Structural Information on the Au-S Interface of. Thiolate-protected Gold Clusters: A Raman. Spectroscopy Study

Transcription

1 Structural Information on the Au-S Interface of Thiolate-protected Gold Clusters: A Raman Spectroscopy Study Supporting Information Birte Varnholt, Patric Oulevey, Sandra Luber, Chanaka Kumara, Amala Dass and Thomas Bürgi* Department of Physical Chemistry, University of Geneva, Quai Ernest Ansermet 30, 1211 Geneva, Switzerland Department of Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland Department of Chemistry and Biochemistry, University of Mississippi, 352 Coulter Hall, Mississippi 38677, United States Corresponding author: thomas.buergi@unige.ch tel:

2 Synthesis and purification All chemicals were used as obtained from commercial sources. Solvents were used in analytical grade. Au 38 (2-PET) 24 and Au 40 (2-PET) 24 were synthesized as described elsewhere. 1 Briefly, HAuCl 4 was dissolved in acetone and glutathione dispersed in acetone was added. The mixture was stirred at 0 C for 30 minutes. A freshly prepared solution of NaBH 4 was added under fast stirring leading to the formation and precipitation of glutathione-stabilized clusters. The black precipitate was dissolved in water; ethanol, toluene and 2-phenylethylthiol were added and etching was performed for 3 hours at 80 C. 0/- Au 25 (2-PET) 18 and Au 144 (2-PET) 60 were synthesized as follows. 2 HAuCl 4 and tetraoctylammonium bromide were dissolved in methanol and stirred vigorously at room temperature for 15 minutes. After addition of 2-phenylethylthiol, the mixture was stirred for another 15 minutes before addition of freshly prepared NaBH 4 solution. The reaction mixture was stirred for 5 hours. For the synthesis of Au 25 (CamS) 18, 3 HAuCl 4 and 1R,4S-camphorthiol were dissolved in tetrahydrofurane and slowly stirred at 0 C for 30 minutes. Freshly prepared NaBH 4 solution was added at 0 C and the reaction was allowed to proceed for 72 hours at room temperature. All clusters were purified and separated by the same process after precipitation with methanol; excess thiol was removed by repeated washing cycles with methanol. Insoluble Authiol polymers were removed by filtration over a PTFE syringe filter (0.2 µm). Size separation was achieved by size exclusion chromatography. 4 The size-separated clusters were washed again with methanol to remove any impurities. Mass spectra were recorded on a Shimadzu Biotech Axima in a positive linear mode using DCTB ([3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile) as matrix. No internal calibration was performed. For the ligand exchange of Au 25 (2-PET) 18 0/-, the respective cluster was dissolved in toluene and a high excess of racemic or S-1,1 -binaphthyl-2,2 -dithiol (ca. 50x excess with respect to the cluster) was added. After stirring for 24 hours at room temperature, the solution was dried and washed with methanol. To remove the free thiols completely, the samples were passed over a short size exclusion chromatography column. For these samples, MALDI mass spectra were recorded on a Bruker Daltonics Autoflex mass spectrometer using DCTB matrix and positive linear mode. The synthesis of S-BINAS protected clusters 5 was performed under nitrogen. HAuCl 4 dissolved in water was transferred to dichloromethane using the phase transfer agent tetraoctylammonim bromide. The organic phase was added to S-BINAS dissolved in dichloromethane. The solution was stirred for 30 min and a freshly prepared NaBH 4 solution was added under vigorous stirring. The reaction was allowed to proceed overnight. For purification, the organic phase was extracted with water and dried over Na 2 SO 4, which was removed by filtering over a syringe filter. Repeated washing cycles with hexane followed to remove excess thiol. Size separation on a size exclusion chromatography yielded three fractions of which the mass could not be determined so far. The first fraction was used for the Raman measurements.

3 Scheme S 1 Structure of the ligands: 2-Phenylethylthiol (A), 1R,4S-camphorthiol (B) and S-1,1 -binaphthyl-2,2 - dithiol (C) Figure S 1 UV-vis (left) and MALDI (right) spectra of Au 25(2-PET) 18 0 (a), Au 25(2-PET) 18 - (b), Au 38(2-PET) 24 (c), Au 40(2-PET) 24 (d), Au 144(2-PET) 60 (e) and Au 25(CamS) 18 (f) proof the purity of the samples. Besides the cluster itself, well-known fragments (mainly belonging to the loss of AuL and Au 4L 4) can be assigned. Figure S 2 UV-vis spectra (left) and anisotropy factor (right) for Au 25(2-PET) 18 clusters exchanged with S- and rac-binas match well with previous reports. 6 A Varian Cary 50 spectrometer (pathlength 5 mm, DCM) was used for recording the UV-vis spectra. CD spectra were recorded on a JASCO J-815 spectrometer (pathlength 5 mm, DCM); four scans were averaged and anisotropy factors g were calculated using the UV-vis spectra measured simultaneously.

