Vibrational Coupling Modulation in n-Alkanethiolate Protected Au25

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Vibrational Coupling Modulation in n-Alkanethiolate Protected Au25(SR)180 Clusters Birte Varnholt,† Matthew J. Guberman-Pfeffer,‡ Patric Oulevey,† Sabrina Antonello,§ Tiziano Dainese,§ José A. Gascón,*,‡ Thomas Bürgi,*,† and Flavio Maran*,§,‡ †

Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, 1211 Geneva, Switzerland Department of Chemistry, University of Connecticut, 55 North Eagleville Road, Storrs, 06269 Connecticut, United States § Department of Chemistry, University of Padova, via Marzolo 1, 35131 Padova, Italy ‡

S Supporting Information *

ABSTRACT: We have studied, both experimentally and theoretically, the Raman vibrational spectra of a series of nalkanethiolate protected Au25(SCnH2n+1)18 clusters, with n = 2, 3, 4, 5, 6, 8, 10, 12, and 14. The C−H stretching region of the infrared spectra reveals that, while shorter chains are flexible, longer chains are more ordered with a propensity toward extended all-trans conformation. The different behavior of long and short chains is also reflected in the low-frequency Raman spectra of the clusters, which are broadened for the longer chains due to interchain interactions and formation of bundles. The experimental low-frequency modes in the Raman spectra, associated with Au−S stretching vibrations, change drastically and in an apparently unsystematic way as a function of chain length. For example, a band around 320 cm−1 associated with tangential Au−S stretching character shifts up in frequency, then down and then up again as the carbon chain is increased. DFT calculations reveal that this behavior is due to a nonlinear coupling of this mode to torsional and bending modes of the alkyl chain. The frequencies of these modes strongly depend on the chain length and, as a consequence, also their coupling with the Au−S stretching modes, which explains the erratic behavior of this band in the spectra. This behavior is well described by calculations on a mimic cluster model that considers only one staple motif. For the ethanethiolate-protected cluster, the entire cluster was included in the calculation of the Raman spectrum, and this allowed for the first time to compare directly experimental and calculated Raman spectra of the same cluster. Furthermore, our study shows that the entire ligand has to be considered for the calculation of the low frequency vibrations of the Au−S interface, as this spectral region is sensitive to coupling with low-frequency ligand modes. eV.18 The molecular features of Au25(SR)18 are particularly evident in its characteristic electrochemical pattern, consisting of a sequence of well-defined voltammetry peaks,18−20 and charge-dependent HOMO−LUMO gap and optical and magnetic behaviors (in both NMR and EPR spectroscopies).8,21,22 Applications of these and larger gold nanoparticles, particularly in the context of electronics, catalysis, and sensors, have been investigated and discussed.23−26 In most cases, understanding the nature of the gold-ligand interactions and the structure of the capping monolayer is essential. Among the methods providing efficient tools in this direction, vibrational spectroscopy can add valuable information on the properties of the thiolates composing the protecting monolayer. Earlier studies showed that it is possible to characterize the ligands’ structure and orientation.27−32 IR

1. INTRODUCTION Small thiolate protected gold clusters are a family of nanoobjects that can be synthesized with atomic precision. Their structures consist of a gold core surrounded by an Au−S interface of S-(Au−S)n staple-like motifs from which the (bio)organic “tails” of the thiolates emanate to constitute the exterior of the protecting monolayer. In recent years several examples have been investigated in detail. In these studies, the successful determination of single-crystal X-ray crystallography structures1−11 has been instrumental to understand their properties and the nature and arrangement of these protecting units on the surface. Within the class of monolayer protected gold clusters, the most stable and best-studied system is Au25(SR)18 (SR = thiolate ligand).12 This cluster, whose structure has been characterized for different ligands and charge states,2,8,13−17 displays a molecular behavior and thus a significant energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO); for the anionic charge state, the gap is 1.30−1.35 © 2016 American Chemical Society

Received: July 28, 2016 Revised: October 18, 2016 Published: October 19, 2016 25378

