Ligand Effects on the Structure and the Electronic Optical Properties

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Ligand Effects on the Structure and the Electronic Optical Properties of Anionic Au25(SR)18 Clusters Alfredo Tlahuice-Flores,* Robert L. Whetten, and Miguel Jose-Yacaman Department of Physics and Astronomy, University of Texas, San Antonio, Texas 78249, United States S Supporting Information *

ABSTRACT: This study addresses how ligands module the structure and the electronic optical properties of a large set of the experimentally known anionic thiolate-protected gold clusters, Au25(SR)18[1−]. Starting from the experimental crystal structure, computational density functional theory calculations reveal that low-polarity R groups do not disturb the Au25S18 framework significantly, such that the inversion symmetry(Ci) of the crystalline state is retained. In the case of p-thiolphenolate ligands, p-SPhX, a major distortion of the Au25S18 framework, destroys the inversion symmetry, the distortion increasing in the order given X = H, Cl, NO2 and CO2H. For branched R groups, linking −CH3 or −NH2 groups at the twoposition of the phenylethylthiolate ligand, the inversion symmetry is retained and lost, respectively; similarly, the N-acetyl-cysteine ligand also distorts the framework. These results demonstrate a systematic preference of inversion-symmetric versus nonsymmetric framework depending on the ligand-type. The more distorted structures also exhibit significantly reduced HOMO−LUMO gap values and affect the optical absorption spectra accordingly. This study correlates the distortion of the Au25S18 framework with the structure, electronic, and optical properties among the studied clusters.

1. INTRODUCTION The earliest synthesis of the smaller thiolate-protected gold clusters1 culminated in the total X-ray determination of the structure of the Au25(S(CH2)2Ph)18− cluster in 2008.2 Subsequent theoretical studies have focused on its electronic, optical, chiroptical, and vibrational properties using preferentially −SH or −SCH3 groups as model ligands to reduce the computational demands.3−7 However, Aikens in 2010 reported a study of SPhX-protected Au25 clusters.4b On the experimental side, there have been several reports regarding the influence of the ligands on the optical properties. Guo and Murray had discerned in 2005 the ligand effects by studying the redox potential and optical gaps of a series of “Au38 clusters”, later determined to be Au25(SR)18−. Their results showed that ligand effects play a crucial role on the electronic levels.8 Negishi et al. in 2007 employed the aliphatic −SC6H13 ligand and discussed its effect in the absorption spectra of the Au25 cluster.9 Shibu and coworkers in 2008 reported the study of ligand exchange of glutathione-protected Au25 clusters, and they determined that the optical absorption band centered at 1.84 eV is characteristic of this size, while slight differences were observed on the bands located at 2.79, 3.92, and 4.53 eV, which can be explained by considering that those electronic transitions held more ligand character.10 It is remarkable that the improvement in synthesis and analytical methods has allowed a better knowledge of properties of the Au25 cluster.11−15 Regarding the influence of the ligands in the optical properties of the thiolated Au25 cluster, there exist a few reports based on DFT calculations, which have determined the principal effects on the energy levels and optical spectra.16 The © 2013 American Chemical Society

study of the chlorine-substituted clusters, by Parker et al. in 2010, reported that significant effects can be observed in the orbitals, charge distribution, and the detachment energies.17 More recently, Jung et al. have explored ligand effects on the stability of Au25, Au38, and Au102 clusters, contrasting aliphatic and aromatic thiols as ligands.18 In addition, that study has established the special stability of the Au25 cluster protected with R = (CH2)2Ph. In the crystalline phase of the thiolated-protected Au25 cluster, the various Au, S, C, and H atom sites are related by an inversion center, reflected by the Ci point group.2,17 Taking into account the fact that the experimental structure holds R= (CH2)2Ph groups, one may logically conclude that symmetric ligands yield Ci structures for the entire cluster structure. A recent report of the substitution on the two-position of the phenylethylthiolate ligand has shown a greatly enhanced circular dichroism (CD) signal by using amino rather than methyl group.19 Moreover, it is well known that Au25 clusters display a weak CD signal with chiral ligands such as glutathionate-protected Au25 clusters. Therefore, it should be interesting to determine how the various ligands generate structures lying further away from the Ci symmetry. The aim is to examine systematically the distortion induced by the ligands to the Au25S18 framework to determine how the electronic and main features of the optical curves are affected. To accomplish this task, one requires parameters capable of Received: July 18, 2013 Revised: September 8, 2013 Published: September 11, 2013 20867

