ARTICLE pubs.acs.org/JPCC
Self-Assembly of Pyridine-Substituted Alkanethiols on Gold: The Electronic Structure Puzzle in the Ortho- and Para-Attachment of Pyridine to the Molecular Chain Hicham Hamoudi,† Katrin D€oring,‡ Frederick Chesneau,† Heinrich Lang,*,‡ and Michael Zharnikov*,† † ‡
Angewandte Physikalische Chemie, Universit€at Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany Institut f€ur Chemie, Lehrstuhl f€ur Anorganische Chemie, Technische Universit€at Chemnitz, Straße der Nationen 62, 09111 Chemnitz, Germany ABSTRACT: X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, infrared reflection absorption spectroscopy, and electrochemistry were applied to monitor the formation and properties of self-assembled monolayers (SAMs) of 4-(aminomethyl)pyridine-11-mercaptoundecanamide (C10AP1), 2-(aminomethyl) pyridine-11-mercaptoundecanamide (C10AP2), and aminomethyl-di(2-pyridyl)-11-mercaptoundecanamide (C10AP3) prepared on Au(111) substrates. While all three precursors formed well-defined SAMs, their structural and electronic properties were found to be noticeably different depending on (i) the attachment of the terminal pyridine moiety to the molecular chain and (ii) the number of pyridine units in the tail group. In particular, whereas the appearance of intramolecular hydrogen bonds is the most likely scenario in the ortho case, the formation of the intermolecular “cross-linking” network could be proposed in the para case. Accordingly, the branching of the characteristic pre-edge absorption resonances in the C K-edge NEXAFS spectra of the target SAMs is distinctly different for the cases of the ortho- or para-attachment of pyridine, which could be tentatively explained by the different couplings of the electronic and vibrational excitations in each case.
1. INTRODUCTION During the last two decades self-assembled monolayers (SAMs) attracted considerable attention of both scientific and industrial communities since they enable to fabricate an organic surface of definite chemical identity or redefine the given surface identity adjusting the surface properties such as wetting, adhesion, lubrication, corrosion, and biocompatibility.16 SAMs are polycrystalline films of chainlike or rodlike molecules that are chemically anchored to the substrate (we refer here to the definition of SAMs given in refs 2 and 3). A SAM precursor consists generally of three parts: a headgroup that binds strongly to the substrate, a tail group that constitutes the outer surface of the film, and a spacer that connects the head and tail groups. Within this general architecture, a flexible combination of different functional moieties is possible, and in particular, a broad variety of different tail groups can be used, depending on the specific application. One of the potentially most useful tail groups is the pyridine moiety. The major characteristic feature of this group is the lone pair of electrons at nitrogen. As result thereof, the pyridine moiety provides a Lewis-base character to the SAM-ambient interface, which makes it suitable for the immobilization of different functional units711 and especially for the coordination of metal atoms and transition metal fragments.1014 r 2011 American Chemical Society
Numerous reports on the preparation and application of different pyridine-terminated SAMs, and in particular SAMs of pyridine-terminated thiolates chemisorbed on Au(111), can be found in literature.711,1321 Whereas the first studies were carried out on 4-mercapto-pyridine films,12,13,2224 in which the pyridine unit was directly attached to the thiolate headgroup, more complex systems were addressed later.15,17,19,20,25 In these studies, the pyridine unit was attached to an aliphatic,10,17 aromatic,15,16 hybrid aliphaticaromatic,19,20 or oligo(phenylen ethynelene)11 spacer or combined with other tail groups.10,18 Some of the studies were devoted to applications, ranging from biology79 to nanofabrication, including the growth of metal nanoparticles,12,14 optically switchable surfaces,11 metalorganic frameworks,26,27 and nanoparticle lithography.21 At the same time, chemical and physical properties of pyridine-terminated surfaces such as the protonationdeprotonation phenomena,17,19 interaction with water,15 and so forth were investigated. In addition, recently, significant attention was paid to the fundamental properties of pyridine-terminated SAMs including their structure and electronic properties.1720 These activities were, however, concentrated on either short-chain aliphatic SAMs17 or aromatic monolayers,1820 while, to the best of our knowledge, no fundamental studies on longchain aliphatic SAMs with a pyridine tail group were reported so far. Received: September 16, 2011 Revised: November 22, 2011 Published: November 28, 2011 861
dx.doi.org/10.1021/jp2089643 | J. Phys. Chem. C 2012, 116, 861–870
The Journal of Physical Chemistry C
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In this context, we studied a series of pyridine-substituted long-chain alkanethiolate SAMs on Au(111) diverging from one another by (i) either the ortho- or para-attachment of the terminal pyridine moiety to the molecular chain and by (ii) the number of the pyridine units in the tail group. The respective SAM precursors, namely, 4-(aminomethyl) pyridine-11-mercaptoundecanamide (C10AP1), 2-(aminomethyl)pyridine-11-mercaptoundecanamide (C10AP2), and aminomethyl-di(2-pyridyl)-11mercaptoundecanamide (C10AP3) are schematically depicted in Figure 1; we will use the abbreviation C10APn for these three films together. The goals of the study were to correlate the specific attachment architecture of the pyridine unit with the structural and electronic properties of the SAMs. To study these properties we used a combination of several complementary experimental techniques (see the next section).