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tangential Au−S vibrations between 220 and 350 cm−1 with the latter occurring at higher wavenumbers and with very low intensities. In that frequency range, two bands consisting of several superimposed modes around 270 and 240 cm−1 were clearly resolved. Additionally, S-CH3 bending and Au−S stretching at 174 and 242 cm−1, respectively, were predicted. In a recent study,43 we reported a Raman spectrum of the bigger Au38(SCH3)24 cluster calculated at the DFT level. In contrast with Au25(SR)18 clusters, protected only by dimeric staples,2 the structure of Au38(SR)24 shows both monomeric and dimeric staples.3 That study allowed identifying contributions of the two types of staples. In particular, bands around 175 and 200 cm−1 belong to Au−S−C bending modes and are sensitive to the staple type. Bending modes belonging to short staples were predicted at lower energies and those of long staples at higher energies. The tangential vibrations of both types are predicted slightly below 300 cm−1 and are associated with low changes in polarizability leading to low intensities. The bands of highest intensity, the radial Au−S vibrations, between 240 and 280 cm−1 are also sensitive to the staple type. The short staples undergo an antisymmetric and a symmetric radial vibration. Vibrations of long staples can be distinguished as Au(core)−S radial and Au(staple center) radial modes and are responsible for the highest intensities around 250 cm−1. The special case of the breathing mode, the symmetric radial movement of all Au−S bonds, is expected to have a high Raman intensity due to the large change in the clusters polarizability. The influence of the ligand was investigated.37 For this purpose, model cyclic tetramers with different ligandsSH, SCH3 and SC2H4Phwere considered. The core breathing mode undergoes only a small change of 2 cm−1, whereas the staple breathing mode is significantly influenced by the ligand with values of 300, 293, and 282 cm−1, respectively. Concerning Au 25 clusters, the calculated spectra of Au25(SH)12(Cl)6− and Au25(SMe)18− showed frequencies of 111 and 104 cm−1 for the core breathing and 245 and 269 cm−1 for staple breathing. This study thus showed that the changing of the ligand influences the core breathing slightly and the staple vibrations significantly. These simplified tetrameric model systems also proved useful for other studies. For example, Au4(SC2H4Ph)4 systems were modeled in order to understand experimental vibrational circular dichroism spectra.45 With the help of DFT calculations, it was shown that the achiral ligand adopts a chiral conformation on the intrinsically chiral Au38(SR)24 cluster. Whereas for other spectral properties (e.g., optical absorption) the specific nature of the ligand has little effect on the spectrum, we show here that chain length has an important effect on the Raman vibrational spectrum of a cluster. Therefore, the standard modeling approach of substituting the organic ligands by SH or SCH3 cannot be applied. So far, the influence of the ligand on the cluster’s vibrational spectrum has not been modeled systematically with quantum detail. For example, to explain the coupling between Au−S stretching and longitudinal acoustic modes, Halas and co-workers used a simplified mechanical approach in which the alkyl chains Cn were modeled as elastic rods with n knots.46 In this work, we present the first direct comparison of DFT calculations with experimental measurements of low-frequency Raman spectra of a large series of Au25(SCn)180 clusters, with n = 2, 3, 4, 5, 6, 8, 10, 12, and 14. Among other findings, we observed that the spectral band at 300−330 cm−1 shows a distinct, nonlinear modulation as a function of the chain length.

