Self-Assembled Monolayers of Perfluoroanthracenylaminoalkane

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Self-Assembled Monolayers of Perfluoroanthracenylaminoalkane Thiolates on Gold as Potential Electron Injection Layers Zibin Zhang,†,¶ Tobias Wac̈ hter,§,¶ Martin Kind,† Swen Schuster,§ Jan W. Bats,‡ Alexei Nefedov,⊥ Michael Zharnikov,*,§ and Andreas Terfort*,† †

Institute of Inorganic and Analytical Chemistry and ‡Institute of Organic Chemistry and Chemical Biology, University of Frankfurt, Max-von-Laue-Straße 7, 60438 Frankfurt, Germany § Applied Physical Chemistry, University of Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany ⊥ Institute of Functional Interfaces, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany S Supporting Information *

ABSTRACT: As a material with relatively small band gap and low lying valence orbitals, perfluoroanthracene (PFA) is of interest for the modification of electrode surfaces, for example, as charge injection layers for n-type organic semiconductors. To covalently attach PFA in the form of self-assembled monolayers (SAMs), we developed a synthesis of derivatives with a sulfur termination, linked to the 2-position of the PFA moieties by an −NH− group and a short alkane chain with two and three methylene groups, respectively. Spectroscopic characterization of the SAMs reveals that the molecules adopt an almost upright orientation on the gold surface, with the packing density mostly determined by the steric demands of the PFA units. The number of the methylene groups in the −NH−alkyl linker has only a minor impact on the SAM structure because of the nonsymmetric attachment of the PFA units, which permits the compensation of the orientational constraints imposed by the bending potential. The investigated SAMs alter the work function of gold by +(0.59−0.64) eV, suggesting comparably strong depolarization effects, affecting the extent of the work function modification. KEYWORDS: self-assembled monolayers, work function, organic electronics, perfluorinated arenes, band gap

1. INTRODUCTION Fluorine is the most electronegative chemical element, which gives fluorinated compounds a variety of specific features. In particular, the introduction of fluorine atoms to polyconjugated systems will not only effectively lower both their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels, facilitating electron acceptance,1,2 but also invert the charge density distribution with respect to the corresponding aromatic hydrocarbons.3 Therefore, perfluorination is an effective way to convert a p-type organic semiconductor to an n-type one, preserving, at the same time, some other basic properties of the system. In this context, it has been reported that the perfluoro-derivatives of pentacene4 and tetracene5 exhibit almost the same charge carrier mobility as the nonfluorinated ones, but with a different charge carrier type (electrons instead of holes). Consequently, fluorinated conjugated compounds may play important roles in electronic devices such as organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs), in particular, because they can be used as ambipolar or n-type semiconductors and be helpful for constructing OFET-based logic circuits.6 The n-type semiconductor character of fluorinated conjugated compounds is also of importance as far as these substances are used as building blocks of functional © 2016 American Chemical Society

intermediate layers in organic electronic devices and heterostructures. Such layers have been employed to optimize the structure and orientation of the adjacent organic semiconductor7 as well as to improve the charge-carrier injection from the electrodes into the semiconductor, typically by level alignment, through work function engineering.8,9 In most cases, these layers are deposited in the form of self-assembled monolayers (SAMs),10,11 the properties of which can be adjusted in a controlled fashion to fit the specific needs for the interfaces in OLEDs, OFETs, and other devices.12−16 Generally, the perfluorination of SAMs leads to an increase in the work function ϕ of the respective surfaces, as has been repeatedly demonstrated for perfluoroalkylated SAMs.17−19 In contrast, only few experimental data on work function changes Δϕ of gold induced by fluorinated aromatic SAMs have been reported so far, namely for perfluorobenzene functionalized SAMs20,21 and for a perfluorinated terphenylthiolate SAM.22 The latter system was particularly interesting, exhibiting a variety of useful properties.22−24 These monolayers were found to be highly oriented and densely packed, exhibiting similar Received: January 15, 2016 Accepted: February 29, 2016 Published: February 29, 2016 7308

DOI: 10.1021/acsami.6b00532 ACS Appl. Mater. Interfaces 2016, 8, 7308−7319

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ACS Applied Materials & Interfaces

exclusion of light. NMR spectra were recorded using dark brown NMR tubes in degassed C6D6. PFA,29 cystamin,35 and bis(3aminopropyl) disulfide36 were prepared according to literature methods. NMR spectra were recorded on a Bruker Avance 300 and a Bruker Avance 500. Because of the strong coupling of the fluorine nuclei to the carbon nuclei, the signals of the latter could often not be found in the regular 13C NMR spectra, so 19F decoupled spectra had to be recorded. For the IR spectroscopy of the bulk samples, a Nicolet 6700 FTIR spectrometer with a diamond attenuated-total reflection (ATR) unit was used. Mass spectra were recorded with a MAT 95 instrument from Finnigan or a Finnigan LTQ-FT from Thermo Fisher Scientific. X-ray diffraction data were collected on a STOE-IPDS-II two circle diffractometer employing graphite-monochromated Mo Kα radiation (0.71073 Å) with ω-scans. Melting points were measured with a Büchi apparatus according to Dr. Tottoli. 2.1.1. Bis[(perfluoroanthracene-2-yl)-2-aminoethyl] disulfide (1). Cystamine (0.33 g, 2.17 mmol) was added to the solution of PFA (1.72 g, 4.80 mmol) in dry THF (150 mL). After stirring at room temperature for 5 days, the solvent was removed. The residue was dissolved in ethyl acetate and washed twice with saturated sodium hydrogen carbonate. The organic layer was dried over anhydrous Na2SO4 and evaporated to afford the crude product, which was purified by flash column chromatography (petroleum ether/ethyl acetate, 5:1). The fractions containing 1 were combined and concentrated under vacuum to give 1 (0.45 g, 25%) as a yellow solid, mp 113−115 °C. Unreacted PFA (0.70 g) could be recovered. 1 H NMR (C6D6, 300 MHz): δ [ppm] = 4.02−3.89 (m, 2H), 3.44− 3.31 (m, 4H), 2.40 (t, J = 6.6 Hz, 4H). 19F NMR (C6D6, 282 MHz): δ [ppm] −124.10 to −125.00 (m, 2F), −126.20 to −127.35 (m, 2F), −145.50 to −145.70 (m, 1F), −145.75 to −146.00 (m, 2F), −146.16 (t, J = 16.4 Hz, 1F), −146.57 (t, J = 16.3 Hz, 1F), −146.82 (t, J = 16.4 Hz, 1F), −148.25 to −148.45 (m, 1F), −145.50 to −148.75 (m, 3F), −155.00 to −155.25 (m, 2F), −155.00 to −155.25 (m, 2F). 13C NMR (C6D6, 125 MHz): δ [ppm] = 43.95, 38.05. 19F-decoupled 13C NMR (C6D6, 125 MHz): δ [ppm] = 144.98, 141.17, 141.13, 140.55, 140.44, 140.30, 138.54, 137.66, 137.48, 137.44, 125.22, 110.09, 108.33, 105.99, 105.83. MS (API−): m/z (%) = 828.0 (100) [M]−. HRMS (API−): m/ z calcd. for [M]− C32H10F18N2S2, 827.9992; found 827.9996, deviation 0.5 ppm. 2.1.2. Bis[(perfluoroanthracene-2-yl)-3-aminopropyl] disulfide (2). A solution of bis(3-aminopropyl) disulfide (0.30 g, 1.67 mmol) in 20 mL of 1,4-dioxane and 10 mL of DMF was added to a solution of PFA (1.40 g, 3.90 mmol) in 100 mL of 1,4-dioxane and 50 mL of DMF. After stirring at room temperature for 5 days, the solvent was removed. The residue was worked up as described above to give 2 (0.19 g, 13%) as a yellow solid, mp 154−155 °C. Unreacted PFA (0.90 g) could be recovered. 1H NMR (C6D6, 300 MHz): δ [ppm] = 3.76− 3.60 (m, 2H), 3.07−3.19 (m, 4H), 2.40 (t, J = 7.1 Hz, 4H), 1.72−1.57 (4H). 19F NMR (C6D6, 282 MHz): δ [ppm] = −124.46 to −125.04 (m, 2F), −126.69 to −127.60 (m, 2F), −145.77 to −146.57 (m, 4F), −146.61 to −147.15 (m, 2F), −148.28 to −149.09 (m, 4F), −155.00 to −155.65 (m, 2F), −156.78 to −157.40 (m, 2F). 13C NMR (C6D6, 125 MHz): δ [ppm] = 44.01, 35.27, 29.95. 19F-decoupled 13C NMR (C6D6, 125 MHz): δ [ppm] = 147.38, 144.88, 141.23, 141.19, 140.56, 140.43, 140.31, 138.51, 137.58, 137.23, 137.19, 125.76, 110.17, 108.29, 105.81, 105.48. MS (API−): m/z (%) = 856.0 (100) [M]−. HRMS (API−): m/z calcd. for [M]− C32H10F18N2S2, 856.0306; found 856.0317, deviation 1.2 ppm. 2.2. Preparation of Self-Assembled Monolayers. SAMs were produced by immersion of the substrates into ethanolic solutions of 1 or 2. We assumed that the disulfide bond of 1 and 2 will be cleaved upon the interaction with the substrate,10 with the resulting thiolate compounds, abbreviated below as PFA-NH-C2 and PFA-NH-C3, respectively, becoming attached to the substrate by sulfur−gold bonds. We will also use a joint abbreviation, PFA-NH-Cn, for PFA-NH-C2 and PFA-NH-C3 together. Gold substrates were obtained by physical vapor-deposition of 200 nm Au layers onto Si(100) wafers (Silicon sense). A chromium layer of 5 nm between the Si surface and the gold layer served as an adhesion promoter. Before SAM deposition, the gold-covered wafer-