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Zunger exchange-correlation functional were selected.26 The relaxed structures by using SIESTA code were used as input in the octopus package, where a ground-state calculation is performed initially to derive the electron density; then, the system is perturbed by a short electric pulse. The perturbation is turned off, and the subsequent temporal evolution of the electric dipole moment of the system is extracted. The absorption spectra are calculated from this autocorrelation function, that is, as the imaginary part of the system’s frequency-dependent dynamical polarizability. A spacing of 0.2 Å was considered in this real-space method. The simulation box is forming by adding spheres of 3.5 Å around each atom. The time-dependent equations were integrated considering a time step of 0.002 ℏ/eV, that is, 1.32 × 10−6 ps.

measuring the degree of distortion. It is proposed that the distribution of bond distances and the statistical deviation (tolerance) from the assignment of the point group are sufficient to quantify the induced distortion by the ligands and to relate this distortion to the optical electronic spectrum.

2. THEORETICAL METHOD The calculations were performed within the framework of density functional theory (DFT) using the generalized gradient approximation (GGA) and the Perdew−Burke−Ernzehorf (PBE) parametrization for the electron−electron exchange and correlation interactions.20 The valence electrons were represented by a double-ζ polarized basis set, while a Troullier Martins scalar relativistic norm-conserving pseudopotentials for the interaction ion-core electrons was used.21 The numerical basis sets represent 5d106s1, 3s23p4, 2s2p4, 2s2sp3, 2s22p2, and s1 valence electrons for Au, S, O, N, C, and H atoms, respectively. In the case of Au atom, it was represented by a scalar relativistic corrected pseudopotential. A real-space integration mesh (implemented in the SIESTA code)22 was employed, and this is equivalent to a plane-wave representation cutoff at 200 Ry. This selection allows a balance between accuracy and computational time. All structural optimizations were performed without constraints using a force tolerance criterion of 0.01 eV/Å in the determination of the energy minimum. Initial structures of the thiolate-protected Au25 clusters, as considered herein, were built as structures where all atoms hold either a Ci or C1 point group. In most cases, the construction of the thiolated Au25 clusters with Ci symmetry employed a previously relaxed structure, derived of the crystalline phase,2a as a parent configuration. In Table 1 is listed the full set of

3. RESULTS AND DISCUSSION A set of Au25 clusters protected with various ligands is discussed regarding their structural and electronic properties. The optical properties are discussed in a separate section; finally, the obtained trends are summarized. 3.1. Au25 Clusters in the Experimental Crystal Structure and As Isolated Anions. Prior to the study of the effect of the ligands, it is mandatory to analyze the structure determined for the crystalline phase. The [Oct4N+][Au25(S(CH2)2Ph)18−] crystal reported by Murray et al.2a shows an Au13 cluster covered by six S−Au−S−Au−S motifs (dimer motifs), which constituted its main Au25S18 framework, where R -groups are attached to the 18 S atoms. In the dimer motifs the Au(dimer)−S bonds are ca. 2.32 Å, while the Au(core)−S bonds are slightly longer (2.38 Å). For a more detailed analysis of the bonding in the experimental structure of the thiolated Au25 cluster, see Figure S1 in the Supporting Information. Starting from the experimental coordinates, optimization yields a structure of the Au25(S(CH2)2Ph)18− cluster of 0.33 eV greater stability associated mainly with the reorientated phenyl rings, but which retains a Ci point group of the entire structure. Subsequent reorientation of the ligands generates a locally stable nonsymmetric C1 structure that is 0.67 eV less stable than the Ci structure used through this report. This result supports the idea that more stable structures are obtained when the phenyl rings are mutually orientated in a more compact, ordered arrangement. 3.2. Au25 Clusters Protected by Other Ligands Holding Low Polarity Substituents. The thiolate ligands with low polarity substituents considered herein are −SH, −SCH3, and −SC6H13 as well as the −S(CH2)2Ph of the crystal structure described above (see Section 3.1), as shown in Figure 1. The substituents are formed from hydrocarbon chains that have small electronegativity differences; therefore, they present very small (local) dipole moments. The relaxed thiolated Au25 clusters show that Au−S bonds (Figure 2a,b) suffer little variation among the four studied ligands, while a major shift in the Au(core)−Au(dimer) bonds was obtained (Figure 2e). If the Au−S and Au(core)−Au(core) bonds are similar, then the different ligands might induce the puckering (deviation from the planarity) of the Au25S18 framework in a different manner. This effect is easily observed by orienting one structure in such a manner that one pair of S− Au−S−Au−S planes (dimer motifs framework) lies horizontally. In the case of the −SC6H13 ligand, the Au25S18 framework is slightly more distorted (tolerance of 0.036 Å), and the distortion might be centered at the neighborhood of the