spectroscopy also provides important information on ligand aggregation, via interchain hydrogen bonds inside the monolayer,31,32 as well as indication of phase-separation in mixed monolayers.33 Concerning ligands that can only interact among each other by van der Waals forces, the IR spectra of alkanethiolateprotected Au nanoclusters (monolayer protected clusters, MPCs) showed that the surface curvature makes these monolayers less packed than those of the corresponding 2D self-assembled monolayers on extended gold surfaces,30,34,35 pointing to weaker interchain interactions toward the monolayer periphery. This mobility effect, observed with clusters of 2.4−2.8 nm, is magnified but also with important differences for the much smaller Au25(SCnH2n+1)180 clusters (for simplicity, Au25(SCn)180), which have a core diameter of only 1 nm. For these molecular clusters, use of various physicochemical approaches, including vibrational spectroscopy, allowed us to detect a transition in the behavior of the clusters protected by aliphatic thiolates with different chain lengths.36 Relatively short alkanethiolates (n < 12) form a monolayer of quite folded ligands resulting in nearly spherical MPCs. On the other hand, alkyl chains of longer ligands form bundles of quite strongly interacting chains to yield more ellipsoidal systems. This study provided the rationale basis to demonstrate that linear alkanethiolate ligands form protecting but not-so-shielding monolayers. Ligand mobility is also responsible for allowing molecule and ion penetration into the monolayer.18 The structure of the Au25(SC2)180 shows that the Au−S bond distances, whether referring to the inner ligands (bond with one of the core Au atoms) or outer ligands (bond only with the staple Au atoms), is virtually the same as in the crystal of Au25(SC2H4Ph)18.8,13,15 Differences may occur because of specific interactions, such as for n = 4 in which a linear polymer forms,16 or upon ligand exchange.14 In addition to a dependency on the nature of the ligand, vibrational spectra are also sensitive to the cluster’s molecular structure, i.e., cluster’s size,36 aggregation state,38 charge,39 excitation state,40 and the arrangement of the ligands on the surface.41−43 The far-infrared region (150−600 cm−1) proved to be especially useful for studying binding properties on the surface. In this context, a first systematic work was performed by Creutz and co-workers,44 who provided far-infrared spectra of alkyl thiolates with varying chain lengths that were attached to about 2 nm size clusters. Signals in the range 150−260 cm−1 were assigned to Au−S vibrations. A band at 350 cm−1 was further assigned to S−C−C vibrations. The observed bands were broad, presumably due to the polydispersity of the investigated clusters and different binding sites. Changes in spectral pattern were observed for different chain lengths. Recently, we performed more detailed infrared42 and Raman43 studies on well-defined clusters whose structure and binding motifs are known. It was shown that the presence of different staple motifs affects the Au−S−C bending and the radial Au−S vibrations. The Raman spectra proved to be useful for identifying bands belonging to monomeric and dimeric staples. Additionally, we reported also spectral variation caused by the use of different protecting ligands. Theoretical predictions from DFT calculations are very valuable in the assignment of spectral bands to normal modes and can therefore provide insights into structure-vibration relationships. Tlahuice-Flores et al.37 conducted theoretical studies on IR and Raman spectra of Au25(SCH3)18− and smaller clusters. The calculated Raman spectra showed radial and 25379

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The Journal of Physical Chemistry C This comparison allows explaining the nature of such modulation pattern. As a byproduct of this analysis, which also includes IR results, we propose a simplified cluster model that can reproduce the Raman spectra quite well.

clusters was carried out according to the silica-gel column chromatography method described previously.36 Both redox forms gave the expected optical spectrum and the oxidized cluster had the expected MALDI-TOF mass-spectrometry pattern (Figures S1 and S2). 2.3. Instrumentation. The UV−vis spectra were obtained at 0.1 mM concentration in DCM with 2 mm cuvettes, using a Thermo Scientific Evolution 60S spectrophotometer and a spectrum resolution of 1 nm. MALDI-TOF mass spectrometry was carried out with an Applied Biosystems 4800 MALDITOF/TOF spectrometer equipped with a Nd:YAG laser operating at 355 nm. We set the laser firing rate at 200 Hz and the accelerating voltage at 25 kV. The cluster was dissolved to form a 0.1 mM solution in dichloromethane containing trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene] malononitrile (DCTB, Sigma-Aldrich, ≥ 98%) as the matrix (1:400 MPC/matrix ratio). A total of 2 μL of this solution was drop casted onto the sample plate and air-dried. The spectra were recorded using the reflectron positive-ion mode. The FTIR absorption spectra were recorded in benzene solution using 1 mm optical path cells, CaF2 windows, 1−2 mg mL−1 MPC concentrations and an argon flushed ThermoFisher Scientific Nicolet 6700 FT-IR spectrometer. The nominal resolution was 1 cm−1 and 16 scans were averaged. Raman spectra of the samples were measured as reported previously on the SCP-ROA (scattered circular polarization Raman optical activity) instrument developed by Hug and coworkers.50,51 Measurements were performed in backscattering geometry. The mean spectral resolution is 7.8 cm−1. The sample was deposited on a glass plate that was rotated at about 3000 rpm to prevent the sample from decomposition from excessive heating at the chosen laser light intensity of 5 mW. For coating of the glass slides (cover slides, 15 mm, MENZELGläser), the clusters were dissolved in dichloromethane, and methanol was added, leading to a more homogeneous coating. To ensure reproducibility of the results, at least two glass cover slides per sample were prepared; each slide was measured at three different positions. The spectra show small signals on top of a strong background which was removed using a rolling circle filter (RCF) with the same parameters, as previously described.43,52 All spectra were treated in the same way. After applying the described filtering procedure to remove the broad background features, the spectra were well reproducible for different samples of the same cluster and at different positions on the same glass slide.

