Ligand Effects of Thiolate-Protected Au102 Nanoclusters

Article pubs.acs.org/JPCC

Ligand Effects of Thiolate-Protected Au102 Nanoclusters Yi Gao* Laboratory of Physical Biology and Division of Interfacial Water, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China S Supporting Information *

ABSTRACT: The Au102(p-MBA)44 (p-MBA = para-mercaptobenzoic acid) nanocluster is an ideal model to study the structures of gold nanoclusters, the motifs of the monolayer ligand groups, and the crystal formation of Au nanoparticles. Based on the partially exchanged Au102(pMBA)40(p-BBT)4 (p-BBT = para-bromobenzene thiol) crystal structure (J. Am. Chem. Soc. 2012, 134, 13316−13322), we employed density functional theory to investigate the ligand effects for different thiolate substitutions. It was found that the intermolecular π−π stacking plays an important role for the crystal’s stability in addition to the increased intrinsic stability from the substituent monomer. Furthermore, we suggested para-(dimethylamino) benzenethiol (N(CH3)2−C6H4−SH) and para-amino benzenethiol (NH2−C6H4−SH) would be more favorable than p-BBT for the stabilities of partially exchanged Au102(p-MBA)44 crystal structures due to their stronger intermolecular π−π stacking. This study provides a theoretical template for surface chemical engineering.

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INTRODUCTION Surface chemical modification is crucial for controlling the structures and growth of nanoparticles1,2 and further affects their electronic and optical properties.3−5 However, the knowledge of ligands’ structures is very limited due to the insufficient information of their single crystal structures, which hinders the controllable synthesis of desired nanoparticles.6−8 Recently, Jadzinsky et al. reported the first single crystal structure of Au102(p-MBA)44, (p-MBA = para-mercaptobenzoic acid) which shed light on the nailing down of the detailed structure of self-assembled monolayers (SAM) on the surface of gold nanoparticles.9 Soon after, Au25(SR)18 and Au38(SR)24 single crystal structures were also synthesized and characterized10−12 and provides more insights into SAM compositions, which arose the excitement of both experimental and theoretical scientists.13−19 Other high-quality “magic” compounds including Au20(SR)16, Au40(SR)24, Au68(SR)34, and Au144(SR)60 have been synthesized, but their structures were not fully characterized yet.20−29 On the other hand, theoretical chemists explored the SAM structures on Au(111) surface and gold nanoparticles using density functional theory extensively. Grönbeck et al. proposed an ordered structure where cis-RS-Au-RS units were energetically preferred over any model involving adatom-thiolate complexes.30 Walter et al.31 and Gao et al.32 theoretically presented the stability of Au102(p-MBA)44. Akola et al.33 theoretically proposed the Au25(SR)18 crystal structure in the same time as the experiment.11 Pei and co-workers applied the core-staple mathematic formula and density-functional theory to predict the core structure of Au38(SR)24 initially,34 and Lopez-Acevedo et al. further proposed its exact chiral structure,35 consistent with Jin’s later experiment.12 Several other theoretical works proposed the structures of Au20(SR)16,36 Au44(SR)282‑,37 and Au144(SR)6038,39 afterward. © 2013 American Chemical Society

Despite all significant progress in the area, the ligand effects have not been fully understood. Song et al. proposed the participation of Au(I)-SR as an active unit in the exchange reaction.40 Guo et al. compared the kinetics of ligand exchanges of thiolate-protected small-sized gold clusters, and proposed it independent of the core size.41 Han and colleagues theoretically investigated the ligand effects of Au25(SR)18−, Au38(SR)24, and Au102(SR)44, and found PhCOOH is more favorable binding than Ph and PhF.42 A very recent paper by Heinecke et al. presented the first single-crystal X-ray structure of a partially exchanged Au102(p-MBA)40(p-BBT)4 (p-BBT = para-bromobenzene thiol) and further provided a significant insight of the reaction pathway for thiol-based ligand exchange in theory.43 Thus, there are still a few questions waiting for addressing based on the first partially exchanged crystal structure: (1) What is the stabilities of these 4 sites in the exchange reaction? (2) What is the dominant factor? (3) Could we design a more stable partially exchange crystal structure? In this paper, we presented a theoretical study to answer above questions.

