Dimerization of Nitrosobenzene Derivatives on an Au(111) Surface

ARTICLE pubs.acs.org/JPCC

Dimerization of Nitrosobenzene Derivatives on an Au(111) Surface Ivana Biljan,*,† Marko Kralj,‡ Tea Mi
sic Radic,§ Vesna Svetli
cic,§ and Hrvoj Van
cik*,† †

Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102A, HR-10000 Zagreb, Croatia Institute of Physics, Bijeni
cka 46, HR-10000 Zagreb, Croatia § Ru{er Bo
skovic Institute, Bijeni
cka 54, HR-10000 Zagreb, Croatia ‡

bS Supporting Information ABSTRACT: In this study, we have investigated the ability of C-nitrosobenzenes to dimerize on an Au(111) surface and form ordered self-assembled bilayers (SABs). Use of scanning tunneling microscopy (STM) revealed that nitrosobenzene derivatives are able to form both well-ordered monolayers (SAMs) and bilayers (SABs). High-resolution STM images showed that within SAM regions molecules are arranged into hexagonal structures. √ Moir √ e-type superstructure with a periodicity of 1.5 nm was observed, indicating a 3 3  3 3 molecular arrangement. Molecularly resolved STM images showed that within bilayers molecules are also arranged into hexagonally ordered structures at intermolecular spacing of ∼0.4 nm. Furthermore, it was observed that the second layer is better ordered than the first one, probably because of the softer background. Use of atomic force microscopy (AFM), through measured height differences between monolayers and bilayers, further confirmed the formation of SABs. The results of the present study demonstrate that nitrosobenzenes represent convenient molecular models that can be used for systematic design and study of bilayers on metal surfaces. Besides the previously known properties of C-nitroso compounds to form dimers in crystals, here we have found a new situation under which these molecules dimerize, which is, in principle, a 2D crystallization.

’ INTRODUCTION C-nitrosobenzenes can exist in two different forms, as monomers or dimers (azodioxides). Broad literature is available on dimerization of nitrosobenzenes.1
10 Because the energy of an azodioxide (O)NdN(O) bond is of the same order of magnitude as a hydrogen bond, its formation strongly depends on the conditions of the environment. In principle, dimers can be formed either in the crystal form or in solution by lowering temperature.11,12 Previous studies revealed that if the molecules are favorably oriented in the crystal lattice with their nitroso groups properly oriented in the vicinity then they would readily form azodioxides in crystals.13,14 Such dimers can undergo photodissociation to monomers in crystals only at low temperatures.15 By warming, redimerization will occur, demonstrating the fundamental role of the 3D crystal packing on the stability of the azodioxide (O)NdN(O) bond. Similarly, dimerization is also possible in polymers.16 In the present work, we expand the current knowledge about the nitrosobenzene dimerization by exploring if it can occur on the 2D metal surface through formation of ordered self-assembled bilayers (SABs). SABs could be formed by vertical interactions of nitroso-groups exposed at the interface of selfassembled monolayer (SAM) and those present in solution via (O)NdN(O) bonds. Numerous reports found in literature are mainly concerned with lateral interactions of molecules adsorbed on a metal surface and their self-assembly into more or less ordered SAMs.17
24 There is a lack of knowledge about the systems where specific vertical intermolecular interactions are being used r 2011 American Chemical Society

for the design of ordered SABs. Apart from presenting an adequate molecular model that can be used for systematic design of both bilayers and multilayers, this class of compounds possesses very interesting properties with potential application in fields such as material science. As it is known from our previous work, nitrosobenzenes exhibit photochromic and thermochromic behavior in the solid state. That is, they act as chemical “off
on” switches that include bond breaking and bond formation between two nitrogen atoms.15 Therefore, layers formed via (O)NdN(O) bonds could in principle be triggered by external stimuli such as light or heat. Moreover, bilayers of nitrosobenzenes provide a convenient way for fabrication of systems with reactive sulfur groups on their interface that could interact with gold nanoparticles or vapor-deposited Au atoms, thus forming top metal contacts for molecular electronics.25
29 To investigate possible formation of ordered SABs, we have prepared nitrosobenzene derivatives 1 and 2 functionalized with sulfur headgroups for adsorption on an Au(111) surface (Chart 1). Whereas compound 2 was assembled on gold via a more traditional disulfide group, for assembling of compound 1, thiocyanates were used. It was recently shown that thiocyanates can be used as alternative precursors for thiolate assemblies on a gold surface with the important advantage of being much more stable regarding the susceptibility of thiols to oxidation.30,31 Both Received: July 11, 2011 Revised: September 7, 2011 Published: September 14, 2011 20267

