Stepwise Growth of Ruthenium Terpyridine Complexes on Au

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Stepwise Growth of Ruthenium Terpyridine Complexes on Au Surfaces Sophia Katharina Peter,† Corinna Kaulen,*,†,∥ Alexander Hoffmann,† Wojciech Ogieglo,‡,⊥ Silvia Karthäuser,§ Melanie Homberger,† Sonja Herres-Pawlis,† and Ulrich Simon† †

JARA-FIT and Institute of Inorganic Chemistry, RWTH Aachen University, 52074 Aachen, Germany DWILeibniz Institute for Interactive Materials, Forckenbeckstr. 50, 52074 Aachen, Germany § JARA-FIT and Peter Grünberg Institut (PGI-7), Forschungszentrum Jülich GmbH, Jülich 52425, Germany J. Phys. Chem. C Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 03/08/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: Self-assembled monolayers (SAMs) of ruthenium-based molecular wires on solid surfaces are of great interest for optoelectronic and nanoelectronic applications. Here, we present a novel reactive Ru precursor, which enabled us to grow SAMs of Ru complex wires on Au surfaces even at room temperature. Thus, the Ru complex wire growth can be performed easily by sequential reaction of the reactive Ru precursor with terpyridine ligands without the harsh reaction conditions needed otherwise. Subsequently, we monitored the stepwise growth using infrared reflection absorption spectroscopy (IRRAS) and surface-enhanced Raman spectroscopy (SERS). A comparison of IRRAS and SERS data with theoretical spectra, derived from density functional theory calculations, enabled us to verify the formation of each individual growth step. Furthermore, we used these data to determine the orientation of the Ru-based molecular wires with respect to the Au surface. Growth step-dependent layer thicknesses obtained from variable angle spectroscopic ellipsometry verify the spectroscopic results. Thus, we provide a room-temperature method to realize Ru complex wire growth based on a reactive Ru precursor.

1. INTRODUCTION Integrating functional molecules into electronic devices represents a next step of miniaturization in electronics and unlocks pathways for novel fields of application.1−4 Molecules that perform electrical tasks like rectifying or switching are available in high purity and quantity.5,6 Moreover, several techniques have emerged to contact these functional molecules reproducibly by forming metal−molecule−metal junctions,1,3,7,8 using scanning probe microscopy,9,10 mechanical break junction systems,11 or nanogaps produced by electromigration.12 However, one feasible technique to implement functional molecules into electronic circuits is based on hybrid molecule−nanoparticle systems formed upon self-assembly processes.13−15 Advanced methods to fabricate such molecular devices combine top-down, e.g., lithographic methods, and bottom-up approaches, e.g., self-assembly processes. This combination enables reliable integration of molecular compounds between preformed electronic contacts, which is a prerequisite for integrated molecular electronic devices. Another promising way to bridge the gap between two electrodes precisely is the stepwise growth of molecular wires.16,17 Within the field of molecular electronics, prominent candidates for molecular wires reported so far are linear bisterpyridine transition-metal complexes because they show superior electronic transport properties.18,19 To control the © XXXX American Chemical Society

molecular wire formation within nanoelectrode gaps, it is of utmost importance to characterize it in detail so that the length and orientation of the molecules with respect to the electrode surface are identified exactly. The same holds for the transition-metal complex wire formation used to build up homogeneous monolayers of highly conducting materials to be used in organic electronics, optoelectronic devices, or sensor applications.17,20,21 Many surface characterization methods like X-ray photoelectron spectroscopy (XPS) or time-of-flight secondary ion mass spectrometry give accurate results on the composition of the adsorbed molecules but destroy their functionality during the measurements.17,21 Therefore, we focus on gentle, nondestructive, spectroscopic methods for surface characterization, such as infrared reflection absorption spectroscopy (IRRAS), surface-enhanced Raman spectroscopy (SERS), or variable angle spectroscopic ellipsometry (VASE). In this work, these characterization methods are used to follow the stepwise formation of surface-bound Ru terpyridine wires on Au surfaces. We choose Ru terpyridine complexes, as they are known to exhibit particular photophysical, chemical, and electrochemical properties.22−24 Tuccitto et al. reported Received: December 16, 2018 Revised: February 16, 2019 Published: February 22, 2019 A

DOI: 10.1021/acs.jpcc.8b12039 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C investigations on the step-by-step formation of Ru bisterpyridine complexes.25 However, the wire growth was performed under harsh reaction conditions, which cannot be applied for wire formation in situ, that is, on-chip between nanoelectrode gaps. Here, we developed a wire formation protocol suited for on-chip preparation and analyzed the Ru terpyridine complex formation in detail using spectroscopic methods. The molecular vibrations, recorded in the IRRA and SER spectra, were unambiguously identified by comparison with density functional theory (DFT) calculations, revealing the theoretical spectra of each intermediate. This, in addition, allows us in combination with bulk IR or Raman spectra of the respective reference compounds to extract information about the orientation of the molecules during growth, to gain full control of the complex wire formation mechanism. Further confirmation of the orientation of the molecules on the Au surface was obtained by determination of the layer thicknesses by VASE. Figure 1 illustrates the experimental procedure of the stepwise wire growth on a Au surface and shows the respective reference compounds.

2. EXPERIMENTAL METHODS The following chemicals were purchased from Sigma-Aldrich Chemie GmbH and used as received if not stated otherwise: methylthiobenzaldehyde, 2-acetylpyridine, sodium, terephthalaldehyde, ruthenium(III)chloride hydrate, N,N-dimethylformamide (DMF), and deuterated solutions of chloroform, dichloromethane, and acetonitrile. Silver hexafluorophosphate was purchased from Alfa Aesar. Acetonitrile and acetone were purchased from Fischer Chemicals AG. Absolute (abs.) ethanol and ammonia solution 25% were purchased from Th. Geyer. Hydrochloric acid was purchased from Grüssing. If necessary, the solvents and chemicals were purified and dried according to standard methods.26 All operations were performed under inert conditions in dry argon atmosphere, using standard Schlenk line techniques. 2.1. Synthesis of Ligands, Ruthenium Precursor, and Model Substances. 4′-(4-Methylthiophenyl)-2,2′:6′,2″-terpyridine (MTP), 4′-mercaptophenyl-2,2′:6′,2″-terpyridine (TP), and 1,4-bis(2,2′:6′,2″-terpyridin-4-yl)benzene (BTP) were synthesized following established methods (see Schemes S1.1−S1.4, Section S1, Supporting Information).27−29 The Ru precursor was synthesized by dechlorination of RuCl3 (Ru) (see Section 1, Scheme 1.2, Supporting Information). RuCl3 (47 mg, 0.18 mmol) and AgPF6 (148.6 mg, 0.59 mmol) were dissolved in acetone (18 mL) and refluxed for 2 h under light exclusion.30 The solution was filtered and evaporated to dryness. The remaining solid was dissolved in abs. ethanol. The model substances (Figure 1) were prepared according to procedures given in the literature.27,31 1H NMR, UV/vis, and mass spectroscopies indicated high purity of the prepared substances (Section S1, Supporting Information). 2.2. Preparation of Au Surfaces. The ⟨100⟩-oriented silicon wafers with dimensions of 2 × 1 cm2 were used to deposit a 10 nm adhesion layer of Ti and a 100 nm Au layer by plasma sputtering technique. Prior to functionalization, the Au surface was cleaned by an oxygen plasma (p(O2) = 0.4 mbar, f = 40 kHz, P = 100 W, t = 2 min). The samples were characterized via atomic force microscopy (AFM), and the root mean square roughness was found to be ∼1 nm (Section 3, Supporting Information). One Au-coated wafer of each batch was kept as reference for the IRRAS and SERS measurements. For reference purposes, spectroscopic and

