J. Phys. Chem. B 2006, 110, 24621-24628
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Self-Assembled Monolayers of Ferrocene-Substituted Biphenyl Ethynyl Thiols on Gold Andrey Shaporenko,† Katrin Ro1 ssler,‡ Heinrich Lang,*,‡ and Michael Zharnikov*,† Angewandte Physikalische Chemie, UniVersita¨t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany, and Lehrstuhl fu¨r Anorganische Chemie, Institut fu¨r Chemie, Technische UniVersita¨t Chemnitz, Strasse der Nationen 62, 09111 Chemnitz, Germany ReceiVed: August 2, 2006; In Final Form: September 21, 2006
Homogeneous and mixed [with biphenylthiol (BPT)] self-assembled monolayers (SAMs) of ferrocenesubstituted biphenyl ethynyl thiols (Fc) were prepared on Au(111) substrates and characterized by several complementary spectroscopic techniques. The mixed films were fabricated either by subsequent immersion of the substrates into the BPT and Fc solutions or by immersion of the substrate into a mixed solution of BPT and Fc. The first procedure resulted in the preparation of high-quality mixed SAMs, in which the Fc molecules were stochastically distributed in the BPT matrix and well-separated from each other. The portion of these molecules in such films could be precisely varied from ca. 7 to 42% by selection of the immersion time in the BPT solution. The films prepared from the mixed solution exhibited a phase separation between the Fc and BPT constituents. These films contained mostly the Fc molecules (∼80-90%), showing, thus, a significant deviation from the relative content of the target molecules in the primary solution (a 1:1 ratio). This finding shows that the Fc molecules, when competing with BPT, preferably adsorb onto Au(111) substrate, suggesting a significant impact of the ferrocene groups onto the structure-building interactions responsible for molecular self-assembly.
1. Introduction Recently, the well-established top-down technology for the fabrication of microelectronic devices and lithographic patterns was complemented by the bottom-up approach, which became especially important in view of new developments made in the design of biological sensors and potential applications of organic materials in microelectronics (see, for example, refs 1-5). The bottom-up approach relies upon the ability of nanoscale structural elements to form a designed structure or pattern on their own, on the basis of mutual interactions, which are mostly governed by the exact composition, structure, and conformation of these elements. A best example of the respective systems are self-assembled monolayers (SAMs), which are 2D polycrystalline films of semirigid molecules, spontaneously assembling on a suitable substrate.5-8 A building block of SAMs consists generally of three essential parts: a headgroup that anchors the molecule to the substrate, a tail group that constitutes the outer surface of the film, and a spacer that separates the head and tail groups and, through the interaction with neighbor molecules, drives the molecular self-assembly. Sticking to this general architecture, one can combine different moieties and functional groups into a simple rodlike molecule, a complex assembly, or even a molecular device, which carries an active element connected to the substrate over the spacer and headgroup. The presence of a specific functional group or an active element, which can be bulky or strongly interacting, can, however, influence the balance of the structure-building forces in the SAM, resulting, under the circumstances, in its poor quality.9 Another problem can be an interaction (or interference) * Authors to whom correspondence should be addressed: E-mail:
[email protected] (H.L.) and Michael.Zharnikov@ urz.uni-heidelberg.de (M.Z.). † Universita ¨ t Heidelberg. ‡ Technische Universita ¨ t Chemnitz.