2. EXPERIMENTAL SECTION 2.1. Calculations. All vibrational modes and Raman intensities for Au25(SCn)18 were obtained using DFT with the generalized gradient approximation (GGA) and the PBE exchange-correlation functional. LANL2DZ (19 valence electrons) was employed for Au atoms, and 6-31g(d) for all other atoms (S, C, and H). Whereas the use of a slightly more accurate and larger basis set (6-31g(d,p)) is tractable for Au25(SMe)18, it quickly becomes considerably more expensive for longer chains. However, the use of 6-31g(d) and 6-31g(d,p) for Au25(SMe)18 gives very similar results, both qualitatively and quantitatively. Thus, as a compromise between accuracy and tractability, we chose to use the slightly smaller basis set for all calculations. Optimizations for the Au25(SCn)18 clusters were carried out exclusively within the Ci symmetry point group, consistent with the recent X-ray structure described by Dainese et al.8 No corrections factors were applied to the calculated frequencies. To all calculated Raman intensities we applied Gaussian broadenings. 2.2. Synthesis of Clusters. The preparation of most of the Au25(SCn)18 clusters (n = 4, 6, 8, 10, 12, and 14) has been already described.16,36 Two more clusters, Au25(SC3)18 and Au25(SC5)18, were prepared to tune more finely the monolayer properties and test the possible occurrence of odd−even effects, as suspected for dry films of Au MPCs47 or observed in 2D SAMs.48,49 For both clusters we used the same synthetic protocol, here described in detail for Au25(SC5)18. The synthesis of Au25(SC5)18 was performed by dissolving 0.50 g (1.27 mmol) of HAuCl4·3H2O and 0.833 g of tetra-noctylammonium (n-Oct4N+) bromide (1.52 mmol, 1.2 equiv) in 40 mL of THF, to form a red solution that was stirred for 15 min at 20 °C. The stirring speed was set to 100 rpm, and 0.840 mL (7.62 mmol, 6 equiv) of 1-pentanethiol in 10 mL of THF was added dropwise over a period of ca. 3 min. The solution quickly became yellow and after ca. 30 min colorless. The stirring speed was increased to 600 rpm and a freshly prepared icy-cold aqueous solution (10 mL) of NaBH4 (0.48 g, 12.7 mmol, 10 equiv) was quickly added to the reaction flask, which was kept at room temperature. The solution immediately became black. The reaction progress was monitored by UV−vis absorption spectroscopy and after a little more than 24 h the reaction mixture could be filtered on paper to remove the white residues insoluble in common solvents. The filtered solution had a dark-brown color with orange hues. THF was removed with a rotary evaporator to leave a reddish-brown oily solid covered by a colorless liquid (residual H2O from aq. NaBH4). After removal of the water phase, the crude product was dissolved in toluene and washed with water (4 × 40 mL) in a separatory funnel. Toluene was evaporated, the solid was dissolved in DCM (50 mL), and the resulting solution left to rest for about 12 h in the dark at 4 °C. By this procedure, the cluster is obtained as [n-Oct4N+][Au25(SC5)18−]. The product was then further purified or oxidized. In the first case, the solid was dissolved in diethyl ether, which leaves undissolved most of the residual tetraoctylammonium salt. The solvent was evaporated and the solid washed thrice with icy-cold methanol to remove the remaining salt. The red-brownish solid was finally dried. Oxidation to form the neutral Au25(SCn)180

