gauche bonds within the chain.24 Thus, azobenzenethiolate molecules with various conformations and dodecanethiolate molecules apparently coexist in the dark regions. D

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the adsorption of the Et-2S molecules, which reached a point of saturation after 24 h. The results of the TDS measurements suggested that the monomer of azobenzenethiolate was not detected. This was because the azobenzene moiety dissociated at either the azo group or the ether bond.34 In addition, the dimer of azobenzenethiolate was also not detected. The thiolate molecules did not dimerize into a bulky structure. This was because of the large distance between the S atoms in thiolates having a loosely packed structure, which is the result of the steric hindrance of the substituents.35,36 Moreover, fragments derived from the azobenzene moieties were also not quantitatively detected. This was probably due to the influences of their electronic properties and packing density.37 To quantify the amount of azobenzenethiolate adsorbed on the surfaces of the gold layers, XPS was performed on the Et-2S and Me-2S SAMs while focusing on the N 1s peak, which is assignable to the azo group. The changes in the N 1s peak areas for various immersion times are shown in Figure 6. The binding

molecules did not produce a large enough volume for them to sway. However, the changes in the surface morphology of the Et-2S SAM were successfully reflected in the irregularities that had a wormlike structure owing to the appropriate amount of volume around the azobenzene molecule. This was due to the weak intermolecular interactions. 3.2. Difference in the Adsorption Behaviors of the Et2S and Me-2S Molecules on Gold Surfaces. The mutual relation between the surface morphologies of the SAMs and the amount of azobenzenethiolate adsorbed was investigated by TDS. The number of adsorbed molecules on a surface that constitute a SAM can be determined by measuring the amount of the species of the adsorbates that are desorbed from the surface.31 On the desorption of alkanethiolate from a gold surface under an ultrahigh vacuum, dimer species formed by contiguous alkanethiolates and monomer species that have dissociated independently from the surface are detected.32,33 Additionally, it is known that the parent species, composed of azobenzenethiolate and alkanethiolate, were not detected in an asymmetrical azobenzene disulfide SAM.34 Thus, the measured fragments were focused on the monomer radical (C12H25S+, mass/charge ratio (m/e) = 201) and the dimer radicals (C12H25S−SC12H252+, m/e = 201, and C12H25S−SC12H25+, m/e = 402) of dodecanethiolate in the Et-2S and Me-2S SAMs. An m/e = 201 corresponds to the monomer radical and a few dimer radicals composed of dodecanethiolate. Figure 5 displays

Figure 6. Changes in the N 1s peak area of the XP spectra of the Et-2S (blue line with rhombuses) and Me-2S (red line with circles) SAMs as a function of the immersion time.

energies of the N 1s orbitals of Et-2S and Me-2S were positioned at 399.9 ± 0.3 and 399.8 ± 0.1 eV, respectively, and these values were in agreement with those reported previously.34,38,39 As can be seen from Figure 6, the peak area of the N 1s orbital in the case of the Et-2S SAM increased after immersion for 2 h and was constant for longer immersions, whereas that of the Me-2S SAM steadily increased over the various immersions and continued to do so for the 336 h long immersion. This behavior is in good agreement with the changes noticed in the peak areas in the TD spectra of the monomer and dimer species that desorbed from the SAMs. Accordingly, the changes in the peak areas seen in the TD and XP spectra represent the number of Et-2S molecules adsorbed onto the gold surface at saturation after immersion for ∼24 h, even though the surface morphology of the SAMs was not wormlike. On the other hand, the adsorption of the Me-2S molecules onto the gold surface continued until the surface was fully covered by the SAM. We compared the surface morphology of the Et-2S SAM formed (Figure 2c) after a 24 h long immersion with the amount of azobenzenethiolate adsorbed over the same period. There was a disagreement that the large lower regions with darker contrast remained in spite of the saturated adsorption state. This disagreement may be due to the existence of

