pubs.acs.org/Langmuir © 2009 American Chemical Society
Structural Analysis of HS(CD2)12(O-CH2-CH2)6OCH3 Monolayers on Gold by Means of Polarization Modulation Infrared Reflection Absorption Spectroscopy. Progress of the Reaction with Bromine Izabella Brand,* Martina Nullmeier, Thorsten Kl€uner, Rajamalleswaramma Jogireddy,† Jens Christoffers, and Gunther Wittstock Carl von Ossietzky University of Oldenburg, Center of Interface Science (CIS) and Department of Pure and e Louis Pasteur, Institut de Science Applied Chemistry, D-26111 Oldenburg, Germany. †New address: Universit et Ing enierie Supramol ecularies, Organic and Bioorganic Laboratory, ISIS-ULP-CNRS (UMR 7006), 8, alle Gaspard Monge, F-67083 Strasbourg Cedex, France Received June 11, 2009. Revised Manuscript Received July 28, 2009 A self-assembled monolayer (SAM) on gold was formed with specifically perdeuterated hexaethylene glycolterminated alkanethiol HS(CD2)12(O-CH2-CH2)6OCH3 (D-OEG). The structure of the d-alkane and the oligoethylene glycol (OEG) parts of the molecule in a SAM was studied by means of polarization modulation infrared reflection absorption spectroscopy. The D-OEG monolayers are highly ordered and exist in a crystalline phase. The d-alkane chain adopts an all-trans conformation. Both, the d-alkane chain and long axis of the OEG part make an angle of 26.0 ( 1.5 with respect to the surface normal, a value characteristic for the tilt of solid n-alkane thiols in the SAMs on Au. The positions of νas(COC) and CH2 wagging and rocking modes indicate a helical arrangement of the OEG chains. The D-OEG SAMs were exposed to 25 μM Br2 in two ways: (i) by immersion into the Br2 solution and (ii) in the galvanic cell Au|D-OEG SAM|25 μM Br2 +0.1 M Na2SO4|| 50 μM KBr+0.1 M Na2SO4|Au. In the galvanic cell, the oxidant (Br2) is scavenged by a heterogeneous electron transfer reaction, slowing the reaction of D-OEG with Br2. The slow progress of the reaction with Br2 allowed us to draw conclusions about molecular rearrangements taking place during this reaction. The reaction with Br2 starts on boundaries and/or defects present in the SAM. First, at the defect place, the R-C atom of the OEG chain reacts with Br2 and the OEG part of the molecule is removed from the monolayer. In consequence an increase in disorder in the OEG part of the SAM is observed. The same mechanism of the reaction with Br2 is applied for the d-dodecane alkanethiol part of the molecule. This reaction is slow, thus the order and the tilt of the hydrocarbon chain changes only a little during the reaction time.
1. Introduction A highly organized monomolecular film built of amphiphilic molecules can be produced by a self-assembly technique.1-4 n-Alkanethiols are one of the most studied molecules that form self-assembled monolayers (SAMs) on gold surfaces.2,5-10 The conformation of the alkane chain, the two-dimensional packing in the monolayer, its stability, the mechanism of chemisorption, and wettability have intensively been investigated. n-Alkanethiol SAMs are uniformly packed with a high degree of order in the hydrocarbon chains.2,6,8,10 Defects on the adsorbing substrate and defects in the monolayer due to domain boundaries between differently oriented molecules contribute to the structural complexity *To whom correspondence should be addressed. E-mail: izabella.zawisza@ uni-oldenburg.de. Phone: 0049-441-798-3975. Fax: 0049-441-798-3979. (1) Ulman, A. An Introduction to Ultrathin Organic Films from LangmuirBlodgett to Self Assembly; Academic Press: Boston, 1991. (2) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105(13), 4481–4483. (3) Roberts, G. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (4) Peterson, I. R. J. Phys. D: Appl. Phys. 1990, 23(4), 379–395. (5) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546–558. (6) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558–569. (7) Naumann, R.; Schiller, S. M.; Giess, S. M.; Grohe, B.; Hartmann, K. B.; Karcher, I.; Koper, I.; Lubben, J.; Vasilev, K.; Knoll, W. Langmuir 2003, 19, 5435–5443. (8) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141–149. (9) Bensebaa, F.; Ellis, T. H.; Badia, A.; Lennnox, R. B. Langmuir 1998, 14, 2361–2367. (10) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990, 93, 767– 773. (11) Atre, S. V.; Liedberg, B.; Allara, D. L. Langmuir 1995, 11, 3882–3893. (12) Guo, L.-H.; Facci, J. S.; McLendon, G.; Mosher, R. Langmuir 1994, 10(12), 4588–4593. (13) Van Patten, P. G.; Noll, J. D.; Myrick, M. L. J. Phys. Chem. B 1997, 101 (39), 7874–7875.
