ARTICLE pubs.acs.org/Langmuir
Azobenzene-Containing Triazatriangulenium Adlayers on Au(111): Structural and Spectroscopic Characterization Ulrich Jung,† Sonja Kuhn,† Ursula Cornelissen,‡ Felix Tuczek,‡ Thomas Strunskus,§ Vladimir Zaporojtchenko,§ Jens Kubitschke,|| Rainer Herges,*,|| and Olaf Magnussen*,† †
Institut f€ur Experimentelle und Angewandte Physik, Christian-Albrechts-Universit€at zu Kiel, Leibnizstr. 19, 24118 Kiel, Germany Institut f€ur Anorganische Chemie, Christian-Albrechts-Universit€at zu Kiel, Olshausenstr. 40, 24098 Kiel, Germany § Technische Fakult€at, Christian-Albrechts-Universit€at zu Kiel, Kaiserstrasse 2, 24143 Kiel, Germany Otto-Diels-Institut f€ur Organische Chemie, Christian-Albrechts-Universit€at zu Kiel, Otto-Hahn-Platz 3, 24098 Kiel, Germany
)
‡
bS Supporting Information ABSTRACT: Adlayers of different azobenzene-functionalized derivatives of the triazatriangulenium (TATA) platform on Au(111) surfaces were studied by scanning tunneling microscopy (STM), X-ray photoelectron scpectroscopy (XPS), gapmode surface-enhanced Raman spectroscopy (gap-mode SERS), and cyclic voltammetry (CV). The chemical composition of the adlayers is in good agreement with the molecular structure, i.e., different chemical groups attached to the azobenzene functionality were identified. Furthermore, the presence of the azobenzene moieties in the adlayers was verified by the vibration spectra and electrochemical data. These results indicate that the molecules remain intact upon adsorption with the freestanding functional groups oriented perpendicularly to the TATA platform and thus also to the substrate surface.
’ INTRODUCTION Surfaces that respond to external stimuli (e.g., light, redox processes, pH, or ion concentration) in a defined manner have received considerable attention because of both fundamental interests and applications.13 A convenient method for the preparation of such functional surfaces is self-assembly of appropriate molecules on metal substrates. However, in molecular adlayers the desired function is often strongly affected or even completely quenched because of the high coverage, i.e., steric hindrances,4,5 or electronic interactions with the substrate.6 To overcome these problems, commonly mixed monolayers of the functional molecules with appropriate spacer molecules are prepared.4,5,716 Alternatively, molecules are employed, which exhibit additional bulky spacer groups.5,1724 Nevertheless, these adlayers exhibit intrinsic disadvantages such as a comparable low structural order or the lack of temporal stability due to phase separation. We recently introduced a novel approach, in which molecular platforms based on the triazatriangulenium (TATA) ion (Figure 1) were employed for the preparation of adlayers.25,26 These compounds exhibit a versatile and modular architecture: Side chains determine by their steric demand the size of the platform and a functional group can be covalently attached to the central carbon atom of the TATA moiety via a customizable spacer. As functional group, derivatives of azobenzene, a prototype system of a molecular switch, were used. Azobenzene derivatives exhibit r 2011 American Chemical Society
cistrans isomerism.27 Isomerization of the thermodynamically preferred trans to the metastable cis isomer can be induced by irradiation with UV light of 365 nm, reisomerization to the trans isomer by irradiation with blue light of 435 nm, or via thermal relaxation. In addition, both trans and cis isomers can be interconverted in two-electron/two-proton redox reactions via hydrazobenzene to the trans isomer only. Adlayers of azobenzene derivatives covalently attached to metal surfaces have been intensively studied,4,5,7,9,10,1317,2864 but in most cases did not exhibit photoswitching, a problem which is commonly ascribed to steric hindrances. Only for mixed monolayers4,5,716 or SAMs of specially designed molecules5,1724,65 was photoswitching demonstrated. The TATA derivatives were shown in STM experiments25,26 to form highly ordered hexagonal adlayers on Au(111) surfaces with intermolecular spacings that can be adjusted by the length of the alkyl side chains attached to the TATA moiety. However, the presence and orientation of the functional group and the side chains could be inferred only indirectly in these studies by their effect on the order and morphology of the adlayers. In this study, we present detailed spectroscopic and electrochemical characterization (by X-ray photoelectron spectroscopy (XPS), gap-mode surface-enhanced Raman spectroscopy (gap-mode Received: November 23, 2010 Revised: February 12, 2011 Published: April 20, 2011 5899
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Figure 1. TATA derivatives used for preparation of adlayers on Au(111).
