Article pubs.acs.org/Langmuir
Molecular Architecture: Construction of Self-Assembled Organophosphonate Duplexes and Their Electrochemical Characterization Anna Cattani-Scholz,† Kung-Ching Liao,‡ Achyut Bora,§ Anshuma Pathak,§ Christian Hundschell,⊥ Bert Nickel,⊥ Jeffrey Schwartz,‡ Gerhard Abstreiter,† and Marc Tornow*,§ †
Walter Schottky Institut, Technische Universität München, München, Germany Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States § Institut für Halbleitertechnik, Technische Universität Braunschweig, Braunschweig, Germany ⊥ Fakultät für Physik & CeNS, Ludwig-Maximilians-Universität, München, Germany ‡
S Supporting Information *
ABSTRACT: Self-assembled monolayers of phosphonates (SAMPs) of 11-hydroxyundecylphosphonic acid, 2,6-diphosphonoanthracene, 9,10-diphenyl-2,6-diphosphonoanthracene, and 10,10′-diphosphono-9,9′-bianthracene and a novel self-assembled organophosphonate duplex ensemble were synthesized on nanometer-thick SiO2-coated, highly doped silicon electrodes. The duplex ensemble was synthesized by first treating the SAMP prepared from an aromatic diphosphonic acid to form a titanium complex-terminated one; this was followed by addition of a second equivalent of the aromatic diphosphonic acid. SAMP homogeneity, roughness, and thickness were evaluated by AFM; SAMP film thickness and the structural contributions of each unit in the duplex were measured by X-ray reflection (XRR). The duplex was compared with the aliphatic and aromatic monolayer SAMPs to determine the effect of stacking on electrochemical properties; these were measured by impedance spectroscopy using aqueous electrolytes in the frequency range 20 Hz to 100 kHz, and data were analyzed using resistance−capacitance network based equivalent circuits. For the 11-hydroxyundecylphosphonate SAMP, CSAMP = 2.6 ± 0.2 μF/cm2, consistent with its measured layer thickness (ca. 1.1 nm). For the anthracene-based SAMPs, CSAMP = 6−10 μF/cm2, which is attributed primarily to a higher effective dielectric constant for the aromatic moieties (ε = 5−10) compared to the aliphatic one; impedance spectroscopy measured the additional capacitance of the second aromatic monolayer in the duplex (2ndSAMP) to be CTi/2ndSAMP = 6.8 ± 0.7 μF/cm2, in series with the first.
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dielectric in pentacene-based organic thin-film transistors.10−12 Because SAMP deposition often yields highly ordered monolayers, with minimal risk of multilayer formation or intramolecular polymerization, superior film properties at the nanoscale level may be attained. Yet, whereas SAMPs on Si/ SiO2 surfaces have been variously characterized by contact angle measurement, X-ray reflectivity (XRR), ellipsometry, and X-ray photoelectron spectroscopy (XPS),6,8,13 electrochemical properties of dense SAMPs in the presence of electrolytes have not been reported heretofore, which contrasts with reports on electrochemical impedance properties of alkyl monolayers grafted in various ways onto silicon or Si/SiO2 surfaces.14−16 Thus, in the context of using SAMPs not just as passive interfaces or linkers but as active layers, we were prompted to
