Nitro-Substituted Arene Sulfenyl Chlorides as Precursors to the

Article pubs.acs.org/Langmuir

Nitro-Substituted Arene Sulfenyl Chlorides as Precursors to the Formation of Aromatic SAMs Hamida Muhammad, Kallum M. Koczkur, Annia H. Kycia, and Abdelaziz Houmam* Electrochemical Technology Centre, Department of Chemistry, University of Guelph, Guelph, Ontario, Canada N1G 2W1 S Supporting Information *

ABSTRACT: The formation of aromatic SAMs on Au(111) using three nitro-substituted arene sulfenyl chlorides (4-nitrophenyl sulfenyl chloride (1), 2-nitrophenyl sulfenyl chloride (2), and 2,4-dinitrophenyl sulfenyl chloride (3)) is studied. The formation of SAMs and their quality are investigated as a function of the position of the nitro substituent(s) on the aromatic ring. The modified surfaces are characterized by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), polarization modulation infrared reflection absorption spectroscopy (PMIRRAS), and cyclic voltammetry (CV). The results show that all three compounds are deposited on Au within very short times. The corresponding coverages are determined using CV. However, only compound 1 forms stable, long-range, well-ordered SAMs. The 4-nitrophenyl thiolate is adsorbed nearly vertically on the Au surface. Compounds 2 and 3 both form lower-quality SAMs where the adsorbed nitro-phenyl thiolates are more tilted. These SAMs are less stable than the ones obtained with the 4-nitrosubsituted precursor and decompose with time, leaving only sulfur on the gold surface.

I. INTRODUCTION Self-assembled monolayers (SAMs) on solid surfaces have been intensively investigated in the last three decades.1,2 This interest stems from their usefulness in tailoring surface properties offering a wide range of potential applications.1,2 Although most of the studies have involved aliphatic SAMs as a result of their ease of formation, aromatic ones are attracting increasing attention because of their unique electronic properties3−15 and their demonstrated relevance in important areas including lithography, 16−22 nanomaterials, 23 and molecular electronics.24−26 Although various precursors have successfully been used to form SAMs on solid surfaces, the archetypal example is that based on the affinity between sulfur-containing precursors and gold surfaces. Thiols, sulfides, and disulfides are the most used precursors in the reported studies.27−30 Thiosulfates,31,32 protected thiols,33,34 and organic thiocyanates35−38 have also been used as alternatives to overcome some of the observed limitations of the traditional precursors. Despite the extensive use of these precursors to obtain high-quality aliphatic SAMs, their ability to form well-ordered aromatic SAMs is very limited.39−42 Aromatic SAMs are much more difficult to obtain, and long-range ordering is believed to suffer from a mismatch between the gold lattice and the organic adsorbate lattice. A number of alternatives have been used to improve the quality of aromatic SAMs, including the displacement of another adsorbate41 and postdeposition annealing.42 A widely used approach that negatively influences the electronic properties of aromatic SAMs is that consisting of using alkane spacers.39,40 Aromatic thiocyanates have also been used as precursors.35−38 The quality of the obtained aromatic SAMs have been shown to © 2012 American Chemical Society

be very sensitive to the purity of the precursors and to the experimental conditions.37,38 Selenol-based precursors have been shown to lead to more ordered and denser aromatic SAMs than sulfur-based ones, indicating that the lower degree of molecular interaction for aromatic groups compared to that for aliphatic chains is only partially responsible for the usual poor quality of aromatic films.43−45 The discovery of new precursors for the formation of aromatic SAMs and the investigation of the parameters affecting their quality are important from both a practical and a fundamental point of view. We have recently introduced 4nitrophenyl sulfenyl chloride as a new precursor for the successful formation of long-range, well-ordered aromatic SAMs.46,47 The effectiveness of arene sulfenyl chlorides, which are structurally similar to arene thiocyanates, is, in our opinion, due to two main characteristics.46,47 The first parameter is the ease of their dissociative reduction leading to the cleavage of the S−Cl bond.48−50 The reduction of the 4nitrophenyl sulfenyl chloride, for example, takes place at −0.05 V versus SCE.48−50 The second parameter is the weakness of the S−Cl chemical bond. For the 4-nitrophenyl sulfenyl chloride, the S−Cl bond dissociation is about 48 kcal/ mol.48−50 The importance of these two parameters can be understood by considering the proposed reductive adsorption mechanism for the sulfenyl chlorides (Scheme 1), which is similar in fact to the one suggested for thiocyanates.35−38,46,47 Received: August 22, 2012 Revised: October 15, 2012 Published: October 18, 2012 15853

