Surface Functionalization of Oxide-Covered Zinc and Iron with

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Surface Functionalization of Oxide-Covered Zinc and Iron with Phosphonated Phenylethynyl Phenothiazine Julian Rechmann,† Adnan Sarfraz,† Alissa C. Götzinger,‡ Elena Dirksen,‡ Thomas J. J. Müller,‡ and Andreas Erbe*,† †

Department of Interface Chemistry and Surface Engineering, Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Strasse 1, 40237 Düsseldorf, Germany ‡ Chair of Organic Chemistry, Heinrich-Heine-Universität Düsseldorf, Universitätsstrasse 1, 40225 Düsseldorf, Germany S Supporting Information *

ABSTRACT: Phenothiazines are redox-active, fluorescent molecules with potential applications in molecular electronics. Phosphonated phenylethynyl phenothiazine can be easily obtained in a four-step synthesis, yielding a molecule with a headgroup permitting surface linkage. Upon modifying hydroxylated polycrystalline zinc and iron, both covered with their respective native oxides, ultrathin organic layers were formed and investigated by use of infrared (IR) reflection spectroscopy, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), contact angle measurement, and ellipsometry. While stable monolayers with upright oriented organic molecules were formed on oxide-covered iron, multilayer formation is observed on oxide-covered zinc. ToF-SIMS measurements reveal a bridging bidentate bonding state of the organic compound on oxide-covered iron, whereas monodentate complexes were observed on oxide-covered zinc. Both organically modified and unmodified surfaces exhibit reactive wetting, but organic modification makes the surfaces initially more hydrophobic. Cyclic voltammetry (CV) indicates redox activity of the multilayers formed on oxide-covered zinc. On the other hand, the monolayers on oxide-covered iron desorb after electrochemical modifications in the state of the oxide, but are stable at open circuit conditions. Exploiting an electronic coupling of phenothiazines to oxides may thus assist in corrosion protection.

1. INTRODUCTION Functional organic molecules exhibit interesting properties and therefore find their application in electronic appliances, such as organic light-emitting devices (OLEDs), thin-film transistors (TFTs), and thin-film organic photovoltaic cells.1−8 As the miniaturization of oxide-based electronic components reaches its limits, new methods are required to obtain materials with desired electronic properties. To that end, bottom-up techniques such as self-assembled monolayers (SAMs) have become an efficient, fast, and economical strategy for the development of molecular organic electronics.9,10 For metals, self-assembly of molecules is a suitable way to functionalize surfaces. Among many heteroaromatic systems such as phenyl derivatives, conjugated biphenyls, porphyrin derivatives, dithiocarbamates, pyridyl complexes, and diketoarylhydrazones, phenothiazines and their derivatives are interesting building blocks for molecular wires for monomolecular electronics (MME).2,10−15 In particular, phenothiazines form stable radical cations, with excellent delocalization, and are thus excellent electron donors.16−19 In combination with other electronic properties, such as tunable redox potentials, and their tendency to form SAMs due to π−π interactions, phenothiazines are suitable compounds for switchable conductive molecular wires.20−22 Phenothiazine can be oxidized by two separate single electron transfer steps.23 The oxidation potential strongly © XXXX American Chemical Society

depends on the substitution pattern of the framework. Thus, a variety of well-known synthesis strategies allow the production of derivatives with desired properties.24,25 While phenothiazines find their application as dyes, redox additives, pharmaceutical reagents, etc., their behavior on metal surfaces and their interfacial chemistry has been rarely reported.26,27 The electronic properties relevant for molecular electronics are on the other hand also relevant for corrosion protection: taking the role of oxides in electronic devices makes phenothiazines interesting accessory components in corrosion protection systems. Phenothiazines undergo redox reactions with metal ion acceptors in aqueous and alcoholic micellar solutions by laser excitation.28 Externally triggered electron transfer motivates an analysis of phenothiazines in contact with oxide-covered metal surfaces susceptible to corrosion. Under atmospheric conditions, non-noble metals spontaneously form “native” oxides that may protect the surface against further oxidation. In the case of iron, naturally grown iron oxide, mainly ferric oxide, is porous and brittle and noninsulating, leading to continuous corrosion processes.29 Ultrathin layers of organic molecules can act as an electronic, physical, and/or Received: April 15, 2015 Revised: June 6, 2015

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DOI: 10.1021/acs.langmuir.5b01370 Langmuir XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis of 3-((4-Bromophenyl)ethynyl)-10-hexyl-10H-phenothiazine (E1, Top), Synthesis of Diethyl(4-((10-hexyl10H-phenothiazin-3-yl)ethynyl)phenyl)phosphonate (1, Middle), and Reaction Conditions of Chemisorption of Compound 1 on Oxide-Covered Transition Metals with Schematic Illustration of Metal−Oxygen−Phosphorus Bond (Bottom)

