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Cite this: Phys. Chem. Chem. Phys., 2013, 15, 19561 Received 10th September 2013, Accepted 4th October 2013
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Far-infrared spectra of well-defined thiolate-protected gold clusters† Igor Dolamic, Birte Varnholt and Thomas Bu ¨ rgi*
DOI: 10.1039/c3cp53845a www.rsc.org/pccp
The far infrared spectra of a series of well-defined gold clusters covered with 2-phenylethanethiolate were studied. The spectra of the clusters are different but the differences are subtle. The Au–S stretching vibrations give rise to bands around 300 cm
1
and below. The relative
intensity of these bands changes but they shift only slightly for different clusters. A low-frequency band was identified, which is sensitive to the conformation (trans/gauche) of the 2-phenylethanethiolate ligand.
Introduction A better understanding of the thiolate–gold interaction is of great importance in different research areas like self-assembled monolayers (SAMs),1 gold nanoparticles and clusters2 and molecular electronics based on gold.3 Since Brust et al. established the synthesis of gold nanoparticles protected by thiolates,4 the latter have been studied widely.2,5 The unique properties of the well-defined thiolate protected gold clusters make them attractive for fields like catalysis, drug delivery and sensing.6,7 The first crystal structure reported for a thiolate-protected cluster Au102(SR)44 by Jadzinsky et al. showed the presence of staple units SR-Au-SR (monomeric) and SRAu-SR-Au-SR (dimeric) that cover a densely packed gold core.8 Such staple units were later also found for Au25(SR)189 and Au38(SR)2410 and proposed for Au40(SR)2411 and Au144(SR)6012 and they might also be ubiquitous on flat thiolate-covered gold surfaces (SAMs).13 The staple units play an important role in determining the structure of the clusters. For example, their arrangement on the surface can impart chirality to the cluster.14–16 Also, the staple units provide stability and influence the chemical properties of the clusters. Despite intense research on thiolate-protected clusters and their characterization, up to now not much attention has been given to their vibrational properties.17–24 In general, the vibrational spectrum is a very characteristic property of a compound. Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland. E-mail: [email protected]; Fax: +41 22 379 61 03; Tel: +41 22 379 65 52 † Electronic supplementary information (ESI) available: Experimental details, UV-Vis spectra of the clusters, MALDI mass spectra of the clusters, and details of DTF calculations. See DOI: 10.1039/c3cp53845a
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In addition, mid-infrared (IR) spectra are easily accessible. However, in the case of thiolate-protected gold clusters and nanoparticles this spectral range is characteristic for the vibrations of the adsorbed thiolates. Extracting structural information from such mid-IR spectra is not trivial, as variations of the spectra due to structural changes of the adsorbate (molecular conformation), of the adsorbate layer (arrangement of adsorbates) and of the cluster itself are subtle. Vibrational circular dichroism (VCD)25 is sensitive to the structure of a molecule, both absolute configuration26 and conformation.27 VCD spectra were recently recorded for thiolate-protected gold clusters providing some information on the conformation of the adsorbed molecules.28–31 An interesting spectral region is the far IR (30–350 cm 1) as in this spectral range Au–Au and Au–S vibrations are expected. The characteristic structural elements (staple motifs) of the gold–thiolate interface should exhibit characteristic vibrational bands. Kawai and coworkers32 studied alkanethiol self-assembled monolayers on Au(111) surfaces by high resolution electron energy loss (HREELS) spectroscopy and assigned bands in the range 210–280 cm 1 to Au–S vibrations. For the shortest alkanethiol investigated (C2) three bands at 210, 250 and 320 cm 1 were reported. Creutz and co-workers studied alkanethiolcapped gold nanoparticles by far-IR spectroscopy.24 The particles were about two nanometers in diameter and were covered with alkanethiols of different lengths. The spectra changed with the length of the alkanethiol and the bands were quite broad, which may be attributed to the fact that the nanoparticles were not perfectly monodispersed. Bands at 260–270 cm 1 and around 180 cm 1 were assigned to Au–S vibrations. Murray and coworkers reported the Raman spectra of [Au25(SCH2CH2Ph)18] 1 and [Au25(SCH2CH2Ph)18]0 and found a band assigned to a Au–S vibration around 290 cm 1 for the anionic cluster. The band shifted to lower energy by 24 cm 1 for the neutral cluster.22 Density functional theory (DFT) calculations identified a Au–S stretching vibration around 300 cm 1 for the Au25 cluster.20 The most detailed analysis of the low-frequency vibrations of small thiolate-protected gold clusters was reported by Tlahuice-Flores and colleagues,17 who used DFT calculations for the prediction
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Communication of far-IR (FIR) and Raman spectra. The largest cluster studied was [Au25(SCH3)18] 1. In general the appearance of the calculated FIR spectra was found to be different for the different clusters considered in that study but some general conclusions could be drawn: Au–Au vibrations were found far below 200 cm 1. Some of these modes were intense in the calculated Raman but weak in the far-IR spectra. Au–S vibrations were classified as tangential vibrations involving the shorter Au(staple)–S(staple) bonds and radial vibrations involving the softer Au(core)–S(staple) bonds. The former were located near 300 cm 1 and slightly above, whereas the latter were found typically below 300 cm 1. The calculated FIR spectrum of [Au25(SCH3)18] 1 showed four bands in the range 220–320 cm 1. The most intense band at 240 cm 1 was attributed to Au(core)–S(staple) stretching modes. Heiz and coworkers reported IR spectra of size-selected gold nanoparticles protected by 2-phenylethanethiol (HSCH2CH2Ph, 2-PET).33 However, they focused on the mid-infrared spectral region and therefore no information on the Au–S vibrations was reported. Surprisingly, in the C–H stretching region the spectrum of the Au25 cluster showed an unexpected red-shift of the aromatic C–H vibrations, which was not observed for the free ligand and the other clusters investigated (Au38 and Au144). To the best of our knowledge up to now no systematic experimental study on the low-frequency vibrations of well-defined thiolate-protected gold clusters was performed. An open question that is addressed in this contribution is whether the low-frequency vibrational spectrum is characteristic for each cluster, and whether one can discriminate between different structural elements, i.e. monomeric and dimeric staple units, which would allow a straightforward characterization of new, unknown samples. For this we prepared samples of composition Aun(2-PET)m with (n,m) equal to (25,18), (38,24), (40,24) and (144,60). The ratio of dimeric to monomeric staple units changes in the series from only dimeric units (Au25) over a mixture of dimeric and monomeric units (Au38, Au40) to only monomeric units (Au144). The structure of the studied clusters is shown in Fig. 1.
