Principles of Optical Spectroscopy of Aromatic Alloy Nanomolecules

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Principles of Optical Spectroscopy of Aromatic Alloy Nanomolecules: Au Ag(SPh-tBu) 36-x

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Shevanuja Theivendran, Le Chang, Aneek Mukherjee, Luca Sementa, Mauro Stener, Alessandro Fortunelli, and Amala Dass J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00556 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 4, 2018

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The Journal of Physical Chemistry

Principles of Optical Spectroscopy of Aromatic Alloy Nanomolecules: Au36-xAgx(SPh-tBu)24. Shevanuja Theivendran1,ǂ, Le Chang2,3,ǂ, Aneek Mukherjee1, Luca Sementa4, Mauro Stener5, Alessandro Fortunelli4,*, Amala Dass1,* 1 Department of Chemistry and Biochemistry, University of Mississippi, Oxford, MS 38677, United States 2 International Research Center for Soft Matter, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China 3 State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China 4 CNR-ICCOM & IPCF, Consiglio Nazionale delle Ricerche, via Giuseppe Moruzzi 1, 56124, Pisa, Italy 5 Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Trieste, Trieste I-34127, Italy

Here we report the synthesis, experimental and theoretical characterization of Au36-xAgx(SPh-tBu)24 alloy nanomolecules to atomic precision. By changing the incoming gold to silver metal ratio during the synthesis of crude mixture, up to eight silver atoms can be incorporated into Au36(SPh-tBu)24, as theoretically confirmed and rationalized in terms of its core and staple structure. Tuning of optical response by Ag doping is strongly affected by aromatic conjugation and qualitatively different with respect to the aliphatic case, with a strikingly non-monotonic behavior of absorption intensity in the low- and high-energy regions, in fair agreement with theoretical predictions, as rationalized via an original analysis tool: the Independent Component Mapping of Oscillatory Strength (ICM-OS) plots.

Gold nanomolecules (also called monolayer-protected clusters, MPC) have a distinct number of gold atoms1 and organo-thiolate ligands, are smaller than 2 nm in metal core diameter, and exhibit unique physical, chemical and optical properties.2-3 These properties can be tuned via ligand chemistry, which is known to have great influence on the stability and response of these nanomolecules. In this field, Au36(SR)24 is a stable nanomolecule and is probably among the best known one among those protected using aromatic thiolate ligands. Au36(SPh-R)24 have been isolated as stable species with both R = H and –tBu.4-7 The properties of gold nanomolecules can also be tuned by alloying with other metals.8 Silver nanomolecules9-11 in fact also have distinct optical properties and have promising antibacterial and antifungal activity.12-13 Au-Ag alloy nanomolecules have then been shown to possess unique properties that are not observed in monometallic Au or Ag particles, and the tuned properties may find applications in catalysis and optics.14 Indeed, Murray et al have studied monolayer-protected metal alloy nanomolecles of size ranging from 3-5 nm by doping Au with Ag, Cu, and Pt.15 El-Sayed et al have synthesized 20nm gold-silver nanoparticles, and showed that silver doping leads to blue shift of the Au SPR peak by ~120nm.16 Au144 is a heavily studied nanomolecule, which has been incorporated with variety of other metals such as silver, copper, and palladium.17-20,21 Au25 nanomolecules have been incorporated with Ag, Cu, Pt, and Pd. 22-23-26.Au38xAgx(SR)24 has been synthesized and its crystal structure has been reported.27-28 Herein, we report the synthesis of Au36-xAgx(SPh-tBu)24 alloy nanomolecules and atomic level incorporation of silver atom into the core of Au36(SPh-tBu)24 as determined by ESI and MALDI mass spectrometry, together with accompanying theoretical analysis. Fan et al have recently reported the crystal structure of Au36-xAgx(SPh-tBu)24 and

revealed that up to 4 dopants the silver atoms are incorporated into 4 of the staple motifs with 50% occupancy29. This result is in contrast with clusters protected with aliphatic thiolates in which Ag dopants preferentially occupy metal core sites24 thus proving that aromaticity (conjugation between the metal-sulfur moiety and the π-system of aromatic groups) brings about qualitatively different phenomena with respect to the aliphatic case. However, Fan et al. did not follow the evolution of optical spectra as a function of the doping level, and performed theoretical analysis on Au36-xAgx(SCH3)24 homologues which contain aliphatic instead of aromatic ligands. At variance, here we show by investigating experimentally a wider doping range and via UV-visible spectroscopy measurements together with theoretical modeling of aromatic rather than aliphatic ligands that the incorporation of silver atoms has a unique effect on the electronic structure and optical properties of these compounds, with doping principles qualitatively different from those established for their aliphatic homologues. Indeed, we find a strong change in absorption intensity in both the low-energy (572-539 nm) and high-energy (366 nm) peaks when the Au:Ag incoming metal ratio is raised from 1:0 to 1:0.2. Moreover, whereas such a change corresponds to a monotonic increase in the intensity of the low-energy peak (with also a blue shift from 572 to 539 nm), a strikingly non-monotonic evolution (to the best of our knowledge unprecedented) is found in the high-energy peak, with a first increase at low Ag doping followed by a decrease at higher levels of doping. Theoretical analysis, also introducing an original analysis tool named Independent Component Mapping of Oscillatory Strength (ICM-OS) plots, rationalizes the increase in absorption intensity in terms of structural and electronic features of Ag doping into the aromatic-thiolate-protected Au36(SPh-R)24