362 DOI: 10.1021/la9020993
of such supramolecular layers.10-14 The n-alkanethiol terminated with an oligoethylene glycol (OEG) chain forms very compact isostructural films.15-18 The OEG chains find application in the following fields: (i) tethering of biospecific groups (e.g., amine, carboxylic) for molecular recognition,19,20 (ii) formation of supported lipid membranes or bilayers,21-23 (iii) design of biosensors and DNA-chips,24-26 and (iv) prevention of protein adsorption.16,17,27,28 The resistance of the OEG to protein adsorption (14) Cygan, M. T.; Dunbar, T. D.; Arnold, J. J.; Bumm, L. A.; Shedlock, N. F.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. J. Am. Chem. Soc. 1998, 120(12), 2721–2732. (15) Faure, M. C.; Bassereau, P.; Desbat, B. Eur. Phys. J. E 2000, 2, 145–151. (16) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chem. B 1998, 102, 426–436. (17) Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10714–10721. (18) Vanderah, D. J.; Gates, R. S.; Silin, V.; Zeiger, D. N.; Woodward, J. T.; Meuse, C. W.; Valincius, G.; Nickel, B. Langmuir 2003, 19(7), 2612–2620. (19) Lahiri, J.; Isaacs, L.; Grzybowski, B.; Carbeck, J. D.; Whitesides, G. M. Langmuir 1999, 15, 7186–7198. (20) Mrksich, M.; Grunwell, J. R.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 12009–12010. (21) Lang, H.; Duschl, C.; Vogel, H. Langmuir 1994, 10, 197–210. (22) Raguse, B.; Braach-Maksvytis, V. L. B.; Cornell, B. A.; King, L. G.; Osman, P. D. J.; Pace, R. J.; Wieczorek, L. Langmuir 1998, 14, 648–659. (23) Williams, L. M.; Evans, S. D.; Flynn, T. M.; Marsh, A.; Knowles, P. F.; Bushby, R. J.; Boden, N. Langmuir 1997, 13, 751–757. (24) Brockman, J. M.; Frutos, A. G.; Corn, R. M. J. Am. Chem. Soc. 1999, 121, 8044–8051. (25) Cornell, B. A.; Braach-Maksvytis, V. L. B.; King, L. G.; Osman, P. D. J.; Raguse, B.; Wieczorek, L.; Pace, R. J. Nature 1997, 387, 580–583. (26) Lahiri, J.; Kalal, P.; Frutos, A. G.; Jonas, S. J.; Schaeffler, R. Langmuir 2000, 16, 7805–7810. (27) Herrwerth, S.; Eck, W.; Reinhardt, S.; Grunze, M. J. Am. Chem. Soc. 2003, 125, 9359–9366. (28) Pertsin, A. J.; Grunze, M.; Garbuzova, I. A. J. Phys. Chem. B 1998, 102, 4918–4926.
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initiated extended studies of those molecules in organized assemblies.16,17,27-33 The contact angle, X-ray photoelectron spectroscopy (XPS), infrared reflection-absorption spectroscopy (IRRAS), electrochemical, and ellipsometric studies15,17,18,34,35 as well as Hartree-Fock calculations29 showed that the OEG SAM is resistant to protein adsorption under the following circumstances: (i) the terminal group is hydrophilic, (ii) the SAM has an hydrophilic interior allowing water molecules to penetrate the monolayer, (iii) the surface of the substrate has some defects, and/ or the monolayer has a relaxed lateral packing density. In order to understand the phenomena of resistance to protein adsorption, the structure of the OEG monolayer must be known in detail. SAMs containing an OEG chain exist in the solid crystalline phase, which was found to be dependent on the substrate used for the self-assembly and on the length of the OEG chain.34 When the OEG-terminated alkanethiols are adsorbed on Au surfaces, the intermolecular distance is equal to 4.97 A˚,29 typical for n-alkanethiols.5,10,34 The alkane chain is tilted by ca. 30 with respect to the surface normal, and molecules are packed hexagonally. One alkane chain occupies an area of 21.4 A˚2.34 In these monolayers the OEG chain adopts the helical conformation.18,34 The surface area per helix is equal to 21.3 A˚2, i.e., practically identical to the area of a hydrocarbon chain of n-alkanethiols in SAMs on Au surfaces.18,34 IRRA spectroscopy is an excellent tool to study conformation, organization, and phase transitions in various organized monolayers adsorbed on surfaces reflecting IR radiation (e.g., Au).36 In a monolayer built of OEG, some band positions can be used as markers, allowing conclusion about the phase and conformation of the OEG chain. They are CH2 wagging, CH2 twisting, νas(COC), and CH2 rocking modes.18,34,37 In SAMs containing six OEG units, strong absorptions are found at 1345, 1243, 1118, and 966 cm-1, indicating that the OEG chain exists in the solid helical conformation.18,34,37 In addition, the transition dipoles of all vibrations listed above are parallel to the chain axis of the OEG chain (A2 symmetry).38 The appearance of those bands in the IR spectrum leads to the conclusion that the OEG chain is oriented perpendicular, or almost perpendicular to the surface.18,34 However, Valiokas et al.37 observed a blue shift of absorption bands in some OEG-terminated alkanethiol SAMs, which is the consequence of increased disorder, resulting either in polymorphism or stronger tilting of the OEG chain. The tilting of the OEG-terminated alkanethiol is supported by quantum chemical calculations of the IRRA spectra of various OEG chains in the SAM on Au.39-42 The calculated IRRA (29) Wang, R. L. C.; Kreuzer, H. J.; Grunze, M. J. Phys. Chem. B 1997, 101, 