Adsorption of Mono- and Divalent 4-(Dimethylamino)pyridines on Gold

Article Cite This: Langmuir 2019, 35, 8667−8680

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Adsorption of Mono- and Divalent 4‑(Dimethylamino)pyridines on Gold Surfaces: Studies by Surface-Enhanced Raman Scattering and Density Functional Theory Xin Gong,† Maurice Taszarek,‡,⊥ Luise Schefzig,‡ Hans-Ulrich Reissig,‡ Steffen Thierbach,† Bernhard Wassermann,† Christina Graf,§ Doreen Mollenhauer,∥,# and Eckart Rühl*,† †

Physikalische Chemie, Institut für Chemie und Biochemie, Freie Universität Berlin, Arnimallee 22, 14195 Berlin, Germany Organische Chemie, Institut für Chemie und Biochemie, Freie Universität Berlin, Takustrasse 3, 14195 Berlin, Germany § Fachbereich Chemie- und Biotechnologie, Hochschule Darmstadt, Stephanstrasse 7, 64295 Darmstadt, Germany ∥ Physikalisch-Chemisches Institut, Justus-Liebig-Universität Gießen, Heinrich-Buff-Ring 17, 35392 Gießen, Germany # Center for Materials Research (LaMa), Justus Liebig University Giessen, 35392 Gießen, Germany

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S Supporting Information *

ABSTRACT: The adsorption thermodynamics of 4(dimethylamino)pyridine (DMAP) and its five divalent derivatives di-DMAP-n (2 ≤ n ≤ 6) with gradually increasing methylene-spacer lengths n binding to planar gold surfaces has been studied by surface-enhanced Raman spectroscopy (SERS) and density functional theory (DFT). SERS intensities of the totally symmetrical breathing mode of the pyridine ring at approximately 1007 cm−1 are used to monitor the surface coverage of the DMAP and di-DMAP-n ligands on gold surfaces at different concentrations. The equilibrium constant as a measure of the binding affinity is obtained from these measurements by using a modified Langmuir isotherm. Due to multivalent binding to the gold substrate, a characteristic enhancement of the binding affinity of di-DMAP-n compared to the monovalent DMAP is observed for all divalent species. First principles calculations of the di-DMAP-n ligands on an ideal Au(111) surface model as well as step terrace models have been performed to understand the adsorption structures and the multivalent binding enhancements. Furthermore, Raman spectra of the adsorbed molecules have been studied by first principles calculations to correlate the binding affinities to experimentally determined adsorption constants. The joint experimental and theoretical investigation of an oscillatory behavior of the binding affinity as a function of the methylene-spacer length in mono- and divalent 4-(dimethylamino)pyridines reveals that the molecular architecture plays an important role for the structure-function interplay of multivalently bound adsorbates.

1. INTRODUCTION Multivalency is defined as the chemical interaction of ligands with several identical functional groups binding to multiply presented acceptor sites.1−3 The stability of the complexes that are formed between multivalent ligands and acceptors is significantly increased when the binding of multivalent systems is compared to the corresponding monovalent species.1,2 Multivalent interactions have been studied by numerous methods, such as NMR spectroscopy,4 mass spectrometry,4 fluorescence spectroscopy,5 and Raman spectroscopy.6 The latter method provides detailed molecular properties, especially if the fingerprint regime is investigated. Specifically, surfaceenhanced Raman spectroscopy (SERS)7 has been applied for investigating adsorbed pyridine and its derivatives.8−10 In particular, the main interest of this work was devoted to studies of adsorption conformations, kinetics, and thermodynamics of pyridine ligands. Pyridines adsorbed on a gold surface may have several possible binding interactions: case (i) vertical © 2019 American Chemical Society

adsorption, which is governed by the nitrogen lone-pair and case (ii) horizontal adsorption via delocalized π-bonds. One of the adsorption structures in case (i), also called end-on adsorption,11 according to Lipkowski et al.,12 is formed by the nitrogen atom coordinated with an atom of the gold surface. The other vertical structure is the edge-on adsorption,13 in which both the nitrogen atom and one of the directly neighboring carbon atoms bind to metal adatoms and the pyridine ring plane is still oriented perpendicular to the surface. For high surface coverage, when each pyridine ligand takes up a relatively small area on the gold surface,12,14,15 the vertical adsorption is mediated mainly by σ states of the Au−N overlap having partially a nitrogen-lone-pair character and a delocalized π-HOMO orbital. The horizontal conformation (ii) is also Received: February 6, 2019 Revised: May 9, 2019 Published: June 7, 2019 8667

DOI: 10.1021/acs.langmuir.9b00371 Langmuir 2019, 35, 8667−8680

Article

Langmuir called flat-on conformation, which appears at low surface coverage. In this case, the pyridine ring is oriented parallel to the metal surface, so that the high-lying π-orbitals of physisorbed pyridine with partial nitrogen-lone-pair character are overlapping with the d-band of the gold surface atoms.13,16 A transition between flat-on and end-on binding has been observed and reported in refs 17−19. 4-(Dimethylamino)pyridine (DMAP) differs from pyridine as the dimethylamino group at C-4 strongly increases the electron density of the π-system of the pyridine ring through a +M effect. As a consequence, DMAP binds stronger to a gold surface than unsubstituted pyridine. DMAP is frequently used as a stabilizing agent for gold nanoparticles.20 Similar to covalently bound thiol ligands, which can be detached by a place-exchange reaction,21,22 the relatively weak adsorption of DMAP can be enhanced by multivalent interactions after carefully designing the molecular structures, such as the spacer length, spacer rigidity, or valency. Therefore, DMAP ligands can provide a balance between the stabilization robustness and reversible ligand exchange.23 The binding kinetics and conformations of DMAP to a gold surface have been investigated by several authors. Gandubert et al.24 investigated the adsorption of DMAP onto a gold surface by means of electrochemistry and surface plasmon resonance and confirmed earlier results of Gittins et al.,20 relating three most likely binding structures. Rosendahl et al. deployed surfaceenhanced infrared absorption spectroscopy to study the influences on adsorption conformation from the substrate potential and solution pH values.25 Barlow et al. proposed that the molecular orientation depends on the surface coverage, pH value relative to the primary pKa of pyridine, and the potential of the metal.9 Two models summarizing the states of DMAP(H+) adsorption on polycrystalline gold were discussed:9 (i) a full monolayer of N-bonded DMAP at high pH and positively charged electrode densities and (ii) a π-bonded arrangement of DMAP(H+) ions observed at negatively charged surfaces and in acidic electrolyte solutions. Iqbal et al.26 reported on the correlation of the measured friction to the vertical binding of N-bonded pyridine to a Au(111) surface. Yet, the most profound first principles analysis of all possible adsorption structures for various pyridine derivatives on a Au(111) surface was given in the work of Mollenhauer et al.27 The role of dispersion effects was emphasized by governing the change from a vertical adsorption structure in pyridine to a parallel one in its derivatives. The adsorption structures on the edges and corners of Au nanoparticles were studied by introducing adatoms. The preceding work of Bilic et al.28 delivered a detailed analysis of flat and atop pyridine adsorption on the Au(111) surface involving bridge sites as well as fcc- or hcp-hollow sites. The highly flexible pyridinegold interaction can serve as a basis for building an effective “alligator clip” for use in molecular electronic application. In this work, we study systematically the adsorption thermodynamics of 4-(dimethylamino)pyridine (DMAP) and its five divalent derivatives di-DMAP-n (2 ≤ n ≤ 6) with gradually increasing methylene-spacer lengths binding to planar gold surfaces by surface-enhanced Raman spectroscopy (SERS) (cf. Figure 1a). Subsequently, first principles calculations of model systems were used to determine the optimal binding structures of these ligands to the Au(111) surface and to assign the obtained Raman spectra. Thus, the purpose of this work is to quantitatively investigate the influences of the multivalency of the ligands and of the

Figure 1. (a) Monovalent 4-dimethylaminopyridine (DMAP) and five divalent derivatives (di-DMAP-2 to di-DMAP-6) with increasing spacer lengths investigated by SERS experiments and (b) synthesis of divalent DMAP-analogs di-DMAP-2 to di-DMAP-6.

methylene-spacer length on the binding behavior of the ligand to a gold surface.