structural order and morphology as the nonfluorinated terphenylthiolate SAMs.25−27 Also, their formation resulted in an increase of the substrate work function by ∼0.6 eV,22 as was intended initially. However, these monolayers turned out to have a substantial band gap of about 5.1 eV, rendering them electrically insulating, similar to partially fluorinated alkanethiolate molecules.22 The most likely reason for this behavior is a significant twist between the individual phenyl rings in the molecular backbone, which is caused by the steric repulsion between the fluorine atoms in the ortho-positions of the adjacent rings, leaving each ring electronically isolated from the others and resulting, in particular, in the localization of the HOMO orbital exclusively at the sulfur atom acting as anchoring group (the HOMO-1 is localized at the phenyl ring next to the alkyl linker).22 An effective method to extend π-systems and, thus, to reduce the band gap is the annelation of aromatic rings, for example, in the form of acene compounds. In this context, we decided to design SAMs that are based on perfluoroanthracene (PFA) because this medium-sized acene already has a moderate band gap (∼3.4 eV)28,29 and is relatively easy to synthesize.29 This band gap is even slightly narrower than that of nonfluorinated anthracene (∼3.6 eV),28 which is a well-established organic semiconductor.30,31 For the attachment of the PFA moieties to the gold surface and their efficient assembly, the thiolate anchor10 was selected. Further, we introduced a short alkyl linker in between the PFA unit and the thiolate anchor (see Figure 1) to improve the structural quality of the mono-

Figure 1. Schematic representation of the investigated perfluoroanthracene-terminated thiolates on gold. The red dotted line represents the main molecular axis that is used in the discussion.

layer32,33 as well as an N−H group due to its potential ability to form a network of hydrogen bonds within the SAM, increasing its stability.34 Additionally, we varied the length of the alkyl linker (see Figure 1) to look for possible odd−even effects, namely dependence on the packing density and molecular orientation in a SAM on the parity of the number of the methylene groups in the alkyl linker. Such effects have been reported in many araliphatic hybrid systems but were absent in anthracene-substituted alkanethiols (due to the specific symmetry of the SAM constituents),33 which, however, could not exclude their appearance in the PFA-based SAMs.

2. EXPERIMENTAL SECTION 2.1. Synthesis of the SAM Precursors. All chemicals were purchased from commercial sources and used without further purification. Because PFA and its derivatives are light and air sensitive, all operations were performed in degassed solvents and under 7309


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reference for the evaluation of the photoemission data in terms of the effective film thickness and packing density. 2.5. Ellipsometry. A Sentech SE 400 ellipsometer equipped with a He/Ne laser (a wavelength of 632.8 nm, a beam diameter of 1−2 mm) was used for ellipsometric characterization of the SAMs. The angle of incidence was set to 70° with respect to the sample surface normal. Complex refractive indices of the substrates were measured before SAM deposition at marked positions. The extinction coefficients and the refractive indices of the monolayers were assumed to be zero and 1.45, respectively. 2.6. IR Spectroscopy. IR spectra of the neat substances and selfassembled monolayers were recorded using a Thermo Nicolet 6700 Fourier transform IR spectrometer with a liquid nitrogen-cooled narrow band cadmium telluride semiconductor detector. During registration of all spectra, the beam path of the spectrometer was purged with dried and CO2-free air. All spectra were recorded at a resolution of 4 cm−1. IR reflection absorption spectroscopy (IRRAS) measurements of the SAMs were carried out with an infrared reflection−absorption unit using p-polarized IR radiation at an incidence angle of 80° relative to the sample surface normal. Perdeuterated dodecanethiolate SAMs on gold served as a reference. For each SAM, at least 2000 scans were averaged. Density functional theory (DFT) calculations were performed to aid the band assignment in the experimental spectra and to obtain the directions of the transition dipole moments (TDMs) for the vibrational modes (see Supporting Information). 2.7. NEXAFS Spectroscopy. The acquisition of the NEXAFS spectra was carried out at the C and F K-edges, in the PEY mode with retarding voltages of −150 and −450 V, respectively. Linear polarized synchrotron light with a polarization factor of ∼95% was used. The energy resolution was ∼60−70 meV at the C edge and slightly lower at the F K-edge. The incidence angle of the light was varied from 90° (Evector in the surface plane) to 20° (E-vector nearly normal to the surface) in steps of 10−20° to monitor molecular orientation and orientational order in the PFA-NH-Cn SAMs. This approach is based on the linear dichroism in X-ray absorption, that is, the strong dependence of the cross-section of the resonant photoexcitation process on the orientation of the electric field vector of the linearly polarized light with respect to the molecular orbital of interest.48 The raw NEXAFS spectra were normalized to the incident photon flux by division through a spectrum of a clean, freshly sputtered gold sample. Afterward, the spectra were reduced to the standard form by subtracting a linear pre-edge background and normalizing to the edge jump (determined by a nearly horizontal plateau 40−50 eV above the respective absorption edges). The energy scale at the C K-edge was referenced to the most intense π* resonance of highly oriented pyrolytic graphite at 285.38 eV.49 The energy scale at the F K-edge was calibrated on the basis of the well-known Δhν ∝ (hν)3/2 behavior of plane grating monochromators. 2.8. Modeling of the NEXAFS Spectra. To provide a reliable basis for the assignment of the features in the experimental NEXAFS spectra, a series of calculations with the quantum chemistry program package StoBe (see Supporting Information) was carried out for the major functional group of the PFA-NH-Cn molecules, namely PFA. We assumed that the contribution of the −NH−Cn linker to the NEXAFS spectra is comparably small so that the calculated spectra should adequately describe the experimental ones, but, of course, some minor differences related to this linker are still possible. The details of the calculation procedure are given in the Supporting Information. 2.9. Work Function Determination. Work function (Φ) measurements were carried out using a UHV Kelvin Probe 2001 system (KP Technology Ltd., UK). The pressure in the UHV chamber was ∼10−10 mbar. The Φ values for the target samples were referenced to those of HDT/Au and freshly sputtered gold.

pieces were rinsed with ethanol, blown dry in N2 stream, and treated for 2 min in an H2 plasma using a Harrick PDC-32 G plasma cleaner at an operation pressure of about 8 mbar and a power of 25 W to remove contaminations from the Au surface. Because of the relatively low solubility of the SAM precursor substances, solutions of only 0.12 μM concentration were used. The solvent was ethanol, which had been purified using gold nanoparticles.37 To avoid oxidation of 1 and 2, the solutions were degassed using the freeze−pump−thaw method, and SAM deposition was carried out under Schlenk conditions. The solutions were kept in the dark to prevent the SAM precursor molecules from light-driven degradation. Immersion times ranged from 22−24 h. After removal from the solutions, the samples were extensively rinsed with ethanol, ethyl acetate, and ethanol again before being blown dry in an Ar stream. Reference SAMs were prepared by immersion of similar gold substrates into 1 mM ethanolic solutions of perdeuterated dodecanethiol, hexadecanethiol (HDT), and anthracene-2-thiol (Ant)38 for 20−36 h. 2.3. Characterization: General Comments. The target and reference SAMs were characterized by high-resolution X-ray photoelectron spectroscopy (HRXPS), ellipsometry, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, infrared (IR) spectroscopy, and work function measurements. All experiments were performed at room temperature. The HRXPS and NEXAFS experiments were carried out at the D1011 beamline at the MAX II storage ring of the MAX-IV synchrotron radiation facility in Lund, Sweden. This is a bending magnet beamline with a plane grating monochromator. A custom-designed experimental station was equipped with a SCIENTA SES200 electron energy analyzer and a partial electron yield (PEY) detector. The above measurements were carried out under UHV conditions at a base pressure < 1.5 × 10−9 mbar. The spectra acquisition time was kept short to minimize damage induced by the primary X-rays.39,40 2.4. High-Resolution X-ray Photoelectron Spectroscopy. The HRXP spectra were acquired in normal emission geometry. Photon energies (PEs) of 350 eV for the S 2p range, 350 and 580 eV for the C 1s range, 580 eV for the N 1s range, and 750 eV for the F 1s range were used. The energy resolution was about 70−80 meV at a PE of 350 eV and only slightly lower at higher PEs so that the energy width of individual emissions, given by full width at half-maximum (fwhm), was mostly determined by the intrinsic energy spread of the photoemission process. The binding energy (BE) scale of the C 1s, S 2p, N 1s, and F 1s spectra was calibrated to the position of the Au 4f7/2 emission of the underlying Au substrate (84.00 eV)41 measured at the same PE. The spectra were decomposed into individual emissions using symmetric Voigt functions and a linear background. The S 2p3/2,1/2 doublets were represented as a combination of two peaks with the same fwhm, the standard71 spin−orbit splitting of ∼1.18 eV (verified by fit), and a branching ratio of 2 (S 2p3/2/S 2p1/2). The same basic fit parameters were used for identical spectral regions of the analogous samples. On the basis of the HRXPS data, effective thickness and packing density of the PFA-NH-Cn monolayers were calculated using a welldefined HDT SAM on Au as the reference. The thickness was determined on the basis of the intensity of the Au 4f signal, assuming its standard, exponential attenuation by the monomolecular film.42 The attenuation lengths reported in ref 43 for a series of well-defined hydrocarbon films were used; these values are well applicable to the fluorocarbon films as well.44 For the thickness of the reference HDT SAM, a value of 1.94 nm was used; this value was estimated on the basis of the alkyl chain length (0.126 nm per CH2 moiety),45 molecular inclination (30−33.5°),46 and Au−S distance (0.24 nm).47 The packing densities were estimated from the intensity ratios of the S 2p and Au 4f emissions, following the approach of ref 47. As a reference system with well-known packing density (4.63 × 1014 molecules/cm2 or 0.216 nm2/molecule),46 a HDT SAM on Au(111) was used. Note that the attenuation lengths for photoelectrons in hydrocarbon and fluorocarbon films are believed to be almost indistinguishable,44 which justifies the use of the HDT SAM as a