Germany). Tetrahydrofuran (THF), ethanol, and ammonia for surface modification were sourced by Sigma-Aldrich (Steinheim, Germany). Synthesis of 3-((4-Bromophenyl)ethynyl)-10-hexyl-10H-phenothiazine (E1). The synthesis of the title compound was performed in analogy to the previously published procedure for the preparation of alkynylated phenothiazines (Scheme 1).33 A solution of 3-ethynyl-10hexyl-10H-phenothiazine34 (1.23 g, 4.00 mmol) in dry THF (12.0 mL) in a flame-dried Schlenk tube under a nitrogen atmosphere was saturated with dry nitrogen for 10 min. Then, 1-bromo-4-iodobenzene (1.13 g, 4.00 mmol), dichlorobis(triphenylphosphine)palladium(II) (112 mg, 0.16 mmol), copper(I) iodide (30.5 mmol, 0.16 mmol), triphenylphosphane (42.0 mg, 0.160 mmol), and piperidine (2.00 mL) were added and the reaction mixture was stirred at room temperature for 2 days. The mixture was then diluted with diethyl ether and water, and the organic layer was extracted three times with diethyl ether. The combined organic phases were dried with anhydrous sodium sulfate, the solvents were removed under reduced pressure, and the crude product was adsorbed on Celite. The product was purified by flash chromatography on silica gel (n-hexane) to give 1.42 g (77%) of 3-((4bromophenyl)ethynyl)-10-hexyl-10H-phenothiazine as a yellow, fluorescent resin. 1 H NMR (acetone-d6, 300 MHz): δ = 0.86 (t, 3JH = 7.1 Hz, 3 H), 1.23−1.34 (m, 4 H), 1.46 (dt, 3JH = 7.0 Hz, 3JH = 7.0 Hz, 2 H), 1.79 (dt, 3JH = 7.5 Hz, 3JH = 7.5 Hz, 2 H), 3.96 (t, 3JH = 7.0 Hz, 2 H), 6.96 (td, 3JH = 7.5 Hz, 4JH= 1.2 Hz, 1 H), 7.03 (d, 3JH = 8.5 Hz, 1 H), 7.04 (dd, 3JH = 8.2 Hz, 4JH = 0.9 Hz, 1 H), 7.14 (dd, 3JH = 7.7 Hz, 4JH = 1.4 Hz, 1 H), 7.21 (ddd, 3JH = 8.2 Hz, 3JH = 7.3 Hz, 4JH = 1.6 Hz, 1 H), 7.28 (d, 4JH = 1.9 Hz, 1 H), 7.36 (dd, 3JH = 8.4 Hz, 4JH = 2.0 Hz, 1 H), 7.42−7.48 (m, 2 H), 7.55−7.61 (m, 2 H). 13 C NMR (acetone-d6, 75 MHz): δ = 14.2 (CH3), 23.3 (CH2), 27.1 (CH2), 27.4 (CH2), 32.1 (CH2), 47.9 (CH2), 88.7 (Cquat), 90.8 (Cquat), 116.5 (CH), 116.9 (CH), 117.3 (Cquat), 122.7 (Cquat), 123.5 (Cquat), 123.8 (CH), 124.6 (Cquat), 125.7 (Cquat), 128.1 (CH), 128.5 (CH), 130.5 (CH), 131.8 (CH), 132.6 (2 CH), 133.9 (2 CH), 145.5 (Cquat), 146.7 (Cquat). MS (MALDI): m/z (%) = 463.1 [M+]. Synthesis of Diethyl(4-((10-hexyl-10H-phenothiazin-3-yl)ethynyl)phenyl)phosphonate (1). The synthesis of the title compound was performed in analogy to a literature procedure for the preparation of arylphosphonates (Scheme 1).34 Palladium(II) acetate (5.64 mg, 2.50 μmol), 1,1′-bis(diphenylphosphano)ferrocene

hydrophobic barrier, separating the metal substrate from a corrosive aqueous environment. Thus, such a layer may retard degradation reactions, which is a key feature in corrosion protection.30−32 Therefore, synthesizing dense and structured ultrathin organic layers with electronic states that can couple to those of the oxides is important to decrease the amount of defects, and may decrease corrosion rates. In the present work, we report the investigation of these aspects by modifying the surface of iron and zinc, both covered with their native oxides, with phosphonated phenylethynyl phenothiazine. The interfacial structure of the resulting layers was investigated using infrared (IR) reflection spectroscopy, Xray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), contact angle measurement, and ellipsometry. Furthermore, the state of the organic layer was investigated after different immersion times. Afterward, the electrochemical properties of the modified surfaces were studied by cyclic voltammetry (CV) in aqueous solution for possible applicability in corrosion sensing and detection.

2. EXPERIMENTAL SECTION 2.1. Materials. All molecule syntheses were carried out in flamedried glassware under nitrogen atmosphere. Reagents and catalyst were purchased reagent-grade and used without further purification. Solvents were dried using an MBraun solvent purification system MBSPS-800. Further purification of the compounds was performed by flash column chromatography (silica gel 60, mesh 230−400). TLC used silica coated aluminum plates (60, F254). 1H, 13C, and DEPT spectra were recorded in CDCl3 on a 300 MHz (Bruker AVIII) spectrometer. The assignments of Cquat, CH, CH2, and CH3 were based on DEPT spectra. Mass spectra were recorded with a Finnigan MAT 8200. High resolution mass spectra were measured on an UHRQTOF maxis 4G (Bruker Daltronics). Infrared spectra were recorded with a Shimadzu IR Affinity-1 with ATR technique. The intensities of the signals are abbreviated as “s” (strong), “m” (medium), and “w” (weak). Zinc sheets (purity 99.95%) with a thickness of 2 mm were obtained from Goodfellow (Cambridge, U.K.). Armco Reineisen Güte 4 (purity 99.87%) was obtained from AK Steel GmbH (Düsseldorf, B