4 Figure S 3 MALDI spectra of the neutral (left) and reduced (right) Au 25(2-PET) 18 cluster exchanged with rac- and S-BINAS. More ligands are exchanged for the reduced form. This might be a concentration effect as the amount of cluster for the reaction was only roughly estimated. For reaction of the cluster of the same oxidation state with rac- and S-BINAS the same concentration was used. Figure S 4 Left: Glass spectrum (b) and the sum of three Raman spectra of Au 38(2-PET) 24 (a) before and (c) after application of the rolling circle filter (RCF). Right: Illustration of the rolling circle filter. The Raman spectrum (of Au 38(2-PET) 24) is converted into dimensionless space, the circle with the chosen radius 40 x n i (n i is the number of points interpolated between those measured) rolls along the spectrum and removes the background. 7

5 Figure S 5 RCF corrected Raman spectra of Au 25(2-PET) 18 0 (a), Au 25(2-PET) 18 - (b), Au 38(2-PET) 24 (c), Au 40(2- PET) 24 (d), Au 144(2-PET) 60 (e) and the free ligand 2-PET (f). The spectra of the clusters were normalized to laser power and measuring time. However absolute values cannot be directly compared because of irreproducible thickness of the coating. The spectrum of the free ligand was scaled in a way, that the ligand vibrations, namely the sharp and intense bands at 1000 and 1600 cm -1, have comparable intensities. Figure S 6 RCF corrected Raman spectra of Au 25(CamS) 18 (a), Au 25(2-PET) 18 0 (b), Au 25(2-PET) 18 - (c), free ligand CamSH (d). The clusters were measured coated on a rotating glass slide, the free ligand was measured in solid state in a rotating glass capillary. The latter allowed higher quantities and higher laser power, resulting in a stronger spectrum which was downscaled for comparison.

6 Figure S 7 RCF corrected Raman spectra of the neutral form of Au 25(2-PET) 18-2x(rac-BINAS) x (b), Au 25(2-PET) 18-2x(S-BINAS) x (c), Au 25(2-PET) 18 (d) and the reference spectrum of the free ligand S-BINAS (e) and an unknown size, protected purely with S-BINAS (a). References (1) Qian, H.; Zhu, Y.; Jin, R. Size-Focusing Synthesis, Optical and Electrochemical Properties of Monodisperse Au38(SC2H4Ph)24 Nanoclusters. ACS Nano 2009, 3, (2) Qian, H.; Jin, R. Ambient Synthesis of Au144(SR)60 Nanoclusters in Methanol. Chem. Mater. 2011, 23, (3) Knoppe, S.; Kothalawala, N.; Jupally, V. R.; Dass, A.; Bürgi, T. Ligand Dependence of the Synthetic Approach and Chiroptical Properties of a Magic Cluster Protected with a Bicyclic Chiral Thiolate. Chem. Commun. 2012, 48, (4) Knoppe, S.; Boudon, J.; Dolamic, I.; Dass, A.; Bürgi, T. Size Exclusion Chromatography for Semipreparative Scale Separation of Au38(SR)24 and Au40(SR)24 and Larger Clusters. Anal. Chem. 2011, 83, (5) Gautier, C.; Taras, R.; Gladiali, S.; Bürgi, T. Chiral 1,1 -Binaphthyl-2,2'-Dithiol- Stabilized Gold Clusters: Size Separation and Optical Activity in the UV-Vis. Chirality 2008, 20, (6) Knoppe, S.; Bürgi, T. The Fate of Au25(SR)18 Clusters upon Ligand Exchange with Binaphthyl-Dithiol: Interstaple Binding vs. Decomposition. Phys. Chem. Chem. Phys. 2013, 15, (7) Brandt, N. N.; Brovko, O. O.; Chikishev, A. Y.; Paraschuk, O. D. Optimization of the Rolling-Circle Filter for Raman Background Subtraction. Appl. Spectrosc. 2006, 60,