dx.doi.org/10.1021/la303395e | Langmuir 2012, 28, 15853−15863

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(EtOH, anhydrous, Commercial Alcohols), and methanol (MeOH, HPLC grade from Caledon) were used as received. Tetrahydrofuran (THF, HPLC grade from Caledon) was dried with sodium metal and distilled under argon just prior to use. For electrochemical studies, potassium hydroxide (KOH 99.99%, semiconductor grade from Sigma-Aldrich) and ultrapure water (Millipore, 18.2 MΩ) were used. Carbon tetrachloride (CCl4, certified ACS grade from Fisher Scientific, dried over molecular sieves) and potassium bromide (KBr, >99% FTIR grade, Sigma-Aldrich) were used as received. Au(111) Preparation. Au from a gold wire (0.762 mm diameter, Premion , 99.999%, Alfa Aesar) was evaporated onto freshly cleaved mica (V1 grade, Ted Pella) in a custom-built evaporation system consisting of a Kurt J. Lesker bell jar and a Varian turbo pump operating at a pressure of 1 × 10−7 Torr. The base plate holding the mica was held at 600 K for 12 h prior to depositing the gold and was kept at 600 K for an additional 3 h after the gold was deposited to help ensure the formation of high-quality Au(111) surfaces. Upon removal from the evaporation chamber, the gold samples were cleaned with chromosulfuric acid, (Chromerge , Bel-Art), anhydrous ethanol (Commercial Alcohols), and ultrapure water (Millipore, 18.2 MΩ) and then dried under a stream of nitrogen and stored in a desiccator until use. Au(111) substrates are routinely imaged using STM before modification to assess their quality. Modification of Au(111) for STM, XPS, and PMIRRAS. Au(111) substrates were immersed in 1 mM solutions of nitrophenyl sulfenyl chlorides in DMF/EtOH (1:9) for compounds 1 and 2 and DMF/EtOH (1:1) for compound 3. After thorough rinsing with a binary solvent mixture, the modified substrate was dried under a gentle stream of nitrogen. The solvent composition was 1:1 for 2,4dinitrophenyl sulfenyl chloride because of the difficulty with respect to solubility. STM. A 5500 SPM system (Agilent) was used for STM imaging. Images were obtained in air using a tungsten tip (0.25 mm diameter, 99.95%, Alfa Aesar) electrochemically etched in 3 M NaOH (99.99%, semiconductor grade, Sigma-Aldrich). STM images were obtained using constant-current mode with a positive bias applied to the tip. XPS. X-ray photoelectron spectroscopy (XPS) studies were conducted in an ultrahigh vacuum (UHV) system (Omicron) operating at a base pressure of 5 × 10−11 Torr. X-rays were generated from an Al Kα source (1486.6 eV). The system contains a hemispherical sector analyzer coupled to a multichannel electron detector. The analyzer was operated in constant analyzer energy (CAE) mode with a pass energy of 20 eV. A takeoff angle of 75° was used for all samples. All XPS spectra presented in this work were referenced to the Au 4f7/2 peak at 84.0 eV and fitted using XPSPEAK 4.1 software. PMIRRAS. IR measurements were obtained using a commercial FTIR system (Thermo Scientific Nicolet 8700) equipped with an external tabletop optical mount, a photoelastic modulator (PEM) (Hinds Instruments PM-90 with a II/ZS50 ZnSe 50 kHz optical head, Hillsboro, OR), and a demodulator (GWC Instruments synchronous sampling demodulator, Madison, WI). Dry air (Balston 74-45) was purged through all atmospheric regions of the system. The angle of incidence of the IR beam was optimized to ∼80° with respect to the surface normal, and PEM was set for half-wave retardation at 1300 cm−1 (for the NO2 stretch). The spectra were obtained using a liquidnitrogen-cooled MCT-A detector with a resolution of 4.0 cm−1, and 10 000 scans were recorded. The output signal was the relative differential reflectivity (ΔR/Rav). (ΔR/Rav = 2(Rp − Rs)/(Rp + Rs)). (Rp and Rs are the reflection coefficients for p and s polarization, respectively.) The spectra were referenced against bare gold, and a spline interpolation technique was used for background correction. Solution IR. Transmittance spectra of the three investigated compounds (1−3) were recorded using a commercial FTIR system equipped with a DTGS detector (Thermo Scientific, Madison MI). The transmittance spectra were collected using a home-designed flow cell between two BaF2 windows and a Teflon spacer with a thickness of ∼25 μm. One hundred scans were recorded for each spectrum with a resolution of 2 cm−1. Spectra were recorded for 4-nitrophenyl sulfenyl chloride (0.011 M), 2-nitrophenyl sulfenyl chloride (0.017