Figure 5. Changes in the peak areas of the thermal desorption spectra of dodecanethiolate (m/e = 201, C12H25S+) and/or didodecane disulfide (m/e = 201, C12H25S−SC12H252+) in the Et-2S (blue line with rhombuses) and Me-2S (red line with circles) SAMs as a function of the immersion time.

the change in the amount of the monomer desorbed as a function of the immersion time. It could be seen from the TD spectra that both the monomer species with an m/e = 201 and the dimer species with an m/e = 402 desorbed at around 380 K. In addition, neither the parent species of Et-2S, which had an m/e value of 859, nor that of Me-2S, which had an m/e = 817, was detected (shown in Figure S6). In Figure 5, the desorbed amount of the monomer and/or dimer with an m/e = 201 derived from Me-2S increased with the immersion time, while that of monomer and/or dimer derived from Et-2S was almost constant after 24 h. Moreover, changes in the amount of dimer desorbed that had an m/e = 402 were similar to those seen in the case of the monomer with an m/e = 201 (Figure S7). These data showed that the adsorption of the Me-2S molecules continued until the SAM covered the gold surface fully, unlike E

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connected to the gold surface. In the case of the Me-2S SAM, initially the area occupied was greater than 0.75 nm2, and this then decreased to ∼0.67 nm2, a value smaller than that determined from the crystal structure. This was because Me-2S experiences steric hindrance to a smaller degree than does Et2S, owing to the lack of bulky substituents and the planar structure of the azobenzene moiety. The azobenzene moiety may thus form densely packed H-aggregates, resulting in a decrease in the area occupied by the molecules of the SAM. 3.4. Effect of the Strength of the Intermolecular Interactions in the SAMs on the Growth Process Models. During the growth of the SAMs, the intermolecular interactions between the adsorbed molecules strongly affect their packing density. Figures 7 and 8 schematically depict the proposed

disordered azobenzene moieties, which apparently form in the lower regions. 3.3. Areas Occupied by the Azobenzene Molecules As Estimated from the UV−Vis Absorption Spectra. The UV−vis absorption spectra provided useful information regarding the formation of aggregates of the azobenzene molecules and the occupied area of azobenzene molecule in SAM. Table 1 lists the wavelengths of maximum absorption Table 1. Wavelengths of Maximum Absorption (λmax) at the π−π* Band and the Estimated Area per Molecule of the Et2S and Me-2S Molecules Et-2S immersion time/h 24 96 168 336

Me-2S

λmax/nm

area per molecule a/nm2

349.7 ± 1.5 350.8 ± 2.4 349.1 ± 3.2

1.03 ± 0.20 0.92 ± 0.07 1.00 ± 0.08

λmax/nm 352.5 352.7 349.8 348.6

± ± ± ±

1.0 2.4 1.9 0.8

area per molecule a/nm2 1.20 1.15 0.97 0.67

± ± ± ±

0.05 0.05 0.04 0.02

(λmax) of the π−π* band and the area per molecule for the azobenzenethiolate molecules in the Et-2S and Me-2S SAMs. The λmax in the case of the Et-2S SAM were almost constant at ∼350 nm for each immersion period, whereas that of the Me2S SAM decreased with an increase in the immersion period. Considering that λmax for the Et-2S and Me-2S solutions were 354 and 368 nm, respectively, the π−π* absorption bands of the Me-2S SAM were significantly blue-shifted by more than 14 nm (Figure S8). The blue-shifted spectra of the Me-2S SAM suggested the following: First, H-aggregates were formed along the long axis of the azobenzene molecule owing to favorable π−π stacking, as reported by us previously.20,21 Second, the number of H-aggregates of the azobenzene molecules formed increased with the immersion period of the sample until the surface of the sample was totally covered. This was associated with the decrease in the area occupied by the Me-2S SAM. The area occupied by the azobenzenethiolate molecules can help elucidate the conformation and packing density of the molecules in the Et-2S and Me-2S SAMs. The area per molecule was calculated from each UV−vis absorption spectrum by applying the Beer−Lambert law.40 As can be seen from Table 1, the area occupied by an Et-2S molecule for various immersion periods was estimated to be ∼1.00 nm2, and this value was almost constant irrespective of the immersion period. On the other hand, in the case of the Me-2S SAM, the area covered by a molecule decreased from 1.20 ± 0.05 to 0.67 ± 0.02 nm2 after a 312 h long immersion. Compared with the area per molecule values for Et-2S and Me-2S (approximately 0.80 and 0.75 nm2, respectively) estimated from X-ray crystal structure data,41 the CPK model, and the absorption spectral data of azobenzene films,23,42,43 the areas occupied by the Et-2S and Me-2S molecules during the growth process were higher. This was true for all the cases except for the Me-2S SAM after a 336 h long immersion. The area occupied by the Et-2S molecule was larger than that estimated on the basis of an X-ray crystal structure analysis. This suggested that the molecules in the SAMs were not densely packed and were in a flexible conformation, owing to there being enough free space. This was because of the steric hindrance of the two ethyl groups at the ortho positions of the azobenzene moiety and the flexible ether bond between the azobenzene moiety and the alkyl chain