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cluster, purporting a decrease of destructive interference and/or increase in long-range off-diagonal single-particle contributions.

and green spectra correspond to Au:Ag ratio of 1:0.1, 1:0.15, and 1:0.2 respectively. MALDI-MS further confirms the presence of silver incorporation into the structure of

aa

b

Figure 1: (a) Electrospray ionization (ESI) mass spectrometry of Au36-xAgx(SPh-tBu)24 nanomolecules for Au: Ag precursor ratios of 1: 0 (Orange), 1: 0.1 (red), 1: 0.15 (blue), and 1: 0.2 (olive green) in the crude mixture. The mass difference between the peaks in nanoalloys corresponds to the Au (196.97 Da) and Ag (107.87 Da) mass difference, ∆m = 89 Da and ∆m = 89/2 Da at +2 charge state. The number of Ag atoms incorporated has been denoted in the figures. (b) MALDI -TOF mass spectra Au36-xAgx(SPh-tBu)24 nanomolecules for Au: Ag precursor ratios of 1: 0 (Orange), 1: 0.1 (red), 1: 0.15 (blue), and 1: 0.2 (olive green) in the crude mixture. The peaks at and before the asterisk mark is from the characteristic fragmentation of the intact species due to the loss of ligands. THF solution of the (Au–Ag) nanoclusters were mixed with DCTB matrix in toluene and air dried in steel plate for MALDI MS analysis.

Figure 1 (a) shows the ESI mass spectrometry zoomed in the mass region of +2 charge state of nanomolecules. The bottom orange spectrum shows the monometallic stable Au36(SPh-tBu)24 that has been heavily studied amongst aromatic nanomolecules. Addition of silver precursor AgNO3 during the synthesis of crude led to the formation of AuxAgy(SCH2CH2Ph)z nanomolecules in the crude mixture. The crude mixture is then etched with HSPht-Bu (tertbutylbenzene thiol) in the presence of toluene resulting in the formation of AuaAgb(SPh-tBu)24, where a+b =36. The red spectra shows the number of silver incorporation into the structure for the incoming Au:Ag ratio of 1:0.1. The envelope of peaks is due to the presence of different number of Au and Ag atoms in the core structure as denoted by the 89 Da mass difference between the peaks (Au = 196.97 Da, Ag = 107.87 Da, ∆m = 89.1 Da and ∆m = 89.1/2 Da at +2 charge state). Similarly, blue and green spectra shows the final product for the incoming Au:Ag ratio of 1:0.15 and 1:0.2 respectively. It could be observed that the number of silver atom incorporation increases with the increase in the Au:Ag incoming ratio. According to ESI-MS, up to 8 silver atoms get incorporated for the Au:Ag ratio of 1:0.2. However, the experiments at Au:Ag ratio higher than 1:0.2 lead to the formation of unstable alloy nanomolecules. A Au:Ag ratio of 1:0.2 is the highest ratio that could be achieved for silver incorporation.

Au36(SPh-tBu)24 and also it could be observed that the MALDI-MS figure of Au:Ag = 1:0.2 metal ratio has a maximum peak at four silver atoms incorporation , thus suggesting a lesser stability of (Ag-Au)36(SR)24 species to MALDI conditions. The peaks that are present at and before the asterisk mark corresponds to the fragmentation in MALDI-MS due to the loss of ligands and atoms. Therefore, even though ESI-MS spectra for Au:Ag incoming metal ratio of 1:0.2 shows up to 8 plausible Ag atoms incorporation, MALDI-MS shows Au32Ag4(SPh-tBu)24 as the most stable species from the intensity of the peak. Figure 2(a) shows the experimental UV visible spectra comparison of pure Au36(SPh-tBu)24 (black) with Au36xAgx(SPhtBu)24 product (green, blue, red) synthesized from different Au:Ag incoming metal ratios. No qualitative differences in the overall pattern of main absorption peaks are observed when the Au:Ag ratio is changed from 1:0 to 1:0.2, and one finds a low-energy peak at 550-570 nm and a highenergy peak at 366 nm. Ag doping provokes a slight blue shift of the peak at 572 nm down to 539. However, what is striking is the evolution of intensity and the interplay between high- and low-energy regions. At low Ag doping, in fact, one observes an increase in intensity in both the lowenergy and high-energy peaks. In striking contrast (and to the best of our knowledge for the first time), at high Ag doping the intensity of the low-energy peak keeps increasing, whereas the intensity of the high-energy peak exhibits a non-monotonic behavior and decreases. To define an unambiguous nomenclature of homotops30 (i.e., isomers with the same structural skeleton but different

Figure 1 (b) shows the corresponding MALDI-MS of the samples showed in ESI-MS. The bottom orange spectra represent +1 charge state of Au36(SPh-tBu)24 with the incoming metal ratio of 1:0. Similar to the ESI spectra, the red, blue,

