Robust, Highly Luminescent Au13 Superatoms Protected by N-Heterocyclic Carbenes

: Gold superatom nanoclusters stabilized entirely by N -heterocyclic carbenes (NHCs) and halides are reported. The reduction of well-defined NHC–Au–Cl complexes produces clusters comprised of an icosahedral Au 13 core surrounded by a symmetrical arrangement of 9 NHCs and 3 chlorides. X-ray crystallography shows that the clusters are characterized by multiple CH– π and π – π interactions, which rigidify the ligand and likely contribute to the exceptionally high photoluminescent quantum yields observed, up to 16.0 %, which is significantly greater than the most luminescent ligand-protected Au 13 superatom cluster. Density functional theory analysis suggests that clusters are 8-electron superatoms with a wide HOMO-LUMO energy gap of 2 eV. Consistent with this, the clusters have high stability relative to all-phosphine clusters.


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ABSTRACT: Gold superatom nanoclusters stabilized entirely by N-heterocyclic carbenes (NHCs) and halides are reported.The reduction of well-defined NHC-Au-Cl complexes produces clusters comprised of an icosahedral Au 13 core surrounded by a symmetrical arrangement of 9 NHCs and 3 chlorides.X-ray crystallography shows that the clusters are characterized by multiple CH-π and π-π interactions, which rigidify the ligand and likely contribute to the exceptionally high photoluminescent quantum yields observed, up to 16.0 %, which is significantly greater than the most luminescent ligand-protected Au 13 superatom cluster.Density functional theory analysis suggests that clusters are 8-electron superatoms with a wide HOMO-LUMO energy gap of 2 eV.Consistent with this, the clusters have high stability relative to all-phosphine clusters.
However, problems remain for real-world application of these nanomaterials, with the most fundamental issue being stability against aggregation.Phosphines are only weakly bound to (Au 13 ) 5+ and can be removed relatively easily, leading to aggregation.Thiols and alkynes have specific, unalterable bonding modes that enable rapid exchange including metal atoms, 3 all of which leads to decreased cluster stability (Fig. 1A).
Recently, we demonstrated that a related superatom, (Au 11 ) 3+ in [Au 11 (PPh 3 ) 8 Cl 2 ] + could be stabilized dramatically by introduction of a single N-heterocyclic carbene (NHC) ligand 16 (Fig. 1B).Single crystal X-ray diffraction (SCXRD) analysis showed that the NHC forms a directional Au-C bond as in the Au-PR 3 bonds.Theoretical calculations indicated that this enhanced stability originates not only from the strong Au-C bond but also interaction of the wingtips of the NHCs with the underlying Au clusters. 17Although NHCs are promising ligands [18][19][20][21] in materials chemistry, and have been valuable in the stabilization of coordination clusters, 22,23 only mixed NHC/phosphine clusters could be prepared by the exchange method described in 1B.Recent computational studies suggest that NHCs will be valuable ligands to tune the photophysical properties of superatom clusters. 24The only other examples of NHCstabilized Au superatom clusters are two Au 3 clusters described in seminal reports by Sadighi and Bertrand. 25Herein, we describe the The NHC-protected (Au 13 ) 5+ superatoms showed much higher thermal stability than related phosphine complexes such as [Au 11 (PPh 3 ) 8 Cl 2 ] + .SCXRD suggests that inter-ligand interactions in addition to a strong Au-C bond contribute to the stability.][28][29] Precursor NHC-Au-Cl complexes 2a-e were produced easily and in high yield by the reaction of commercially available Me 2 S-Au-Cl with benzimidazolium salts (1a-e) in the presence of K 2 CO 3 (Fig. 2A). 30Optimized conditions for cluster synthesis with 2a employed NaBH 4 as the reducing agent at 0 °C, followed by warming to room temperature and treatment with HCl (Fig. 2B). 10 Examining the clusters by UV-vis absorbance spectroscopy and electrospray ionization mass spectrometry (ESI-MS) before and after HCl treatment showed that the crude material contained only molecular species and Au 13 clusters, with no other significant cluster species detected.No significant changes were detected by ESI-MS after HCl treatment; the absorbance spectra, however, were significantly sharper (see Supporting Information). 31he molecular formula of cluster 3a was determined to be [Au 13 (NHC Bn ) 9 Cl 3 ] 2+ , by ESI-MS analysis (Fig. 2C).This formula yields a gold core with eight delocalized electrons, in agreement with predictions from superatom theory, 4 and closely related to the cluster [Au 13 (NHC) 10 Cl 2 ] 3+ predicted by DFT. 17 Crystallization of cluster 3a was achieved after anion exchange to yield the PF 6 - cluster [3a][PF 6 ] 2 .As shown in Fig. 2D, the cluster is comprised of an icosahedral Au 13 core with one gold atom at the center, with the remaining gold atoms bound to either chloride or NHC ligands.
Through the use of 13 C labeled NHC precursors, we were able to identify the C-Au bonds as occurring in the 200-214 ppm range of the 13 C NMR spectrum.The number of signals observed in the cluster region is indicative of the presence of other regioisomers, that were not isolated by crystallography.This illustrates another advantage of the use of NHCs as ligands, namely that they provide a spectroscopic handle that is highly indicative of the precise structure, employing a bulk spectroscopic method.The precise structure of these regioisomers is currently under investigation.
