Article pubs.acs.org/Langmuir
Intermixed Terpyridine-Functionalized Monolayers on Gold: Nonlinear Relationship between Terpyridyl Density and Metal Ion Coordination Properties Christoph H.-H. Traulsen,† Erik Darlatt,‡ Sebastian Richter,† Johannes Poppenberg,† Santina Hoof,† Wolfgang E. S. Unger,*,‡ and Christoph A. Schalley*,† †
Institut für Chemie und Biochemie der Freien Universität Berlin, Takustrasse 3, 14195 Berlin, Germany BAM Bundesanstalt für Materialforschung und -prüfung, Unter den Eichen 44-46, 12203 Berlin, Germany
‡
S Supporting Information *
ABSTRACT: Aiming at the functionalization of surfaces with terpyridine anchors for the coordinative deposition of additional layers, mixed self-assembled monolayers (SAMs) were prepared from binary solutions of 12-(2,2′:6′,2″-terpyridine-4′-yl)dodecane-1-thiol (TDT) and 1decanethiol (DT). The SAMs and the order of the constituting molecules were analyzed by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and time-of-flight-secondary ion mass spectrometry (ToF-SIMS). The composition of the (TDT/DT)-SAMs and with it the surface density of terpyridyl groups correlates linearly with the relative concentrations of the two compounds in the solution used for depositing them. In marked contrast, the amount of terpyridine-coordinated PdII ions significantly deviates from this trend with an optimum at a 1:3 ratio of TDT/DT. This indicates a major fraction of the terpyridines in TDT-rich SAMs not to be accessible for PdII ion coordination. In agreement, NEXAFS spectroscopy reveals the alkyl backbones in TDT-rich SAMs not to be ordered, while they are preferentially upright oriented in the optimal 1:3-(TDT/DT)-SAMs. We interpret this in terms of terpyridine backfolding in TDT-rich SAMs, while they are located in accessible positions on top of the SAM in the 1:3(TDT/DT)-SAM. While the alkyl backbones in the 1:3-(TDT/DT)-SAM are ordered, NEXAFS spectroscopy shows the terpyridyl groups not to have a preferential orientation in this SAM and thus retain enough flexibility to adjust to molecules that are deposited on top of the mixed SAM. In conclusion, the novel SAM does not undergo phase separation and consists predominantly of intermixed phases with adjustable surface density of quite flexible terpyridine anchor groups. The terpyridine− PdII anchors are not only available for a future deposition of the next layer, but the metal ions also represent a sensitive probe for the accessibility of the terpyridyl groups.
1. INTRODUCTION A current topic in chemical research is the transfer of molecular processes from solution to solid supports, aiming at the preparation of ordered arrays of molecules that finally exhibit coherent function and directional processes.1,2 Especially, the chemisorption of organothiols on gold surfaces generating welldefined self-assembled monolayers (SAMs) has been extensively investigated over the past decades.3−11 Besides monomolecular SAMs, mixed monolayers have recently gained attention because of the possibility to introduce sterically demanding molecules into organized monolayers.12−15 To enable the growth of further layers subsequent to SAM formation, several methodologies have been established which are either based on covalent or noncovalent interactions.16−25 Besides electrostatic interactions, metal−ligand coordination is one of the most important noncovalent interactions mediating a layer-by-layer deposition of the molecules of interest.26−29 Widely used ligands in this context are pyridines and terpyridines; several examples exist for pyridine/transition metal-ion complexes on surfaces.30−33 A main feature of 2,2′:6′,2″-terpyridine−metal complexes is their relatively high © 2012 American Chemical Society
stability due to the strong metal-to-ligand back-donation and due to dynamic chelating effects.34 A strong binding is most likely preferable to construct long-living and robust molecular assemblies on solid supports. Recently, some examples for multilayers based on monomolecular and mixed terpyridine SAMs were reported, which predominantly focus on rigid, conjugated systems and/or aim at molecular conductivity.35−42 In view of the broad variety of potential molecules available for immobilization, for example biomolecules or sterically demanding supramolecular structures, it is important to have a broad diversity of well-characterized SAMs available as platforms to build on.43−45 A system which has been previously used by several groups for the construction of coordination-chemistry-based (multi)layers on solid supports is (4-mercaptophenyl)-2,2′:6′,2″terpyridine (TPT). Particularly, mixed monolayers consisting of TPT and mercaptobenzene (PhT) have been used Received: April 23, 2012 Revised: June 15, 2012 Published: June 28, 2012 10755