Experimental Gold nanoclusters were prepared and purified as described in previous reports.34–36 All samples were characterized using UV-vis spectroscopy and MALDI-mass spectrometry. More details about preparation and characterization of size-selected gold nanoclusters can be found in the ESI.† FIR spectra were recorded on a Bruker Vertex 80sv Fourier transform IR spectrometer. Measurements were performed in vacuum at a resolution of 4 cm 1 by averaging 200 scans. A Mylar 6 mm multilayer beamsplitter was used. The samples were dissolved in dichloromethane, spread over a silicon window substrate and dried. Alternatively the sample was dried on the window of a diamond cell.
Results and discussion Fig. 2 shows FIR spectra of five gold clusters: (a) Au144(2-PET)60, (b) Au40(2-PET)24, (c) Au38(2-PET)24, (d) [Au25(2-PET)18] 1 TOA+
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Fig. 1 Structure of (a) Au144(2-PET)60 (30 monomeric staples); (b) Au40(2-PET)24 (6 monomeric and 4 dimeric staples); (c) Au38(2-PET)24 (3 monomeric and 6 dimeric staples); (d) Au25(2-PET)18 (6 dimeric staples). Green, gold core atoms; yellow, gold ad-atoms; orange, sulphur. For clarity –CH2CH2Ph units were removed.
(TOA+, tetraoctylammonium counterion) and (e) [Au25(2-PET)18]0. A detailed view is shown in Fig. 3. Fig. 2(a) shows the FIR spectrum of the Au144(2-PET)60 cluster. The structure of the cluster is not yet confirmed but it has been proposed that the Au144(2-PET)60 cluster consists of an Au114 icosahedral core protected by 30 RS-Au-SR monomeric staple motifs only.12 Ackerson and coworkers used NMR spectroscopy to show that all the 60 2-PET ligands are symmetry equivalent, in agreement with the proposed model.37 Bands at 321 cm 1 and 284 cm 1 fall in the spectral range expected for Au–S vibrations. Following the calculations of Tlahuice-Flores17 the former can be assigned to tangential Au(staple)–S(staple) vibrations whereas the latter might be due to Au(core)–S(staple) modes or Au(staple)–S(staple) modes. Recently the isolation of the Au40(2-PET)24 cluster as a side product of the etching process in the Au38(2-PET)24 cluster synthesis has been reported.15,34 The structure of this cluster is not yet solved. High resolution transmission electron microscopy (TEM) shows an elongated cluster.11 Furthermore, the cluster was shown to be chiral and the circular dichroism (CD) spectra were measured after separation of the enantiomers by high performance liquid chromatography (HPLC).15,38 A structure (more precisely a family of structures) was proposed where a bi-icoshaedral Au26 core is protected by six monomeric and four dimeric staple units. Density functional theory (DFT) was used to calculate the absorption and CD spectra of the proposed structure, which are in fair agreement with experiment. Another model for Au40(2-PET)24 was also proposed which, according to calculations, should have similar energy (or be even slightly more stable).39 In this model the cluster
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Fig. 2 Far infrared spectra of size selected gold nanoclusters: (a) Au144(2-PET)60, (b) Au40(2-PET)24, (c) Au38(2-PET)24, (d) [Au25(2-PET)18] TOA+, (e) [Au25(2-PET)18]0.
Fig. 3 Far infrared spectra of: (a) Au144(2-PET)60, (b) Au40(2-PET)24, (c) Au38(2-PET)24, (d) [Au25(2-PET)18] TOA+, (e) [Au25(2-PET)18]0.