The SAM precursors C10AP1 (2), C10AP2 (4), and C10AP3 (6) described by the general formula RNHCO(CH2)10SH (2, R = p-C5H4NCH2; 4, R = o-C5H4NCH2; 6, R = (o-C5H4N)2CH) were prepared by the synthesis methodology as outlined in Figure 2 and described in detail for every compound separately below (we use numbers instead of the abbreviation in the synthesis part). In short, the addition of the amine RNH2 (1.2 equiv) (1, R = p-C5H4NCH2; 3, R = o-C5H4NCH2; 5, R = (o-C5H4N)2CH) and N,N0 dicyclohexylcarbodiimide (1.2 equiv) to 11-mercaptoundecanoic acid (1 equiv) and 1-hydroxybenzotriazole (1.2 equiv) afforded by concomitant precipitation of (NH2)2CO the products 2, 4, and 6, respectively. After appropriate workup, the reaction products 2, 4, and 6 could be isolated as colorless solid materials in 7479% yield. Please note that the yield for compound 2 is stated in literature as ∼20%.29 All reactions were carried out under an atmosphere of purified nitrogen using standard Schlenk techniques. Dichloromethane was purified by distillation from phosphorus pentoxide. Compounds 2, 4, and 6 were characterized by elemental analysis, spectroscopy (IR; 1H, 13C{1H} NMR), and mass spectrometry (ESI-TOF). Infrared spectra were recorded with a Nicolet FT-IR 200 spectrometer. NMR spectra were recorded with a Bruker Avance 250 spectrometer. 1H NMR spectra were recorded at 250.130 MHz (internal standard, relative to CDCl3, δ 7.26) and 13 C{1H}NMR spectra at 62.902 MHz (internal standard, relative to CDCl3, δ 77.16). Chemical shifts are reported in δ units (parts per million) downfield from tetramethylsilane with the solvent as the reference signal. Elemental analyses were performed with a Flashea analyzer, Thermo Electron Corporation. Melting points were determined using sealed nitrogen purged capillaries on a Gallenkamp MFB 595 010 M melting point apparatus. The ESITOF (electrospray-ionizationtime-of-flight) mass spectra were recorded with a micrOTOF QII from Bruker Daltonik in dichloromethane. Synthesis of p-C5H4NCH2NHCO(CH2)10SH (2). 11Mercaptoundecanoic acid (1.20 g, 5.49 mmol) and 1-hydroxybenzotriazole (0.89 g, 6.59 mmol) were added to 25 mL of dichloromethane at 25 °C. After 4-(aminomethyl)pyridine (1, 0.71 g, 0.66 mL, 6.59 mmol) was added in a single portion, a colorless solution was obtained. N,N0 -Dicyclohexylcarbodiimide (1.36 g, 6.59 mmol) dissolved in 10 mL of dichloromethane was added at 0 °C. After 45 min of stirring at 0 °C, a colorless precipitate was obtained, and stirring was continued for 48 h at 25 °C. Afterward the reaction mixture was filtered through silica gel with diethyl ether. All volatiles were removed in an oil-pump vacuum, and the residue was dissolved in dichloromethane (20 mL). The reaction mixture was extracted three times with
2. EXPERIMENTAL SECTION 2.1. Synthesis of the SAM Precursors. Di(2-pyridyl)methylamine was prepared following published procedures.28 All other chemicals are commercially available, namely, 11-mercaptoundecanoic acid (99%, Aldrich), 1-hydroxybenzotriazole (97%, Aldrich), N,N0 -dicyclohexylcarbodiimide (99%, Acros Organics), 4-(aminomethyl)pyridine (98%, Sigma), and 2-(aminomethyl)pyridine (99%, Aldrich). These chemicals were used as received.
Figure 1. A schematic drawing of the SAM precursors along with their acronyms. The pyridine units are coupled to the alkyl chain over the amide group. This coupling chemistry was used to simplify the synthesis procedure.