of the densely packed active elements with each other, which can disturb their performance. A suitable strategy to avoid these problems is to prepare a mixed SAM, in which the active elements are incorporated in a passive matrix, promoting a proper self-assembly and serving as a separator between the former species. The standard approach to fabricate the respective systems is the immersion of the substrate into a solution containing both active and matrix species. However, due to the competitive desorption of these components, the film composition can differ significantly from the relative contents of the primary molecules in the solution.10,11 In addition, an agglomeration of the active species can occur, which can lead to the above-mentioned interference and mutual disturbance problems. In this study, we are trying to avoid this problem by not using immersion into a mixed solution but subsequent immersion into the solutions of the matrix and active species, going through the preparation of a poor-quality matrix SAM at the first stage to the formation of a high-quality mixed SAM at the second stage. The latter process occurs via the incorporation of the active species into the poor-quality matrix film, which takes place preferably at the positions of defects, stochastically distributed over the primary film. A similar procedure was used before, e.g., to incorporate the phenylene-ethynylene oligomers, which are prototypes of the molecular switches, in an alkanethiolate (AT) matrix.12,13 The molecule of this study is a ferrocene-substituted biphenyl ethynyl thiol, ferrocene-C≡CC6H4C6H4SH (see Scheme 1),14 which we will denote as Fc below. Ferrocenes are very robust compounds and excellent organometallic one-electron reservoirs, which makes them quite valuable for electrochemical applications. A homogeneous or mixed SAM comprising of ferrocenesubstituted molecules can, thus, serve as an active electrochemical template, for which factors governing the rate of electron
10.1021/jp064943f CCC: $33.50 © 2006 American Chemical Society Published on Web 11/07/2006
24622 J. Phys. Chem. B, Vol. 110, No. 48, 2006 SCHEME 1: Molecules of This Studya
Shaporenko et al. represents a barrier for the electron transfer, with the rate of electron transfer being the same for covalently attached and solution dissolved redox moieties.46 In the following section we describe the experimental procedure and techniques. The results are presented and briefly discussed in section 3. An extended analysis of the data is given in section 4, followed by a summary in section 5. 2. Experimental Section
a
See text for the nomenclature and detailed information.
transfer across interfacial barriers can be precisely monitored and controlled. To promote an effective electron transfer between the ferrocene unit and the metal substrate, it should be connected to the headgroup by a molecular spacer with possibly high electric conductivity. In this regard, a oligophenyl or oligo(phenylene-ethynylene) chain are the best choice, considering that these moieties have much lower resistance as compared to the aliphatic chain frequently used in SAM design.15,16 This is the reason for the use of the biphenyl moiety as spacer in our study. At the same time, this spacer, together with the ethynyl unit, is long enough to guarantee that the ferrocene group and the metal substrate are decoupled. As the matrix molecules we used a nonsubstituted biphenyl thiol, C6H5C6H4SH (BPT, see Scheme 1), which, at a sufficiently long immersion time, forms good-quality SAMs on noble metal substrates.17-22 In addition, BPT suits exactly to the molecular foot of the Fc molecule, which should promote a proper selfassembly at a prolonged immersion of a poor-quality matrix BPT film into a Fc solution. Finally, the thiol headgroup of both Fc and BPT molecules has a high affinity for many metal and semiconductor surfaces and is characterized by a quite low contact resistance, which is of a special importance for the Fc molecules, since the contact resistance between a molecule and substrate depends strongly on its bonding.23,24 For this study, we have selected Au(111) as most reproducible and studied substrate for the SAM preparation.5-8,25 Note that ferrocene-substituted SAMs have already been extensively studied previously, above all, in view of their electrochemical properties and possible applications as components of memory devices and biological sensors.2,26-45 Also, results on mixed SAMs2,37,43-45 and SAMs containing a combination of ferrocene and other active moieties, e.g., another ferrocene,42 fullerene,33,41 porphyrin,2,28,41 and azobenzene,31 either as parts of different SAM constituents2 or united in a single molecule,28,31,33,41,42 were reported. The emphasis was, however, put on aliphatic SAMs,27,29-32,34-37,39,40,42,44,45 and only few studies dealt with aromatic ferrocene-substituted SAM, viz. films of diphenylethyne- and phenyl-linked ferroceneporphyrins on Au28 and layers of 4-ferrocenylbenzyl alcohol on Si.2 In this regard, the systems of this study are more promising, since the biphenyl ethynyl backbone is a much better electrical conductor than the aliphatic chain. Aliphatic matrix