Table 1. Tolerance Necessary to Yield a Ci Point Group of the Au25 Clusters and Their Calculated HL Gap Values ligand −SH −SCH3 −S(CH2)2Ph −SC6H13 −SCH2C*H(CH3)Ph −S(NAC) −SPh −SCH2C*H(NH2)Ph −SPhCl −SPhNO2 −SPhCO2H

tolerance (Å)

HL gap (eV)

0.001a 0.001a 0.0021a 0.47a 0.036 0.04a 0.16 0.24 0.31 0.35 0.39 0.44

1.36 1.32 1.23 1.25 1.22 1.20 1.16 1.21 1.17 1.06 1.04

a Tolerance to get a Ci point group of the entire structure; the other values refer to 43-atom inorganic framework only.

ligands used in this report, and the comparison between crystalline and relaxed structure is provided in Figure S1 in the Supporting Information. It is noteworthy that the Ci symmetry is held by the crystalline Au25(S(CH2)2)18− clusters independently of their charge state (neutral vs anionic). Table S1 in the Supporting Information contains a complete analysis of the point group of the crystalline structures. For the calculation of the optical spectra, a real-time method within time-dependent DFT (TD-DFT) formalism,23 as implemented in “octopus” code, is used.24,25 To calculate singlet excitations in the 1.0−5.0 eV region, the time-dependent local-density approximation (TD-LDA) and the Perdew− 20868

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Figure 1. Relaxed structures of anionic Au25 clusters with ligands including low-polarity substituents. (1a) Au25(SH)18−, (1b) Au25(SCH3)18−, (1c) Au25(SCH2CH2Ph)18−, and (1d) Au25(SC6H13)18−. Au, S, C, and H atoms are in yellow, orange, gray, and white, respectively. 1a−1c are entire Ci structures, while 1d exhibits a loose Ci structure.

Au25(SH)18− and Au25(SCH3)18− clusters have the large calculated HL gap values, 1.36 and 1.32 eV, respectively. The Au25 clusters with ligands constituted by R = (CH2)2Ph and C6H13 groups present the smaller HL gap values (1.23 and 1.25 eV, respectively). It is found that the Au25 cluster with −SH ligands displays Kohn−Sham (K−S) levels shifted toward more negative energies (Figure 4a). Nevertheless, the Au25 cluster

Figure 2. Calculated bond lengths of four anionic Au25 clusters that hold a Ci point group. The distances in the Au25(SH)18− , Au25(SCH3)18−, Au25(SCH2CH2Ph)18−, and Au25(SC6H13)18− clusters are colored as green, blue, red, and black, respectively. (a) Au(dimer)− S bonds, (b) Au(core)−S bonds, (c) Au(core)−Au(core) bonds of the Au12 shell mixed with Au(center)−Au(core) bonds, (d) Au(core)− Au(core) bonds on the Au12 shell, and (e) Au(core)−Au(dimer) bonds. Vertical line separates approximately the Au−Au bonds forming the Au13 core from the aurophilic bonding.