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Langmuir (27.8 mg, 5.00 μmol), and potassium acetate (10.7 mg, 25.0 μmol) were placed in a 20 mL Schlenk tube which was then evacuated and flushed with nitrogen three times. The starting materials were dissolved in dry THF (4.00 mL), triethylamine (0.174 mL, 1.26 mmol) was added dropwise, and the solution was stirred at 68 °C for 15 min. Diethyl phosphonate (0.135 mL, 1.05 mmol) and 3-((4bromophenyl)ethynyl)-10-hexyl-10H-phenothiazine (387 mg, 0.837 mmol),34 dissolved in a small amount of THF, were added dropwise, and the reaction was stirred at 68 °C for 3 days. The product was adsorbed on silica gel and purified by flash chromatography (hexane/ diethyl ether 20:1 to 0:1) to give 382 mg (0.735 mmol, 88%) of diethyl(4-((10-hexyl-10H-phenothiazin-3-yl)ethynyl)phenyl)phosphonate as a yellow, fluorescent resin. 1 H NMR (CDCl3, 300 MHz): δ = 0.85−0.89 (m, 3 H), 1.27−1.35 (m, 4 H), 1.32 (t, 3JH = 6.9 Hz, 6 H), 1.38−1.47 (m, 2 H), 1.74−1.84 (m, 2 H), 3.81−3.86 (m, 2 H), 6.78−6.86 (m, 2 H), 6.89−6.95 (m, 1 H), 7.09−7.18 (m, 2 H), 7.26−7.32 (m, 2 H), 7.54−7.58 (m, 2 H), 7.78−7.80 (m, 2 H). 13 C NMR (CDCl3, 75 MHz): δ = 14.2 (CH3), 16.5 (d, 3JP−C = 6.5 Hz, CH3), 22.8 (CH2), 26.8 (CH2), 27.0 (CH2), 31.6 (CH2), 47.8 (CH2), 62.4 (d, 2JP−C = 5.4 Hz, CH2), 88.5 (d, 5JP−C = 1.7 Hz, Cquat), 92.0 (Cquat), 115.2 (CH), 115.7 (CH), 116.4 (Cquat), 123.0 (CH), 124.2 (Cquat), 125.1 (Cquat), 127.6 (CH), 127.67 (CH), 127.72 (d, 1 JC−P = 189.4 Hz, Cquat), 128.0 (d, 4JC−P = 3.5 Hz, Cquat), 130.5 (CH), 131.2 (CH), 131.5 (d, CH, 2JP−C = 15.3 Hz), 131.9 (d, 3JP−C = 10.0 Hz, CH), 144.7 (Cquat), 146.0 (Cquat). MS (EI, 70 eV): m/z (%) = 519 ([M+], 100), 448 ([M+ − C5H11]+, 29), 434 ([M+ − C6H13]+, 49), 374 ([C21H13NO2PS]+, 12), 360 ([C 2 0 H 1 1 NO 2 PS] + , 53), 342 ([C 2 1 H 1 3 NO 2 P] + , 12), 297 ([C20H11NS]+, 11), 187 ([C21H13NO2PS]2+, 10). IR (ATR): ṽ = 3057 cm−1, 2978, 2955, 2928, 2857, 2201, 1593, 1574, 1501, 1460, 1443, 1393, 1366, 1335, 1294, 1248, 1196, 1161, 1121, 1099, 1049, 1018, 961, 880, 854, 831, 812, 789, 746, 712, 691, 667, 615. HRMS Calcd for C30H34NO3PS: 519.1995; Found: 519.1992. Preparation of Metal Sheets. To achieve a homogeneous and smooth surface, substrates (15 mm × 20 mm) were ground with SiC paper up to 4000 grit, followed by polishing with an oxide polishing suspension (OPS; SiO2, diameter 50 nm). Prior to functionalization, the substrates were immersed in 0.1 M NaOH for 1 min to increase the amount of hydroxyl groups.35 Surface Modification. The overall chemisorption of 1 is illustrated in Scheme 1. In a Schlenk tube, 1 mL of a 7.7 mM stock solution of 1 in THF was diluted to a final concentration of 0.19 mmol/L and degassed at room temperature with argon for 10 min using a long thin syringe needle. The substrate was mounted on a holder made out of Teflon and added to the reaction mixture with 0.4 mL of a 0.1 M aqueous solution of NH4OH. To ensure a thorough mixing of the aqueous solution with the organic solvent, the degassing was continued for 10 min. The substrate was left in the solution for different immersion times. Afterward, the sample was taken out of the solution and rinsed with an excess of THF and ethanol. Finally, the substrate was dried with a stream of dry nitrogen and stored under vacuum until further analysis. 2.2. Characterization Methods. Ellipsometry. To obtain the thickness of the organic layer, samples have been measured with an UV/vis spectroscopic ellipsometer (SE 800, Sentech Instruments GmbH, Berlin/Krailling, Germany) with a xenon lamp as source. Data in a wavelength range of 320−800 nm at angle of incidence of 70° obtained with a polarizer fixed at 45° was measured at three spots and used for the analysis. For data analysis, a simplified three-layer model (air/organic layer/substrate) was applied with a refractive index of 1.5 for the organic layer.36 Substrate data was measured in a previous measurement of a clean, unmodified sample. This analysis procedure corresponds to an analysis of the changes in ellipsometric parameters upon surface modification. It cannot distinguish between changes in the thin native oxide layer and the organic layer; hence layer thickness values are reported under the assumption that, during surface modification, no change in the layer thickness of the oxide occurs. The thickness of the native oxide is typically 2−3 nm. Results on the