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The Journal of Physical Chemistry C

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Chart 1. Molecular Structures of Nitrosobenzene Derivatives 1 and 2

compounds were isolated in dimeric (azodioxide) forms. The resulting mono- and bilayers were characterized by means of scanning tunneling microscopy (STM) and atomic force microscopy (AFM).

’ EXPERIMENTAL METHODS Synthesis. General Information. 1H and

13

C NMR spectra were recorded on a Bruker AV600 spectrometer. The chemical shifts (δ) are reported as ppm relative to SiMe4. IR spectra were recorded on a Bruker Equinox FT-IR spectrometer under 4 cm
1 resolution. The course of the reactions was monitored by TLC (Merck silica gel 60-F254-coated plates). Column chromatography was performed with silica gel 60 (Fluka, 0.063 to 0.200 mm). All chemicals were used as received from supplier (Aldrich). The solvents used were purified or dried according to literature procedures. All reactions were carried out under an inert atmosphere of nitrogen or argon. Nitrosobenzene derivatives were prepared from corresponding nitrobenzenes by standard procedures based on Zn reduction, followed by oxidation with FeCl3.32,33 Thiocyanates were prepared by simple routes from corresponding bromides.34 Compounds 2a, 2b, and 2c were prepared by adaptation of literature procedure previously described.35,36 3-Thiocyanatopropyl-4-nitrosobenzoate (1). 3-Bromopropyl-4-nitrobenzoate (1a). To a solution of 4-nitrobenzoic acid (6 mmol) in DMF (35 cm3) cooled to 0 °C, NaH (0.3 g, 60% dispersion in mineral oil) was added. The suspension was stirred at 0 °C for 1 h; then, 1,3-dibromopropane (36 mmol) was added. The mixture was stirred at room temperature for 24 h, quenched by addition of saturated NH4Cl solution, and extracted with several portions of EtOAc. Organic extracts were dried over Na2SO4. Evaporation of solvent under reduced pressure, followed by column chromatography (silica gel, starting with petrolether/DCM 3/1 increasing to DCM) yielded 3-bromopropyl4-nitrobenzoate as a yellow oil that solidified upon refrigeration (78%). δH(600 MHz; CDCl3; Me4Si): 2.4 (2H, m, CH2), 3.6 (2H, t, CH2
Br), 4.5 (2H, t, CH2
O), 8.2 (2H, m, Ph), 8.3 (2H, m, Ph). δC(150.90 MHz; CDCl3; Me4Si): 28.7 (CH2), 31.2 (CH2
Br), 63.2 (CH2
O), 123.1 (Ph), 130.3 (Ph), 134.9 (C
CdO), 150.2 (C
NO2), 163.9 (CdO). IR (NaCl) νmax/cm
1: 1724 (CdO), 1523 (NO2 asymmetric), 1348 (NO2 symmetric), 1274 (C
O). 3-Thiocyanatopropyl-4-nitrobenzoate (1b). The bromo group was converted to thiocyanate by reaction with KSCN. A mixture of 3-bromopropyl-4-nitrobenzoate (2.2 mmol) and KSCN (3.3 mmol) in ethanol (5 cm3) was refluxed for 4 h. After cooling to room temperature, the reaction mixture was filtered.