Figure 4. Energy levels diagram (Kohn−Sham orbitals) of Au25 clusters with various ligands. (a) Au25(SH)18−, (b) Au25(SCH3)18−, (c) Au25(S(CH2)2Ph)18−, and (d) Au25(SC6H13)18−. Three-fold degenerate HOMO level is shown in red, while two-fold degenerate LUMO is shown is blue color.

Au(core)−Au(dimer) bonds. See Figure 3 for illustration of the puckering of the Au25S18 framework. The frontier orbitals of the thiolated Au25 cluster are threefold (HOMO) and two-fold degenerate (LUMO),2b,3a with a HL gap value reported in the range 1.20 to 1.36 eV.5,14,27−29 Consistent with the experimental values, in this report, the

with −S(CH2)2Ph ligands exhibits more negative K−S levels than −SC6H13 ligands, although their HL gap values are similar. For a general view of the frontier levels of the Au25 cluster protected with the 11 distinct ligands, see Figure S2 in the Supporting Information. Table 1 shows the dependence of HL-gap values with the polarity of the substituents. The decrement in the HL-gaps can be correlated with the major distortion of the Au25S18 framework found in the Au25 cluster protected with −SC6H13 ligand (Figure 3). Further discussion on the mentioned dependence will be extended in the next sections. 3.3. Au25 Clusters Protected with p-Thiophenolates. The Au25 clusters protected with p-thiophenolates (R = PhX) are formed by X substituents linked to the phenyl ring in para position, where X = H, Cl, COOH, and NO2 groups. The NO2 substituent is a strong electron-withdrawing group, and its presence has an inductive effect on the polarization of the C−N bond in such a manner that this ligand can be considered as

Figure 3. Puckering of the Au25S18 framework is depicted. (3a) Au25(SH)18− and (3b) Au25(SC6H13)18− clusters. The major puckering of the horizontal dimer motifs in the Au25 cluster protected with the aliphatic ligand is remarkable. 20869

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Figure 5. Relaxed structures of Au25 clusters protected with p-thiophenolates, which include electron-withdrawing sustituents. (5a) Au25(SPh)18−, (5b) Au25(SPhCl)18−, (5c) Au25(SPhNO2)18−, and 5d) Au25(SPhCO2H)18−. Au, S, Cl, O, N, C, and H atoms are in yellow, orange, green, red, blue, gray, and white, respectively. 5a−5d are entire C1 structures.

more polar than the four ligands discussed in the previous section. After the relaxation of initial nonsymmetric C1 and Ci structures, it was found that the former structures are preferred (Figure 5). The SPh-protected Au25 cluster favored a C1 structure over the Ci symmetry form by 0.2 eV, and this preference increases to 0.43 and 0.94 eV when considering −SPhCl and −SPhNO2 as ligands, respectively. This result supports the idea that ligands with more electron-withdrawing groups induce more distortion, resulting in less symmetric structures. Further analysis of the Au−S and Au−Au bond lengths reveals that indeed the optimal structures have distorted Au12 cores, and it is not possible to distinguish between characteristic Au(center)−Au(core) and Au(core)−Au(core) bonds (Figure 6c,d). In addition, Au25 clusters with R = Ph,

Figure 7. Two different Au25S18 frameworks are depicted. (7a) Au25(SPh)18− and (7b) Au25(SPhCO2H)18− clusters. Vertical dimer motifs on 7b show tilting and rotation more clearly.

The influence on the HL-gap values of the p-substituted phenyl rings is not significant, as they range between 1.04 and 1.17 eV. This finding is in agreement with a recent study by Aikens,4b who studied S6 structures. We have estimated the energy difference between the SPh-protected Au25 cluster with all of its atoms holding an S6 symmetry and our no-symmetric isomer as 0.44 eV. Further analysis of the K−S energy levels shows that the degeneracy of the HOMO and LUMO levels is maintained, but in the case of −SPh and SPhCl ligands, the levels below the HOMO level are three-fold degenerate, in contrast with the two-fold degeneracy displayed by Au25 clusters protected with SPhNO2 and −SPhCO2H ligands (Figure 8). Another interesting finding is that the more negative levels are displayed by the Au25 cluster protected with −SPhNO2 ligand (Figure 8c).