core is protected by six dimeric and three monomeric staples. However, there are no calculated spectra reported for this model, which would allow a comparison with the available experimental data. Fig. 2(b) shows the FIR spectrum of Au40(2-PET)24. In the spectral region where the Au–S stretching vibrations are expected three bands are observed at 329 cm 1, 285 cm 1 and 216 cm 1. According to the calculations by TlahuiceFlores17 the band above 300 cm 1 may be assigned to tangential Au(staple)–S(staples) modes whereas the one at 216 cm 1 may be due to radial Au(core)–S(staple) modes. The band at 285 cm 1 may be assigned to both types of Au–S modes. The structure of the Au38(2-PET)24 was solved by X-ray crystallography.10 The cluster consists of a bi-icosahedral Au23 core and of three monomeric and six dimeric staple units. The achiral 2-PET ligand itself is arranged in a chiral fashion on the cluster surface and both enantiomers of Au38(2-PET)24 are contained in the unit cell of the crystal. Recently, enantiomers of Au38(2-PET)24 were separated using chiral HPLC.14 Fig. 2(c) shows the far infrared spectrum of Au38(2-PET)24. Overall the spectrum looks similar to the one of Au40(2-PET)24 with different relative intensities of the bands. Bands associated with Au–S modes are found at 315 cm 1 and 284 cm 1. Compared to Au40 the former band (tangential Au–S mode) is more intense and is
shifted to lower wavenumbers (14 cm 1 shift). A weak band at 217 cm 1 cm 1 is also observed. The structure of Au25(2-PET)18 is also known.9 The cluster consists of an Au13 icosahedra core and is protected by six dimeric staple motifs. The cluster exists in neutral and anionic form. In the charged cluster tetraoctylammonium (TOA+) is present as a counter ion. Fig. 2(e) shows the FIR spectrum of the neutral form of Au25(2-PET)18, whereas in Fig. 2(d) the spectrum of the anionic form is shown. Bands at 523 cm 1 and 536 cm 1 with a shoulder at 542 cm 1 in the spectrum of the anionic cluster are associated with the TOA+ counter ion and are missing in the spectrum of the neutral [Au25(2-PET)18]0 cluster. The bands assigned to Au–S vibrations are found at 317 cm 1 and 282 cm 1 for the neutral cluster and at 313 cm 1 and 281 cm 1 for the anionic cluster. Note that for the same cluster a band was observed around 280 cm 1 in the Raman spectrum of the anionic compound, which shifted to lower frequencies by about 24 cm 1 for the neutral cluster.22 The band observed in the present study around 280 cm 1, which we assign to Au–S radial and/or tangential modes, hardly shifts when changing the oxidation state of the cluster. This discrepancy between Raman and IR is likely related to the fact that the two techniques probe different modes. Likely the mode in the reported Raman spectrum is related to the staple breathing mode
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Communication where all the radial Au–S bonds elongate in phase. This mode should have a large Raman cross-section but it should be silent in the IR spectrum. The FIR spectrum of [Au25(2-PET)18] 1 can be compared to the one calculated for [Au25(SCH3)18] 1,17 as the structure of the two clusters is the same except for the thiolate ligand. The Au–S vibrations should be similar. However, different ligands could affect the Au–S vibrational spectra through their different mass. The calculated spectrum shows four bands at around 320, 300, 270 and 240 cm 1, with the latter one being the most intense. Below 240 cm 1 only very weak bands were calculated. The prominent band at 240 cm 1 (and also the band at 270 cm 1) was assigned to Au(core)–S(staple) vibrations (radial modes), whereas the bands at 300 and 320 cm 1 were assigned to tangential Au(staple)–S(staple) vibrations. In the experimental FIR spectrum of [Au25(2-PET)18] 1 (Fig. 2 and 3) two quite prominent bands are observed at 311 and 282 cm 1 (for exact numbers, see table). The former one could be assigned to tangential Au–S modes, following the calculations by TlahuiceFlores and colleagues.17 The latter band may be due to radial or tangential modes. At lower wavenumbers, in the predicted range for the radial Au- vibrations two further bands at 250 cm 1 and 215 cm 1 may be observed. These bands are very broad and more distinct in the spectrum of the neutral cluster (see Fig. 3). The intensity of the corresponding bands in the reported calculations and the measured FIR spectrum of the Au25 cluster do not agree well. This might have several reasons, one is a temperature effect not considered in the calculations. The intensity of the low frequency vibrations is damped because these vibrations are populated at room temperature. Another reason might be the different ligands used (2-PET vs. SCH3). A comparison shows that the FIR spectra of the clusters are different although the same ligand is used in all cases. This has certainly to do with the fact that the vibrations involving the staple units fall in this range and that the structure of the Au–S interface (arrangement and relative abundance of short and long staples) is different for the clusters. For example, the FIR spectra of Au38(2-PET)24 and Au144(2-PET)60 are clearly different in the FIR region whereas their spectra cannot be distinguished in the mid-IR spectral region.33 However, the differences in the FIR are subtle and mostly concern the relative intensity of bands. For example, all the clusters show bands at around 280 cm 1 and 315 cm 1 but their relative intensity differs for the different clusters. As the size of the clusters decreases, the number of monomeric staples decreases as well, whereas the number of dimeric staples increases. However, there is no clear systematic variation of the FIR spectra, which could be traced to characteristic vibrations of the short or long staples. In fact, the spectra of the Au25(2-PET)18 and the Au144(2-PET)60 clusters are quite similar. The former cluster is covered exclusively by dimeric staples, whereas the latter is presumably covered only by monomeric ones. It seems therefore difficult to estimate the relative abundance of long and short staples based on the FIR spectrum of a cluster. Some of the bands observed in the spectral range considered here (600–100 cm 1) are associated with modes related to the 2-PET ligand. In order to better understand these modes we
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PCCP performed DFT calculations of isolated staple units. For this the geometry of one staple was taken from the crystal structure of Au38(2-PET)24, including three Au core atoms. The latter were fixed and the staple relaxed before calculating the normal modes (for more information see ESI†). Based on these calculations we can assign the prominent band around 560 cm 1 observed for all clusters to in plane deformation modes of the phenyl rings coupled with C–S stretching modes. The band at around 490 cm 1 is due to coupled out of plane deformation modes of the phenyl rings coupled with C–C–S bending modes. This mode strongly depends on the conformation of the 2-PET ligand (see below). The bands at around 400 cm 1 are due to out of plane modes of the phenyl ring coupled to CH2 rocking vibrations. The bands found at around 180 cm 1 may be due to Au–S–C bending modes. Table 1 lists the observed modes and provides an assignment. The band at around 490 cm 1 deserves some more attention as its frequency shifts with the conformation of the 2-PET ligand. The 2-PET ligand has some conformational freedom, particularly around the C–C bond of the ethyl group. More precisely, the sulphur atom and the phenyl ring can adopt anti or gauche positions. In the crystal structure of Au38(2-PET)2410 we counted 14 anti and 10 gauche conformations, whereas in Au25(2-PET)189 14 anti and 4 gauche conformations are found. For the gauche conformers the vibration mentioned above is shifted to lower frequencies by about 12 cm 1 according to the calculations. This shift can also be rationalized based on the calculations. As mentioned above the vibration around 490 cm 1 is mainly an out of plane phenyl vibration, which has also some C–C–S bending character. However, this is only true for the trans conformation. In the gauche conformation the out of plane phenyl vibration is coupled to a torsion around the C–S bond. This mode is softer than the C–C–S bending coordinate and therefore the vibration shifts to lower frequency for the gauche conformation. This gives rise to the band (shoulder) at around 470 cm 1 in the experimental spectra. A comparison shows that the relative intensity of the main band at around
Table 1 Observed bands (wavenumbers) in the far-IR spectra of size-selected gold nanoclusters covered with 2-PET (Aun(2-PET)m, (n,m))
(25, 18) (144, 60) (40, 24) (38, 24) (25, 18)0 TOA+ 560
562
561
561
490 468 404 390
493 471 402 389
492 468 401 390
321 284
329 285
315 284
216
217 180 158
492 467 403 383 354 317 282 250 215 180
175
1
Assignmenta
563 537 523 492 471 403 383
i.p. def. + nC–S TOA+ TOA+ o.o.p. def. + C–C–S b. o.o.p. def. + t. o.o.p. def. + CH2 r. o.o.p. def. + CH2 r.