Figure 7. Schematic diagram showing the growth mechanism of the Me-2S SAM on a Au(111) surface: (a) the formation of dendritic islands, followed by (b) space being made available for further adsorption, (c) the adsorption of Me-2S molecules into this available space, and (d) the formation of a densely packed structure, composed of H-aggregates, whose morphology that did not exhibit photoswitchability at the saturated adsorption stage as observed after the 336 h long immersion.

growth mechanism for the Me-2S and Et-2S SAMs, respectively. When a SAM forms, there are several possible mechanisms for the desorption/adsorption of molecules at the SAM−solution interface. It has been known that asymmetrical disulfide SAMs that include both longer and shorter alkyl chains mainly consist of the longer alkyl chains owing to the thermodynamically favored van der Waals interaction between the alkyl chains.44−46 During the formation of such SAMs, the longer moieties replace the shorter ones through the desorption of thiolates or disulfides, composed only of shorter chains, and the succeeding readsorption of disulfides with longer and shorter chains. If one assumes that these conditions are applicable to the Me-2S SAM as well, they may result in a decrease in the amount of dodecanethiolate adsorbed, accompanied by an increase in amount of azobenzene F

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molecules being closely packed. This is followed by Haggregation and the creation of voids in the SAM. Therefore, it is assumed that the successive adsorption of Me-2S molecules onto the surfaces of the voids results in the formation of a flat SAM that fully covers the gold surface. In addition, because the Me-2S SAM has a concavities- and convexities-free smooth morphology, pronounced changes in its morphology cannot be observed. On the other hand, in the case of the Et-2S SAM, disulfide molecules did not apparently get adsorbed onto the gold surface when saturated and the occupied area per molecule was ∼1.00 nm2 (Figure 8b). When considering the formation of Et2S SAM, the most obvious explanation is that the twisted azobenzene moiety of Et-2S prevents the excessive adsorption of the Et-2S molecules. In the case of the Et-2S SAM, it appears that the steric hindrance of the substituents on the azobenzene moieties prevents the further adsorption of Et-2S molecules, which have a large volume, after a 24 h long immersion. Moreover, it is reported that the azobenzene moiety of Et-2S adopted a distorted conformation around the azo group with the dihedral angles C−C−NN of 156.32° due to the steric hindrance of the ortho substituents.41 Because the azobenzene moieties do not form densely packed H-aggregates owing to the steric hindrance and the twisted structures as is clear from Figure S8a, the weak interaction between the azobenzene moieties cannot create enough vacant space for additional adsorption. Although the specific molecular orientations at every growth stage are not evident, the changes in the surface morphology of the Et-2S SAM at the saturated adsorption state, shown in Figures 2c−e, can be attributed to the molecular conformation changes in the structure of the azobenzene moiety, which changed from a bent or “lying-down” one to a “standing-up” one, rather than the additional adsorption of the Et-2S molecules (Figures 8b,c). This may be due to the characteristics of the Et-2S molecules, which cannot aggregate in a dense manner, and the very slow rate of exchange of molecules in the SAM. In the former case, it has been shown that the azobenzenethiolate of Et-2S cannot assemble closely in a SAM because of weak interactions between the nonplanar azobenzene moieties, twisted by the steric hindrance of the ortho-positioned ethyl substituents.20,21,41 Although the twisted azobenzenethiolate and dodecanethiolate could result in sparsely populated SAMs because of weak interactions, large voids for the adsorption of Et-2S molecule were not created in the SAM. Therefore, sufficient space was not available for the bulky disulfide molecules to penetrate into the Et-2S SAM, in contrast to the Me-2S SAM. For the latter case, prior studies have shown that the exchange between thiolates and thiols could be induced via the dissociation of thiolates from a surface.32,51 In addition, it has been inferred that the dissociation is a rate-determining step of the exchange reaction. This inference was supported by experimental data showing that the exchange rate depends to a lesser extent on the concentration of the thiol solution.52 The exchange can occur at defects such as domain boundaries in crystalline monolayers and step edges in the gold substrate where thiolates diffuse into the defect sites from the interior of the domains.53 Moreover, the exchange rate is limited by the diffusion of thiols into defects; therefore, the exchange of molecules in SAMs requires sufficient time. If azobenzenethiolates or dodecanethiolates were exchanged by the other half of the Et-2S molecule at defects in the SAM, we should have observed higher or lower outlines of the azobenzene moiety or the dodecyl chain along