Figure 6. Calculated bond lengths of four anionic Au25 clusters protected with p-thiophenolates. The distances in the Au25(SPh)18−, Au25(SPhCl)18−, Au25(SPhNO2)18−, and Au25(SPhCO2H)18− clusters are colored as green, blue, red, and black, respectively. Labels a−e represent bonds described in Figure 2. The major distortion displayed is in the Au(core)−Au(core) distances (c,d). The similarity of Au(core)−Au(dimer) bonds is opposed to the trend obtained for lowpolarity ligands.

PhCl, and PhNO2 ligands share a similar bonding pattern. However, the Au25 cluster protected with −SPhCO2H (Figure 7b) shows a major puckering, and its dimer motifs exhibit tilting and rotation. The obtained major distortion of the Au25S18 framework is in agreement with a study by the Dass group, who use flexible aliphatic ligands for testing adsorption sites on the Au25 cluster and estimating the number of ligands that incorporate after ligand exchange reaction. The mentioned study has found that the electronic and optical features of the Au25 cluster were preserved after ligand exchange reaction with aliphatic ligands, which is opposed to the results obtained with ligands including phenyl rings.28

Figure 8. Energy levels diagram (Kohn−Sham orbitals) of Au25 clusters with various ligands. (a) Au25(SPh)18−, (b) Au25(SPhCl)18−, (c) Au25(SPhNO2)18−, and (d) Au25(SPhCO2H)18−. Three-fold degenerated HOMO level is shown in red, while two-fold degenerate LUMO is shown is blue color. 20870

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Figure 9. Relaxed structures of anionic Au25 clusters protected with chiral ligands. (9a) Au25(SCH2C*H(CH3)Ph)18−, (9b) Au25(SCH2C*H(NH2)Ph)18−, and (9c) Au25(S-NAC)18−. Au, S, O, N, C, and H atoms are in yellow, orange, red, blue, gray, and white, respectively. 9a holds an entire Ci symmetry, while 9b and 9c are C1 structures.

The four studied ligands include electron-withdrawing groups, which are symmetric, in such a manner that any chiroptical signal cannot be attributed to themselves, but it might arise from their chiral arrangement. However, this study has shown an induced distortion in the Au25S18 framework after linking of ligands that include electron-withdrawing groups. Thus, our geometric analysis might correlate to an enhanced CD signal with the entire structure of the studied clusters. 3.4. Au25 Clusters with Chiral −SCH2C*H(X)Ph or NAC Ligands. Recently, the synthesis of Au25 clusters protected with amino19 and methyl30 groups, which are attached to the two-position of the phenylethylthiolate ligand, respectively, has been reported. That substitution yielded carbon atoms in the two-position as chiral centers. Herein are considered the −SCH2C*H(X)Ph ligands, where phenylethylthiolate ligand has been modified by X = NH2 and CH3 groups. From a geometric viewpoint, the arrangement and configuration of the chiral carbon atoms might determine the entire symmetry of the Au25 clusters. The first scenario to be considered is an Au25S18 framework, which maintains the same orientation, and it is linked by ligands with a number of carbon atoms with S configuration that equals those with R configurations (i.e., a racemic mixture), then the Ci point group will be preserved. The second scenario considers that the number of carbon atoms with R or S configurations are different, with an inversion symmetry destroyed (C 1 symmetry). A third scenario is when all carbon atoms are in an S or R configuration, also resulting in an absence of symmetry (C1). It was found that the −SCH2C*H(CH3)Ph cluster (Figure 9a) prefers a Ci (scenario 1) rather than a C1 point group (scenario 3 and R configuration) by 0.36 eV. This finding is interesting because it was not expected that chiral ligands might yield one structure where all of its atoms held a Ci symmetry. In contrast, the Au25 cluster protected with −SCH2C*H(NH2)Ph ligands (Figure 9b) does exhibit a nonsymmetric C1 preference over the Ci symmetry by 0.39 eV. These findings are in agreement with a recent study of Zhu et al., who have reported a CD signal enhanced by substitution with amino groups.19 Further analysis of the relaxed clusters with these CH3 or NH2 side groups shows that the Au25 clusters displayed a different Au25S18 framework (Figure 10), yet their HL-gap values are closely related with no appreciable difference. Moreover, the Au25S18 framework of the Au25 cluster covered