Complexes 2b-2f were subjected to the same reducing conditions.The procedure was found to be general, but sensitive to the steric constraints of the NHCs.For example, 2-naphthylCH 2substituents were tolerated, with complex 2b giving Au The thermal stability of these new clusters was assessed in hot acetonitrile.All-phosphine cluster [Au 11 (PPh 3 ) 8 Cl 2 ]Cl (4) 32 was used as a benchmark, as it is one of the best characterized and most stable 8 electron superatom gold clusters stabilized by monodentate phosphines.As shown in Figs.3A-C, 4 underwent full decomposition after 3 h, while NHC cluster 3a showed only slight decomposition after 20 h, and cluster 3b shows virtually no decomposition to nanoparticles, as judged by UV-vis absorbance spectroscopy.NMR analysis suggests that there may be some rearrangement of ligands on the surface, but ESI-MS analysis of cluster 3a after heating showed that the cluster was essentially unchanged, consistent with UV-vis absorbance studies.This improved stability is consistent with thermogravimetric analysis of the clusters, which demonstrates the loss of a single NHC at 180 °C and complete ligand loss at 585 °C.In contrast, ligand loss begins at 150 °C and is complete at 245 °C for cluster 4 (see Supporting Information).
The structural and electronic properties of 3a and 3b were examined by density functional theory (DFT), using the crystal structure of 3a as a starting point (technical details in Supporting Information).The binding energy of the NHC ligand in 3a was found to depend on the bonding site.A higher binding energy of about 2.3 eV was found to a gold site such as Au(2) in Fig. S54, which has chlorides bound to two neighboring Au sites.A lower binding energy of about 2.0 eV was found at Au( 11) that has only one neighboring Au-Cl bond.These values were obtained by using the PBE functional; we found that the van der Waals corrections increased the binding energy by about 0.6 eV.For reference, PBE bonding energies of phosphine ligands to a related [Au 11 (PPh 3 ) 8 Cl 2 ] + cluster 16 were found to vary from 1.1 to 1.4 eV.This demonstrates that NHC binds to such gold clusters clearly stronger than phosphine.
When van der Waals interactions in the ligand layer were accounted for by using the BEEF-vdW functional, the predicted Au-NHC bond strength increased to 2.62 eV in 3a.Compounds 3a and 3b have very similar HOMO-LUMO energy gaps of ca.2.0 eV, reflecting the expected electronic stability of the 8-electron configuration (Figs.3D-F, Supporting Information).Kohn-Sham orbitals near the Fermi level show distinct symmetry properties when projected to the Ih point group in the volume of the gold core (Figs.3D and E).The HOMO, HOMO-1 and HOMO-2 states have triply degenerate T 1u symmetry corresponding to the p-type spherical orbitals as expected for an 8-electron superatom.The first few unoccupied states have e g symmetry corresponding to five-fold degenerate d-states in spherical representation.Chlorine ligands break both the T 1u and e g states by lowering the symmetry to C 3 (see Supporting Information).
The calculated UV-vis spectra of 3a and 3b were in good agreement with experimental data concerning the location of the optical gap and the visible absorption peaks (see Supporting Information).The superatom electronic structure predicts a dipoleallowed HOMOLUMO transition as the lowest optical transition, which is confirmed in the spectral analysis.First excited states were examined by exciting an electron from the HOMO to the LUMO state by forcing the occupation numbers accordingly in the spin-polarized DFT calculation and relaxing the system with the PBE functional.The excited states of 3a and 3b were 1.68 eV and 1.61 eV higher in energy compare d to the ground state, respectively.These energies correspond to 738-770 nm in wavelength, in excellent agreement with observed emission spectra.
Fluorescence excitation-emission matrix (EEM) spectroscopy 33 studies of 3b are shown in Figs.3G and H.The excitation spectrum of 3b matches very well with the absorbance spectrum for the same compound (Figs.2E and 3H), indicating that the clusters are of high optical purity, and confirming that de-excitation of the superatom excited-state is responsible for the intense emission observed. 33The quantum yield of fluorescence was determined from the EEM spectra (Fig. 3H) with excitation matched to the standard, zinc phthalocyanine.Clusters 3b and 3a boast impressive emission quantum yields of 16% and 7%, respectively, making these among the highest quantum yields ever recorded for Au 13 superatoms. 10,12ore commonly, quantum yields for thiol-stabilized gold nanoclusters are <1%. 6,7,12High emission quantum yields and blue shifted emission maxima, have been linked with electronic factors, although structural rigidity is also an important factor. 24emtosecond pump/probe spectroscopy (Fig. S67) confirms theoretical calculations that show the HOMO/LUMO transitions leading to the emissive excited state of 3b.These preliminary results show a complete lack of dynamics for 3b, consistent with a very rigid structure induced by the ligands, which have many internal π-stacking interactions.These interactions restrict the vibrational and rotational motion of ligands, limiting pathways for non-radiative decay processes.Cluster 3a shows significant changes in the excited state following laser pulse excitation, as expected from a cluster with a lower emission quantum yield.
In conclusion, we have reported a simple, straightforward method to prepare NHC-stabilized superatom Au clusters.These clusters were shown to have higher stability than the corresponding phosphine clusters.Their HOMO-LUMO gaps, measured spectroscopically and computed with DFT, are approximately 2 eV, which is consistent with the high stability determined by thermal treatment and analysis by UV-vis absorbance and electrospray ionization mass spectrometry.In fluorescence studies, naphthyl-containing cluster 3b was found to have a quantum yield of 16.0%, which is over twice that of the best reported Au 13 superatom gold cluster. 10This high quantum yield was attributed to a rigid NHC core, supported by a detailed analysis of molecular arrangement in benzyl derivative 3a, and bodes well for future applications of these exciting, novel clusters.