Figure 1. Schematic illustration of the stepwise wire growth on a Au surface upon complex formation using 4′-mercaptophenyl-2,2′:6′,2″terpyridine (TP), Ru(TP, Ac3), and 1,4-bis(2,2′:6′,2″-terpyridin-4yl)benzene (BTP) (left). Model substances for the bulk spectra TP, Ru(MTP, Cl3), and Ru(MTP)2 (right).

ellipsometry measurements of all Au surfaces were performed before immersion in TP solution. 2.3. Stepwise Growth of Ru Terpyridine Complexes on Au Surfaces. Samples that allow the in situ observation of the stepwise growth and complexation by IRRAS, SERS, and VASE were prepared as follows. For the first step, TP was immobilized on pristine Au surfaces as a self-assembled monolayer (SAM), which was performed in several invesB


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The Journal of Physical Chemistry C tigations before.21,25,32,33 The Au surfaces were immersed in TP (c = 1 mM) in ethanol for 12 h for this step, rinsed five times with dry ethanol and DMF, and dried in flowing argon. Subsequently, the reactive metal-ion precursor Ru and BTP were added to the surface-bound TP alternatingly to grow unidimensional molecular wires. For that, the functionalized Au surfaces were deposited into a solution of the Ru precursor (c = 1 mM) in ethanol for 12 h and washed analogously. For the next growth step, the substrates were immersed into a solution of BTP (c = 1 mM) in chloroform for 12 h and rinsed with chloroform and DMF. After each functionalization step, the Au samples were characterized via IRRAS, SERS, and VASE. Between the measurements, the functionalized Au surfaces were stored under inert conditions. Figure 1 depicts our approach for the assembly of the TP SAMs on the Au surface and the following growth steps upon complexation.34 2.4. Preparation of IR and Raman Samples of the Model Substances. Fourier transform infrared (FT-IR) measurements of the model substances were taken from a pellet with 250 ± 10 mg of dry KBr and 1−2 mg of the sample. Raman spectra were obtained from solid-state samples on microscope slides (dimension, 26 × 76 × 1 mm2; Thermo Fisher). 2.5. Instrumentation. FT-IR and IRRA spectra were measured with a Vertex 70 FT-IR spectrometer (Bruker Optics) equipped with a MCT detector. IRRAS was performed on an A513/Q variable angle reflection accessory equipped with an automatic rotational holder for MIR polarizer. The setup for the measurements was reported previously.35 The spectral resolution was 4 cm−1 and taken from 800 to 1500 scans. The spectra were corrected with a rubber band correction under exclusion of the carbon dioxide band. UV/ vis measurements were performed with a JASCO V-630 spectrophotometer. Raman and SER spectra were obtained using an Horiba Jobin Yvon Raman spectrometer at a laser wavelength of 633 nm and 180° backscattering arrangement with Olympus objectives at 10- and 50-fold magnifications. A spectral resolution of 7 cm−1 was achieved. VASE measurements were performed on a RC2 variable angle spectroscopic ellipsometer (J.A. Woollam Co., Inc.) operating in the wavelength range of 193−1000 nm. Spectroscopic ellipsometry is based on measuring the change in the polarization state of light reflected from a thin-film sample. For each measured wavelength, the data are depicted as a pair of psi (Ψ) and delta (Δ) values related to the rp (inplane) and rs (out-of-plane) reflection coefficients represented rp by ρ = r = tan(Ψ) e iΔ. Film properties, such as the film

with the CCDC-No. 673952. All optimized geometries were characterized as stationary points on the potential energy, and no imaginary frequencies were obtained. The nonlocal hybrid meta-GGA TPSSh functional40 and def2-SVP basis set41 were used. All calculations were performed with Grimme dispersion and Becke−Johnson damping GD3BJ,42−44 as implemented in Gaussian 09. The IR frequencies were scaled according to the literature.45 The dipole derivative unit vectors (DDUVs) and the transition dipole moment (TDM) vectors were calculated as implemented in Gaussian 09 program suite.