3. RESULTS 3.1. Synthesis and Crystal Structure of the SAM Precursors. Because of the very underdeveloped chemistry of PFA, two questions had to be addressed during the 7310


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compounds 1 and 2. The crystal structure of 1 also reveals another interesting feature, that is the formation of intramolecular hydrogen bonds. Although organically bound fluorine atoms are only weak hydrogen bonding acceptors, they still can form hydrogen bonds in the absence of better acceptors.53 The F···HN distance is 0.219 nm (a and b in Figure 2), and the N-H···F angle is 115°. Both values fall into the range of the criterion for a F···H-N hydrogen-bond, that is, the F···HN distance < 0.23 nm and the N-H···F angle > 90°.53 This bond can stabilize the alignment of the alkyl linker and the PFA unit, but the extent of this stabilization is difficult to evaluate. In addition, there are also N-H···S hydrogen-bonds since the N···S distances are shorter than 0.35 nm (c = 0.264 nm and d = 0.260 nm in Figure 2).54 Because of these bonds, the whole disulfide molecule is slightly contracted. The latter interaction is, however, of no importance for the densely packed SAMs where the disulfide bonds are presumably cleaved upon the adsorption, which should permit the complete extension of the alkyl chains. 3.2. Ellipsometry. After deposition of the molecules 1 and 2 onto Au(111) substrates, the thicknesses of the respective PFA-NH-Cn monolayers were determined by ellipsometry (see Experimental Section). The values for the PFA-NH-C2 and PFA-NH-C3 SAMs were 1.75 ± 0.09 nm and 1.88 ± 0.04 nm, respectively (Table 1). These values are comparable to the sum of the molecular length (1.48 and 1.60 nm, respectively) and the length of the S−Au bond (0.24 nm),47 suggesting an extended conformation and almost upright orientation of the molecules in the PFA-NH-C2 and PFA-NH-C3 films. 3.3. HRXPS. C 1s, S 2p, N 1s, and F 1s HRXP spectra of the PFA-NH-Cn SAMs are shown in Figure 3, along with the respective fits and decomposition of the C 1s spectra. The S 2p spectra in Figure 3, panel a exhibit a single S 2p3/2,1/2 doublet at a BE of ∼162.0 eV (S 2p3/2) for both PFA-NH-C2/Au and PFA-NH-C3/Au. The above BE value corresponds to the thiolate species bound to gold,40,55 suggesting that the PFANH-Cn are indeed well defined SAMs, with all molecules bound to the substrate in a proper fashion, via the headgroup. No species associated with atomic sulfur, disulfides, dialkylsulfides, physisorbed thiol, or oxidized sulfur were observed. The C 1s HRXP spectra of the PFA-NH-Cn SAMs in Figure 3, panel a exhibit three emissions at BEs of ∼284.4 eV (1), ∼285.35 eV (2), and ∼286.9 eV (3) assigned to the carbon atoms in the aliphatic linker (1) and the PFA moiety (2 and 3). The emission 3 can be associated with the carbon atoms bound directly to fluorine, whereas the emission 2 can be related to the residual carbon atoms of the PFA moiety, including the one bound to the NH-alkyl linker. This assignment is supported by the intensity relations between the emissions, which reproduce

development of the molecules: (i) how to (chemo)selectively attach the side chain with the nitrogen atom and (ii) how to attain the (regio)selectivity in the favored 2-position at the PFA. To achieve the chemical selectivity, we wanted to refrain from using a separate protective group for the very nucleophilic sulfur atoms as this would have required additional steps during the synthesis. Therefore, we rather used the respective disulfide compounds (Scheme 1), in which the nucleophilicity of the Scheme 1. Synthetic Route to 1 (n = 1) and 2 (n = 2) Based on the Nucleophilic Substitution at 2-Position of PFA

sulfur atoms is considerably reduced, while the oxidative addition to the gold surface under formation of the gold− thiolate bond is well established.10,50 Indeed, the respective bis(ω-aminoalkyl) disulfides turned out to be suitable for the substitution reactions if care was taken that the reactions were performed at moderate temperatures (room temperature) and that the reactions partners were completely dissolved before the reaction. While for the reaction with bis(2-aminoethyl) disulfide (cystamine) it was possible to use THF as solvent, the synthesis of PFA disulfide 2 required a mixture of 1,4dioxane and DMF to make sure that both PFA and bis(3aminopropyl) disulfide were completely dissolved at room temperature. Under these conditions, the reaction proceeded within five days with acceptable yields of 30−35%. It has been controversially discussed in the literature at which position of the PFA molecules the nucleophilic reactions proceed. While Burdon et al. reported the selective substitution at the 2-position based on 19F NMR results,51 Muir and Baker predicted a preferred substitution at the position 9 based on theoretical calculations.52 As we were able to obtain single crystals of compound 1 by slow evaporation of a solution of 1 in benzene, we could address this problem directly. Because of the strong quadrupole interactions, benzene became incorporated into the crystals, although obviously not all possible sites were occupied, presumably due to a loss of part of the solvent molecules during crystal handling. The X-ray crystallographic analysis clearly demonstrates that the nucleophilic substitution by the nitrogen nucleophiles occurred at the 2-position of PFA, thus supporting the results of Burdon et al. (Figure 2). We can assume that the substitution in compound 2 is at the same position as the 19F NMR spectra are basically identical for

Figure 2. X-ray crystal structure of 1. The green dotted lines indicate the N-H···F hydrogen bonding. The red dotted lines indicate the N-H···S hydrogen bonding. 7311


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Table 1. Ellipsometry- and HRXPS-Derived Effective Thicknesses as Well as HRXPS-Derived Packing Density of the PFA-NHC2 and PFA-NH-C3 SAMs. Molecular Lengths of PFA-NH-C2 and PFA-NH-C3 Are Given for Comparison (Note That the Length of the S−Au Bond (0.24 nm)47 Must Be Added To Obtain the Maximum Theoretical Layer Thickness) monolayer

molecular length (nm)

effective thickness ellipsometry (nm)

effective thickness HRXPS (nm)

packing density (molecules cm−2)

PFA-NH-C2 PFA-NH-C3

1.48 1.60

1.75 ± 0.09 1.88 ± 0.04

1.82 ± 0.05 1.91 ± 0.05

3.6 × 1014 3.7 × 1014

and 3.7 × 1014 molecules/cm2, respectively, corresponding to molecular footprints of ∼0.28 and ∼0.27 nm2/molecule. Such footprints are noticeably larger than those for hydrocarbon SAMs on Au(111) (0.216 nm2/molecule)46 but typical of the monolayers containing fluorocarbon moieties.24,56,57 The difference is mostly related to the larger volume of fluorine atoms as compared to the analogous parameter for hydrogen atoms. The above data are summarized in Table 1, along with the results of the ellipsometry measurements. 3.4. IR Spectroscopy. The IRRA spectra of the PFA-NHCn SAMs, the IR spectra of the neat substances 1 and 2, and the calculated (DFT) IR spectra of the 1- and 2-derived thiols are displayed in Figures 4 and 5.