INTRODUCTION Self-assembled organic monolayers on semiconductor surfaces are interesting as interfaces for chemical and biological sensing, for integrated molecular nanoelectronics and for hybrid photovoltaics. In this context strategies have been reported for functionalizing silicon surfaces using alkenes or organosilicon compounds1−5 either by direct C−Si bond formation or by covalent attachment of siloxanes to a thin silicon oxide surface layer, respectively. Self-assembled monolayers of phosphonates (SAMPs) are robust building blocks for surface functionalization; they are attractive alternatives to organosilicon-based coatings,6,7 especially where hydrolytic stability and highly ordered functional interfaces are needed. In this regard we have reported that SAMPs are stable and organized platform systems for target molecule binding detection in electrolyte-gated field-effect nanowire transistor devices,8 for applications in DNA microarrays,9 and for modifying the gate © 2012 American Chemical Society
Received: April 20, 2012 Published: April 25, 2012 7889
dx.doi.org/10.1021/la301610a | Langmuir 2012, 28, 7889−7896
Langmuir
Article
2,6-Bis(diethylphosphono)-9,10-diphenylanthracene.17 Yield 94%. 1H NMR (CDCl3, 500 MHz, 298 K): δ = 8.29 (d, 3J(H,P) = 17.0 Hz, 2H), 7.80 (d, 3J(H,H) = 5.2 Hz, 2H), 7.65−7.55 (m, 8H), 7.45 (d, 3 J(H,H) = 6.4 Hz, 4H), 4.10−4.04 (m, 8H), 1.24 ppm (dd, 3J(H,H) = 6.6 Hz, 12H). 13C NMR (CDCl3, 125 MHz, 298 K): δ = 139.3, 137.4, 133.5, 133.4, 131.1, 131.0, 130.0, 129.9, 128.8, 128.2, 128.1, 128.0, 126.6, 125.3, 125.2, 125.1, 62.3, 62.3, 16.3, 16.3 ppm. 31P NMR (CDCl3, 202 MHz, 298 K): δ = 18.48 ppm. HRMS (ESI-TOF) for C34H37O6P2: calcd 603.2060 (M + H)+; found m/z 603.2064. 10,10′-Bis(diethylphosphono)-9,9′-bianthracene. 9,10-Dibromoanthracene (Aldrich, 99%, 1.34 g, 4.0 mmol) was dissolved in flash distilled THF (150 mL) under dry air at −78 °C. tert-Butyllithium (Aldrich, 1.7 M, 5.0 mL, 8.5 mmol, 2.1 equiv) was slowly injected through a rubber stopper via a glass syringe, and the resulting orange solution was allowed to stir first for 30 min at −78 °C and then for 2 h at 0 °C. The solution was cooled to −78 °C, and diethyl chlorophosphonate (1.3 mL, 8.9 mmol, 2.2 equiv) was then added; the reaction mixture was slowly stirred first for 1 h at −78 °C and then for 8 h at room temperature. The resulting yellow suspension was concentrated under reduced pressure, and the recovered solid was purified by chromatography on a silica column (15% ethyl acetate/ hexane eluent) affording the title product (1.90 g, 3.0 mmol, 76% yield) as a yellow solid. 1H NMR (CDCl3, 500 MHz, 298 K): δ = 9.51 (d, 3J(H,H) = 9.0 Hz, 4H), 7.57 (dd, 3J(H,H) = 7.5 Hz, 4J(H,H) < 2 Hz, 4H), 7.16 (dd, 3J(H,H) = 8.0 Hz, 4J(H,H) < 2 Hz, 4H), 7.03 (d, 3 J(H,H) = 8.5 Hz, 4H), 4.47−4.39 (m, 4H), 4.28−4.20 (m, 4H), 1.42 ppm (t, 3J(H,H) = 7.0 Hz, 12H). 