Filtrate was extracted with diethyl ether, and combined organic extracts were dried over Na2SO4. After evaporation of solvent, 3-thiocyanatoproply-4-nitrobenzoate was isolated as a yellow oil (80%). δH(600 MHz; CDCl3; Me4Si): 2.4 (2H, m, CH2), 3.2 (2H, t, CH2
SCN), 4.6 (2H, t, CH2
O), 8.2 (2H, m, Ph), 8.3 (2H, m, Ph). δC(150.90 MHz; CDCl3; Me4Si): 29.2 (CH2), 30.8 (CH2
SCN), 63.2 (CH2
O), 123.8 (Ph), 130.8 (Ph), 134.9 (C
CdO), 164.5 (CdO). IR (NaCl) νmax/cm
1: 2151 (SCN), 1725 (CdO), 1524 (NO2 asymmetric), 1350 (NO2 symmetric), 1275 (C
O). 3-Thiocyanatopropyl-4-nitrosobenzoate (1). 3-Thiocyanatopropyl-4-nitrobenzoate (1.75 mmol) was dissolved in 1,2-dimethoxyethane (10 cm3); then, a solution of NH4Cl (2.9 mmol) in H2O (2.3 cm3) was added. After that, finely powdered zinc (6.8 mmol) was added in small portions to the reaction mixture while stirring vigorously. After all of the zinc was added, the mixture was further stirred at room temperature for another 30 min, then filtered and washed with 1,2-dimethoxyethane. An ice-cold solution of FeCl3 (3.3 mmol) in H2O (6.4 cm3) and ethanol (2.9 cm3) was added to the cold filtrate (
5 °C). The mixture was extracted with EtOAc (three times), and combined organic extracts were dried over Na2SO4. After evaporation of solvent, a dark orange solid was obtained that was purified by column chromatography eluting with DCM to afford a product as a yellow solid with a yield of 23%. δH(600 MHz; CDCl3; Me4Si): 2.4 (2H, m, CH2), 3.1 (2H, t, CH2
SCN), 4.5 (2H, t, CH2
O), 7.9 (2H, m, Ph), 8.3 (2H, m, Ph). δC(150.90 MHz; CDCl3; Me4Si): 29.2 (CH2), 30.8 (CH2
SCN), 63.1 (CH2
O), 123.7 (Ph), 130.5 (Ph), 165.5 (CdO). IR (KBr) νmax/cm
1: 2153 (SCN), 1725 (CdO), 1278 (C
O), 1264 (ONdNO). Elemental analysis: Found: C, 53.03; H, 3.93; N, 11.04; O, 19.39; S, 12.61. Calculated for C11H10N2O3S: C, 52.79; H, 4.03; N, 11.19; O, 19.18; S, 12.81. Bis(4-nitrosophenylethyl)disulfide (2). 4-Nitrophenylethylthioacetate (2a). To a solution of 4-nitrophenylethyl-bromide (3.5 mmol) in ethanol (30 cm3) and tetrahydrofuran (20 cm3), potassium thioacetate was added (4.4 mmol). Reaction mixture was heated under reflux for 4 h, allowed to cool to room temperature, and then extracted with CH2Cl2 (80 cm3). Combined organic extracts were washed with two portions of 5% hydrochloride acid (50 cm3) and one portion of brine solution (80 cm3) and then dried over Na2SO4. Evaporation of solvent yielded 2a as a yellow oil (90%). δH(600 MHz; CDCl3; Me4Si): 2.3 (3H, s, CH3), 3.0 (2H, t, CH2
S), 3.2 (2H, t, CH2
Ph), 7.4 (2H, m, Ph), 8.1 (2H, m, Ph). δC(150.90 MHz; CDCl3; Me4Si): 29.7 (CH3), 30.7 (CH2), 35.7 (CH2), 123.8 (Ph), 129.5 (Ph), 147.5 (C-NO2), 195.2 (CdO). IR (NaCl) νmax/cm
1: 1677 (CdO), 1514 (NO2 asymmetric), 1346 (NO2 symmetric). 4-Nitrophenylethyl-thiol (2b). To a solution of 2a (1.7 mmol) in ethanol (20 cm3) and tetrahydrofuran (15 cm3), a solution of potassium hydroxide (7.3 mmol) in water (7.5 cm3) was added. The mixture was stirred for 3 h. Reaction mixture was washed two times with 5% hydrochloride acid (60 cm3) and after that with brine solution (60 cm3). Organic phase was dried over Na2SO4. Evaporation of solvent yielded 2b as a yellow oil (90%). δH(600 MHz; CDCl3; Me4Si): 1.4 (1H, t, SH), 2.9 (2H, q, CH2
SH), 3.1 (2H, t, CH2
Ph), 7.4 (2H, m, Ph), 8.2 (2H, m, Ph). δC(150.90 MHz; CDCl3; Me4Si): 25.3 (CH2), 39.8 (CH2), 123.8 (Ph), 129.6 (Ph), 147.3 (C-NO2). IR (NaCl) νmax/cm
1: 2572 (S
H), 1514 (NO2 asymmetric), 1344 (NO2 symmetric). Bis(4-nitrophenylethyl)disulfide (2c). Catalytic amount of sodium iodide and 30% hydrogen peroxide (120 μL) were added 20268

