Sharp Transition from Nonmetallic Au 246 to Metallic Au 279 with Nascent Surface Plasmon Resonance

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1 Supporting Information: Sharp Transition from Nonmetallic Au 246 to Metallic Au 279 with Nascent Surface Plasmon Resonance Tatsuya Higaki,,# Meng Zhou,,# Kelly J. Lambright, Kristin Kirschbaum, Matthew Y. Sfeir, Rongchao Jin*, Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States Department of Chemistry and Biochemistry, University of Toledo, Toledo, Ohio 43606, United States Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States Chemicals Tetrachloroauric(III) acid (HAuCl 4 3H 2 O, >99.99% metals basis, Aldrich), tetraoctylammonium bromide (TOAB, 98%, Fluka), 2-phenylethanethiol (PET, C 8 H 9 SH, 98%, Aldrich), 4-tert-butylbenzyl mercaptan (TBBM, C 11 H 15 SH, Aldrich), 4-tert-butylbenzenethiol (TBBT, C 10 H 13 SH, >97.0%, TCI), sodium borohydride (NaBH 4, Aldrich). Solvents: Methanol (HPLC grade, 99.9%, Aldrich), Dichloromethane (ACS reagent, 99.5%, Aldrich), Toluene (HPLC grade, 99.9%, Aldrich), 2-Methyltetrahydrofuran (anhydrous, inhibitor-free, 99%, Aldrich). All chemicals were used without further purification. Nanopure water was prepared with a Barnstead NANOpure Diamond system. Synthesis of Au 279 (TBBT) 84 nanoclusters Our synthesis consists of multiple steps as follows: Typically, 100 mg of HAuCl 4 3H 2 O was dissolved in 5 ml nanopure water and 160 mg of TOAB was dissolved in 10 ml toluene. They were mixed in a 100 ml round bottom flask. Then, the solution was vigorously stirred for 15 minutes to allow all the Au(III) salt in the aqueous phase to be transferred into the organic phase with the aid of TOA +. The colorless aqueous phase (no Au(III) salt left) was removed by a glass pipet. The flask was cooled to ~0 ºC in an ice bath (~30 minutes). Then, 80 μl of PET was added to the flask using a syringe. The color of the toluene phase changed from dark red-orange to pale yellow and eventually colorless. The solution was kept stirring for 60 minutes. Subsequently, 98 mg of NaBH 4 (dissolved in 5 ml of ice cold nanopure water) was rapidly added to the reaction flask. The color of the reaction mixture immediately turned black, indicating the formation of gold nanoclusters. After 30 minutes, the aqueous phase was discarded by a glass pipet, and the organic phase was dried by rotary evaporation. The as-obtained polydisperse Au x (PET) y nanoparticles were washed with methanol 3 times to remove excess thiol and other side-products. Of note, the size distribution in this precursor determines the purity and yield of the final product. s1