3. RESULTS AND DISCUSSION The molecular building blocks for the synthesis of the Ru terpyridine wires, TP and BTP, were synthesized as reported earlier.29,30,32 1H NMR, UV/vis, and mass spectroscopies indicated high purity of the prepared substances (Section S1, Supporting Information). Tuccitto et al. already reported investigations on the step-by-step formation of Ru bisterpyridine complexes. However, the formation of the complexes took some weeks using RuCl3 as metal precursor.25 By dechlorination of RuCl3·xH2O with AgPF6, we synthesized a Ru precursor that was expected to demand lower reaction times and temperatures and further prevent the formation of AgCl on the Au surfaces during the complexation. Herein, only weakly coordinated acetone molecules stabilize the metal center.30 Using this labile Ru precursor, we observed significantly accelerated Ru terpyridine complex formation over 12 h at room temperature. The monolayers TP, Ru(TP, Ac3), and Ru(TP, BTP) were prepared as displayed in Figure 1 and analyzed by IRRAS, SERS, and VASE. To confirm the results, the model compounds Ru(MTP, Cl3) and Ru(MTP)2 were synthesized ex situ and analyzed by FT-IR and Raman spectroscopies.27,30 Additionally, we performed DFT calculations of TP, Ru(TP, Ac3), and Ru(TP, BTP) to obtain the theoretically expected IR and Raman spectra. In the following, the IRRA and SER spectra measured after each step will be compared to the FT-IR and Raman spectra of the corresponding model substances Ru(MTP, Cl 3 ) and Ru(MTP)2 and the calculated spectra of TP, Ru(TP, Ac3), and Ru(TP, BTP). 3.1. First Step: Characterization of TP SAM by IRRAS and SERS. The IRRA, FT-IR, and calculated spectra of TP are depicted in Figure 2, and the vibrational assignments are listed in the Supporting Information (Section S2, Table S2.1). The FT-IR spectra were transformed into absorbance to allow comparison with the IRRA spectra. The bands at 3085, 3078, 3057, and 3031 cm−1 in the calculated spectrum of TP; the bands at 3014 and 3051 cm−1 in the FT-IR spectrum of TP; and a single weak band at 3054 cm−1 in the IRRA spectrum are assigned to the aromatic stretching vibrations ν(C−H) of the pyridine and phenyl rings (Figure 2, blue). The ν(C−H) vibrations will not be considered in further growth steps, as the Fermi resonances of the ν(C−H) with the γ(C−H) vibrations lead to complex band patterns and shoulders in this region.46,47 The bands between 2850 and 2962 cm−1 in the IRRA and FTIR spectra (Figure 2, red and black spectra) are assigned to the symmetric and asymmetric ν(CH) vibrations of residual ethanol.17,21,46 In the FT-IR spectrum of TP (Figure 2, black spectrum), the band at 2552 cm−1 for the ν(S−H) vibration is visible, while it is not observed in the IRRA spectrum (Figure 2, red spectrum). This band is observed at 2602 cm−1 in the calculated spectrum, while the ν(S−H) vibration was observed

s

thickness, were estimated with an optical model suitable to represent the optical features of the sample. The simulated data from the model were then numerically fit to the measured data. More details on the technique and optical modeling can be found elsewhere.36 In this study, a B-spline model was used to fit the Au surface sputter deposited on a silicon wafer ⟨100⟩. Optical constants of the respective surface were taken from the literature.37 The modeling was done in the wavelength range of 550−1000 nm, at which the sample could be assumed to be transparent, applying a simple Cauchy-type method.38 2.6. DFT Calculations. All calculations were performed based on the Kohn−Sham density functional theory (DFT) as implemented in the Gaussian 09 program suite (revision D.01).39 The starting geometries of the complexes for optimization were generated from the X-ray crystal structure C


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at 2568 cm−1 for benzenethiol before.48 The deformation vibrations of the thiol group δ(S−H) with contributions of the out-of-plane vibrations γ(C−H) are visible at 889 and 874 cm−1 in the calculated spectrum of TP.46 In the FT-IR spectrum, only one broad band at 887 cm−1 is found, which can be assigned to these vibrations, while no band is observed in the IRRA spectrum. Both observations are consistent with the fact that the chemisorption of TP to the Au surface was successful and a Au−S bond was formed.46,49 The band at 1729 cm−1 in the IRRA spectrum (Figure 2, red spectrum) is assigned to an overtone of the out-of-plane deformation γ(C−H) bands of the 1,4-substituted phenyl ring at 576 cm−1.50 The aromatic stretching vibrations ν(CC) and ν(CN) in the region between 1604 and 1537 cm−1 appear in an ascending and descending pattern in all three spectra, with the second band exhibiting the highest intensity. The band at 1604 cm−1 in the IRRA and FT-IR spectra corresponds to the ν(CC) vibration of the 1,4-substituted phenyl ring and is relatively high in intensity, due to an asymmetric substitution pattern with different directing effects.50 The band with the highest intensity at 1587 cm−1 in IRRA and FT-IR spectra is assigned to the ν(CC) and ν(CN) vibrations of the 2monosubstituted pyridines of the terpyridine group according to the DFT calculations, which is in agreement with literature.51−53 The bands at 1568 and 1537 cm−1 also result from ν(CC) and ν(CN) vibrations. The position, the structure, as well as the intensities of these bands are in good accordance with the calculated band pattern (Figure 2). The four bands in the IRRA and FT-IR spectra arising in the region between 1500 and 1400 cm−1 in all three spectra are equally assigned to the ν(CC) and ν(CN) vibrations of the 2monosubstituted pyridines. The band at 1383 cm−1 in the IRRA spectrum corresponds to the ν(CC) vibrations of the 1,4-substituted phenyl ring.46,53 According to Schneider et al. and the DFT calculations, we assign this band to a vibration resulting from the linear combination of the phenyl ring vibrations with a deformation mode of the central pyridine ring modified by the coupling with the outer rings.51 As the coupling between the pyridine rings is characteristic for the terpyridines, we abbreviate this band at 1383 cm−1 with ν(C−C).51 The region between 1261 and 1076 cm−1 shows several weak bands in the IRRA and FT-IR spectra due to the in-plane deformation vibrations δ(C−H)i.p. of the rings with one strong band at 1261 cm−1. We attribute this band to a combined vibration that contains contributions of the ν(CC) and ν(CN) vibrations and δ(C−H)i.p. that is only weakly visible in the simulated spectrum at 1294 cm−1.51,46 The bands at 822 and 792 cm−1 in the IRRA and FT-IR spectra arise from the out-of-plane γ(C−H) vibrations with different contributions of the aromatic rings and the characteristic doublet pattern, according to the theoretical spectrum and literature-known values.46,53 In contrast to IRRAS, Raman spectroscopy depends on the polarization change of the molecule during vibration54 We used the same substrates as for the SERS and IRRAS measurements, to exclude additional optical effects arising from sample variations. For most SERS investigations reported so far, spectra were measured on metal colloids, which show a distinct high roughness to support the enhancement of the light scattering.51,52,54−56 We assume that the bands result

Figure 2. Comparison of measured and calculated IR and Raman spectra. (a) Magnification of ν(CC), ν(CN), and γ(C−H)o.o.p. vibrations, corresponding to different dipole derivatives (see Section 2, Supporting Information). (b) IR spectra of TP in KBr pellet (black), IRRAS of TP SAM (red), and theoretical IR spectra of TP (blue) based on DFT calculations. Intensities differ due to different orientations. Also see Table 1. The ν(CC) and ν(CN) vibrations in the region between 1568 and 1604 cm−1 result in a triplet vibration. (c) Raman of TP (black), SERS of TP SAM (red), and DFTcalculated Raman spectra of TP (blue). The δ(C−H)i.p. vibrations of the aromatic rings show two bands at (a1) 1078 cm−1 and (b2) 1034 cm−1. The ν(CC) and ν(CN) vibrations at 1580 cm−1 result in a band with the highest intensity in all spectra. The ν(CC) and ν(CN) vibrations between 1400 and 1500 cm−1 originate from combinatory vibrations of the pyridine rings and phenyl ring. The band at 825 (b1) cm−1 is assigned to an δ(C−H)o.o.p. vibration of the phenyl ring. D