Figure 10. Calculated bond lengths of two anionic Au25 clusters that hold a chiral carbon atom. The distances in the Au25(SCH2C*H(CH3)Ph)18−, Au25(SCH2C*H(NH2)Ph)18− clusters are colored as black, and blue, respectively. Labels a−e are described in Figure 2. Note that Au(dimer)−S bonds show more structure in the Au25(SCH2C*H(CH3)Ph)18− cluster.

by −SCH2C*H(NH2)Ph ligands (Figure 9b) is very close to that obtained with aliphatic −SC6H13 ligands (Figure 3b and Figure S3 in the Supporting Information). This coincidence implies that even when the Au25 cluster covered by SCH2C*H(NH2)Ph ligands exhibits a total C1 symmetry, its Au25S18 framework retains a quasi Ci symmetry. Conversely, the bonding in the Au25 cluster protected with −SCH2C*H(CH3)Ph ligands is similar to the bonding displayed by a total structure with a Ci point group (Figure S4 in the Supporting Information). Another interesting chiral ligand is the L-cysteine amino acid, which under experimental conditions exhibits its zwitterion form (HS−CH2−CH−NH3+−COO−). It represents the more simple structure that is built by amino (NH2), thiol (SH), and carboxyl (COOH) groups. However, calculations in the gas phase are not able to find the zwitterion form, and it is necessary to include solvent effects.31 Explored instead in this report is the modified N-acetyl-cysteine (NAC), which resembles more approximately the glutathionate ligand. The initial Ci structure for the Au25(S-NAC)18−, upon the relaxation evolved into a nonsymmetric C1 structure (Figure 9c). The rotation of the ligands setting the carboxyl O atoms close to the N−H atoms of vicinal ligands is identified as causing the symmetry lowering. However, the distortion induced by the NAC group influences Au−S and Au−Au bonds also. It is interesting to notice that the bond distribution of the Au25(S-NAC)18− cluster resembles that obtained by considering −SPhNO2 as ligands but differing into the Au(core)−Au(dimer) distances (Figure 11). This supports 20871

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Figure 11. Comparison of the calculated bond lengths of Au25(SNAC)18− and Au25(SPhNO2)18− cluster (black dots). a−e labels represent bonding described in Figure 2. The main difference is in the Au(core)−Au(dimer) distances, which are labeled with e.

the idea that when NAC is used as ligand, the cluster suffers more distortion in its Au25S18 framework. This result sustains a possible explanation to the observed experimental CD signal, which cannot be attributed merely to the chiral carbon atoms included in the ligands. The analysis of the K−S energy levels, of the three studied clusters in this section, shows that Au25 clusters protected with −SCH2C*H(CH3)Ph and −SCH2C*H(NH2)Ph have similar degeneracy of the HOMO and LUMO levels, and their position is almost maintained independently of the group linked to the two-position of the phenylethylthiolate ligand (Figure 12 and

Figure 12. Energy levels diagram (Kohn−Sham orbitals) of Au25 clusters with various ligands. (a) Au25(SCH2C*H(CH3)Ph)18−, (b) Au25(SNAC)18−, and (c) Au25(SCH2C*H(NH2)Ph)18−. Red color stands for three-fold degenerated HOMO level, while two-fold degenerate LUMO is shown is blue color.

Figure 13. Experimental low-temperature Au25(SC6H13)18− (a) and calculated optical spectra of (b) Au25(SCH3)18−, (c) Au25(S(CH2)2Ph)18−, (d) Au25(SPh)18−, (e) Au25(SCH2C*H(CH3)Ph)18−, and (f) Au25(S-NAC)18−.