13C NMR (CDCl3, 125 MHz, 298 K): δ = 140.8, 140.8, 134.9, 134.8, 131.1, 131.1, 128.0, 127.9, 127.6, 127.5, 126.0, 121.3, 119.9, 62.4, 62.4, 16.7, 16.7 ppm. 31P NMR (CDCl3, 202 MHz, 298 K): δ = 19.47 ppm. HRMS (ESI-TOF) for C36H37O6P2: calcd 627.2060 (M + H)+; found m/z 627.2056. General Procedure for the Preparation of Diphosphonoacenes. The bis(diethylphosphono)acene (1 mmol) was suspended in anhydrous methylene chloride (15 mL) under argon, and bromotrimethylsilane (Aldrich, 97%, 1.2 mL, 6 mmol, 6 equiv) was added. The reaction mixture was stirred overnight at room temperature; anhydrous methanol (1.2 mL) was then added, and the mixture was stirred for an additional 6 h. The resulting suspension was concentrated under reduced pressure, and the residue was dissolved in methanol (10 mL). The solution was filtered through a filter aid on a fritted disk. The filtrate was concentrated under reduced pressure, and the recovered solid was recrystallized from ethanol, affording the product as a bright yellow solid. 2,6-Diphosphonoanthracene (2). Yield 94%. 1 H NMR (CD3OD, 500 MHz, 298 K): δ = 8.68 (s, 2H), 8.59 (d, 3J(H,P) = 16.0 Hz, 2H), 8.19 (dd, 3J(H,H) = 8.5 Hz, 4J(H,H) = 3.5 Hz, 2H), 7.79 ppm (dd, 3J(H,H) = 10.0 Hz, 4J(H,H) < 2 Hz, 2H). 13C NMR (CD3OD, 125 MHz, 298 K): δ = 134.3, 134.2, 133.9, 133.0, 132.8, 131.8, 130.3, 129.9, 129.8, 129.1, 126.7, 126.6 ppm. 31P NMR (CD3OD, 202 MHz, 298 K): δ = 15.59 ppm. HRMS (ESI-TOF) for C14H13O6P2: calcd 339.0182 (M + H)+; found m/z 339.0180. 2,6-Diphosphono-9,10-diphenylanthracene (3).17 Yield 98%. 1 H NMR (CD3OD, 500 MHz, 298 K): δ = 8.31 (d, 3J(H,P) = 17.0 Hz, 2H), 7.76 (dd, 3J(H,H) = 9.0 Hz, 4J(H,H) = 3.7 Hz, 2H), 7.69−7.61 (m, 8H), 7.48 ppm (d, 3J(H,H) = 7.6 Hz, 4H). 13C NMR (CD3OD, 125 MHz, 298 K): δ = 140.4, 139.1, 132.9, 132.8, 132.3, 132.1, 131.5, 131.1, 131.0, 130.0, 130.0, 129.3, 128.5, 128.4, 126.4, 126.3 ppm. 31P NMR (CD3OD, 202 MHz, 298 K): δ = 15.42 ppm. HRMS (ESITOF) for C26H21O6P2: calcd 491.0808 (M + H)+; found m/z 491.0806. 10,10′-Diphosphono-9,9′-bianthracene (4). Yield 76%. 1H NMR (CD3OD, 500 MHz, 298 K): δ = 9.53 (d, 3J(H,H) = 9.5 Hz, 4H), 7.55 (dd, 3J(H,H) = 7.5 Hz, 4J(H,H) < 2 Hz, 4H), 7.18 (dd, 3 J(H,H) = 8.0 Hz, 4J(H,H) < 2 Hz, 4H), 6.98 ppm (d, 3J(H,H) = 8.5 Hz, 4H). 13C NMR (CD3OD, 125 MHz, 298 K): δ = 135.2, 135.1, 132.2, 132.1, 129.5, 129.5, 128.1, 127.6, 126.8 ppm. 31P NMR (CD3OD, 202 MHz, 298 K): δ = 15.24 ppm. HRMS (ESI-TOF) for C28H21O6P2: calcd 515.0808 (M + H)+; found m/z 515.0812. Self-Assembled Monolayers of Phosphonates (SAMPs). Silicon oxide-coated Si electrode surfaces were cleaned in acetone
begin the systematic characterization of their electrical properties;17 we now address these properties in detail. In the course of these investigations, we also learned how to prepare organized stacks of dense phosphonate monolayers; in principle, this discovery could enable us to build structurally organized, active devices with vertical dimensions far greater than can be attained by simple monolayer coatings alone.
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EXPERIMENTAL SECTION
General. 11-Hydroxyundecylphosphonic acid (1) was synthesized as previously reported.18 2,6-Dibromoanthracene19 and 2,6-dibromo9,10-diphenylanthracene20,21 were synthesized according to published procedures. Titanium tetra(tert-butoxide) (7) was purchased from Strem. Common reagents and organic solvents were purchased from Sigma-Aldrich and were used without further purification. Ultrapure water (Millipore) was used in conjunction with standard electrolyte 50 mM Tris HCl (pH 7.2), 100 mM NaCl solutions. Silicon wafers (4 in. diameter), either ⟨100⟩ oriented highly p-doped (boron, 15−25 mΩ·cm, or boron,