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The Journal of Physical Chemistry

a

b

Figure 2: a) Experimental optical spectra of (AuAg)36(SPh-tBu)24 for different Au:Ag precursor ratios 1:0 (black), 1:0.1 (green), 1:0.15 (blue), and 1:0.2 (red) in the starting material. b) Calculated optical spectra of Au36(SPh)24, Ag4Au32(SPh)24 [4Ex-St-near], and Ag8Au32(SPh)24 [4ExTd&4Ex-St-near].

doping content) we decompose the Au36(SR)24 structure as described in Figs.S1,S2,S3 of the Supplementary Information (SI) – note that an alternative nomenclature has also been used in the literature5, see Fig.S4. In detail, a Au20 core in the interior of the cluster is generated from a 20-atom tetrahedron by misplacing the 4 vertexes (Fig.S1) however still keeping their tetrahedral relative positions, so that we distinguish: 4 external tetrahedral atoms (Ex-Td, the four misplaced vertexes), 4 internal tetrahedral atoms (In-Td, the atoms in the 111 facets of the Td), and 12 atoms on the edges of the tetrahedron (Edge). This Au20 core is surrounded by two different types of dimeric staples, each constituted by 4 units: 4 “external” dimeric staples (Ex-St) and 4 “internal” dimeric staples (In-St) – we name the staples “external” or “internal” according to the average distance from the core. It is important to note that in the external staples Ex-St, 4 of the 8 Au atoms (named Ex-St-near) are closer and bind to Ex-Td core Au atoms (a representative one of these is labelled as 12 in Fig. S2) while other 4 of the 8 Au atoms (named Ex-St-far) do not bind to core Au atoms and are plotted in dark green (a representative one of these is labelled as 13 in Fig. S2). Density-Functional Theory (DFT) full geometry minimizations and Time-Dependent DFT (TDDFT) optical response simulations on the relaxed geometries were performed on (Ag-Au)36(SPh)24 clusters at selected compositions: Au36(SPh)24, Ag4Au32(SPh)24[4Ex-Td], Ag4Au32(SPh)24[4ExSt-near], and Ag8Au28(SPh)24[4Ex-Td&4Ex-St-near], where in square brackets the Ag-doping sites are listed. We use full geometry relaxation which has been shown to appreciably affect geometry and symmetry of aromatic-protected MPC31. As for the choice of doping sites, we take from Ref. 29 that at low doping level the preferred homotop is Ag4Au32(SPh)24[4Ex-St-near], and we conduct simulations also on Ag4Au32(SPh)24[4Ex-Td] for comparison. At high doping level we select the Ag8Au28(SPh)24[4Ex-Td&4Ex-St-

near] homotop on the basis of total energy considerations, as described in the SI. The resulting spectra are plotted in Fig. 2(b) (see also fig. S5), and should be compared with experimental spectra in Figure 2(a). An inspection of these figures shows that there are two regions: one around 360 nm and one around 550 nm, exhibiting distinct differences between doped and undoped cases. At low doping, Ag4Au32(SPh)24[4Ex-St-near] exhibits an increase in absorption intensity around 500 nm and below 390 nm. The optical spectrum of the Ag4Au32(SPh)24[4Ex-St-near] is therefore in fair agreement with the experimental features at Au:Ag incoming ratios of 1:0.1 and 1:0.15, with their increase in absorption intensity around 366 nm (high energy region) and around 539 nm (low energy region). At high doping (modeling the experimental high-doping case of Au:Ag incoming ratio of 1:0.2), Ag8Au28(SPh)24[4Ex-Td&4Ex-St-near] exhibits a significant increase in intensity in the whole region beyond 440 nm, and a near coincidence with the low doping case below 400 nm. Theory thus reasonably predicts the experimental observation of a large intensity increase above 500 nm but it does not manage to capture the non-monotonic behavior below 450 nm. To understand the origin of absorption intensity variations in such complex aromatic and doped nanomolecules, we introduce here an original tool – that we name Independent Component Mapping of Oscillatory Strength (ICM-OS) plots – which allow us to investigate the connection between absorption and single-particle excitations. In detail, the z component of the oscillator strength at each given frequency (energy) can be calculated as the imaginary part of the zz diagonal element of the dynamical polarizability tensor32: occ virt

 zz     i z  a Pi a (z ) i


a

(1)


Figure 3: Two-dimensional ICM-OS plots for X and Y components of (Ag-Au)36(SPh)24 nanomolecules at: (a) 4.0 eV; (b) 2.0 eV.

where Pia is a density-matrix element due to electric field in the Z-direction and

i z a

given energy is proportional to the integral of the ICMOS(ω) at the given energy over the (  i ,  a ) plane. Since the

is a dipole matrix element,

ICM-OS plots have a positive and negative part, they identify both the nature of the excited state and also the relative sign of the contribution of the dipole vs. density-matrix elements on the oscillator strength. ICM-OS plots at 2 and 4 eV are reported in Fig. 3 for Au36(SPh)24, Ag4Au32(SPh)24[4Ex-Stnear], and Ag8Au28(SPh)24[4Ex-Td&4Ex-St-near]. The most striking feature at 4 eV – Fig. 3(a) – is the reduced presence of negative off-diagonal single-electron contributions in doped clusters with respect to Au36(SPh)24. This proves that

both evaluated over a pair of occupied(=i)/virtual(=a) single-particle molecular orbitals. We then plot as ICM-OS(ω) the individual

 i z  a Pi a components

(smeared with a

Gaussian function to make them visually clearer) as functions of the single-particle energies of occupied (  i ) and virtual (  a ) orbitals. Optical absorption intensity at each