SERS), and electrochemical experiments (cyclic voltammetry)) of the attached functional groups in adlayers of TATA derivatives on Au(111). In addition, the previous scanning tunneling microscopy (STM) studies are complemented by results for additional compounds with different terminal groups at the azobenzene moiety. On the basis of the spectroscopic and structural data, the adlayer composition and vertical structure will be discussed.
’ EXPERIMENTAL SECTION Materials. The employed triazatriangulenium (TATA) derivatives (Figure 1) were synthesized following the procedure described in our previous publications.25,26,66 The quality of all substances was verified by NMR and mass spectroscopy, indicating a purity better than 99%. As substrates for the STM and the electrochemical measurements Au(111) single crystals with a surface diameter of 10 or 9 mm oriented within 0.3° (MaTecK GmbH) were used. XPS and gap-mode SERS measurements were performed on (111)-oriented Au films (250 nm) with Cr adhesion promotor (5 nm) on glass (”Arrandees”, Fa. Dr. D. Schroer). The Au(111) single crystals were cleaned by immersion in ”piranha” solution and subsequent extensive rinsing with 18.2 MΩ cm water (Elga LabWater). Afterward, they were annealed in a butane gas flame for 2 to 5 min. The same annealing was also performed for the Au films on glass. The cleaned Au substrates were immersed into solutions of the TATA derivatives in ethanol or toluene (Merck, p.a.) with concentrations of 100 or 1 μM for 30 to 60 min at room temperature or 80 °C. After self-assembly of the adlayers, the samples were immersed into the pure solvent for several seconds up to 15 min to rinse excess TATA molecules. For the TATA derivatives with a terminal group at the azobenzene moiety (compounds 2d, 2e, 2f; see Figure 1), very similar adlayers were obtained independent of preparation conditions, as was confirmed by STM and gap-mode SERS measurements. In contrast, for the compound 2c without a terminal group at the azobenzene moiety, the adlayer structure strongly depends on the preparation conditions26 (see below). Since for this compound bilayer formation anyway could not be entirely avoided, the preparation conditions were chosen to
obtain almost perfect bilayers (100 μM solution, 30 min immersion time, room temperature). The gap-mode SERS samples were prepared by subsequent immersion of the freshly fabricated adlayer-modified substrates into aqueous gold colloid solution for typically 4872 h. Afterward, the samples were dried in air. The gold colloid was prepared from HAuCl4 in sodium citrate solution following the procedure67,68 and exhibited an absorption maximum at 522 nm.42 Instrumentation. STM measurements were performed in air using a PicoPlus STM (Agilent, Inc.) with mechanically cut Pt/Ir (70%/30%) tips in constant current mode at tunneling currents of 3060 pA and bias voltages of 200600 mV. Lateral drift in the STM images was corrected by a dedicated software. The XPS measurements were performed using a Al KR X-ray source (Eph = 1486.6 eV) in an Omicron Full Lab vacuum system with a base pressure of 3 109 mbar. The spectra were energy-corrected using the Au 4f7/2 peak at a binding energy of 84.0 eV as reference. The peaks were fitted after subtraction of a linear background with symmetrical Voigt profiles. For the Raman spectroscopy measurements, two different spectrometers were used: (i) A Dilor XY-Raman spectrometer (Horiba) with an ArKr mixed gas laser (RM 2018, Spectra Physics) at a wavelength of λex = 647.1 nm (Pex = 10 to 30 mW, size of laser spot on the sample: 0.5 mm 1 mm) and (ii) an ISF66/FRA106 Fourier transform Raman spectrometer (Bruker AXS GmbH) with a Nd:YAG laser at a wavelength of λex = 1064 nm. All Raman spectroscopy measurements were carried out at ambient conditions. The electrochemical measurements were performed in a homemade electrochemical cell69 using an Autolab PGSTAT12 potentiostat (Eco Chemie). The adlayer-modified single-crystalline Au(111) working electrode was mounted in hanging-meniscus geometry. Counter electrode was a platinum wire and reference electrode a saturated calomel electrode (SCE). As electrolyte, either 0.1 M H2SO4 or 0.1 M NaClO4 (suprapure, SigmaAldrich Chemie GmbH) with Britton-Robinson buffer at pH = 5 was used, which was deaerated with Ar (5.0 purity, Messer Griesheim). Density functional theory (DFT) calculations of the free molecules were performed employing B3LYP hybrid functional and 6-31þþG(d,p) 5900
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Figure 2. STM images (30 30 nm2) of various functionalized TATA adlayers on Au(111): (a) 2a, (b) 2c, (c) 2d, (d) 2e, (e) 2f, and (f) 2h. basis set with Gaussian 03, revision D.02.70 From these calculations, Raman spectra were obtained assuming Lorentzian peaks and a width parameter of 5 cm1. These spectra were scaled by 0.945v þ 36.150 cm1 to fit with the experimental data.