Figure 3. (a) C 1s, (b) S 2p, (c) N 1s, and (d) F 1s HRXP spectra of the PFA-NH-Cn SAMs (open circles) as well as respective fits (thin solid lines). The C 1s spectra are decomposed into several components, which are color coded and marked by numbers.

coarsely the molecular composition, as well as by the increase of the relative intensity of emission 1 with increasing PE (not shown). A higher PE results in a higher kinetic energy of photoelectrons, associated with smaller attenuation of the respective signal and larger spectral weight of the deeper parts of the sample.42 Finally, in the spectra of both compounds, the well accounted emissions 1−3 are accompanied by very weak peaks at BE of ∼288.9 eV, which are presumably related to minor COOH contamination. The N 1s HRXP spectra of the PFA-NH-Cn SAMs in Figure 3, panel c exhibit a single emission at a BE of 399.2−399.3 eV assigned to the nitrogen atom in the NH-Cn linker. The presence of a single peak suggests the same chemical state for all nitrogen atoms in the monolayers. The F 1s HRXP spectra of the PFA-NH-Cn SAMs in Figure 3, panel d exhibit a single emission as well. This emission, appearing at a BE of ∼687.2− 687.3 eV, is assigned to the fluorine atoms in the PFA moiety of PFA-NH-Cn. On the basis of the photoemission data (see Experimental Section), the effective thickness of the PFA-NH-C2 and PFANH-C3 monolayers could be estimated. The resulting values of 1.82 ± 0.05 nm and 1.91 ± 0.5 nm, respectively, agree well within the margin of error with the ones obtained by ellipsometry (see Table 1). Further, the packing density in the PFA-NH-Cn monolayers was evaluated, following the procedure described in the Experimental Section. The packing densities of the PFA-NHC2 and PFA-NH-C3 monolayers were estimated at 3.55 × 1014

Figure 4. IR data for PFA-NH-C2. Top curve: IRRA spectrum of the PFA-NH-C2 SAM. Middle curve: spectrum of the neat substance 1 (disulfide). Bottom curve: calculated spectrum of the 1-derived thiol. The most important bands are labeled with numbers. For the experimental spectra, absorbance scale bars are given. See text for discussion.

The wavenumber positions and assignments of the most prominent vibrational bands in the spectra are listed in Table 2. Generally, the IRRA spectra feature the same bands as the spectra of the neat substances. These bands include mode 1, typical of aromatic secondary amines, as well as several C−F stretching and bending modes. Note that the wavenumber position of band 1 points to possible formation of hydrogen bonds, which is in line with the crystal structure of 1 that suggests the formation of a hydrogen bond between the amine proton and an F atom of the anthracene unit (see section 3.1). Some of the bands (e.g., 2, 7, 8, and 12) are almost or completely extinguished in the SAM spectra. The TDMs of these modes are perpendicular to the main molecular axis as defined in Figure 1. According to the surface selection rules for metal substrates, the attenuation of these bands points to alignment of the main molecular axis in the monolayers with the surface normal, that is, to the PFA units standing nearly 7312


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Figure 5. IR data for PFA-NH-C3. Top curve: IRRA spectrum of the PFA-NH-C3 SAM. Middle curve: spectrum of the neat substance 2 (disulfide). Bottom curve: calculated spectrum of the 2-derived thiol. The most important bands are labeled with numbers. For the experimental spectra, absorbance scale bars are given. See text for discussion.

Figure 6. Representative C K-edge NEXAFS data for the PFA-NH-C2 SAM (two top curves) and PFA-NH-C3 SAM (two bottom curves). The data for both SAMs include the spectra acquired at an X-ray incidence angle of 55° (black curves) and the difference between the spectra acquired at X-ray incident angles of 90° and 20° (gray curves). The horizontal dashed lines correspond to zero. Individual resonances are marked by numbers.

upright on the gold surface. Note that because of the lack of intense out-of-plane bands (i.e., a third independent direction of the TDMs, additional to ∥ and ⊥, see Table 2), a quantitative evaluation of the IRRA spectra to yield the orientation of the PFA-NH-Cn moieties in the respective SAMs is impossible. 3.5. NEXAFS Spectroscopy. Representative C K-edge NEXAFS data for the PFA-NH-Cn SAMs are compiled in Figure 6. The data for these SAMs include the spectra acquired at the so-called “magic angle” of X-ray incidence (55°) as well as the difference between the spectra collected at X-ray incident angles of 90° and 20°. The magic angle spectra are exclusively representative of the electronic structure of the SAMs studied (unoccupied molecular orbitals), whereas the difference spectra are a convenient fingerprint for the linear dichroism effects in X-ray absorption (see Experimental Section).48

The magic angle spectra of the PFA-NH-C2 and PFA-NHC3 SAMs are very similar, which suggests, as expected, that they are dominated by the absorption resonances associated with the joint structural unit, namely PFA. This conclusion is also supported by the similarity of these spectra to the analogous spectrum of perfluorinated naphthalene (PFN).58 The spectra exhibit a variety of sharp resonances (1−7 in Figure 6) in the vicinity of the absorption edges as well as several broader features at higher PEs. The positions of the sharp resonances are listed in Table 3. The pattern of these

Table 2. Positions (Given in cm−1) and Assignments of Selected Vibrational Modes in the IR Spectra of the 1- and 2-Derived Thiols (PFA-NH-C2 and PFA-NH-C3, Respectively) along with the Orientation of Their TDMs and Calculated (DFT) Positions PFA-NH-C2 vibrational modea 1 2 3 4 5 6 7 8 9 10 11 12 13 14

ν ν ν δ ν ν δ δ δ δ δ ν δ δ

NH CC δ NH CC δ NH NH ν CC C1C2 ν C2N C6C7 δ NH NH ν CC NH ν CC NH δ CCC ν CF CCC ν CF CCC ν CF CF CF δ CCC CCC

PFA-NH-C3

TDMb

DFT

neatc

SAMc

DFT

neatc

SAMc

⊥ ∥ ∥ ∥ ∥ ⊥ ⊥ / ∥ / ⊥ ∥ ∥

3522 1655 1636 1557 1521 1501 1433 1428 1147 1113 1076 929 665 643

3398 w 1679 m 1662 w 1579 w 1526 m 1497 m 1423 m 1406 vs 1141 m 1102 m 1076 m 920 m 680 w 657 m

3364 w 1679 w 1664 w 1578 m 1528 m 1508 vs

3492 1655 1636 1558 1517 1499 1433 1424 1138 1112 1086 931 663 669

3386 w 1680 m 1663 m 1577 m 1526 m 1496 s 1424 s 1407 vs 1140 m 1100 m 1078 m 923 s 678 m 659 m

3361 w 1678 m 1659 m 1575 m 1530 s 1505 vs 1443 m 1419 m 1141 w 1109 m 1082 m 935 w 676 w 659 w

1145 w 1110 m 1078 w 923 w 679 vw 661 w

ν, stretching mode; δ, in-plane bending mode. b∥, parallel to main molecular axis; ⊥, perpendicular to main molecular axis and in aromatic plane; /, in aromatic plane yet neither ∥ nor ⊥. cvs, very strong; s, strong; m, medium; w, weak; vw, very weak. a

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Table 3. PE Positions (eV) of the Most Prominent Pre- and Near-Edge Absorption Resonances in the C K-Edge NEXAFS Spectra of the PFA-NH-Cn SAMs resonance

1

2

3

4

5

6

7

PE

284.95

285.7

286.3

287.05

288.2

289.15

290.6

resonances, which mostly have π* and σ*(C−F) character,58 is quite complex, which is typical of polyacenes as compared to poly-p-phenylenes.3,33,38,59−62 This is mostly related to nonequivalency of different carbon atoms in polyacene backbones, with three nonequivalent positions for naphthalene and four in the case of anthracene.60,61,63 A further important aspect is the fluorination of anthracene, resulting, for example, in the case of benzene,64,65 naphthalene,58 and terphenyl24 in significant modification of the resonance pattern as compared to the respective parent compounds. Since, to the best of our knowledge, there are neither experimental nor theoretical data for NEXAFS spectra of PFA, we performed the respective calculations, as described in the Experimental Section and the Supporting Information. The results of these calculations are presented in Figure 7. In Figure 7, panel a, the joint theoretical spectrum is compared to the experimental spectrum of PFA-NH-C2/Au,