The synthesis of the BPT and Fc molecule is described elsewhere.14,19 The gold substrates were prepared by thermal evaporation of 200 nm of gold (99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense) primed with a 5-nm titanium adhesion layer. Such films are standard substrates for thiol-derived SAMs. They are polycrystalline, with a grain size of 20-50 nm, as observed by atomic force microscopy. The grains predominantly exhibit a (111) orientation, which is, in particular, supported by the observation of the corresponding forward-scattering maxima in the angular distributions of the Au 4f photoelectrons47 and by the characteristic binding energy (BE) shift of the Au 4f surface component.48 The homogeneous BPT and Fc SAMs were formed by immersion of freshly prepared substrates into a 1 mM solution of the respective substance in DMF at room temperature for 24 h. The mixed BPT/Fc films were also prepared at room temperature, either by subsequent immersion of the substrates into 1 mM BPT (variable time) and Fc (24 h) solutions in DMF or, for comparison, by immersion of the substrates in a mixed solution of BPT and Fc in DMF (24 h; 0.5 mM BPT and 0.5 mM Fc). For further comparison, we have also prepared homogeneous SAMs formed from the acetylprotected Fc molecules (abbreviated as FcP; see Scheme 1). The same procedure as for the Fc films was used, but an appropriate amount of triethylamine (TEA) and water was added to the solution to remove the protection acetyl groups. In our experience, TEA is the best deprotection agent for the fabrication of SAMs from acetyl-protected thiols.49 After immersion, the samples were carefully rinsed with pure ethanol, blown dry with argon, and kept, in some cases, for several days in argon-filled glass containers until characterization. No evidence for impurities or oxidative degradation products was found. Note that the strategy selected by us for the fabrication of mixed SAMs comprised of the ferrocene-substituted and matrix molecules has not been used before for similar systems. The previous approaches included a coadsorption,2,43 two-step adsorption with incorporation of only individual ferrocenesubstituted molecules in highly ordered matrix SAM,45 and covalent attachment chemistry.37,44 In addition to the SAM samples, we have also prepared a powder sample of solid-state ethynyl ferrocene (see Scheme 1), which served as a reference for some of the spectroscopic measurements. The detailed description of the preparation procedure can be found elsewhere.50 The fabricated films were characterized by X-ray photoelectron spectroscopy (XPS), high-resolution XPS (HRXPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. All experiments were performed at room temperature. The measurements were carried out under ultrahigh vacuum conditions at a base pressure better than 1.5 × 10-9 mbar. The spectra acquisition time was selected in such a way that no noticeable damage by the primary X-rays occurred during the measurements.51-54
SAMs of Substituted Biphenyl Ethynyl Thiols The XPS measurements were performed with a Mg KR X-ray source and a LHS11 analyzer. The spectra acquisition was carried out in normal emission geometry with an energy resolution of ≈0.9 eV. The X-ray source was operated at 260 W power and positioned ≈1 cm away from the samples. The spectra were acquired mostly for the Fe 2p range. The HRXPS experiments were performed at the bending magnet beamline D1011 at the MAX II storage ring of the MAX-lab synchrotron radiation facility in Lund, Sweden. The HRXPS spectra were acquired in normal emission geometry at photon energies (PE) of 350 and 650 eV for the C 1s range and a PE of 350 eV for the S 2p region. In addition, Au 4f, O 1s, and Fe 3p spectra were collected. The BE scale of every spectrum was individually calibrated using the Au 4f7/2 emission line of AT-covered Au substrate at 83.95 eV. The latter value is the latest ISO standard.55 It is very close to a value of 83.93 eV, which has been obtained by us for Au 4f7/2 using a separate calibration to the Fermi edge of a clean Pt-foil.48 The energy resolution was better than 100 meV, which is noticeably smaller than the full widths at half-maximum (fwhm) of the photoemission peaks addressed in this study. Thus, these fwhms are representative for the natural widths of the respective lines. Both XPS and HRXPS spectra were fitted by symmetric Voigt functions and either a Shirley-type or linear background. To fit the S 2p3/2,1/2 doublet we used a pair of such peaks with the same full width at half-maximum (fwhm), the branching ratio of 2 (2p3/2/2p1/2), and a spin-orbit splitting (verified by fit) of ≈1.18 eV.56 The fits were performed self-consistently: the same peak parameters were used for identical spectral regions. For the HRXPS data, the accuracy of the resulting BE/ fwhm values is 0.02-0.03 eV. The NEXAFS measurements were performed at the HE-SGM beamline of the synchrotron storage ring BESSY II in Berlin, Germany. The spectra acquisition was carried out at the C K-edge and Fe L-edge in the partial electron yield mode with a retarding voltage of -150 and -450 V, respectively. Linear polarized synchrotron light with a polarization factor of ≈82% was used; this factor has been separately measured using a reference sample of highly oriented pyrolytic graphite (HOPG). The energy resolution was ≈0.40 eV. The incidence angle of the light was varied from 90° (E vector in surface plane) to 20° (E vector near surface normal) in steps of 10°-20° to monitor the orientational order of the SAM constituents. The raw NEXAFS spectra were normalized to the incident photon flux by division through a spectrum of a clean, freshly sputtered gold sample. The PE scale was referenced to the pronounced π1* resonance of HOPG at 285.38 eV.57 The relative PE shift at the Fe L-edge was calibrated using the Au 4f photoemission from a bare Au sample.