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The Journal of Physical Chemistry the optical response of Au36(SPh)24 in the near-UV is damped by a destructive interference among contributions coming from off-diagonal high-energy excitations, which disappears in the doped systems in which such excitations interfere constructively with the main peaks thus increasing oscillator strength. At 2 eV the situation is more complex, and an intense additional peak arises in the HOMOLUMO region of the ICM-OS in the Y-direction, associated with a red-shift of optical excitations due to conjugation effects of the aromatic residues in the thiolates with Ag atoms in the staples. This additional peak – which prevails over an accompanying interference damping also apparent in Fig. 3(b) – finally results in a doubled absorption intensity at low energy in good agreement with experiment. In this connection we can recall that Ag-doping has been shown to modify the HUMO-LUMO gap33-35 however usually entailing a blue-shift of optical absorption in aliphatic-ligand-protected nanomolecules14, 36: the red-shift here theoretically predicted and experimentally observed underlines again the peculiarity of aromatic-protected systems.

We present here the synthesis, mass-spectrometric and optical characterization of Ag-Au alloy nanomolecules with aromatic thiolate ligands. (Ag-Au)36(SPh-tBu)24 clusters are prepared through a core-size conversion process, and then thoroughly characterized via ESI-MS and MALDI-MS spectrometry, UV-vis absorption spectroscopy and theoretical analysis. A peculiar evolution of absorption enhancement with an interplay between low- and high-energy regions as a function of doping level is observed at the experimental level, with a strikingly non-monotonic behavior of absorption intensity as a function of the level of Ag doping in the high-energy (366 nm) peak. Theory correctly predicts the strong absorption intensity increase at low energy and at high energy at low doping levels, however misses the nonmonotonic behavior at high energy and high doping. An ICM-OS analysis tool here purposely introduced clarifies general effects due to the coupling of aromatic conjugation and Ag doping onto optical features, such as a decrease of destructive interference due to long-range off-diagonal single-particle contributions at high energy and a red-shift of excitation intensity at low energy. The basic principles of optical spectroscopy of aromatic monolayer-protected alloyed clusters are thus here explored and demonstrated to be uniquely different from those of aliphatic systems.

The previous ICM-OS analysis shows the complex interplay among aromaticity and doping phenomena. Such effects are very sensitive to the precise relative positions of energy levels and spatial extent of fragment orbitals between doped metal core, doped staple and organic residues, whence the difficulty of theory in catching the non-monotonic behavior of absorption intensity in near-UV at high doping content (work is in progress to explore alternative methods such as other TD xc-functionals that might improve predictivity of our theoretical approach). Nevertheless, theory explains that these unique phenomena are associated with the aromatic character of the HSPh thiols here employed37-38: as illustrated in Fig. S5(b) – consistent with ref. 29, the optical spectra of aliphatic (Ag-Au)36(SCH3)24 nanomolecules present a completely different pattern of evolution, with a nearly constant increase in intensity in the whole energy range below 530 nm for Ag4Au32(SCH3)24[4Ex-Td] and Ag8Au28(SCH3)24[4Ex-Td&4Ex-St-near], whereas Ag4Au32(SCH3)24[4Ex-St-near] presents a modest increase only around 380 nm: the overall pattern is thus at complete variance with the present experimental and theoretical results with conjugated ligands. Although the replacement of SPh-tBu with SCH3 is a common practice in the literature35 here we show that such a replacement can qualitatively affect theoretical modeling in the case of mixed Ag-Au systems, even more substantially than in the known case of pure Au MPC.39 Finally, we note that it would be very interesting to study the effect of Ag doping on photoluminescence, following exploration of pure Au aliphatic-protected MPC.40 Indeed, recent experimental work demonstrates that mixed Ag-Au MPC systems protected by aromatic ligands can exhibit highly enhanced photoluminescence quantum yield.41 However, we conjecture that inter-system crossing effects recently investigated for bare Au cluster 42 are likely to be even stronger in aromatic-protected clusters, which severely complicates theoretical analysis, so that we defer this study to future work.