9767–9773. (30) Kaji, H.; Hashimoto, M.; Nishizawa, M. Anal. Chem. 2006, 78, 5469–5473. (31) Kaji, H.; Kanada, M.; Oyamatsu, D.; Matsue, T.; Nishizawa, M. Langmuir 2004, 20, 16–19. (32) Kaji, H.; Tsukidate, K.; Matsue, T.; Nishizawa, M. J. Am. Chem. Soc. 2004, 126, 15026–15027. (33) Zhao, C.; Witte, I.; Wittstock, G. Angew. Chem., Int. Ed. 2006, 45, 5469–5471. (34) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chem. B 1998, 102, 426–436. (35) Zolk, M.; Eisert, F.; Pipper, J.; Herrwerth, S.; Eck, W.; Buck, M.; Grunze, M. Langmuir 2000, 16, 5849–5852. (36) Lippert, R. J.; Lamp, B. D.; Porter, M. D. Specular reflection spectroscopy. In Modern Techniques in Applied Molecular Spectroscopy; Mirabella, F. M., Ed.; John Wiley and Sons, Inc.: New York, 1998; pp 83-126. (37) Valiokas, R.; Svedhem, S.; Svensson, S. C. T.; Liedberg, B. Langmuir 1999, 15, 3390–3394. (38) Miyazawa, T.; Fukushima, K.; Ideguchi, Y. J. Chem. Phys. 1962, 37(12), 2764–2776. (39) Malysheva, L.; Klymenko, Y.; Onipko, A.; Valiokas, R.; Liedberg, B. Chem. Phys. Lett. 2003, 370, 451–459. (40) Malysheva, L.; Onipko, A.; Valiokas, R.; Liedberg, B. J. Phys. Chem. B 2005, 109, 13221–13227. (41) Malysheva, L.; Onipko, A.; Valiokas, R.; Liedberg, B. Appl. Surf. Sci. 2005, 246, 372–376. (42) Malysheva, L.; Onipko, A.; Valiokas, R.; Liedberg, B. J. Phys. Chem. A 2005, 109, 7788–7796.
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spectrum of OEG-akanethiols having a tilt angle of the long axis of the molecule equal to 26 showed that the νas(COC) mode has two absorptions: a strong band at 1114 cm-1 and a weak band at 1140 cm-1.42 Similarly, the calculated spectrum of randomly distributed molecules in the monolayer shows two absorptions in the νas(COC) mode.42 It is of technological interest to remove locally the proteinresistant and cell-repellent properties of organic thin films in order to create templates for protein adsorption and cell adhesion.30-33,43 Using a SAM system as cell-repellent layers is particularly attractive because of the large range of soft lithographic and electrochemical techniques applicable for patterning. The electrochemical desorption of thiols was used in order to increase protein adsorption properties of an OEG SAM.44 The cell-repellent properties of adsorbed proteins (albumin, fibronectin) on insulating surfaces have been locally “etched” by microelectrochemical generation of HBrO.30-32,43 In a similar setup, the cell repellent properties of OEG SAMs on Au can be changed by electrochemical formation of Br2.33,45 The Br2-treated areas of the OEG SAM showed an increased permeability, as evident from feedback imaging using scanning electrochemical microscopy (SECM). Bromine was not detected by XPS in modified areas. The modified areas allowed protein adsorption and cell adhesion.33,45 The reaction of the OEG SAM with Br2 is fast (several seconds) and irreversible and leads to a preferential removal of the OEG part, while the n-alkanethiol part degrades with a slower rate.45 However, the mechanism of the reaction with Br2 and the structural changes taking place in the monolayer have not yet been elucidated. This study provides a detailed structural and conformational analysis of the OEG and d-alkane chains of the HS(CD2)12(O-CH2-CH2)6OCH3 (D-OEG) molecule in the SAM before and after reaction with Br2. It makes use of a specifically deuterated D-OEG that facilitates the distinction of IR absorption bands originating from both parts of the molecule.46 In addition the Br2 treatment is performed in two configurations to mimic the situation in a microelectrochemical experiment and to compare this with a conventional immersion technique.
2. Experimental Section 2.1. Materials. The synthesis of specifically perdeutereated hexaethylene glycol-terminated alkanethiol D-OEG was reported previously.46 Na2SO4 (Merck, Darmstadt, Germany), Br2 (Fluka, Steinheim, Germany), ethanol (VWR International, Darmstadt, Germany) were used without further purification. KBr (Fluka, Steinheim, Germany) was dried before use. CCl4 (Merck, Darmstadt, Germany) was purified according to a standard procedure.47 2.2. Substrate Preparation. Microscope glass slides (MenzelGl€aser, Braunschweig, Germany, 2.5 2.5 cm2) were rinsed with water (PureLab Classic, Elga LabWater, Celle, Germany, 18.2 MΩ cm-2) and ethanol and cleaned in an ozone cleaner (Bioforce Nanosciences Inc., Ames, IA) for 10 min. On the cleaned glass surface, 5 nm of an adhesive Cr layer and 200 nm of gold were evaporated using a Tectra MiniCoater (Tectra GmbH, Frankfurt/ Main, Germany). Gold slides were immersed in a 1 μM D-OEG (43) Kaji, H.; Tsukidate, K.; Hashimoto, M.; Matsue, T.; Nishizawa, M. Langmuir 2005, 21, 6966–6969. (44) Jiang, X.; Ferrigno, M.; Mrksich, M.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 2366–2367. (45) Zhao, C.; Zawisza, I.; Nullmeier, M.; Burchardt, M.; Tr€auble, M.; Witte, I.; Wittstock, G. Langmuir 2008, 24, 7605–7613. (46) Jogireddy, R.; Zawisza, I.; Wittstock, G.; Christoffers, J. Synlett 2008, 1219–1221. (47) Schwetlick, K. Organikum, 21st ed.; Wiley-VCH: Weinheim, Germany, 2001; p 760.