J. Phys. Chem. B, Vol. 110, No. 48, 2006 24623
Figure 1. S 2p HRXPS spectra of BPT, mixed BPT/Fc, Fc, and FcP SAMs on Au. The mixed BPT/Fc film was prepared by subsequent immersion of the substrate into BPT (30 s) and Fc (24 h) solutions.
3. Results
Figure 2. C 1s HRXPS spectra of BPT, mixed BPT/Fc, Fc, and FcP SAMs on Au. The mixed BPT/Fc film was prepared by subsequent immersion of the substrate into BPT (30 s) and Fc (24 h) solutions. The dotted and dashed lines are a guide for the eyes. The effective thicknesses of the SAMs are given at the respective curves.
3.1. XPS and HRXPS. S 2p and C 1s HRXPS spectra of BPT, BPT/Fc, Fc, and FcP SAMs on Au are presented in Figures 1 and 2, respectively. The BPT/Fc film was prepared by subsequent immersion of the substrate into BPT (30 s) and Fc (24 h) solutions. The respective spectra are representative for all mixed films of this study. The S 2p spectra of both homogeneous and mixed films in Figure 1 exhibit a single S 2p3/2,1/2 doublet at 162.0 eV (S 2p3/2) commonly assigned to the thiolate species,48,58 with no evidence for disulfides, alkylsulfides, or oxidative products. This means that all molecules in these films are attached to the substrate over the thiolate anchor. The fwhm of the S 2p3/2,1/2 peaks increases on going from the BPT SAM to the BPT/Fc layer
and especially to the Fc and FcP films, which suggests a slight inhomogeneity in the mixed film and an even larger inhomogeneity in the Fc and FcP films as compared to the BPT SAM. At the same time, the latter film seems to be noticeably thinner than the Fc-containing layers, which follows from the lower intensity of the S 2p signal for these layers. The C 1s spectra of both homogeneous and mixed films in Figure 2 exhibit a single emission, with a BE of 284.18 and 284.40 eV for the BPT and Fc (or FcP) SAMs, respectively. The BE positions of the C1s emission for the mixed films are somewhere between these values, in accordance with their (assumed) composition. The C 1s peak for the Fc-containing films is noticeably broader than that for the BPT SAM, which
24624 J. Phys. Chem. B, Vol. 110, No. 48, 2006
Figure 3. Fe 2p3/2 XPS spectra of BPT, mixed BPT/Fc, Fc, and FcP SAMs on Au. The mixed BPT/Fc films were prepared by subsequent immersion of the substrates into BPT (variable time given at the respective curves) and Fc (24 h) solutions.