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The Journal of Physical Chemistry C rather from the polarization of the incident light than from the scattered beam, as the surfaces used were not particularly roughened. Therefore, an excitation by the normal component of the field results in an induced dipole whose component perpendicular to the surface is measured.54 The selection rules are therefore in close relation to those applied in IRRAS.54,57 Figure 4 shows the SER (red), Raman (black), and calculated (blue) spectra of TP. Similar to the IR spectra, ν(CC) and ν(CN) vibrations at 1592, 1570, 1537, 1497, 1465, 1442, and 1408 cm−1 are observed in the Raman spectrum, of which the one at 1592 cm−1 exhibits the highest intensity.51,52 The SER spectrum shows only four bands at 1580, 1468, 1430, and 1408 cm−1 in this region, of which the one at 1580 cm−1 shows the highest intensity and is enclosed by two shoulders on both sides. The calculated spectrum shows two bands of high intensity at 1594 and 1581 cm−1 and further bands at 1557, 1535, 1496, 1469, 1430, and 1397 cm−1 that agree with the bulk spectrum of TP. Compared to the bulk spectrum, the bands at 1592, 1570, 1537, and 1497 cm−1 are not visible in the SER spectrum. The other three bands at 1468, 1430, and 1408 cm−1 for the ν(CC) and ν(CN) vibrations in the SER spectrum match well with the calculations and the Raman measurements, but show a lower relative intensity. In the FT-IR spectrum of TP, the band at 1385 cm−1 was assigned to the asymmetric ν(C−C) vibration between the aromatic rings (see Figure 2).51,58 In the Raman spectra, we can assign the band at 1357 cm−1 to the symmetric band of the stretching vibration ν(C−C) between the rings, which is low in intensity for the calculated IR spectrum but high in intensity for the calculated Raman spectrum (0.60− 57.81 esu2 cm2). The pattern corresponding to the in-plane ring deformation δ(C−H)i.p. vibrations in the SER spectra between 1287 and 1034 cm−1 agrees well with the pattern observed in the Raman and calculated spectra, apart from slight shifts in the range of few wavenumbers.51 Further bands at 397, 472, 708, 825, and 990 cm−1 are assigned to out-of-plane vibrations γ(C−H)o.o.p. and stretching vibrations of the aromatic rings.51 The most distinct difference between both the experimental and theoretical Raman spectra and the SER spectrum is observed for the band at 1078 cm−1, as the intensity is distinctly higher in the SER spectrum. 3.2. Second Step: Characterization of Ru(TP, Ac3) SAM by IRRAS and SERS. Following the characterization line described above, we performed IRRA and SER spectroscopies in the next step in wire growthcomplexation of the immobilized TP with Ru. Due to this complexation, the conformation of the pyridine rings of the terpyridine changes from all trans to all cis, and additionally acetone ligands are coordinated to the Ru to meet the octahedral coordination geometry (Figure 1). We used acetone as further ligands that show strong signals in IR spectroscopy and PF6−, which is a common counterion for Ru terpyridine complexes.27,30,46 As this compound is a labile intermediate in the syntheses of full complexes, we compared the IRRAS and SERS results with the FT-IR and Raman spectra of a model substance for this step, which is commonly used for the synthesis of Ru terpyridine complexes, short Ru(MTP, Cl3) (Figure 1). Furthermore, we performed DFT calculations of Ru(TP, Ac3) to identify the vibrational features in the respective experimental spectra. The experimentally obtained FT-IR (black) and IRRA (red) spectra as well as the theoretical spectrum (blue) are depicted in Figure 3. One prominent change in the IRRA spectrum of

Figure 3. Comparison of measured and calculated IR and Raman spectra. (a) IR spectra of Ru(MTP, Cl3) in KBr (black), IRRAS of Ru(TP, Ac3) SAM (red), and theoretical IR spectra of Ru(TP, Ac3) (blue). The ν(CC) and ν(CN) vibrations in the region around 1600 cm−1 result in a single band in the calculations. In the region between 1320 and 1500 cm−1, several bands contribute to the combinatory ν(CC) and ν(CN) vibrations of the phenyl ring and the pyridine rings, which are attenuated in the IRRA spectra. (b) Raman spectrum of the model substance Ru(MTP, Cl3) (black), SERS of Ru(TP, Ac3) SAM (red), and DFT-calculated Raman spectra of Ru(TP, Ac3) (blue). The bands at (a1) 1077, (b2) 1034, and (b1) 834 cm−1 are used to calculate the orientation as for TP. The ν(CC) and ν(CN) vibrations at 1584 cm−1 result in a band with the highest intensity in all spectra. The ν(CC) and ν(CN) vibrations between 1400 and 1500 cm−1 originate from combinatory vibrations of the pyridine rings and the phenyl ring and are in good agreement.

the Ru(TP, Ac3) compared to TP SAM is the occurrence of new bands at 1643 and 1614 cm−1. The calculated absorption bands at 1635 and 1618 cm−1 are assigned to the ν(CO) vibrations of acetone. While the ν(CO) band appears at around 1725−1705 cm−1 in acetone,59 it is shifted here to lower wavenumbers due to the interaction of the carbonyl groups with the metal center. From the calculations, it is evident that both bands also contain contributions of ν(CC) and ν(CN) vibrations. Since the model substance contains chloride ligands instead of acetone (Figure 1), the carbonyl bands are not observed in the FT-IR spectrum of Ru(MTP, Cl3). The band at 1603 cm−1 in the IRRA spectrum is assigned to the ν(CC) and ν(CN) vibrations of TP. The intensity is comparable to the E