Scheme 1. Proposed Mechanism for the Deposition of Arene Sulfenyl Chlorides

The quality of aliphatic SAMs is well known to depend on the structure of the employed precursors.1,2 Attention has been given to a number of characteristics including the headgroup− substrate bonding and the alkyl chain composition, orientation, and conformation.51 These have been shown to depend mainly on the adsorption time, the chain length, the temperature, the substrate quality, and the nature of the terminal group.1,2 The quality of SAMs has been shown to decrease with the introduction of hydrophilic/metalophilic groups (such as −SH, −COOH, −OH, and −NH2)52 compared to that of a −CH3 terminal group.1,2 Here we report on the investigation of three commercially available nitro-substituted arene sulfenyl chlorides. These include the 4-nitro (1), the 2-nitro (2), and the 2,4-dinitro (3) phenyl sulfenyl chlorides (Chart 1). The first objective is to Chart 1. Structures of Investigated Nitro-Substituted Arene Sulfenyl Chlorides (1−3)

test the efficiency of the sulfenyl chloride (−SCl) functional group in attaching organic molecules to solid surfaces. A second objective is to investigate the effect of the position of the NO2 group on the quality of the formed SAMs. The presence of a nitro group at the ortho position may be hindering, but it has also been shown that this group can effectively interact with the gold surface.53−58 Keep in mind that 4-nitrophenyl sulfenyl chloride has led, under certain conditions, to the formation of a double-row structure with a distance between double rows of up to 12 Å that is in principle large enough to accommodate a phenyl ring with a NO2 substituent at the para position. The present investigation will be conducted under these conditions. Another important characteristic of the ortho nitro-substituted sulfenyl chlorides and other related compounds is the welldocumented through-space S···O interactions.50 It would be interesting to see if this interaction affects SAM formation. It is important to note that most of the investigations regarding terminal groups and substituent effects have been performed using aliphatic SAMs. This study will provide insight into various aspects in relation to aromatic SAM formation including the interaction of the S−Cl group with the Au surface as well as the adsorbate/substrate and adsorbate/ adsorbate interactions that determine the quality of the produced SAMs.

2. EXPERIMENTAL SECTION Chemicals and Substrates. 4-Nitrophenyl sulfenyl chloride (Sigma-Aldrich), 2-nitrophenyl sulfenyl chloride (Lancaster), and 2,4-dinitrophenyl sulfenyl chloride (Lancaster) were purchased from Aldrich and were used without further purification. N,N-Dimethyl formamide (DMF, certified ACS from Fisher Scientific), ethanol 15854


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Figure 1. XPS signals in the S 2p, C 1s, N 1s, and O 1s regions for SAMs on Au prepared using (a1−d1) 4-nitrophenyl sulfenyl chloride, (a2−d2) 2nitrophenyl sulfenyl chloride, and (a3−d3) 2,4-dinitrophenyl sulfenyl chloride. M), and 2,4-dinitrophenyl sulfenyl chloride (0.014 M) solutions in CCl4. Cyclic Voltammetry. Electrochemical measurements were conducted in a three-electrode glass cell thermostatted at 25 °C under dry nitrogen. A 2-mm-diameter solid-gold electrode (Ω Metrohm) was used. The electrode was carefully polished and ultrasonically rinsed with ethanol and then electrochemically cleaned in a 0.5 M KOH aqueous solution. The reference electrode was an SCE. The counter electrode was a platinum wire. The electrochemical instrument used was an Autolab PGSTAT30 (Eco Chemie).

The analysis of the XPS data shows that the signals are more pronounced for the 4-nitrophenyl sulfenyl chloride-modified substrate. The S 2p region shows an unresolved doublet at binding energies of 162.0 and 163.2 eV for S 2p3/2 and S 2p1/2, respectively, which corresponds to the gold-bound sulfur (Figure 1a1). The C 1s signal is observed at 285.5 eV for the C−N and C−S carbons and at 284.1 eV for the aromatic carbons (Figure 1b1). The N 1s signal is observed at 405.0 eV and corresponds to the nitro group (Figure 1c1). A peak at 399 eV was occasionally seen, accompanied by a decrease or sometimes a complete loss of the peak at 405 eV. This suggests a reduction of the nitro group to the amino group as has been previously reported for SAMs of nitroaromatic thiolates formed from their corresponding thiols59−64 and thioacetates.65,66 A signal is also observed at 531.7 eV and corresponds to the O 1s (Figure 1d1) as expected. The 2-nitrophenyl sulphenyl chloride-modified substrate also showed signals corresponding to C, N, O, and S. A very important difference concerns the S 2p signal that can now be fitted by up to three doublets with the S 2p3/2 components located at 162.4, 163.4, and 164.4 eV (Figure 1a2). Similar signals were observed in previous studies when only S was deposited on Au or when nonbonded sulfur-containing species were present.67−69 In these cases, the observed signals were assigned to chemisorbed monatomic sulfur, gold−thiolate bonded species, and weakly bound, unbound, or polymeric S on the surface.67−69 This is the first indication of the potential presence of a nonadsorbed precursor or sulfur on the surface. Because no signal corresponding to Cl is observed, this data may indicate some decomposition of the initially deposited nitro-substituted arene thiolate. This will be further investigated below. Signals for C 1s are clearly observed at 285.1 eV for the C−N and C−S carbons and at 284.1 eV for the aromatic carbons (Figure 1b2). In the N 1s region, a signal is observed at 399.2 eV in addition to the signal observed at 405.0 eV (Figure 1c2). All signals are less intense than the one observed with the 4nitrophenyl sulfenyl chloride precursor. As discussed earlier, the