’ RESULTS AND DISCUSSION Scanning Tunneling Microscopy. STM images of adlayers of different functionalized triazatriangulenium (TATA) derivatives on Au(111) are shown in Figure 2. The data for 2a, 2c, 2d, and 2e adlayers already have been presented in our previous publications,25,26 but are reproduced here to complement the spectroscopic studies discussed below. TATA molecules with propyl as well as octyl side chains form highly ordered hexagonal adlayers with lattice √ constants √ that are in good agreement with commensurate √ ( 13 √ 13)R13.9° (d = 10.4 Å, Θ = 1/13 ML) or ( 19 19)R23.4° (d = 12.6 Å, Θ = 1/19 ML) superstructures, respectively. The typical domain size is in the range 10100 nm. As discussed previously,25,26 these findings indicate that the TATA platforms adsorb flat on Au(111) surfaces, with the alkyl side chains being tilted partially away from the surface. On the basis of these observations, we have proposed that the platforms are bound not only via van der Waals forces and interactions between the π electrons and the metal substrate, but also via local bonds between the N and the Au atoms.26 The latter is in agreement with a tetrahedral coordination of the N atoms. For the TATA derivatives exhibiting an ethynylbenzene or ethynylazobenzene functionality (substances 2a and 2c, Figure 2a,b), self-assembly always results in the formation of mixed mono- and bilayer phases (the latter appear in the according STM images as white protusions), which may be attributed to π electron stacking between the functional groups. In our previous systematic studies,26 it was clearly demonstrated that it is not possible to completely prevent bilayer formation by variation of the preparation conditions (e.g., concentration, immersion time, temperature, and rinsing). However, introduction of a terminal group at the azobenzene moiety (substances 2d, 2e, and 2f) or an additional phenyl spacer (substance 2h) leads to formation of almost perfect monolayers where only e10% of the surface is
covered by bilayer aggregates (Figure 2cf). The significantly reduced tendency toward bilayer formation is most probably due to steric hindrance or repulsive electrostatic interactions between the terminal groups at the azobenzene moiety. Furthermore, in all STM measurements the adlayers were found to cover the entire surface. Although these STM measurements yield important information on the adlayer structure and clearly reveal that it is possible to prepare monolayers of functional TATA derivatives, they do not provide direct data on the functional groups and the side chains. Nevertheless, the influence of the functional groups on the tendency toward bilayer formation strongly suggests that these compounds adsorb intact with the functional group remaining attached to the TATA platform. X-ray Photoelectron Spectroscopy. The XPS measurements were performed to check the chemical composition and to estimate the thickness of the TATA adlayers. Spectra of adlayers of different (functionalized) TATA derivatives (compounds 2, 2c, 2d, and 2f) on Au(111) films on glass are shown in Figure 3. The overall quality of the spectra was very similar for all samples and is exemplarily illustrated for a 2 adlayer in Figure 3a. Beam damages could be largely excluded, since the peak intensities were almost constant during the experiments (typically for more than 5 h). First, the spectral contributions of the substrate are described: The most intense peaks were expectedly due to Au (i.e., 5d at EB = 4 eV, 5p3/2 at 57 eV, 5p1/2 at 74 eV, 4f7/2 at 84 eV, 4f5/2 at 87 eV, 5s at 111 eV, 