3. RESULTS AND DISCUSSION 3.1. Experimental Vibrational Spectra. All clusters were prepared and characterized as described in the Experimental Section. The as-prepared clusters are anions stabilized by tetran-octylammonium as the counterion. In this charge state, the clusters may undergo some minimum, spontaneous oxidation even during purification. On the other hand, the neutral clusters Au25(SCn)180 are indefinitely stable and carry no counterion that could have affected some of the measurements. For these reasons, in this study we consistently used the neutral forms for the spectroscopic studies; hereafter, the absence of a superscript in formulas Au25(SCn)18 indicates a neutral cluster. Oxidation was carried out by passage of a DCM solution of the anionic cluster through silica-gel chromatographic column using DCM as eluent and compressed air as the pushing gas.36 This rapidly causes the orange solution of the anionic cluster to turn green. After evaporation of the DCM solution, the oxidized cluster 25380

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antisymmetric (d-) mode; the symmetric (d+) mode shows the same effect (Figure S5). The gold−sulfur interface can be studied in the low wavenumber range, i.e., 600−100 cm−1, by using Raman spectroscopy. Figure 2 shows the experimental spectra (after

appears as a black-brownish powder. Comparison of the optical behavior of each of the investigated clusters before and after oxidation shows the expected transition in spectral features, and no evidence of further oxidation to the +1 charge state.21 The UV−vis spectral results36 including those obtained for the newly prepared clusters, show that the electronic structure is virtually unaffected by the variation of chain length. On the other hand, the vibrational spectra are quite sensitive to the ligand length. Information about the interaction between the chains can be found in the mid-IR range, as illustrated in Figure 1. For both free thiols and Au25(SCn)18, the effect of n on the

Figure 2. Raman spectra of Au25(SCn)18 showing strong chain length dependence.

Figure 1. FTIR spectra of Au25(SCn)18 in benzene. The spectra were normalized for the methyl antisymmetric stretch at 2954−2961 cm−1. The inset shows the dependence of the IR antisymmetric methylene stretching (d-) on n.

background correction) of the series of Au25(SCn)18 with n = 2, 3, 4, 5, 6, 8, 10, 12, and 14. The chain length has a strong impact on the vibrational spectrum. The higher mass of longer chains should generally cause a red shift in the spectra but no such trend is evident. Variations of the relative intensity and position of the bands can be observed throughout the whole range of the spectrum in Figure 2. The most prominent features are the vanishing of the band at 220−225 cm−1, which is present in C2 and C3 but not in longer chains; the fluctuating shift for the band at 320 cm−1; the shift of the signal at 380 cm−1. To understand the normal modes of these bands and their shifts, we relied on specific DFT calculations, which will be discussed later. Additionally, the spectra allow distinguishing between the different behaviors displayed by various chain lengths. As already discussed,36 whereas short aliphatic chains distribute in a more isotropic way around the cluster, long chains form bundle-like structures. For the latter, the spectra show less defined signals but exhibit a broad feature due to broadening and overlapping of signals. This behavior can be attributed to two causes. On one hand, van der Waals interactions between the chains cause an alignment, leading to collective vibrations and inhomogeneous line broadening. On the other hand, coupling between the longitudinal acoustic modes of the long chains and the Au−S vibrations complicates the spectra.45 To understand the details of the spectral changes in the low frequency range, we calculated the spectra of Au25, protected with methyl thiolates, commonly used to simplify calculations, and with ethyl thiolates, the smallest ligand used experimentally. This is the first direct comparison between calculated and experimental vibrational spectra of gold MPCs. 3.2. Calculations. The DFT structure for Au25(SEt)180 is in good agreement with the X-ray structure described by Dainese

methyl antisymmetric stretch band is minimum, with a frequency value of 2954−2961 cm−1. On the other hand, the methylene C−H stretching modes are very sensitive to the variation of n. For both symmetric (d+) and antisymmetric (d-) modes, which are known to be diagnostic for obtaining information about the conformation of alkyl chains, the free aliphatic thiols show a red shift as n increases, until a virtually constant frequency is attained for n > 10 (Figures S3 and S4). The behavior of the same chains in the monolayer capping Au25, however, is more complex. A similar red shift is observed up to C8, followed by a slight blue shift for C12-C14 (inset to Figure 1). The C5 case represents an exception, both for the intensity and the frequency of the antisymmetric methylene stretching. Longer thiolates show36 strongly red-shifted signals and frequency values similar to those observed for 2D-SAMs and larger nanoparticles.30,53 These results indicate a different behavior for short and long chains, the latter eventually displaying the frequency expected for a fully extended all-trans conformation. As aforementioned, previous analysis36 including 1 H NMR spectroscopy results, the effect of distance on the electron-transfer rate across the monolayer, and specific molecular dynamics simulations, showed that the different behavior is caused by a structural transition of the thiolates in the monolayer as one goes from shorter to longer ligands. Whereas the shorter ligands form a fluid monolayer structure of folded chains, longer alkyl chains (n > 12) self-organize into bundles of chains interacting by van der Waals forces. This behavior has never been detected or suspected for larger MPCs. Figure 1 and its inset also highlight the very interesting different behavior displayed by the C5 case. The inset refers to the 25381