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The Journal of Physical Chemistry C intensity of the corresponding band in the TP monolayer (see Figure 3 and Section S2 Tables S2.1 and S2.3 in the Supporting Information), which indicates that not all TP molecules chemisorbed on the Au surface form Ru complexes. Overall, the ν(CC) and ν(CN) vibrations around 1600 cm−1 in the calculated spectrum lead to one strong absorption band, differing from the complex band pattern observed for the TP monomer. The band at 1585 cm−1 exhibits the highest intensity in the IRRA spectrum and corresponds to these ν(CC) and ν(CN) vibrations. It agrees well in terms of position to the corresponding band from the DFT calculations and the model substance Ru(MTP, Cl3) spectrum, but its intensity is lower (Figure 3). The high intensity in the region between 915 and 745 cm−1 can be explained by an overlap of the γ(CH)o.o.p. and the stretching vibrations of the counterion PF−6 .45,59 Additional bands at 1495, 1441, 1414, 1253, 1181, and 1064 cm−1 in the IRRA spectra were assigned in an analogous manner to the assignment in TP, as can be seen in Table S2.3 in the Supporting Information. Consistent with the combined analysis of TP by means of IRRAS and SERS, we also recorded SER and Raman spectra of Ru(TP, Ac3) and Ru(MTP, Cl3), respectively, and compared the results with the DFT calculations of Ru(TP, Ac3) (Figure 3). The data on the assignment of all bands are given in Table S2.4 in the Supporting Information. Distinct changes compared to the TP SAM occur for the bands at 1077 (a1), 1034 (b2), and 990 cm−1. The band at 990 cm−1 arising from the stretching vibration of the aromatic ring is bathochromically shifted to 1018 cm−1, which agrees with the shift of this band in the calculated spectra from 969 to 1022 cm−1 from TP to Ru(TP, Ac3). 3.3. Third Step: Characterization of Ru(TP, BTP) SAM by IRRAS and SERS. For the third step, the Au surface with Ru(TP, Ac3) was immersed in a BTP solution to exchange acetone coordinated to Ru. IRRA, FT-IR, and SER spectroscopies were performed on the respective sample. As model compound, the homoleptic Ru terpyridine complex Ru(MTP)2 was synthesized, according to an established protocol.27 The IRRA spectrum of Ru(TP, BTP) (red) and the calculated spectrum of Ru(TP, BTP) (blue) are shown in Figure 4a together with the FT-IR spectrum of the model compound Ru(MTP)2 (black). The ν(CO) band, which was observed in the spectra of the Ru(TP, Ac3) complex cannot be observed in the IRRA spectrum anymore, indicating that acetone has been replaced by BTP. Two bands at 1602 and 1615 cm−1 are observed for the ν(CC) and ν(CN) vibrations that are assigned to vibrations of TP and Ru(TP, Ac3), respectively, indicating that parts of the preexisting SAM have not reacted with the BTP. The rise in intensity of the band at 1615 cm−1, which is now the band with the highest intensity around 1600 cm−1, indicates that the band also contains contributions of the ν(CC) and ν(CN) vibrations of Ru(TP, BTP). Further, ν(CC) and ν(CN) vibrations were observed in the IRRA bands at 1574, 1427, and 1407 cm−1, which agree with the calculations and the model substance. The bands at 1585, 1495, 1476, 1442, 1389, and 791 cm−1 were assigned to the same vibrations as in TP and Ru(TP, Ac3), as can be seen in Tables S1 and S5 in the Supporting Information. A shift toward lower wavenumbers in Ru(TP, BTP) is visible for the δ(C−H)i.p. vibration at 1263 cm−1, but with a lower relative intensity. The same band occurs amplified in the TP spectra, but not in the Ru(TP, Ac3) complex. According to the calculations, it is assigned to the uncomplexed terpyridine

Figure 4. Comparison of measured and calculated IR and Raman spectra. (a) FT-IR spectrum of the model substance Ru(MTP)2 in KBr (black), IRRA spectrum of Ru(TP, BTP) immobilized on the Au surface (red), and DFT-calculated IR spectra of Ru(TP, BTP) (blue). The ν(CC) and ν(CN) vibrations in the region between 1500 and 1620 cm−1 still show a triplet vibration, although calculations predicted a single band. The band at 1161 cm−1 is assigned to the uncomplexed side of BTP. The ν(CO) band decreases through the complexation. (b) SER spectrum of Ru(TP, BTP) SAM (red) and DFT-calculated Raman spectra of Ru(TP, BTP) (blue). The calculations agree well with the SERS in terms of the ν(CC) and ν(CN) vibrations. A band of high intensity at 1078 cm−1 can be assigned to the δ(C−H)i.p. vibration of Ru(TP, BTP). SER spectrum is shifted to higher wavenumbers, indicating a strong interaction with the substrate.61

group, and therefore indicates a successful formation of the full complex wire. Further shifts are observed of Ru(TP, Ac3) to Ru(TP, BTP) for the δ(C−H)i.p. vibrations from 1181 to 1161 cm−1, 1064 to 1053 cm−1, and the γ(CH)o.o.p. from 822 to 833 cm−1. Similar shifts are observed in the calculated spectra, with an exception for the γ(CH)o.o.p. The band at 1161 cm−1 corresponds to the uncomplexed terpyridine of BTP and therefore indicates a successful complexation. As noted in the previous section, the bands arising from the outof-plane vibrations γ(CH)o.o.p. around 833 cm−1 are influenced by the vibrations of the PF 6− . 46,60 Other δ(C−H)i.p. vibrations are observed at 1309, 1263, and 1020 cm−1, which is in agreement with the calculated spectra and the spectrum of the model substance. It was not possible to obtain a Raman spectrum of the model complex Ru(MTP)2 due to intense fluorescence bands. Thus, F