cannot, however, be unambiguously associated with the lower homogeneity of the former films but is related to the combined effect of differently coordinated carbon atoms in the Fc and FcP molecules. In addition to the BE and fwhm analysis, the effective thickness of the BPT, BPT/Fc, Fc, and FcP SAMs was calculated using a standard procedure,59 taking the intensity ratios of the C 1s and Au 4f signals and standard attenuation lengths for these emissions.60 The resulting values are given at the respective curves; the accuracy of these values is (5%. According to these values, the thickness of the BPT film is significantly smaller than that of the Fc-containing films, and the thickness of the mixed SAMs, in accordance with their assumed composition, lies between the respective values for the BPT and Fc layers. The comparably low effective film thickness for FcP suggests that the packing density in this film is lower than that in the Fc SAM. Additional information is provided by the Fe 2p3/2 XPS spectra presented in Figure 3. Whereas only a homogeneous inelastic background is observed for the BPT SAM in this spectral region, the spectra of the Fc-containing films exhibit a characteristic emission related to Fe in the ferrocene moiety. Since this moiety comprises the SAM-ambient interface, the respective signal is practically unaffected by attenuation effects, and its intensity is a measure of the Fc content in the film. As clearly seen in Figure 3, this content continuously increases with decreasing BPT immersion time. 3.2. NEXAFS Spectroscopy: Electronic Structure. C K-edge NEXAFS spectra of BPT, mixed BPT/Fc, Fc, and FcP SAMs on Au as well as the spectrum of the solid-state ethynyl ferrocene are presented in Figure 4; these spectra were acquired at the magic angle of X-ray incidence (the spectrum is independent of the molecular orientation).61 The mixed BPT/ Fc films were prepared either by subsequent immersion of the substrates into BPT (variable time given at the respective curves) and Fc (24 h) solutions or by immersion of the substrate in a mixed solution of BPT and Fc. The spectrum of the BPT SAM (bottom curve) is dominated by the intense π1* resonance of the phenyl rings at ≈285.1 eV,
Shaporenko et al.
Figure 4. Normalized C K-edge NEXAFS spectra of BPT, mixed BPT/ Fc, Fc, and FcP SAMs on Au as well as the spectrum of the solid-state ethynyl ferrocene. The mixed BPT/Fc films were prepared either by subsequent immersion of the substrates into BPT (variable time given at the respective curves) and Fc (24 h) solutions or by immersion of the substrate into a mixed solution of BPT and Fc. Prominent absorption resonances are indicated. The dotted lines are a guide for the eyes.
which is accompanied by the respective π2* resonance at ≈288.8 eV, the R*/C-S* resonance at ≈287.3 eV, and several broad σ* resonances at higher photon energies (the assignment has been performed in accordance with refs 18, 25, 62, and 63). The spectrum of the solid-state ethynyl ferrocene (top curve) exhibits the characteristic π* resonances at 285.6 and 287.2 eV, related to the 4e1g and 3e2u orbitals of ferrocene, respectively, and a broad σ-resonance of ferrocene around 292 eV (the assignment has been performed in accordance with refs 6466). A relatively strong resonance at 288.6 eV is presumably related to CdO contamination in the powder sample, while the intensity at ≈285 eV can be associated with CdC contamination. Alternatively, the resonance at 288.6 eV can originate from interaction of the π* orbitals of ferrocene with the triple bond of the ethynyl unit.61 The π* resonance of the ethynyl group should be observed at ∼286 eV and is practically not distinguishable in the spectrum. The spectra of both Fc and FcP films are quite similar. As expected, they represent a superposition of the spectra of BPT and solid-state ethynyl ferrocene (apart from contamination). They are dominated by the π1* resonance of biphenyl at ≈285.1 eV and π* resonances of ferrocene at 285.6 and 287.2 eV; the higher PE region is dominated by the σ-resonances of biphenyl. The spectra of the mixed Fc/BPT films represent a superposition of the BPT and Fc spectra. As seen in Figure 4, for the films prepared by the subsequent immersion, the Fc-related contribution (both π* resonances of ferrocene) continuously increases with decreasing BPT immersion time. The spectrum of the film prepared from a mixed solution looks more like that of the Fc SAM than like the spectrum of the BPT layer. The relative portion of Fc molecules in the mixed films can be easily obtained by a decomposition of the spectra into individual contributions related to these and the BPT molecules, respectively, as illustrated by Figure 5. The results of this decomposition, in which we used two symmetric peaks to approximate
SAMs of Substituted Biphenyl Ethynyl Thiols
Figure 5. A decomposition of the C K-edge NEXAFS spectra (circles) of BPT, mixed BPT/Fc, and Fc SAMs on Au. Individual absorption resonances are drawn by thin solid lines. The asymmetric π1* resonance of BPT and high-energy π1* resonance of ferrocene are approximated by two symmetric peaks.