311 282
n Au–S, tangential n Au–S, tangential/radial
176
Au–S, radial Au–S–C b.
a
i.p.: phenyl in plane; o.o.p.: phenyl out of plane; def.: deformation; t.: torsion; r.: rocking; b.: bending.
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490 cm 1 and the shoulder at around 470 cm 1 differs for the clusters studied, indicating that the relative abundance of trans and gauche conformations of the 2-PET ligand is changing. The most intense shoulder is observed for the Au40(2-PET)24 cluster indicating that this cluster has the most gauche conformations of all the samples. Also, for the Au38(2-PET)24 cluster the shoulder associated with the gauche conformations of 2-PET is more pronounced than for the Au25(2-PET)18 cluster, in agreement with the crystal structure, which shows more gauche conformations for the former cluster. It is important to note that the conformation (trans/gauche) might be different in the single crystal and in the state observed here (amorphous solid) or in the liquid state.
Conclusions A series of well-defined gold nanoclusters protected by 2-phenylethanethiolate were prepared and their far IR spectra measured. The clusters of different size contain ligand shells of different composition with respect to monomeric and dimeric staple units. The far IR spectra are characteristic for each cluster but spectral differences are subtle. No clear systematic variation could be found which would allow one to quantify the relative amount of monomeric and dimeric staple units. A band near 490 cm 1, which is associated with an out of plane mode of the phenyl moiety coupled to a C–C–S bending mode, is indicative of the conformation of the phenylethylthiolate. Most of the ligands adopt a trans conformation of the S–C–C-phenyl unit. However, some units are gauche, which leads to a shift towards lower wavenumbers of this band.
Acknowledgements This work is supported by the Swiss National Science Foundation. We thank Dr Sophie Michalet (SMS, UniGE) for the MALDI measurements.
Notes and references 1 J. C. Love, L. A. Estroff, J. K. Kriebel, R. G. Nuzzo and G. M. Whitesides, Chem. Rev., 2005, 105, 1103–1169. 2 H. Hakkinen, Nat. Chem., 2012, 4, 443–455. 3 M. A. Reed, C. Zhou, C. J. Muller, T. P. Burgin and J. M. Tour, Science, 1997, 278, 252–254. 4 M. Brust, M. Walker, D. Bethell, D. J. Schiffrin and R. Whyman, J. Chem. Soc., Chem. Commun., 1994, 801–802. 5 A. C. Templeton, M. P. Wuelfing and R. W. Murray, Acc. Chem. Res., 2000, 33, 27–36. 6 C. F. Shaw, Chem. Rev., 1999, 99, 2589–2600. 7 Y. Zhu, H. F. Qian and R. C. Jin, J. Mater. Chem., 2011, 21, 6793–6799. 8 P. D. Jadzinsky, G. Calero, C. J. Ackerson, D. A. Bushnell and R. D. Kornberg, Science, 2007, 318, 430–433. 9 M. W. Heaven, A. Dass, P. S. White, K. M. Holt and R. W. Murray, J. Am. Chem. Soc., 2008, 130, 3754–3755. 10 H. F. Qian, W. T. Eckenhoff, Y. Zhu, T. Pintauer and R. C. Jin, J. Am. Chem. Soc., 2010, 132, 8280–8281.