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Article solution in ethanol for 24 h to form a SAM. The modified surface was rinsed with ethanol and dried in an Ar stream. 2.3. Reaction with Bromine. For each reaction time, the experiment was repeated for two independently prepared D-OEG SAMs. In one set of experiments, the SAM-modified gold slides were placed into the solution of 25 μM Br2 and 0.1 M Na2SO4. The time of the reaction varied from 1 to 120 s. In this set of experiments, each sample was used once. In the second set of experiments, the SAM-modified gold served as an electrode in a galvanic cell Au|D-OEG SAM| 25 μM Br2 + 0.1 M Na2SO4||50 μM KBr+0.1 M Na2SO4|Au. The reaction time varied from 10 to 900 s. After the PM IRRAS experiment was performed on samples exposed to the galvanic cell for short times (0-100 s), the reaction was continued for longer time (150-900 s). In parallel, a new sample was placed into the galvanic cell for a given long time (150-900 s). We did not observe differences between the two spectra. After the reaction with Br2, each sample was rinsed with water and dried in an Ar steam, and next the PM IRRA spectra were measured.
2.4. Fourier Transform Infrared (FT-IR) Measurements. Neat D-OEG was placed between two BaF2 windows separated by a 25 μm thick Teflon spacer (Aldrich, Steinheim, Germany). Thirty-two spectra with a resolution of 4 cm-1 were recorded with a Vertex 70 spectrometer (Bruker, Ettlingen, Germany). Additionally, 32 IR spectra of CCl4 (background) and 100 spectra of 0.5% (mass/mass) D-OEG in CCl4 were collected with a resolution of 4 cm-1. The isotropic optical constants of D-OEG were obtained according to the procedure described in the literature.48-50 The optical constants in the CH stretching, CD stretching and CH deformation, and COC stretching mode regions are shown in the Supporting Information. The attenuation coefficient k was determined from the transmission spectrum. Next, the refractive index n was calculated from k values using Kramers-Kronig transformation.48 In the CH and CD stretching mode regions, the refractive index at infinite frequency n¥ is equal to 1.41.51 For the COC stretching modes region of the OEG part, n¥ is equal 1.40.52 2.5. PM IRRAS Measurements. The polarization modulation infrared reflection absorption (PM IRRA) spectra were recorded using a Vertex 70 spectrometer equipped with a polarization modulation set (PMA 50 Bruker, Ettlingen, Germany) containing a photoelastic modulator and a demodulator (Hinds Instruments, USA). All spectra were recorded with a resolution of 2 cm-1. The maximum proton exchange membrane (PEM) efficiency was set for the half-wave retardation at 2900 cm-1 for the analysis of the CH stretching bands at 2200 cm-1 for CD stretching bands, and at 1500 cm-1 for the CH2 deformation and COC stretching modes. Each spectrum results from 8000 averaged spectra. The incident angle of light was set to 80. All IR spectra were collected in dry air atmosphere. Before reaction with Br2, each sample was tested. 1000 p.m. IRRA spectra were collected. The number and intensities of absorption bands in 3050-2700, 2200-2000, and 1600-900 cm-1 were checked. The reproducibility of this experiment was very high. From all 32 different D-OEG SAMs, the error in the integrated intensities of the strongest bands (asymmetric COC stretching mode, asymmetric and symmetric O-CH2 modes, symmetric OCH3 mode, CH2 twisting mode, and asymmetric CD2 stretching mode) was ( 3%. The D-OEG SAM was immersed either directly into the electrolyte solution containing 25 μM Br2 or in the galvanic cell. Each experiment was repeated two times. (48) Allara, D. L.; Baca, A.; Pryde, C. A. Macromolecules 1978, 11(6), 1215– 1220. (49) Li, N.; Zamlynny, V.; Lipkowski, J.; Henglein, F.; Pettinger, B. J. Electroanal. Chem. 2002, 524-525, 43–53. (50) Popenoe, D. D.; Stole, S. M.; Porter, M. D. Appl. Spectrosc. 1992, 46, 79–87. (51) Flach, C. R.; Gericke, A.; Mendelsohn, R. J. Phys. Chem. B 1997, 101(1), 58–65. (52) Leger, J. M.; Carter, S. A.; Ruhstaller, B. J. Appl. Phys. 2005, 98(12), 124907/1–124907/7.
