Adsorption of Azothiophene Dye Having an N-Bridging Bidentate Tail

Jul 15, 2009 - Tour , J. M., Jones , L. , II, Pearson , D. L., Lamba , J. J. S., Burgin , T. P., Whitesides , G. M., Allara , D. L., Parikh , A. N. an...
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Adsorption of Azothiophene Dye Having an N-Bridging Bidentate Tail Group on Gold Gyeong Sook Bang,† Junghyun Lee,† Hee Yoel Baek,†,§ Hyoyoung Lee,*,† Kun Jun,‡ and Seung Rim Shin‡ †

National Creative Research Initiative, Center for Smart Molecular Memory, Department of Chemistry, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon 440-746, South Korea, and ‡Korea Research Institute of Chemical Technology, P.O. Box 107, Sinseongno, Yuseong-gu, Daejeon 305-600, South Korea. §Present address: Department of Chemistry, Hanyang University, Seoul, South Korea. Received April 15, 2009. Revised Manuscript Received June 18, 2009 The novel azothiophene derivatives [AT-di(CnSAc) (n = 6 and 12)], an azo linkage (NdN) that bridges one phenyl ring with an N-bridging dialkylthioacetate tail and one thiophene ring, are synthesized and characterized. The azothiophene derivatives substituted with electron-withdrawing groups are blue in color, exhibit a bathochromic shift of a longer wavelength, and are electrochemically active. The formation and characterization of AT-di(CnSAc) SAMs with a bidentate tail group of the N-bridging dialkylthioacetate have been studied by surface sensitive techniques such as grazing angle Fourier transform infrared (FT-IR), a quartz crystal microbalance (QCM), spectroscopic ellipsometry, X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV). It has been shown that the adsorption reaction of the thioacetate group is almost spontaneous and the N-bridging dialkylthioacetate SAMs with longer methylene length have a packing density higher than that with a shorter methylene length. However, the sulfur tethers of the N-bridging dialkylthioacetate show incomplete binding of sulfur atoms in AT-di(CnSAc) SAMs.

*To whom correspondence should be addressed: National Creative Research Initiative, Center for Smart Molecular Memory, Department of Chemistry, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon 440-746, South Korea. Telephone: (82)-31-299-4566. Fax: (82)-31290-5934. E-mail: [email protected].

used for holographic and digital storage of information.4-8 The absorption characteristic of azobenzene derivatives can be easily controlled by the addition of polar groups on the azo molecule.4,9,10 It has been noted that azo dyes with one phenyl ring and a thiazole unit or electron donor and acceptor groups give lower excitation energy than azobenzene.10 In this work, we introduce azothiophene derivatives having a thiophene ring, including two nitro groups to the azo moiety for a lower excitation energy than azobenzene and a bridging aminophenyl containing a bidentate dialkylthioacetate group. Compared with azobenzene chromophores, the azothiophene chromophore has significant bathochromicity of a longer wavelength shift of the absorbance maximum, which is empirically relative to the higher nonlinear optic (NLO) activity. It has been suggested on the basis of both experimental results and theoretical considerations that the azothiophene chromophores with strong electron donor and acceptor groups possess substantially large polarizabilities and a fairly large dipole moment.11 Multidentate SAMs are also a good choice because of their enhanced stability for exchange and desorption and better control of molecular orientation. It was reported that the stability of bidentate (or tridentate) SAMs was superior to that of single dentate SAMs.12 These azothiophene derivatives, an azo linkage (NdN) that bridges one phenyl ring with an N-bridging dialkylthioacetate tail and one thiophene ring, are synthesized by our previously reported method.13 We name these azothiophene derivatives