2. EXPERIMENTAL SECTION 2.1. Materials. DMAP p.a. was purchased from ACROS Organics. Methanol was purchased from Sigma-Aldrich. All chemicals were used as received. Ultrapure water with a measured residual resistivity >18.2 MΩ cm was obtained from a Milli-Q Academic water purification system. Gold wire with a purity of 99.99% was purchased from Goodfellow. Glass plates with a size of 1 × 18 × 18 mm were obtained from Biosuplar. 2.2. Synthesis of the Divalent DMAP Analogs. The required divalent DMAP analogs di-DMAP-n (2 ≤ n ≤ 6) were prepared by two different methods (Figure 1b). The aromatic nucleophilic substitution29−31 using 4-chloropyridine hydrochloride as precursor with the corresponding diamine required fairly drastic reaction conditions and provided after careful purification di-DMAP-2, diDMAP-3, and di-DMAP-6 in low yields but sufficient quantities. All other attempts to achieve higher efficiency were not successful.32 The second method involves the deprotonation of 4-(methylamino)pyridine with n-butyllithium in THF to provide a reactive nucleophile. This intermediate was treated with 1,4-dibromobutane or 1,5dibromopentane, respectively, to afford di-DMAP-4 and di-DMAP-5 in good yields. In all cases, the products had to be purified by reversed-phase HPLC to remove side-products (e.g., the corresponding monosubstituted aminopyridine derivatives with an N-alkenyl group formed by HBr elimination in ca. 10% yield). This type of nucleophilic substitution could not be applied to the corresponding 1,2- or 1,3-dihaloalkanes.32 For di-DMAP-2, an alternative multistep route was developed involving a catalytic hydrodefluorination of the easily available tetrafluoro-substituted compound.33 2.3. SERS Experiments. The samples were prepared for SERS experiments as follows: glass plates were cleaned with aqua regia and then washed thrice with ultrapure water. Subsequently, they were coated by a thin gold film using a home-built electron beam 8668


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Langmuir Table 1. Concentrations of DMAP and Divalent Di-DMAP-n (2 ≤ n ≤ 6) Ligands in the SERS Experiments ligand DMAP di-DMAP-2 to di-DMAP-6

concentration (μmol/L) 0.2 0.1

1 0.5

2 1

10 5

20 10

evaporation apparatus. During the deposition process, the film thickness was monitored by a quartz crystal microbalance (Maxtek, model: MDC-360) with a systematic error of 0.1 nm. The deposition rate was kept constant at 0.01 nm/s. When the monitor quartz crystal microbalance indicated a gold film thickness of 50 nm, the evaporation was stopped. The topography of the gold surface was characterized by AFM (Neaspec, cf. ref 34), and the recorded data were analyzed by the free and open source software Gwyddion to determine the surface roughness of the samples.35 The surface roughness was determined from 30 randomly chosen profile lines yielding a surface roughness of 2.4 ± 0.4 nm. Before the SERS experiments, the gold-coated substrates were washed with piranha solution and then with ultrapure water three times. Subsequently, the substrates were immersed for 1 h in a solution of the mono- or divalent 4-(dimethylamino)pyridine derivatives in methanol. The concentrations are listed in Table 1. The concentration of monovalent DMAP was always twice as high as that of the divalent derivatives so that the number of functional groups binding on the gold surface was kept constant in all SERS experiments. Subsequently, the samples were dried in a gentle nitrogen flow and placed on a rotating disc spinning at approximately 60 rpm to minimize the thermal load by the focused laser beam that was used for SERS experiments and to ensure data reproducibility. SERS measurements of these samples were performed on a HORIBA Jobin-Yvon Raman microscope, (Olympus BX 41), which was equipped with a 50× objective. The SERS spectra were then recorded by a Dilor XY-800 Spectrometer from HORIBA Jobin-Yvon with a 2 cm−1 spectral resolution.36 The width of the entrance slit and pinhole was set to 100 and 1000 μm, respectively. The excitation wavelength was 632.8 nm [He−Ne cw laser, power: 30 mW, Melles Griot (25LHP-925−230)]. The instrument was calibrated against the 520.6 cm−1 line of crystalline silicon. The temperature was kept constant at 25 °C. For each measurement, the collection time was set to 10 s and each spectrum was averaged from five individual scans. SERS spectra were recorded in the range between 200 and 1800 cm−1. The substrates were cleaned after the measurements for subsequent use in an ultrasonic bath (Bandelin Sonorex, RK 512H, 215 W) using 1 M HCl solution, 1 M NaOH solution, and ultrapure water for three times and then they were dried in a nitrogen flow. Before reuse of the Raman spectrometer, a SERS measurement of the purified substrate was carried out to make sure that no traces of the previously adsorbed pyridine derivatives were present. All measurements were repeated at least five times using independent samples at the given concentrations compiled in Table 1. 2.4. Spontaneous Raman Measurements. Spontaneous Raman spectra were performed for all six DMAP ligands directly casted on a glass plate in the wavenumber range between ca. 100 and 3600 cm−1. We used the same backscattering geometry and experimental conditions mentioned above. However, the recording times were increased to 600 s for di-DMAP 5 and 60 s for the other samples (cf. Table 1). 2.5. Density Functional Theory Computations. 2.5.1. Free DMAP and Di-DMAP-n Ligands. Density functional theory (DFT) calculations of the free ligands DMAP and di-DMAP-n (2 ≤ n ≤ 6) analogs have been done by using the Gaussian09 software package.37 The density functional PBE38 has been applied for structure optimizations and calculations of vibrational frequencies. The correlation-consistent Dunning basis set has been applied for all calculations.39 All structures have been identified as minima structures by frequency calculations. Start structures for DMAP and di-DMAP-2 ligands have been taken from ref 40. Different conformers of diDMAP-3 and di-DMAP-4 have been determined by a force field-

50 25

100 50

200 100

500 250

1000 500

2000 1000

based conformer search. The 20 structures of lowest energy have been reoptimized at the DFT level of theory, and six conformers for diDMAP-3 and three conformers for di-DMAP-4 have been determined. On the basis of the di-DMAP-4 conformer structures, various di-DMAP-5 and di-DMAP-6 conformers have been generated by adding an extra CH2 group into the methylene-spacer and optimizing the structures. The PBE/cc-pVZT optimized structures have served as initial geometries for adsorption studies of the molecules on a gold surface. 2.5.2. DMAP and Di-DMAP-n in Interaction with a Gold Surface Model. Density functional theory (DFT) calculations of the ligands adsorbed on a gold surface have been performed by using the CP2K software package.41 The Au(111) surface was represented by three layers of 60 gold atoms. The maximum influence of the second layer has been estimated for the dissociation energy to be smaller than 12% and of the third to be smaller than 2−3%. The gold atoms have been fixed at distances determined by the experimental lattice constant (d = 4.08 Å).42 The cluster approach has been chosen to be able to perform the calculations for all ligand sizes avoiding moleculemolecule interactions. Furthermore, reasonable agreement of this approach has been obtained for DMAP and di-DMAP-2 regarding the geometric structure and dissociation energy compared to periodic surface calculations. The average size of periodic box was taken as a slab of 17 × 14 Å with the vacuum region of 25 Å between the slabs. The plane wave cutoff energy was selected as 280 Ry with the number of multigrids equal to 5 and was tested for convergence. Free ligands, optimized at the PBE/cc-pVTZ level of theory, have been placed at different starting orientations on the fixed gold surface cluster, and just the coordinates of the ligands have been relaxed. The optimization of the adsorption structures has been performed in three steps. (i) The initial search of the adsorption structure has been done by a semiempirical structure optimization using the PM6 method appended by dispersion correction43 and by utilizing a minimal basis set. (ii) The structures gained in (i) have been reoptimized at the DFT level of theory by using the PBE functional in combination with D3(BJ) correction.38,44 The pseudo potential GTH-PBE(q) (qH = 1, qC = 4, qN = 5, and qAu= 11)45,46 combined with the corresponding basis set DZVP-GTH-PBE for hydrogen, carbon, and nitrogen and with the TZ-GTH-PBE set for gold atoms have been employed. All calculations have been corrected for the basis set superposition error (BSSE). (iii) In order to include dispersion corrections for an extended surface, the dimension of the gold cluster surface has been extended to 330 atoms (containing 3 layers) and the dispersion correction [D3(BJ)] has been determined for the ligand adsorbed structure (ii) with the extended surface (iii) similar as in ref 40, whereas the DFT contribution to the energy has been taken from the results obtained in (ii). The dissociation energies determined in (iii) vary by about 6% from the values gained in (ii). The desired values of dissociation energies (DE) have been calculated by subtracting the sum of the plane surface and the lowest energy conformer from the total energy of the optimized adsorption structure. Furthermore, a rough surface has been modeled by taking three gold layers of the ideal (111)-surface and introducing a step in the two upper gold layers. The hexagonal slab contained 78 Au atoms with lateral cell dimensions of 17.3 × 14.4 Å along the directions of the lattice vectors. Facet edges were simulated by shaping the location of the atoms in the two uppermost layers as well. Also, adatoms and cluster simulating vertexes were added to the simulation cell of the ideal surface in order to model a rough surface. Surface relaxation effects have been neglected. It is assumed that with this simplified approach, the results, i.e., dissociation energies of the different adsorption structures, allow for a qualitative comparison. 8669