364 DOI: 10.1021/la9020993
Brand et al. The spectra were calculated for the air-gold-D-OEG SAM interface at an incident angle equal to 80. The length of the D-OEG molecule was calculated taking into account the thickness of a single methylene unit of 0.126 nm, the thickness of the ethylene glycol unit forming a helical structure of 0.285 nm, and the thickness of the terminal methyl group of 0.153 nm.16 The calculated length of the D-OEG molecule is equal to 3.4 nm. This value agrees well with thickness of 3.3 nm measured for a SAM of HS(CH2)11(O-CH2-CH2)6OCH3 by ellipsometry containing only 11 methylene groups compared to 12 methylene groups in our study.16 2.6. Computation. The IR-spectra for the adsorbed oligomer were obtained by first principles calculations of the CH3-O(-C2H4-O-)6C12H24-SH molecule within density functional theory (DFT) using the Gaussian 03 program package.53 A full geometry optimization of the species was performed using the Perdew-Burke-Ernzerhof (PBE) functional for exchange and correlation,54,55 applying a cc-pVDZ basis set56,57 containing 726 basis functions. In order to simulate IR spectra for various tilt angles of the hydrocarbon chain with respect to the surface normal, transition dipole moments vectors were calculated for a fixed molecular orientation using the “nosymmetry” keyword. The surface normal was defined to be the z-axis of the coordinate system. In accordance with surface selection rules, relative IR-intensities I(ν)/I0 for each normal mode with frequency νi were simulated by taking only the z-components of the transition dipole moment vector μiz into account. Assuming a Lorentzian line shape with a full width at half-maximum (fwhm) σ of 15 cm-1, we obtained spectra for N normal modes according to58 N IðνÞ σX ðμiz Þ2 ¼ I0 π i ¼1 ν - γ 3 νi þ σ 2
ð1Þ
Each normal-mode frequency νi was scaled by a correction factor of γ=0.9911. This factor was derived by frequency calculation of our molecule on the PBE/6-31G(d) level of theory, for which correction factors of 0.987559 and 0.987460 have been reported. From the scaled results, the correction factor γ = 0.9911 for the PBE/cc-pVDZ level of theory was obtained.
3. Results and Discussion 3.1. Spectral Characteristics of D-OEG. The IR spectrum of the neat D-OEG molecule shown in Figure 1 has three strong absorptions centered around 2870, 2200, and 1115 cm-1. These (53) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, J. T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (54) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77(18), 3865–3868. (55) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1997, 78(7), 1396. (56) Dunning, T. H. J. Chem. Phys. 1989, 90(2), 1007–1023. (57) Woon, D. E.; Dunning, T. H. J. Chem. Phys. 1993, 98(2), 1358–1371. (58) Shinohara, H.; Kasahara, T.; Kadokura, K.; Uryu, Y.; Itoh, K. J. Phys. Chem. B 2004, 108, 3584–3592. (59) Merrick, J. P.; Moran, D.; Radom, L. J. Phys. Chem. A 2007, 111(45), 11683–11700. (60) Jimenez-Hoyos, C. A.; Janesko, B. G.; Scuseria, G. E. Phys. Chem. Chem. Phys. 2008, 10(44), 6621–6629.
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Figure 1. The IR spectrum of neat D-OEG pressed between two BaF2 windows separated by a 25 μm thick spacer.
absorptions are due to the CH stretching modes in the OEG, the CD stretching modes in the d-alkane chain, and the COC stretching modes in the OEG part of the molecule, respectively. Figure 2 shows a PM IRRA spectrum of D-OEG SAM on Au (panel a) and the calculated spectrum of D-OEG monolayer obtained from optical constants existing in the liquid state (panel b) in the CH and CD stretching modes region. The broad absorptions in 3000-2700 cm-1 spectra region correspond to CH3 and CH2 stretching modes arising from seven overlapping fundamental and combination vibrations.38 In order to find positions of the overlapping absorption bands, the second derivative was used. Table 1 lists the positions and the relative intensities of the ν(CH) and the ν(CD) bands of D-OEG in the native form, dissolved in CCl4, and in the SAM on gold. As listed in Table 1, which provides the positions and intensities of νas(OCH2), three νs(OCH2) modes are strongly dependent on the analyzed sample. Namely, in neat D-OEG and D-OEG dissolved in CCl4 mentioned above, bands are found at 2-8 cm-1 higher frequencies as compared to the SAM. A blue-shift of the methylene stretching modes is due to the liquid phase of the two analyzed D-OEG samples and statistical random distribution. Moreover, Figure 2a shows a very weak absorption around 2925 cm-1, suggesting that, in the alkane chain of D-OEG, few CHbonds in CHD groups are present (see Supporting Information).61 As listed in Table 1, the absorption maxima of the νas(CD2) and νs(CD2) modes of the SAM have lower wavenumbers than the neat D-OEG and D-OEG dissolved in the CCl4. Concluding, positions of CH2 and CD2 stretching modes indicate that the OEG and alkane thiol parts of the D-OEG molecules in the SAM exist in a solid state. The low-frequency region of the IR spectrum of D-OEG is dominated by a strong absorption arising from the νas(COC) stretching mode and weaker absorptions originating from the methylene and the methyl bending, wagging, twisting, and rocking modes in the OEG part of the molecule. The IR spectrum of neat D-OEG is shown in Figure 1, and that of the D-OEG in the SAM on Au (panel a) and of the D-OEG dissolved in CCl4 (panel b) are shown in Figure 3. Positions and relative intensities of IR absorption bands of the analyzed D-OEG molecule are presented in Table 2. Not all IR absorptions observed in neat D-OEG and D-OEG dissolved in CCl4 are visible in the PM IRRA spectrum of the D-OEG monolayer. The number and positions of the CH wagging modes and of the νas(COC) are dependent on (i) the packing and crystalline phases of the OEG part of the molecule in the SAM, and (ii) the orientation of the molecule in the monolayer.16,28,62
In the PM IRRA spectrum of the D-OEG SAM, the νas(COC) stretching mode has a strong and sharp absorption at 1117.2 cm-1 with a weak shoulder at 1130.6 cm-1 (Figure 3a). In the SAM, the CH2 wagging mode is expressed by a sharp absorption band (fwhm equal to 5.5 cm-1) centered at 1347.4 cm-1. Moreover, the presence of the CH2 rocking absorption at 963.7 cm-1 indicates that the D-OEG SAM exists in the crystalline phase and has a helical conformation in which the O-C, C-C- and C-O bonds have trans, gauche, and trans conformation, respectively.16,62 The νas(COC) stretching mode located around 1130 cm-1 (SAM) and around 1140-1145 cm-1 (neat D-OEG and D-OEG dissolved in CCl4) are assigned to the amorphous phase of the OEG, where the C-C bond has trans-gauche-trans or gauche-gauche-trans arrangements.16,63 The absorption around 1143-1148 cm-1 together with the CH2 wagging mode located at 1326 cm-1 are assigned to all-trans conformation of the OEG.37 The spectrum presented in Figure 3a has practically no intensity around 1326 cm-1 and around 1149 cm-1, indicating that the all-trans conformation of the OEG part does not exists in the SAM on the Au surface. The CH2 wagging mode has a very weak absorption at 1357.5 cm-1, indicating that a small portion of the D-OEG molecules may exist in an amorphous phase. This is also expressed by the presence of the νas(COC) mode around 1130.6 cm-1. However, the absorption around 1130 cm-1 can also arise from the νas(COC) mode having the transition dipole directed perpendicularly to the long axis of the OEG.16 The very low intensity of the CH2 wagging mode at 1357.5 cm-1 and the relatively large intensity of the band
(61) Fringeli, U. P. Z. Naturforsch. 1977, 32c, 20–45. (62) Valiokas, R.; Svedhem, S.; Ostblom, M.; Svensson, S. C. T.; Liedberg, B. J. Phys. Chem. B 2001, 105, 5459–5469.