4d5/2 at 335 eV, 4d3/2 at 353 eV, 4p3/2 at 547 eV, 4p1/2 at 643 eV, and 4s at 761 eV). In addition, peaks due to Si (2p3/2 at 103 eV), O (1s at 531 eV), and Cr (3s at 74 eV, 2p3/2 at 577 eV, 2p1/2 at 586 eV, and 2s at 697 eV) were observed. As already stated in the Experimental section, Au films with Cr adhesion promotor on borosilicate glass were used as substrates. In AFM/STM studies, these substrates were found to consist of Au islands exhibiting (111) texture and a typical diameter of >1 μm with several 10 nm deep trenches in between.71 Therefore, it is most probable that the contributions due to Si, O, and Cr are not impurities on top of the Au(111) islands, which could affect the TATA adlayer structure, but rather originate from the trenches, in which the oxidized Cr adhesion layer and the glass substrate is exposed. 5901
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Figure 3. X-ray photoelectron spectra of various functionalized TATA adlayers on Au(111): (a) Overview spectrum, (b) C 1s peak, (c) N 1s peak, (d) I 3d5/2,3/2 peak in the shoulder of the Au 4p1/2 peak, and (e) F 1s peak.
In comparison to these contributions of the substrate, the peaks which can be attributed to the TATA adlayers (i.e., due to C, N, I, and F) have rather small intensities. Detailed spectra of the C 1s, N 1s, I 3d5/2þ3/2, and F 1s peaks are shown in Figure 3be. These spectra are in good qualitatitve agreement with the chemical structure of the TATA molecules: All compounds exhibit contributions due to C (1s peak at 285.0 eV) and N (1s peak at 400.4 eV), whereas the I 3d5/2þ3/2 peaks (at 619.4 and 630.9 eV, respectively) and the F 1s peak (at 687.7 eV) were observed only for adlayers of molecules containing I (2d) or CF3 (2f) terminal groups, respectively. Moreover, for nominally clean Au substrates (not shown), only the peak due to C but no peaks due to N, I, or F could be identified. Hence, the XPS measurements confirm the presence of the TATA adlayer, including that of the functional groups. However, it was not possible to distinguish between the different C and N atoms in the TATA molecules due to the small chemical shifts for the involved species.72 Noteworthy is also the absence of the F 1s peak for the bare platform 2, which indicates that the BF4 counterions are desorbed. The latter would be unlikely for a fully cationic TATA adsorbate and hence suggests a significant charge transfer to the substrate.
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To clearly demonstrate that the peaks due to C, N, I, and F are contributions of a surface adlayer and not of the bulk sample, spectra were recorded at different incidence angles (i.e., under normal incidence and tilted by 50°) as shown for a 2c adlayer in Figure 4. By tilting the sample, the relative intensities of the Au 4f7/2þ5/2 peaks decrease by 0.77 and 0.78, respectively, whereas that of the C 1s peak increases by 1.19 and that of the N 1s peak by 1.31. Since with increasing tilt angle the escape depth of the emitted electrons decreases, this observation is in agreement with the behavior expected in the presence of an organic adlayer. A more quantitative discussion of the chemical composition of the adsorbate layers is complicated by three factors: First, for all of the studied adlayers comparably high C concentrations are observed (see Table 1), which most probably partially originate from contaminations on top of the adlayers due to sample handling between preparation of the adlayers and the transfer into the vacuum system (requiring ∼1 min under air). This is supported by measurements on