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11 S. Malola, L. Lehtovaara, S. Knoppe, K. J. Hu, R. E. Palmer, T. Burgi and H. Hakkinen, J. Am. Chem. Soc., 2012, 134, 19560–19563. 12 O. Lopez-Acevedo, J. Akola, R. L. Whetten, H. Gronbeck and H. Hakkinen, J. Phys. Chem. C, 2009, 113, 5035–5038. 13 O. Voznyy, J. J. Dubowski, J. T. Yates and P. Maksymovych, J. Am. Chem. Soc., 2009, 131, 12989–12993. 14 I. Dolamic, S. Knoppe, A. Dass and T. Burgi, Nat. Commun., 2012, 3, 798. 15 S. Knoppe, I. Dolamic, A. Dass and T. Burgi, Angew. Chem., Int. Ed., 2012, 51, 7589–7591. 16 O. Lopez-Acevedo, H. Tsunoyama, T. Tsukuda, H. Hakkinen and C. M. Aikens, J. Am. Chem. Soc., 2010, 132, 8210–8218. 17 A. Tlahuice-Flores, R. L. Whetten and M. Jose-Yacaman, J. Phys. Chem. C, 2013, 117, 12191–12198. 18 A. Tlahuice-Flores, Mol. Simul., 2013, 39, 428–431. 19 R. C. Price and R. L. Whetten, J. Phys. Chem. B, 2006, 110, 22166–22171. 20 J. Akola, K. A. Kacprzak, O. Lopez-Acevedo, M. Walter, H. Gronbeck and H. Hakkinen, J. Phys. Chem. C, 2010, 114, 15986–15994. 21 S. A. Miller, J. M. Womick, J. F. Parker, R. W. Murray and A. M. Moran, J. Phys. Chem. C, 2009, 113, 9440–9444. 22 J. F. Parker, J. P. Choi, W. Wang and R. W. Murray, J. Phys. Chem. C, 2008, 112, 13976–13981. 23 E. Hulkko, O. Lopez-Acevedo, J. Koivisto, Y. Levi-Kalisman, R. D. Kornberg, M. Pettersson and H. Hakkinen, J. Am. Chem. Soc., 2011, 133, 3752–3755. 24 J. Petroski, M. Chou and C. Creutz, J. Organomet. Chem., 2009, 694, 1138–1143. 25 L. A. Nafie, T. A. Keiderling and P. J. Stephens, J. Am. Chem. Soc., 1976, 98, 2715–2723. 26 T. Burgi, A. Urakawa, B. Behzadi, K. H. Ernst and A. Baiker, New J. Chem., 2004, 28, 332–334. 27 R. Schweitzer-Stenner, F. Eker, K. Griebenow, X. L. Cao and L. A. Nafie, J. Am. Chem. Soc., 2004, 126, 2768–2776. 28 C. Gautier and T. Burgi, Chem. Commun., 2005, 5393–5395. 29 C. Gautier and T. Burgi, J. Am. Chem. Soc., 2006, 128, 11079–11087. 30 C. Gautier and T. Burgi, J. Phys. Chem. C, 2010, 114, 15897–15902. 31 H. Yao, N. Nishida and K. Kimura, Chem. Phys., 2010, 368, 28–37. 32 H. S. Kato, J. Noh, M. Hara and M. Kawai, J. Phys. Chem. B, 2002, 106, 9655–9658. 33 M. Farrag, M. Tschurl, A. Dass and U. Heiz, Phys. Chem. Chem. Phys., 2013, 15, 12539–12542. 34 S. Knoppe, J. Boudon, I. Dolamic, A. Dass and T. Burgi, Anal. Chem., 2011, 83, 5056–5061. 35 S. Knoppe, A. C. Dharmaratne, E. Schreiner, A. Dass and T. Burgi, J. Am. Chem. Soc., 2010, 132, 16783–16789. 36 H. F. Qian and R. C. Jin, Chem. Mater., 2011, 23, 2209–2217. 37 O. A. Wong, C. L. Heinecke, A. R. Simone, R. L. Whetten and C. J. Ackerson, Nanoscale, 2012, 4, 4099–4102. 38 B. Varnholt, I. Dolamic, S. Knoppe and T. Burgi, Nanoscale, 2013, 5, 9568–9571. 39 D.-E. Jiang, Nanoscale, 2013, 5, 7149–7160.
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Cite this: Phys. Chem. Chem. Phys., 2013, 15, 19561 Received 10th September 2013, Accepted 4th October 2013
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Far-infrared spectra of well-defined thiolate-protected gold clusters† Igor Dolamic, Birte Varnholt and Thomas Bu ¨ rgi*
DOI: 10.1039/c3cp53845a www.rsc.org/pccp
The far infrared spectra of a series of well-defined gold clusters covered with 2-phenylethanethiolate were studied. The spectra of the clusters are different but the differences are subtle. The Au–S stretching vibrations give rise to bands around 300 cm
1
and below. The relative
intensity of these bands changes but they shift only slightly for different clusters. A low-frequency band was identified, which is sensitive to the conformation (trans/gauche) of the 2-phenylethanethiolate ligand.
Introduction A better understanding of the thiolate–gold interaction is of great importance in different research areas like self-assembled monolayers (SAMs),1 gold nanoparticles and clusters2 and molecular electronics based on gold.3 Since Brust et al. established the synthesis of gold nanoparticles protected by thiolates,4 the latter have been studied widely.2,5 The unique properties of the well-defined thiolate protected gold clusters make them attractive for fields like catalysis, drug delivery and sensing.6,7 The first crystal structure reported for a thiolate-protected cluster Au102(SR)44 by Jadzinsky et al. showed the presence of staple units SR-Au-SR (monomeric) and SRAu-SR-Au-SR (dimeric) that cover a densely packed gold core.8 Such staple units were later also found for Au25(SR)189 and Au38(SR)2410 and proposed for Au40(SR)2411 and Au144(SR)6012 and they might also be ubiquitous on flat thiolate-covered gold surfaces (SAMs).13 The staple units play an important role in determining the structure of the clusters. For example, their arrangement on the surface can impart chirality to the cluster.14–16 Also, the staple units provide stability and influence the chemical properties of the clusters. Despite intense research on thiolate-protected clusters and their characterization, up to now not much attention has been given to their vibrational properties.17–24 In general, the vibrational spectrum is a very characteristic property of a compound. Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland. E-mail: [email protected]; Fax: +41 22 379 61 03; Tel: +41 22 379 65 52 † Electronic supplementary information (ESI) available: Experimental details, UV-Vis spectra of the clusters, MALDI mass spectra of the clusters, and details of DTF calculations. See DOI: 10.1039/c3cp53845a