(63) Parikh, A. N.; Beers, J. D.; Shreve, A. P.; Swanson, B. I. Langmuir 1999, 15, 5369–5381.
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Figure 2. The IR spectra of D-OEG in the (A) CH and (B) CD stretching modes region: (a) the experimental PM IRRA spectrum and (b) the calculated PMIRRA spectrum calculated from optical constants determined from D-OEG dissolved in CCl4. The absorption modes are marked in the figure. Filled area of the deconvoluted modes marked with vertical (horizontal) lines denotes absorption bands with transition dipole oriented parallel (perpendicular) to the chain long axis.
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Table 1. Description, Position, Relative Intensity, and Direction of the Transition Dipole of the CH and CD Stretching Modes in Neat D-OEG, D-OEG Dissolved in CCl4, and in a SAM on Aua band position/cm-1b band description
neat
in CCl4
SAM
polarization/direction of the transition dipole
ref
2884.9 w 2979.4 w 2981.0 w 38 νas(OCH3) 2950.2 w 2947.0 w 2941.8 w 38 νas(OCH2) 2927.0 w 2925.6 w 2929.7 vw ^ vs helical axis 61 νas(CH2) 2900.0 m 2907.2 m 2892.0 vs || vs helical axis 38 νs(OCH2) 2882.5 m 2881.9 m 2880.0 vw ^ vs helical axis 38 νs(OCH2) -1 2866.4 m 2864.7 m 2862.8 s || vs helical axis 38 νs(OCH2) + combination tone δ(CH2) at 1460 cm -1 + CH2 wagg. at 1415 cm 38 combination tone δ(CH2) at 1460 cm-1 + CH2 wagg. at 1355 cm-1 2827.4 w 2827.0 w 2825.2 w ^ vs helical axis 2816.1 w 2816.5 w 2814.3 w ^ vs helical axis 38 νs(OCH3) -1 -1 2730.4 vw 2731.4 vw 2742.3 w || vs helical axis 38 combination tone δ(CH2) at 1460 cm + CH2 twisting at 964 cm 2198.3 s 2200.6 s 2194.3 w ^ to the bisector of the methylene group 74 νas(CD2) 2097.0 s 2100.3 s 2087.8 w || to the bisector of the methylene group 74 νs(CD2) a References are provided for band descriptions and their polarizations. b Description of intensities of absorption bands: vs - very strong, s - strong, m - medium, w - weak, and vw - very weak.
Figure 3. The IR spectra of D-OEG in the COC stretching and CH deformation modes region: (a) the experimental PM IRRA spectrum and (b) the calculated PMIRRA spectrum calculated from optical constants determined from D-OEG dissolved in CCl4. The absorption modes are marked on the figure. Filled area of the deconvoluted modes marked with vertical (horizontal) lines denotes absorption bands with transition dipole oriented parallel (perpendicular) to the chain long axis.