nominally clean Au substrates, where the C 1s peak was found to exhibit an integrated intensity of 0.04 of the Au 4f7/2 peak. Second, due to the poor signal-to-noise ratio the peak fitting errors were comparably large (especially for N, I, and F), even for long measurement times. Third, due to their molecular structure, the adlayers do not form compact films with a homogeneous density in surface normal direction, but the density gradually decreases from the TATA platforms near the Au surface to the functional groups (see also Figure 8), complicating quantitative modeling of the data. This is even true for the bare platforms (compounds 1 and 2), since the alkyl side chains are expected to be partially oriented above the TATA platforms due to space requirements. For these reasons, it is not possible to conclude wether there is a good quantitative agreement with the chemical composition of the adsorbate molecules. However, the substantially smaller C and N concentrations identified for the adlayers of all other TATA derivatives than for those of compound 2c show that the coverage of the latter significantly exceeds one monolayer, which is in very good agreement with the results of the previous STM measurements26 (see above). Other aspects, e.g., why the N 1s peaks of the functionalized adlayers 2d and 2f are weaker than that of the bare platform 2, are currently not understood, however. The thickness of the TATA adlayers was estimated from the integrated intensities of the Au 4f7/2 and C 1s peaks employing eq 1:73,74 IC, s =IAu, s 1 expð ds =λC Þ expð dr =λAu Þ ¼ expð ds =λAu Þ 3 1 expð dr =λC Þ IC, r =IAu, r
ð1Þ
Here, IAu and IC denote the integrated intensities of the Au 4f7/2 and C 1s peaks, respectively, d the adlayer thickness and λ the escape depths of the electrons at the appropriate kinetic energies (λAu = 20.2 Å, λC = 30.2 Å75). The indices s and r denote the sample and the reference. As reference, a n-dodecanethiol SAM with a known thickness of dr = 17 Å76 was employed. The results of this calculation are summarized in Table 2. For adlayers of the bare TATA platform 2 as well as for the functionalized TATA derivatives 2d and 2f, a thickness of 1113 Å was obtained (see Table 1). For comparison, the maximum adlayer thickness as well as an effective thickness are included in this table. The former was obtained from the molecular height of the DFT calculations for the free molecules assuming a distance between the topmost Au atoms and the N atoms in the TATA platform of 2.2 Å and upward tilted linear octyl side chains (based on a tetrahedral coordination of 5902
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Figure 4. Comparison between the relative XPS intensities measured at normal (0°) and tilted incidence (50°) for a 2c adlayer on Au(111): (a) Au 4f7/2þ5/2 peaks, (b) C 1s peak, and (c) N 1s peak.
Table 2. Thickness dexp of TATA Adlayers on Au(111) Derived from eq 1 Employing a n-Dodecanethiol SAM as Reference as well as Distance dmax of the Topmost Atom in the TATA Molecules from the Au Surface Obtained from DFT Calculations and Effective Thickness deff Given via eq 2a
Table 1. Results of the Quantitative analysis of the XPS Spectra for Adlayers of Various (Functionalized) TATA Derivatives on Au(111)a 2 peak
EB/eV
I/IAu
(I/σ)/(IAu/σAu)
ΔE/eV
compound
dexp/Å
Au 4f7/2
84.1
1.000
1.000
1.6
2
13
10.4
Au 4f5/2
87.7
1.164
1.480
1.6
2c
18
17.1 (21.6)
13.9 (27.7)
285.2 400.5
0.058 0.004
0.562 0.023
2.4 1.6
2d 2f
11 11
18.2 18.2
13.9 14.1
C 1s N 1s
a
dmax/Å
deff/Å 10.3
The values in brackets for compound 2c correspond to an ideal bilayer.