while in Figure 7, panel b, the spectral contributions of the symmetry inequivalent atoms of PFA are shown, along with the theoretical positions of the absorption edges, which, as can be expected,58 are distinctly different for the carbon atoms bound (C1, C2, and C4) and unbound (C3) to fluorine. The experimental spectrum is satisfactorily reproduced by the theoretical one even though there are certain differences both in the positions and intensities of individual features, especially in the pre-edge range where the oscillator strengths of the π*like resonances are overestimated in the theoretical spectrum. These differences are presumably related to the limitations of the used program package as well as to the neglect of the contribution of the -NH-Cn linker and effect of the intermolecular interaction in the densely packed PFA-NH-Cn monolayers. Tentative assignments of the individual resonances (Table 3), on the basis of the theoretical calculations, are compiled in Table S1 in the Supporting Information. As could be expected (see above), most of these resonances have π* or mixed π*/σ*(C−F) character, including the most prominent resonance 4 at 287.05 eV. Note that the analogous resonance in the spectrum of PFN has been mostly ascribed to the b2gπ* orbital58 so that significant π*-like contributions can be assumed in our case as well. The contributions of the alkyl linkers, which should be located above the R* resonance at ∼287.7 eV,55 are expected to be comparably weak, both because of the small size of these moieties and attenuation of the PEY signal by the PFA overlayer. Otherwise, the linker represents a substitution for the PFA moiety in the PFA-NH-Cn backbone so that certain contributions and deviations from the only-PFA resonance pattern are possible, as indeed observed in Figure 7, panel a. Significantly, the TDMs of the π* resonances associated with the PFA moiety are directed perpendicular to its main axis (see Figure 1) and to the molecular plane. The TDMs of the σ*(C− F) resonances are oriented perpendicular to this axis as well, but within the molecular plane and with some minor contributions along the axis, associated with the directions of the C−F bonds at the 3-, 6-, and 7-positions of the PFA moiety.24,38,64 This allows determining the molecular orientation of the PFA units in the framework of the linear dichroism in X-ray absorption (see Experimental Section). Indeed, at normal incidence, the intensities of the π*/σ*(C−F) resonances are markedly higher than those at grazing incidence, which, in agreement with the HRXPS and IR data, suggests an upright molecular orientation in the PFA-NH-Cn monolayers. By using the entire sets of the C K-edge NEXAFS spectra, average tilt angles of these molecular orbitals, α, could be calculated within the standard theoretical framework for vectorlike orbitals.24,48 According to this evaluation, performed for the most prominent resonance 4, which presumably has predominant π*-like character (see above), these angles were estimated at 72.5° and 75° for PFA-NH-C2/Au and PFA-NH-C3/Au, respectively (with an accuracy of ± 3°). On the basis of the above values, the average tilt angles β of the molecular axes of the PFA-NH-Cn moieties in the respective SAMs were calculated using the formula cos α = sin β × cos γ.66 The twist angle γ of the PFA units in the monolayer was assumed to

Figure 7. (a) Calculated C K-edge NEXAFS spectrum of the PFA moiety as well as the experimental, magic angle spectrum of the PFANH-C2 SAM given for comparison. The theoretical spectrum was shifted to some extent (see the Supporting Information) to align the most intense resonances with those in the experimental spectrum. (b) Calculated C K-edge NEXAFS spectra for the symmetry inequivalent carbon atoms of the PFA moiety. The color-coded vertical bars show the ionization potential for the respective carbon atoms. The corresponding atoms are marked by numbers in the schematic cartoon (inset). Note that this notation only reflects the symmetry nonequivalence and is different from the standard notation of carbon atoms positions in anthracene. 7314


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Table 4. Work Function Shifts, Δϕ, with Respect to HDT/ Au (Reference) and Clean Golda (in Parentheses). Calculated Dipole Moments μ of the 1- and 2-Derived Thiols Are Also Listed, Together with the Dipole Moments μ⊥ along the Surface Normal (Calculated Using the Obtained Tilt Angles of the SAMs)

be identical with that in the bulk packing of perfluoroanthracene, which amounts to 25°.29 Consequently, the average molecular tilt angles in PFA-NH-C2/Au and PFA-NH-C3/Au were estimated at 19.5° and 16.5°, respectively. Note, however, that these angles are presumably even smaller in reality due to the admixture of the σ*(C−F) states, with the TDMs partly directed along the PFA axis, into the joint resonance 4. Note also that the above formula is only strictly valid at the pure π*like character of the molecular orbitals involved. The corrections due to the admixture of the σ*(C−F) states cannot however be large at the given, almost vertical molecular orientation. In addition to the C K-edge data, F K-edge spectra of the PFA-NH-Cn monolayers were measured. The respective data are presented in Figure 8, in the same form as the C K-edge

SAM

Δϕ (meV)

μ (D)

μ⊥ (D)

Ant PFA-NH-C2 PFA-NH-C3

15 ± 5 (−785) 1390 ± 5 (590) 1440 ± 5 (640)

5.0 5.0

4.7 4.8

a

The work function shift of HDT/Au necessary to calculate these quantities was assumed to be − 0.8 eV.19 This value has been reproduced by our own experiments as well.

anthracene-based monolayer (Ant/Au; reference sample), the calculated dipole moments of the thiols corresponding to 1 and 2, and the parts of the dipole moments, which become effective in the monolayers (projection to the surface normal at the given molecular inclinations). The Φ values are given with respect to two systems, HDT/Au and clean, freshly sputtered gold.

4. DISCUSSION The synthesis of compound 1 is the first example of a substitution product of PFA of which a crystal structure could be obtained. This structure clearly shows the substitution in position 2, which is the desired one for molecules capable of forming dense monolayers. The structure additionally shows the presence of intramolecular hydrogen-bonds of which the ones toward the fluorine atoms can be expected to be maintained within the monomolecular films on Au(111) substrates. These films were characterized by a variety of complementary spectroscopic techniques, which provided unequivocal evidence for the formation of well-defined SAMs. First, the S 2p HRXP spectra exhibited only a sole S 2p3/2,1/2 doublet, characteristic of the thiolate-gold bond, suggesting that all PFA-NH-Cn molecules were attached to the substrate via this anchor group, generated by cleavage of the disulfide bond. Second, HRXPS, IRRAS, and NEXAFS spectroscopy showed further specific spectroscopic features of the PFA-NH-Cn moieties, namely the PFA unit, the NH group, and the alkyl linker. According to the HRXPS and ellipsometry data, the PFANH-Cn monolayers are densely packed. The grafting densities in the PFA-NH-C2 and PFA-NH-C3 SAMs on Au(111) were estimated at 3.55 × 1014 and 3.70 × 1014 molecules/cm2, respectively, corresponding to molecular footprints of ∼0.28 and ∼0.27 nm2/molecule. These densities are markedly smaller than the analogous value for n-alkanethiolate SAMs on gold, namely 4.63 × 1014 molecules/cm2,46 and even smaller than the density found for nonfluorinated anthracene-substituted alkanethiolate (Ant-Cn) monolayers on the same substrate, for example, ∼4.2 × 1014 molecules/cm2 for n = 3.33 This difference reflects the increased space requirement for the perfluorinated anthracene moieties in comparison to the nonperfluorinated ones. Interestingly, the ratio of the Ant-Cn SAM packing density to the average of that for the PFA-NHCn films amounts to 1.17, which matches exactly the analogous ratio for bulk anthracene68 and PFA,29 indicating that the packing in the PFA-NH-Cn monolayers is mostly dictated by the interaction between the PFA moieties. Note that molecular footprints of ∼0.28 nm2/molecule are typically found in SAMs

Figure 8. Representative F K-edge NEXAFS data for the PFA-NH-C2 SAM (two top curves) and PFA-NH-C3 SAM (two bottom curves). The data for both SAMs include the spectra acquired at an X-ray incidence angle of 55° (black curves) and the difference between the spectra acquired at X-ray incident angles of 90° and 20° (gray curves). The horizontal dashed lines correspond to zero. Individual resonances are marked by numbers.

spectra in Figure 6. The 55° spectra exhibit several sharp resonances close to the absorption edge, namely at 689.0 eV (1), 671.25 eV (2), and 696.6 eV (3), as well as a variety of broader features at higher PEs. On the basis of the literature data for PFN that exhibits a similar spectrum,58 resonances 1, 2, and 3 can be tentatively assigned to the b2gπ*/σ*(C−F), auπ*/ σ*(C−F), and σ*(C−C) orbitals, respectively. These assignments are supported by the difference spectra in Figure 8, where the resonances with the proposed π* and σ*(C−C) character exhibit positive and negative difference peaks, respectively, corresponding to the upright molecular orientation in the SAMs, in agreement with all other experimental data. We refrained, however, from quantitative evaluation of the F K-edge spectra in view of the principal problems of this procedure for fluorinated planar aromatic molecules.67 3.6. Work Function. Values of the work function Φ for PFA-NH-C2/Au and PFA-NH-C3/Au, measured with a Kelvin probe (see Experimental Section), are compiled in Table 4, together with the values for a similar, nonfluorinated 7315