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Langmuir 2.5.3. Raman Spectra. The wavenumbers and intensities of Raman transitions for the free molecules and SERS spectra for adsorbed ligands have been calculated with the help of the software package Amsterdam Density Functional (ADF).47 Therefore, we have employed the PBE functional and the augmented triple-ζ polarized Slater type AUG/ATZP basis set, see ref 47. The dispersion D3 method with Becke-Johnson correction has been used, as before. These calculations are based on the application of the DIM/QM (discrete interaction model/quantum mechanics) method48 for SERS. In the current work, the PIM (polarizability interaction model) description of this method was chosen. The substrate is considered to be a collection of interacting atoms, each characterized by the atomic polarizability and atomic capacitance. In this way, optical properties of DMAP are described by taking the local environment of the gold surface into account.48 The calculated intensities of Raman transitions were slightly adjusted by changing the potential parameters given by Payton et al.48 for pyridine on a silver surface.

This is a so-called lightning rod-effect in SERS and other surface plasmonic phenomena.50 As the substrate used in this study is a polycrystalline gold film, such nanostructures are randomly distributed and might form fractal networks, which give rise to an inhomogeneous enhancement.51 Other side effects caused by the highly amplified laser field are nonlinear optical phenomena, such as surface-enhanced second harmonics generation, which can lead to photodecomposition of adsorbed organic species. SERS spectra of 4-methoxypyridine and its divalent derivatives were measured under similar conditions (results not shown here). In this case, photodecomposition took place so fast that the prominent breathing mode at 1007 cm−1 decreases and disappeared within at most 1 min after repeated scans of the samples, so that it was not possible to take proper SERS measurements. With regard to the adsorbates, those nanostructures are preferable binding sites. In order to consider the contribution from those nanostructures, the parameter b in eq 1 is introduced into the Langmuir isotherm to account for such contributions. Adsorption isotherms and corresponding fits according to eq 1 of the monovalent DMAP and the other five divalent diDMAP-n (2 ≤ n ≤ 6) ligands under study are displayed in Figure 2.

3. RESULTS AND DISCUSSION 3.1. Binding Affinity of Ligands on a Gold Surface. In order to extract a quantitative relationship between the pyridine concentration and the SERS signal intensity, the intensities of the breathing and trigonal modes at ca. 1007 and 1035 cm−1, respectively, were recorded for different ligands comprising of DMAP and di-DMAP-2 to di-DMAP-6. Typical fits of line shapes from such calculation are shown for two ligands in Figure S1. In the case of the di-DMAP-4, di-DMAP5, and di-DMAP-6 spectra, this analysis required first to deconvolute these peaks and to fit both modes by Voigt profiles. The SERS spectra of the three other ligands have no substantial overlap of these modes so that a direct Voigt fitting was performed (cf., Figure S1). The band intensity corresponds to the integrated area under the bands obtained from this analysis. The intensities of the breathing mode as a function of the pyridine concentration of all six DMAP derivatives under study are summarized in Table S1. Adsorption isotherms of the ligands on gold are monitored by the SERS intensities of the pyridine ring breathing mode occurring at approximately 1007 cm−1. It is assumed that the concentration-dependent surface coverage θ(c) is proportional to the intensity I(c) of this breathing mode. On the basis of this assumption, the SERS intensity of the breathing mode of the pyridine ring as a function of the pyridine concentration can be fitted by a modified Langmuir isotherm (see eq 1), which is justified due to the concentration of the ligands in submonolayers ≤ 1 ML (see Table 1), so that interactions between the adsorbates are avoided and the adsorption sites are considered to be equivalent to each other, similar to previous work.5 Here, KL is the equilibrium constant of the adsorption/desorption equilibrium of DMAP or the divalent DMAP ligands adsorbed on the gold surface. θ(c) ∝ I(c) = θmax

KLc +b 1 + KLc

Figure 2. SERS intensities of the totally symmetric breathing mode of pyridine moieties at approximately 1007 cm−1 (as indicated in Figure S1) of DMAP (■) and the divalent di-DMAP-n (2 ≤ n ≤ 6) derivatives [di-DMAP-2 (red ●), di-DMAP-3 (blue ▲), di-DMAP-4 (magenta ▼), di-DMAP-5 (green ◆), and di-DMAP-6 (dark blue left-pointing triangle)] bound on a planar gold surface as a function of the concentration (from 0.2 to 200 μM, cf. Table 1) of pyridine groups in solution. When the concentration exceeds 200 μM (up to 2000 μM), the SERS intensities from all six ligands are almost constant and are not shown here for clarity. The dashed lines are fits according to eq 1.

Evidently, the binding constants of the DMAP ligands, listed in Table 2 and displayed in Figure 2, exhibit an oscillatory odd−even behavior as a function of spacer length, saturating for long ligand chains (n ≥ 4). This saturating value for diDMAP-4 and di-DMAP-6 is enhanced by 6 to 8 times, as compared to DMAP and di-DMAP-3. This oscillatory trend in the curve, followed by saturation, is surprisingly close to a related study on a different system published by Walczak et al.52 There, the dependence of infrared absorption intensity of several CH3-modes as a function of the spacer length in nalkenothiolates adsorbed on a silver surface was communicated. First principles calculations have been performed in

(1)

The constant b in eq 1 corresponds to a ligand-independent background of the SERS spectra and is slightly different from the similar KL formulation in ref 49 for SERS from mercaptobenzimidazole (MBI) on gold nanoparticles. On a polycrystalline gold surface exist lots of gold clusters, terraces, and facets in the nanometers range. Unlike mesoscopic structures with sizes between 50 and 100 nm, which provide size-independent amplification of the incoming laser light, those nanostructures may act as extraordinary antennas: the sharper the nanostructures are, the higher the intensification. 8670