dx.doi.org/10.1021/la301644r | Langmuir 2012, 28, 10755−10763
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Scheme 1. (a) Synthesis of (4-Mercaptophenyl)-2,2′:6′,2″-terpyridine (TPT) and Mixed SAM Formation with Mercaptobenzene (PhT);a (b) Synthesis of 12-(2,2′:6′,2″-Terpyridine-4′-yl)dodecane-1-thiol (TDT), ABCV = 4,4′-Azobis(4cyanopentanoic acid); (c) Mixed SAM Formation with TDT by Coadsorption with 1-Decanethiol (DT) and PdII Ion Deposition
a
For comparison with our results, we repeated the synthesis and the preparation of the (TPT/PhT)-SAM and obtained results consistent with those in the literature.47
successfully for this purpose (Scheme 1a).46 Because of the conjugated π-system of TPT, its advantages for electrontransfer processes between the surface and the molecules deposited on top of the SAM are obvious. The rigidity of this binary SAM has also proven useful for building well-defined multilayers.38−40 However, the lack of flexibility of the TPT/ PhT monolayer may also hamper self-organization processes of subsequently complexed molecules with a higher steric demand than the terpyridine itself. To enable self-organization of sterically demanding molecules, a platform with the opposite propertieselectrical isolation through an alkyl backbone and higher flexibilitymight nicely complement the (TPT/PhT)SAM. It avoids electron-transfer processes where problematic and presents terpyridyl anchor groups able to adjust to the spatial requirements of the layer deposited on top of the SAM. In this contribution, we develop and comprehensively characterize a mixed SAM with flexible terpyridyl anchor groups and ordered aliphatic backbones. Spatial separation of the large terpyridines was achieved by coadsorption of the terpyridine-terminated thiol with an unfunctionalized alkylthiol in different molar ratios. The composition of the monolayer is
related to the molar ratio of the depositing binary thiol solution which can be tuned linearly. Additionally, metal-ion coordination experiments on these mixed SAM templates were performed to determine the effects of terpyridine density at the interface on the coordination behavior. These experiments showed the surface composition and surface distribution of the terpyridyl groups to be of crucial importance for the coordination of metal ions. A lower terpyridine density at the surface yielded a higher amount of coordinated metal ions. The PdII ions can be used to probe the existence of a predominantly intermixed SAM phase.
2. EXPERIMENTAL SECTION General Methods. Synthetic reaction steps were conducted under a dry argon atmosphere. Chemicals were purchased from Alfa Aesar, Sigma-Aldrich, or HetCat and used without further purification. Dry dimethylformamide (DMF) and tetrahydrofuran (THF), isopropanol, and methanol (MeOH) were purchased from ACROS Organics and used as received. Diethyl ether (Et2O), hexane, and ethyl acetate were purchased from VWR and destilled prior to use by usually laboratory methods. Ethanol (EtOH) and acetonitrile (ACN) used for SAM preparation and metal deposition experiments were purchased from 10756
dx.doi.org/10.1021/la301644r | Langmuir 2012, 28, 10755−10763
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surface of HOPG (highly ordered pyrolytic graphite, Advanced Ceramic Corp., Cleveland, OH) at 285.4 eV.52 Spectra are shown with the pre-edge count rate subtracted and after normalization in units of the absorption edge jump.51 C K-edges were measured at 30°, 55°, and 90° incident angle of the linearly polarized synchrotron light beam. ToF-SIMS spectra were obtained on a time-of-flight-secondary ion mass spectrometer, TOF-SIMS IV manufactured by Ion-Tof GmbH (Münster, Germany). Targets were bombarded by 25 keV Bi32+ cluster primary ions. The total ion dose of 5.5 × 1011 ions/cm2 was below the static limit of SIMS (∼1.0 × 1013 ions/cm2).53 For each analysis, the