Experimental section: Materials: Phenylethane thiol (SAFC, >99%), 4-tert-butylbenzenethiol, TBBT (TCI, >97%), hydrogen tetrachloroaurate(III) trihydrate (Alfa Aesar, 99.99%), Silver nitrate (Fisher), sodium borohydride (Acros Organics), trans-2-[3[(4-tertbutylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB matrix)(TCI ≥ 99%), tetrahydrofuran (THF) HPLC grade (Fisher), toluene (Fisher), methanol (Fisher), and acetone (Fisher) were purchased and used as received. Instruments: Matrix-assisted laser desorption ionization time of fight (MALDI-TOF) mass spectra were collected on a Voyager DE Pro mass spectrometer in linear positive using 20 mM DCTB as matrix spotted from a THF solution. ESI-MS spectra were acquired on Waters Synapt HDMS instrument in positive mode by dissolving the title compound in HPLC grade THF, and UV-visible absorption spectra were recorded in toluene solution on a Shimadzu UV-1601 UV-vis spectrophotometer. Synthesis of (AuAg)36(SPh-tBu)24: Followed a similar protocol reported by our group for the synthesis of crude mixture6. Firstly, HAuCl4 and AgNO3 in a specific ratio (Au:Ag = 1:0, 1:0.1, 1:0.15, and 1:0.2) were dissolved in THF (10mL THF for 0.1g of HAuCl4, Sonication might be required for the dissolution of AgNO3), then phenylethane thiol (Au:Thiol=1:6) was added into the mixture and stirred for 40-60 minutes until the yellow solution turned to white turbid mixture. Thereafter, NaBH4 (Au:NaBH4=1:10) dissolved in 2mL of cold water is added to the mixture. The reaction was stirred for 10 minutes before the solvent is removed via rotary evaporation. The resulting product was washed with methanol three times to remove the excess thiol and NaBH4. The second step was the thermochemical treatment with tertbutylbenzenethiol. The crude product (obtained from 0.1g of HAuCl 4) was dissolved in 0.5 mL of THF and 0.75 mL of tert-butylbenzenethiol and etched for 7 days at 60-65⁰C under stirring. The resulting product was washed with methanol five times to remove excess thiol. The final product could be further purified using size-exclusion chromatography if needed. Computational Details 1.DFT



We are grateful for the support from NSF-CHE-1255519. L.C. is grateful to the China Scholarship Council for a scholarship support of an internship at CNR-ICCOM. Computational research was performed in part using EMSL, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory, and PNNL Institutional Computing at Pacific Northwest National Laboratory. Support from CINECA supercomputing center within the ISCRA program is also gratefully acknowledged.

Calculations on (Ag-Au)36(SCH3)24 systems were performed using the Quantum Espresso package43 and ultrasoft pseudopotentials44. A semiempirical correction (Grimme-D2) was added to the Perdew-BurkeErnzerhof [PBE] exchange and correlation (xc-) functional45 to account for dispersio interactions46-47. A cubic unit cell measuring 35 Å in length and 40 Ry and 400 Ry, respectively, as the cutoff for the plane-wave representation of the wave function and the density were employed. Calculations were performed spin-restricted and at the Gamma point only. Starting from experimental crystal geometry of Au36(SPh)246, Ph residues were transformed into CH2CH2Ph and then into CH3 residues. The Cartesian coordinates of the resulting Au36(SCH3)24 species are provided in the SI. To reduce computational effort, further geometry relaxation was not considered in this first step. The relative energy differences among the “homotops”48 (isomers sharing the same skeletal structure and composition but differing in the mixing pattern) thus produced are reported in Table S1.

Associated content Supporting information. Simulated TD-DFT spectra included. This material is available free of charge via the internet at http://pubs.acs.org.

Fully relaxed optimizations were instead performed on (AgAu)36(SPh)24 systems, i.e., Au36(SPh)24, two Ag4Au32(SPh)24 homotops, and one Ag8Au28(SPh)24 homotop. The experimental geometry of Au36(SPh)246 was taken as a starting point, and full geometry relaxation on each M36(SPh)24 species was performed using the CP2K package49 whose DFT algorithms are based on a hybrid Gaussian/Plane-Wave scheme (GPW)50. Pseudopotentials derived by Goedecker, Teter, and Hutter (GTH)51 were chosen to describe the core electrons of all atoms and DZVP basis sets52 to represent the DFT Kohn-Sham orbitals. The semiempirical Grimme-D3 correction53 was added to the PerdewBurke-Ernzerhof (PBE)45 exchange and correlation (xc-) functional to take into account the dispersion interaction between organic ligands. The cutoff for the auxiliary plane-wave representation of the density was 300 Ry.

REFERENCES 1. 2.

3.

4. 5.

2. TDDFT 6.

The complex polarizability TDDFT method as well as its implementation have been described in detail previously 32, 54, therefore we refer the reader to the original work for more information about the algorithm and how to calculate the corresponding matrix elements (a description of its salient features is also given in the SI).

7. 8.

The method is implemented in a local version of the ADF program, in which the LB9455 exchange-correlation (xc-) functional is employed to solve the KS equations. It can be noted that LB94 has an asymptotically correct Coulomb tail and is thus superior to purely gradient-correct xcfunctionals (such as PBE) in describing low-lying unoccupied electronic states. The exchange-correlation kernel is approximated according to the Adiabatic Local Density Approximation (ALDA) 56 in the TDDFT part. A basis set of Slater Type Orbitals (STO) included in the ADF database of triple-zeta polarized (TZP) quality has been employed which has proven to provide fully accurate results57. A subset of the ADF STO fitting functions is employed to solve the TDDFT equations (see matrix system (3) in the SI)) to save computer time without loss of accuracy, as checked by preliminary test calculations. Moreover, the Zero Order Regular Approximation(ZORA)58 is employed to include relativistic effects, which are important for heavy elements such as gold and, to less extent, silver.

9.

10.

11.

12.

13.

14.

Finally, few more technical details on the analysis tool here introduced, named Independent Component Mapping of Oscillatory Strength (ICMOS) plots, are given in the SI.