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ACS Applied Materials & Interfaces of perfluorinated alkanethiolates, in which the fluorine to carbon ratio is even higher than that in the aromatic systems.24,56,57 Within the aromatic series, it is of interest to compare the footprints of the PFA-NH-Cn molecules with those found for perfluorinated terphenyl substituted ethaneand propanethiolate SAMs, which amount to 0.29 nm2/ molecule and 0.26 nm2/molecule, respectively.24 Such differences in molecular packing (as well as in the inclination, see below) are usually a result of odd−even effects related to the presence of the so-called bending potential, which defines the orientation of the substrate−headgroup−C joint and, subsequently, the orientation of the alkyl linker, for which according to IR studies an all-trans conformation can be assumed.24,26,27 Consequently, depending on the parity of the number of CH2 groups in this linker, the functional moiety attached to it is either slightly or strongly inclined, accompanied by the respective changes in the packing density.24,26,27 Yet, for systems with a nonsymmetric attachment of the aromatic moiety to the alkyl linker, the odd−even effect can be widely compensated by rotation of this moiety, permitting the adoption of an energetically favorable, almost upright orientation of the aromatic part regardless of the number of the methylene groups in the linker, as has been found for alkanethiolate SAMs with a (nonfluorinated) anthracene termination.33 For these monolayers, very small differences in the packing densities with odd- and even-numbered spacers were reported (∼2% throughout 2−6 spacer units), which are well within the limits of the typical range of error in packingdensity determination by XPS (5−10%). The slightly greater difference obtained in the given case, for the PFA-terminated molecules (∼4%) might be related to the effect of hydrogen bonding between a fluorine atom in the “bottom” ring of the PFA unit and the amine hydrogen. This bond promotes a certain “azimuthal” orientation of the PFA moiety with respect to the linker, hindering, to some extent, its optimal adaptation to the most densely molecular packing. The existence of such hydrogen bonds in the monolayers follows from the crystallographic data for the disulfide precursors and is corroborated by the wavenumber positions of the N−H stretch bands in the IR spectra of the SAMs, which are red-shifted relative to the respective band in the neat molecule spectra, pointing to an even stronger hydrogen-bond interaction in the SAMs as compared to the neat substances. The dense molecular packing is only possible at an upright molecular orientation and high orientational order in the monolayers. This is indeed supported by the HRXPS, ellipsometry, IRRAS, and NEXAFS data. In particular, HRXPS and ellipsometry derived effective thicknesses of both PFA-NH-C2 and PFA-NH-C3 SAMs are very close to the sum of molecular length and the length of the S−Au bond, which points, even though indirectly, to an upright molecular orientation. Further evidence is provided by IRRAS: the attenuation of particular bands in the IRRA spectra (see section 2.6) implies that the main molecular axes (as defined in Figure 1) are only slightly tilted against the surface normal. Finally, on the basis of the NEXAFS spectroscopy data (section 2.7), average tilt angles of 19.5° and 16.5° were obtained for the PFA-NH-C2 and the PFA-NH-C3 SAMs, respectively, within a reasonable assumption about the molecular twist. The relative similarity of these values for both monolayers implies that altering the number of methylene units has only a small impact on the orientation of the PFA moieties in the films, suggesting, in accordance with the HRXPS data, that the packing and

alignment of the PFA-NH-Cn molecules on the gold substrate is mainly governed by the space requirement of the PFA moieties. Such a situation is typical for partially fluorinated SAMs as well as for other SAMs with bulky substitutions such as (nonfluorinated) anthracene-terminated alkanethiolates.33 The arrangement of the PFA-NH-Cn molecules in the respective SAMs follows presumably a herringbone motif, as could be found for unsubstituted PFA29 and perfluoropentacene.5 The work function change (Δϕ) of the Au(111) substrate upon the formation of the PFA-NH-C2 and PFA-NH-C3 SAMs was found to be +0.59 eV and +0.64 eV, respectively. These Δϕ values are positive and in the same range as the analogous values for other SAMs with perfluorinated aromatic terminations, namely C6F5-S/Au (0.79 eV),21 C6F5-C6H4-S/Au (0.66 eV),21 C6F5-C6H4-NN-C6H4-S/Au (0.29−0.38 eV),20 and C6F5-C6F4-C6F4-(CH2)3-S/Au (0.6 eV).22 Note that a positive Δϕ observed for all above films was expected because of a considerable negative dipole moment of the SAM constituents, associated with the high electronegativity of the fluorine atoms. As to the slightly higher Δϕ value for PFA-NHC3/Au as compared to PFA-NH-C2/Au, it is presumably related to the somewhat denser molecular packing and smaller molecular inclination in the former SAM, resulting in a stronger contribution of the molecular dipole to the entire work function. In this context, it is worth discussing a correlation of Δϕ with the dipole moment of the SAM constituents. Generally, a negative dipole moment is expected to cause an increase of the work function.69 Moreover, it has to be kept in mind that not the dipole moment of the individual molecules but the dipole moment of the molecular layer is crucial for the alteration of the work function. Because of depolarization effects,70 the layer dipole moments can be distinctly lower than the ones of the isolated molecules. Interestingly, experimentally obtained work function changes induced by partially fluorinated alkanethiolate SAMs17−19 are similarly strong as or even stronger than the ones of the PFA-NH-Cn SAMs. At the same time, the calculated dipole moments of these partially fluorinated alkanethiol molecules are markedly smaller than the calculated dipole moments of the PFA-NH-Cn thiols along the surface normal. While 4.7 and 4.8 Debye induce Δϕ values of about 0.59 and 0.64 eV, respectively (see Table 4), a work function change of a similar magnitude was achieved with, for example, CF3(CF2)8(CH2)2SH, the calculated dipole moment parallel to the surface normal of which is reported to be ∼2.2 Debye only.19 Although a direct comparison of the calculated dipole moments is hampered by differences in the calculation methods, it seems that fluorinated aromatic SAMs generally induce smaller work function changes than fluorinated aliphatic SAMs when both aromatic and aliphatic molecules have identical dipole moments. This might be explained by a stronger depolarization effect in aromatic SAMs as compared to aliphatic ones, due to a significantly higher polarizability of aromatic in comparison to aliphatic molecules. Thus, while fluorinated aromatic SAMs carry advantages of smaller band gap (a higher conductivity) and a better structural match to typical organic semiconductors as compared to partly fluorinated aliphatic monolayers, they bear a disadvantage that high Δϕ values are more difficult to achieve. 7316



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5. CONCLUSIONS We could show that PFA, a perfluorinated acene with a relatively small band gap, can be suitably functionalized with bis(ω-aminoalkyl)disulfides to form SAMs on Au(111). The utilization of two and three CH2 groups in the alkyl chain permitted a comparison of the film-forming properties of these two molecules. As could be demonstrated by a suitable combination of several analytical techniques, both precursors form well-defined and densely packed monomolecular films, which were anchored to the substrate via thiolate-Au bonds. The molecular packing in these films was mostly governed by the space requirement of the bulky PFA moieties, resulting, along with the nonsymmetrical attachment of the PFA moiety to the NH-alkyl linker, in almost complete compensation of the odd−even effects which are typically found for oligophenylsubstituted alkanethiolate SAMs. Presumably, the presence of the −NH− group in the linker resulted in additional stabilization of the film structure, mediated by the hydrogen bond to the terminal fluorine atom of the PFA moiety. The films with a longer linker exhibited, however, slightly higher packing density and smaller molecular inclination than the monolayers with the shorter one. Functionalization of the gold substrate with the PFAterminated monomolecular films resulted in an increase of their work function by 0.59−0.64 eV. This value is comparable with the analogous values for other SAMs with perfluorinated aromatic terminations but is somewhat low in view of the considerable dipole moment of the PFA-terminated thiols, suggesting strong depolarization effects in the respective monomolecular films. In addition to the work function modification, PFAterminated monomolecular films are presumably useful as templates for structurally similar n-type organic semiconductors such as perfluorinated annelated arenes (perfluorotetracene, perfluoropentacene, etc.). This can open new opportunities to tune the electronic characteristics of organic electronic devices utilizing n-type organic semiconductors. In this context, the smaller band gap of the PFA moieties compared to molecules with perfluorinated phenyl units will be of a clear advantage. Further studies of the electronic properties of PFAterminated SAMs, including their electronic structure and conductive properties, are under way. We also play with molecular structure, looking for alternative ways to link the sulfur head groups to PFA, for example, without an intermediate −NH− group or with shorter linkers. The latter can be of particular advantage to improve the electric conductivity, important in context of applications. A very promising strategy to obtain high structural quality as well as strong electronic coupling would be to bind the PFA units directly via a selenium atom,62 an approach for which the chemistry still has to be developed.

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ¶

These authors contributed equally to the given work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Max-IV staff, and A. Preobrajenski in particular, for the technical support during the synchrotronbased experiments. This work has been supported financially by the Deutsche Forschungsgemeinschaft (German Research Foundation, DFG), within the Grant Nos. TE 247/12-1 and ZH 63/14-2. The generous donations of anhydrous CsF by Rockwood Lithium and anhydrous HF by Solvay Fluor GmbH are very gratefully acknowledged.