2c peak
EB/eV
I/IAu
(I/σ)/(IAu/σAu)
ΔE/eV
Au 4f7/2
84.0
1.000
1.000
1.6
Au 4f5/2
87.7
1.183
1.503
1.6
C 1s
285.0
0.097
0.941
2.4
N 1s
400.5
0.012
0.090
2.4
peak
EB/eV
I/IAu
(I/σ)/(IAu/σAu)
ΔE/eV
Au 4f7/2
84.1
1.000
1.000
1.6
Au 4f5/2
87.7
1.148
1.459
1.6
C 1s
284.9
0.060
0.584
2.3
N 1s I 3d5/2
400.0 619.4
0.002 0.005
0.009 0.004
1.4 2.1
I 3d3/2
630.9
0.002
0.001
1.4
peak
EB/eV
I/IAu
(I/σ)/(IAu/σAu)
ΔE/eV
Au 4f7/2
83.8
1.000
1.000
2.3
Au 4f5/2 C 1s
87.4 284.8
1.220 0.047
1.550 0.456
2.8 3.4
N 1s
400.6
0.004
0.019
3.1
F 1s
687.7
0.009
0.020
4.1
2d
2f
EB, binding energy; I, intensity; σ, photoionization cross section;72 and ΔE, full-width at half maximum. a
the N atoms in the TATA platform). The latter is given via def f ¼ dr
Ns Θs Nr Θr
ð2Þ
where Ns and Nr are the total number of C atoms per molecule and Θs and Θr the coverages in the sample and the reference, respectively. This definition of an effective thickness tries to account for the gradual density profile of the adlayer in the surface normal direction by comparing the average number of C atoms per Au surface atom for the sample and the reference. It consequently should be better suited for comparison with the experimental thickness obtained from eq 1. Indeed, this rather rough estimate is in reasonable agreement with the experimental thickness obtained from the XPS data for molecules 2d and 2f, which form very homogeneous adlayers according to the STM studies, indicating that these compounds form monolayers. In contrast, the 2c adlayers exhibit a significantly higher thickness, which supports the presence of bilayers (see above). Surface-Enhanced Raman Spectroscopy. In addition to the STM and XPS measurements, adlayers of the TATA derivatives were characterized by gap-mode surface-enhanced Raman spectroscopy. For this, an adlayer of metallic nanoparticles is deposited from solution on top of a conventionally prepared molecular adlayer on a metallic substrate.42,77 By excitation of localized plasmons in the gaps between the nanoparticles and the substrate (i.e., in the region of the molecular adlayer), the electromagnetic field and thus the Raman-scattered radiation are strongly enhanced, allowing measurement of Raman spectra of highly ordered molecular adlayers on well-defined, smooth substrates. In our previous publication, we employed gap-mode SERS for vibrational spectroscopic characterization of azobenzene-containing alkanethiol SAMs.42 Here, the gap-mode SERS technique was used to study the structure of the TATA adlayers, employing the same preparation methods as in ref 42. Examples of gap-mode SERS spectra of different TATA adlayers on Au(111) and corresponding Raman spectra of the bulk compounds 5903
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Figure 5. (a) Bulk Raman spectra of various TATA derivatives (λex = 1064.0 nm) and (b) gap-mode SERS spectra of adlayers of these substances on Au(111) (λex = 647.1 nm). The numbers denote normal vibrations of the benzene moieties, T indicates contributions of the TATA platform and A of the azobenzene moiety.
are shown in Figure 5 (spectra of all the studied substances are provided in the Supporting Information). Obviously, the gapmode SERS spectra are in close agreement with those of the bulk samples for all of the studied compounds. In particular, the same spectral lines with similar relative intensities were found. Since it was not possible to resolve individual spectral lines in the gapmode SERS spectra at higher Raman shifts (i.e., in the range 28003150 cm1) under the experimental conditions, these regions were omitted. The intensities in the bulk Raman spectra were normalized to the spectral line at 2870 cm1 corresponding to the CH3 stretching modes, for which only contributions of the alkyl side chains are expected. In the Raman spectrum of the bare TATA platform (substance 2), several spectral lines are visible between 450 and 1650 cm1, which are mainly due to CdC deformation of the TATA platform. Between 2800 and 3000 cm1 bands due to CH2 stretching of the alkyl side chains are found. Furthermore, between 3000 and 3150 cm1 bands due to CH stretching of the TATA platform are observed. Hence, the spectrum of the bulk substance is in very good agreement with the molecular structure. Also, the corresponding gap-mode SERS spectrum of the TATA adlayer shows an overall very good agreement. However, because of the poor signal-to-noise ratio and the broad background between 500 and 1600 cm1, it is not possible to resolve all spectral lines, only the most prominent. For the compound with a ethynylbenzene functionality (substance 2a), a very similar bulk Raman spectrum was obtained as for the bare TATA platform. However, the relative intensities of the spectral lines at 450 and 490 cm1 were significantly reduced and those at 1595 and 1615 cm1 enhanced. In addition, two spectral lines were identified at 2200 and 2220 cm1, which are attributed to stretching of the ethynyl moiety. In the gap-mode
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Figure 6. Comparison between the measured spectra of bulk 2 and 2c (λex = 1064.0 nm) with the calculated spectra of TATAH3 and of the trans and cis isomers of phenyl-(4-phenylethynyl-methyl)-diazene (trans- and cis-Azo).