15. 16.

Author information

17.

Corresponding authors 18.

* [email protected] * [email protected] ǂ

19.

S.T. and L.C. equally contributing first authors.

20.

Acknowledgments

Qian, H.; Zhu, M.; Wu, Z.; Jin, R., Quantum Sized Gold Nanoclusters with Atomic Precision. Acc. Chem. Res. 2012, 45, 1470. Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U., Nanocrystal gold molecules. Adv. Mater. 1996, 8 (5), 428-433. Chaki, N. K.; Negishi, Y.; Tsunoyama, H.; Shichibu, Y.; Tsukuda, T., Ubiquitous 8 and 29 kDa Gold:Alkanethiolate Cluster Compounds: MassSpectrometric Determination of Molecular Formulas and Structural Implications. J. Am. Chem. Soc. 2008, 130, 8608. Nimmala, P. R.; Dass, A., Au36(SPh)23 Nanomolecules. J. Am. Chem. Soc. 2011, 133 (24), 9175-9177. Zeng, C.; Qian, H.; Li, T.; Li, G.; Rosi, N. L.; Yoon, B.; Barnett, R. N.; Whetten, R. L.; Landman, U.; Jin, R., Total Structure and Electronic Properties of the Gold Nanocrystal Au36(SR)24. Angew. Chem. Int. Ed 2012, 51 (52), 13114-13118. Nimmala, P. R.; Knoppe, S.; Jupally, V. R.; Delcamp, J. H.; Aikens, C. M.; Dass, A., Au36(SPh)24 Nanomolecules: X-ray Crystal Structure, Optical Spectroscopy, Electrochemistry, and Theoretical Analysis. J. Phys. Chem. B 2014, 118 (49), 14157-14167. Zeng, C.; Liu, C.; Pei, Y.; Jin, R., Thiol Ligand-Induced Transformation of Au38(SC2H4Ph)24 to Au36(SPh-t-Bu)24. ACS Nano 2013, 7 (7), 6138-6145. Yang, S.; Chai, J.; Song, Y.; Fan, J.; Chen, T.; Wang, S.; Yu, H.; Li, X.; Zhu, M., In Situ Two-Phase Ligand Exchange: A New Method for the Synthesis of Alloy Nanoclusters with Precise Atomic Structures. J. Am. Chem. Soc. 2017, 139 (16), 5668-5671. Joshi, C. P.; Bootharaju, M. S.; Alhilaly, M. J.; Bakr, O. M., [Ag25(SR)18]−: The “Golden” Silver Nanoparticle. J. Am. Chem. Soc. 2015, 137 (36), 1157811581. Yau, S. H.; Ashenfelter, B. A.; Desireddy, A.; Ashwell, A. P.; Varnavski, O.; Schatz, G. C.; Bigioni, T. P.; Goodson, T., Optical Properties and Structural Relationships of the Silver Nanoclusters Ag32(SG)19 and Ag15(SG)11. J. Phys. Chem. C 2017, 121 (2), 1349-1361. Chakraborty, I.; Kurashige, W.; Kanehira, K.; Gell, L.; Häkkinen, H.; Negishi, Y.; Pradeep, T., Ag44(SeR)30: A Hollow Cage Silver Cluster with Selenolate Protection. J. Phys. Chem. Lett. 2013, 4, 3351. Yang, H.; Wang, Y.; Huang, H.; Gell, L.; Lehtovaara, L.; Malola, S.; Häkkinen, H.; Zheng, N., All-thiol-stabilized Ag44 and Au12Ag32 nanoparticles with singlecrystal structures. Nat. Commun. 2013, 4, 2422. Desireddy, A.; Conn, B. E.; Guo, J.; Yoon, B.; Barnett, R. N.; Monahan, B. M.; Kirschbaum, K.; Griffith, W. P.; Whetten, R. L.; Landman, U.; Bigioni, T. P., Ultrastable silver nanoparticles. Nature 2013, 501 (7467), 399-402. Barcaro, G.; Sementa, L.; Fortunelli, A.; Stener, M., Optical properties of nanoalloys. Phys. Chem. Chem. Phys. 2015, 17 (42), 27952-27967. Templeton, A. C.; Wuelfing, M. P.; Murray, R. W., Monolayer protected cluster molecules. Acc. Chem. Res. 2000, 33 (1), 27-36. Link, S.; Wang, Z. L.; El-Sayed, M. A., Alloy Formation of Gold−Silver Nanoparticles and the Dependence of the Plasmon Absorption on Their Composition. J. Phys. Chem. B 1999, 103 (18), 3529-3533. Dharmaratne, A. C.; Dass, A., Au144-xCux(SC6H13)60 nanomolecules: effect of Cu incorporation on composition and plasmon-like peak emergence in optical spectra. ChemComm 2014, 50 (14), 1722-1724. Kothalawala, N.; Kumara, C.; Ferrando, R.; Dass, A., Au144-xPdx(SR)60 nanomolecules. ChemComm 2013, 49 (92), 10850-10852. Kumara, C.; Dass, A., (AuAg)144(SR)60 alloy nanomolecules. Nanoscale 2011, (3), 3064-3067. Malola, S.; Häkkinen, H., Electronic Structure and Bonding of Icosahedral Core–Shell Gold–Silver Nanoalloy Clusters Au144–xAgx(SR)60. J. Phys. Chem. Lett. 2011, 2, 2316.