3. RESULTS AND DISCUSSION Au(111) substrates were modified using compounds 1−3 in DMF/EtOH mixtures for 24 h as described in the Experimental Section. After thorough rinsing, the modified substrates were characterized by a series of techniques including XPS, PMIRRAS, and STM. Polycrystalline gold electrodes were modified for various times from 1 min to 18 h and were also characterized using stripping cyclic voltammetry. XPS Results. The X-ray photoelectron results are shown in Figure 1, and the signals assignments are reported in Table 1. Signals corresponding to the main elements S, C, N, and O are clearly observed for all three substrates. No peak was observed in the Cl region, indicating the adsorption of the nitrosubstituted phenyl thiolates moieties through the dissociation of the S−Cl chemical bond as previously suggested.46,47 Table 1. XPS Peak Positions in Electronvolts for SAMs on Au(111) Obtained Using the Nitro-Substituted Arene Sulfenyl Chlorides assignment

compound 1

compound 2

compound 3

S 2p (S1) (S2) (S3) N 1s O 1s C 1s

2p3/2, 2p1/2

2p3/2, 2p3/2 161.2, 162.4 162.2, 163.4 163.2, 164.4 405.0, 399.2 531.8, 529.8 285.1, 284.1

2p3/2, 2p1/2 161.2, 162.4 162.2, 163.4 163.2, 164.4 405.0, 399.2 531.8, 529.8 285.1, 284.1

162.0, 163.2 405.0 531.7, 529.5 285.5, 284.1

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Figure 2. Solution (in CCl4) and SAM IR spectra of (a) 4-nitrophenyl sulfenyl chloride, (b) 2-nitrophenyl sulfenyl chloride, and (c) 2,4dinitrophenyl sulfenyl chloride.

signal located at 529.8 eV than does the substrate modified with the 4-nitrophenyl sulfenyl chloride (Figure 1d1). The 2,4-dinitrophenyl sulfenyl chloride-modified substrate shows signals very similar to the ones observed for the 2nitrophenyl-substituted precursor. S 2p, N 1s, and O 1s all show multiple contributions (Figure 1a3−d3). The S signal again indicates the presence of species resulting from the decomposition of the deposited 2,4-dinitrophenyl thiolate on the Au(111) surface. This data suggests that the NO2 group at the ortho position seems to hinder the adsorption of the thiolate moiety, leading to its decomposition. It is important to note the absence of any Cl signal, indicating the fact that the observed signals all result from the

second signal is assigned to the nitrogen of the NH2 group obtained by the reduction of the NO2 group. For this compound, this signal may also result from an interaction between the NO2 group (located here at the ortho position) and the substrate. Such an interaction has been studied before using nitrobenzene, and it has been shown that on Cu(100),70 Ni,71 and Fe71 the initial precursor partially decomposed to nitrosobenzene, showing N 1s signals similar to the ones observed here in addition to a more pronounced shoulder for the O 1s signal at 529.9 eV, assigned to the chemisorbed oxygen resulting from the reduction of the nitro group. The O 1s signal (Figure 1d2) indeed makes a larger contribution to the 15856


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Table 3. CC−δCH and δCH IR Data for Compounds 1−3 in Solution and on Au(111)a

nitro-substituted phenyl thiolates initially adsorbed through the dissociation of the S−Cl bond. PMIRRAS Results. PMIRRAS was also used to characterize the produced SAMs and to gain insight into the spatial orientation of the adsorbed thiolates on the Au surface through a comparison of IR measurements with bulk samples.72−74 For a molecule adsorbed on a metal surface, only vibrational modes with a component of the IR vector perpendicular to the surface are probed (surface selection rule).72−74 IR measurements were performed for SAMs of the three investigated compounds along with measurements of the bulk samples both in KBr (Supporting Information) and in CCl4. To identify the main vibrational modes and the associated vectors, the structures of the three investigated nitro-substituted arene sulfenyl chlorides were first optimized using Gaussian 03 at the B3LYP level.75 The optimized structures show planar nitrophenyl moieties with the nitro groups in the plane of the phenyl ring in each case. No imaginary vibrations were observed. The optimized structures and the calculated IR spectra were used in determining the main vibrational modes discussed below. These vibrations modes are in accordance with the previously reported experimental and theoretical ones in various studies of molecules with similar nitrophenyl moieties.76,77 The IR spectra recorded for the three nitro-substituted arene sulfenyl chlorides, 1−3, both in solution (CCl4) and after deposition on Au(111) surfaces, are shown in Figure 2. The signals are reported in Tables 2 and 3 along with their assignments based on previous theoretical and experimental IR studies.76,77