(1) Ray, S. G.; Cohen, H.; Naaman, R.; Liu, H.; Waldeck, D. H. J. Phys. Chem. B 2005, 109, 14064. (2) Kumar, G. S.; Neckers, D. C. Chem. Rev. 1989, 89, 1915. (3) Moniruzzaman, M.; Talbot, J. D. R.; Sabey, C. J.; Fernando, G. F. J. Appl. Polym. Sci. 2006, 100, 1103. (4) Astrand, P.-O.; Sommer-Larsen, P.; Hvilsted, S.; Ramanujam, P. S.; Bak, K. L.; Sauer, S. P. A. Chem. Phys. Lett. 2000, 325, 115. (5) Berg, R. H.; Hvilsted, S.; Ramanujam, P. S. Nature 1996, 383, 505. (6) Ikeda, T.; Tsutsumi, O. Science 1995, 268, 1873. (7) Liu, Z. F.; Hashimoto, K.; Fujishima, A. Nature 1990, 347, 654. (8) Natansohn, A.; Rochon, P.; Gosselin, J.; Xie, S. Macromolecules 1992, 25, 2268.

(9) Abd-El-Aziz, A. S.; Afifi, T. H. Dyes Pigm. 2006, 70, 8. (10) Morley, J. O.; Guy, O. J.; Charlton, M. H. J. Phys. Chem. A 2004, 108, 10542. (11) Moreley, J. O.; Hutchings, M. G.; Zyss, J.; Ledoux, I. J. Chem. Soc., Perkin Trans. 1997, 2, 1139. (12) Wei, L.; Padmaja, K.; Youngblood, W. J.; Lysenko, A. B.; Lindsey, J. S.; Bocian, D. F. J. Org. Chem. 2004, 69, 1461. (13) Lee, H.; Lee, J.; Baek, H. Y.; Bang, G. S.; Park, J.; Choi, N.-J.; Jun, K.; Shin, S. R. Compound having thiol anchoring group. U.S. Pat. Appl. 2007073058, 2007.

Introduction Recently, in the fields of nanoscience and nanotechnology, functional molecular materials have been applied to magnetic storage media, electronic and optical devices, and bioanalytical devices using self-assembled monolayers (SAMs). Organic SAMs have been intensively studied as a powerful means of surface modification and functionalization with molecular-level control. Of particular note, the chemical functionality has a crucial influence on the surface properties, both structurally and electronically. It was already shown that organic SAMs can tune the electronic properties of the interface by modifying the adsorbate molecules with a dipole moment.1 Furthermore, organic SAMs with electrical and optical properties are of great interest for device miniaturization, reduced cost, and lighter and more efficient devices. Azobenzene derivatives are photoactive isomers and are also electrochemically active. Because of their simple structure and reversible trans-cis isomerization under photoillumination,2,3 they have been widely studied as candidates for molecular switches or storage media. Thin films of these materials can be

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Published on Web 07/15/2009

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Figure 1. Structure of AT-di(CnSAc).

azothiophene-di(CnSAc), AT-di(CnSAc) (n = 6 or 12) (Figure 1). The azothiophene derivatives substituted with electron-withdrawing groups are blue in color, show a bathochromic shift9 of a longer wavelength shift, and are electrochemically active. Furthermore, it was reported that the sulfur atom of the thiophene ring plays a decisive role by acting as an efficient electron sink, based on valence band theory.9,14 These azothiophene derivatives give rise to intense optical absorption and may have potential applications in molecular-based information storage and electronic devices.15 Thus, this bidentate structure with an Nbridging dialkylthioacetate tail and a single electro-active group has some advantages, including being thermally more robust than monodentate SAMs on gold.16 Here, we report the formation and characterization of the ATdi(CnSAc) SAMs with an N-bridging dialkylthioacetate by using surface sensitive techniques, including grazing angle Fourier transform infrared (FT-IR) spectroscopy, a quartz crystal microbalance (QCM), spectroscopic ellipsometry, X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV). The surface coverage, film thickness, and chemical composition of the molecular layer were determined by QCM and CV, spectroscopic ellipsometry, and XPS, respectively. The SAMs of N-bridging dialkylthioacetate with two binding sites are expected to bind in a different manner and thus exhibit properties different from those of mono alkanethiols that have only one binding site. This lends insight into the interfacial structure and properties of N-bridging dialkylthioacetate groups on the structure of the SAMs and the electrochemical behaviors of the azothiophene redox center in the SAMs.