layer thickness differed by 0.2−0.3 nm between the different spots of one measurement. Contact Angle Measurement. Contact angles at three spots were measured by the sessile drop method using deionized water with a goniometer (OCA 20, Dataphysics, Germany). The drop volume was 5 μL. Agreement between data from the different spots was always better than 5°. As evolution with time will be analyzed, data from the middle of the sample is presented exclusively. IR Spectroscopy. Infrared spectra were taken with a Bruker Vertex 70v Fourier transform IR spectrometer (Bruker Optik GmbH, Ettlingen, Germany), operated under vacuum. The absorption spectra were recorded with a spectral resolution of 4 cm−1 using p-polarized light at an angle of incidence of 80°. The spectrometer was equipped with a middle band mercury cadmium telluride detector which was cooled with liquid nitrogen 30 min before the measurements. Prior to surface modification, background spectra were obtained from freshly cleaned metal samples. The absorbance spectra shown in this work were recorded against these backgrounds. Each sample was measured with 250 scans. Calculation of the Tilt Angle between Phenothiazines and Oxide-Covered Iron Surface by IR Spectroscopy. The tilt angle α between the molecular main axis (defined as the axis between the C2 atom of the phenothiazine and the C atom bound to phosphorus) of the organic molecule and surface normal on oxide-covered iron has been calculated according to37

R

ref

Rref = cos2 α + 2RTR

(1)

after immersion times of 30 min and 6 h. The ratio Rref = Ar(1)/Ar(2) can be obtained by integrating the respective bands in the reflectance absorbance spectrum, whereas Ar(1) represents γsym and Ar(2) is γas. The same is done for RTR, which is taken from a spectrum of the substance in (solid) solution, where the molecules of the species are randomly distributed. For calculation, the peaks at 1132 cm−1 (γsym) and 1573 cm−1 (γas) were integrated. XPS. XPS was performed to investigate the chemical composition of the sample surface (Quantera II, Physical Electronics, Chanhassen, MN, USA) applying a monochromatic Al Kα X-ray source (1486.6 eV) operated at a pass energy of 55 eV and a step size of 0.1 eV. The takeoff angle was 45°. The binding energy scale was referenced to the C−C signal at 284.8 eV. The quantitative analysis was carried out with CasaXPS.38 A postmeasurement reduction of resolution was carried out by Fourier transform low pass filtering of the N 1s, P 2p, and S 2p spectra for noise removal, effectively reducing the energy resolution in these spectral regions to 1 eV. Filtering was performed such that the full width at half-maximum of the peaks appeared unaffected within measurement uncertainty. The C 1s spectrum was not analyzed, as the difference between the expected signals of the expected organic molecule and those of the adventitious carbon contamination is too small for a reasonable interpretation. XPS spectra of N 1s, P 2p, S 2p, and C 1s were not fitted as the low intensities do not allow an accurate fitting. For interpretation, commonly used online databases were used as starting points (http://www.lasurface.com/accueil/index.php, http://xpssimplified.com/elements, accessed April 1, 2015). Calculation of Surface Coverage from XPS Data. Surface coverage N(P) per unit area of compound 1 on oxide-covered iron was obtained from39−41

N (P) =

AP SFe (ρ(Fe, FexOy ) λ(Fe, FexOy ) sin θ) AFe SP et /[λ(P,org)sin θ] et /[λ(Fe,org)sin θ]

(2)

In eq 2, the ratio AP/AFe represents the ratio of peak areas of P 2p and Fe 2p peaks, respectively, SFe/SP is the ratio of atomic sensitivity factors, ρ(Fe,FexOy) is the number of iron atoms per unit volume in FexOy, λ(Fe,FexOy) is the inelastic mean free path (IMFP) of Fe photoelectrons in FexOy, λ(P,org) and λ(Fe,org) are the IMPFs of the C


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Figure 1. (a) XPS spectra of modified oxide-covered iron with compound 1 after an immersion time of 1 h. Detected elements of interest are O 1s, S 2p, P 2p, and N 1s. (b) Secondary ion mass spectrum of an oxide-covered iron surface modified with compound 1, and as vertical red lines with an offset of −0.3 m/z expected natural isotope distribution. (c) Contact angles between deionized water and substrate. Blue, bare oxide-covered iron; red, oxide-covered iron modified with compound 1. respective photoelectrons in the organic layer, t represents the thickness of the organic layer, and θ = 45° is the takeoff angle. The thickness of the organic layer was obtained by ellipsometry. An atomic density of 9.862 × 1022 cm−1 for the iron substrate was used.42 For exact estimations, the stoichiometry of the uppermost iron oxide layer first has to be characterized. The IMFP of characteristic Fe 2p photoelectrons in FexOy amounts to roughly 0.306 nm.37 For phosphorus and iron photoelectrons, the IMFP in the organic layer was calculated according to the empirical relation34