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In addition, mid-infrared (IR) spectra are easily accessible. However, in the case of thiolate-protected gold clusters and nanoparticles this spectral range is characteristic for the vibrations of the adsorbed thiolates. Extracting structural information from such mid-IR spectra is not trivial, as variations of the spectra due to structural changes of the adsorbate (molecular conformation), of the adsorbate layer (arrangement of adsorbates) and of the cluster itself are subtle. Vibrational circular dichroism (VCD)25 is sensitive to the structure of a molecule, both absolute configuration26 and conformation.27 VCD spectra were recently recorded for thiolate-protected gold clusters providing some information on the conformation of the adsorbed molecules.28–31 An interesting spectral region is the far IR (30–350 cm 1) as in this spectral range Au–Au and Au–S vibrations are expected. The characteristic structural elements (staple motifs) of the gold–thiolate interface should exhibit characteristic vibrational bands. Kawai and coworkers32 studied alkanethiol self-assembled monolayers on Au(111) surfaces by high resolution electron energy loss (HREELS) spectroscopy and assigned bands in the range 210–280 cm 1 to Au–S vibrations. For the shortest alkanethiol investigated (C2) three bands at 210, 250 and 320 cm 1 were reported. Creutz and co-workers studied alkanethiolcapped gold nanoparticles by far-IR spectroscopy.24 The particles were about two nanometers in diameter and were covered with alkanethiols of different lengths. The spectra changed with the length of the alkanethiol and the bands were quite broad, which may be attributed to the fact that the nanoparticles were not perfectly monodispersed. Bands at 260–270 cm 1 and around 180 cm 1 were assigned to Au–S vibrations. Murray and coworkers reported the Raman spectra of [Au25(SCH2CH2Ph)18] 1 and [Au25(SCH2CH2Ph)18]0 and found a band assigned to a Au–S vibration around 290 cm 1 for the anionic cluster. The band shifted to lower energy by 24 cm 1 for the neutral cluster.22 Density functional theory (DFT) calculations identified a Au–S stretching vibration around 300 cm 1 for the Au25 cluster.20 The most detailed analysis of the low-frequency vibrations of small thiolate-protected gold clusters was reported by Tlahuice-Flores and colleagues,17 who used DFT calculations for the prediction
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Communication of far-IR (FIR) and Raman spectra. The largest cluster studied was [Au25(SCH3)18] 1. In general the appearance of the calculated FIR spectra was found to be different for the different clusters considered in that study but some general conclusions could be drawn: Au–Au vibrations were found far below 200 cm 1. Some of these modes were intense in the calculated Raman but weak in the far-IR spectra. Au–S vibrations were classified as tangential vibrations involving the shorter Au(staple)–S(staple) bonds and radial vibrations involving the softer Au(core)–S(staple) bonds. The former were located near 300 cm 1 and slightly above, whereas the latter were found typically below 300 cm 1. The calculated FIR spectrum of [Au25(SCH3)18] 1 showed four bands in the range 220–320 cm 1. The most intense band at 240 cm 1 was attributed to Au(core)–S(staple) stretching modes. Heiz and coworkers reported IR spectra of size-selected gold nanoparticles protected by 2-phenylethanethiol (HSCH2CH2Ph, 2-PET).33 However, they focused on the mid-infrared spectral region and therefore no information on the Au–S vibrations was reported. Surprisingly, in the C–H stretching region the spectrum of the Au25 cluster showed an unexpected red-shift of the aromatic C–H vibrations, which was not observed for the free ligand and the other clusters investigated (Au38 and Au144). To the best of our knowledge up to now no systematic experimental study on the low-frequency vibrations of well-defined thiolate-protected gold clusters was performed. An open question that is addressed in this contribution is whether the low-frequency vibrational spectrum is characteristic for each cluster, and whether one can discriminate between different structural elements, i.e. monomeric and dimeric staple units, which would allow a straightforward characterization of new, unknown samples. For this we prepared samples of composition Aun(2-PET)m with (n,m) equal to (25,18), (38,24), (40,24) and (144,60). The ratio of dimeric to monomeric staple units changes in the series from only dimeric units (Au25) over a mixture of dimeric and monomeric units (Au38, Au40) to only monomeric units (Au144). The structure of the studied clusters is shown in Fig. 1.
Experimental Gold nanoclusters were prepared and purified as described in previous reports.34–36 All samples were characterized using UV-vis spectroscopy and MALDI-mass spectrometry. More details about preparation and characterization of size-selected gold nanoclusters can be found in the ESI.† FIR spectra were recorded on a Bruker Vertex 80sv Fourier transform IR spectrometer. Measurements were performed in vacuum at a resolution of 4 cm 1 by averaging 200 scans. A Mylar 6 mm multilayer beamsplitter was used. The samples were dissolved in dichloromethane, spread over a silicon window substrate and dried. Alternatively the sample was dried on the window of a diamond cell.
Results and discussion Fig. 2 shows FIR spectra of five gold clusters: (a) Au144(2-PET)60, (b) Au40(2-PET)24, (c) Au38(2-PET)24, (d) [Au25(2-PET)18] 1 TOA+
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Fig. 1 Structure of (a) Au144(2-PET)60 (30 monomeric staples); (b) Au40(2-PET)24 (6 monomeric and 4 dimeric staples); (c) Au38(2-PET)24 (3 monomeric and 6 dimeric staples); (d) Au25(2-PET)18 (6 dimeric staples). Green, gold core atoms; yellow, gold ad-atoms; orange, sulphur. For clarity –CH2CH2Ph units were removed.