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The Journal of Physical Chemistry C to a solution of 2b (1.1 mmol) in ethyl acetate (3 cm3). After stirring reaction mixture for 30 min, a saturated solution of sodium thiosulfate (5 cm3) was added. The mixture was extracted three times with EtOAc (5 cm3). Combined organic extracts were washed with brine solution and dried over Na2SO4. Evaporation of solvent yielded 2c as a yellow solid (54%). δH(600 MHz; CDCl3; Me4Si): 2.9 (2H, t, CH2
S), 3.1 (2H, t, CH2
Ph), 7.4 (2H, m, Ph), 8.2 (2H, m, Ph). δC(150.90 MHz; CDCl3; Me4Si): 35.2 (CH2), 39.1 (CH2), 123.8 (Ph), 129.5 (Ph), 147.3 (C-NO2). IR (NaCl) νmax/cm
1: 1514 (NO2 asymmetric), 1344 (NO2 symmetric). Bis(4-nitrosophenylethyl)disulfide (2). 2c (0.55 mmol) was dissolved in 1,2-dimethoxyethane (3 cm3), and a solution of NH4Cl (0.93 mmol) in H2O (1 cm3) was added. After that finely powdered zinc (2.3 mmol) was added in small portions to the reaction mixture while stirring vigorously. After all of the zinc was added, the mixture was further stirred for 30 min at room temperature and then filtered. An ice-cold solution of FeCl3 (1 mmol) in H2O (2 cm3) and ethanol (1 cm3) was added to the cold filtrate (
5 °C). The mixture was extracted with EtOAc (two times) and combined organic extracts were dried over Na2SO4. After evaporation of the solvent a brown solid was obtained which was purified by column chromatography eluting with DCM. Compound 2 was isolated as a yellow solid with a yield of 35%. δH(600 MHz; CDCl3; Me4Si): 3.0 (2H, t, CH2
S), 3.1 (2H, t, CH2
Ph), 7.4 (2H, m, Ph), 7.8 (2H, m, Ph). δC(150.90 MHz; CDCl3; Me4Si): 36.7 (CH2), 37.2 (CH2), 120.7 (Ph), 128.7 (Ph), 163.1 (C-NO). IR (NaCl) νmax/cm
1: 1260 (ONdNO). Elemental analysis: Found: C, 57.49; H, 4.96; N, 8.38; O, 9.70; S, 19.47. Calculated for C12H17NO2S: C, 57.81; H, 4.85; N, 8.43; O, 9.62; S, 19.29. Preparation of Self-Assembled Layers. Previously flameannealed37 Au(111)/mica substrates (Molecular Imaging) were immersed in a 1 mM solution of compounds 1 and 2 in a 1:1 mixture of ethanol and chloroform at room temperature for ∼24 h. After being removed from solution, Au(111)/mica substrates samples were rinsed with a copious amount of solvent and dried in the stream of argon. Surface Characterization Methods. Scanning Tunneling Microscopy (STM). The measurements were carried out in the constantcurrent mode under ambient conditions at room temperature using a home-built STM.38 The STM tips were prepared by mechanically cutting a Pt/Ir wire (0.25 mm diameter). Lateral STM distances were calibrated by precision of ∼10%. The obtained data were analyzed using the WSxM processing software.39 Atomic Force Microscopy (AFM). AFM imaging was performed using a Multimode Scanning Probe Microscope with Nanoscope IIIa controller (Veeco Instruments, Santa Barbara, CA) and a vertical engagement (JV) 125 μm scanner. Measurements were done in air at ambient temperature and humidity of 50
60%, using tapping mode with silicon tips (TESP, Veeco, nom. freq. 320 kHz, nom. spring constant of 42 N/m). The linear scanning rate was optimized between 1.0 and 1.5 Hz for the tapping mode and between 0.5 and 1.5 Hz with scan resolution of 512 samples per line. Processing and analysis of images were carried out using the NanoScope software (Digital Instruments, version V614r1) and WSxM processing software.39

