Au36(SR)22 Nanocluster and a Periodic Pattern from Six to Fourteen Free Electrons in Core Size Evolution

An Au36(S-tBu)22 nanocluster (NC) is synthesized using the bulky tert-butyl thiol as the ligand. Single-crystal X-ray crystallography reveals that it has an Au25 core which evolves from the Au22 core in the previously reported Au30(S-tBu)18, and the Au25 core is protected by longer staple-like surface motifs. The new Au36 NC extends the members of the face-centered cubic structural evolution by adding an Au3 triangle and an Au4 tetrahedron unit. Additionally, it is found that Au36 emits near-infrared photoluminescence at 863 nm with a quantum yield (QY) of 4.3%, which is five times larger than that of Au30(S-tBu)18—the closest neighbor in the structural evolution pattern. The higher QY of Au36 is attributed to a larger radiative relaxation (kr), resulting from the structural effect. Finally, we find that the longer staple motifs lead to enhanced stability of Au36(S-tBu)22 relative to Au30(S-tBu)18.


Synthesis
HAuCl4•3H2O 98.74 mg (0.25 mmol) and 65.5 L tert-butyl thiol were dissolved in 15 ml of THF under rapid stirring (550 rpm).The solution turned to deep orange in 30 minutes.At this point, a freshly prepared NaBH4 solution (57 mg, 1.5 mmol) was rapidly added to the reaction mixture.Upon mixing, the solution immediately turned black.The reaction was allowed to proceed for three hours.Then, the reaction solution was evaporated to dryness.The black solid was washed three times with 50% aqueous methanol to remove excess thiol and extracted by 3 ml toluene.The vial containing Au NCs solution was then sealed and put in dark for incubation.After one week, the solution was concentrated and further purified by thin layer chromatography (TLC, developing solvent: hexane:dichloromethane = 3:2(v/v)).The yield of Au36(S-tBu)22 is 6% based on gold atoms.The TLC separation of Au36(S-tBu)22 is displayed in Figure S1 and the reproduciblilty of the synthesis of this compound is high.Of note, we found that the majority of NCs remained stable (judging by the optical absorption spectra) after 2 hours of etching with excess tert-butyl thiol at 60 o C, with a small amount transform into Au30(SR)18 and Au nanoparticles of larger sizes.

Steady-State and Time-Resolved Photoluminescence Measurements
Steady-state photoluminescence (PL) spectra were measured on an FLS-1000 spectrofluorometer (Edinburgh).PL lifetimes were measured by time-correlated single photon counting (TCSPC) on the same instrument.Visible PL was measured using a photomultiplier (PMT) as the detector.Near-infrared PL was measured using a wide-range InGaAs detector (600-1600 nm) cooled to -80 ℃ with liquid nitrogen.The PLQY of Au NCs in toluene and DCM were determined by using [Au25(PPh3)10(SC2H4Ph)5Cl2] 2+ nanocluster (PLQY: 8%) 1 as a reference.

Computational Methods
Nanocluster Model and Structural Optimization.The DFT calculations with the Perdew-Burke-Ernzerhof (PBE) functional and projector augmented wave (PAW) pseudopotential and projector augmented wave (PAW) pseudopotential were performed using the Vienna Ab Initio Simulation Package (VASP) 5.4.4. 2 The Kohn-Sham one electron valence eigenstates were expanded in terms of plane-wave basis sets with cutoff energy of 520 eV.The ionic and electronic convergence limit was set to 0.03 eV/Å and 110 -5 eV, respectively, while the Methfessel-Paxton scheme was utilized with a smearing width of 0.2 eV. 3 The Au36(SR)22 (R=tert-butyl) nanocluster model is derived from the experimentally-solved crystal structures.Following previous works on atomically precise gold nanoclusters, the organic fragment of the ligands is modeled using a -CH3 moiety in order to generate a computationally tractable model while accurately capturing the geometrical structure on the nanocluster. 4The resulting 146-atom Au36(SCH3)22 model was inserted into a three-dimensional 30 Å × 30 Å × 30 Å periodic cubic box to exclude periodic interaction between them.The sampling of the Brillouin zone was conducted with a Γ-point k-point mesh.The DFT calculations yield optimized geometries that are in very good agreement with experiment.
Electronic Structure and Photo-Absorption Spectra.In the present study, the time dependentdensity functional theory (TDDFT) as implemented in TURBMOLE package (version 7.4) 5 was used to describe the electronic structure and photo-absorption spectra of Au36(SCH3)22.The PBE functional together with the def2-SV(P) basis set 6 and respective effective core potentials to describe the inner electrons were chosen.Quadrature grids were of multiple grids m3 quality. 7The resolution of identities approximation 8 were used to calculate 500 lowest singlet-to-singlet vertical energies.Simulated spectra were generated by convolving the calculated absorption energies and intensities with a Gaussian function of sigma set to 40 nm, and sampling the energy over 500 points within a 400-900 nm range.

X-ray Crystallography
A thin black plate of Au36(S-tBu)22 was used for data collection on a Bruker Duo diffractometer with a PHOTON II detector and Iμs CuKα radiation (1.54178 Å) at 200 K.The structure was solved in the triclinic space group P.Integration and scaling of the data yielded 104,900 reflections, of which 32,307 were unique (and 23,324 unique data with I > 2σ(I)), to a maximum of θ = 70.75°(d = 0.82 Å) with a completeness of 94.6% (99.5% out to θ = 50.00°;d= 1.00 Å) and an Rint of 10.34%.
The quality of numerous screened crystals was poor.Eventually, a single crystal of high quality was found and used for data collection.All Au and S atoms were refined with anisotropic displacement parameters, while all carbon atoms were treated with an isotropic displacement parameter.Two disordered lattice toluene molecules were identified and subjected to similarity restraint (SIMU) and the six phenyl carbon atoms constrained at the corners of a regular hexagon (AFIX 66), their occupancies were initially refined, and were constrained to 65% and 35% in the final refinement.The toluene methyl groups were subjected to similar distance restraints (SADI) and restrained to be coplanar with the aromatic ring within 0.1 Å (FLAT).Hydrogen atoms were placed in idealized positions and treated with a riding model.Thus, the final model consisted of 907 parameters with 22 restraints.
The final refinement on F 2 converged at R1 = 8.81% (I > 2σ(I)) and wR2 = 27.68%(all data).The goodness-of-fit was 1.354.The largest residual density extrema were 7.41 and -3.39 (e -/Å 3 ) with an RMS deviation of 0.57 e -/Å 3 .The highest two residual density maxima were located close to Au33 and Au31, a corresponding maximum was located in the vicinity of S2.Attempts were made to model this staple portion with a disorder model, occupancies refined to ~90% and 10%, but did not result in meaningful improvements to the model and the displacement parameters for the minor component were unusual when compared to other atoms in the model.Therefore, we have chosen not to include this potential disorder in our final model.

Figure S2 .
Figure S2.Ball-and-stick model of the experimental structure determined by single crystal X-ray crystallography for Au36(S-tBu)22.The contents of one unit cell are depicted in two viewing directions (a and b) and H atoms have been omitted for clarity.