in a freshly prepared piranha solution for 10 min, followed by several washes with deionized water and ethanol, and finally dried with a stream of nitrogen gas. The average roughness factor was 1.1. A QCM (SHIn EQCN 2000, Korea) with a Teflon-based cell17 was used to monitor changes in the frequency of adsorption of the thioacetate compounds under ambient conditions with no potential applied to the quartz crystal (part II of the Supporting Information). Cyclic voltammograms (CVs) were obtained using a CHI 660A electrochemical analyzer. An Ag(s)|AgCl(s)|KCl(saturated) reference electrode and a platinum wire auxiliary electrode were used. All electrochemical measurements were taken in a completely deaerated solution. The thickness of the SAMs was measured using a spectroscopic ellipsometer (model M-2000, J. A. Woollam VASE) at an incident angle of 75° in a wavelength range between 400 and 900 nm. The ellipsometric data were fitted with commercial software (WVASE32) by J. A. Woollam Co., Inc. UV-vis spectra were recorded using a Hitachi U-3501 UV/vis/NIR spectrophotometer. A quartz cell with a path length of 1.0 cm was used for absorbance. The contact angle with water was measured using a goniometer (Phoenix, model 300). FT-IR spectra were recorded in a single-reflection mode by using a dry N2-purged Thermo Nicolet Nexus grazing angle, FT-IR apparatus equipped with the smart apertured grazing angle (SAGA) accessory. In absorption mode, a bare gold background spectrum was first obtained, followed by those of the modified gold films. All spectra were recorded at 2 cm-1 resolution with 2000 scans. Elemental analysis from XPS was conducted using a VG Scientific ESCALAB-200R analyzer with an Al Ka source. The XPS spectra were fitted using Gaussian-Lorentzian lines with XPSPEAK. Overlapping 2p spin-split doublets of the S signal were fitted by fixing the peak widths at half-maximum values. Preparation of AT-di(CnSAc) Monolayers. Monolayers of AT-di(CnSAc) were prepared by direct formation through the thioacetate functional group containing a long bidentate tail group on a gold surface (Scheme 1). The AT-di(CnSAc) monolayer was prepared by soaking the clean gold films in the solution containing only 3 mM AT-di(CnSAc) without a catalyst of H2SO4 in DMF for 24 h. The films were removed from solution, via a thorough rinse with DMF and subsequent drying with a stream of nitrogen gas. The modified films were immediately used in all measurements.

Results and Discussion Experimental Section Materials. The synthesis of azothiophene derivatives [ATdi(CnSAc) (n = 6 and 12)] is reported in part I of the Supporting Information. AT-di[(CH2)6SAc] and AT-di[(CH2)12SAc] are abbreviated AT-diHT and AT-diDT, respectively. Potassium ferrocyanide(III) {K3[Fe(CN)6], >99%, Aldrich}, ethanol (99.5%, HPLC-grade, Aldrich), sulfuric acid (H2SO4, electronicgrade), hydrogen peroxide (H2O2, electronic-grade), and DMF (>99.9%, HPLC-grade, Aldrich) were used without further purification. Other chemicals were of analytical reagent grade. The deionized water with a resistivity of 18 MΩ cm was used throughout the experiment. Phosphate buffer (pH 7.2, I = 0.1 M) was used as the supporting electrolyte. Apparatus. Gold films with 800 A˚ thicknesses on a Ti/Si wafer were used for FT-IR, ellipsometric, and electrochemical experiments. The gold film was evaporated with an E-beam evaporator. Before chemisorption, the gold film was cleaned with piranha solution [98% H2SO4 and 30% H2O2 (50:50, v/v)]. Caution: piranha solution reacts violently with most organic materials and should be handled with extreme care. The gold film was immersed (14) Gordon, P. F.; Gregory, P. Organic Chemistry in Color; Springer: Berlin, 1983; p 139. (15) Matharu, A. S.; Jeeva, S.; Huddleston, P. R.; Ramanujam, P. S. J. Mater. Chem. 2007, 17, 4477. (16) Shon, Y.-S.; Lee, T. R. J. Phys. Chem. B 2000, 104, 8192.

