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Photodynamics of a Molecular Water-Soluble Nanocluster Identified as Au130(pMBA)50 Satu Mustalahti,† Pasi Myllyperkiö,† Tanja Lahtinen,† Sami Malola,‡ Kirsi Salorinne,† Tiia-Riikka Tero,† Jaakko Koivisto,† Hannu Hak̈ kinen,†,‡ and Mika Pettersson*,† †

Department of Chemistry and ‡Department of Physics, Nanoscience Center, P.O. Box 35, FI-40014, University of Jyväskylä, Finland

Downloaded by KTH ROYAL INST OF TECHNOLOGY on September 11, 2015 | http://pubs.acs.org Publication Date (Web): August 17, 2015 | doi: 10.1021/acs.jpcc.5b07672

S Supporting Information *

ABSTRACT: Photodynamics of a highly monodisperse sample of a water-soluble gold nanocluster tentatively identified as Au130(pMBA)50 (pMBA = p-mercaptobenzoic acid) was studied by mid-IR transient absorption spectroscopy with visible excitation. The observed long-lived excited states (>1 ns) indicate a molecular behavior of the cluster. By combining the transient absorption data with DFT calculation results the observed relaxation dynamics could be fully explained by identifying several relaxation processes involving singlet and triplet manifolds. The results indicate that the cluster may have interesting transient magnetic properties due to a long-lived triplet population.

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INTRODUCTION Monolayer-protected gold nanoclusters with atomically precise composition provide an ideal class of compounds to study the size dependence of cluster properties.1,2 Due to recent developments in nanocluster synthesis it is now possible to study highly monodisperse samples of various cluster sizes (see for example ref 3). One of the intriguing size-dependent features of metal-containing particles in nanoscale that can now be addressed is the transition from molecular systems to metallic systems. A key property when considering the molecular or metallic behavior is the electronic energy level structure and especially the existence or nonexistence of an energy gap or a HOMO−LUMO gap.4 The small molecular clusters have a distinct energy gap which closes for metallic clusters.1,2,4 Optical spectroscopy is well suited for the experimental studies of cluster energy level structures since absorption spectroscopy can be used to study the electronic transitions and to determine the onset for electronic absorption.5−7 However, in some cases a detectable optical gap is not identical to an energy gap due to very low oscillator strength for low-lying transitions. This has been shown for example for the Au144(SR)60 cluster.6,7 Time-resolved transient absorption experiments are especially well suited to reveal the existence of an energy gap and hence the molecular or metallic nature of clusters.4,8−15 This is due to the fact that the relaxation dynamics after electronic excitation is greatly affected by differences in electronic energy level structure of the cluster, and a drastic difference in relaxation dynamics is seen between molecular and metallic systems if the energy gap is larger than a typical phonon energy.8,11 Long-lived excited species observed in transient absorption spectra are therefore an exceptionally © 2015 American Chemical Society

clear indication of the molecular nature of the cluster as has been shown for Au25(SR)18 clusters and Au102(pMBA)44.8,12−15 Complementary information about the electronic energy structure of the clusters can also be obtained computationally by using DFT calculations.1,5 The transition region between molecular and metallic behavior has been significantly narrowed due to recent research efforts utilizing transient absorption experiments.8−11 A Au144 cluster has been identified as the smallest known metallic system,9−11 while the Au102 species has been identified as a molecular system, having an energy gap of 0.45 eV.5,8,11 The results of two different research groups agree well for these two clusters. To further study the transition, cluster core sizes between 102 and 144 gold atoms are of great interest. Recently, a Au133(SR)52 cluster was studied by using transient absorption experiments and determined to be a nonmetallic cluster.16 However, the observed relaxation dynamics of the cluster indicated no long-lived components, and relaxation time constants were comparable with those of a metallic Au144 cluster. Clusters with approximate core sizes of Au115 and Au117 have also been studied, and no long-lived components were reported.11 In addition to the existence of an energy gap the transient absorption spectroscopy provides information about the electronic energy levels and their role in the relaxation processes.8 The possible involvement of triplet states in the relaxation has been discussed for Au102(pMBA)44 based on the transient absorption results and the calculated energy levels and Received: August 7, 2015 Revised: August 10, 2015 Published: August 10, 2015 20224