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The Journal of Physical Chemistry C Figure 4b shows the calculated spectrum of Ru(TP, BTP) (blue) and the SER spectrum of Ru(TP, BTP) (red) obtained by the stepwise growth process. By comparing the SER spectrum obtained from the second growth step, the complexation of immobilized TP with Ru (Figure 4b and Section 3.2), to the one we obtained here, it is evident that only very slight changes in intensity can be observed. While most bands are attenuated upon the complexation with BTP, the band for the ν(CC) and ν(CN) vibrations at 1583 cm−1 becomes more intense (Table S2.6 in the Supporting Information). The calculated spectrum (Figure 4b) also shows an amplification of this band, while the in-plane deformation vibration at 1364 cm−1 for the ν(C−C) is comparably low in intensity, as detected in the spectra before. The δ(C−H)i.p. vibrations at 1078 (a1), 1038 (b2), and 1022 cm−1 are slightly shifted to higher wavenumbers, while the pattern stays the same as in the Ru(TP, Ac3) complex. In the previous sections, we discussed the IRRA and SER spectra of each growth step TP, Ru(TP, Ac3), and Ru(TP, BTP) in comparison to theoretical and bulk spectra, obtained from model substances. At this point, we want to summarize how a discrimination between the spectra corresponding to the different growth steps is possible. For this purpose, we plotted all IRRA spectra in comparison to all simulated spectra and highlighted the relevant differences (Figure 5a,b). There is an overall agreement of the DFT calculations with the IRRA spectra in terms of position of the bands of each step, while the intensities differ due to the dependence of the intensity in IRRA spectra on the orientation of the molecules. The low signal-to-noise ratio in the region between 1800 and 1400 cm−1 corresponds to the compensation of the water vapor bands in this region. Nevertheless, we managed to record full spectra, due to an analogous treatment of the samples and the wafers used as background. The ν(CC) and ν(CN) vibrations are observed in all three spectra in the region from 1600 to 1400 cm−1, as well as δ(C−H)i.p. vibrations in the region from 1400 to 950 cm−1 and γ(CH)o.o.p. vibration in the region between 830 cm−1 and below. From TP to Ru(TP, Ac3), there is a clear change due to the ν(CO) vibrations of acetone as a ligand at 1634 and 1618 cm−1, while from Ru(TP, Ac3) to Ru(TP, BTP) the ν(CO) vibration decreases again. Also the δ(C−H)i.p. vibrations of the ring show a shift from TP to Ru(TP, Ac3) from 1161 to 1181 cm−1 and from Ru(TP, Ac3) to Ru(TP, BTP) from 1181 to 1161 cm−1. The same trend can be observed in the calculations. The vibration at 1181 cm−1 originates from the uncomplexed terpyridine unit, while the band at 1161 cm−1 corresponds to the complexed unit. The shifts from the first to the second step and from the second to the third step therefore indicate a stepwise growth. The γ(CH)o.o.p. vibrations were observed at 822 and 833 cm−1 and at 791 and 793 cm−1 in all IRRA spectra. Unlike the KBr spectra, they seem unaffected by the PF−6 counterion and correspond very well to the DFT spectra. Further, we plotted all SER spectra together (Figure 5c), applied a baseline correction, and normalized them to the band at 1085 cm−1 (Figure 5). Apart from the changes of the δ(C−H)i.p. vibration already discussed in the previous sections, a change of the band around 508 cm−1 was observed, which was assigned to the γ(CH)o.o.p. vibration of the 1,4disubstituted phenyl ring (Figure 5).46 In the SER spectrum of Ru(TP, BTP), we observe both the bands resulting from the γ(CH)o.o.p. vibrations of the uncomplexed terpyridine at 476 cm−1 and the γ(CH)o.o.p. vibration of complexed terpyridine

Figure 5. Comparison of measured and calculated IR and Raman spectra. (a) Details of the calculated IR spectra (left) and IRRA spectra (black) of TP, Ru(TP, Ac3) (red), and Ru(TP, BTP) (blue). (b) DFT calculation of TP (black), Ru(TP, Ac3) (red), and Ru(TP, BTP) (blue). (c) Baseline-corrected SER spectra of TP (solid line), Ru(TP, Ac3) (dot-dashed line), and Ru(TP, BTP) (dotted line). Enlarged is an area for the γ(CH)o.o.p. vibration of the 1,4disubstituted phenyl rings.

at 508 cm−1, as in this compound both molecular groups are present. 3.4. Growth-Dependent Molecular Orientation. In the following, we determine the orientation of TP relative to the Au substrate, comparing the intensities of the IRRAS bands with the respective intensities in the FT-IR spectra, which correspond to the vibrational modes in the free molecule. This is possible, as the intensities of the bands in IRRAS bear a relation to the orientation of the transition dipole relative to the normal of the plasmonic substrate. Generally, the intensities of bands in the IRRA spectra depend on the direction of the vibration, the orientation of the monolayer, and its density.62 For the interpretation of the orientation, it is important that only those vibrations are detected that employ a nonzero transition dipole moment (TDM) in the direction of the surface normal.47 The DFT calculations allowed exporting G


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moment derivative of the mode, dμ⃗/dQ, the intensities of the a1, b1, and b2 modes can be written as follows48

the dipole derivative unit vector (DDUV), which is a vector parallel to the TDM. These TDMs can adopt three different orientations due to the different vibrational modes (Figure 6).

I(a1) ∝ cos2 θI 0(a1)

(1)

I(b1) ∝ sin 2 θ cos2 χI 0(b1)

(2)

I(b2 ) ∝ sin 2 θ sin 2 χI 0(b2 )

(3)

0

where I represents the intrinsic intensity of the corresponding mode without surface effect. We use the ν(CC) and ν(CN) vibrations in the region between 1350 and 1620 cm−1 and the γ(CH)o.o.p. vibrations under 830 cm−1 for the determination of their orientation as the IRRA and FT-IR spectra of TP match well with respect to the position of the wavenumbers (see Figure 2).47 We observed that in the IRRA spectra, the vibrations at 1604 (a) and 1587 (b) cm−1 show increased intensity compared to the respective FT-IR vibrations and exhibit an a1 mode considering a C2v symmetry for the TP molecule (Table S2.7.1, Supporting Information). As these bands are visible, we know that θ is unequal to 90°, the z axis is not parallel to the surface, in agreement with the surface selection rules, and the TP molecule does not lie flat on the Au surface. The intensities of the IRRA spectra at 1568, 1537, 1469, 1412, and 1383 cm−1 (c, d, e, g, and h, respectively) that exhibit a b2 mode are attenuated compared to those of the a1 mode (a, b). These vibrations depend on the tilt angle θ and the twist angle χ (eq 2). Further, the b1 modes visible at 1442 and 792 cm−1 (f and i, Table 1) obtain intensities comparable to those of the b2 mode. According to eqs 2 and 3, the intensities of the b1 and b2 modes are distinguished by sin2 χ and cos2 χ, leading to the conclusion that both vibrations are equally visible when χ is close to 45° and the zy-plane is twisted around the z axis. Wan et al. determined the tilt angle of benzenethiol on Au(111) from the ratio of intensities in the FT-IR and SEIRA spectra of the δ(C−H)i.p. vibration of the phenyl ring and the γ(C−H)o.o.p. vibrations. From eqs 1 to 3, two equations are deduced to calculate θ and χ.