λ (Å) = 9.0 + 0.022E (eV)

the presence of aqueous ammonia to promote the saponification of the phosphonate. The in situ liberation of the phosphonic acid prevents a precondensation of the organic molecules and thus a higher degree of cross-linkage. This allows the formation of a well-defined and structured monolayer. Furthermore, in slightly alkaline solution, iron or zinc corrosion is inhibited, which is important to avoid side reactions.44 In addition, the metal substrates were immersed in 0.1 M NaOH to increase the number of hydroxyl groups, which is a key parameter for surface modification.45 1 is well soluble in THF and therefore it is used as solvent. In addition, THF exhibits a low dielectric constant which is assumed to be important for high quality monolayers.46 After the sample was removed, it was washed with an excess of THF and ethanol to remove physisorbed molecules. 3.2. Verification of Successful Surface Modification. Iron. To characterize the ultrathin organic layer on the modified oxide-covered iron surface with compound 1, IR, XPS, and secondary ion mass spectra were recorded. Surface composition was estimated by conducting XPS measurements. Figure 1a shows the presence of O 1s between 527 and 534 eV, S 2p around 169.0 eV, P 2p at 133.1 eV, and N 1s at 399.6 eV. These elements are characteristic for 1. The P 2p3/2 peak of phosphonic acid molecules is typically found at 133.0 eV binding energy, indicating the formation of stable covalent bonds between the oxidic surface and the deprotonated headgroup.47 The peak of sulfur in organic components (e.g., sulfidic or disulfic sulfur) is typically observed around 162 eV. The shift to higher binding energy indicates an oxidized state. The signal in the O 1s spectrum was fitted with four different components.48 The signal at 532.5 eV is attributed to POC groups and adsorbed water, which points toward partially

(3)

where E was the XPS-measured kinetic energy in electronvolts. The ratios AP/AFe and SFe/SP were obtained from the XPS measurements. ToF-SIMS. The mass spectra were recorded with a PHI TRIFT CE (Physical Electronics) with a gallium ion gun with acceleration energy of 15 kV on a spot size of 100 μm × 100 μm. Positive ions were measured in the mass range of 0−1000 amu and the mass resolution was m/Δm = 600. Masses of C, CH3, and Fe/Zn were used for calibration. CV. All CV experiments were carried out under ambient atmosphere in a borate buffer solution (0.075 mol of Na2B4O7·10H2O and 0.3 mol of H3BO4, pH 8.6) as electrolyte with a PHE200 potentiostat (Gamry Instruments Inc., USA). The scan rate was 25 mV/s and the step size was 10 mV. The working electrode was a modified iron or zinc sample with an area of 0.785 cm2. The counter electrode was a graphite rod, and the reference electrode was an Ag/AgCl/3 M KCl electrode (E0 = +210 mV).43

3. RESULTS AND DISCUSSION 3.1. Preparation and Investigation of Sample Films of 1. For preparing ultrathin films, the phosphonated phenothiazine derivative was subjected to chemisorption experiments with previously prepared metal substrates. As the headgroup is present as protected acid, the experiments were carried out in D


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Langmuir hydrolyzed phosphonic ester groups.49 The peak at a binding energy of 531.3 eV is assigned to POH groups and free OH groups, indicating an incomplete reaction between surface hydroxyl groups and phosphonic acid. At a binding energy of 530.4 eV, the PO bond can be detected. The signal at 529.0 eV corresponds to the FeO bond of the substrate. Due to the modification procedure, samples are contaminated by organic C−O and CO moieties, mainly dust as well as solvents (Supporting Information, Figure SI3). Thus, the O 1s spectrum always contains signals of these compounds, which cannot be quantified. Detailed information about the bonding between organic molecule and inorganic substrate was obtained by ToF-SIMS analysis. A full analysis of the spectrum is not presented, because of a high degree of fragmentation and the limited use for interface analysis. However, the fragment [PO3Fe2]+ with m/z = 190.67 was detected, as shown in Figure 1b. The isotopic abundance is consistent with the calculated distribution. Higher intensities than calculated have been observed at several m/z. These higher intensities can be explained by the presence of other fragments of the molecule in the respective region. Although mass spectrometry does not give quantitative information on surface concentrations, it gives a clue for the presence of covalent bonds between substrate and adsorbate. In addition, the headgroup has two possibilities to bind via hydroxyl groups to the substrate. As iron complexation to the phosphoryl moiety is improbable and the fragment contains two iron atoms, the formation of bridging bidentate complexes is obvious.50 The results will be summarized visually in section 4. Modifying metal surfaces with organic molecules leads to a lowering in hydrophilicity. To evaluate such in the system studied here, contact angle measurements have been carried out. The angle between a drop of deionized water and substrate has been recorded until the water wetted the entire surface. The measured values in comparison to data obtained on an unmodified iron surface are shown in Figure 1c. As opposed to the situation for organically modified noble metals, the oxide-covered iron exhibits reactive wetting, as the amount and possibly state of the oxide change in contact with water.51−53 Whereas the initial contact angle of bare iron amounts to 49° and complete wetting of the surface is recorded after 20 min, the initial angle on the modified sample is 74°, i.e., significantly higher. The oxide-covered iron modified with 1 is completely wetted after 45 min. The comparison proves that the surface became less hydrophilic after chemisorption of the phenothiazine. With increasing time, water penetrates from the hydrophobic end of the molecule through defects in the layer to the surface and reacts with oxide-covered iron, causing complete wetting. Furthermore, IR reflection spectra were recorded. The positions of the signals were compared with those from the literature.54 The peaks of a sample with an immersion time of 30 min (Figure 2a, IR2) and their tentative assignment are listed in Table 1. A number of vibrational modes present in the bulk are also observed after adsorption to the surface, and can clearly be assigned, e.g., the characteristic aromatic peaks between 1450 and 1600 cm−1. The bands between 1263 and 1012 cm−1 might be attributed to different bonds. On one hand, the bands at 1263, 1178, and 1132 cm−1 may be attributed to C−C stretching modes and those at 1049 and 1012 cm−1 attributed to C−H in-plane bending modes from the aromatic system. On the other hand, the band at 1263 cm−1

Figure 2. (a) IR spectra of chemisorbed 1 on oxide-covered iron after different immersion times. IR1, 10 min; IR2, 30 min; IR3, 4 h; and IR4, 6 h. (b) Layer thickness obtained by ellipsometry (blue) and surface coverage obtained by XPS (red) for films of compound 1 on oxide-covered iron after different immersion times, respectively.