assignment rCC−δCH (a) rCC−δCH (b) δCH (a)

compound 1 compound 2 compound 3 compound 1 compound 2 compound 3 compound 1 compound 2 compound 3

transition dipole direction

solid (KBr)

NO2 r+ 1341 1334 1347 NO2 r− x 1509 x, z 1507 x + x, z 1522 NO2 Ir (IR+/IR−) z/x 1.1 x, z/x, z 1.0 z + x, z/x + x, z 1.1 z x, z z + x, z

solution (CCl4)

on Au(111)

1342 1329 1342

1342 1327b 1346

1525 1520 1531

1518 1524 1531

1.0 0.8 0.8

5.1

1570 1468

compound 2

compound 3

1581 1564

1583

IR frequencies in cm−1 for rCC−δCH (vibrations corresponding to the superposition of CC stretching and CH in-plane bending of the phenyl ring) and δCH (vibrations corresponding to CH bending of the phenyl ring) modes of three nitro-substituted arene sulfenyl chlorides on Au(111). a

axis and that of the asymmetric mode being in the phenyl ring plane (Figure 2a). Other signals include the superposition of the CC stretchings with the CH in-plane bendings of the phenyl ring. These are the rCC−δCH(a), observed at 1583 cm−1 (very small), with an associated vector perpendicular to the molecular axis, and the rCC−δCH(b), observed at 1570 cm−1, with an associated vector parallel to the molecular axis and in the plane of the phenyl ring (Figure 2a). Another signal that is clearly observed is that corresponding to a CH bending mode (δCH(a)) at 1468 cm−1, with a vector parallel to the molecular axis. The other CH bending modes, if present, are within the noise level. These signals are indeed small (1293 and 1304 cm −1 ) even in the bulk spectrum (Supporting Information). The assignment of these signals is done in accordance with the calculated ones using Gaussian 03 at the B3LYP level (Supporting Information) and with the previously reported IR data for the bulk sample and real and calculated SAMs.76,77 Although r+NO2 is observed at the same position in the bulk and in the SAM, all other bands (r−NO2, rCC−δCH(a), rCC−δCH(b), and δCH(a)) are slightly shifted to lower wavenumbers by about 6−8 cm−1 in the SAM. An important observation is the decrease in the signals with vectors perpendicular to the molecular axis (r−NO2 and rCC−δCH(a)) compared to those with vectors parallel to the molecular axis (r+NO2, rCC−δCH(b), and δCH(a)). The intensity ratio of the symmetric to the asymmetric NO2 stretching modes is 5 for the 4-nitrophenyl thiolate SAM but 1 in the bulk sample. This clearly indicates that the 4-nitrophenyl thiolate is not very tilted and is nearly vertically adsorbed on the gold surface. We have previously deduced from STM data a tilt angle for the 4nitrophenyl thiolate on Au(111) of 9.35°.47 This is also in agreement with previously reported data based on the use of 4nitrophenyl thiol suggesting a small tilt angle on both Au and Ag (14 and 8°, respectively).78 The spectra obtained for the 2-nitrophenyl sulfenyl chloride are reported in Figure 2b. For the SAM, only three main signals are observed, including the r−NO2 signal at 1524 cm−1, the rCC−δCH(a) at 1581 cm−1, and the rCC−δCH(b) at 1564 cm−1. The suppression of the r+NO2 signal is an indication that the vector associated with this mode (Figure 2b) is parallel to the Au surface. This suggests that the 2-nitrophenyl thiolate adsorbate is more tilted than the 4-nitrophenyl thiolate. Figure 2c shows the spectra for the 2,4-dinitrophenyl sulfenyl chloride. The spectrum for the 2,4-dinitrophenyl thiolate SAM shows three main signals that are very similar to those observed for the 4-nitrophenyl sulfenyl chloride. These signals are similar to those determined for the 1,3-dinitrobenzene77 and include a symmetric r+NO2 signal at 1346 cm−1 that is shifted 4 cm−1 to higher wavenumbers. A second signal (r−NO2) is observed at 1531 cm−1 and shows an important decrease by comparison to