Page 6 of 7

Page 7 of 7 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry 21. Bhattarai, N.; Black, D. M.; Boppidi, S.; Khanal, S.; Bahena, D.; TlahuiceFlores, A.; Bach, S. B. H.; Whetten, R. L.; Jose-Yacaman, M., ESI-MS Identification of Abundant Copper–Gold Clusters Exhibiting High Plasmonic Character. J. Phys. Chem. C 2015, 119 (20), 10935-10942. 22. Negishi, Y.; Kurashige, W.; Niihori, Y.; Iwasa, T.; Nobusada, K., Isolation, structure, and stability of a dodecanethiolate-protected Pd1Au24 cluster. Phys. Chem. Chem. Phys. 2010, 12 (23), 6219-6225. 23. Negishi, Y.; Iwai, T.; Ide, M., Continuous modulation of electronic structure of stable thiolate-protected Au25 cluster by Ag doping. ChemComm 2010, 46 (26), 4713-4715. 24. Kumara, C.; Aikens, C. M.; Dass, A., X-ray Crystal Structure and Theoretical Analysis of Au25–xAgx(SCH2CH2Ph)18– Alloy. J. Phys. Chem. Lett. 2014, 5 (3), 461-466. 25. Kwak, K.; Tang, Q.; Kim, M.; Jiang, D.-e.; Lee, D., Interconversion between Superatomic 6-Electron and 8-Electron Configurations of M@Au24(SR)18 Clusters (M = Pd, Pt). J. Am. Chem. Soc. 2015, 137 (33), 10833-10840. 26. Qian, H.; Jiang, D.-e.; Li, G.; Gayathri, C.; Das, A.; Gil, R. R.; Jin, R., Monoplatinum Doping of Gold Nanoclusters and Catalytic Application. J. Am. Chem. Soc. 2012, 134 (39), 16159-16162. 27. Kumara, C.; Dass, A., AuAg alloy nanomolecules with 38 metal atoms. Nanoscale 2012, 4 (14), 4084-4086. 28. Kumara, C.; Gagnon, K. J.; Dass, A., X-ray Crystal Structure of Au38– xAgx(SCH2CH2Ph)24 Alloy Nanomolecules. J. Phys. Chem. Lett. 2015, 6 (7), 1223-1228. 29. Fan, J.; Song, Y.; Chai, J.; Yang, S.; Chen, T.; Rao, B.; Yu, H.; Zhu, M., The solely motif-doped Au36-xAgx(SPh-tBu)24 (x = 1-8) nanoclusters: X-ray crystal structure and optical properties. Nanoscale 2016, 8 (33), 15317-15322. 30. Jellinek, J.; Krissinel, E. B., NinAlm alloy clusters: analysis of structural forms and their energy ordering. Chem. Phys. Lett. 1996, 258 (1–2), 283-292. 31. Tlahuice-Flores, A.; Whetten, R. L.; Jose-Yacaman, M., Ligand Effects on the Structure and the Electronic Optical Properties of Anionic Au25(SR)18 Clusters. J. Phys. Chem. C 2013, 117 (40), 20867-20875. 32. Baseggio, O.; Fronzoni, G.; Stener, M., A new time dependent density functional algorithm for large systems and plasmons in metal clusters. J. Chem. Phys. 2015, 143 (2), 024106. 33. Guidez, E. B.; Mäkinen, V.; Häkkinen, H.; Aikens, C. M., Effects of Silver Doping on the Geometric and Electronic Structure and Optical Absorption Spectra of the Au25–nAgn(SH)18– (n = 1, 2, 4, 6, 8, 10, 12) Bimetallic Nanoclusters. J. Phys. Chem. C 2012, 116 (38), 20617-20624. 34. Lugo, G.; Schwanen, V.; Fresch, B.; Remacle, F., Charge Redistribution Effects on the UV–Vis Spectra of Small Ligated Gold Clusters: a Computational Study. J. Phys. Chem. C 2015, 119 (20), 10969-10980. 35. Molina, B.; Tlahuice-Flores, A., Thiolated Au18 cluster: preferred Ag sites for doping, structures, and optical and chiroptical properties. Phys. Chem. Chem. Phys. 2016, 18 (3), 1397-1403. 36. Xiang, J.; Li, P.; Song, Y.; Liu, X.; Chong, H.; Jin, S.; Pei, Y.; Yuan, X.; Zhu, M., X-Ray crystal structure, and optical and electrochemical properties of the Au15Ag3(SC6H11)14 nanocluster with a core-shell structure. Nanoscale 2015, 7 (43), 18278-18283. 37. Sementa, L.; Barcaro, G.; Baseggio, O.; De Vetta, M.; Dass, A.; Aprà, E.; Stener, M.; Fortunelli, A., Ligand-Enhanced Optical Response of Gold Nanomolecules and Its Fragment Projection Analysis: The Case of Au30(SR)18. J. Phys. Chem. C 2017, 121 (20), 10832-10842. 38. Tlahuice-Flores, A., Ligand effects on the optical and chiroptical properties of the thiolated Au18 cluster. Phys. Chem. Chem. Phys. 2016, 18 (40), 2773827744. 39. Tlahuice-Flores, A., Ligand effects on the structure and vibrational properties of the thiolated Au18 cluster. Progress in Natural Science: Materials International 2016, 26 (5), 510-515. 40. Weerawardene, K. L. D. M.; Aikens, C. M., Theoretical Insights into the Origin of Photoluminescence of Au25(SR)18– Nanoparticles. J. Am. Chem. Soc. 2016, 138 (35), 11202-11210. 41. Soldan, G.; Aljuhani, M. A.; Bootharaju, M. S.; AbdulHalim, L. G.; Parida, M. R.; Emwas, A.-H.; Mohammed, O. F.; Bakr, O. M., Gold Doping of Silver Nanoclusters: A 26-Fold Enhancement in the Luminescence Quantum Yield. Angew. Chem. Int. Ed. 2016, 55 (19), 5749-5753. 42. Yu, C.; Harbich, W.; Sementa, L.; Ghiringhelli, L.; Aprá, E.; Stener, M.; Fortunelli, A.; Brune, H., Intense fluorescence of Au20. J. Chem. Phys. 2017, 147 (7), 074301. 43. Paolo Giannozzi; Stefano Baroni; Nicola Bonini; Matteo Calandra; Roberto Car; Carlo Cavazzoni; Davide Ceresoli; Guido L. Chiarotti; Matteo Cococcioni; Ismaila Dabo; Andrea Dal Corso; Stefano Fabris; Guido Fratesi; Stefano de Gironcoli; Ralph Gebauer; Uwe Gerstmann; Christos Gougoussis; Anton Kokalj; Michele Lazzeri; Layla Martin-Samos; Nicola Marzari; Francesco Mauri; Riccardo Mazzarello; Stefano Paolini; Alfredo Pasquarello; Lorenzo Paulatto; Carlo Sbraccia; Sandro Scandolo; Gabriele Sclauzero; Ari P. Seitsonen; Alexander Smogunov; Paolo Umari; Renata M.Wentzcovitch, QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials. J. Phys.: Condens. Matter 2009, 21, 395502. 44. Vanderbilt, D., Soft self-consistent pseudopotentials in a generalized eigenvalue formalism. Phys. Rev. B 1990, 41 (11), 7892-7895. 45. John P. Perdew; Kieron Burke; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868.