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Adsorption Kinetics. First, the tendencies of the self-assembling process of AT-di(CnSAc) were observed in a pure DMF solution using a QCM. The QCM is very sensitive to mass changes as small as several nanograms.18 The sensitivity to mass change is determined by the crystal resonant frequency, according to the Sauerbrey equation.19 Figure 2 shows the QCM results (solid line) for binding the dialkylthioacetate tail group to the gold surface as a thiolate20 and the Langmuir fitting curve (dashed line) for the adsorption kinetics in a 0.1 mM AT-di(CnSAc) solution. When the frequency change was stabilized within (0.1 Hz with time for ca. 7 min, the AT-di(CnSAc) solution was added and the frequency data were obtained. The frequency rapidly decreased immediately after the sample was added, followed by a gradual decrease. The adsorption kinetics was observed by monitoring the frequency decrease, which is equivalent to the increase in mass on the gold surface. From the frequency change (ΔF ), we estimate the surface density of AT-di(CnSAc). The total frequency change was (17) Bang, G. S.; Jeon, I. C. Bull. Korean Chem. Soc. 2001, 22, 281. (18) Pan, W.; Durning, C. J.; Turro, N. J. Langmuir 1996, 12, 4469. (19) Buttry, D. A.; Ward, M. D. Chem. Rev. 1992, 92, 1355. (20) Tour, J. M.; Jones, L.II; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529.

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Scheme 1. (A) Direct Immobilization through the N-Bridging Dialkylthioacetate Group of AT-di(CnSAc) on a Gold Surface and (B) EnergyMinimized Geometry

7.3 Hz for AT-diHT and 13.8 Hz for AT-diDT, which corresponds to 5.4  10-11 and 9.6  10-11 mol cm-2, respectively. If the roughness factor of the gold surface was 1.2,17 the coverage would be 4.5  10-11 and 8.0  10-11 mol cm-2 for AT-diHT and AT-diDT, respectively. The surface density of AT-diDT is higher than that of AT-diHT. From the QCM results, the adsorption kinetics can also be fitted with the adsorption model of the Langmuir isotherm,21,22 which leads to θ ¼

Cf1 -exp½ - Nkao ðCþKÞtg CþK

ð1Þ

where θ is the fraction of the monolayer coverage and C is the bulk concentration of the solution in molarity. The adsorption equilibrium constant K=kd/ka, where kd and ka are the desorption and adsorption rate constants, respectively, No is the surface adsorbent concentration at full coverage, and t is the adsorption time. Equation 1 can be simplified by the substitution kobs = (ka/No)C þ kd and K0 =C/(C þ K ), giving θ ¼ K 0 ½1 -expð -kobs tÞ

ð2Þ

The curves were fitted by an exponential decay function, and the dashed line in Figure 2 shows a fitting curve according to eq 2. We obtained the kobs values by fitting the raw data to eq 2. Then, by the linear regression fit of kobs versus the concentration of AT-di(CnSAc) (inset of Figure 2), the values of ka and kd were obtained. The free energy of adsorption of the AT-di(CnSAc) monolayer is obtained from K (ΔG = RT ln K). Table 1 summarizes the resulting Langmuir fits and the Gibbs energies of adsorption. The adsorption of AT-di(CnSAc) on the gold surface is an equilibrium system since kd is greater than zero. This adsorption reaction is almost spontaneous according to the ΔG (21) Ulman, A. An Introduction to Ultrathin Organic Films from LangmuirBlodgett to Self-Assembly; Academic Press: San Diego, 1991; p 243. (22) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 3315.