Figure 2. Synthesis of mercaptoundecanamides C10AP1, C10AP2, and C10AP3 from mercaptoundecanoic acid and 1, 3, and 5, respectively. 862
dx.doi.org/10.1021/jp2089643 |J. Phys. Chem. C 2012, 116, 861–870
The Journal of Physical Chemistry C
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water (25 mL) and once with a saturated NaCl solution (25 mL), and the combined layers were dried over magnesium sulfate. After removal of all volatiles in oil-pump vacuum, the residue was purified by column chromatography (column size: 20 3 cm, silica gel, dichloromethane/methanol = 50:2 (v/v)). The title compound was eluted at first (colorless band) followed by nonreacted 1 (colorless band). The evaporation of all volatile materials in oil-pump vacuum gave a colorless solid. Yield: 1.25 g (4.05 mmol, 74% based on 11-mercaptoundecanoic acid). Elemental analysis: Calcd for C17H28N2OS (308.47): C, 66.19; H, 9.15; N, 9.08. Found: C, 65.98; H, 9.20; N, 10.31%. m.p.: 65 °C. IR (KBr): ν [cm1] 3299 (vs) (νNH), 3071 (m), 3024 (w), 2919 (vs) (νCH2), 2849 (vs) (νCH2), 2498 (s) (νSH), 1945 (w), 1639 (vs) (νCdO), 1604 (s), 1552 (s) (νNH), 1460 (m), 1429 (m), 1416 (m), 1364 (m), 1338 (m), 1318 (m), 1294 (m), 1285 (m), 1269 (m), 1245 (m), 1218 (m), 1190 (w), 1119 (w), 1088 (w), 1065 (w), 1045 (w), 1026 (w), 993 (w), 799 (m), 765 (w), 745 (m), 734 (m), 726 (m), 695 (m), 665 (w), 646 (w), 602 (m), 487 (w), 479 (w), 460 (w). 1H NMR (CDCl3): δ 1.271.38 (m, 12 H, CH2), 1.571.70 (m, 4 H, CH2), 2.26 (m, 2 H, COCH2), 2.52 (m, 2 H, SCH2), 4.46 (d, 3JHH = 6.1 Hz, 2 H, CH2C5H4N), 5.83 (s, 1 H, NH), 7.19 (d, 3JHH = 6.0 Hz, 2 H, CHN/C5H4N), 8.54 (dd, 3JHH = 4.5 Hz, 4JHH= 1.6 Hz, 2 H, CHC/C5H4N). 13C{1H}NMR (CDCl3): δ 24.7 (CH2), 25.0 (CH2), 25.8 (CH2), 28.4 (CH2), 29.1 (CH2), 29.3 (CH2), 29.4 (CH2), 29.5 (CH2), 34.1 (CH2), 36.7 (CH2), 42.3 (CH2NH), 122.5 (CH/C5H4N), 148.2 (iC/C5H4N), 149.7 (CH/C5H4N), 173.6 (CdO). ESI-MS [m/z]: 309.19 [M + H]+. Synthesis of o-C5H4NCH2NHCO(CH2)10SH (4). For the synthesis of 4 the same procedure was used as described for the preparation of 2 [11-mercaptoundecanoic acid (1.17 g, 5.39 mmol), 1-hydroxybenzotriazole (0.87 g, 6.47 mmol), 2-(aminomethyl)pyridine (3, 0.70 g, 0.66 mL, 6.47 mmol), and N,N0 -dicyclohexylcarbodiimide (1.33 g, 6.47 mmol)]. After appropriate workup, compound 4 was obtained as a colorless solid. Yield: 1.26 g (4.08 mmol, 76% based on 11-mercaptoundecanoic acid). Elemental analysis: Calcd for C17H28N2OS (308.47): C, 66.19; H, 9.15; N, 9.08. Found: C, 66.09; H, 9.29; N, 9.23%. m.p.: 69 °C. IR (KBr): ν [cm1] 3316 (vs) (νNH), 3071 (w), 3013 (w), 2923(vs) (νCH2), 2849 (vs) (νCH2), 2559 (w) (νSH), 1636 (vs) (νCdO), 1590 (s), 1570 (s), 1547(s) (νNH), 1472 (m), 1437 (m), 1421 (m), 1385 (w), 1352 (m), 1339 (w), 1311 (w), 1296 (w), 1266 (w), 1239 (m), 1213 (w), 1049 (w), 1033 (w), 992 (w), 758 (m), 729 (w), 719 (w), 695 (w), 606 (m). 1H NMR (CDCl3): δ 1.231.34 (m, 12 H, CH2), 1.531.66 (m, 4 H, CH2), 2.24 (m, 2 H, COCH2), 2.48 (m, 2 H, SCH2), 4.53 (d, 3 JHH = 5.0 Hz, 2 H, CH2C5H4N), 6.81 (s, 1 H, NH), 7.16 (ddd, 3 JHH = 7.5 Hz, 3JHH = 4.9 Hz, 4JHH = 0.9 Hz, 1 H, C5H4N), 7.23 (d, 3JHH = 7.8 Hz, 1 H, C5H4N), 7.63 (td, 3JHH = 7.7 Hz, 4JHH = 1.8 Hz, 1 H, C5H4N), 8.50 (ddd, 3JHH = 5.0 Hz, 4JHH = 1.5 Hz, 5 JHH = 0.9 Hz, 1 H, C5H4N). 