the asymmetric π1* resonance of BPT and high-energy π1* resonance of ferrocene, will be discussed in the next section. C K-edge NEXAFS data are complemented by the Fe L-edge ones, which are presented in Figure 6. Whereas the spectrum of the BPT SAM represents a homogeneous inelastic background, the spectra of all ferrocene-containing films exhibit the characteristic π* resonances related to the 4e1g and 3e2u orbitals of ferrocene at both L2- and L3-edges.66,67 Note that the intensity of the ferrocene-related resonances looks quite similar for all these samples due to the normalization procedure. The spectra of the Fc and FcP look almost identical to the spectrum of the solid state ethynyl ferrocene, whereas the spectra of the mixed films prepared by the subsequent immersion are distinctly different. For the latter films, there is a pronounced upward shift (by ∼0.7 eV) in the PE position of the π*(4e1g) resonance at the Fe L3-edge, and the intensity of the π*(3e2u) resonance at this absorption edge is quite low. Some differences are also observed at the Fe L3-edge, but they are less pronounced, because of a lower intensity of the resonant absorption features at this edge. The spectrum of the film fabricated from the mixed Fc/BPT solution looks intermediate between the spectra of Fc and FcP and the spectra of the mixed films prepared by the subsequent immersion, although it has a much more similarity with the spectra of Fc and FcP. Considering the characteristics of the preparation procedures and the assumed film structure, we relate the observed difference in the spectra to the presence (Fc, FcP, Fc/BPT from mixed solution) or absence (Fc/BPT by the subsequent immersion) of the intermolecular interaction between the neighboring ferrocene moieties. This tentative explanation suggests that the Fc species in the Fc/BPT films prepared by the subsequent immersion are individually incorporated into the BPT matrix, and the respective ferrocene units are well-separated from each other.
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Figure 6. Normalized Fe L-edge NEXAFS spectra of BPT, mixed BPT/Fc, Fc, and FcP SAMs on Au as well as the spectrum of the solid ferrocene. The mixed BPT/Fc films were prepared either by subsequent immersion of the substrates into BPT (variable time given at the respective curves) and Fc (24 h) solutions or by immersion of the substrate into a mixed solution of BPT and Fc. Prominent absorption resonances are indicated. The dotted lines are a guide for the eyes.
3.3. NEXAFS Spectroscopy: Molecular Orientation. Along with the insight into the electronic structure of the investigated films, NEXAFS data provide information on the orientation of the SAM constituents, since the cross-section of the resonant photoexcitation process depends on the orientation of the electric field vector of the linearly polarized synchrotron light with respect to the molecular orbital of interest (so-called linear dichroism in X-ray absorption).61 As far as there is a molecular orientation in a film, the intensity of the absorption resonances changes at a variation of the incidence angle of the synchrotron light. A convenient way to monitor the respective change is to calculate the difference between the spectra acquired at different X-ray incidence angles, optimally, between the spectra collected at normal (90°) and grazing (20°) incidence. Such C K-edge difference spectra of the BPT, mixed BPT/Fc, Fc, and FcP SAMs on Au are presented in Figure 7. Considering the amplitude of the π1* difference peak (biphenyl), which is the measure of molecular orientation in the films, one can say that the average molecular inclination (or orientational disorder) in both Fc and FcP films is larger than that in the mixed layers, in which the degree of the orientational order continuously increases with the decreasing BPT immersion time, going (but not achieving) to the value for the BPT SAM. The orientational order of the Fc/BPT film prepared from the mixed solution looks intermediate between those of the Fc and Fc/BPT films (a short BPT immersion time). Note that the transition dipole moment (TDM) of the π1* orbital is perpendicular to the plane of the phenyl ring. Apart from these qualitative conclusions, the average tilt angles of the biphenyl backbone in the investigated SAMs was derived by a standard numerical evaluation of the NEXAFS data, taking the entire set of spectra acquired at different X-ray incidence angles for every particular sample.61 The exact description of the procedure for the case of aromatic SAMs can be found elsewhere.18,49,68 The major assumptions made at the evaluation of the NEXAFS data are a herringbone molecular
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Figure 8. Portion of the ferrocene-substituted molecules in the mixed BPT/Fc SAMs as function of immersion time of the substrates into BPT solution. The SAMs were prepared by subsequent immersion of the substrates into BPT (variable time) and Fc (24 h) solutions. The portion was calculated on the basis of XPS (1) and NEXAFS (4) data.
