Temperature-Dependent Absorption and Ultrafast Luminescence

Article pubs.acs.org/JPCC

Temperature-Dependent Absorption and Ultrafast Luminescence Dynamics of Bi-Icosahedral Au25 Clusters Mary Sajini Devadas, Viraj Dhanushka Thanthirige, Semere Bairu, Ekkehard Sinn, and Guda Ramakrishna* Department of Chemistry, Western Michigan University, Kalamazoo Michigan 49008, United States S Supporting Information *

ABSTRACT: Temperature-dependent absorption and ultrafast luminescence dynamics of [Au25(PPh3)10(SC6)5Cl2 ]2+ (Au25-rod) was studied and compared with [Au25(SC6)18]− (Au25-sphere) and Au38(SC2Ph)24 (Au38-rod) to understand the influence of the crystal structure on the optical properties of monolayer protected gold clusters. The temperaturedependent absorption of Au25-rod shows a shift in the absorption maximum to high energies and a small increase in the oscillator strength with decrease in temperature. The energy shift was modeled via the O’Donnell and Chen relationship, which yielded average phonon energy of 160 ± 80 cm−1, quite smaller than the 350 cm−1 observed for Au25-sphere and Au38-rod. There is an increase in the oscillator strength with a decrease in temperature of about 40% for Au25-rod while it is nearly 250% for Au25-sphere and more than 180% for Au38-rod. The oscillator strength increase is attributed to the coupling of core-gold exciton and shell-gold phonons. The smaller increase in the oscillator strength for Au25-rod is consistent with its structure that possesses no shell-gold. Femtosecond luminescence measurements carried out on Au25-rod clusters show wavelength-independent ultrafast luminescence decay traces. The lifetimes from the analysis are consistent with the relaxation of higher energy states. In contrast, Au25-sphere and Au38-rod clusters show specific wavelength-dependent luminescence growth and decay, representing the relaxation of core-gold states to shell-gold states. structures of −SR−Au−SR− and −SR−Au−SR−Au−SR− covering the inner core gold atoms (Figure 1). This inner core is an electron rich environment covered by an outer shell of Au atoms that is electron deficient due to the charge transfer from Au to thiol groups. This charge difference between core and shell Au atoms, and corresponding core−shell interactions are crucial in understanding the catalytic activity of gold clusters.32 Although enormous research attention is focused on the synthesis and electrochemical and catalytic activity of the atomically precise monolayer protected gold clusters, the understanding of the core-gold and shell-gold interactions is still limited. Moran and co-workers43,44 have shown via transient absorption measurements that core-gold to shellgold decay dominates the early time excited-state relaxation of Au25(SR)18− clusters. In an earlier work, we have demonstrated wavelength-dependent ultrafast luminescence dynamics45,46 for Au25(SR)18− that is attributed to the relaxation of core-gold to shell-gold states.45 Also, we have shown with temperaturedependent absorption measurements on Au25(SC6)18− (Au25sphere) and Au38(SC2Ph)24 (Au38-rod) clusters that core−shell electron−phonon interactions play a major role on the optical properties of clusters.47

1. INTRODUCTION Ligand-protected gold clusters with sizes less than 2 nm show quantum confinement behavior, and they have garnered enormous research attention both for their fundamental interest and applications in interdisciplinary areas of sciences.1−7 Quantum confinement leads to nonmetallic molecular behavior for these clusters and interesting electrochemical, catalytic, linear and nonlinear optical behavior.3−17 Of particular interest is the catalytic activity of the quantum sized gold clusters. Haruta et al.18−20 initiated interest in the chemistry of gold catalysts and demonstrated that bare gold clusters with sizes less than 3 nm act as remarkable catalysts. The synthesis of atomically precise monolayer-protected gold clusters has ignited research on gold nanocatalysis as it is now possible to correlate cluster size, shape and functionality with catalytic activity.21−36 Several recent investigations on the catalytic properties of monolayer-protected gold clusters (MPCs) report success in catalyzing many chemical reactions.22−27 The catalytic activity depends on size, shape and crystal structure.29−32,36 Jin and co-workers36 have shown that [Au25(PPh3)10(SC2Ph)5Cl2]2+ with a bi-icosahedral structure has lower catalytic activity when compared to Au25(SC2Ph)18− and indicated that the core−shell structures of clusters play a major role in their catalytic activities. The crystal structures of Au 1 0 2 (p-MBA) 4 4 , 3 7 , 3 8 Au25(SC2Ph)18−,39−41 and Au38(SR)2442 helped explain the catalytic activity of the clusters, based on having staple motif © 2013 American Chemical Society

Received: August 20, 2013 Revised: October 1, 2013 Published: October 3, 2013 23155

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Figure 1. Crystal structures of Au25-sphere,39,40 Au25-rod48,49 and Au38-rod.42 Here Auc and Aush are core-gold and shell-gold respectively. S refers to the thiolate ligands, Cl is chlorine and P refers to the phosphine ligands. The carbon tails of the ligands are all omitted for clarity.