13C{1H}NMR (CDCl3): δ 24.7 (CH2), 25.1 (CH2), 25.8 (CH2), 28.5 (CH2), 28.9 (CH2), 29.4 (CH2), 29.5 (CH2), 29.6 (CH2), 34.1 (CH2), 36.8 (CH2), 44.5 (CH2NH), 122.3 (CH/C5H4N), 122.5 (CH/C5H4N), 136.9 (CH/C5H4N), 149.1 (CH/C5H4N), 156.6 (iC/C5H4N), 173.3 (CdO). ESI-MS [m/z]: 309.19 [M + H]+. Synthesis of (o-C5H4N)2CHNHCO(CH2)10SH (6). For the synthesis of 6 the same procedure was used as for the preparation of 2 [11-mercaptoundecanoic acid (0.96 g, 4.39 mmol), 1-hydroxybenzotriazole (0.71 g, 5.28 mmol), di(2-pyridyl)methylamine (5, 0.98 g, 5.28 mmol), and N,N0 -dicyclohexylcarbodiimide
(1.09 g, 5.28 mmol)]. After appropriate workup, complex 6 was obtained as a colorless solid. Yield: 1.33 g (3.47 mmol, 79% based on 11-mercaptoundecanoic acid). Elemental analysis: Calcd for C22H31N3OS (385.55): C, 68.53; H, 8.10; N, 10.90. Found: C, 68.37; H, 8.50; N, 10.94%. m.p.: 70 °C. IR (KBr): v [cm1] 3325 (m) (νNH), 3296 (m) (νNH), 2917 (vs) (νCH2), 2849 (vs) (νCH2), 2545 (w) (νSH), 1637 (vs) (νCdO), 1588 (m), 1569 (m), 1540 (m) (νNH), 1466 (m), 1432 (m), 1411 (w), 1384 (w), 1312 (w), 1233 (w), 1149 (w), 1088 (w), 1048 (w), 994 (w), 878 (w), 765 (m), 719 (w), 651 (w), 615 (w), 586 (w). 1H NMR (CDCl3): δ 1.251.38 (m, 12 H, CH2), 1.561.71 (m, 4 H, CH2), 2.34 (m, 2 H, COCH2), 2.51 (m, 2 H, SCH2), 6.22 (d, 3JHH = 6.7 Hz, 1 H, CH), 7.15 (ddd, 3JHH = 7.5 Hz, JHH = 4.9 Hz, 4JHH = 1.1 Hz, 2 H, C4H5N), 7.41 (d, 3JHH = 7.9 Hz, 2 H, C4H5N), 7.63 (td, 3 JHH = 7.7 Hz, 4JHH = 1.8 Hz, 2 H, C4H5N), 7.89 (d, 3JHH = 7.3 Hz, 1 H, NH), 8.52 (ddd, 3JHH = 4.8 Hz, 4JHH = 1.5 Hz, 5JHH = 0.7 Hz, 2 H, C4H5N). ESI-MS [m/z]: 386.22 [M + H]+. 13C{1H}NMR data could not be obtained due to the low solubility of 6. 2.2. Preparation of the SAMs. The gold substrates were prepared by thermal evaporation of 100150 nm of gold (99.99% purity) onto either polished single-crystal silicon (100) wafers (Silicon Sense) primed with a 5 nm titanium adhesion layer or on freshly cleaved mica (rate 2 nm/s, temp. 340 °C),30 The resulting metal substrates were polycrystalline, with a predominant (111) orientation and a grain or terrace size of either 2050 (Si) or 100200 nm (mica) as observed by atomic force microscopy and scanning tunnelling microscopy. The SAMs were prepared by immersion of the freshly prepared substrates into a 1 mM solution of the SAM precursors in ethanol at ambient temperature for 24 h, with identical results for either type of the substrates. After immersion, the samples were carefully rinsed with the pure solvent and dried with N2. In addition to the target films, we have also prepared a reference SAM of nonsubstituted alkanethiols, using 1-dodecanethiol (DDT) as a representative compound. DDT SAMs were formed by the same procedure as the target films. 2.3. Characterization of the SAMs. The C10APn films were characterized by high-resolution X-ray photoelectron spectroscopy (HRXPS), ellipsometry, angle-resolved near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, infrared reflection absorption spectroscopy (IRRAS), and cyclovoltammetry. All experiments were performed at ambient temperature. The HRXPS and NEXAFS spectroscopy measurements were carried out under UHV conditions at a base pressure of