2 The size focusing methodology was applied to convert the initially polydisperse gold nanoclusters into specific-sized gold nanoclusters (containing Au 144 (PET) 60, Au 333 (PET) 79, and Au ~519 (PET) ~104 ). S1 Typically, ~60 mg of polydisperse Au x (PET) y nanoclusters obtained from step I were dissolved in 1 ml toluene and 1 ml PET. The solution was heated to 80 ºC and maintained at this temperature under air atmosphere with gentle stirring. After ~3 days of etching, Au 144 (PET) 60, Au 333 (PET) 79, and Au ~519 (PET) ~104 survived as stable gold nanocluster species (left in the solution). Then, the product nanoclusters were separated from the thiol/toluene solution by adding excess methanol and further centrifugation at 10,000 rpm for 3 min. The supernatant (which contains excess thiol) was discarded, and the black precipitate (containing the product nanoclusters) was washed with methanol 3 times. Finally, the product nanoclusters were extracted from the precipitate by methylene chloride. Further size focusing & ligand-exchange (PET to TBBM) were applied to convert the mixture of multiple sized gold nanoclusters into single-sized gold nanoclusters with molecular purity. Typically, ~24 mg of gold nanoclusters obtained from step II was dissolved in 1 ml toluene and 1 ml TBBM. The solution was heated to 80 ºC and maintained at this temperature under air atmosphere with gentle stirring. After ~5 days of etching, Au 333 (TBBM) 79 was the only stable gold nanocluster species left in the solution. Then, Au 333 (TBBM) 79 nanoclusters were separated from the thiol/toluene solution by adding excess methanol and further centrifugation at 10,000 rpm for 3 min. The supernatant which contains excess thiol was discarded, and the black precipitate containing Au 333 (TBBM) 79 nanoclusters was washed with methanol 3 times. Finally, Au 333 (TBBM) 79 nanoclusters were extracted from the precipitate by methylene chloride. The product yield is 7-14 mg (10-20% in gold atom basis). To perform ligand-exchange-induced size transformation of Au 333 (TBBM) 79 to Au 279 (TBBT) 84, ~3 mg of Au 333 (TBBM) 79 nanoclusters was dissolved in a mixture of 0.5 ml of toluene and 0.5 ml of TBBT. Then, the solution was heated to 80 C and maintained at this temperature for ~48 hours. After that, methanol was added to the reaction mixture to precipitate the product, followed by centrifugation at 10,000 rpm, and the solid product was washed with methanol 3 times to remove excess thiol. Finally, Au 279 (TBBT) 84 nanoclusters were extracted from the precipitate by methylene chloride. The product yield is ~40% in gold atom basis. Matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) MALDI-MS was performed with a PerSeptive-Biosystems Voyager DE super-str time-of-flight (TOF) mass spectrometer. Trans-2-[3-(4-tert-Butylphenyl)-2-methyl-2-propenyldidene]malononitrile (DCTB, 99.0%, Sigma-Aldrich) was used as the matrix. Typically, 0.5 mg of matrix and mg of sample (i.e., a 100:1 mass ratio between DCTB and sample) were mixed in 50 μl of CH 2 Cl 2. A 10 μl portion of solution was applied to the steel plate and air-dried. All the mass spectra were obtained with positive ion mode. s2

3 Electrospray ionization mass spectrometry (ESI-MS) The ESI mass spectra were recorded using a Waters Q-TOF mass spectrometer equipped with a Z-spray source. The source temperature was kept at 70 C. The cluster sample was directly infused into the chamber at 5 μl/min. The spray voltage and the cone voltage were kept at 2.20 V and 60 V, respectively. The ESI sample was dissolved in dichloromethane (~1 mg/ml). All the mass spectra were obtained with positive ion mode. Calibration was performed using CsI clusters. Steady-state UV-Vis-NIR and Cryogenic spectroscopy UV-Vis-NIR spectra of Au nanoclusters were acquired on a UV-3600 Plus UV-VIS-NIR spectrophotometer (Shimadzu) at room temperature. The temperature-dependent experiments were carried out on a home-built system comprising the UV3600Plus spectrometer, an Optistat CF2 cryostat (Oxford Instruments), a temperature controller, and a vacuum pump. 2-Methyltetrahydrofuran was used as the solvent in the temperature range between 60 and 296 K for a clear glass measurement. Ultrafast spectroscopy Femtosecond transient absorption spectroscopy was carried out using a commercial Ti:Sapphire laser system (SpectraPhysics, 800 nm, 100 fs, 3.5 mj, 1 khz). Pump pulse was generated using a commercial optical parametric amplifier (LightConversion). A small portion of the laser fundamental was focused into a sapphire plate to produce supercontinuum in the visible region, which overlapped in time and space with the pump. The diameter of the pump beam was 0.75 mm and the pump power was varied between 0.2 and 4.0 mw using neutral density filter. Multiwavelength transient spectra were recorded using dual spectrometers (signal and reference) equipped with array detectors whose data rates exceed the repetition rate of the laser (1 khz). Solutions of both clusters in 1 mm path length cuvettes were excited by the tunable output of the OPA (pump). All data shown in this manuscript were performed in dilute solutions using dichloromethane and toluene as solvent. During the experiments, all the samples were continuously stirred by a magnetic bar coated by Teflon. UV-Vis-NIR absorption spectra and MALDI mass spectra remained the same before and after the femtosecond experiments. Single Crystal X-ray Diffraction Analysis A specimen of C 840 H 1092 Au 279 S 84, approximate dimensions 0.01 mm x 0.06 mm x 0.07 mm, was used for the X-ray crystallographic analysis. The total exposure time was hours. The frames were integrated with the Bruker SAINT software package using a wide-frame algorithm. The integration of the data using an orthorhombic unit cell yielded a total of 851,645 reflections to a maximum θ angle of (1.03 Å resolution), of which 109,670 were independent (average redundancy 7.77, completeness = 91.6%, R int = 14.86%, R sig = 14.47%) and 51,979 (47.40%) were greater than 2σ(F 2 ). The final cell constants of a = (3) Å, b = (4) Å, c = (6) Å, volume = 127,990.(20) Å 3, are based upon the refinement of s3