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Langmuir Table 2. Equilibrium Constant KL for DMAP and DiDMAP-n (2 ≤ n ≤ 6) Ligands, as Derived from the SERS Intensity of the Totally Symmetric Breathing Mode at 1007 cm−1 species DMAP di-DMAP-2 di-DMAP-3 di-DMAP-4 di-DMAP-5 di-DMAP-6

species, the motion amplitude of all carbon atoms in the pyridine ring can deliver the vibrational energy to the spacer, whereas its nitrogen counterpart on the opposite side of the ring is barely moving. Consequently, in this case, the adsorption effects on the SERS intensity will be mostly described by the projection of horizontal polarization vector of the incoming light on the C2v symmetry axis of the pyridine ring and thus solely by the type of adsorption structure. In contrast, for the trigonal mode, shown in Figure 3b, the vibration is yielding a strong displacement of the ring nitrogen locally bound to gold and two other carbon sites, whereas the carbon site directly bound to the spacer is hardly moving. In this case, the intensity of the SERS line will depend not only on the projection of the polarization vector on the pyridine ring but in addition it gets modulated by the angle of this vector to the Au−N bond. As a result, the SERS intensity will strongly oscillate, depending on the lateral position of the ligand on the surface, thus quenching useful information on the vertical orientation of the adsorption structure. In addition, the intensity of the trigonal mode is significantly weaker than the totally symmetric breathing mode (in most cases less than 25%), which leads to a smaller signal-to-noise ratio and to a lower fitting accuracy. As a result, solely the totally symmetric breathing mode can be reliably chosen as a fingerprint for spacer effects on adsorption for the studied ligands, whereas the trigonal mode does not meet this requirement. The equilibrium constants KL derived from the totally symmetric breathing mode intensity via eq 1 are listed for DMAP and di-DMAP-n (2 ≤ n ≤ 6) ligands in Table 2. These are plotted against the number of methylene units in the spacer between the two pyridine rings, as shown in Figure 3c. 3.2. First Principles Calculations of the Adsorption of DMAP Ligands on a Gold Surface. In order to explain the distinct dependence of the binding constant KL on the length of the spacer chain, one needs to gain detailed information of binding structures of DMAP and all divalent di-DMAP-n ligands on a gold surface. Therefore, first principles calculations have been performed to understand the adsorption structures at the surface and to compare the dissociation energies of different possible structures. The ligands have been investigated in interaction with an ideal Au(111) surface model as well as with different models of stepped surfaces. The goal of introducing a stepped surface into the model was to demonstrate the importance of surface roughness as a key element to obtain agreement between theoretical and experimental results. In our calculations, we considered a single molecule adsorption and neglected coverage effects. In addition, first principles calculations of measured SERS spectra in the low (200−500 cm−1) and midwavenumber ranges (500−1800 cm−1) have been performed in order to distinguish between atop chemisorbed ligands and flat physisorption of the ligands. 3.2.1. Model for the Interaction of DMAP and Di-DMAP-n Ligands with an Ideal Au(111) Surface. For DMAP two main adsorption modes at an ideal Au(111) surface have been reported earlier,27 the vertically oriented one (1N), and the parallel to the surface plane oriented one (1R). For di-DMAPn conformers adsorbed on a Au(111) surface cluster, five main adsorption structures have been found. These are shown in their binding geometry in Figure 4: (i) a parallel or nearly parallel adsorption structure (2R) with the molecule-surface interaction by the π-system of the two pyridine rings and the spacer unit (see Figure 4a, ref 40); (ii) a vertical adsorption

equilibrium constant KL derived from breathing mode (μM−1) 0.028 0.130 0.031 0.170 0.140 0.160

± ± ± ± ± ±

0.006 0.020 0.008 0.040 0.020 0.020

order to understand the oscillations in the binding constants as a function of the spacer length of DMAP and di-DMAP-n ligands. These results are discussed in the following section. In order to make the correct choice of vibrational mode for the analysis of spacer effects, one can compare the atomic displacements of the totally symmetric breathing (1007 cm−1) and trigonal breathing (1035 cm−1) modes shown for diDMAP-2 in Figure 3a,b, respectively (cf. Figure S1). Both modes are expected to deliver a reasonable gain in SERS spectra, by using previous work on the adsorption of 4methoxypyridine on gold nanoparticles.53 Figure 3a indicates that for the totally symmetric breathing mode in adsorbed

Figure 3. (a) Main atomic displacements of the ring atoms for diDMAP-2 adsorbed on gold (yellow spheres) in the totally symmetric breathing mode at νSERS = 1007 cm−1; (b) main atomic displacements of the ring atoms for di-DMAP-2 adsorbed on gold (yellow spheres) in the trigonal breathing mode at νSERS = 1035 cm−1; and (c) binding affinity of divalent DMAP derivatives on a rough gold surface quantified by the binding constant KL fitted from the totally symmetric breathing mode as a function of the number of CH2 groups in the spacer between the two pyridine rings (see Figure 1). Error limits can be found in Table 2, which are omitted here for clarity. The dotted line is a guide to the eye. For a comparison, the value of KL for monovalent DMAP is also shown. 8671


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Figure 4. Calculated adsorption structures of di-DMAP-n molecules on an ideal Au(111) surface: (a) parallel adsorption structure 2R (shown for di-DMAP-2); (b) vertical adsorption structure 2N (shown for di-DMAP-2); (c) combined adsorption structure RN (shown for di-DMAP-2); (d) vertical adsorption structure 1N (shown for diDMAP-3); and (e) parallel adsorption structure 1R (shown for diDMAP-3).

structure, in which two nitrogen atoms of the pyridine rings bind to two gold surface atoms (2N) with the normals of the pyridine ring planes oriented almost parallel to the gold surface (see Figure 4b, ref 40); (iii) a combined adsorption structure with one pyridine unit binding via the π-system and the other one binding via the nitrogen atom to the surface (RN, see Figure 4c); (iv) a vertical adsorption structure with one endocyclic nitrogen atom binding to the surface (1N), whereas the other unit is not interacting with the surface (see Figure 4d, ref 40); and (v) a parallel adsorption structure with one pyridine unit binding via the π-system (1R), while the other unit is not interacting with the surface (see Figure 4e). The direction of the normal to the plane of the pyridine ring is marked by a red arrow, color code: Au (yellow), C (gray), N (blue), and H (white). Adsorption on a rough surface is shown in Figure 5. For clarifying the adsorption structures and the used terminology, we have also derived from these results a simplified sketch to show more clearly the adsorption modes of DMAP and di-DMAP-n on a flat as well as a rough gold surface, as shown in Figure 6. Table 3 summarizes for the Au(111) surface the dissociation energies (DE) per DMAP unit for all ligands bound to the Au(111) surface model. The results for DMAP and di-DMAP-

Figure 6. Schematic overview on basic adsorption structures of DMAP and di-DMAP-n (2 ≤ n ≤ 6) ligands, listed in Tables 3−5 and Figures 4 and 5: (a) structures bound to the flat Au(111) surface; (b) structures bound to steps or terraces by either nitrogen atoms or pyridine rings; and (c) structures bound to steps or terraces by nitrogen atoms and pyridine rings. Pyridine is symbolized by a rectangle; nitrogen N is only shown if it binds to the gold surface (yellow shaded areas). The alkyl chain is symbolized by a black line between the pyridine moieties.

2 on the Au(111) surface model are in qualitative agreement with their adsorption structures and dissociation energies of periodic DFT calculations with dispersion correction.27,40 For the different di-DMAP-n ligands, all five adsorption structures can be found. However, in contrast to the other divalent ligands, di-DMAP-3 is not stable in the RN structure and transforms easily into the 2R structure. For di-DMAP-2, the 2N adsorption structure exists in two conformers, differing in their Nring−Nring distances of 5.26 and 7.04 Å, respectively40 (see Table 3).