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REFERENCES

(1) Bredas, J. L.; Heeger, A. J. Influence of Donor and Acceptor Substituents on the Electronic Characteristics of Poly(paraphenylene vinylene) and Poly(paraphenylene). Chem. Phys. Lett. 1994, 217, 507− 512. (2) Lei, T.; Dou, J.-H.; Ma, Z.-J.; Yao, C.-H.; Liu, C.-J.; Wang, J.-Y.; Pei, J. Ambipolar Polymer Field-Effect Transistors Based on Fluorinated Isoindigo: High Performance and Improved Ambient Stability. J. Am. Chem. Soc. 2012, 134, 20025−20028. (3) Reichenbächer, K.; Süss, H. I.; Hulliger, J. Fluorine in Crystal Engineering-″The Little Atom that Could″. Chem. Soc. Rev. 2005, 34, 22−30. (4) Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Inoue, Y.; Tokito, S. Perfluoropentacene and Perfluorotetracene: Syntheses, Crystal Structures, and FET Characteristics. Mol. Cryst. Liq. Cryst. 2006, 444, 225−232. (5) Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Fukai, Y.; Inoue, Y.; Sato, F.; Tokito, S. Perfluoropentacene: High-Performance p-n Junctions and Complementary Circuits with Pentacene. J. Am. Chem. Soc. 2004, 126, 8138−8140. (6) Martínez Hardigree, J. F.; Katz, H. E. Through Thick and Thin: Tuning the Threshold Voltage in Organic Field-Effect Transistors. Acc. Chem. Res. 2014, 47, 1369−1377. (7) Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willett, R. L.; Someya, T.; Gershenson, M. E.; Rogers, J. A. Elastomeric Transistor Stamps: Reversible Probing of Charge Transport in Organic Crystals. Science 2004, 303, 1644−1646. (8) Heimel, G.; Romaner, L.; Zojer, E.; Bredas, J.-L. The Interface Energetics of Self-Assembled Monolayers on Metals. Acc. Chem. Res. 2008, 41, 721−729. (9) Yu, S.-Y.; Huang, D.-C.; Chen, Y.-L.; Wu, K.-Y.; Tao, Y.-T. Approaching Charge Balance in Organic Light-Emitting Diodes by Tuning Charge Injection Barriers with Mixed Monolayers. Langmuir 2012, 28, 424−430. (10) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, 1103−1170. (11) Kind, M.; Wöll, C. Organic Surfaces Exposed by Self-Assembled Organothiol Monolayers: Preparation, Characterization, and Application. Prog. Surf. Sci. 2009, 84, 230−278. (12) Campbell, I. H.; Rubin, S.; Zawodzinski, T. A.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. Controlling Schottky Energy Barriers in Organic Electronic Devices using SelfAssembled Monolayers. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, R14321−14324. (13) Hamadani, B. H.; Corley, D. A.; Ciszek, J. W.; Tour, J. M.; Natelson, D. Controlling Charge Injection in Organic Field-Effect

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00532. 1 H, 19F, 13C, and 2D F-F COSY NMR spectra; X-ray crystallographic data of disulfide 1 in CIF format (CCDC 1443111); detailed information on modeling of the C Kedge NEXAFS spectra of PFA; information on the DFT calculation of single molecule IR spectra (PDF) 7317


Research Article

ACS Applied Materials & Interfaces Transistors Using Self-Assembled Monolayers. Nano Lett. 2006, 6, 1303−1306. (14) Fracasso, D.; Muglali, M. I.; Rohwerder, M.; Terfort, A.; Chiechi, R. C. Influence of an Atom in EGaIn/Ga2O3 Tunneling Junctions Comprising Self-Assembled Monolayers. J. Phys. Chem. C 2013, 117, 11367−11376. (15) Jiang, L.; Yuan, L.; Cao, L.; Nijhuis, C. A. Controlling Leakage Currents: The Role of the Binding Group and Purity of the Precursors for Self-Assembled Monolayers in the Performance of Molecular Diodes. J. Am. Chem. Soc. 2014, 136, 1982−1991. (16) Liao, K.-C.; Bowers, C. M.; Yoon, H. J.; Whitesides, G. M. Fluorination, and Tunneling across Molecular Junctions. J. Am. Chem. Soc. 2015, 137, 3852−3858. (17) Evans, S. D.; Urankar, E.; Ulman, A.; Ferris, N. Self-Assembled Monolayers of Alkanethiols Containing a Polar Aromatic Group: Effects of the Dipole Position on Molecular Packing, Orientation, and Surface Wetting Properties. J. Am. Chem. Soc. 1991, 113, 4121−4131. (18) Alloway, D. M.; Hofmann, M.; Smith, D. L.; Gruhn, N. E.; Graham, A. L.; Colorado, R., Jr.; Wysocki, V. H.; Lee, T. R.; Lee, P. A.; Armstrong, N. R. Interface Dipoles Arising from Self-Assembled Monolayers on Gold: UV - Photoemission Studies of Alkanethiols and Partially Fluorinated Alkanethiols. J. Phys. Chem. B 2003, 107, 11690− 11699. (19) de Boer, B.; Hadipour, A.; Mandoc, M. M.; van Woudenbergh, T.; Blom, P. W. M. Tuning of Metal Work Functions with SelfAssembled Monolayers. Adv. Mater. 2005, 17, 621−625. (20) Crivillers, N.; Osella, S.; Van Dyck, C.; Lazzerini, G. M.; Cornil, D.; Liscio, A.; Di Stasio, F.; Mian, S.; Fenwick, O.; Reinders, F.; Neuburger, M.; Treossi, E.; Mayor, M.; Palermo, V.; Cacialli, F.; Cornil, J.; Samorì, P. Large Work Function Shift of Gold Induced by a Novel Perfluorinated Azobenzene-Based Self-Assembled Monolayer. Adv. Mater. 2013, 25, 432−436. (21) Fenwick, F.; Van Dyck, C.; Murugavel, K.; Cornil, D.; Reinders, F.; Haar, S.; Mayor, M.; Cornil, J.; Samori, P. Modulating the Charge Injection in Organic Field-Effect Transistors: Fluorinated Oligophenyl Self-Assembled Monolayers for High Work Function Electrodes. J. Mater. Chem. C 2015, 3, 3007−3015. (22) Kong, L.; Chesneau, F.; Zhang, Z.; Staier, F.; Terfort, A.; Dowben, D. A.; Zharnikov, M. Electronic Structure of Aromatic Monomolecular Films: The Effect of Molecular Spacers and Interfacial Dipoles. J. Phys. Chem. C 2011, 115, 22422−22428. (23) Schüpbach, B.; Bolte, M.; Zharnikov, M.; Terfort, A. Grafting Organic n-Semiconductors to Surfaces: (Perfluoro-p-terphenyl-4yl)alkanethiols. Eur. J. Org. Chem. 2010, 2010, 3041−3048. (24) Chesneau, F.; Schüpbach, B.; Szelągowska-Kunstman, K.; Ballav, N.; Cyganik, P.; Terfort, A.; Zharnikov, M. Self-Assembled Monolayers of Perfluoroterphenyl-Substituted Alkanethiols: Specific Characteristics and Odd-Even Effects. Phys. Chem. Chem. Phys. 2010, 12, 12123−12127. (25) Ishida, T.; Mizutani, W.; Choi, N.; Akiba, U.; Fujihira, M.; Tokumoto, H. Structural Effects on Electrical Conduction of Conjugated Molecules Studied by Scanning Tunneling Microscopy. J. Phys. Chem. B 2000, 104, 11680−11688. (26) Shaporenko, A.; Brunnbauer, M.; Terfort, A.; Grunze, M.; Zharnikov, M. Structural Forces in Self-Assembled Monolayers: Terphenyl-Substituted Alkanethiols on Noble Metal Substrates. J. Phys. Chem. B 2004, 108, 14462−14469. (27) Azzam, W.; Bashir, A.; Terfort, A.; Strunskus, T.; Wöll, C. Combined STM and FTIR Characterization of Terphenylalkanethiol Monolayers on Au(111): Effect of Alkyl Chain Length and Deposition Temperature. Langmuir 2006, 22, 3647−3655. (28) Milián Medina, B.; Beljonne, D.; Egelhaaf, H.-J.; Gierschner, J. Effect of Fluorination on the Electronic Structure and Optical Excitations of pi-Conjugated Molecules. J. Chem. Phys. 2007, 126, 111101. (29) The perfluoroanthracene was obtained via a newly established synthesis route, which will be described elsewhere. (30) Käfer, D.; Witte, G.; Cyganik, P.; Terfort, A.; Wöll, Ch. A Comprehensive Study of Self-Assembled Monolayers of Anthracene-