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values. This kinetic data are similar to the values obtained from monothiol compounds having a bulky terminal group.17,23 Our data provide direct evidence that the thioacetate-gold system is an equilibrium system. Cyclic Voltammograms of AT-di(CnSAc) SAM. The ATdi(CnSAc) with an N-bridging bidentate dialkylthioacetate group spontaneously forms a self-assembled monolayer on gold, and it exhibits an electrochemically active reaction with unique reduction-oxidation behavior. The CVs of the AT-di(CnSAc) SAM are shown in Figure 3, and the electrochemical data are summarized in Table 2. The CVs shows a reversible redox peak at -0.24 V. The ratio of the redox peak current is ∼1.0, and the peak separation (ΔEp =Epa -Epc =20-30 mV) is considerably smaller than the reported value of azobenzene SAM with a monoalkylthiol group. The small peak separation is a sign of reversibility of the faradaic process with fast electron transfer between the redox center and the electrode surface. The ΔEp of AT-diDT SAM with a long alkyl chain is smaller than that of the AT-diHT SAM, which means packed and ordered SAMs for ATdiDT. The ΔEfwhm provides information about the environments and interaction of the redox center in the AT-di(CnSAc) SAMs. The ΔEfwhm value of AT-diHT SAMs is larger than those of ATdiDT SAMs and the theoretical value of 90.6/n mV,24 where n is the mole number of electrons transferred. It is indicative of a higher degree of organization in the monolayer, whereas smaller values indicate attractive or stabilizing interactions. Thus, the broad wave of the AT-diHT SAM can be understood as a higher degree of organization, the change in structure, and the environment of the monolayer. The peak currents increased quite linearly (R2=0.999) with an increasing scan rate (inset of Figure 3 A,B), a typical characteristic curve of immobilized species. This strongly supports the possibility that the AT-di(CnSAc) compounds are immobilized on the (23) Bang, G. S.; Park, J.; Lee, J.; Choi, N.-J.; Baek, H. Y.; Lee, H. Langmuir 2007, 23, 5195. (24) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons: New York, 2000.

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Figure 2. Representative curves (black solid line) and fitting curves (red dashed line) for adsorption of 0.1 mM (A) AT-diHT and (B) AT-diDT in DMF. The fits correspond to Langmuir adsorption isotherms. The inset shows plots of kobs vs the concentration of AT-di(CnSAc). Table 1. Langmuir Fit Data of QCM Results for the Adsorption of AT-di(CnSAc) on Gold adsorbate

ka (M-1 s-1)

AT-diHT AT-diDT

97 26

kd (s-1) 3.0  10-4 3.8  10-5

ΔGads (kcal/mol) -7.5 -7.9

gold surface. The surface coverage of the AT-di(CnSAc) SAM was then determined by integration of the reduction peak current of the CVs. Considering the roughness factor (1.1), the coverage value for AT-diHT SAM was 3.110-11 mol/cm2 and the value for AT-diDT was 1.3  10-10 mol/cm2. The coverage of AT-diDT SAM shows a value higher than that of AT-diHT SAM. These coverage values correspond to the surface areas (602 and 129 A˚2) for immobilized AT-diHT and AT-diDT, respectively. That is, AT-diDT has a more closely packed SAM structure than ATdiHT on gold. The surface coverage in QCM and CV results shows a slight difference, although absolute values of the CV and QCM results cannot be compared. The difference between the CV and QCM results can be understood by considering the sources of possible errors in the respective measurements. The CV technique is sensitive to only electroactive molecules bound to the metal surface to allow electron transfer. The QCM is used to monitor adhesion of molecules on the gold electrode surface in real time, and the QCM experiments are complicated by the unknown solvation state of the adsorbed species, density changes in the liquid phase, and the possibility of trapping of the solution in the adsorbed film. The surface coverage of AT-diHT with N-bridging bidentate groups is substantially lower than the coverage observed for Langmuir 2009, 25(18), 10788–10793

Figure 3. CVs for (A) AT-diHT SAM and (B) AT-diDT SAM in phosphate buffer and (C) 0.5 mM Fe(CN)63- in 0.1 M phosphate buffer at 0.1 V/s. Table 2. CV Data for AT-di(CnSAc) SAM on Gold in 0.1 M Phosphate Buffer at 0.1 V/s SAM

Ep (V)