Table 2. NO2 Stretching Mode IR Data for Compounds 1−3 in Solution and on Au(111)a assignment

compound 1

2.0

Comparison of IR frequency (in cm−1) and intensity ratios of the symmetric (r+) and asymmetric (r−) stretching modes for the NO2 group for three nitro-substituted arene sulfenyl chlorides in a solid (KBr), in a solution (CCl4), and on Au(111). bNot visible in the spectrum and assigned arbitrarily. a

Figure 2a shows the PMIRRAS spectrum of the 4nitrophenyl sulfenyl chloride-modified Au surface as well as the solution IR spectrum of the precursor compound. A number of clear and nonoverlapping signals can readily be assigned for the 4-nitrophenyl thiolate SAM. These include the symmetric (r+NO2) stretch at 1342 cm−1 and the asymmetric (r−NO2) stretch at 1518 cm−1. The vectors associated with these vibrational modes are perpendicular to each other, with that of the symmetric mode being parallel to the 1,4 molecular 15857


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Figure 3. Cyclic voltammetry, in a 0.5 M KOH aqueous solution at 25 °C and a scan rate of 200 mV/s, of modified Au electrodes at different deposition times using (a) 4-nitrophenyl sulfenyl chloride, (b) 2-nitrophenyl sulfenyl chloride, and (c) 2,4-dininitrophenyl sulfenyl chloride. (d) Comparison of the deposition of the three compounds after 30 min.

modification time to 5 min shows an important increase in the stripping peak area. This is a good indication that the modification process is very fast. The peak area increases again for longer modification times of up to 30 min, where a maximum coverage is reached, confirming the fast deposition of the 4-nitrophenyl thiolate. Keeping in mind the stability of the nitro-substituted arene sulfenyl chlorides in solution, the rapidity of the deposition process rules out the potential formation of other intermediate precursors that could lead to the formation of SAMs such as disulfides. Electrodes modified using 2-nitrophenyl sulfenyl chloride (2) and 2,4-dinitrophenyl sulfenyl chloride (3) were similarly investigated, and their results are reported in Figure 3b,c. Similar behavior is observed for these two precursors, which show some differences from the results observed with the 4nitrophenyl sulfenyl chloride (1). For short modification times (of up 3 h), a reductive stripping peak, similar to the one seen for compound 1 is obtained. For the 2-nitrophenyl sulfenyl chloride-modified electrodes, a reduction peak at −0.83 V versus SCE is observed. For the 2,4-dinitrophenyl sulfenyl chloride-modified electrodes, the reduction peak is located at −0.80 V versus SCE . For longer modification times (greater than 3 h), a dramatic change in the reductive stripping peak is observed for both precursors. A considerable decrease in the initial reduction peak (which now appears as a shoulder) is noticed along with a new sharp peak at a slightly more negative potential (around −0.96 V versus SCE for both compounds). Such a change in the CV shape may be interpreted as due to a change in the structure of the film (through a change in orientation, density, intermolecular interactions, etc.) or the complete decomposition of the deposited molecule on the surface. Interestingly, this new sharp peak is very similar to and is located at the same position as that previously reported for the reductive stripping peak of sulfur adsorbed on Au (details in

the symmetric mode in the SAM compared to the solution. A signal is also observed at 1583 cm−1 corresponding to the rCC−δCH(a) mode, which also shows an important decrease in the SAM compared to the solution. The IR data support the XPS data and show the deposition of the nitro-substituted phenyl thiolates using all three sulfenyl chloride precursors. It also confirms the previously suggested nearly vertical orientation of the 4-nitrophenyl thiolate in the produced SAM and shows that despite the lower quality of the SAMs of 2-nitrophenyl and 2,4-dinitrophenyl thiolates as suggested by XPS, these SAMs show good IR spectra indicating well-defined orientations of these adsorbates on the surface. Cyclic Voltammetry Results. Stripping cyclic voltammetry is another technique that was used to characterize the modified surfaces using the nitro-substituted arene sulfenyl chlorides (1− 3). For all precursors, after modification in a DMF/EtOH solution of the precursor, the electrode is rigorously rinsed with EtOH and then with water prior to its introduction into the electrolyte solution for the stripping CV measurements. Figure 3a shows the cyclic voltammograms, in a 0.5 M KOH aqueous solution, for a bare gold electrode as well as for electrodes previously modified in a 1 mM solution of 4nitrophenyl sulfenyl chloride in DMF/EtOH (1:9) for times ranging from 1 min to 18 h. For the modified electrodes, a clear reductive stripping peak is observed at around −0.87 V versus SCE, whose area and shape are dependent on the modification time. This peak is observed only in the first cyclic voltammetric scan and disappears totally in the second scan, which provides a cyclic voltammogram similar to that of the bare gold electrode. This is an indication that the SAM formed at the surface of the electrode during the modification process is then removed upon reductive scanning in the first cyclic voltammetry sweep. It is important to note that the stripping peak at −0.87 V/SCE is observed even after 1 min of modification. Increasing the 15858