Figure 8. Schematic diagram showing the growth mechanism of the Et-2S SAM on a Au(111) surface: (a) the formation of dendritic islands, (b) the prevention of further adsorption owing to unavailability of sufficient space at the saturated adsorption stage, and (c) formation of the wormlike structure, which exhibited photoswitchability and transformed into a smoother structure owing to molecular conformation changes in the SAM.

adsorbed. However, given that the amounts of both azobenzenethiolate and dodecanethiolate adsorbed in the case of the Me-2S SAM increased over the 336 h immersion, as demonstrated by the TDS and XPS results, Me-2S molecules were continuously adsorbed onto the gold surface without resulting in the formation of an azobenzenethiolate-dominated SAMs. The results suggest that intermolecular interactions between dodecanethiolates and especially those between azobenzene moieties facilitate the adsorption of the Me-2S molecules on the gold surface. In general, it is known that planar azobenzenethiolates form relatively densely packed monolayers on gold surfaces.47,48 In such dense monolayers, azobenzene moieties are arranged in distorted hexagonal or oblique structures owing to strong chromophoric interactions between the moieties.49 It is elucidated that the azobenzene moiety of Me-2S has a planar conformation with the dihedral angles C−C−NN of 179.61° like a nonsubstituted azobenzene from the X-ray crystallographic analysis.50 In fact, the formation of H-aggregates by Me-2S (and Me-SH) molecules can be explained by the parallel packing of the long axes of the azobenzene molecules and the mutually nonparallel conformation of their short axes, as shown by the absorption spectra in Figure S8b. Since it seems that the strong interactions do affect the azobenzene molecules that have just been adsorbed onto a surface, their conformation may afford them wiggle room and allow them to be flexible in the Me-2S SAM. However, once the azobenzene moieties experience the strong interactions, the wiggle room is no longer available for the azobenzene molecules. This leads to the creation of vacant spaces in the SAM instead. The interaction between the azobenzene moieties with a plain structure is much stronger than that between dodecyl chains, which results in the G

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possible owing to insufficient space. Our findings will help facilitate the construction of smart surfaces that are reactive to light radiation.

the step edges on the surface. However, the immersion of the SAM in the Et-2S solution for a longer duration did not lead to the formation of outlines or change the width or the morphology of the wormlike structure (not shown in figure). This suggests that the exchange between the Et-2S SAM and the disulfide molecule is very small, and thus, the changes in the surface morphology of the SAM with time at the saturated state resulted from the molecular conformation changes in the SAM. Consequently, the phase-separated wormlike structure, induced by molecular conformation changes, seen in sparsely packed SAMs permits the isomerization of azobenzene molecules and the surface morphology of the SAMs to vary, following exposure to UV and visible light.