4 the XYZ-centroids of 621 reflections above 20 σ(i) with < 2θ < Data were corrected for absorption effects using the Multi-Scan method (SADABS). The ratio of minimum to maximum apparent transmission was The structure was solved and refined using the Bruker SHELXTL Software Package, using the space group P , with Z = 4 for the formula unit, C 840 H 1092 Au 279 S 84. The final anisotropic full-matrix least-squares refinement on F 2 with 3,591 variables converged at R1 = 27.59%. The goodness-of-fit was The largest peak in the final difference electron density synthesis was e - /Å 3 and the largest hole was e - /Å 3 with an RMS deviation of 1.3 e - /Å 3. The Au 279 S 84 nanoparticle crystallizes on a general position. All 279 Au- and 84 S- crystallographically independent atoms were obtained by direct methods and subsequent difference Fourier syntheses. The Au279 S84 structure is the best model from the collected data. Disordered S-atoms were constrained (DFIX, SADI, SIMU, ISOR, FLAT) to be chemically reasonable. Disordered Au-atoms were also restrained (SADI, ISOR, SIMU) to maintain a chemically reasonable model. Supporting Figures and Tables: Figs S1-S4 Tables S1-S2 s4

5 Figure S1. Electrospray ionization mass spectrometry (ESI-MS) characterization of the Au 333 (TBBM) 79. The peaks correspond to the 6+, 5+, and 4+ ion sets of Au 333 (TBBM) 79. Figure S2. The ligand-exchange process monitored by positive mode MALDI-MS for the synthesis of Au 279 (TBBT) 84 from Au 333 (TBBM) 79. s5

6 Figure S3. Power dependence of Au 279 nanoclusters. (A)-(C) Transient absorption data map with pump power varying from 75 to 150 to 250 nj/pulse; (D) TA spectra measured with different pump power; (E) Kinetic traces probed at GSB at 530 nm measured with different pump power; (F) GSB maxima intensity as a function of pump power; the linear relationship rules out multiphoton effects or photo damage to samples. Figure S4. Transient absorption spectra of Au 279 (in toluene) with 360 nm excitation by a visible detector (black line) and a NIR detector (red line), respectively. The broad ESA extends to ~1600 nm. s6

7 Table S1. Sample and crystal data for Au 279 S 84. Identification code Au 279 S 84 Chemical formula C 840 H 1092 Au 279 S 84 Formula weight Temperature Wavelength Crystal size Crystal system g/mol 100(2) K Å x x mm orthorhombic Space group P Unit cell dimensions a = (3) Å α = 90 b = (4) Å β = 90 c = (6) Å γ = 90 Volume (20) Å 3 Z 4 Density (calculated) g/cm 3 Absorption coefficient mm -1 F(000) s7

8 Table S2. Data collection and structure refinement for Au 279 S 84. Theta range for data collection 1.08 to Index ranges -31<=h<=32, -43<=k<=45, -69<=l<=69 Reflections collected Independent reflections [R(int) = ] Coverage of independent reflections 91.6% Absorption correction Multi-Scan Max. and min. transmission and Structure solution technique direct methods Structure solution program XS, VERSION 2013/1 Refinement method Full-matrix least-squares on F 2 Refinement program SHELXL-2016/6 (Sheldrick, 2016) Function minimized Σ w(f 2 o - F 2 c ) 2 Data / restraints / parameters / 3078 / 3591 Goodness-of-fit on F Δ/σ max Final R indices data; I>2σ(I) R1 = , wr2 = all data R1 = , wr2 = Weighting scheme w=1/[σ 2 (F 2 o )+(0.2000P) 2 ] where P=(F 2 o +2F 2 c )/3 Absolute structure parameter 0.489(17) Largest diff. peak and hole and eå -3 R.M.S. deviation from mean eå -3 Supporting reference: S1. Qian, H.; Zhu, Y.; Jin, R. Proc. Natl Acad. Sci. U.S.A. 2012, 109, 696. s8