Figure 5. Examples of calculated adsorption structures of DMAP and divalent DMAP ligands bound to a step on a Au(111) surface: (a) tilted 1N adsorption structure of DMAP; (b and c) two different 2N adsorption structures shown for di-DMAP-2; (d) 2R adsorption structure shown for diDMAP-2; (e) 1ST (RN) combined adsorption structure of DMAP; (f) tilted 1ST (RN) adsorption structure of DMAP; (g) 2ST (RN) adsorption structure shown for di-DMAP-3; and (h) 2ST (RN) adsorption structure shown for di-DMAP-5. The Nring−Nring connection line for the divalent ligands in (b−d) is oriented toward the step edge, and in (g and h) it is oriented along the step edge. See also Figure 6 for a schematic view. 8672


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Table 3. Dissociation Energies (DE) Calculated per DMAP Unit for the Different Adsorption Structures of DMAP and DiDMAP-n (2 ≤ n ≤ 6) on a Au(111) Surface Model Calculated at the PBE-D3/DZVP/TZ-GTH-PBE Level of Theory adsorption structure

ligand type

DE per DMAP unit (kcal/mol)

adsorption structure

ligand type

DE per DMAP unit (kcal/mol)

1N

DMAP di-DMAP-2 di-DMAP-3 di-DMAP-4 di-DMAP-5 di-DMAP-6 di-DMAP-2

−18.3 (−18.1)a −16.1 −15.2 −17.4 −16.9 −20.6 −11.9 (−8.5)b −15.0 −19.2 −16.0 −17.2 −18.2 −19.8 not stable −22.4

1R

2R

DMAP di-DMAP-2 di-DMAP-3 di-DMAP-4 di-DMAP-5 di-DMAP-6 di-DMAP-2

−29.1 (−24.8)a −29.4 −28.1b −32.9 −32.6 −39.8 −24.0 (−15.3)b

RN

di-DMAP-3 di-DMAP-4 di-DMAP-5 di-DMAP-6 di-DMAP-5

−26.8 −29.2 −31.6 −37.8 −22.2

di-DMAP-6

−20.3

2N

RN

di-DMAP-3 di-DMAP-4 di-DMAP-5 di-DMAP-6 di-DMAP-2 di-DMAP-3 di-DMAP-4

a

The result was taken from ref 27 for a comparison. bThe result was taken from ref 40 for a comparison.

The rapid increase in dissociation energy for the 2N conformer of di-DMAP-3 is caused by a better accommodation to the Au lattice resulting in a short distance of both N atoms to gold (dAu−N = 2.44 Å). For all ligands, the divalent parallel orientation (2R) of the molecules on the surface is preferred compared to the divalent vertical one. Thus, the binding nature of these ligands to the gold surface is predominantly dispersive. A separation of the DFT-D3 energy parts for the horizontally adsorbed ligands shows that the DFT contribution is smaller than 10%, and the dispersion correction determines the dissociation energy. In contrast, the vertical adsorption structures exhibit a DFT contribution of about 25%, indicating a small covalent bonding contribution between the N lone pair orbital and the Au d orbitals. For the preferred parallel adsorption structures, the dissociation energies per DMAP unit are smaller for di-DMAP-(2−4) compared to the monovalent DMAP. However, due to the increasing length of the spacer unit that also interacts with the gold surface, the adsorption energies increase from di-DMAP-2 to di-DMAP-4. The ligands di-DMAP-(5,6) show an even larger increase in the dissociation energies per DMAP unit as compared to the monovalent DMAP. The vertical N-binding adsorption structure is for all ligands less preferred and shows for most of the divalent molecules smaller dissociation energies than for the monovalent DMAP. The reason is the tilted adsorption structure, in which the divalent molecules cannot bind ideally to the gold surface.40 However, with increasing spacer length the dissociation energy converges to the one of DMAP. Furthermore, the adsorption studies of DMAP and the divalent di-DMAP-n ligands binding to an ideal Au(111) surface cannot explain the experimental findings depicted in Figure 3c. We assume that the surface roughness can lead to qualitatively different adsorption structures27 compared to that of an ideal Au(111) surface. Thus, in the next step, we have modeled the interactions of DMAP and the divalent ligands to a stepped surface model in order to account for possible contributions to ligand binding due to the surface roughness. 3.2.2. Model of the Interaction of DMAP and Di-DMAP-n Ligands with a Stepped Gold Surface. In order to model the roughness of the gold surface, a surface step model has been employed.54 Mollenhauer et al. have shown that by switching from a flat Au(111) surface to gold adatoms on the surface the

binding nature of pyridine derivatives is changing from van der Waals (vdW) to chemical binding.27 Therefore, DMAP and the divalent DMAP ligands have been investigated in interaction with two vicinal surfaces: (i) {111}-type step (miscut angle along the [2̅11] direction) and (ii) {100}-type step (miscut angle along the [21̅1̅] azimuth). An overview of the fundamental adsorption structures, i.e., three for DMAP and five for the divalent DMAP ligands, to the stepped surface are shown Figure 5a−h. The most preferred adsorption mode for the monovalent DMAP combines maximal binding via the pyridine nitrogen atom and the π ring system (1ST) (see Figure 5e). The adsorption structures of the divalent ligands connected to surface steps can be divided into two main categories: the first group shows an almost perpendicularly aligned Nring−Nring connection line which is aligned almost perpendicular to the step boundary. The corresponding results are shown in Table 4. An overview of the fundamental adsorption structures to the stepped surface are schematically shown in Figure 6. The dissociation energies of divalent 2N adsorption structures are decreased compared to the monovalently adsorbed DMAP except for di-DMAP-3, see Table 4 and the adsorption structure in Figure 5c. The 2N adsorption structures for diDMAP-4, -5, and -6 derivatives are not stable at the step and convert easily into a 2ST adsorption structure, which is a RNlike structure with a parallel binding to the surface step (see Figure 5h and Figure 6). The divalent 2R bound ligands are connected to the step via dispersive interactions of the πsystem (see Figure 5d and Figure 6). Both rings of the 2R adsorption structures nearly accommodate the shape of the step (see Figure 5d). The distances of the pyridine units to the stepped surface are slightly increased, i.e., up to 8%, compared to the one of the Au(111) surface. The dissociation energies per DMAP unit of the 2R adsorption structures of the divalent ligands are less strong than the 1R adsorption structure and the 1ST adsorption structure of the monovalent DMAP. This indicates no multivalent enhancement for the 2R bound ligands to the stepped surface. In summary, for all adsorption structures with an Nring−Nring connection line perpendicular to the step boundary, the spacer length is too short for a flexible ring accommodation. Moreover, the dissociation energies of 8673


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dispersive binding of the spacer to the planar surface region. For the ligands di-DMAP-2 and di-DMAP-3, the spacer length is too short to result in a parallel alignment of both pyridine rings relative to the surface, so that one ring is tilted (see Figure 5g). In contrast, both pyridine units of the ligands diDMAP-4, -5, and -6 are almost parallel aligned to the surface (see Figure 5h). The pyridine ring orientation, its flexibility with increasing spacer length, and the additional increase of the dispersive interaction of the spacer with the gold surface explain the increasing dissociation energies from di-DMAP-2 to di-DMAP-6. A comparison of all adsorption modes to the modeled gold step shows that nearly all ligands under study energetically prefer the ST adsorption structure. With consideration of the adsorption to the Au{111} type step, the di-DMAP-4, -5, and -6 ligands show similar or slightly higher dissociation energies per DMAP unit than the monovalent DMAP. In contrast, the dissociation energies per DMAP unit of di-DMAP-2 and -3 are decreased compared to the monovalent ligand. The 1R adsorption structure of di-DMAP-3 is energetically similar to the ST adsorption structure. A similar picture is obtained for the adsorption of the ligand binding to the Au{100} type step with the difference that the dissociation energies are slightly higher and that for di-DMAP-2 the 2R adsorption structure is energetically preferred. In summary, for the adsorption of the divalent ligands to gold steps, there is partially a small multivalent enhancement for large spacer lengths observed. The monovalent and the divalent di-DMAP-ligands exhibit the ST adsorption structure at the stepped gold surface with a strong contribution of the nitrogen-gold interaction, ring-gold interaction, and spacergold interaction. Considering the preferred adsorption structures to both modeled steps, multivalent, i.e., electronic, enhancement is obtained for long spacer lengths (n > 4 for a Au{111} type step and n = 6 for a Au{100} type step. For clarifying the adsorption structures and the used terminology, we have also derived from the modeling results a simplified sketch to show more systematically the adsorption structures of DMAP and di-DMAP-n on a flat as well as a rough gold surface, as shown in Figure 6. Figure 6a is presenting a complete set of all possible combinations of ligand binding to the flat Au(111) surface formed by Au−N bonds and the πsystem of pyridine rings (see Table 3). The structures on a stepped surface are grouped into the weakly bound modes (see Figure 6b and Table 4) and strongly bound ones (see Figure 6c and Table 5). The first group (R, N type) is bound to the step either by pure Au−N bonds or solely by π-electrons. The latter group (ST type) is bound by all possible Au−N bonds and the π-systems simultaneously. In the former case, the spacer of divalent ligands is oriented perpendicularly to the line of the step, whereas in the latter case the ligands are bound parallel to the step. This part of the investigation clearly shows that for increasing spacer length the interaction to the surface steps, as well as to the ideal surface, is increasing due to the interactions of the spacer to the surface and an optimal matching of the pyridine moieties to the surface due to higher spacer flexibility, which can explain the experimental results shown in Figure 3c, except for di-DMAP-2. Note that the coverage under experimental conditions is in general higher than the one considered in the model calculations using a periodic cell, where the size of the cell was chosen to avoid van der Waals interactions between the ligands.