Figure 6. Schematic representation of the model used to estimate the molecular orientation; the directions of the dipole moment changes of a1, b1, and b2 modes in C2v symmetry. The surface electric field, E⃗ , has one normal component, which is parallel to the Z direction. XYZ fix the Au surface and the electric filed tensor E⃗ and the xyz axes the molecular-fix axis system. θ, χ, and ϕ are the Eulerian angles, which correlate the molecular fixed axis system xyz with the substrate XYZ.48

These are (i) oriented in the plane of the pyridine ring and parallel to the 4,4′-axis, a1, (ii) oriented in the plane of the pyridine ring but perpendicular to the 4,4′-axis, b2, and (iii) oriented perpendicular to the pyridine ring and perpendicular to the 4,4′-axis, b1. With those directions, the molecular axis system can be fixed, as (i) correlates to the z axis, (ii) to the y axis, and (iii) to the x axis of the TP molecule. The Eulerian angles θ, χ, and ϕ thereby correlate the molecule-fixed axis system with the Au substrate and the surface electric field E⃗ .48 θ is the tilt angle of the z axis of the molecular system from the surface normal Z, which is 0°, when the molecule is parallel to the Z axis. χ is the twist angle of the molecular plane around the S-phenyl bond, which is 0° when the y axis is parallel to the surface. ϕ is not represented in Figure 6 because it is not in relation to the intensities, as it is the rotation angle of the molecular system around the Z axis. Considering that the intensity of a vibrational mode is proportional to the square of the scalar product of the electric field, E⃗ , and the dipole

tan 2 χ =

I(b2 ) I 0(b1) I(b1) I 0(b2 )

(4)

0

tan 2 θ =

I(b1) I (a1) 1 I(a1) I 0(b1) cos2 χ

(5)

We applied the same procedure to the Raman and SER spectra of TP (Figure 2), as the TP molecule on the Au surface is well suited for this approach. It also contains the mercaptophenyl moiety with analogous vibrations, whose geometrical requirements were checked with the DFT calculations. The

Table 1. Correlation of Vibrations of TP with DDUVs and the Resulting Intensity Ratio selected vibration (indication in Figure 3)

vibrational mode

wavenumber (cm−1)

IIRRAS (10 arb. u.)/IFT‑IR (arb. u.)

a b c d e f g h i

a1 a1 b2 b2 b2 b1 b2 b2 b1

1604 1587 1568 1537 1469 1442 1412 1383 792

1.11 1.21 0.54 0.4 0.5 0.4 0.4 0.38 0.55

H


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The Journal of Physical Chemistry C Table 2. Correlation of Vibrations of Ru(TP, Ac3) with DDUVs and the Resulting Intensity Ratio selected vibration (indication in Figure 3)

vibrational mode

wavenumber (cm−1)

IIRRAS (10 arb. u.)/IFT‑IR (arb. u.)

a b c d e f g

a1 b2 a1 b2 b2 b1 b1

1585 1495 1477 1425 1385 822 793

0.68 0.36 0.52 0.38 0.31 0.25 0.18

δ(C−H)i.p. vibration at 1083 (a1) and 1029 (b2) and the γ(C−H)o.o.p. vibration at 822 (b1) cm−1 were used to calculate θ to be 44° and χ to be 46°. The value of θ corresponds well to the tilt that was found by Karipidou and Tuccitto et al., who calculated θ from AFM and XPS measurements of Au surfaces covered with ∼40 nm long Fe BTP wires to be ∼45°.17,25 To the best of our knowledge, χ has not been reported yet. However, the found twist of the phenyl ring of 46° around the z axis agrees with our observation of the b1 and b2 modes in the IRRA spectra. A major difference between the second step Ru(TP, Ac3) and the TP SAM is that the terpyridine moiety changes its conformation from all trans to all cis upon complexation, and thus the torsion angle of about 10° to the central pyridines to the outer pyridines is lowered to 0°, as described by Constable et al. for the 2,2′:6′,2″-terpyridine motif.49,63 Hence, we expect the complex to be more rigid and to obtain dipole moment derivatives differing from those found for the TP (see Tables S2.7.1 and S2.7.2, Supporting Information). Disregarding the acetone molecules and the counterions, a C2v symmetry is also evident for Ru(TP, Ac3). For the Ru(TP, Ac3) SAM, the ν(CC) and ν(CN) vibrations at 1585 (a) and 1477 (c) cm−1 are set parallel to the z axis of the molecule, as described for TP in an a1 mode (Figures 6, 2 (top), and Table 1), while the ν(CC), ν(CN), and ν(C−C) vibrations at 1495 (b), 1425 (d), and 1385 (e) cm−1, respectively, are parallel to the y axis in a b2 mode. The γ(C−H)o.o.p. vibrations with bands at 822 (f) and 793 (g) cm−1 were also identified here to have a dipole moment derivative along the x axis of the molecular axis system, which means they obtain a b1 mode, as in TP. As the bands at 1585 (a) and 1477 (c) cm−1 with the a1 mode are visible in the IRRA spectra of Ru(TP, Ac3), we know that θ is not equal to 90° and the Ru(TP, Ac3) complex is thereby not lying flat on the Au surface. The intensity ratios of the IRRA spectra of Ru(TP, Ac3) and FT-IR spectra of Ru(MTP, Cl3) at 1495, 1425, and 1385 cm−1 (b, d, e) that exhibit an b2 mode are attenuated compared to the intensities of (a) and (c) (a1 mode). Further, the vibrations with b1 mode obtain slightly lower intensities than those of the b2 mode, as observed for the bands 822 (f) and 793 (g) cm−1 (Table 2). This is an indication that the x axis of the molecular axis system is twisted a little more around the z axis than the χ = 46°. We are aware that the model substance Ru(MTP, Cl3) is not identical to the surface-bound Ru(TP, Ac3) complex (Figure 1); however, according to the ratio of the intensities of the bands, the respective orientation is supported. In general, all intensities we observe in the IRRA spectra are in the same range as those observed in the case of the surface-bound TP, which implies that the orientations with respect to the Au surface are similar for both substances (Figures 1 and 2).17,21,62,64 The model substance Ru(MTP, Cl3) differs from the Ru(TP, Ac3) in its ligand proximity and the counterion, but