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COMPUTATIONAL METHODOLOGY All geometries were fully optimized using density functional theory with the generalized gradient approximate Perdew− Burke−Ernzerhof functional.44 The DZVP-MOLOPT-GTH basis sets45 with the pseudopotentials of Goedecker−Teter− Hutter (GTH)46−48 were adopted for all atoms. Energy cutofff was set as 280 Ry. Grimme long-range dispersion correction was applied to evaluate the weak interactions.49 The unit cell was set as 45 Å × 45 Å × 45 Å for monomers and 65 Å × 55 Å × 65 Å for dimers, which ensured 10 Å vacuum for each directions to avoid neighboring interactions. The SCF Received: November 19, 2012 Revised: April 7, 2013 Published: April 8, 2013 8983

dx.doi.org/10.1021/jp311415d | J. Phys. Chem. C 2013, 117, 8983−8988

The Journal of Physical Chemistry C

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convergence was set as 10−7 Hartree. The convergence for the RMS step and RMS gradient was 0.0015 and 0.0003 in the geometrical optimizations, respectively. The basis-set superposition error (BSSE) was corrected using the counterpoise method. The calculations were carried out using the CP2K software package.50 In order to examine the reliability of PBE with Grimme dispersion correction (PBE-D) in dealing with π−π interactions, the binding energy of the benzene dimer (face to face) was investigated. The PBE functional without Grimme dispersion correction and CCSD(T) results were used for comparison. The configuration of benzene was optimized using PBE method. The CCSD(T) calculations were performed with Gaussian 09 software,49 and PBE-D and PBE calculations were carried out using CP2K software. All binding energies were BSSE corrected. As shown in Figure 1, the PBE-D and

Figure 2. Graphic anatomy of the Au102 structure (based on the crystal structure from ref 43). The gold atoms attached with the exchanged thiolate groups are highlighted as red. Other gold atoms in the innercore, outer-shell, and staples of the nanoclusters are depicted in yellow, coral, and blue. The sulfur atoms are green. All ligands are omitted for clarity.

clearly in Figure 2) and four p-MBA ligands replaced by p-BBT in Heinecke’s experiment,40 the 4 out of 44 exchange sites could be reduced to 2 out of 22. In order to explore the relative stabilities of the ligand substitution in each adsorption site, we examined the reaction energy of monosubstitution based on the equation below:

Figure 1. Binding energies (kcal/mol) of the benzene dimer with CCSD(T), PBE (without Grimme correction), and PBE (with Grimme correction) methods. The geometry is optimized using the PBE/6-31+G(d,p) method.

Au102(p‐MBA)44 + Br−C6H4−SH

CCSD(T) results gives a similar trend. When the distance between benzene dimers is ∼4.0 Å, it corresponds to the largest binding energies, although PBE-D method slightly overestimates the binding energies with respect to CCSD(T) method. The PBE-D results give a shorter distance (4.0 Å) with a slightly larger binding energy of −1.59 kcal/mol, whereas the largest binding energy of CCSD(T) results (−0.57 kcal/mol) corresponds to the π−π distance of 4.3 Å. We also notice the binding energy curve is quite flat when the π−π distance is more than 4.0 Å, indicating a geometrical flexibility of the benzene dimer. The binding energies by PBE (without Grimme correction) decrease monotonously with an increase in the distance, which is completely different from the results of CCSD(T) and PBE-D methods. Thus, it is reasonable to use the PBE-D method to evaluate π−π binding energies of weak interaction systems.