SERS spectrum of 2a adlayers, again only the most prominent spectral lines were observed. In particular, the spectral line at 2220 cm1 due to stretching of the ethynyl group was also found. In contrast, for the compounds with ethynylazobenzenecontaining functionalities (substances 2c and 2d) distinctively different Raman spectra were obtained. In addition to the spectral lines due to the TATA platform mentioned above, in the range between 1100 and 1650 cm1 spectral lines with much higher intensities were found, which are mainly due to valence vibrations of the aromatic rings in the azobenzene moiety. The spectral lines at 1140 and 1180 cm1 exhibit contributions due to C—N stretching, whereas those in the range between 1400 and 1500 cm1 exhibit contributions due to NdN stretching. Also, in the spectral range between 3000 and 3150 cm1 distinct differences due to C—H stretching of the azobenzene moiety are identified. Although the spectra of the two compounds 2c and 2d are very similar, there exist clear differences (predominantly in the spectral range between 1400 and 1500 cm1), which can be attributed to the effect of the I terminal group on the vibrations of the azobenzene moiety. Similar differences were observed for all azobenzene-functionalized TATA derivatives, which allow one to distinguish between them (see Supporting Information). The gap-mode SERS spectra are again in close agreement with the corresponding bulk Raman spectra, verifying the presence of the azobenzene moieties in the adsorbate layers. To achieve a more detailed understanding of these Raman spectra, DFT calculations of a simplified TATA platform (with hydrogen atoms as side chains) and of the attached azobenzene derivative (phenyl-(4-phenylethynyl-methyl)-diazene) were performed and theoretical spectra were derived (Figure 6). In the 5904
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Langmuir range between 450 and 1650 cm1, the bulk spectrum of 2 and the calculated spectrum of the TATA platform are in close agreement. Major differences appear in the range between 2800 and 3000 cm1, because the alkyl side chains were omitted in the calculations. In the spectral range between 3000 and 3150 cm1, the agreement is better again. Comparison of the spectrum of bulk 2c with the calculated spectra of phenyl-(4phenylethynyl-methyl)-diazene reveals that the experimental data are clearly more consistent with the calculated spectrum of the compound in trans than in cis configuration. In particular, for the trans isomer of phenyl-(4-phenylethynyl-methyl)-diazene in the range between 1000 and 1650 cm1 basically the same spectral lines with similar relative intensities are observed as in the measured spectrum. In contrast, for the cis isomer the spectral lines are considerably shifted (e.g., from 1145 to 1135 cm1, with C—N stretching contributions, or from 1463 to 1547 cm1, with NdN stretching contributions) and exhibit distinctively lower intensities. Even more characteristic is the absence of a spectral line at 602 cm1 due to NdN deformation in the measured spectrum, since this vibration is Raman-inactive (symmetryforbidden) for the trans isomer. These differences are analogous to those previously reported for bulk and gap-mode SERS spectra of azobenzene-containing alkanethiols42 and strongly suggest that the azobenzene moieties in the TATA molecules are in trans configuration. The results of the DFT calculations together with previous literature allow a complete assignment of the spectral lines, as exemplarily presented for substance 2c in the Supporting Information. Since both the TATA platform and the functionality contain aromatic moieties, a definite assignment is often not possible. However, there exist distinct spectral lines, which solely originate from one of these groups and thus can be used as markers. For example, the spectral line at 490 cm1 due to CdC deformation is found in the spectra of all TATA derivatives but not in those of azobenzene-containing alkanethiols.42 Thus, it is a clear indicator for the TATA platform. Furthermore, the spectral lines at 2200 and 2220 cm1 due to stretching of the ethynyl group are markers for the spacer between the TATA platform and the functionality. The vibrations in the range between 1100 and 1600 cm1 mainly originate from the azobenzene moiety in trans configuration and accordingly are markers for this functionality. The spectral intensities obtained for different gap-mode SERS samples of the same TATA derivatives (typically for 2 to 4 independent samples) exhibited deviations of on average