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the Supporting Information).79 This data provides further support for the discussed XPS results suggesting the decomposition of the adsorbed nitro-substituted thiolates for compounds 2 and 3 on the surface. The newly observed sharp reduction peak is more likely due to the adsorbed sulfur resulting from the decomposition of these adsorbates. These results indicate that the presence of the nitro group at the ortho position hinders the molecular packing of the thiolates of both compounds 2 and 3. It is important to note that despite this hindering substituent the deposition of both thiolates is still taking place and is fast, which is further confirmation of the efficiency of the sulfenyl chloride functional group as a precursor to thiolate deposition. These precursors allow the investigation of precursors with excellent adsorption ability but with unfavorable intermolecular and adsorbate/ substrate interactions for SAM formation. Another important result is that when comparing the stripping cyclic voltammograms for the three studied compounds for the same modification time (30 min), compound 1 provides a much larger peak than the other two compounds, 2 and 3 (Figure 3d). This is a further indication that the modification is much more efficient with the former compound than with the latter ones, which is confirmed by the deduced coverage values using the stripping signals after a modification time of 30 min. Compound 1 provides an area per molecule of 28 Å2, and compounds 2 and 3 provide an area per molecule of around 40 Å2. The electrochemical data is in accordance with the XPS and IR results and indicates the efficient formation of 4-nitrosulfenyl thiolate SAMs, using compound 1, with a greater stability and a higher coverage than those obtained using 2nitro-subtituted compounds 2 and 3. To gain more insight into the quality of the obtained films on the molecular level, the modified surfaces were studied by STM. STM Results. We have already reported two different structures for the 4-nitrophenyl thiolate SAMs on Au(111) depending on the modification conditions.46,47 An STM image of a Au(111) substrate, modified using compound 1 in 1 mM in DMF/EtOH (1:9) for 24 h is reported in Figure 4a. It shows the formation of a molecular double-row structure. A 31/2 × 4 rectangular unit cell (Figure 4b) and a packing density of 27.8 Å2/molecule are deduced.46 The double-row structure shows bright and dark rows. The difference in brightness is usually attributed to a difference in the tilt angle or in the adsorption sites. A mixed SAM structure (in which molecules were bound either through sulfur or the NO2 group) was proposed for vertically oriented molecules containing a terminal nitro group in the gas phase under low-temperature conditions.53−58 It was suggested that in such an orientation the dipole−dipole lateral repulsions are avoided and the unfavorable net surface dipole moment is minimized.54 The formation of a bilayer structure with an antiparallel arrangement of nitro groups in two monolayers was also suggested as an option for compensating dipoles.54 However, neither of these arrangements was stable at room temperature. Our XPS data that shows only one type of sulfur (corresponding to the Au−S bond) for the 4-nitrophenyl sulfenyl chloride precursor (1), along with the STM-deduced thickness of the SAM (1.2 nm),47 rules out the latter two possibilities. Extensive imaging of Au(111) substrates modified using 1 mM solutions of compounds 2 in DMF/EtOH (1:9) and 3 in DMF/EtOH (1:1) for various times up to 24 h did not allow the observation of any well-ordered areas. Various modification

Figure 4. (a) 10 × 10 nm2 STM image of a Au(111) substrate modified using 4-nitrophenyl sulfenyl chloride. The inset shows a slightly filtered 5 × 5 nm2 image. (b) Proposed model with the corresponding √3 × 4 rectangular unit cell (a = 4.75 ± 0.24 Å and b = 11.70 ± 0.43 Å). STM images of a Au(111) substrate modified using 2-nitrophenyl sulfenyl chloride: (c) 28 × 28 nm2 and (d) 10 × 10 nm2. STM images of a Au(111) substrate modified using 2,4-nitrophenyl sulfenyl chloride: (e) 10 × 10 nm2 and (f) 10 × 10 nm2. Imaging conditions: I = 0.8 nA, V(bias) = 0.1 V, and scan rate = 4.1 lines/s.