ΔE (mV)

ΔEfwhm (mV)

Γ (mol/cm2)

AT-diHT AT-diDT

-0.240 -0.240

30 20

70 56

3.1  10-11 1.3  10-10

azobenzene hexanethiol with a monoalkylthiol group (1.7  10-10 mol/cm2).25 This implies that the structure of AT-diHT is hindered for a closely packed SAM due to the N-bridging dialkylthioacetate group. We next observed the response of Fe(CN6)3-/4- to obtain the blocking properties of these SAMs. Figure 3C shows the cyclic voltammogram for 0.4 mM Fe(CN6)3-/4- in a 0.1 M phosphate buffer on bare and modified gold electrodes. For the bare gold electrode, the redox peak of Fe(CN6)3-/4- shows a welldocumented reversible curve. For the modified electrode with AT-diHT SAM, the Fe(CN6)3-/4- response is seen as an irreversible redox peak, and the current density was ∼50% lower in comparison with that of the bare gold electrode. However, for the (25) Zhang, W.; Li, H.; Liu, L.; Xie, J.; Lu, C.; Zhou, Y.; Ren, X.; Meng, Q. J. Colloid Interface Sci. 2003, 261, 82.

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modified electrode with AT-diHT SAM, the redox response of Fe(CN6)3-/4- was severely hindered and the current density was 5-fold smaller than that on the bare gold electrode. This indicates that the packing density of AT-diDT SAMs is higher than that of AT-diHT SAMs. From these results, it is found that the N-bridging dialkylthioacetate SAMs with a longer methylene length have a packing density higher than that with a shorter methylene length, and this is in agreement with the results of monoalkylthiol SAMs.21 In the monoalkylthiol SAM, the selfassembled molecule is defined by three parts, the headgroup, the space group of alkyl chain, and the terminal group.21 It was generally known that the interplay among the three parts within the monoalkylthiol molecule determines the order and packing density of SAM and the van der Waals forces between these methylene (-CH2-) chains provide a driving force for the selfassembly.21 It has been reported that a chain length of >10 tends to form more ordered SAMs.26 In the case of AT-diHT and ATdiDT, the headgroup and the terminal group have the same structure as the N-bridging dialkylthioacetate group and thiophene ring with the NO2 group. The only difference is the length based on the number of the methylenes. Therefore, the AT-diDT with a longer methylene length leads to higher ordered and packing density, which is in reasonable agreement with the surface coverage obtained with QCM and CV. A similar feature was previously observed for FcCO(CH2)5SH and Fc[CO(CH2)5SH]2 SAMs.27 Thickness and Wettability. Spectroscopic ellipsometry is used to determine the thickness of the AT-di(CnSAc) SAM. From the film thickness, the quality and the degree of surface coverage of the SAM can be estimated. The refractive index (n) value was between 1.0 and 1.5 over a wavelength range of 400-800 nm, and the absorption band near 640 nm in the extinction (k) curve was attributed to the maximum electronic transition of AT-di(CnSAc) (part IV of the Supporting Information). The curve of extinction coefficient k is similar to the absorption spectrum, and the λmax of AT-di(CnSAc) in THF is at 636 nm. This characteristic absorption band is mainly attributed to the π f π* transition of the conjugated chromophore containing an NdN group and a thiophene and benzene ring. The thicknesses were approximately 1.12 ( 0.08 and 2.61 ( 0.05 nm for AT-diHT and AT-diDT SAMs, respectively. The thickness value of AT-diHT SAM is ∼1 nm lower than the estimated molecular length (Scheme 1B), and that of the AT-diDT SAM is similar to the estimated value. Molecular length was estimated using Cambridge Scientific Chem3D. On the basis of the molecular structure of the lowestenergy configuration, the molecular length is the distance from the terminal NO2 group of the thiophene ring to the S plus the Au-S bond length (0.23 nm). The smaller thickness of the ATdiHT SAM would be expected for a loosely packed SAM. This also suggests that the AT-diDT SAM with a longer methylene length has a packing density higher than that with a shorter methylene length. The wetting behavior is one of the monolayer properties and yields information about the ordering of the monolayer. The water contact angle (WCA) for bare gold was 53.7 ( 1.3°, and the angles for AT-diHT and AT-diDT SAM on gold were 72.5 ( 1.1° and 77.1 ( 0.2°, respectively. These values are quite similar to the value of the NO2-terminated surface (θ=75 ( 3°).28 (26) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (27) Viana, A. S.; Jones, A. H.; Abrantes, L. M.; Kalaji, M. J. Electroanal. Chem. 2001, 500, 290. (28) Marten, J.; Erbe, A.; Critchley, K.; Bramble, J. P.; Weber, E.; Evans, S. D. Langmuir 2008, 24, 2479.