it also obtains the conformational change of the terpyridine moiety from all trans to all cis. Figure 2 shows that the SER and Raman spectra of Ru(TP, Ac3) and Ru(MTP, Cl3) are in good agreement. As in TP, χ is the twist angle of the phenyl ring around the z axis, which was calculated to be 44° from the intensities of δ(C−H)i.p. at 1034 cm−1 (b2) and γ(C−H)o.o.p. at 824 cm−1 (b1) (eqs 4 and 5). The tilt angle θ from the surface normal was calculated to be 48° from the intensities of the δ(C−H)i.p. vibrations at 1077 (a1) and γ(C−H)o.o.p. at 824 (b1) cm−1 in the Raman and SER spectra. The signal-to-noise (S/N) ratio has been described for both IRRAS and SERS to have an influence on the error of the angles. Although it was found to exceed 10° for biphenyl-thiols on Au for IRRAS measurements, it was found to have an influence as little as 2° for χ and 1.5° for θ for benzenethiol on Au.65,66 We have calculated the error resulting from the S/N ratio of the Raman bulk and surface spectra with Gaussian error propagation (Section S2.8, Supporting Information). For TP, the error from the S/N ratio results in ±1.46° for χ and ±2.49° for θ. For Ru(TP, Ac3), those errors are ±1.59° for χ and ±2.13° for θ. For the third step, Ru(TP, BTP), the model substance Ru(MTP)2 we used is not identical to the Ru(TP, BTP) complex on the surface. Due to large fluorescence, we could not obtain a Raman spectrum; the orientation of the last step could thus not be calculated. 3.5. Stepwise Growth Followed by VASE. Ellipsometry has been successfully used to establish the thickness of monolayers on noble metals, while spectroscopic ellipsometry is known for its high thickness sensitivity (∼0.1 Å).67,68 To verify the layer thickness derived from the above-mentioned spectroscopic data, we measured the layer thicknesses of each growth step by means of VASE. Based on the tilt angle of 44° and the crystal structure of Ru(MTP)2 reported by Constable et al.,27 we identified the carbon atom in para position of the outer pyridine rings as the most protruding atom for TP. Taking into account that the distance between this C atom and the S atom is 1.16 nm and the C atom is tilted 22.5° from the z axis of the molecule, the expected layer thickness is 1.07 nm. The VASE of the first layer resulted in a thickness of 0.98 ± 0.02 nm for TP (Figure 7), which is in good agreement. Furthermore, this layer thickness is also consistent with a thickness of 1.3 nm derived from surface plasmon resonance measurements.66 In the second step, when Ru(TP, Ac3) is formed, the C atom in para position is still the outermost atom, leading to the unchanged layer thickness of 1.07 nm. The VASE measurements on Ru(TP, Ac3) layers give in a thickness of 1.1 ± 0.05 nm, being consistent with the orientation deduced from the spectra of Ru(TP, Ac3). In the literature, the layer thickness of Ru(TP, Ac3) is reported to be around 1.09 nm from calculations, which is also in good agreement with our findings.69 For the theoretical thickness of the third layer I


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reactive Ru precursor leaves the tilt angle nearly unaffected. This is also reflected in the VASE measurements. Further, the third growth step with BTP does not significantly affect the tilt angle. The thickness of the layer from VASE confirmed our expectations. In summary, our investigations document a nondestructive approach allowing the detailed investigation of the stepwise formation of linear terpyridine Ru complex wire structures. The applied combination of analysis methods can be easily extended to other wires based on pyridine metal complex formation, thus not only giving a detailed understanding and control over the complex wire formation mechanism, but also enabling exact identification of the length and orientation of the molecules with respect to the electrode surface. The latter is of utmost importance in the context of integrating these functional molecules into electronic devices.

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Figure 7. Layer thickness derived from variable angle spectroscopic ellipsometry of each step: Au (0.01 ± 0.00 nm (substrate)), TP (0.99 ± 0.02 nm), Ru(TP, Ac3) (1.11 ± 0.13 nm), and Ru(TP, BTP) (1.82 ± 0.24 nm).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b12039. Assignments of IR, Raman, SER, IRRA, and calculated spectra; DDUVs, VASE, and AFM data; NMR, UV/vis, and mass spectroscopy data of the ligands; and model substances and their synthesis schemes (PDF)

Ru(TP, BTP), we considered the thickness of the Ru(TP, Ac3) SAM and added the length of one Fe-BTP unit.21 Based on the tilt angle for the molecular units of 48° determined by SERS, the thickness of the third layer should be approximately 1.80 nm. The thickness experimentally obtained by VASE is with 1.8 ± 0.2 nm close to the expected value. A larger standard deviation is obtained for the measurement of the last growth step, as the presence of TP and Ru(TP, Ac3) needs to be taken into account and leads to a larger deviation, which is consistent with the results from the vibrational spectra. Hence, the experimentally derived average values for the layer thicknesses of TP, Ru(TP, Ac3), and Ru(TP, BTP) on Au are 0.99 ± 0.02, 1.11 ± 0.13, and 1.82 ± 0.24 nm, respectively, and agree with our orientation analysis from SERS and IRRAS. Figure 7 illustrates the comparison of the spectroscopically determined molecular orientation and the resulting layer thicknesses with the layer thicknesses obtained from the VASE measurements.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +49-241-600953895. ORCID

Corinna Kaulen: 0000-0003-1194-5192 Silvia Karthäuser: 0000-0003-3953-6980 Sonja Herres-Pawlis: 0000-0002-4354-4353 Present Addresses ⊥

Functional Polymer Membranes Group, Advanced Membranes and Porous Materials Center, King Abdullah University of Science and Technology, 23955 Thuwal, Kingdom of Saudi Arabia (W.O.). ∥ FH Aachen, University of Applied Science, 52428 Jülich, Germany (C.K.).

4. CONCLUSIONS Here, we present a protocol for the formation of wires immobilized on Au surfaces consisting of terpyridine Ru complexes suited for on-chip preparation and the detailed analysis of the stepwise wire formation by combining spectroscopic means, DFT calculations, and VASE measurements. We focused our investigations on the first three steps in wire growth, involving the immobilization of the thiolsubstituted terpyridine derivative TP, the complexation with a respective Ru precursor, and further complexation with the bis-terpyridine derivative BTP. Spectroscopic analyses of each step involved IRRAS and SERS measurements of the formed Ru complexes immobilized on Au substrates as well as IR and Raman bulk measurements of respective reference compounds, representing each intermediate step. Furthermore, we obtained theoretical spectra of the respective compounds through DFT calculations. This combination allowed us to unambiguously assign the specific molecular vibrations of each intermediate. Moreover, this combination allowed us to derive the orientation of the molecules during growth. We found that the TP molecule itself is bound via the thiol anchor group with a tilt angle of 44° with respect to the surface normal and that the subsequent complexation of TP upon reaction with the

Author Contributions

S.K.P., C.K., S.K., M.H., and U.S. conceived and designed the experiments. S.K.P. synthesized the molecular building blocks and performed IRRAS, SERS, and VASE measurements. W.O. supervised the VASE measurements and data evaluation. A.H. performed the DFT calculations. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The German Research Foundation (DFG Si 609/16-1, Ka 1819/7-1) supported this work. The authors thank Seung-Jae Chong for AFM measurements. They acknowledge the OCuLUS Cluster at the PC2 Paderborn for permitting calculation.

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Stepwise Growth of Ruthenium Terpyridine Complexes on Au

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