conditions (concentration, solvent, and modification time) were used to that end, and only at high concentrations (10 mM or higher) and long modification times (48 h or longer) were images showing nonuniform areas obtained. Figure 4c,d shows images obtained for a Au(111) surface modified using compound 2 (10 mM in THF/MeOH 1:1) for 48 h. The large-scan image shows various areas where bright spots are clearly observed. Enlarged views of these areas show nonordered structures with no well-defined phase. Approximate measurements provided periodicities of the bright spots within rows of about 8.8 Å and between rows of about 12.9 Å, again indicating a much lower film density than that obtained with the 4-nitrophenyl thiolate SAM. These measurements indeed correspond to the 3 × √20 phase (Supporting Information). Such a phase is associated with a coverage value of 38 Å2/ molecule, which is very close to the value determined by electrochemistry. The nitro group at the ortho position does not allow well-ordered packing on the surface, as is the case for compound 1. This was also suggested by the XPS, PMIRRAS, and electrochemical data. 15859


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adsorbed nitrophenyl thiolates are more tilted. These SAMs are less stable than the ones obtained with the 4-nitrosubsituted precursor and decompose to leave only sulfur on the gold surface. Despite the lower-quality SAMs formed by compounds 2 and 3, their adsorption, as observed by all of the techniques used in this study, shows the importance of the SCl functional group for the efficient deposition of organic compounds onto Au surfaces.

For the 2,4-dinitrophenyl sulfenyl chloride (3)-modified Au(111) substrates, imaging under various conditions did not show any well-ordered areas. The only structures that were observed are rectangular structures as shown in Figure 4e,f. These were obtained using a 10 mM concentration of compound 3 in a THF/MeOH (1:1) solution and a modification time of 48 h. Each rectangular structure shows eight bright spots. The unit cell dimensions of this rectangular structure are a = 8.2 Å and b = 8.2 Å. These rectangular features are characteristics of S-modified Au(111) surfaces (Supporting Information). They have been widely investigated in order to understand the S/Au interaction and to answer important fundamental questions in relation to SAM formation such as the adsorption sites and the mobility of the thiolate adsorbates.67−69 The observation of these rectangular structures confirms the electrochemical data. This suggests that, at least in these areas, decomposition of the precursor takes place, ejecting the aromatic moiety and leaving only sulfur on the Au surface. The deposition of S through the decomposition of organic selfassembled monolayers (SAMs) is a well-known process.80−82 Studies have been performed to understand the decomposition mechanism that leads to S and its dependence on the experimental conditions. These studies have shown the initial formation of the SAMs, which have been characterized using a number of techniques, and then its subsequent decomposition to yield S through the dissociation of S-alkyl or S-aryl chemical bonds, similar to the present case. The STM data confirms the results of the other characterization techniques used in this investigation. 4-Nitrophenylsulfenyl chloride leads to the formation of a stable, reproducible, well-ordered, nearly vertically oriented, densely packed molecular structure on Au(111). With compounds 2 and 3, adsorption does take place. However, the nonfavorable intermolecular and adsorbate/substrare interactions as a result of the bulky ortho substituent for compounds 2 and 3 lead to less stable, disordered, lower-quality films on Au(111). The decomposition of the adsorbed molecules leaving only sulfur on the surface was confirmed by XPS, electrochemistry, and STM.

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ASSOCIATED CONTENT

S Supporting Information *

Additional theoretical, IR, and STM data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 1-519-766-1499. Tel: 1519-824-4120. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the Natural Sciences and Engineering Research Council (NSERC), the Ontario Graduate Scholarship (OGS), the Canada Foundation for Innovation (CFI), and the Ontario Innovation Trust (OIT) for funding. We are also very grateful to Dr. Jacek Lipkowski for the use of PMIRRAS and for the discussion regarding the related data and to Dr. Paul Rowntree for the use of the evaporator system.

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REFERENCES

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4. CONCLUSIONS The formation of SAMs using three nitro-substituted arene sulfenyl chlorides as precursors was investigated. The extent of modification and the quality of the obtained SAMs were monitored using a series of techniques. XPS showed the successful deposition of all compounds, and signatures corresponding to all elements were clearly observed. The electrochemical characterization showed that 4-nitrophenyl sulfenyl chloride leads to a much higher coverage than the other two precursors, providing the initial clues into the effect of the NO2 group at the ortho position. The PMIRRAS data provided additional information because it showed that the 4nitrophenyl sulfenyl thiolate is nearly vertically orientated on the gold surface and the ortho-substituted phenyl thiolates are more tilted. This indicates that the hindering aspect of this substituent counteracts its ability to interact with the gold surface, yielding a much lower quality SAM. The STM investigation confirmed the results obtained using the other techniques. Only the 4-nitrophenyl sulfenyl chloride leads to the formation of stable, long-range, well-ordered SAMs. A √3 × 4 phase is obtained where the adsorbed 4-nitrophenyl thiolates are densely packed and are nearly vertically oriented. The 2-nitrophenyl sulfenyl chloride and the 2,4-dinito phenyl sulfenyl chloride both form lower-quality SAMs where the 15860


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Nitro-Substituted Arene Sulfenyl Chlorides as Precursors to the

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