Table 1. Structural Groups and Their Corresponding Peak Positions in the IR Spectrum of an Oxide-Covered Iron Sample Modified with Compound 1 after an Immersion Time of 30 min

a

peak position [cm−1]

structural group

modea

CH3(as) CH2(as) CH3(sym) CH2(sym) CCb CHb CCb/PO CH in-planeb/POCc/POH CH out-of-planeb

νasCH3 νasCH2 νsymCH3 νsymCH2 rg νCC rg βCH rg νCC/νPO rg βCH/νPOC/νPOH rg βCH

2964 2929 2873 2856 1587, 1494, 1263, 1049, 877

1573 1442 1178, 1132 1012

rg = ring. bAromatic. cAliphatic.

indicates the presence of the PO bond and the signals at 1049 and 1012 cm−1 correlate with the POC and POH bonds. Due to broad signals, a clear assignment is not possible. Furthermore, broad peaks suggest a disordered monolayer as expected, because of the n-hexyl side chain, which is preventing the formation of crystalline layers.28 Besides detected signals, some peaks are missing compared to the bulk spectrum (Supporting Information, Figure SI1). The absence of signals, e.g., the alkyne moiety, or the signal of the aromatic amine is consistent with the literature and may correlate with the alignment of the molecule on the surface of the substrate.27 E


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Figure 3. (a) XPS spectra of oxide-covered zinc modified with compound 1 after an immersion time of 1 h. Detected elements of interest are P 2p, O 1s, N 1s, and S 2p. (b) Secondary ion mass spectrum of an oxide-covered zinc surface modified with compound 1 and as vertical lines with an offset of −0.3 m/z expected natural isotope distribution. (c) Contact angles between deionized water and substrate. Blue, bare oxide-covered zinc; red, oxide-covered zinc modified with compound 1.

of immersion time belong to modes with transition dipole moment parallel (γas = 90°) to the molecular axis. This observation leads to the conclusion that the organic molecules “stand up” with increased immersion time. The tilt angle α between the molecular main axis and surface normal has been calculated after different immersion times (Figure 2b). After 30 min the angle amounts to 65°. After 6 h, this angle decreased to 12°. A decrease in the tilt angle of the phenothiazine molecules should be accompanied by an increase of the organic layer thickness. Especially after longer immersion times, ellipsometry data show an increase, while initially the thickness remains constant within the error of the experiment. These results support the conclusion of a monolayer being formed. It needs to be stressed again that this conclusion relies on the fact that possible changes in the thickness of the oxide and the surface roughness are small, so that the ellipsometric analysis using the oxide-covered substrate without organic monolayer remains valid. Here, however, there is no hint to changes in the oxide during surface modification. Due to steeper molecular arrangement, additional chemisorption of phenothiazine molecules is expected. To confirm such additional adsorption, the surface coverage was estimated by XPS (Figure 2b). After immersion of the substrate for 30 min in the solution, a surface coverage of 1.18 × 1014 molecules· cm−2 was calculated, which increases to 1.37 × 1014 molecules· cm−2 after 6 h. This increase of 16% in molecular density supports the conclusion that the molecules reorient with increasing adsorption time. Zinc. Zinc samples modified with compound 1 were characterized using the same procedure as for iron samples. Figure 3a illustrates the XPS spectra O 1s, S 2p, P 2p, and N 1s of a modified zinc sample. The signal of phosphorus at 133.6

However, the observation of the significant signals indicates a successful functionalization of the surface. To observe the structural evolution of the organic layer during formation, experiments with varied immersion times have been carried out. Figure 2a displays IR spectra of iron samples immersed into the respective solution for 10 min (IR1), 30 min (IR2), 4 h (IR3), and 6 h (IR4). After 10 min of immersion into the reaction solution, adsorption of organic molecules has been detected. However, the IR spectrum shows broad and unresolved peaks. After 30 min, clear absorptions can be detected, suggesting an increase in order in the organic layer. In subsequent spectra, changes are still observed, indicating a continuing reorientation of the organic molecules. The absorbance of the symmetric CH stretching modes of the aliphatic side chain at 2873 and 2854 cm−1 are weak. Nevertheless, there is a change in the intensity ratio of the asymmetric CH stretching modes at 2964 and 2929 cm−1, indicating a change of conformation of the aliphatic chain. Furthermore, the peak assigned to the PO bond at 1265 cm−1 diminishes until it almost disappears after 6 h. The peaks at 1178, 1132, and 877 cm−1 belonging to aromatic C−C and CH bonds increase in absorbance with time of immersion. However, the broadening of the peaks after approximately 30 min points toward increasing disorder in the monolayer due to interrupted layer formation by the n-hexyl side chains. On a metallic substrate as used here, the surface selection rule determines which absorptions can be observed in the spectrum. Orientations of the transition dipole moments of this class of molecules have been published.28 The transition dipole moments of the modes with decreasing absorbance are oriented perpendicular (γsym = 0°) to the main molecular axis. Furthermore, those absorptions which intensify as a function F


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Figure 4. (a) IR spectra of chemisorbed 1 on oxide-covered zinc after different immersion times. IR1, 30 min; IR2, 1 h; and IR3, 4 h. (b) Layer thickness determined by ellipsometry for films of compound 1 on oxide-covered zinc after different immersion times, respectively.