(TOA+, tetraoctylammonium counterion) and (e) [Au25(2-PET)18]0. A detailed view is shown in Fig. 3. Fig. 2(a) shows the FIR spectrum of the Au144(2-PET)60 cluster. The structure of the cluster is not yet confirmed but it has been proposed that the Au144(2-PET)60 cluster consists of an Au114 icosahedral core protected by 30 RS-Au-SR monomeric staple motifs only.12 Ackerson and coworkers used NMR spectroscopy to show that all the 60 2-PET ligands are symmetry equivalent, in agreement with the proposed model.37 Bands at 321 cm 1 and 284 cm 1 fall in the spectral range expected for Au–S vibrations. Following the calculations of Tlahuice-Flores17 the former can be assigned to tangential Au(staple)–S(staple) vibrations whereas the latter might be due to Au(core)–S(staple) modes or Au(staple)–S(staple) modes. Recently the isolation of the Au40(2-PET)24 cluster as a side product of the etching process in the Au38(2-PET)24 cluster synthesis has been reported.15,34 The structure of this cluster is not yet solved. High resolution transmission electron microscopy (TEM) shows an elongated cluster.11 Furthermore, the cluster was shown to be chiral and the circular dichroism (CD) spectra were measured after separation of the enantiomers by high performance liquid chromatography (HPLC).15,38 A structure (more precisely a family of structures) was proposed where a bi-icoshaedral Au26 core is protected by six monomeric and four dimeric staple units. Density functional theory (DFT) was used to calculate the absorption and CD spectra of the proposed structure, which are in fair agreement with experiment. Another model for Au40(2-PET)24 was also proposed which, according to calculations, should have similar energy (or be even slightly more stable).39 In this model the cluster
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Fig. 2 Far infrared spectra of size selected gold nanoclusters: (a) Au144(2-PET)60, (b) Au40(2-PET)24, (c) Au38(2-PET)24, (d) [Au25(2-PET)18] TOA+, (e) [Au25(2-PET)18]0.
Fig. 3 Far infrared spectra of: (a) Au144(2-PET)60, (b) Au40(2-PET)24, (c) Au38(2-PET)24, (d) [Au25(2-PET)18] TOA+, (e) [Au25(2-PET)18]0.
core is protected by six dimeric and three monomeric staples. However, there are no calculated spectra reported for this model, which would allow a comparison with the available experimental data. Fig. 2(b) shows the FIR spectrum of Au40(2-PET)24. In the spectral region where the Au–S stretching vibrations are expected three bands are observed at 329 cm 1, 285 cm 1 and 216 cm 1. According to the calculations by TlahuiceFlores17 the band above 300 cm 1 may be assigned to tangential Au(staple)–S(staples) modes whereas the one at 216 cm 1 may be due to radial Au(core)–S(staple) modes. The band at 285 cm 1 may be assigned to both types of Au–S modes. The structure of the Au38(2-PET)24 was solved by X-ray crystallography.10 The cluster consists of a bi-icosahedral Au23 core and of three monomeric and six dimeric staple units. The achiral 2-PET ligand itself is arranged in a chiral fashion on the cluster surface and both enantiomers of Au38(2-PET)24 are contained in the unit cell of the crystal. Recently, enantiomers of Au38(2-PET)24 were separated using chiral HPLC.14 Fig. 2(c) shows the far infrared spectrum of Au38(2-PET)24. Overall the spectrum looks similar to the one of Au40(2-PET)24 with different relative intensities of the bands. Bands associated with Au–S modes are found at 315 cm 1 and 284 cm 1. Compared to Au40 the former band (tangential Au–S mode) is more intense and is
shifted to lower wavenumbers (14 cm 1 shift). A weak band at 217 cm 1 cm 1 is also observed. The structure of Au25(2-PET)18 is also known.9 The cluster consists of an Au13 icosahedra core and is protected by six dimeric staple motifs. The cluster exists in neutral and anionic form. In the charged cluster tetraoctylammonium (TOA+) is present as a counter ion. Fig. 2(e) shows the FIR spectrum of the neutral form of Au25(2-PET)18, whereas in Fig. 2(d) the spectrum of the anionic form is shown. Bands at 523 cm 1 and 536 cm 1 with a shoulder at 542 cm 1 in the spectrum of the anionic cluster are associated with the TOA+ counter ion and are missing in the spectrum of the neutral [Au25(2-PET)18]0 cluster. The bands assigned to Au–S vibrations are found at 317 cm 1 and 282 cm 1 for the neutral cluster and at 313 cm 1 and 281 cm 1 for the anionic cluster. Note that for the same cluster a band was observed around 280 cm 1 in the Raman spectrum of the anionic compound, which shifted to lower frequencies by about 24 cm 1 for the neutral cluster.22 The band observed in the present study around 280 cm 1, which we assign to Au–S radial and/or tangential modes, hardly shifts when changing the oxidation state of the cluster. This discrepancy between Raman and IR is likely related to the fact that the two techniques probe different modes. Likely the mode in the reported Raman spectrum is related to the staple breathing mode
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Communication where all the radial Au–S bonds elongate in phase. This mode should have a large Raman cross-section but it should be silent in the IR spectrum. The FIR spectrum of [Au25(2-PET)18] 1 can be compared to the one calculated for [Au25(SCH3)18] 1,17 as the structure of the two clusters is the same except for the thiolate ligand. The Au–S vibrations should be similar. However, different ligands could affect the Au–S vibrational spectra through their different mass. The calculated spectrum shows four bands at around 320, 300, 270 and 240 cm 1, with the latter one being the most intense. Below 240 cm 1 only very weak bands were calculated. The prominent band at 240 cm 1 (and also the band at 270 cm 1) was assigned to Au(core)–S(staple) vibrations (radial modes), whereas the bands at 300 and 320 cm 1 were assigned to tangential Au(staple)–S(staple) vibrations. In the experimental FIR spectrum of [Au25(2-PET)18] 1 (Fig. 2 and 3) two quite prominent bands are observed at 311 and 282 cm 1 (for exact numbers, see table). The former one could be assigned to tangential Au–S modes, following the calculations by TlahuiceFlores and colleagues.17 The latter band may be due to radial or tangential modes. At lower wavenumbers, in the predicted range for the radial Au- vibrations two further bands at 250 cm 1 and 215 cm 1 may be observed. These bands are very broad and more distinct in the spectrum of the neutral cluster (see Fig. 3). The intensity of the corresponding bands in the reported calculations and the measured FIR spectrum of the Au25 cluster do not agree well. This might have several reasons, one is