DOI: 10.1021/acs.jpcc.5b07672 J. Phys. Chem. C 2015, 119, 20224−20229

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The Journal of Physical Chemistry C also suggested for Au25 and Au28 species.8,15,17 The role of triplet states can be further tested by studying the relaxation dynamics of other molecular clusters. The potential involvement of excited triplet states that are lower in energy than the excited singlet states has interesting implications related to the magnetic properties of the clusters. For molecular clusters in which the energy gap is small, the low-lying triplet states can be thermally populated which leads to weakly magnetic systems. Interestingly, it is also possible that for some cluster size, having an energy gap smaller than singlet−triplet splitting, a triplet state becomes the ground state thus making the cluster magnetic. Thus, it is of great interest to study clusters close to the energy gap closing size. In this work a water-soluble cluster with core size between 102 and 144 gold atoms was synthesized and preliminarily identified as Au130(pMBA)50. To study the photodynamics of the system transient mid-IR spectroscopy was used. According to the results, the cluster shows molecular behavior with respect to the existence of an energy gap. The relaxation dynamics was also analyzed, and results from DFT calculations were used to explain the observed relaxation time constants. The interpretation of the results is fully consistent with results previously obtained for the Au102(pMBA)44 cluster and further indicates the involvement of the triplet state as a key species in relaxation dynamics.8

reported relative to the energy of the relaxed singlet-state configuration, which is the lowest-energy state of all calculated configurations. The first excited state of singlets and triplets was estimated directly based on the HOMO−LUMO gap separately for each configuration in question. Calculations with the Perdew−Burke−Ernzerhof (PBE) functional were also performed, and the results were in good agreement with those obtained by using the LDA functional. Optical absorption spectra were calculated using the linear response form of timedependent density functional theory (LR-TDDFT) together with the PBE functional and 0.25 Å grid spacing.22

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RESULTS AND DISCUSSION Several methods were first used to characterize the cluster. The results of SDS-PAGE analysis are shown in Figure 1A. This

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EXPERIMENTAL METHODS The sample used in this study was obtained as a byproduct of Au102(pMBA)44 synthesis18 and isolated by precipitation. A more detailed description of the synthesis is given in the Supporting Information. In all of the experiments D2O solution of the sample was used with Na+ as the countercation. The size and monodispersity of the obtained sample was analyzed by using SDS-PAGE. The gel electrophoresis visualization of the sample was run on 15% polyacrylamide gel (29:1, acrylamide:bis(acrylamide)) using 1X TBE buffer in a Bio-Rad Mini-Protean Tetra System gel electrophoresis apparatus at 130 V for 2.5 h. The UV/vis spectrum of the sample was measured with a PerkinElmer lambda 850-UV/vis spectrophotometer, and the FTIR spectrum was obtained with a Nicolet Magna 760 FTIR spectrometer in a flow cell19 with 80 μm Teflon spacer. The 1H NMR spectra were measured with a Bruker Avance 400 MHz spectrometer at 299−302 K. The transient absorption experiment was performed similarly to the previously reported experiments for Au102 species by using 652 nm visible excitation and the same vibrational mode of the pMBA ligand for probing.8 The excitation energy dependence of the system kinetics was tested by using excitation pulse energies between ∼100 and ∼350 nJ. The used laser setups and performed measurements are described in more detail in refs 8 and 9. Computational Methods. We used density functional theory (DFT) as implemented in the code-package GPAW.20 GPAW uses projector-augmented waves (PAWs) and realspace grids and has relativistic corrections included in the Auatom setup. We used a reduced model for Au130(SR)50 with SH groups describing the protecting layer and the overall structure that was reported in ref 21. For the calculations of different magnetic states we fixed the total magnetic moment of the cluster and relaxed the structure of the cluster and at the same time the individual magnetic moments of the atoms for each state separately. In structural relaxation we used the 0.2 Å grid spacing and LDA functional. Energies of the clusters were