To observe the evolution of the organic layer of compound 1 on oxide-covered zinc, IR spectra of samples with immersion times of 30 min (IR1), 1 h (IR2), and 4 h (IR3) were compared and are shown in Figure 4a. The absorbance of the signals of the n-hexyl side chain between 2965 and 2854 cm−1 intensifies with increasing reaction time. The same observation is made for the signal of the aromatic CC stretching modes at 1587 cm−1 and the signals between 1178 and 1132 cm−1. On the other hand, the signals of the PO stretching mode at 1265 cm−1 and POC/POH groups around 1037 cm−1 decrease. The broadening of the signals originates from a molecular reorientation within the ultrathin organic layer. While after 30 min multilayer formation as a consequence of intermolecular condensation of the free phosphonic headgroups was observed, the upper layer of the organic molecules gets lost due to saponification of the P−O−P bonds. This loss leads to more flexibility of the molecules which are bound to the substrate, reordering within the thin film. With upstanding molecules, the n-hexyl side chain points toward the organic layer, disturbing the molecules to form a dense and close packed monolayer. To investigate the chemical behavior of compound 1 on oxide-covered zinc, the layer thicknesses of samples after different immersion times were determined by ellipsometry. As illustrated in Figure 4b, the thickness of the layer of compound 1 on oxide-covered zinc is bigger than the total length of the organic molecule, proving the formation of multilayers by a monodentate bonding state. Former publications report the investigation of phosphonic acid molecules on zinc oxide and single-crystal ZnO(0001), concluding a bi/tridentate bonding state to the solid substrate.55−57 However, as the grafting conditions differ a lot compared to this work, the observation of an alternative reaction behavior is possible.55−57 3.3. Investigation of the Electrochemical Behavior of the Ultrathin Organic Layers. After ensuring a successful preparation route for organic layers of compound 1 on oxidecovered zinc and iron, the electrochemical properties of the phenothiazine derivative were examined by CV. The experiments shown here were carried out under air atmosphere, but very similar curves are obtained under argon atmosphere. Results did not vary significantly with the immersion time used to prepare the monolayer.

eV is suppressed, due to the intense and broad Zn 3s signal. The O 1s spectrum was fitted with three components. The signal at a binding energy of 529.3 eV is attributed to ZnO. The signal at 531.0 eV is assigned to POH groups present in this sample, supporting the interpretation of the IR spectrum of an incomplete binding of the phosphonate headgroup. The peak at 532.8 eV binding energy can be attributed to POC groups and adsorbed water.48 The peak at 399.6 eV shows the presence of nitrogen. The S 2p signal around 168.1 eV suggests an oxidized state of the organic sulfur, as observed on the iron samples. Due to low intensity of the sulfur and nitrogen signals, quantification was not possible. However, the detection of all heteroatoms indicates the presence of the organic compound and thus a successful functionalization. Secondary ion mass spectra were recorded to gain insight into the bonding state of the organic compound (Figure 3b). The fragment [HO3PZnNa]+ with m/z = 168.35 was detected, where the isotopic abundance is again consistent with the calculated isotope distribution. The sodium cation originates from the pretreatment of dipping the sample in a 0.1 M NaOH solution. This observation supports the qualitative interpretation of free hydroxyl groups of the phosphonic acid and thus a different bonding state than on oxide-covered iron. Figure 3c shows the measured contact angles on a modified zinc surface. With a higher initial contact angle and a longer wetting time, the functionalized zinc surface is less hydrophilic than the bare substrate and even as the modified iron surface. In terms of the IR spectrum of the modified zinc sample, the spectrum after an immersion time of 30 min (Figure 4a, IR1) looks similar to the spectrum of modified iron, except for higher absorbance. A significant change is observed in the intensity ratio of the stretching modes of the CH2 and CH3 groups, suggesting a different conformation of the aliphatic side chain. In addition, the peaks at 744 and 707 cm−1 could not be detected in the IR spectrum of modified iron samples. The peaks at 952 and 927 cm−1 are assigned to phosphonic acid, showing an uncompleted reaction with surface hydroxyl groups. These changes in the IR spectrum suggest a different conformation of compound 1 on modified oxide-covered zinc compared with functionalized oxide-covered iron. G


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Figure 5. (a) CV of oxide-covered iron modified with compound 1 (immersion time 30 min) and bare oxide-covered iron as inset graph in borate buffer solution at a scan rate of 25 mV/s. Blue, first cycle; green, second cycle; red, third cycle. (b) IR spectra of an oxide-covered iron sample after surface modification with compound 1 (IR1), after dipping in electrolyte under open circuit conditions for 2 min (IR2) and after CV (IR3).

Figure 6. (a) CV of bare oxide-covered zinc (inset: scan in restricted electrode potential range). (b) CV of oxide-covered zinc modified with compound 1 after 30 min immersion (inset: scan in restricted electrode potential range). Blue, first cycle; green, second cycle; red, third cycle.