ion beam was digitally scanned within a 128 × 128 array over an analysis area of 100 μm × 100 μm. Two measurements at adjacent areas were performed using the same settings. Internal calibrations were conducted using H+, H2+, Na+, Ca+, and Au+ peaks for positive and H−, C−, C2−, C3−, and Au− for negative mass spectra. Synthesis of 4-(Dodec-11-enyl)-2,2′:6′,2″-terpyridine (TD). To a solution of diisopropylamine (0.49 mL, 3.5 mmol) in 40 mL of dry tetrahydrofuran, n-butyllithium (1.6 M in hexane, 2.21 mL, 3.5 mmol) was added. After 15 min, the solution was cooled to −20 °C and 4′-methyl-2,2′:6′,2″-terpyridine (601 mg, 2.43 mmol) was added. The solution was stirred for 30 min before 11-bromo-1-undecene (0.53 mL, 2.43 mmol) was added dropwise. The solution was stirred at room temperature overnight. Afterward, 80 mL of deionized water was added, and the resulting layer was extracted with 3 × 70 mL of diethyl ether. The combined organic layers were dried over MgSO4 and filtered, and the solvent removed under reduced pressure. Purification by column chromatography on aluminum oxide with diethyl ether/ hexane (1:3) as the eluent afforded 385 mg (0.97 mmol) of a colorless oil. Yield: 40%. 1H NMR (400 MHz, CDCl3, δ): 8.70 (2H, m, CHtpy), 8.63 (2H, d, J = 7.9, CHtpy), 8.30 (2H, s, CHtpy), 7.85 (2H, m, CHtpy), 7.32 (2H, ddd, J = 7.4, 4.8, 1.1, CHtpy), 5.80 (1H, ddt, J = 16.9, 10.2, 6.7, CHalkene), 4.98 (1H, ddt, J = 17.1, 2.2, 1.6, CH2,alkene), 4.92 (1H, ddt, J = 10.1, 2.0, 0.9, CH2,alkene), 2.78 (2H, t, J = 7.9, CH2), 2.03 (2H, q, J = 7.0, CH2), 1.76 (2H, m, CH2), 1.35 (6H, m, CH2), 1.26 (8H, s, CH2) ppm. 13C NMR (101 MHz, CDCl3, δ): 156.58, 155.36, 154.19, 149.16, 139.39, 137.00, 123.78, 121.48, 121.29, 114.21, 35.98, 33.94, 30.76, 29.70, 29.63, 29.60, 29.58, 29.56, 29.26, 29.06 ppm. HRMS (ESI-ToF, CH2Cl2/MeOH, m/z (%)): [M + H]+ calcd 400.2747, found 400.2747 (100); [2M + Na]+ calcd 821.5241, found 821.5219 (5). Synthesis of 12-(2,2′:6′,2″-Terpyridin-4-yl)dodecyl-ethanethioate (TDTAc). To a solution of TD (272 mg, 0.68 mmol) dissolved in 12 mL dry THF thioacetic acid (1.23 mL, 15.2 mmol) and 4,4-́ azobis(4-cyanovaleric acid) (80.0 mg, 0.28 mmol) were added. The mixture was irradiated with UV-light (366 nm) for 24 h while stirring at room temperature. Afterward, 4,4′-azobis(4-cyanovaleric acid) (80 mg, 0.28 mmol) was added and irradiation was continued. After another 24 h, thioacetic acid was removed by vacuum distillation (120 °C; 0.1 mbar). Purification by column chromatography on aluminum oxide with ethyl acetate/hexane (1:12) as the eluent afforded 223 mg (0.47 mmol) of a pale yellow solid. Yield: 69%. 1H NMR (400 MHz, CDCl3, δ): 8.69 (2H, m, CHtpy), 8.62 (2H, d, J = 8.0, CHtpy), 8.29 (2H, s, CHtpy), 7.84 (2H, m, CHtpy), 7.31 (2H, ddd, J = 7.5, 4.8, 1.2, CHtpy), 2.84 (2H, t, J = 7.3, CH2), 2.78 (2H, t, J = 8.0, CH2), 2.30 (3H, s, CH2), 1.75 (2H, m, CH2), 1.54 (2H, m, CH2), 1.34 (6H, m, CH2), 1.23 (10H, s, CH2) ppm. 13C NMR (101 MHz, CDCl3, δ): 196.16, 156.57, 155.34, 154.15, 149.14, 136.96, 123.76, 121.45, 121.26, 35.96, 30.75, 30.74, 30.73, 29.68, 29.64, 29.61, 29.59, 29.56, 29.54, 29.28, 29.21, 28.92 ppm. HRMS (ESI-ToF, CH2Cl2/MeOH, m/z (%)): [M + H]+ calcd 476.2736, found 476.2736 (100); [M + Na]+ calcd 498.2555, found 498.2550 (70). Synthesis of 12-(-(2,2′:6′,2″-Terpyridin-4′-yl)dodecane-1thiol (TDT). To a solution of TDTAc (150 mg, 0.32 mmol) dissolved in 10 mL of MeOH concentrated HCl (1 mL) was added. The solution was stirred under reflux for 24 h. Afterward, MeOH was removed under reduced pressure. The residue was dissolved in 60 mL of ethyl acetate and washed with deionized water (2 × 60 mL). The organic layer was dried over MgSO4. Purification by column chromatography on aluminum oxide with ethyl acetate/hexane (1:3) as the eluent afforded 102 mg (0.24 mmol) of the product. Yield: 74%.
Carl Roth and VWR in HPLC grade and used as received. Thin-layer chromatography was carried out on precoated silica gel 60/F254 plates (Merck KGaA) and aluminum oxide N/UV254 plates (MachereyNagel). Aluminum oxide (neutral; 0.05−0.20 mm; ACROS) was used for column chromatography. Gold substrates used for XPS, NEXAFS, and ToF-SIMS were prepared onto polished single-crystal Si(100) wafers which have been precoated with a 9 nm titanium adhesion layer. The substrates roughness is rms