DOI: 10.1021/acs.jpcc.6b07592 J. Phys. Chem. C 2016, 120, 25378−25386

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The Journal of Physical Chemistry C et al.8 In the crystal structure the average distance between the central Au atom and the 12 icosahedral core atoms is 2.8 Å. That same distance varies from 2.86 to 2.89 Å in the calculated structure. Another important distance is that between the 12 icosahedral core Au atoms and the Au atoms of the staple motifs. In the crystal structure, the average distance varies from 3.07 to 3.35 Å (average = 3.18 Å). In the calculated structure, that distance varies from 3.18 to 3.48 Å (average = 3.30 Å). Our Raman calculations for Au25(SMe)18 in the far-infrared region are also in very good agreement with previous calculations by Tlahuice-Flores et al.37 However, the first aspect emerging from the present calculations is that using SMe ligands as models for longer ligands can lead to misinterpretation of peak assignments. For instance, the radial Au-SR stretching modes in SEt are red-shifted by 40 to 50 cm−1 when compared with equivalent peaks in SMe (Figure 3). Relative intensities for the

bending and torsional modes. This coupling plays an important role in the modulation of bands as the chain length increases. From Figure 3, it is also evident that even core modes differ considerably in the 100 cm−1 region. Such difference is due to core breathing modes coupled to chain torsional modes. In conclusion, calculations carried out for a specific ligand type are not transferable to other ligands for the regions of the spectrum that involve coupling between the staple motifs and the protecting ligands. Another interesting aspect worth mentioning is the comparison of the vibrational spectra of the neutral Au25(SR)18 and the anionic Au25(SR)18− clusters. Whereas here we describe the results for the neutral cluster (as used in the experiments), we also calculated the Raman spectrum for Au25(SMe)18− and Au25(SEt)18−. In general, some differences between the neutral and anionic clusters are detectable (Figure S6 and S7); however, they are small, especially for the tangential Au−S stretching band analyzed below. Interestingly, such differences are much less pronounced in SEt than in SMe. Having established that the alkyl chains should be modeled without approximations, in Figure 4 we start by making a

Figure 4. Calculated and experimental Raman spectrum of Au25(SEt)18. To help in the assignment of vibrational modes, the calculated spectrum was shifted by a constant value of 20 cm−1. A Gaussian line shape with full width at half height of 12 cm−1 was applied to the calculated frequencies.

comparison between theory (cf., Figure 3, bottom panel) and experiment for Au25(SEt)18 (cf. Figure 2). We associate the theoretical peak at 300 cm−1 with that located at 320 cm−1 in the experimental spectrum. Thus, at the level of theory here employed, we underestimate the experimental peak by about 20 cm−1. Nonetheless, the main features of the experimental spectrum are reproduced very well, particularly in the area involving staple vibrations and their coupling with the protecting ligands. To guide the eye in matching the theoretical and the experimental spectrum, and to assign vibrational modes to the later, Figure 4 shows the superposition of the two spectra with the calculated frequencies shifted by a constant value of 20 cm−1. As shown for Au25(SEt)18 in Figure 3, both the peak at ∼320 cm−1 and the peak at ∼375 cm−1 correspond to tangential Au−S stretching modes coupled with alkyl vibrations. The lower-frequency Au−S stretch couples with an ethyl torsional mode, whereas the higher-frequency Au−S

Figure 3. Calculated Raman spectra of Au25(SMe)18 (top) and Au25(SEt)18 (bottom). The line shape was constructed by adding Gaussians with full width at half-maximum of 4.7 cm−1 to each calculated spectral line.