’ RESULTS Figures 1a,b show low-resolution STM images of adlayers of compound 1 on an Au(111) surface. The morphology of the

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Figure 1. (a) 117 nm  117 nm and (b) 57 nm  57 nm large-scale STM images of adlayers of compound 1 on an Au(111) surface. The scale bars are 11 (a) and 23 nm (b). (c) Height profile along line A shown in panel b.

Figure 2. (a) 19 nm  19 nm and (b) 16 nm  16 nm high-resolution images of mono- and bilayer domains of compound 1 formed on an Au(111) surface. The white arrow in part a marks individual molecules of dimers and their nucleation into 2D crystals. The scale bars are 3.8 (a) and 3.2 nm (b).

surface is characterized by the presence of separate domains that differ in heights. Three different domains could be distinguished as depicted in Figure 1b. These are a lower appearing domain labeled as 1, presumably corresponding to monomolecular layer (SAM) and two higher appearing domains labeled as 2 and 20 . The appearance of higher domains suggested that in addition to SAM formation molecules of nitrosobenzene derivative 1 probably tend to dimerize on an Au(111) surface and form bilayers. However, domains 2 and 20 evidently differ in height, and according to the height profile shown in Figure 1c, this difference is ∼2.5 Å. The latter value agrees well with that of monatomic steps on Au(111) (2.4 Å), thus implying that the formation of the second, higher domain 20 could be related to Au adatom islands. In literature, the formation of adatom islands is rationalized by a lower mobility of Au atoms that appear during the chemisorption 20269

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Figure 3. (a) Large-scale (117 nm  117 nm) STM image of adlayers of compound 2 on Au(111) surface. The scale bar is 23 nm. (b) Fourier filtered high-resolution STM image (9.5 nm  9.5 nm) of SAM of 2 on an Au(111) surface. The scale bar is 1.9 nm. The white arrow in part a marks the presence of higher appearing domains.