→ Au102(p‐MBA)43 (p‐BBT) + HOOC−C6H4−SH (1)

The reaction energies are evaluated as ΔE = E[Au102(p‐MBA)43 (p‐BBT)]+E[HOOC−C6 H4−SH]−E[Au102(p‐MBA)44 ]−E[Br−C6H4−SH]

In this study, nine distinct sites with monosubstitution were considered, eight locating between the outer-shell and secondorder staple and one on the third-order staple, as shown in Figure 3. Among these, sites 1 and 2 correspond to the exchange sites in the Au102(p-MBA)40 (p-BBT) 4 crystal structure.43 After full geometrical optimization, the reaction energies of eq 1 were listed in Table 1 for the monosubsitution of different sites. Their corresponding structures could be found in Figures S1−S9. The substitution energy of site 2 is −5.68 kcal/mol, notably lower than all other sites, which indicates this site is most energy favorable for the p-BBT exchange reaction. This is quite similar to the theoretical results by Jung et al.42 Similarly, Heinecke et al. proposed that this active site is the most solvent accessible, which makes it more exchangeable.43 However, it is surprising to find out that the pBBT substitution of site 1 is highest in energy (−0.50 kcal/ mol) among all subsitutional sites, which implies site 1 is the least favorable site of p-BBT substitution for Au102(p-MBA)44. Then there arises a very interesting question: Why is the least

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RESULTS AND DISCUSSIONS In previous literature, resolving the Au102 structure according to the crystal structure of Au102(p-MBA)44 has been widely discussed.6,9,16,31,32,51 The most common way is to anatomize the crystal structure into the Au79 core with D5h symmetry and a Au23(p-MBA)44 monolayer, which has been proposed by Kornberg, Whetten, and other scientists.6,9,31 As shown in Figure 2, the Au79 core could be further split into Au39 in the inner-core (in yellow) and Au40 in the outer-shell (in coral). Considering there is C2 symmetry for Au102 (we could find it 8984



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Figure 3. Nine substitution sites for Au102(p-MBA)44; ligands are omitted for simplicity. Sites 1 and 2 correspond to the crystal structure partially exchanged by p-BBT. Green: S; yellow: Au.

favorable substitutional site replaced in the experimental exchange reaction? It should be noted that the above theoretical analysis is based on the ligand exchange on Au102(p-MBA)44 monomer. Thus, we need to consider the further reaction relating to the crystal formation:

Figure 4. Crystal structure of the Au102(p-MBA)44 dimer with 4 p-BBT substitutions for each monomer (ref 43). (a) The ligand at site 1 and corresponding ligand is displayed using a ball and stick model. (b) Enlarged picture of the structure in the red rectangle of (a) red, O; gray: C; white: C; green: S; gold: Au; pink: Br (in a) or R (in b).

n Au102(p‐MBA)44 − x (p‐BBT)x → [Au102(p‐MBA)44 − x (p‐BBT)x ]n

energies is −N(CH3)2 > −NH2 > −Br > −I ≈ −OH > −CH3 ≈ −Cl ≈ −OCH3 > −CHO > −CN > −COOH > −CF3 > −H ≈ −F. Meanwhile, the perpendicular distance between the phenol groups for the −Br ligand is 3.43 Å, very close to the value of the crystal structure (3.40 Å), which means this simple model would be reliable for estimating the π−π interactions for large systems. Furthermore, the ranking order of the perpendicular distance between the phenol groups is −N(CH3)2 < −H < −Cl < −OH < −NH2 < −Br < −I ≈ −F < −OCH3 < −CH3 < −CHO < −CN < −COOH < −CF3. As shown in Figure 5, it could be concluded the smaller distances correspond to the stronger binding energies with exception of −H and −F. Next, we evaluate the reaction energies for the monosubstitution of Au102(p-MBA)44 dimer with PBE-D method, as shown below:

(2)

Here, [Au102(p-MBA)44‑x(p-BBT)x]n represents the aggregation of Au102(p-MBA)44‑x(p-BBT)x into the crystal. For simplicity, a dimer model (n = 2) was applied to study the interactions in the initial step of the cluster aggregations in this paper, as shown in Figure 4a. The original geometry comes from the crystal structure of ref 43. The ligand at site 1 is displayed with a ball and stick model in Figure 4a and enlarged in Figure 4b. According to the crystal structure (Figure 4b), the perpendicular distance between the phenol groups of the left (site 1) and right clusters is 3.40 Å, in the range of π−π stacking.52−56 For evaluating this weak interaction, a very simple model is employed. We fully optimized the ball-andstick structure shown in Figure 4b, where the −SC6H4R group was protected by H atoms. This structure is very similar to the orientation 4 in Hunter’s paper.52 There are 14 substitutional groups considered, including −COOH and −Br. The fully optimized structures could be found in Figure S10. The calculated binding energies and the perpendicular distance between phenol groups are collected in Table 2 using the PBED functional with BSSE corrections. All binding energies are negative, indicating the favorable binding between two phenol groups for all substitutions. The ranking order of the binding