Table 4. Dissociation Energies (DE) per DMAP Unit for the Different Adsorption Structures of DMAP and Di-DMAP-n (2 ≤ n ≤ 6) on a Stepped Gold Surface Model, for which the Nring−Nring Connection Line Is Almost Perpendicular to the Step Boundary, Calculated at the PBE-D3/DZVP (TZGTH-PBE) Level of Theory DE/DMAP unit (kcal/mol) adsorption structure

ligand type

1N tilted 2N

DMAP di-DMAP-2

1R 2R

di-DMAP-3 DMAP di-DMAP-2 di-DMAP-3 di-DMAP-4 di-DMAP-5 di-DMAP-6

Au {111} type step

Au {100} type step

−14.4 −16.8 −16.5 −21.8 −29.1 −16.2 not found −18.4 −23.5 −25.8 −25.4

−17.6 (−18.8)a −15.0 −18.6 −21.0 −29.1b −17.6 −24.3c −24.0 −26.4 −23.8 −23.1

a

The result of the adsorption of DMAP to a single adatom at a Au(111) surface was taken from ref 27 for a comparison. bThe dissociation energy of DMAP to the stepped surface has the same value as the one to the ideal surface, see Table 3. cDouble step 2R conformer, bound to three adjacent terraces.

the 2R adsorption structure to the stepped surface are not exceeding the ones of the ideal Au(111) surface (see Table 4). The second group (see Table 5) contains a Nring−Nring connection line of the ligands, which is almost parallel to the Table 5. Dissociation Energies (DE) Calculated per DMAP Unit for the Different Adsorption Structures of DMAP and Di-DMAP-n (2 ≤ n ≤ 6) on a Stepped Gold Surface Model for Which the Nring−Nring Connection Line Is Almost Parallel to the Step Boundary Calculated at the PBE-D3/ DZVP (TZ-GTH-PBE) Level of Theory DE/DMAP unit (kcal/mol) adsorption structure 1ST tilted 1ST 2ST

ligand type

Au {111} type step

Au {100} type step

DMAP DMAP di-DMAP-2 di-DMAP-3 di-DMAP-4 di-DMAP-5 di-DMAP-6

−27.2 −31.6 (−32.1)1 −21.5 −30.0 −33.6 −32.9 −32.3

−25.5 (−25.5)a −37.7 −21.6 −32.2 −36.0 −35.4 −39.1

a

The result of the adsorption of DMAP to a single adatom at a Au(111) surface was taken from ref 27 for a comparison.

step. In these adsorption structures, the nitrogen atoms of the pyridine ring system interact with gold atoms of the step with a gold nitrogen (ring) distance dAu−N of about 2.4 Å. Furthermore, the π-systems of the pyridine moieties interact with the lower part of the modeled step. This 2ST adsorption structure is the energetically most favorable one for all divalent ligands. The reasons for this result could be that these adsorption structures combine the binding of nitrogen atoms in the pyridine ring to low coordinated gold atoms of the step, strong dispersive interactions due to the planar orientation of the pyridine units to the planar gold surface region, and 8674


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Langmuir 3.3. Analysis of the Adsorption Structures of DMAP Ligands. It is well-known that not only the value but also the sign of the shift for the breathing mode is sensitive to the surface roughness. This effect was explained for pyridine adsorbed on silver in ref 55 by changes in the force constant κ considered for two different cases. It turned out that κ drops from 5.7 × 10−9 N/m on a rough surface to essentially zero on a smooth surface. This can be rationalized by high index facets occurring on rough surfaces, which tend to attract electrons and form stronger bonds. In the following, we have used the results from model calculations for assigning the results of the SERS spectra. 3.3.1. Behavior of the Binding Constant KL. In order to explain the experimentally observed anomalies in the binding constant KL, we conducted the analysis of the whole calculated SERS spectra for different di-DMAP-n ligands. To determine the KL curve, the SERS intensities of the totally symmetric breathing mode (1007 cm−1) was used. This mode belongs to a′ species of the Cs symmetry group listed for DMAP for the pyridine ring-metal systems.56,57 In C2v symmetry of the pyridine ring, a′ → a1 + b1, and this sum is governed by its a1 component. Since the experiment was done using s-polarized laser radiation, the SERS intensity is delivered solely by the |αyy|2 and |αzy|2 components of the polarizability tensor for pyridine rings positioned in the y−z plane vertically to the gold surface (cf. refs 58 and 59). For the horizontal orientation of the molecular y−z plane, the contributions of |αzz|2 and |αyz|2 should be added. Thus, the difference between the scattered field, coming from flat-on (R-type) and vertical (N-type) pyridine ring adsorption geometries for breathing vibration can be reliably estimated by the tilt angle β between the molecular plane and the gold surface, containing the field vector of spolarized light. For divalent ligands its value should be averaged for both pyridine rings. Although the local field enhancement factors I/Iincident are usually taken as |Etotal|4/| Eincident|4 (cf. ref 48), there are strong limitations to this law imposed by the chosen SERS model, as noted before.60 Note that the best fit to a KL curve in this work is achieved by using 3 the third power of the electric field, Iligand SERS ∼ cos β. The ligand expression for ISERS is neither dependent on the surface coverage nor on the ligand concentration, as the experimental SERS intensity in eq 1 does. Here, Iligand SERS reflects the adsorption properties of a single molecular species and therefore can be linked directly to the binding constant KL. Due to the elevated experimental surface coverage conditions of adsorbed molecules, as pointed out above, neither the flat 2R structures of ligands shown in Figures 5d and 6a nor the flat 1R structure for DMAP (see Figures 4e and 6a) were needed to be considered as the starting geometries for optimization due to steric hindrance arguments. As a result, the vertical arrangement of the ligands is preferred at high surface coverage.61−63 This assumption is further supported by ref 49 reporting on the vertical orientation of ligands on gold at packing conditions close to 1 ML. A perpendicular orientation for the heteroatomic ring system of pyridine derivatives (npyridinemethanol and n-pycolylamine) relative to the surface of silver nanoparticles was also reported, as derived from the vibrational analysis of SERS spectra.64 Finally, SERS studies on derivatives of benzothiazole-2-thione adsorbed on gold indicate chemisorption by N and exocyclic S atoms, with the molecular plane perpendicular to the metal surface.65 Therefore, in accordance with the present experimental work, the optimizations were started solely from vertical 1N (2N)

structures in order to simulate the migration and accommodation of ligands on terraces, and facets, as well as formation of 1ST (2ST) structures bound to surface steps. Low KL values are observed for the monovalent DMAP in Figure 7, which is associated mostly with vertical 1N