Figure 7. C K-edge NEXAFS difference spectra of BPT, mixed BPT/ Fc, Fc, and FcP SAMs on Au. The curves represent the difference between the spectra acquired at X-ray incidence angles of 90° and 20°. The mixed BPT/Fc films were prepared either by subsequent immersion of the substrates into BPT (variable time given at the respective curves) and Fc (24 h) solutions or by immersion of the substrate in a mixed solution of BPT and Fc. The dotted lines are a guide for the eyes.
arrangement with a twist angle of the biphenyl backbone of 32°, which is characteristic of aromatic bulk systems69-72 and is believed to be the case for aromatic SAMs as well.68,73-75 In addition, we assumed the coplanar arrangement of the individual rings within the biphenyl backbone, which is the case for at least simple aromatic SAMs.49 The resulting values of the average tilt angle of the biphenyl moieties in the investigated SAMs obtained by the evaluation of the π1* resonance intensity are given at the respective difference spectra in Figure 7; the accuracy of these values are (5°. The analogous evaluation performed for the π*(4e1g) resonance of ferrocene in the Fc and FcP films gives very similar values, in agreement with a rodlike conformation of these molecules. For the mixed Fc/BPT films, the average tilt angles of the biphenyl moieties (both matrix and Fc) are slightly (by several degrees) smaller than the respective values for the ferrocene moieties. This suggests a slightly larger average inclination of the Fc molecules as compared to the matrix moieties. Apart from this general phenomenon, the orientational order in the mixed films obtained by the subsequent immersion improves with decreasing BPT immersion time. Note that, according to the average tilt angle of the SAM constituents, the orientational order in the Fc SAM is higher than that in the FcP film. 4. Discussion According to the XPS and NEXAFS data, which are mutually consistent, well-ordered and densely packed SAMs of ferrocenesubstituted biphenyl ethynyl thiols can be prepared on Au(111) substrate. These SAMs are contamination free. All SAM constituents are attached to the substrate by the thiolate anchor. These constituents have an upward orientation with an average tilt angle of about 32.3°. Thus, at the assumed rodlike conformation of the Fc molecules, the ferrocene moieties do not significantly affect self-assembly, apart from a lower orientational order (or a larger molecular inclination) of the Fc
films than in the reference SAM of nonsubstituted biphenyl thiols. The presumable reasons for this relatively small effect of the ferrocene tailgroup are the above-mentioned rodlike conformation of the Fc molecules, the similar orientation of the biphenyl (spacer) and cyclopentadienyl (ferrocene) rings, and the relatively small size of ferrocene. On the basis of the Fc and FcP data only, one cannot say to what extent the ferrocene tailgroups contribute to the structure-building intermolecular interaction in the respective SAM, but they seem to interact with each other, as seen from the analysis of the NEXAFS data. Similar SAM can also be prepared from the acetyl-protected ferrocene-substituted biphenyl ethynyl thiols, but the respective packing density is somewhat (∼20%) lower than that of the Fc films. Also, the orientational order in the FcP SAMs is somewhat worse than that in the Fc films, as manifested by the C K-edge NEXAFS difference spectra (Figure 7) and the higher value of the average tilt angle of the SAM constituents. These results are understandable, considering that the presence of the protection group and introduction of the deprotection substance usually affect the molecular assembly.49,76,77 Obviously, a further optimization of the deprotection procedure for the FcP molecules is necessary, since the quality of the Fc film has not been achieved so far. The subsequent immersion of the gold substrate into the BPT and Fc solution results in the formation of the mixed film, in which the Fc moieties are stochastically incorporated into the BPT matrix. The portion of the Fc molecules in the mixed film was calculated on the basis of the Fe2p3/2 XPS intensity (see Figure 3) and using the decomposition of the C K-edge spectra (Figure 4), as is shown in Figure 5. The Fc film was taken as the reference (100%). The results are presented in Figure 8, with the XPS- and NEXAFS-derived values exhibiting a good