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Figure 4. Grazing angle FT-IR spectra of (A) the AT-diHT SAM and (B) the AT-diDT SAM on a gold surface.

Grazing Angle FT-IR. The FT-IR spectra for the AT-di(CnSAc) SAM on gold and bulk AT-di(CnSAc) (film on NaCl, transmission) are shown in Figure 4. Asymmetric and symmetric NO2 stretching bands of the thiophene ring are observed at 1520 and 1327 cm-1 for AT-diHT and AT-diDT SAMs, recpectively. The aromatic CdC stretching bands (νΦCdC) of benzene and the thiophene ring are observed near 1602 and 1548 cm-1 for ATdiHT SAM and 1602 and 1547 cm-1 for AT-diDT SAM, respectively, and the benzene ring-N-dialkyl stretching is observed at 1126 cm-1 for AT-diHT and AT-diDT SAM. The C-H stretching bands of the AT-diHT SAM are lower in intensity than that of the AT-diDT SAM. Two peaks in the C-H stretching region of the AT-diDT SAM, 2929 and 2857 cm-1, are characteristic asymmetric and symmetric vibrations, respectively. In general, the position and intensity of the C-H stretching bands (νsym-CH2 and νasym-CH2) are sensitive to the phase state and packing density of the methylene chain in organic thin films.29-32 In the AT-diHT SAM, the intensity of the C-H stretching bands is considerably lower than that of the AT-diDT SAM. The difference in this peak intensity could reflect different tilted orientations for the methylene chains or different amounts of the methylene chains on the gold surface. We infer hereby that the difference in the intensity of the C-H stretching bands indicates different amounts of methylene chains of AT-diHT and AT-diDT, and the AT-diHT molecule forms the more disordered SAM than the AT-diDT molecule. The frequencies of the antisymmetric and symmetric bands of methylene are known to be strongly influenced by the crystallinity and packing density of the organic monolayer.21,32 The wavenumber value for (29) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (30) Park, J.-S.; Vo, A. N.; Barriet, D.; Shon, Y.-S.; Lee, T. R. Langmuir 2005, 21, 2902. (31) Shon, Y.-S.; Lee, T. R. J. Phys. Chem. B 2000, 104, 8182. (32) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145.

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Article

Figure 5. XPS S 2p and N 1s spectra of bare gold and AT-diHT and AT-diDT SAMs on a gold surface: raw data (red and blue lines) and fits (black line).