of thiols on Au(111) surfaces. The mobility reflects the diffusion barrier, which is governed by S
Au binding at the interface.40 This will be discussed in more detail later in the text. From high-resolution STM images shown in Figure 2 it is observed that both lower appearing domains representing monolayer regions and higher domains attributed to bilayers are molecularly resolved. Moreover, organization of molecules within SAM regions seems to be different in comparison with those assigned to SABs. Interestingly, in the middle of the SAM domain, isolated molecules of dimers and their nucleation into the 2D crystals is visible (white arrow in Figure 2a). The precise analysis revealed that within SAMs molecules are organized into a hexagonal structure with two adjacent molecules spaced at the distances of 0.36 ( 0.03 nm. Upon closer inspection, we could observe a Moire-type superstructure characterized by an additional height variation of molecular layer (Figures 2b and Figure S1 of the Supporting Information). The measured corrugation and periodicity were about 0.02 and 1.5 nm, respectively. These periodic modulations could be attributed to a structural mismatch between the molecular layer of compound 1 and the underlying Au substrate. It is known from the literature that the geometric height difference between different adsorption sites of sulfur atoms on an Au(111) lattice (on-top, three-hollow, and bridge) is ∼0.02 nm,41 which is in agreement with the apparent corrugation. The structural mismatch between the registry of molecules 1 and the lattice of the Au(111) substrate could be induced by dominant intermolecular interactions of nitrosobenzene residues within the molecular layer, which then may lead to occupation of different adsorption sites of the Au(111) √ lattice by S atoms. Value of 1.5 nm corresponds very well to 3 3 times that of the Au(111) lattice (a = 0.29 nm). Therefore, we propose that the of nitrosobenzene derivative 1 on Au(111) exhibits a √ SAM √ 3 3  3 3 structure. Detailed analysis of molecularly resolved images of bilayer domains enabled us to determine specifics of molecular ordering within those areas. The arrangement of molecules within domains 2 and 20 is similar but different in comparison with domain 1 representing SAM. This further supports our assumption that regions 2 and 20 in fact represent bilayers of compound 1. The structure of SABs regions seems to be somewhat more complex in comparison with the structure of SAM of the same compound. The molecules within these regions also pack into a hexagonally ordered structure with the distance between adjacent molecules of 0.40 ( 0.02 nm. In addition, we could again observe the

Figure 4. AFM image (500 nm  500 nm) of adlayers of compound 2 on an Au(111) surface. The scale bar is 100 nm.

superstructure but with somewhat lower periodicity than in the case of SAMs. For every ∼0.7 nm there is a variation in height probably caused by molecules occupying different adsorption sites on Au surface. It is interesting that the second layer shows better ordering with respect to the first one. A possible explanation for this could be in the “softer background” in comparison with the gold surface on which the first layer has been formed. Large-scale STM images of adlayers of compound 2 on Au(111) surface show similar morphology, as observed for compound 1 (Figure 3). The Au(111) terraces separated by monatomic steps are visible together with the appearance of higher domains (white arrow in Figure 3a). The presence of higher appearing domains indicated that molecules of nitrosobenzene derivative 2 also form bilayers on an Au(111) surface. High-resolution STM images, as the one displayed in Figure 3b, show that molecules of nitrosobenzene derivative 2 form wellordered SAMs on an Au(111) surface. The precise analysis revealed that, similarly as in the case of compound 1, they show hexagonal arrangement with two adjacent molecules separated at a distance of 0.37 ( 0.03 nm. The Moire-type corrugation with height variations of 0.02 nm, assigned to structural mismatch between the registry of molecules and the lattice of the Au(111) substrate, is also visible here (Figures 3b and Figure S2 of the Supporting Information). √ √ The measured periodicity of 1.5 nm highlights the 3 3  3 3 molecular arrangement. Unfortunately, in this case, we were not able to achieve molecular resolution within the regions attributed to bilayer films. The impression is that going down to a smaller scale STM tip simply removes the more weakly bonded second layer. The invasive nature of tip
sample interactions is well-described in literature.42,43 AFM further confirmed formation of SABs of compounds 1 and 2. Figure 4 shows typical AFM height image for molecules of nitrosobenzene-derivative 2 adsorbed on an Au(111) surface. The morphology of the surface is characterized by the presence of Au(111) terraces covered by close-packed islands. Height profile analysis of the AFM images of molecular adlayers of compounds 1 and 2 revealed that the heights of islands, standing for the height differences between monolayers and bilayers, are approximately 0.9 and 0.8 nm, respectively. When compared with the lengths of molecules, which are 1.1 (compound 1) and 20270

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The Journal of Physical Chemistry C 0.9 nm (compound 2) as determined from molecular models, the measured values support the formation of bilayers in both cases.

’ DISCUSSION In this study, we have investigated whether nitrosobenzeneterminated molecules adsorbed on an Au(111) surface could dimerize through vertical interactions of exposed nitroso-groups and those present in solution. In that way, ordered SABs could be formed. The results of STM and AFM studies on molecular adlayers of nitrosobenzene derivatives 1 and 2 on Au(111) surfaces revealed several points. First of all, both compounds form SAMs of very good quality with crystalline packing over an extended area. It

Figure 5. Proposed structural model for the SAM of nitrosobenzene derivatives 1 and 2. Yellow circles represent Au atoms and red circles represent S atoms in molecules 1 and 2 occupying different adsorption sites on an Au(111) surface.