Figure 1. (A) PAGE visualization of Au102(pMBA)44 (lane 1), the studied cluster (lane 2), and Au144(pMBA)60 (lane 3) showing the size difference and monodispersity of the samples. (B) A comparison between the experimental UV/vis spectrum of the studied cluster (black) and calculated spectrum (blue) for Au130(SH)50. The e x p e r im e n t a l U V / v is s p e c tr u m f o r a n o r g a no s o lu b l e Au130(SC12H25)50 cluster3 is shown in green for comparison. The data for Au130(SC12H25)50 were reproduced with permission from ref 3.

analysis shows that the sample is highly monodisperse. A comparison to known cluster sizes also indicates that the size of the cluster core is between Au102 and Au144 species. Further characterization was done with UV/vis spectroscopy. The obtained spectrum is shown in Figure 1B together with a computational spectrum for Au130(SH)50 and an experimental spectrum from an organosoluble Au130(SC12H25)50.3 An 1H NMR spectrum of the sample was also measured, and clearly distinct spectral features were observed in comparison to corresponding spectra of Au102 and Au144 species.18,23 The comparison of the 1H NMR spectra is shown in Figure S1 in the Supporting Information. To determine the exact cluster composition a mass spectrometry experiment was attempted without success. Similar difficulty in determining the mass spectrum for larger water-soluble clusters has also been previously reported.24 Due to the unsuccessful attempt to identify the cluster composition by mass spectrometry, an indirect identification based on the electrophoresis analysis and UV/vis spectroscopy is used. Because the size of the cluster core is narrowed to be between 102 and 144 gold atoms by SDS-PAGE analysis, the obtained UV/vis spectrum was compared to known stable cluster sizes in this region. Water-soluble clusters with an approximate size of Au115 have been reported with the pMBA ligand.11,24 However, the UV/vis spectrum of the sample does 20225


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The Journal of Physical Chemistry C

modes of D2O and the OH stretching mode of residual H2O (∼3000−3500 cm−1) observed in the region of interest, and no accurate value for the optical gap can be obtained. However, based on the spectrum it can be narrowed to the 2000−4300 cm−1 region (0.25−0.53 eV). The FTIR spectrum is also compared to the computational electronic spectrum for Au130(SH)50 as shown in Figure 2. For further comparison, a spectrum showing the calculated individual optical transitions is shown in Figure S2 in the Supporting Information. By studying the individual transitions it can be clearly seen that the first transitions have low oscillator strength, while the stronger transitions occur at energies above 0.55 eV (4400 cm−1). This observation matches the experimental spectrum. The transient absorption spectrum clearly shows a groundstate bleach signal and a hot band signal. The bleach signal seemingly disappears after ∼20 ps, while the hot band signal decreases in different time scales ranging from a fast picosecond component to long-lived nanosecond components. A contour plot of the transient spectrum is shown in Figure S3 in the Supporting Information. The kinetics for bleach band and the maximum of the hot band are shown in Figure 3. The kinetics