The calculated optical spectrum of the Au25 cluster protected by R = CH3 group (Figure 13b) shows two bands centered at 1.46 and 2.18 eV, which might correlate with the splitting of the band centered at 1.81 eV. The other band located at 3.16 eV seems to be in agreement with the features reported before. The optical spectrum calculated by considering −S(CH2)2Ph as ligands (Figure 13c) shows an improved comparison with the experiment, and a significant peak located at 1.58 eV and the other two peaks located at 2.49 and 2.79 eV are found. Clearly those values seem to match better with the main features on the thiolated Au25 cluster. The optical spectrum using −SPh as ligand (Figure 13d) shows peaks located at 1.8 and 2.55 eV that correlate to peaks located at 1.9 and 2.57 eV of the experimental profile given by Ramakrishna et al. Previously, Aikens had reported the UV−vis spectrum of the thiolate-protected Au25 cluster by using −SPh ligand and considering an S6 point group structure. The profile shown in Figure 13d is in good agreement with the mentioned report, and the spectrum is depicted in the range of 1.0 to 2.7 eV in the Figure S5 in the Supporting Information. It was also found that the calculated optical absorption spectrum of the Au25 cluster protected with chiral −SCH3C*H-

Figure S2 in the Supporting Information for a more general view). The more characteristic difference is that their HOMO and LUMO levels are slightly shifted toward more negative energies with respect to the Au25 cluster protected with −S(CH2)2Ph ligand. In the case of the Au25 cluster protected with −SNAC ligand, it is evident that K−S levels below the HOMO level show a three-fold degeneracy in a similar manner as the clusters protected with −SPh ligands (Figure 8). 4. Optical Properties of Au25 Clusters Protected with Various Ligands. This section concerns the optical spectral profiles of the various Au25 clusters. The experimental optical profile of thiolated Au25 clusters shows three characteristic features located at 1.81, 2.75, and 3.1 eV.2b,4a In 2011, Ramakrishna et al.32 reported the measurement of the better resolved optical spectra of the Au25 cluster protected with −SC6H13 ligand as well as the Au38 cluster at low temperatures (Figure 13a) and determined that at T = 77 K two peaks located at 1.81 and 2.75 eV shift toward 1.90 and 2.87 eV, respectively. Moreover, the peak located at 1.81 eV can be considered as to be separated in two peaks located at 1.67 and 1.90 eV. 20872

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(CH3)Ph ligands shows a profile that is in better agreement with the reported spectrum by Zhu et al.2b Thus, displayed peaks in Figure 13e are located at 1.9, 2.7, and 3.1 eV being near to the found splitting of the peak located at 1.8 eV and the other peak located at 2.75 and 3.1 eV previously reported. Therefore, it seems that ligands including the phenyl ring improve the agreement among experimental and calculated optical spectrum of the Au25 clusters. Conversely, by considering −SNAC as ligand (Figure 13f), a distinct profile was obtained. It shows peaks located at 1.9, 2.81, 3.0, and 3.6 eV that are in according with the experimental values. One interesting feature of the calculated optical absorption profile is the presence of structured and welldefined peaks. This study has established the influence of the ligand on the structure of the Au25 clusters for ligands classified into three sets: low-polarity, electron-withdrawing, and chiral ligands. The following paragraphs summarize the obtained results. (a) The Au 25 (SR)18 − clusters protected with ligands constituted by low polarity substituents retain the Ci symmetry. Even after relaxation of Au25 clusters with R = H, CH3 or (CH2)2Ph ligands, they exhibit a Ci symmetry of the total structure. In contrast, the aliphatic −SC6H13 ligand exhibits a quasi-Ci symmetry. (b) Even when the low-polarity substituents decrease the HL gap of the thiolated Au25 cluster, this effect is more evident in the case of the electron-withdrawing substituents. The trend in the reduction of the HL gap values, can be correlated with a more distorted Au25S18 framework (mixed puckering, tilting, and rotation effects) after the substitution with electron-withdrawing groups. Consistently, the calculated HL gap values for this type of ligands are below 1.20 eV. (c) The Au25 clusters protected by chiral ligands show a point group that depends on the group substituting the two-position of the phenylethylthiolate ligand. It was found that amino group induces a major preference by the nonsymmetric C1 structures, which might explain the experimental results where one observes an enhanced CD signal with respect to that calculate for the methyl group. Moreover, the calculated HL gaps do not show a dependence on the group linked to the chiral carbon, but the distortion that induces into the Au25S18 framework is distinct for the Au25 cluster protected with −SCH2C*H(NH2)Ph (Figure 13h) and −SCH2C*H(CH3)Ph (Figure 13e) ligands, respectively. It was determined that ligands including the NAC group induce more distortion to the Au25S18 framework comparable to ligands including electron-withdrawing groups. Figure 14, displays the correlation between the Au25S18 framework distortion and the HL gap values of Au25 clusters protected with various studied ligands. Clearly, the general trend is that a large distortion of the Au25S18 framework results in reduced HL gap values.