agreement. According to this figure, the portion of the Fc molecules was varied from ≈7% at a BPT immersion time of 150 min to ≈42% at a BPT immersion time of 5 s. Note that the Fc portion is presumably slightly lower in reality, since the packing density of our reference (Fc) is lower than that of the BPT matrix, which constitutes a noticeable part of the mixed film. Apart from this correction, we think that 7-42% is a dynamical range in which the portion of the Fc molecules in the mixed film can be precisely adjusted. Considering the relatively fast kinetics of the first step of self-assembly,7,8 the further decrease of the BPT immersion time will not result in a noticeable increase of the Fc portion. Also, the immersion time below 5 s is difficult to control precisely. In a similar way,
SAMs of Substituted Biphenyl Ethynyl Thiols an increase of the BPT immersion time beyond 150 min will presumably not result in a noticeable decrease of the Fc portion, even though some reduction can be achieved. In contrast to the mixed film prepared by subsequent immersion, the portion of the Fc molecules in the film prepared from the mixed (a 1:1 ratio) Fc/BPT solution is much higher. According to the evaluation of the XPS and NEXAFS data, it achieves about 80-90%, i.e., is significantly larger than the relative portion of the Fc molecules in the primary solution. This suggests a preferable adsorption of these molecules on Au substrates as compared to the BPT moieties, which means that both ethynyl and especially ferrocene units contribute to the intermolecular structure-building interaction, making the assembly of the Fc molecules thermodynamically more favorable than that of the BPT species. Under these conditions, it can be expected that the former species agglomerate together, in full agreement with the Fe L-edge NEXAFS data, showing a big similarity between the Fc SAM and the Fc/BPT film prepared from the mixed solution. 5. Summary Homogeneous and mixed (with BPT) SAMs of ferrocenesubstituted biphenyl ethynyl thiols were prepared on Au(111) substrates and characterized by XPS, HRXPS, and NEXAFS spectroscopy at the C K- and Fe L-edges. The mixed films were fabricated either by subsequent immersion of the substrates into the BPT and Fc solutions or by immersion of the substrate into a mixed solution of BPT and Fc. The subsequent immersion resulted in the preparation of densely packed and well-oriented films, in which the Fc molecules were separated from each other and stochastically distributed in the BPT matrix. The portion of the Fc molecules in such films could be varied in a controlled way from 7 to 42% by selection of the immersion time in the BPT solution. In contrast, the film prepared from the mixed solution contained about 80-90% of the Fc molecules, which differed significantly from the relative content of the target molecules in the primary solution (a 1:1 ratio), suggesting a preferable adsorption of the Fc molecules and a significant impact of the ferrocene groups onto the structure-building interactions, responsible for molecular self-assembly. According to the spectroscopic data, the Fc molecules in these films were not separated from each other but agglomerated. For the homogeneous films, the introduction of the protection acetyl group to the Fc molecule causes a noticeable (by 20%) lowering of the packing density and a slight decrease of the orientational order of the resulted films as compared to the SAM prepared from the nonprotected molecules. Both homogeneous and mixed Fc films represent active electrochemical templates. The advantages of the films prepared by the subsequent immersion are controlled density of the “active” Fc species and the separation between these moieties, which excludes interference effects. Electrochemical experiments with these templates are in preparation. Another interesting issue is a scanning tunneling microscopy or atomic force microscopy characterization of the homogeneous and mixed Fc films. We plan such experiments in the near future. Acknowledgment. A.S. and M.Z. thank M. Grunze, for the support of this work; A. Ku¨ller, for providing us with BPT; Ch. Wo¨ll (Universita¨t Bochum), for providing us with the equipment for the NEXAFS measurements; E. Moons, S. Watcharinyanon, and L. S. O. Johansson (Karlstad University), for the cooperation at MAX-lab; and the BESSY II and MAX-
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