at 1130.6 cm-1 indeed suggest a spectral contribution from the νas(COC) mode having its dipole moment perpendicular to the helical axis (Figure 3a). 3.2. Orientation of D-OEG Molecule in the SAM Exposed to Bromine. OEG Part of D-OEG. The progress of the reaction of D-OEG SAM with Br2 was monitored by means of the PM IRRAS. Figure 4 shows the PM IRRA spectra of D-OEG monolayer exposed directly to 25 μM Br2 solution (Figure 4A,B) and in the galvanic cell Au|D-OEG SAM| 25 μM Br2 þ 0.1 M Na2SO4|| 50 μM KBr þ 0.1 M Na2SO4|Au (Figure 4C,D). We have shown that the reaction of the OEG monolayer with Br2 occurs rapidly, destroying the monolayer, while, in the galvanic cell, Br2 may be scavenged at partially damaged and thus permeable SAMs by the heterogeneous electron transfer reaction Br2þ2e- f 2Br-. The electrons are provided by the reverse reaction at the anode of the galvanic cell, preserving the structure of the monolayer for longer time (see Supporting Information). As shown in Figure 4A,B the ν(CH) stretching, the νas(COC) stretching, and the CH2 wagging, twisting, and rocking modes of the OEG part of the molecule in the SAM disappear after 120 s of the reaction with 25 μM Br2 solution. Already after 5 s of the reaction, the intensity of all absorption bands has decreased. The rate of disappearance of absorption bands originating from the terminal methyl group and from the methylene groups in the 366 DOI: 10.1021/la9020993
OEG is the same. This indicates that the reaction with Br2 affects the entire OEG part of the molecule. In the galvanic cell, however, the reaction with Br2 is slower. Even after 900 s of the reaction, the IR absorption bands typical for the OEG part of the molecule are present in the PM IRRA spectrum (Figure 4C,D). During the reaction absorptions centered at 2892.0, 1347.4, 1244.1, 1117.2, and 963.7 cm-1 slowly decrease in intensity. Absorption bands at 2880.0, 1257.5, 1130.6, and 1080.0 cm-1 arising from νs(CH2), CH2 twisting, νas(COC), and νs(COC) modes increase in intensity during the reaction in the galvanic cell. As listed in Table 2, the νas(COC) mode is deconvoluted into three absorption bands. In the galvanic cell, PM IRRA spectra are not affected so significantly. After 900 s, this band still had 45% of its initial intensity. During the reaction with Br2, the integral intensity of the νas(COC) band at 1130.6 cm-1 does not change much. The intensity of the very weak νs(COC) mode at 1080.0 cm-1 slightly increases. Changes in the integral intensities may be due to (i) very slow removal of the OEG part of the molecule from the SAM, and (ii) to reorientation of the molecule during reaction. Intensities of parallely polarized modes decrease, while intensities of perpendicularly polarized modes slightly increase or remain constant during the reaction. As a consequence of the reaction with Br2, the tilt of the OEG chain changes. The solution spectra of the D-OEG molecule, as demonstrated above, do not correspond to a spectrum of a crystalline helical form of the SAM on Au. A calculated spectrum, obtained from optical constants cannot be used to quantitatively analyze the orientation of the SAM during the reaction with Br2. In order to discuss the molecular structure and its changes in the D-OEG monolayer, an approach based on the DFG calculations was used. The IR spectrum of the crystalline OEG molecule existing in helical conformation was calculated at the PBE/cc-pVDZ level of theory. The molecular orientation was frozen in such a way that the z axis (perpendicular to the surface normal) was overlapped with the direction of the fully expanded chain of the alkane thiol part of the molecule. In order to calculate an IR spectrum, fulfilling surface selection rules,58 only normal components of the transition dipoles were taken into account. In the next steps, the molecule was tilted so that the calculated spectra corresponded to the tilt of the long axis of the helix varying from 20 to 60 with respect to the normal to the surface. Figure 5 shows calculated spectra in the νas(COC) stretching region of the OEG molecule in which the tilt angle of the helical axis changes from 20 to 60 with respect to the surface normal. The fwhm is set to 15 cm-1, similarly to experimental results. In the calculated spectrum, the Langmuir 2010, 26(1), 362–370
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Table 2. Description, Position, Relative Intensity, and Direction of the Transition Dipole in the CH Deformation and COC Stretching Modes in Neat D-OEG, D-OEG Dissolved in CCl4, and in SAM on Aua band position/cm-1 band description
neat
in CCl4
SAM
δ(CH2) CH2 wagg. δs(CH3) CH2 wagg. amorph. CH2 wagg. gauche CH2 wagg. trans CH2 twisting CH2 twisting CH3 rock. νas(C-O-C)
1458.1 m 1402.9 vw 1378.2 vw 1351.5 m
1459.8 w 1378.1 vw 1352.2 m
1326.8 vw 1300.0 w 1248.0 w 1198.6 w 1141.9 vs
1326.6 vw 1297.5 vw 1245.6 vw 1198.9 vw 1146.5 s
1461.8 w 1415.5 vw 1388.9 vw 1357.9 vw 1347.4 vs 1329.3 vw 1244.1 w 1201.5 w
1130.6 m 1113.8 vs 1118.6 s 1117.2 vs νas(C-O-C) 1034.0 w 1036.6 w 1030.6 vw νs(C-O-C) 963.7 m CH2 rock. gauche a References are provided for band descriptions and their polarizations.
polarization/direction of the transition dipole
ref
|| to the bisector of methylene group ^ vs helical axis || C3v axis in methyl group ^ vs helical axis || vs helical axis
61 38 61 38 38
^ vs helical axis || vs helical axis
38 38
^ vs helical axis
38
|| vs helical axis ^ vs helical axis || vs helical axis
38 38 38
Figure 4. PM IRRA spectra of D-OEG monolayers on Au after different reaction times with 25 μM Br2 solution (times are given in the figure) (A,B) and in the galvanic cell Au|D-OEG SAM| 25 μM Br2 þ 0.1 M Na2SO4|| 50 μM KBr þ 0.1 M Na2SO4|Au (C,D) in the CH stretching (A,C) and CH2 deformation and COC stretching modes region (B,D).