not directly match the spectrum published for the Au115 cluster, and a visual comparison of the SDS-PAGE analyses for these clusters also shows a slightly different result.24 This indicates that the studied cluster is not the Au115 species. The obtained UV/vis spectrum is also different from the spectrum of the recently identified Au133(SR)52 cluster.16,25 One of the stable core sizes in this region is Au130 species, which has been reported with different organosoluble ligands.21,26−28 The UV/ vis spectra for different Au130 clusters were compared in a recent paper by Chen et al.27 and were shown to have slightly different spectral features for clusters with different ligands. The UV/vis spectrum of our sample resembles the reported UV/vis spectra for different Au130 species, at least when regarding the most prominent features of the spectrum.3,21,26−28 On the basis of this, the UV/vis spectrum of our sample was also compared to a computational spectrum calculated for the Au130(SH)50 cluster based on a model proposed in ref 21. Very recently, this model was shown to be correct as the crystal structure of Au130(p-MBT)50 (p-MBT = 4-methylbenzenethiolate) was solved.29 The comparison to the computational spectrum and to an organosoluble Au130(SC12H25)50 cluster3 is shown in Figure 1B. Some of the differences between the spectra may be attributed to the different ligands, especially since the pMBA shows strong electronic absorbance below 300 nm.5 On the basis of this comparison the studied cluster is tentatively identified as Au130(pMBA)50. A FTIR spectrum of the cluster in D2O solution was measured to determine the absorption of vibrational modes of the ligands and to study the optical gap. The spectrum is shown in Figure 2. Due to strong solvent absorption the sample

Figure 3. Hot band maximum signal kinetics at 1387 cm−1 (red) and bleach signal kinetics at 1400 cm−1 (blue) obtained with 652 nm excitation and 350 nJ excitation energy. Fits to the experimental data are shown in black.

was more carefully analyzed by fitting, and five different relaxation times corresponding to different relaxation processes were obtained. The bleach data could be fitted with a single exponential component convoluted with a Gaussian instrument response function, while for the hot band a sum of three exponential components and a long-lived constant component convoluted with a Gaussian instrument function were used. The obtained time constants were 5.6 ps for bleach kinetics and 1.2 ps, 20 ps, and 1.2 ns for the hot band. The longest time component was estimated to have a time constant longer than 10 ns. No excitation energy dependence for the time constants was observed in the 100−350 nJ range, which is consistent with molecular behavior. The normalized kinetics obtained with different excitation energies are shown in Figure S4 in the Supporting Information. The apparent disappearance of the bleach component was further studied by fitting the transient spectrum with different time delays to reveal bleach recovery kinetics hidden by the overlapping bleach and hot band. A similar observation was previously reported for the Au102(pMBA)44 cluster.8 The transient absorption results indicate that the studied cluster has a significant HOMO−LUMO gap indicating

Figure 2. Experimental FTIR spectrum of the studied cluster (black) in D2O and the computational electronic spectrum of a Au130(SH)50 cluster (blue). The region showing the continuous rise of the signal is enlarged in the inset. The oscillation of the signal is due to interference from the cuvette. The peak marked with an asterisk (*) is induced by OH and HOD absorption of residual water.

spectrum in the spectral region from ∼2000 to ∼3000 cm−1 cannot be measured in D2O. The vibrational absorptions were found to be typical for the pMBA ligand and essentially indistinguishable from the FTIR spectrum for the Au102(pMBA)44 cluster.5,8 This is reasonable because the vibrational modes localized in the ligands are expected to be similar for both clusters. The rising continuous absorption in the higher-energy region of the spectrum shown in the inset of Figure 2 clearly shows the electronic absorption of the cluster. The determination of the onset of electronic absorption for the studied cluster is hindered by the strongly absorbing vibrational 20226


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Figure 4. Calculated electronic energy levels of Au130(SH)50 and the assigned time constants for different relaxation processes. The excitation is shown in red. The different relaxation pathways are indicated with different colors. T* is the triplet state in the relaxed singlet-state geometry, and S* is the singlet (ground) state in the relaxed triplet state geometry.