peaks in the 300−350 cm−1 region are also quite different for the two systems. This is particularly important because these peaks show strong modulation as a function of the chain length (Figure 2). The reason the SEt spectrum is considerably different from the SMe spectrum in this region is due to the interaction of the Au−S stretching modes with the ethyl 25382

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The Journal of Physical Chemistry C stretch couples with an ethyl bending. As we show in the next section, the nature and spectral position of the coupled vibration at 320 cm−1 changes in a chain-length dependent fashion from C3 and beyond. This observation is crucial to understand the evolution of this band as a function of the chain length. 3.3. Analysis of Normal Modes and Chain Length Dependence. Calculating the spectra at the DFT level for the Au25 cluster with all chain lengths used experimentally would be exceedingly computationally expensive. Therefore, model systems that could reasonably reproduce the experimental observation at a significantly reduced computational cost were explored. We considered several models, including AuSEt, H2Au2SEt−, Au2(SEt)3−, Au3SEt, Au5(SEt)3, Au9(SEt)6−, and Au17(SEt)12−. For each model system we calculated the Raman spectrum. We then selected the minimal model that reproduced qualitatively and semiquantitatively the calculated Raman spectrum of Au25(SEt)18 in the region of interest (200−500 cm−1). We found that Au5(SEt)3 is the smallest system that reproduces the simulated Raman spectral region of interest for Au25(SEt)18. This model system was constructed by extracting one staple from Au25(SEt)18 together with three core gold atoms that belong to the same plane as the staple. The model was then fully relaxed, leading to the structure shown in Figure 5 (inset, upper figure).

Figure 6. Experimental Raman spectra for Au25(SCn)18 (n = 2 to 6) with the chain-length dependent spectral feature indicated with vertical segments.

Figure 7. Theoretical Raman spectra for the model system Au5(SCn)3 (n = 2 to 6) with the chain-length dependent spectral feature indicated with vertical segments. A Gaussian line shape with full width at half height of 4.7 cm−1 was applied to each frequency.

Figure 5. Comparison of the simulated Raman spectral region of interest for Au25(SEt)18 and the model system, Au5(SEt)3. A Gaussian line shape with full width at half height of 4.7 cm−1 was applied to the calculated frequencies.

For longer alkyl chains, we proceeded to increase the length of the chain while maintaining the Au5 unit motif constant (Au5(SR)3, R = ethyl through hexyl). As Figure 6 shows, a marked chain-length dependence is observed experimentally for the position of the Raman spectral features in the region 280− 330 cm−1 (indicated with vertical segments). The calculated spectra for the model system reproduce quite well all these features (Figure 7). In particular, theory reproduces the behavior of the main band in the 300−330 cm−1 region (corresponding to Au−S tangential stretching). The chain length dependent frequency shifts for the 300−330 cm−1 band are summarized in Figure 8. Despite the overall shift between the two curves, theory clearly follows the experimental pattern for this particular band. We have also included in this

Figure 8. Comparison between experiment and theory of the band in the region 300−330 cm−1.

figure the peak for C8, which experimentally and theoretically shows a blue shift with respect to C6. 25383

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The Journal of Physical Chemistry C

4. CONCLUSIONS Raman spectroscopy is a valuable technique for investigating the structure and properties of monolayer protected gold clusters. Au25 clusters protected by a series of linear alkanethiolates show features that are fluctuating in position and relative intensity. The spectral shape allows discriminating the interligand interactions leading to the bundle-like behavior of long chains from the free spherical orientation of short ligands. By including a systematic minor offset in the frequency values, the DFT spectral patterns show an excellent match with the experiment outcome. DFT calculations show that torsional and bending ligand motions couple with the Au−S stretching modes, and this has a clear influence on the Raman spectra. Such coupling is modulated by the intrinsic alkyl chain fluctuations of torsional and bending vibrations around the Au−S stretching frequency. These couplings are thus responsible for the noticeable shifting of the experimental signals. The C5 case stands out because the coupling is minimal. The C5 case is also special for the frequency of the antisymmetric methylene stretch mode. Calculations also reveal that simplified ligands should not be used to predict the spectrum. Interestingly, however, proper selection of small models, such as the proposed Au5 cluster that includes one single staple, is suitable to provide a very good correlation with experiments, as long as ligands are not simplified.