Au102(p‐MBA)44 − Au102(p‐MBA)44 + R−C6H4−SH →Au102(p‐MBA)44 − Au102(p‐MBA)43 (R−C6H4−S) + HOOC−C6H4−SH

(3)

Note that only one p-MBA was exchanged in eq 3, where the site was enlarged in Figure 4b. All other p-MBA ligands are not

Table 1. Calculated Reaction Energies (kcal/mol) for the Mono-Substitution Exchange Reaction from Au102(p-MBA)44 to Au102(p-MBA)43(p-BBT) (Reaction 1) with PBE-D Functional ΔE

iso1

iso2

iso3

iso4

iso5

iso6

iso7

iso8

iso9

−0.50

−5.68

−3.44

−5.47

−3.62

−4.21

−5.26

−4.88

−3.80

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Table 2. Calculated Binding Energies (kcal/mol) of π−π Interaction (Ball-and-Stick Model Shown in Figure 4b and Fully Optimized Structures in Figure S10) using PBE-D Method with BSSE Correctionsa −N(CH3)2

a

PBE-D D (Å)

−7.47 3.36 −OCH3

PBE-D D (Å)

−5.03 3.51

−NH2

−Br

−6.46 3.42 −CHO

−5.47 3.43 −CN

−4.62 3.57

−4.51 3.59

−OH

−CH3

−Cl

−5.33 3.46 −COOH

−I

−5.30 3.41 −CF3

−5.08 3.52 −H

−5.05 3.40 −F

−4.35 3.65

−4.02 3.87

−3.74 3.38

−3.72 3.46

The perpendicular distance between phenol groups are presented as D (Å).

This could be confirmed by the crystal structure of the p-BBT substitution in this exchange site.43 The larger the reaction energy, the more stable the dimer structure. It is quite clear that −N(CH3)2 has the highest binding energy, followed by −NH2. Both are more favorable for ligand-exchange than −Br. Then they are followed by −OCH3, −OH, −I, −Cl, and −CH3. All of these ligand-exchange reactions are exothermic. The total ranking order of the reaction energies is −N(CH3)2 > −NH2 > −Br ≈ −OCH3 > −OH > −I > −Cl > −CH3. This ranking order is also consistent with that of the simple model shown in Table 2, which implies the binding preferences of the substitutions could be evaluated with the simple model. Above we have discussed the monosubstituent effects for sites 1 and 2, respectively. Now we consider more complex situations using the dimer model: (1) For Au102(p-MBA)43(SR)-Au102(p-MBA)43(SR) (each monomer has one substitution in site 1 and none of 2 is substituted), the distance between the SR ligands of each monomer is 11.22 Å (Figure S20) that we could easily neglect the steric effects between the substituent ligands even for the bulky −N(CH3)2 ligand. Thus, the ranking order should be same as the monosubsitution: −N(CH3)2 > −NH2 > −Br ≈ −OCH3 > −OH > −I > −Cl > −CH3. (2) For Au102(p-MBA)42(SR)2-Au102(p-MBA)44 (sites 1 and 2 are substituted for one monomer, and another monomer is not substituted), the steric effects between the ligand at site 2 and its neighboring ligands might be considered. As shown in Figure S21, the minimum distance between R and neighboring ligands is 3.45 Å, which is large enough to neglect the steric effects for small ligands (like −NH2, −-Br, −OH, −CH3, −Cl, −H, and −I). However, for large ligands like −N(CH3)2 and −OCH3, the steric effects could destabilize the crystal at site 2. Thus, we could expect the ranking order is −NH2 > −Br > −OH > −I > −Cl > −CH3. (3) For Au102(p-MBA)42(SR)2-Au102(p-MBA)42(SR)2 (both sites 1 and 2 are substituted for each monomer), it could be treated as the combination of the above situations. Considering −Br ligand’s intrinsic stability at site 2 and favorable π−π binding at site 1, it could easily explain Heinecke’s experimental observation.43 Finally, it is worth noting that the crystallization is a slow step compared to the ligand-exchange reaction,43 which means the final crystal structure is mainly determined by the