6.8 ps vibrational cooling time constant for Au102 species. Therefore, this time constant can be assigned to the vibrational cooling of the system, i.e., dissipation of heat to the environment. The assignment of the longer time constants by just comparing them to Au102(pMBA)44 cluster dynamics is not as simple. Intuitively one would assume that the relaxation time is shortened as the energy gap decreases from 0.45 eV observed for Au102 species5 to 0.22 eV calculated for Au130(SH)50. The calculated energy difference between the ground state and the lowest triplet state is also decreased from 0.37 eV (Au102)8 to 0.13 eV (Au130). However, according to the present results the longest relaxation time (>10 ns) appears to be much longer than the longest time constant reported for the Au102 species (∼3.5 ns). By taking into account the differences in the electronic energy level structure for the two clusters, the observed kinetics can be explained. The calculated 0.22 eV energy difference between the S1 and ground state is presumably large enough to lead to a long relaxation time constant since the relaxation via cluster core phonons and gold−ligand interface vibrational modes requires high-order multiphonon processes. By comparing the longer relaxation time constants to those of Au102 species, the second longest relaxation time of 1.2 ns is assigned to the relaxation from the lowest excited singlet state to the ground state. This assignment is intuitively acceptable when comparing the result to the ∼3.5 ns time constant assigned to a similar process for Au102 species. The two remaining relaxation time constants can then be attributed to relaxation involving the triplet manifold. For the Au102 cluster the 84 ps relaxation time constant has been assigned to the transition from the first excited triplet state to the ground state via a potential surface crossing with the ground state close to the triplet state energy.8 However, according to DFT calculations the singlet ground state at relaxed triplet geometry is about 0.24 eV higher in energy than the T1 state for the Au130 cluster. Therefore, an energy barrier is presumed to exist between the T1 and S0 states as shown in Figure 4. This barrier is expected to significantly slow down the relaxation from the T1 state because of the required activation energy and hence increase the triplet state relaxation time compared to Au102. On the basis of this the longest time constant (>10 ns) is attributed to the relaxation from the T1 state to the ground state. Due to the existence of an energy barrier, the remaining 20 ps time constant can then be attributed to relaxation from the T1 state to the ground state during the vibrational relaxation, when the system is vibrationally hot and the excess vibrational energy can facilitate the crossing of the barrier and a fast relaxation to the ground state.

molecular behavior. This can be clearly seen by the existence of long-lived relaxation components. The results can also be used to further confirm that the studied cluster is different from the previously studied cluster sizes Au115 and Au133 because the observed relaxation dynamics differ from the ones previously reported for these clusters.11,16 A clear difference in the overall kinetics can also be seen between the studied cluster and the previously reported Au102(pMBA)44 cluster since the corresponding time constants reported for Au102 species were 6.8 ps, 1.5 ps, 84 ps, and ∼3.5 ns.8 To further confirm the difference between the two cluster sizes, the transient absorption measurement was repeated for the previously used Au102(pMBA)44 sample with exactly the same experimental conditions. The repeated experiment confirmed the previous results for the Au102 species. To help understand the relaxation processes and the energy states involved in the relaxation, the energies of the electronic states in singlet and triplet electronic configuration were calculated for the Au130(SH)50 cluster by using DFT and the cluster model shown in ref 21. This cluster was used as a model system based on the assignment of the cluster composition. The computational HOMO−LUMO gap for Au130 species was determined to be 0.22 eV, which matches reasonably well with the limits determined from the experimental spectrum. We considered both singlet and triplet electronic states of the cluster. As previously reported for the Au102 cluster8 the Au130 species also has a slight change in the spatial arrangement of the cluster core atoms associated with a change in the electronic spin state from singlet to triplet. The ground-state geometry was used to calculate the energies of the first excited singlet state and the triplet state. In addition, the triplet-state geometry was relaxed, and the energies of the states were calculated by using this geometry. The calculated energies of the lowest singlet and triplet states are shown in Figure 4. On the basis of the calculated energy level structure an extensive picture of the relaxation dynamics that explains the observed kinetics can be proposed. A summary of the assigned kinetics is shown in Figure 4. The fastest observed time constant 1.2 ps is comparable to the 1.5 ps time constant for the Au102 cluster and can be assigned to initial electronic relaxation that occurs from the initially excited states to lower excited states. Due to the strong spin−orbit coupling of gold atoms this relaxation process is expected to involve multiple internal conversions and intersystem crossings in both the singlet and the triplet manifolds. This also leads to population in both manifolds on the lower energy states. The 5.6 ps time constant obtained from the bleach kinetics is also comparable to the similarly obtained 20227