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was found that in both cases molecules are arranged into a hexagonally ordered structure with a distance between adjacent molecules of ∼0.37 nm. This value is somewhat smaller than expected for SAMs of aromatic thiols. Nevertheless, it is known from literature that because of specific intermolecular interactions between terminal groups, SAMs with unique molecular arrangement can be formed.44
46 For example, it was found that spacing of molecules within SAMs of alkyl thiols with azobenzene terminal groups is also 0.37 nm.46 In addition, a similar molecular arrangement as in our case was found for SAM of 1-mercaptopyrene on Au(111) surface.47 Upon closer inspection, we could observe a hexagonal Moiretype superstructure with corrugation of ∼0.02 nm. The appearance of the superstructure was attributed to structural mismatch between the ideal structure for nitrosobenzenes 1 and 2 monolayer and the underlying Au substrate. The measured height variations observed by STM are in good agreement with geometric height differences between on-top, three-hollow, and bridge sites known from literature.41 A possible explanation of such a mismatch between the registry of adsorbed molecules and the lattice of the Au(111) substrate may be in dominant intermolecular interactions of nitrosobenzene residues leading to molecules occupying different adsorption sites. The measured periodicity of the √ superstructure is 1.5 nm, which is in good agreement with 3 3 times that of the Au(111) √ lattice. √Accordingly, a structural model corresponding to 3 3  3 3 molecular arrangements for SAMs of molecules 1 and 2 is proposed, as shown in Figure 5. It is assumed that the variation of STM contrast within the Moire cell relates to different adsorption geometry of individual molecules within the cell. Interestingly, in both cases, the presence of higher appearing domains attributed to bilayers was detected. Whereas for compound 2 such islands could be observed only on large-scale images, for compound 1, molecularly resolved images were obtained, revealing organization of molecules within those regions. Detailed analysis showed that within SABs molecules pack into a

Figure 6. Proposed model of molecular adlayers of compound 1 on an Au(111) surface with the appearance of mono- (SAM) and bilayer (SAB) domains together with Au adatom islands covered by bilayer. 20271

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The Journal of Physical Chemistry C hexagonally ordered structure at intermolecular spacing of ∼0.4 nm. We could also observe a hexagonal superstructure with periodicity of ∼0.7 nm. It is also interesting to find two higher appearing domains that display similar molecular organization but differ in heights of ∼0.25 nm, present on an Au(111) surface. The measured difference in height is in very good agreement with the Au(111) monatomic step, which led us to conclude that these higher islands are in fact Au adatoms covered by bilayer of compound 1. Moreover, such a conclusion is additionally supported by the hexagonal shape of higher islands (domain 20 , Figure 2a). It is not quite clear why adatoms appear only in this specific case. Their formation is associated with lifting of Au(111) reconstruction during adsorption.40 Depending on the mobility of released extra Au atoms they can diffuse rapidly and merge at neighboring step edges, in which case no Au adatom islands are visible, or, as a result of lower mobility, Au adatoms islands can be formed. For some reason the diffusion barrier of surface Au atoms is higher in the presence of a bilayer than in the case of a monolayer where similar features were not observed. Interactions between molecules and a surface are responsible for the mobility of Au surface atoms. However, it is plausible to think that there is no significant difference in an S
Au bond at monolayer domain in comparison with a bilayer domain. Presumably, lower mobility of Au atoms covered by bilayer could be a consequence of specific intermolecular interactions that prevent their diffusion. In any case, further studies are necessary to resolve this issue completely. Figure 6 depicts a proposed model of molecular adlayers of compound 1 on an Au(111) surface with the appearance of SAM and SAB domains together with Au adatom islands covered by SAB.