νas(COC) mode at 1110 cm-1 has the transition dipole parallel to the helical chain of the OEG part of the molecule. In the experimental spectrum, this band corresponds to the νas(COC) mode located at 1117.2 cm-1. In the calculated spectrum, the band at 1133 cm-1 has the transition dipole vector perpendicular to the helix direction and corresponds to the experimental band observed at 1130.6 cm-1. As seen in Figure 5, increasing the tilt of the helix axis causes the intensity of the νas(COC) mode at 1110 cm-1 to gradually decrease. In contrast, the intensity of the νas(COC) mode at 1133 cm-1 slowly increases with tilting of the helix of the OEG molecule toward the surface. In order to find the tilt of the D-OEG helix in the SAMs, the ratio of the νas(COC) mode at 1110 cm-1 to the mode at 1133 cm-1 was compared between the calculated and experimental results (Figure 6). Large Langmuir 2010, 26(1), 362–370
points shown in Figure 6 correspond to the experimental data, namely, the ratio after various times of the reaction with Br2. In the SAM before reaction with Br2, the tilt of the helix is close to 27 with respect to the surface normal. During the reaction with Br2, this tilt progressively increases to ca. 50 with respect to the surface normal after 900 s of the reaction in the galvanic cell. This increase of the tilt of the OEG helix points to an increase in disorder in the SAM and gradual inclination of the OEG chains. d-Alkane Chain. Figure 7 shows the PM IRRA spectra in the CD stretching mode region of the d-alkane chain of D-OEG molecules in a SAM after reaction with 25 μM Br2 solution and in the galvanic cell Au|D-OEG SAM| 25 μM Br2 þ 0.1 M Na2SO4|| 50 μM KBr þ 0.1 M Na2SO4|Au. The blue-shift and narrowing of the ν(CD2) bands in the SAM monolayer compared to neat DOI: 10.1021/la9020993
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Figure 5. Calculated IR spectra of an OEG molecule with tilt of the OEG helix axis varying from 20 to 60 with respect to the surface normal.
Figure 7. PM IRRA spectra of D-OEG monolayers on Au after different reaction times, in 25 μM Br2 (times are given in the figure) (A) and reaction in the galvanic cell Au|D-OEG SAM| 25 μM Br2 þ 0.1 M Na2SO4|| 50 μM KBr þ 0.1 M Na2SO4|Au (B) in the CD stretching mode region.
Figure 6. Ratio of the COC stretching mode at 1110 cm-1 to 1133 cm-1 in the calculated spectra (line) with marked ratio of the same modes in the SAM during reaction with Br2. Time of the reaction marked on the figure.
D-OEG and D-OEG dissolved in CCl4 solution indicate a solidification, ordering, and reduction of the mobility of the dalkane chain (Table 2).64,65 Indeed, positions of the νas(CD2) and νs(CD2) modes are characteristic for the d-alkane chain existing in the solid all-trans conformation.64,65 The νas(CD2) and νs(CD2) modes are present in the spectrum even after 120 s of reaction with 25 μM Br2 solution. Contact of the D-OEG monolayer with 25 μM Br2 solution causes gradual red-shift of ν(CD2) modes (Figure 7A). After 120 s of the reaction, the position of the νas(CD2) shifts from 2194.3 to 2198.5 cm-1 and that of the νs(CD2) shifts from 2087.8 to 2098.0 cm-2. The fwhm values increase from 15.3 to 17.8 cm-1 and from 16.8 to 20.3 cm-1, respectively. These changes point to a gradual fluidification of the d-alkane chains in the monolayer. The integral intensities of the ν(CD) modes decrease during the reaction, indicating a removal of this part of the molecule from the monolayer and/or its reorientation. The transition dipoles of both asymmetric and symmetric d-methylene stretching modes are orthogonal to each other and lie in the plane of the methylene group.61 On one hand, (64) Cameron, D. G.; Gudgin, E. F.; Mantsch, H. H. Biochemistry 1981, 20, 4496–4500. (65) Pastrana-Rios, B.; Flach, C. R.; Brauner, J. W.; Mautonr, A. J.; Mendelsohn, R. Biochemistry 1994, 33, 5121–5127.
368 DOI: 10.1021/la9020993
Figure 8. Tilt angle of the d-alkane chain (θtilt) in the D-OEG monolayer as a function of reaction time in the galvanic cell.
a decrease in integral intensities of the ν(CD2) stretching modes may indicate that the θ angles increase, thus alkane chains adopt more vertical orientation with respect to the surface normal.6,36 On the other hand, a blue-shift and broadening of the ν(CD) modes point on fluidification and an increased mobility of the alkane chains. These two conclusions contradict each other. Summarizing, a reduction of the CD stretching bands intensities is most likely due to a decrease of the surface concentration. As shown in Figure 7B, the d-alkane part of the molecule in the SAM behaves differently when exposed to Br2 solution in the galvanic cell. Independently of the reaction time, the position and fwhm of νas(CD2) is constant. The νs(CD2) mode is observed at constant wavenumber but it becomes broader. The integral intensities of both CD2 stretching modes increase during reaction in the galvanic cell. As shown above, the d-alkane chains exist in all-trans conformation, thus vectors of the transition dipoles of Langmuir 2010, 26(1), 362–370
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Article Scheme 1. Proposed Mechanism of the Reaction of the D-OEG SAM with Br2
νas(CD2) and νs(CD2) are perpendicular to each other and form a right angle to the axis of the chain. Values of the angles θ2195 and θ2090 are used to calculate the tilt of the alkane chain θtilt.66 A plot of the calculated θtilt versus reaction time is shown in Figure 8. At t