Figure 14. Correlation between calculated HL gaps values and Au25S18 framework distortion of studied anionic Au25 clusters. Labels indicate various ligands: (a) −SH, (b) −SCH3, (c) −S(CH2)2Ph, (d) −SC6H13, (e) −SCH2C*H(CH3)Ph, (f) −S-NAC, (g) −SPh, (h) −SCH 2 C*H(NH 2 )Ph, (i) −SPhCl, (j) −SPhNO 2 , and (k) −SPhCO2H. Au25 clusters under horizontal blue line hold entire C1 symmetry. The distortion is given as the tolerance (Å) necessary to obtain a Ci point group.

We have found that the major distortion induced in the Au25S18 framework is by those p-thiophenolates ligands that include electron-withdrawing groups, and this behavior is reflected by a major preference by nonsymmetric C1 rather than Ci structures. Chiral ligands show a preference by C1 or Ci structures, which depends on the specific substituent located at the twoposition of the phenylethylthiolate ligand. It was found that the C1 structure yielded by using an amino group is reversed when methyl group was considered (Ci structure). Further analysis of the bonding of the Au25 cluster including the amino group reveals that its Au 2 5 S 1 8 framework resembles the Au25(SC6H13)18− cluster (Ci). Therefore, the C1 symmetry might be conferred by all 18 ligands, which exhibit a R configuration. This supports the idea that the CD signal observed experimentally might be attributed to the ligands mainly for this specific cluster.29 In contrast, NAC group induces more distortion to the Au25S18 framework, which is comparable to the effect of the ligand including electronwithdrawing groups. This major distortion is expected to explain the observed chiroptical response in the case of gluthathionate-protected clusters. Finally, the obtained results suggest that the CD signal of the Au25(SR)18− cluster might be due to the presence of stereogenic centers in the ligands, but also the induced distortion and the arrangement of the ligands need to be considered.

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ASSOCIATED CONTENT

S Supporting Information *

Bond distances of crystal structures including neutral and anionic Au25 clusters. Extended analysis of point group of crystalline structures and symmetry analysis of all Au25 clusters. K−S energy levels of all studied clusters. 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]. Tel: (210) 458 8173.

4. CONCLUSIONS Both inversion-symmetric Ci and nonsymmetric C1 structures of various Au25 clusters have been considered. This study has included a set of 11 thiolate-protected gold clusters spanning low-polarity, p-thiophenolates including electron-withdrawing groups and chiral ligands.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Ramakrishna for providing the UV-vis spectrum used for comparison along this report and to Prof. Aikens for 20873

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the Cartesian coodinates of the S6 symmetry of the SPhprotected Au25 cluster. A.T.-F. acknowledges Consejo Nacional de Ciencia y Tecnologiá (CONACyT) and the National Science Foundation (NSF) for support with grants DMR1103730, “Alloys at the Nanoscale: The Case of Nanoparticles Second Phase and PREM: NSF PREM Grant # DMR 0934218; “Oxide and Metal Nanoparticles- The Interface Between Life Sciences and Physical Sciences”. All calculations were done in the Texas Advance Computing Center and by using resources of the Computational Biology Initiative.

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