The ensuing questions now are what is the nature of this band that exhibits a nonlinear behavior and what is the reason for the observed unusual shift as a function of chain length? The answers have to do with the resonances between the Au−S stretching modes and the alkyl chain modes. For Au5(SEt)3, the peak at 312 cm−1 is an Au−S stretch coupled to an ethyl torsion. This coupling stems from the fact that the two lowest energy normal modes for ethane are a torsion at 311 cm−1, and a bending vibration at 808 cm−1. Tangential Au−S stretching occurs, e.g., in Au5(SMe)3, in the 300 cm−1 region. Therefore, due to the close proximity in energy of tangential Au−S stretching and the ethyl torsion, these normal modes couple well. As the chain length increases, alkyl torsions continuously red shift. For instance, alkyl torsional modes fall into the 250 cm−1 region for C6. On the other hand, the C−C−C bending motions emerge in the 250−450 cm−1 range from C4 through C6. In contrast to the ethylthiolate case, both alkyl torsional and bending vibrations are similar enough in energy to mix with tangential Au−S stretching for C3 through C6 chain lengths. Thus, the ethylthiolate ligand is unique in that the alkyl motion that couples with tangential Au−S stretching is purely torsional, whereas for propyl through hexyl chain lengths, the alkyl motion that couples with tangential Au−S stretching is a blend of torsional and bending vibrations. For chain lengths in which the alkyl vibration coupled with tangential Au−S stretching is similar in nature (e.g., propyl through hexyl), the frequency of the spectral feature of interest likely decreases (with respect to the ethyl chain) because of the greater reduced mass involved in the normal mode with increased chain length. Notice that both theory and experiment show that such red shifting reaches a maximum value for C5. Indeed, for the C5 case, the frequency of the coupled vibration is ∼302 cm−1, which is essentially identical to the tangential Au−S stretching in Au5(SMe)3 at ∼304 cm−1. The nearest alkyl torsional modes are ∼60 cm−1 less, and the nearest alkyl bending modes are ∼90 cm−1 higher in frequency. Coupling with an alkyl torsion or bending mode is so poor for C5 that the frequency of Au−S stretching is practically unperturbed. Importantly, for no other chain length is the energy mismatch between tangential Au−S stretching and both alkyl torsional and bending vibrations as great as for the C5 case. In other words, the decoupled Au-SCH3 system has a larger effective mass because it is decoupled from a lighter atom chain. Note that also in the IR spectra of the methylene stretch modes (cf r. inset in Figure 1) the C5 case is special. As the chain is made longer, the position of the band blueshifts. This shift is attributable to a greater resonance of tangential Au−S stretching with an alkyl bending vibration, rather than with lower-frequency alkyl torsion. Whereas for the C6 and C8 chain lengths alkyl torsional modes are consistently in the 240 cm−1 region, an alkyl bending vibration falls from 452 cm−1 in C6 to 325 cm−1 in C8, thereby permitting strong resonance of this bending vibration with tangential Au−S stretching in the 300 cm−1 region. Visualization of the normal modes shows that this resonance is sufficiently strong that the octyl vibration that couples with Au−S stretching is entirely bending in nature. In summary, the chain length dependence of the Raman spectral feature in the 300−330 cm−1 region corresponds to coupling of tangential Au−S stretching with an ethyl torsion, a complex blend of torsional and bending vibrations for propyl- through hexyl-chain lengths, and an octylbending vibration.

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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.jpcc.6b07592. MALDI-TOF mass spectrometry, UV−vis absorption spectroscopy, further IR absorption spectroscopy and calculated Raman spectra, and Cartesian coordinates of the optimized structures. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: (860) 486 0591. *E-mail: [email protected]. Tel: +41 (0)22 3796552. *E-mail: fl[email protected]. Tel: +39 049 827 5147. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Italian Association for Cancer Research, AIRC (FM, Grant 12214: Innovative Tools for cancer risk assessment and early diagnosis −5 per mille), the Swiss National Science Foundation (TB, Grant 200020_152596), and the Office of the Vice President of Research at the University of Connecticut (JG).

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REFERENCES

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