Figure 7. Dependence of the experimental KL (right ordinate) and calculated SERS signal intensities (left ordinate) per pyridine ring on the number of pyridine ring spacers in DMAP ligands for the adsorption structures on the {111}-step of the Au(111) surface (red line). The experimental KL calculated from SERS studies (cf. Figure 3c) is shown as a solid blue line for the totally symmetric breathing mode (ν1 = 1007 cm−1). The expected signal for 2R structures on an ideal surface is indicated for a comparison by a black line.

conformers distributed over the terraces as well as bound to steps, as shown in Figures 5a,f and 6. In both cases, the plane of the molecule is aligned almost perpendicular to the surface. The significant increase in KL for di-DMAP-2 is due to the 2N conformer, as shown in Figure 5b, with the largest Nring−Nring distance of 10.3 Å. Since one of its rings is almost parallel to the upper terrace, the SERS signal is expected to be moderately large. The step-bound 2ST structure is not reached from the initial stable 2N adsorption conformer, as follows from test MD simulations. Further optimization attempts, starting directly from the flat 2ST structure, were not successful, as well. This is explained by the short di-DMAP-2 spacer length, being unable to accommodate the in-plane geometry of both rings on the lower part of a terrace. The contribution to the SERS signal of another di-DMAP-2 conformer, which is oriented with both ring planes and nitrogen gold bonds almost perpendicular to the terraces (see Figure 5c and Table 4a), is expected to be moderate. In this case, only the nondiagonal polarizability components are active, since the incident radiation is polarized almost perpendicularly to the main symmetry axis of both pyridine rings. The decrease in the binding constant for di-DMAP-3 can be explained by the same 2N conformer shown in Figure 5c with the ring planes and nitrogen gold bonds oriented almost perpendicular to both step terraces. As for di-DMAP-2, the 2ST structure was never reached in the course of MD simulations starting from the initial stable 2N adsorption structure. All adsorption structures for di-DMAP-2 and -3 considered above belong to the adsorption group with the Nring−Nring connection line perpendicular to the step boundary (see Table 4a) and have adsorption energies, which are comparable to the ideal surface case. 8675


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Figure 8. Comparison of theoretical Raman and SERS spectra computed for several considered free and adsorbed di-DMAP-n (2 ≤ n ≤ 6) ligands in the harmonic approximation with the experimental results: (a) DMAP; (b) di-DMAP-2; (c) di-DMAP-3; (d) di-DMAP-4; (e) di-DMAP-5; and (f) di-DMAP-6 (PBE-D3/AUG-ATZP level of theory). Experimental SERS spectra are marked by solid black lines, and calculated SERS spectra correspond to solid blue lines. The experimental SERS spectrum of pyridine on polycrystalline Au is shown in (a), indicated by a dashed red line. Raman spectra of the free molecules are indicated by black (experiment) and blue (calculated) dashed lines. The intense spectral features of all species indicate the dominance of the breathing mode, as is typical for vertical adsorption structures (see text for further details).

structure at the step. In this case, the Nring−Nring connection line is aligned along the step boundary, with horizontal Austep− N bonds being almost in one line with the C2v axis of the relevant pyridine rings. The resulting Iligand SERS dependences on the spacer length, calculated by using the given relations, fully reproduce the oscillatory behavior and saturation in the experimental KL curve shown in Figure 7. The upper black

A strong increase in KL detected for di-DMAP-4, -5, and -6 can be unequivocally related to the appearance of the strongly bound 2ST adsorption structure (see Figures 5 and 6) and Table 4b). All conformers starting from the 2N structure, typical for the initial vertical attachment of molecules to the surface under high coverage conditions, are rapidly transformed in the course of geometry optimization to a flat 2ST 8676


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Langmuir curve, calculated for the flat adsorption structure on an ideal gold surface for comparison, delivers practically the same KL value for all types of ligands. The slight disagreement between the experimental and modeled curve for a stepped surface solely for di-DMAP-5 (cf. Figure 7) is within the error limits of the model. This is due to the uncertainty in the average of the lateral position of its different conformers on the surface. As discussed in section 3.1, there is still some dependence of the SERS intensity as a function of the angle of the local Au-N bond to the surface. This is specifically the case for the breathing mode, which is smearing out the information on the vertical orientation of the ligands. 3.3.2. Calculation of SERS Spectra of DMAP and DiDMAP-n (2 ≤ n ≤ 6) Ligands at Gold Surfaces. The general spectral features of SERS spectra taken for DMAP and diDMAP-n (2 ≤ n ≤ 6) ligands at a gold surface, corresponding to the mid-IR regime, i.e., 400−1800 cm−1, see Figure 8, are not exhibiting a pronounced dependence on the type of the ligand. Similarly, the odd-even oscillations in IR intensity of the methylene modes, as shown in ref 52, do not exceed 30% on top of a constant background, as well. This background, described by the constant b in eq 1, is produced by all N type conformers vertically adsorbed on steps, facets, and terraces. Figure 8a−f clearly indicates that there is a significant enhancement in intensity of the breathing mode a′ (950−1000 cm−1) for all four considered ligands, which is accompanied by damping of almost all other spectral components. A strong molecular mode a′ at 750 cm−1, reflecting the symmetric ringN-methyl vibration, is getting weak or even vanishes. This can be explained by the complete absence of an axial αzz polarizability component in the vertically positioned pyridine ring for s-polarized light and is an indication for the N type adsorption geometry. In contrast, the a′ vibration at 1600 cm−1, which is mostly given by the a1-type of CC deformations in pyridine rings (called “semicircle stretch” in ref 56), remains strong for all ligands, which is due to the αyy and αyz tensor elements. The detailed assignments of all modes are given in Tables S2−S4). The intense spectral features of all species shown in Figure 8 indicate the dominance of the breathing mode, proving the expected preference of vertical adsorption structures due to high di-DMAP-n (2 ≤ n ≤ 6) coverage in the experimental spectra, mostly corresponding to 2N adsorption. The average of all these spectra is contributing to the constant b in eq 1, whereas the oscillations in intensity of the breathing mode reflect different binding strengths. The dominance of the breathing mode in all SERS spectra of Figure 8 can be partially related to the overlapping contribution of the out-of-plane N-H-Au vibration delivered by the nitrogenbound proton for some part of protonated ligands present on the surface. An exemplary analysis was initially outlined by Payton et al.48 for SERS of pyridine adsorption on face, edge, and top sites of Ag nanoparticles and by Zhao et al.66 for small Ag aggregates (ADF code). The average blue-shift of 25 cm−1 for the breathing mode observed in the present work for DMAP and di-DMAP-n (2 ≤ n ≤ 6) on Au closely matches the value of 17 cm−1 given in ref 48 for pyridine on Ag. This behavior clearly correlates with the SERS curves depicted in Figure 8a−f (blue solid lines) and is typical for adsorption of pyridine at edge sites,66 serving as a prototype for an adsorption at steps on the Au surfaces. In addition, Figure 9 demonstrates another typical case for an optimized structure of di-DMAP-2 on the facet edge of the

Figure 9. Typical vertical adsorption geometry of di-DMAP-2 adsorbed on a facet edge of a (1̅1̅2)− (112) Au vicinal surface boundary. There is no direct π-coupling between the vertically oriented rings and the gold surface, explaining the dominance of the breathing mode in Figure 8b and the contribution to the constant b in eq 1. The experimental surface roughness corresponds to 6 to 7 of such steps.