a temperature effect not considered in the calculations. The intensity of the low frequency vibrations is damped because these vibrations are populated at room temperature. Another reason might be the different ligands used (2-PET vs. SCH3). A comparison shows that the FIR spectra of the clusters are different although the same ligand is used in all cases. This has certainly to do with the fact that the vibrations involving the staple units fall in this range and that the structure of the Au–S interface (arrangement and relative abundance of short and long staples) is different for the clusters. For example, the FIR spectra of Au38(2-PET)24 and Au144(2-PET)60 are clearly different in the FIR region whereas their spectra cannot be distinguished in the mid-IR spectral region.33 However, the differences in the FIR are subtle and mostly concern the relative intensity of bands. For example, all the clusters show bands at around 280 cm 1 and 315 cm 1 but their relative intensity differs for the different clusters. As the size of the clusters decreases, the number of monomeric staples decreases as well, whereas the number of dimeric staples increases. However, there is no clear systematic variation of the FIR spectra, which could be traced to characteristic vibrations of the short or long staples. In fact, the spectra of the Au25(2-PET)18 and the Au144(2-PET)60 clusters are quite similar. The former cluster is covered exclusively by dimeric staples, whereas the latter is presumably covered only by monomeric ones. It seems therefore difficult to estimate the relative abundance of long and short staples based on the FIR spectrum of a cluster. Some of the bands observed in the spectral range considered here (600–100 cm 1) are associated with modes related to the 2-PET ligand. In order to better understand these modes we
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PCCP performed DFT calculations of isolated staple units. For this the geometry of one staple was taken from the crystal structure of Au38(2-PET)24, including three Au core atoms. The latter were fixed and the staple relaxed before calculating the normal modes (for more information see ESI†). Based on these calculations we can assign the prominent band around 560 cm 1 observed for all clusters to in plane deformation modes of the phenyl rings coupled with C–S stretching modes. The band at around 490 cm 1 is due to coupled out of plane deformation modes of the phenyl rings coupled with C–C–S bending modes. This mode strongly depends on the conformation of the 2-PET ligand (see below). The bands at around 400 cm 1 are due to out of plane modes of the phenyl ring coupled to CH2 rocking vibrations. The bands found at around 180 cm 1 may be due to Au–S–C bending modes. Table 1 lists the observed modes and provides an assignment. The band at around 490 cm 1 deserves some more attention as its frequency shifts with the conformation of the 2-PET ligand. The 2-PET ligand has some conformational freedom, particularly around the C–C bond of the ethyl group. More precisely, the sulphur atom and the phenyl ring can adopt anti or gauche positions. In the crystal structure of Au38(2-PET)2410 we counted 14 anti and 10 gauche conformations, whereas in Au25(2-PET)189 14 anti and 4 gauche conformations are found. For the gauche conformers the vibration mentioned above is shifted to lower frequencies by about 12 cm 1 according to the calculations. This shift can also be rationalized based on the calculations. As mentioned above the vibration around 490 cm 1 is mainly an out of plane phenyl vibration, which has also some C–C–S bending character. However, this is only true for the trans conformation. In the gauche conformation the out of plane phenyl vibration is coupled to a torsion around the C–S bond. This mode is softer than the C–C–S bending coordinate and therefore the vibration shifts to lower frequency for the gauche conformation. This gives rise to the band (shoulder) at around 470 cm 1 in the experimental spectra. A comparison shows that the relative intensity of the main band at around
Table 1 Observed bands (wavenumbers) in the far-IR spectra of size-selected gold nanoclusters covered with 2-PET (Aun(2-PET)m, (n,m))
(25, 18) (144, 60) (40, 24) (38, 24) (25, 18)0 TOA+ 560
562
561
561
490 468 404 390
493 471 402 389
492 468 401 390
321 284
329 285
315 284
216
217 180 158
492 467 403 383 354 317 282 250 215 180
175
1
Assignmenta
563 537 523 492 471 403 383
i.p. def. + nC–S TOA+ TOA+ o.o.p. def. + C–C–S b. o.o.p. def. + t. o.o.p. def. + CH2 r. o.o.p. def. + CH2 r.
311 282
n Au–S, tangential n Au–S, tangential/radial
176
Au–S, radial Au–S–C b.
a
i.p.: phenyl in plane; o.o.p.: phenyl out of plane; def.: deformation; t.: torsion; r.: rocking; b.: bending.
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490 cm 1 and the shoulder at around 470 cm 1 differs for the clusters studied, indicating that the relative abundance of trans and gauche conformations of the 2-PET ligand is changing. The most intense shoulder is observed for the Au40(2-PET)24 cluster indicating that this cluster has the most gauche conformations of all the samples. Also, for the Au38(2-PET)24 cluster the shoulder associated with the gauche conformations of 2-PET is more pronounced than for the Au25(2-PET)18 cluster, in agreement with the crystal structure, which shows more gauche conformations for the former cluster. It is important to note that the conformation (trans/gauche) might be different in the single crystal and in the state observed here (amorphous solid) or in the liquid state.
Conclusions A series of well-defined gold nanoclusters protected by 2-phenylethanethiolate were prepared and their far IR spectra measured. The clusters of different size contain ligand shells of different composition with respect to monomeric and dimeric staple units. The far IR spectra are characteristic for each cluster but spectral differences are subtle. No clear systematic variation could be found which would allow one to quantify the relative amount of monomeric and dimeric staple units. A band near 490 cm 1, which is associated with an out of plane mode of the phenyl moiety coupled to a C–C–S bending mode, is indicative of the conformation of the phenylethylthiolate. Most of the ligands adopt a trans conformation of the S–C–C-phenyl unit. However, some units are gauche, which leads to a shift towards lower wavenumbers of this band.
Acknowledgements This work is supported by the Swiss National Science Foundation. We thank Dr Sophie Michalet (SMS, UniGE) for the MALDI measurements.
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