Published: February 24, 2011 r2011 American Chemical Society 3752 dx.doi.org/10.1021/ja111077e | J. Am. Chem. Soc. 2011, 133, 3752–3755 COMMUNICATION pubs.acs.org/JACS Electronic and Vibrational Signatures of the Au 102 (p-MBA) 44 Cluster Eero Hulkko, † Olga Lopez-Acevedo, † Jaakko Koivisto, † Yael Levi-Kalisman, ‡ Roger D. Kornberg, ‡ Mika Pettersson,* , † and Hannu H € akkinen* , †, § † Nanoscience Center, Department of Chemistry, P.O. Box 35, FI-40014 University of Jyv € askyl € a, Finland ‡ Department of Structural Biology, Stanford University School of Medicine, Stanford, California 94305, United States § Nanoscience Center, Department of Physics, P.O. Box 35, FI-40014 University of Jyv € askyl € a, Finland b S Supporting Information ABSTRACT: Optical absorption of a gold nanocluster of 102 Au atoms protected by 44 para-mercaptobenzoic acid (p-MBA) ligands is measured in the range of 0.05-6.2 eV (mid-IR to UV) by a combination of several techniques for purified samples in solid and solution phases. The results are compared to calculations for a model cluster Au 102 (SMe) 44 based on the time-dependent density functional theory in the linear-response regime and using the known structure of Au 102 (p-MBA) 44 . The measured and calculated molar ab- sorption coefficients in the NIR-vis region are comparable, within a factor of 2, in the absolute scale. Several character- istic features are observed in the absorption in the range of 1.5-3.5 eV. The onset of the electronic transitions in the mid-IR region is experimentally observed at 0.45 ( 0.05 eV which compares well with the lowest calculated transition at 0.55 eV. Vibrations in the ligand layer give rise to fingerprint IR features below the onset of low-energy metal-to-metal electronic transitions. Partial exchange of the p-MBA ligand to glutathione does not affect the onset of the electronic transitions, which indicates that the metal core of the cluster is not affected by the ligand exchange. The full spectroscopic characterization of the Au 102 (p-MBA) 44 reported here for the first time gives benchmarks for further studies of manipulation and functionalization of this nanocluster to various applications. T hiol-stabilized gold nanoclusters are robust particles that are currently under intense research owing to their fascinating size-dependent electronic, optical, chiroptical, photolumines- cent, and bioconjugate properties. 1,2 Recent total-structure determinations of Au 102 (p-MBA) 44 , 3 Au 25 (SPhC2) 18 -1 , 4,5 and Au 38 (SPhC2) 24 6 clusters have shed light on the atomistic structure of the gold-thiolate interface and the nature of the surface-covalent Au-S bond. All these clusters have molecule- like electronic structures, with a stabilizing HOMO-LUMO gap that can be correlated to observed optical and/or electrochemical gaps. 7 Density functional theory has yielded insights into the electronic structure of these particles and has given predictions on their apparent energy gaps and specific shell closings of delocalized electrons in the gold core. 5,8-12 Among the “generations” 13 (stable sizes) of the thiolate protected gold clusters in the mass range of 5-29 kDa, the cluster with the gold mass of about 21 kDa attracted special attention when the total-structure-determination of its water- soluble variant synthesized by using p-MBA thiols succeeded in 2007, yielding a definite assignment as Au 102 (p-MBA) 44 . 3 The analysis of the structure revealed a central decahedral Au 79 core protected by 19 RSAuSR and 2 RS(AuSR) 2 units (Supporting Information Figure S1). 3,9 Earlier computations by using density functional theory had predicted bonding motifs and composi- tions where Au 0 atoms form the metallic core and Au I atoms are chemically bound in the protecting thiolate layer. 14 Experimental data on optical or electrochemical gap of this cluster has not been available, mainly due to the fact that it was first obtained (and crystallized) from a mixture in which it was present in a trace amount. 3 However, a procedure that yields the compound in abundant, essentially pure form was very recently reported. 15 Here, we report, for the first time, a complete spectroscopic characterization of pure Au 102 (p-MBA) 44 samples in the spectral region of 0.05-6.2 eV (mid-IR to UV) by using a combination of several techniques in solid and solution phases, and study the electronic structure and electronic transitions by linear-response time-dependent density functional theory (LR- TDDFT). We will show the vibrational signatures of the p-MBA layer in the mid-IR region, give a definite demonstration of the quantum size effects in the electronic structure of this nanoclus- ter by reporting the onset of electronic transitions in the NIR region where the calculated and measured absolute molar absorption agree quantitatively, and discuss signature features of absorption in the VIS region. Furthermore, we show that ligand exchange to glutathionate does not affect the electronic structure of the Au 79 core. Au 102 (p-MBA) 44 was synthesized as recently described. 15 Briefly, p-MBA and HAuCl 4 (3:1 ratio of p-MBA/gold) are combined in water and 47% methanol at a final gold concentra- tion of 3 mM as follows: To 3.94 mL of water, we add 5.64 mL of methanol, 1.286 mL of 28 mM HAuCl 4 solution, and 1.134 mL of 95 mM p-MBA, 300 mM NaOH solution. The mixture is kept for 1 h at room temperature on a rocking platform. A total of 0.48 mL of 150 mM fresh NaBH 4 solution is added (2:1 ratio of BH 4 - :gold) and the reaction is allowed to proceed from a minimum of 5 h to as long as overnight at room temperature. The product is precipitated with ammonium acetate (80 mM final concentration) and methanol (80% v/v) and the pellet is then dissolved in a minimum amount of water. Further Received: December 9, 2010