’ CONCLUSIONS In summary, we have investigated the ability of nitrosobenzene derivatives to dimerize on an Au(111) surface by forming ordered SABs. Bilayers can be formed by vertical interactions of nitroso-groups exposed at the interface of SAM and those present in solution via (O)NdN(O) bonds. The high-resolution STM images revealed that nitrosobenzene derivatives form wellordered SAMs with molecules arranged into hexagonal structures. A hexagonal Moire-type superstructure with a periodicity of 1.5 √ nm and √a corrugation of ∼0.02 nm was observed indicating a 3 3  3 3 molecular arrangement. The appearance of the superstructure was attributed to a structural mismatch between the ideal structure for the nitrosobenzene monolayer and the underlying Au substrate. Besides ordered SAM regions, STM images revealed the presence of additional, higher appearing domains that were attributed to SABs. Molecularly resolved images of SABs regions of compound 1 revealed that within bilayers molecules also pack into hexagonally ordered structures. The distances between adjacent molecules are ∼0.4 nm, whereas the periodicity of the observed hexagonal superstructure is ∼0.7 nm. The formation of SABs was further confirmed by AFM showing that the height difference between monolayers and bilayers is in very good agreement with the lengths of molecules as determined from molecular models. The results presented in this work provide new insights into nitrosobenzenes dimerization (a sort of 2D crystallization) which are very important contributions to the chemistry of this class of compounds. We have shown that nitrosobenzene dimerization is also possible at the metal
solution interface. Moreover, it is evident that nitrosobenzenes present good model systems that

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can be used for the controlled design and study of ordered SABs on metal surfaces. Such systems could provide numerous applications, one of which is fabrication of metallic layers through the presence of reactive sulfur groups on their interface.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional STM images of adlayers of compounds 1 and schematic illustration of SAM of compound 2. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (H.V.); [email protected] (I.B.). Phone: (+) 385-1-4606416. Fax: (+) 385-1-4606401.

’ ACKNOWLEDGMENT We thank Ida Dela
c for the assistance with STM measurements. We gratefully acknowledge the financial support to this work from the Ministry of Science, Education, and Sports of the Republic of Croatia, nos. 119-1191342-1334, 098-09829342744, and 035-0352828-2840. M.K. thanks the Alexander von Humboldt Foundation for supporting development of STM setup used in this work. ’ REFERENCES (1) Fletcher, D. A.; Gowenlock, B. G.; Orrell, K. G. J. Chem. Soc., Perkin Trans. 2 1998, 797. (2) Fletcher, D. A.; Gowenlock, B. G.; Orrell, K. G.; Sik, V. Magn. Reson. Chem. 1995, 33, 561. (3) Fletcher, D. A.; Gowenlock, B. G.; Orrell, K. G.; Sik, V.; Hibbs, D. E.; Hursthouse, M. B.; Malik, A. K. M. J. Chem. Soc., Perkin Trans. 2 1996, 191. (4) Gowenlock, B. G.; L€utke, W. Quart. Rev. 1958, 12, 321. (5) Greene, F. D.; Gilbert, K. E. J. Org. Chem. 1975, 40, 1409. (6) Greer, M. L.; Sarker, H.; Medicino, M. E.; Blackstock, S. C. J. Am. Chem. Soc. 1995, 117, 10460. (7) Orrell, K. G.; Sik, V.; Stephenson, D. Magn. Reson. Chem. 1987, 25, 1007. (8) Orrell, K. G.; Stephenson, D.; Rault, T. Magn. Reson. Chem. 1989, 27, 368. (9) Snyder, J. P.; Heyman, M. H.; Suciu, E. J. Org. Chem. 1975, 40, 1395. (10) Wajer, T. A. J.; De Boer, T. J. Recueil 1972, 91, 565. (11) Azoulay, M.; Fischer, E. J. Chem. Soc., Perkin Trans. 2 1982, 637. (12) Fletcher, D. A.; Gowenlock, B. G.; Orrell, K. G. J. Chem. Soc., Perkin Trans. 2 1997, 2201. (13) Van
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