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ASSOCIATED CONTENT

* Supporting Information S

AFM topographic images with cross-sectional profiles of azobenzene disulfides and thiols SAMs upon UV and visible light exposure; STM images of initial growing phase of Et-2S SAM and the schematic model of the possible molecular alignment in the striped phase; TD spectra and UV−vis absorption spectra of azobenzene disulfides SAMs and solutions. This material is available free of charge via the Internet at http://pubs.acs.org.

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4. CONCLUSIONS The effect of interactions between azobenzene moieties on the light-triggered morphologies of SAMs was investigated using two azobenzene disulfides: Et-2S with a sterically bulky structure and Me-2S with a plain structure. We followed the growth process of the SAMs and proposed a molecular aggregation model by the topographic imaging of the Au(111) surface using STM and AFM. The amount of Et-2S and Me-2S adsorbed in the SAMs (measured by TDS and XPS) and the areas occupied by the azobenzene molecules (measured by UV−vis absorption spectroscopy) were determined as well. The STM and AFM images showed the cleavage of the S−S bond of the asymmetrical disulfide and the formation of phaseseparated domains. This was eventually followed by the formation of a photoswitchable wormlike structure in the Et2S SAM via the formation of a dendritic domain. This was in contrast to the Me-2S SAM, which had a completely flat morphology that showed no change on photoirradiation. The TDS and XPS measurements revealed that the amount of Et-2S adsorbed reached a point of saturation within ∼24 h, whereas that of the Me-2S increased until 336 h, at which point the gold surface was completely covered with the SAM. In addition, it was estimated that the area occupied by the azobenzene moiety in the Et-2S SAM was almost constant irrespective of the immersion time, whereas that in the Me-2S SAM decreased with the immersion time. The differences in characteristics of the two SAMs were attributed to the aggregate-forming abilities of the azobenzenes. As could be seen from their UV−vis absorption spectra, although no aggregates could form in the Et-2S SAM, Haggregates along the long axis of azobenzene formed in the Me2S SAM. The formation of the H-aggregates by strong intermolecular interactions caused the area occupied by the azobenzene molecules to decrease dramatically. Our proposed growth mechanism for the azobenzene SAMs was based on the differences in their intermolecular interactions, which, in turn, were attributable to the differences in their molecular structures. Surface observations and spectroscopy analyses suggested that the azobenzene molecules exhibited stronger interactions in the Me-2S SAM than they did in the Et-2S SAM. Regarding the growth process of the Me-2S SAM, strong interactions between planar azobenzene moieties can create adequate space on the substrate and induce the successive adsorption of disulfide molecules. Regarding the growth of the Et-2S SAM, the interactions weaken because of the steric hindrance of the two ethyl groups at ortho positions, and thereby, the twisted structure of the azobenzene moiety can result in the sparse assembly of azobenzene molecules, on which the further adsorption of bulky disulfide molecules is not

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Tel +81-45-924-5447, Fax +81-45-924-5447 (M. Han); e-mail [email protected]. jp, Tel +81-45-924-5437, Fax +81-45-924-5437 (M. Hara). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Global Centers of Excellence (COE) program of the Ministry of Education, Culture, Sports, Science, and Technology of Japan. We are grateful to Mr. Takumu Honda for synthesizing the azobenzene derivatives.

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ABBREVIATIONS SAMs, self-assembled monolayers; STM, scanning tunneling microscopy; AFM, atomic force microscopy; TDS, thermal desorption spectroscopy; XPS, X-ray photoelectron spectroscopy; UV−vis, ultraviolet−visible; Et-2S, bulky-substituted azobenzene disulfide; Me-2S, nonsubstituted azobenzene disulfide; Et-SH, ortho-diethylated azobenzene thiol; Me-SH, nonsubstituted azobenzene thiol; NIR, near-infrared; DLA, diffusion-limited aggregation; CPK, Corey−Pauling−Koltun.

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REFERENCES

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