Figure 5. Correlation between the perpendicular distance (Å) of the phenol groups and their binding energies (kcal/mol) with the simple dimer model in Table 2.

exchanged in this study. The reaction energies of eq 3 are evaluated as ΔE = E[Au102(p‐MBA)44 − Au102(p‐MBA)43 (R−C6 H4−S)]+E[HOOC−C6H4−SH] −E[Au102(p‐MBA)44 − Au102(p‐MBA)44 ] −E[R−C6H4−SH]

Eight substitution groups with relative strong binding energies are considered. All structures are fully optimized and given in the Supporting Information (S11−S19) and the binding energies are listed in Table 3. It should be noted that these are very large molecular structures containing over 1500 atoms each, which took us 30 days (or more) to optimize one structure using 64 CPUs. As shown in the table, the perpendicular distance between two phenol groups for −Br monosubstitution is 3.45 Å. Comparing the distance of 3.40 Å of the crystal structure shown in Figure 4b, our calculations would be reliable. The reaction energy for −Br monosubstitution gives −1.18 kcal/mol, which indicates p-BBT substitution is more favorable than the original p-MBA ligand.

Table 3. Calculated Reaction Energies (kcal/mol) for the Mono-Substitution Exchange Reaction (R−C6H4−SH) in thte Au102(p-MBA)44 Dimer (Reaction 3) with the PBE-D Functionala

a

−R

−Br

−NH2

−N(CH3)2

−OH

−Cl

−I

−OCH3

−CH3

ΔE D (Å)

−1.18 3.45

−3.26 3.39

−4.73 3.37

−0.81 3.44

−0.35 3.44

−0.60 3.48

−1.10 3.42

−0.13 3.39

The perpendicular distance between phenol groups are presented as D (Å). 8986



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crystallization step rather than the ligand-exchange reaction. Thus, the enhanced intermolecular π−π interaction with the mono p-BBT substitution at site 1 would be important for the cluster aggregation (nucleation), although it is not the favored exchange sites in the monomer (shown in Figure 3 and Table 1). In addition, considering the large substitution energies of −N(CH3)2 and −NH2 compared to those for the −Br ligand, we suggest that the ligand-exchange rections with p(dimethylamino) benzenethiol (N(CH3)2-C6H4−SH) and pamino benzenethiol (NH2−C6 H4−SH) might be more favorable exchange ligands than p-BBT to enhance the crystallization based on our calculations.

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CONCLUSION In summary, we systematically studied the ligand effects for Au102(p-MBA)44 using density functional theory. It was found that the PBE functional with long-range dispersion corrections method could give reasonable results to evaluate weak interactions. Based on our calculations, the increased intrinsic stability from the substituent monomer is not the only factor in the partial exchange reactions. The intermolecular π−π interaction plays an important role for the stability of the partial ligand-exchange crystal structure Au102(p-MBA)40(pBBT)4. Detailed investigation of different thiolate substitutions suggested p-(dimethylamino) benzenethiol (N(CH3)2-C6H4− SH) and p-amino benzenethiol (NH2−C6H4−SH) could potentially enhance the aggregation of Au102 nanoparticles rather than p-BBT, which provides viable theoretical design waiting for experimental confirmation.

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ASSOCIATED CONTENT

S Supporting Information *

All geometrical coordinates and energies of nine isomers for Au102(p-MBA)43(p-BBT), fully optimized simple dimer model structures, original and eight monosubstituted Au102(p-MBA)44 dimers, and enlarged structures of ligand exchange sites. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The computational resources utilized in this research were provided by Shanghai Supercomputer Center, Supercomputing Center of Chinese Academy of Sciences in Beijing, and National Supercomputing Center Shenzhen. This work is supported by the startup funding from Shanghai Institute of Applied Physics, Chinese Academy of Science (Y290011011), National Natural Science Foundation of China (21273268), and “Hundred People Project” from the Chinese Academy of Sciences.

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REFERENCES

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Ligand Effects of Thiolate-Protected Au102 Nanoclusters

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