thiol on Gold: Solvent Effects, Structure, and Stability. J. Am. Chem. Soc. 2006, 128, 1723−1732. (31) Karl, N.; Ziegler, J. Generation and Transport of Charge Carriers in the Charge-Transfer Complex Anthracene-PyromelliticDianhydride. Chem. Phys. Lett. 1975, 32, 438−442. (32) Shaporenko, A.; Brunnbauer, M.; Terfort, A.; Johansson, L. S. O.; Grunze, M.; Zharnikov, M. Odd-Even Effects in Photoemission from Terphenyl-Substituted Alkanethiolate Self-Assembled Monolayers. Langmuir 2005, 21, 4370−4375. (33) Dauselt, J.; Zhao, J.; Kind, M.; Binder, R.; Bashir, A.; Terfort, A.; Zharnikov, M. Compensation of the Odd-Even Effects in Araliphatic Self-Assembled Monolayers by Nonsymmetric Attachment of the Aromatic Part. J. Phys. Chem. C 2011, 115, 2841−2854. (34) Mirjani, F.; Thijssen, J. M.; Whitesides, G. M.; Ratner, M. A. Charge Transport across Insulating Self-Assembled Monolayers: NonEquilibrium Approaches and Modeling to Relate Current and Molecular Structure. ACS Nano 2014, 8, 12428−12436. (35) Jiang, X.; Liu, J.; Xu, L.; Zhuo, R. Disulfide-Containing Hyperbranched Polyethyleneimine Derivatives via Click Chemistry for Nonviral Gene Delivery. Macromol. Chem. Phys. 2011, 212, 64−71. (36) Hamacher, K.; Hanuš, J. Synthesis of 1-[11C]-D,L-Homocysteine Thiolactone: A Potential Tracer for Myocardial Ischemia Using PET. J. Labelled Compd. Radiopharm. 1989, 27, 1275−1283. (37) Barbe, K.; Kind, M.; Pfeiffer, C.; Terfort, A. Purification of Ethanol for Highly Sensitive Self-Assembly Experiments. Beilstein J. Nanotechnol. 2014, 5, 1245−1260. (38) Frey, S.; Stadler, V.; Heister, K.; Eck, W.; Zharnikov, M.; Grunze, M.; Zeysing, B.; Terfort, A. Structure of Thioaromatic SelfAssembled Monolayers on Gold and Silver. Langmuir 2001, 17, 2408− 2415. (39) Zharnikov, M.; Grunze, M. Modification of Thiol-Derived SelfAssembling Monolayers by Electron and X-ray Irradiation: Scientific and Lithographic Aspects. J. Vac. Sci. Technol., B: Microelectron. Process. Phenom. 2002, 20, 1793−1807. (40) Zharnikov, M. High-Resolution X-ray Photoelectron Spectroscopy in Studies of Self-Assembled Organic Monolayers. J. Electron Spectrosc. Relat. Phenom. 2010, 178−179, 380−393. (41) Moulder, J. F.; Stickle, W. E.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastian, J., Ed.; PerkinElmer Corp.: Eden Prairie, MN, 1992. (42) Ratner, B. D.; Castner, D. G. Electron Spectroscopy for Chemical Analysis. Surface AnalysisThe Principal Techniques; Vickerman, J. C., Ed.; Wiley & Sons: Chichester, 1997. (43) Lamont, C. L. A.; Wilkes, J. Attenuation Length of Electrons in Self-Assembled Monolayers of n-Alkanethiols on Gold. Langmuir 1999, 15, 2037−2042. (44) Colorado, R., Jr.; Lee, T. R. Attenuation Lengths of Photoelectrons in Fluorocarbon Films. J. Phys. Chem. B 2003, 107, 10216−10220. (45) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. Structure and Interfacial Properties of Spontaneously Adsorbed n-Alkanethiolate Monolayers on Evaporated Silver Surfaces. J. Am. Chem. Soc. 1991, 113, 2370−2378. (46) Schreiber, F. Structure and Growth of Self-Assembling Monolayers. Prog. Surf. Sci. 2000, 65, 151−256. (47) Kondoh, H.; Iwasaki, M.; Shimada, T.; Amemiya, K.; Yokoyama, T.; Ohta, T.; Shimomura, M.; Kono, K. Adsorption of Thiolates to Singly Coordinated Sites on Au(111) Evidenced by Photoelectron Diffraction. Phys. Rev. Lett. 2003, 90, 066102. (48) Stöhr, J. NEXAFS Spectroscopy; Springer-Verlag: Berlin, 1992. (49) Batson, P. E. Carbon-1s Near-Edge-Absorption Fine-Structure in Graphite. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 48, 2608−2610. (50) Satoh, N.; Yamamoto, K. Self-Assembled Monolayers of MetalAssembling Dendron Thiolate Formed from Dendrimers with a Disulfide Core. Org. Lett. 2009, 11, 1729−1732. (51) Burdon, J.; Childs, A. C.; Parsons, I. W.; Tatlow, J. C. Nucleophilic Replacement in Decafluoroanthracene. J. Chem. Soc., Chem. Commun. 1982, 10, 534−535. 7318


Research Article

ACS Applied Materials & Interfaces (52) Muir, M.; Baker, J. A. Simple Calculational Model for Predicting the Site for Nucleophilic Substitution in Aromatic Perfluorocarbons. J. Fluorine Chem. 2005, 126, 727−738. (53) Dunitz, J. D.; Taylor, R. Organic fluorine hardly ever accepts hydrogen bonds. Chem. - Eur. J. 1997, 3, 89−98. (54) Lautié, A.; Novak, A. N-H···S Hydrogen Bonds. Chem. Phys. Lett. 1980, 71, 290−293. (55) Zharnikov, M.; Grunze, M. Spectroscopic Characterization of Thiol-Derived Self-Assembled Monolayers. J. Phys.: Condens. Matter 2001, 13, 11333−11365. (56) Tamada, K.; Ishida, T.; Knoll, W.; Fukushima, H.; Colorado, R., Jr.; Graupe, M.; Shmakova, O. E.; Lee, T. R. Molecular Packing of Semifluorinated Alkanethiol Self-Assembled Monolayers on Gold: Influence of Alkyl Spacer Length. Langmuir 2001, 17, 1913−1921. (57) Lu, H.; Zeysing, D.; Kind, M.; Terfort, A.; Zharnikov, M. Structure of Self-Assembled Monolayers of Partially Fluorinated Alkanethiols with a Fluorocarbon Part of Variable Length on Gold Substrate. J. Phys. Chem. C 2013, 117, 18967−18979. (58) Robin, M. B.; Ishii, I.; McLaren, R.; Hitchcock, A. P. Fluorination Effects on the Inner-Shell Spectra of Unsaturated Molecules. J. Electron Spectrosc. Relat. Phenom. 1988, 47, 53−92. (59) Horsley, J.; Stöhr, J.; Hitchcock, A. P.; Newbury, D. C.; Johnson, A. L.; Sette, F. Resonances in the K-shell Excitation Spectra of Benzene and Pyridine: Gas Phase, Solid and Chemisorbed States. J. Chem. Phys. 1985, 83, 6099−6107. (60) Yokoyama, T.; Seki, K.; Morisada, I.; Edamatsu, K.; Ohta, T. XRay Absorption Spectra of Poly-p-Phenylenes and Polyacenes: Localization of π* Orbitals. Phys. Scr. 1990, 41, 189−192. (61) Ågren, H.; Vahtras, O.; Carravetta, V. Near-Edge Core Photoabsorption in Polyacenes: Model Molecules for Graphite. Chem. Phys. 1995, 196, 47−58. (62) Ossowski, J.; Wächter, T.; Silies, L.; Kind, M.; Noworolska, A.; Blobner, F.; Gnatek, D.; Rysz, J.; Bolte, M.; Feulner, P.; Terfort, A.; Cyganik, P.; Zharnikov, M. Thiolate versus Selenolate: Structure, Stability and Charge Transfer Properties. ACS Nano 2015, 9, 4508− 4526. (63) Klues, M.; Hermann, K.; Witte, G. Analysis of the Near-Edge Xray-Absorption Fine-Structure of Anthracene: A Combined Theoretical and Experimental Study. J. Chem. Phys. 2014, 140, 014302. (64) Breuer, T.; Klues, M.; Witte, G. Characterization of Orientational Order in π-Conjugated Molecular Thin Films by NEXAFS. J. Electron Spectrosc. Relat. Phenom. 2015, 204, 102−115. (65) Hitchcock, A. P.; Fischer, P.; Gedanken, A.; Robin, M. B. Antibonding π* Valence MOs in the Inner-Shell and Outer-Shell Spectra of the Fluorobenzenes. J. Phys. Chem. 1987, 91, 531−540. (66) Ballav, N.; Schüpbach, B.; Dethloff, O.; Feulner, P.; Terfort, A.; Zharnikov, M. Direct Probing Molecular Twist and Tilt in Aromatic Self-Assembled Monolayers. J. Am. Chem. Soc. 2007, 129, 15416− 15417. (67) de Oteyza, D. G.; Sakko, A.; El-Sayed, A.; Goiri, E.; Floreano, L.; Cossaro, A.; Garcia-Lastra, J. M.; Rubio, A.; Ortega, J. E. Inversed Linear Dichroism in F K-edge NEXAFS Spectra of Fluorinated Planar Aromatic Molecules. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 075469. (68) Brock, C. P.; Dunitz, J. D. Temperature Dependence of Thermal Motion in Crystalline Anthracene. Acta Crystallogr., Sect. B: Struct. Sci. 1990, 46, 795−806. (69) Abu-Husein, T.; Schuster, S.; Egger, D. A.; Kind, M.; Santowski, T.; Wiesner, A.; Chiechi, R.; Zojer, E.; Terfort, A.; Zharnikov, M. The Effects of Embedded Dipoles in Aromatic Self-Assembled Monolayers. Adv. Funct. Mater. 2015, 25, 3943−3957. (70) Cornil, D.; Olivier, Y.; Geskin, V.; Cornil, J. Depolarization Effects in Self-Assembled Monolayers: A Quantum-Chemical Insight. Adv. Funct. Mater. 2007, 17, 1143−1148.

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Self-Assembled Monolayers of Perfluoroanthracenylaminoalkane

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