Iron. The inset graph in Figure 5a shows the CV with three cycles on bare oxide-covered iron. Two cathodic current peaks at −0.67 and −0.60 V and one anodic current peak at −0.02 V can be observed, according to formation and reduction of iron oxide.58 In comparison, Figure 5a shows the CV of functionalized iron with compound 1. No further peaks can be observed in addition to those related to iron oxidation. Similar measurements between −0.3 and 0.9 V were performed (not shown), also without observation of noticeable changes. The lack of characteristic peaks of compound 1 could be explained by an oxidized state of the sulfur atom, shifting the oxidation potential of the phenothiazine to higher energy, thus making reversible oxidation impossible due to beginning oxygen evolution. In addition, possible oxidation of phenothiazine by metal ion acceptors is known, which results in positively charged molecules. Further oxidation, e.g., of the sulfur atom is impossible in the given potential window.28 On the other hand, the bond between transition metal oxide and organic molecule might not be strong enough, leading to desorption with electrode polarization. Therefore, IR spectra after surface functionalization (IR1), after dipping the sample in electrolyte under open circuit conditions for 2 min (IR2), and after CV experiment (IR3) were recorded and compared, illustrated in Figure 5b. IR1 shows small peaks after surface modification. In IR2, peaks, e.g., at 2917 and 1575 cm−1 are clearly visible, indicating a molecular reorientation. However, IR3 shows no

characteristic peaks of compound 1, indicating desorption of the organic molecules. Subsequently, XPS measurements show no presence of phosphorus, nitrogen, and sulfur. According to the reaction mechanism, the phenothiazine derivative is bound via a covalent bond to the transition metal oxide. By polarization of the surface, the oxide layer changes, inevitably leading to destruction of the ultrathin organic layer. Zinc. Figure 6a shows the CV for bare oxide-covered zinc. One anodic current peak A1, one anodic current plateau A2, and one cathodic current peak C1 were observed. The signals originate from the formation and degradation of ZnO (A1 and C1) and Zn(OH)2 (A2), respectively.59 The inset graph in Figure 6a illustrates a CV of unmodified oxide-covered zinc in a smaller electrochemical window, showing the evolution of oxygen. Figure 6b shows the CV of an oxide-covered zinc surface functionalized with compound 1. Compared to the unmodified substrate, no peaks of the oxide formation and reduction could be detected. Furthermore, a cathodic current peak C2 around −0.87 V appeared. To identify the new signal, the electrochemical window was reduced, shown as an inset graph in Figure 6b. One anodic current peak and one cathodic current peak could be observed, giving rise for redox activity of the organic layer, due to the absence of any signals in the inset graph in Figure 6a. At the same time, the formation of surface H


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Langmuir

After developing a route for surface modification, the interfacial structure of functionalized samples was investigated (Figure 8). On iron, stable monolayers were obtained. Molecular reorientation was observed as a function of immersion time. After 30 min, densely packed monolayers with a tilt angle of 25° of the organic molecules were formed. Surface binding is realized in a bidentate bridging bonding state, with stable covalent bonds between deprotonated headgroup and substrate. Contact angle measurements show reactive wetting of the iron surface, with less hydrophilicity of modified samples with compound 1 than on bare oxide-covered iron.The same modification protocol leads to monodentate bonding and thus free hydroxyl groups leading to multilayer formation on oxidecovered zinc. Molecular rearrangement and reorientation is also observed as a function of immersion time. Contact angle measurements show less hydrophilicity of the modified oxidecovered zinc to bare oxide-covered zinc and oxide-covered iron modified with compound 1 as a consequence of multilayer formation. Reactive wetting again leads to a full wetting of the interface after a certain time. For both transition metals, XPS data point toward an oxidized state of the sulfur atom, indicating that not the native organic molecule as shown in Figure 8 was chemisorbed. It remains unclear when oxidation took place. Upon dipping the samples in an aqueous electrolyte, molecular reorientation was observed on both transition metals. Electrode polarization of the modified oxide-covered iron leads to desorption of the phenothiazine, likely together with reduction of the oxide. On oxide-covered zinc on the other hand, the formed multilayers show sufficient stability. The CV shows anodic and cathodic current peaks, suggesting redox activity of the organic layer.

oxide after polarizing to oxygen evolution still makes the curves not fully reversible. After polarization, the sample was investigated by IR spectroscopy and XPS. Figure 7 shows the IR spectra after

Figure 7. IR spectra of an oxide-covered zinc sample after surface modification with compound 1 (IR1) and after CV (IR2).

functionalization (IR1) and after CV (IR2). Besides changes in absorbance attributed to molecular reorientation, the signals of the aliphatic side chain between 2850 and 2960 cm−1, of the aromatic framework between 1600 and 1300 cm−1, and of the PO stretching mode at 1263 cm−1 are detected. In addition, XPS spectra (Supporting Information, Figure SI2) show the presence of all heteroatoms, proving sufficient stability against electrochemical treatment.

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ASSOCIATED CONTENT

S Supporting Information *

4. CONCLUSION Palladium-catalyzed cross-coupling reaction of 3-((4bromophenyl)ethynyl)-10-hexyl-10H-phenothiazine with diethyl phosphonate furnishes phenylethynyl phenothiazinyl phosphonate, a redox-active, fluorescent molecule with a group permitting surface linkage. The phosphonic ester headgroup can in situ form reactive phosphonic acid molecules, which chemisorb on hydroxylated, oxide-covered transition metals.

Further XPS and IR spectra from additional analyses. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01370.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected].

Figure 8. Overview of the main results presented in this work. I


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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We cordially thank Dr. Dominik Urselmann for providing 3((4-bromophenyl)ethynyl)-10-hexyl-10H-phenothiazine. A.E. acknowledges Prof. M. Stratmann for continuous support. A.E. and J.R. acknowledge support from the Cluster of Excellence RESOLV (EXC 1069) funded by the Deutsche Forschungsgemeinschaft.

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Surface Functionalization of Oxide-Covered Zinc and Iron with

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