46. Barone, V.; Casarin, M.; Forrer, D.; Pavone, M.; Sambi, M.; Vittadini, A., Role and effective treatment of dispersive forces in materials: Polyethylene and graphite crystals as test cases. J. Comput. Chem. 2009, 30 (6), 934-939. 47. Grimme, S., Semiempirical GGA-type density functional constructed with a long-range dispersion correction. J. Comput. Chem. 2006, 27 (15), 1787-1799. 48. Jellinek, J.; Krissinel, E. B., NinAlm alloy clusters: analysis of structural forms and their energy ordering. Chem. Phys. Lett. 1996, 258 (1), 283-292. 49. Hutter, J.; Iannuzzi, M.; Schiffmann, F.; VandeVondele, J., cp2k: atomistic simulations of condensed matter systems. Wiley Interdiscip Rev Comput Mol Sci 2014, 4 (1), 15-25. 50. Gerald Lippert; Jürg Hutter; Parrinello, M., The Gaussian and augmented-planewave density functional method for ab initio molecular dynamics simulations. Theor. Chem. Acc. 1999, 103, 124-140. 51. Goedecker, S.; Teter, M.; Hutter, J., Separable dual-space Gaussian pseudopotentials. Phys. Rev. B 1996, 54 (3), 1703-1710. 52. VandeVondele, J.; Hutter, J., Gaussian basis sets for accurate calculations on molecular systems in gas and condensed phases. J. Chem. Phys. 2007, 127 (11), 114105. 53. Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H., A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010, 132 (15), 154104. 54. Baseggio, O.; De Vetta, M.; Fronzoni, G.; Stener, M.; Fortunelli, A., A new time-dependent density-functional method for molecular plasmonics: Formalism, implementation, and the Au144(SH)60 case study. Int. J. Quantum Chem. 2016, 116 (21), 1603-1611. 55. van Leeuwen, R.; Baerends, E. J., Exchange-correlation potential with correct asymptotic behavior. Phys. Rev. A 1994, 49 (4), 2421-2431. 56. Gross, E. K. U.; Kohn, W., Time-Dependent Density-Functional Theory. Adv. Quantum Chem. 1990, (21), 255-291. 57. Baseggio, O.; De Vetta, M.; Fronzoni, G.; Stener, M.; Sementa, L.; Fortunelli, A.; Calzolari, A., Photoabsorption of Icosahedral Noble Metal Clusters: An Efficient TDDFT Approach to Large-Scale Systems. J. Phys. Chem. C 2016, 120 (23), 12773-12782. 58. Lenthe, E. v.; Baerends, E. J.; Snijders, J. G., Relativistic regular two‐ component Hamiltonians. J. Chem. Phys. 1993, 99 (6), 4597-4610.


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