(1̅1̅2)− (112) Au vicinal surface boundary. The dissociation energy is similar to the values listed in Table 3, corresponding to −28.3 kcal/mol per DMAP unit. The facet adsorption structure is evidently another example of a vertical adsorption structure yielding the Raman spectra shown in Figure 8b. As a result, the large distance to the surface prevents a direct πelectron coupling to the Au surface, which can be traced to the dominance of breathing mode in Figure 8b. It should be mentioned here that the experiments are averaging the signal over a large sample area. This area may also contain ligands adsorbed on edge sites of surface steps of facets, as shown in Figure 9, or even on single Au clusters. The effect of such fractal clusters on SERS intensity and the role of surface carbonization were discussed before.67,68 An opposite effect has been observed by Lange et al.53 for the SERS dependence of 4-methoxypyridine (4-MP) adsorbed on Au(111). The absence of a spectral shift and a moderately selective enhancement in the totally symmetric breathing to the trigonal mode ratio, Ibreath/Itrigon, from 7 to 10 was related to the direct coupling of 4-MP to the gold surface through π-electrons of the pyridine ring, implying a flat orientation on the gold surface. The integral SERS intensity enhancements observed both in this work and in ref 53 are yielding a factor of 6, which is inconsistent with a plasmon model, yielding gain factors on the order of 105. A charge transfer model with a typical gain factor of 103 is also unlikely to be active in the present case (see the relevant discussion in ref 66). As a result, it can be concluded that the chemical enhancement mechanism is dominant for di-DMAP-n, thus allowing us to use the static polarizability for calculations of SERS spectra. 3.3.3. Spacer Effect on the Adsorption of DMAP and DiDMAP-n (2 ≤ n ≤ 6) Ligands on a Au(111) Surface Detected by SERS. Here, we summarize the arguments given above, so that it is concluded due to steric reasons at high surface coverage resembling the experimental conditions that most studied ligands are preferentially adsorbed in a vertical orientation (2N), even on a rough gold surface. Still, even for light that is polarized in the surface plane, a sufficiently strong SERS signal from the breathing mode can be detected, since the planes of the adsorbed pyridine rings are angled at 60°−70° relative to the surface. The observed intense breathing mode in both the experimentally observed and the 8677


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Langmuir calculated SERS spectra is excluding the direct π-coupling between the pyridine rings and the gold surface for most of the adsorbed species. Yet, the experimentally observed differences between various types of di-DMAP-n ligands cannot be clearly separated by the means of spectroscopic modeling. For this reason, a detailed analysis of the totally symmetric mode intensities was conducted in this work. The oscillating intensity of the breathing mode as a function of spacer length between the pyridine moieties, calculated from eq 1, is related solely to the ligands, which are efficiently bound to the edges of surface steps. The highest binding energies are found for the diDMAP-4, -5, and -6 ligands since they have a sufficient spacer length for flexible arrangement, so that both pyridine rings are free to bind almost parallel to the planes of step terraces. At the same time, both nitrogen atoms of the rings are locked to the gold atoms of the step, with Au−N bonds practically coinciding with the symmetry axis of the pyridine rings. On the other hand, for di-DMAP-2 and di-DMAP-3, the short length of the spacer is not allowing for the flexible conversion into the favorable, completely flat 2ST arrangement of both rings at a step. Rather one pyridine ring remains inclined to the terrace (see Figure 5g). This results in a smaller contribution of di-DMAP-2, and DMAP-3 to the breathing mode in SERS spectra, as compared to di-DMAP-4, -5, and -6 (see Figure 7). The second factor of this result is that 2ST structures of diDMAP-2 and di-DMAP-3 were never reached in the course of the simulations by starting from the vertical 2N conformers, which makes the 2ST structure for these ligands rather unlikely to occur.

gain in dissociation energy, as expected from the accommodation of the molecular spacer to the step, was detected, as compared to the ideal surface. The divalent species have shown no bond strengthening relative to the monovalent DMAP. The ligands of the second moiety were bound via an adsorption structure 2ST (1ST) with both ring nitrogens, forming wellmatching Au−N bonds with atoms of the step and with the πsystem of both pyridine rings coupled flatly to the lower terrace of the step (plane gold surface). The insufficient spacer length in the case of di-DMAP-2 and di-DMAP-3 allowed for a completely flat 2ST arrangement only for a single pyridine ring. This results in a smaller contribution to the breathing mode in SERS spectra of the n = 2, 3 ligands, as compared to the ligands with longer spacer lengths (n = 4, 5, and 6). The calculated adsorption energies for these 2ST conformers were increased up to 50%, as compared to the ideal gold surface. For di-DMAP-(4, 5, 6) ligands, even a multivalent bond strengthening was observed. The resulting oscillatory dependence of the SERS intensity on the spacer length, based on the variation in the ring orientation to the Au surface, is observed for N-type conformers at the step. This is matched by the changes in binding constant KL, as experimentally estimated for the breathing mode. This effect was explained by the proper choice of conformers adsorbed at surface steps. The determined influence of the spacer length to the adsorption properties by SERS is in close agreement with previous results on nalkenothiolates adsorbed on Ag, as reported before.52 The dominant role of the vertical adsorption structure of 2N (1N) type, partially converting to 2ST (1ST) geometries in the current simulations, is supported by arguments of steric hindrance.61−63 These are specifically valid at high surface coverage (θ > 0.1 ML), as is typical for the discussed experimental conditions. The observed enhancement of the totally symmetric breathing mode at 1007 cm−1 in SERS spectra is accompanied by the complete damping of the ring-N-methyl vibration at 750 cm−1. This can be solely related to ligand adsorption bound to edge sites of surface steps or facets. This assumption is supported by large spectral blue-shifts of the breathing mode in the SERS signal, which are detected for all DMAP and diDMAP-n ligands.

4. CONCLUSIONS The adsorption structures of DMAP and di-DMAP-n (2 ≤ n ≤ 6) ligands, bound to a rough Au(111) surface, have been studied by means of SERS spectroscopy and first principles calculations. The calculated adsorption isotherms, monitored by the measured SERS intensity of the totally symmetric breathing mode, has shown strong odd-even oscillations in the binding constant KL for monovalent DMAP and for di-DMAPn ligands saturating for longer spacer chains (n ≥ 4). All SERS spectra of ligands, corresponding to the mid-IR regime, exhibit a strong enhancement in intensity for the breathing mode, accompanied by damping of the other spectral features. In order to explain the observed phenomena two limiting cases of binding were considered by first principles calculations of single molecule adsorption: (i) an ideal Au(111) surface and (ii) a stepped surface to model a rough surface. Binding of the DMAP and di-DMAP-n (2 ≤ n ≤ 6 ligands is systematically explored and schematically visualized in Figure 6, yielding the following results: (i) the optimized di-DMAP-n-ligand structures on an ideal surface reveal five main adsorption structures: two parallel adsorption structures, 1R and 2R, two vertical adsorption structures 1N and 2N, and two combined to one RN. A strong dispersive interaction is dominating both the coupling of the π-system with the gold surface and the Au− N bond. The dissociation energies of the 2R adsorption structure are energetically preferred and gradually increasing with the length of the divalent spacer. (ii) By repeating the calculations for two alternative vicinal Au(111) stepped surfaces, as a model for a rough surface, the adsorption structures were grouped into ligands with the Nring−Nring connection line aligned either perpendicular or parallel to the step boundary. The first group included ligands adsorbed in the conventional R and N type of structures. In this case, no

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b00371. Chemical syntheses and results from SERS experiements and model calculations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Christina Graf: 0000-0002-3308-5640 Eckart Rühl: 0000-0002-0451-8734 Notes

The authors declare no competing financial interest. ⊥ Dr. Maurice Taszarek died on February 24, 2018. 8678


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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (SFB 765, A2, C5, and C7) and Freie Universität Berlin. We thank Dr. Piotr Patoka (Physical Chemistry, Freie Universität Berlin) and the group of Prof. Dr. Fumagalli (Department of Physics, Freie Universität Berlin) for the evaporation of the gold films on glass substrates. The authors are indebted to Prof. Dr. B. Paulus (Freie Universität Berlin) as well as Dr. Ladislav Benda and Prof. Dr. Martin Kaupp (Technische Universität Berlin) for valuable discussions. The CPU assistance of ZEDAT (FU Berlin) and ZIB (Zuse Institute Berlin) is greatly appreciated. The communication with the SCM Group (Amsterdam) is kindly acknowledged.

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REFERENCES

This work is dedicated to the memory of Dr. Maurice Taszarek.

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Adsorption of Mono- and Divalent 4-(Dimethylamino)pyridines on Gold

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