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Once the system is cooled, the population is trapped to the T1 state and relaxes much slower by a thermally activated process. The presented interpretation of the relaxation dynamics is consistent with the energy level calculations for the Au130(SH)50 cluster and is also fully consistent with the results previously reported for the Au102(pMBA)44 cluster.8 The structure-related electronic level structure of the cluster is further confirmed to play a vital role in the relaxation dynamics. In addition, the present results indicate that the vibrational cooling and the involvement of triplet states have an important role in the electronic relaxation of the molecular clusters, as already implied by the results reported for Au102 species.8 On the basis of the presented results the studied cluster has a significant energy gap and shows molecular behavior, hence providing new information about the size-dependent transition from molecular to metallic clusters. By comparing this information with previously reported results, it can be seen that pinpointing a single cluster size that represents the borderline in the transition region may be complicated. Due to size-induced changes to geometric structures of the gold core the electronic structure of the clusters and the size of the energy gap does not change monotonously with the number of gold atoms; for example, the water-soluble Au68 species has an energy gap comparable to Au102.30 This type of structural dependence of the relaxation dynamics might also explain the fast relaxation dynamics reported for Au133 and Au115.11,16 This might also indicate that clusters with certain compositions and structures larger than 144 gold atoms might also be molecular. A further interesting implication of our interpretation of the relaxation dynamics for Au130 is that after electronic excitation there may be a relatively long-lived population in the lowest triplet state, thus making the cluster sample transiently magnetic. The relatively small energy difference between the ground state and the T1 state is calculated to be 0.13 eV which also implies that the triplet state might be slightly thermally populated at room temperature. This idea was tested by measuring the EPR spectrum of the sample, but no EPR signal assignable to the cluster was obtained in the measurement. Further experiments exploring this possibility are potentially of interest for future work. The interpreted magnetic behavior for the studied cluster also allows a prediction that possibly for some cluster size close to the transition region, for which an energy gap is smaller than singlet−triplet splitting, a triplet state becomes the ground state thus making the cluster magnetic. We predict that such a situation may occur for cluster sizes between Au130 and Au144.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b07672. A more detailed description of the cluster synthesis, a comparison of the 1H NMR spectrum of the studied sample to that of Au102 and Au144, calculated electronic absorption spectrum showing individual electronic transitions, a contour plot of the transient absorption spectrum, and a comparison of normalized hot band kinetics obtained with different excitation energies (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Email: mika.j.pettersson@jyu.fi. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the NGS-NANO (S.M.) and LASKEMO (J.K.) graduate schools as well as the Academy of Finland (H.H.). Dr. Elina Kalenius and Dr. Ian Morgan are gratefully acknowledged for performing the mass spectrometry experiments and the EPR measurements, respectively. DFT computations were performed using CSC-IT center computational facilities.

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REFERENCES

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CONCLUSIONS In conclusion, a highly monodisperse sample of water-soluble cluster tentatively identified as Au130(pMBA)50 was studied by vis-pump/mid-IR probe transient spectroscopy to distinguish between the molecular or metallic behavior and to study the relaxation dynamics. The observed relaxation dynamics confirmed the existence of a HOMO−LUMO gap and thus the molecular behavior of the cluster. This helps to further elucidate the transition region between molecular and metallic systems. An extensive picture of the relaxation dynamics, which is fully consistent with the calculations and results previously reported for the Au102(pMBA)44 cluster is presented. Also an implication of a long-lived triplet state population suggests that the studied cluster may have interesting magnetic properties. 20228


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