the C-H stretching of AT-diDT SAM is much higher than that of a well-packed monoalkanethiol monolayer (2920 and 2850 cm-1). This observation shows disorder in the conformational structure of the methylene chains in the SAM. It is assumed that the N-bridging dialkylthioacetate group prevents the closest packing, and considerable space exists around the methylene chains of the AT-di(CnSAc) SAM. Therefore, these results suggest that the AT-di(CnSAc) SAM generates poorly ordered films. In both AT-diHT and AT-diDT SAMs, the CdO peak is observed at 1695 cm-1, which can be attributed to the carbonyl vibration (νCdO) of the unbound thioacetate group. This is a residual signal for the carbonyl vibration of S-acetyl-protected molecules after monolayer formation. This band is a sign for unbound sulfurs of their unreacted thioacetate group and indicates that the unbound sulfur of the AT-di(CnSAc) SAM was present after monolayer formation. XPS Investigations. XPS can also provide important information about the quality and structure of SAMs. To characterize the bonding state of the AT-di(CnSAc) SAM with an N-bridging dialkylthioacetate group on a gold surface in greater detail, XPS spectra of the AT-di(CnSAc) SAMs were examined and the spectra of the N 1s and S 2p core levels are shown in Figure 5. The N 1s spectrum shows two signals at 400 and 406 eV for NdN- and -N-C- parts of the benzene ring and the NO2 part of the thiophene ring, respectively. The oxidized nitrogen occurs at a higher N 1s binding energy, and these values are in good agreement with the reported results for nitroazobenzene.33,34 The S 2p region can provide evidence of bond formation between the sulfurs of the dialkylthioacetate tail group and the gold surface. From the reported data, the binding energy for bound sulfur on gold is known to be 162 and 163.2 eV with an intensity ratio of 2:135,36 and the binding energy for unbound sulfur on gold is known to be between 163 and 164 eV. Oxidized sulfur is known to appear above 165 eV. When the data are fit, the AT-di(CnSAc) SAMs with an N-bridging dialkylthioacetate group contained two sulfur forms, bound and unbound, S 2p peaks were indicated at 162.2 and 163.1 eV for the AT-diHT SAM and 162.1 and 163.3 eV (33) Liu, Y.-C.; McCreery, R. L. Anal. Chem. 1997, 69, 2091. (34) Sharma, L.; Matsuoka, T.; Kimura, T.; Matsuda, H. Polym. Adv. Technol. 2002, 13, 481. (35) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (36) Ishida, T.; Choi, N.; Mizutani, W.; Tokumoto, H.; Kojima, I.; Azehara, H.; Hokari, H.; Akiba, U.; Fujihara, M. Langmuir 1999, 15, 6799.

Langmuir 2009, 25(18), 10788–10793

for the AT-diDT SAM. On the basis of the reported data, the peak at ∼162 eV is assigned to typical S-Au species and the peak at ∼163 eV is assigned to the unbound sulfur. SAMs on gold through disulfide and multipodal thiol have also reported the unbound sulfur peak at 163.5 or 164 eV.37 Thus, the unbound sulfur can likely be attributed to the unreacted thioacetate of the tail group. From the clear XPS and FT-IR evidence, unbound thioacetate species are present in the AT-di(CnSAc) SAM and the sulfur tethers of the N-bridging dialkylthioacetate show incomplete binding of sulfur atoms.

Conclusion We have synthesized and described the preparation and characterization of a new azothiophene with an N-bridging dialkylthioacetate anchor group. These AT-di(CnSAc) molecules form electrochemically detectable SAMs. The N-bridging bidentate SAM with a longer methylene length has a packing density higher than that with a shorter methylene length. As can be expected on the basis of the molecular structure of the bidentate, however, the surface coverage density is smaller than the reported data for a monoalkyl azobenzene SAM. The structure of ATdi(CnSAc) molecules prevents close packing, and as a result, empty space exists around the methylene chains in their SAMs. In the AT-di(CnSAc) SAMs, unbound thioacetate species are present and the sulfur tethers of the N-bridging dialkylthioacetate show incomplete binding of sulfur atoms. Acknowledgment. This work was supported by the Creative Research Initiatives research fund (Project Smart Molecular Memory) of the Ministry of Education Science and Technology (MEST)/Korea Science and Engineering Foundation (KOSEF). We thank Professor Insung S. Choi and Mr. Young Shik Ghi of the Department of Chemistry at KAIST for grazing angle FT-IR measurements. Supporting Information Available: Detailed syntheses of the new AT-di(CnSAc) compounds, information about quartz crystals, and ellipsometric spectra of the AT-diDT SAM on gold. This material is available free of charge via the Internet at http://pubs.acs.org. (37) Shirai, Y.; Cheng, L.; Chen, B.; Tour, J. M. J. Am. Chem. Soc. 2006, 128, 13479.

DOI: 10.1021/la901323d

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