clusters. The reaction mixture was stirred for 3 h at 298 K. After the completion of the reaction, the solution was rotary evaporated to near dryness. Four milliliters of acetonitrile was added to the product to dissolve Au25-rod clusters. The acetonitrile solution was rotary evaporated, and the dried product was thoroughly washed with copious amounts of ethyl acetate, hexane, and then ethyl ether. Finally, ∼20 mg of product of Au25-rod was obtained and the composition was determined to be [Au25(PPh3)10(SC6H13)5Cl2]2+ (Cl− is the counterion of the cationic cluster) by positive ion ESI-MS (see Supporting Information). 2.2. Methods. Ground state electronic absorption and temperature-dependent absorption measurements were carried out using a Shimadzu UV 2101 PC absorption spectrometer. Toluene for Au25-rod and methylcyclohexane:methylcyclopentane (1:1 v/v forms a clear glass at low temperature) for Au25 sphere were used as the solvents for temperature-dependence measurements. Optistat DN cryostat (Oxford instruments), temperature controller and a pressure gauge were used to conduct the temperature-dependent experiments from 78 to 313 K. Time-resolved fluorescence measurements of the gold clusters were studied using the femtosecond fluorescence upconversion technique described elsewhere.55 Briefly, the upconversion system used in our experiments was obtained from CDP Instruments, Inc., Russia. In the present investigation, studies were carried out with the second harmonic (400 nm) of the fundamental Ti:sapphire laser at 800 nm, as the excitation source. Polarization of the excitation beam for the magic-angle fluorescence and anisotropy measurements was controlled using a Berek compensator, and the sample was continuously rotated in a rotating cell of 1 mm thickness. Horizontally polarized fluorescence emitted from the sample was upconverted in a nonlinear crystal of β-barium borate using a pump beam at 800 nm, which first passed through a variable delay line. Instrument response function (IRF) was measured using Raman scattering from water. Fitting the Gaussian peak from the Raman scattering yielded a sigma value of ∼130 fs, which gave a full width half-maximum (fwhm) of ∼280 fs. Spectral resolution was achieved using double monochromator and photomultiplier tube. The excitation average power varied during experiments but was mostly in the range of 21 ± 1 mW. As the luminescence quantum efficiency of the clusters is quite small (10−8), longer collection times (around 5 s) were used for the measurements. Even at such higher laser powers, the degradation of the clusters was small as observed from the optical absorption spectra before and after the measurements.

Among several investigated MPCs, [Au 25 (PPh 3 ) 10 (SR)5Cl2]2+ (Au25-rod) clusters are interesting as the majority of its 25 Au atoms form the core while shell-gold is minimum.48−52 The investigations on Au25-rod clusters will provide contrast information on the core−shell interactions in atomically precise MPCs. Femtosecond transient absorption measurements53 carried out on Au25-rod clusters have shown two major relaxation components: one is the internal conversion from higher excited states to lower excited state, followed by microsecond relaxation of the lowest excited state. Also, Lee and co-workers52 have observed markedly different electrochemical properties in Au25-rod and Au25-sphere. Apart from these measurements, there are no reports yet elucidating the optical properties of Au25-rod clusters. Thus, to understand the excited state relaxation and possibly address the lower catalytic activity of Au25-rod clusters, temperature-dependent absorption and ultrafast luminescence dynamics on Au25-rod clusters (R = hexanethiol) are performed in the current work and further compared with that of Au25-sphere and Au38-rod. Au38-rod clusters are similar in shape to Au25-rod (i.e. both are rod-shaped), but they possess additional shell-gold over the biicosahedral core-gold. (Figure 1) The measurements carried out on these systems can differentiate the role of the cluster shape and core−shell interactions.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Chloro(triphenylphosphine)gold(I) ([(C6H5)3P]AuCl, ≥99.9% trace metals basis), hydrogen tetrachloroaurate trihydrate (HAuCl4· 3H 2 O, reagent grade), tetraoctylammonium bromide (Oct4NBr, 98%), 1-hexanethiol (HSC6, 98%), sodium borohydride (NaBH4, 99%) and sodium sulfate (99% ACS reagent, anhydrous) were obtained from Aldrich and used as received. HPLC grade toluene, acetone, absolute ethanol, acetonitrile, dichloromethane (DCM), and dimethyl sulfoxide (DMSO) were purchased from Aldrich and were used as received. LCMS grade acetonitrile used for the ESI-MS spectrometry was purchased from Aldrich and used as received. All the glassware used in the experiments was washed with aqua-regia. The synthesis Au25(SC6)18− was carried out following a procedure published by Kim et al.54 Au38-rod clusters were synthesized using a procedure published elsewhere.42 2.1.1. Synthesis of Bi-Icosahedral Au25 [Au25(PPh3)10(SC6)5Cl2 ]2+ Clusters. The clusters were synthesized using a one-step, one-phase method. In a typical synthesis, 0.124 g (0.25 mmol) of AuPPh3Cl and 74 μL (0.50 mmol) of hexanethiol were mixed in 20 mL of 3:1 v/v chloroform/ ethanol at 298 K. To this clear solution was added 0.217 g (2.5 mmol) of tertbutylamine−borane complex. The solution became dark brown immediately, indicating the formation of 23156

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3. RESULTS AND DISCUSSION Shown in Figure 2 are the optical absorption spectra of Au25rod in toluene and Au25-sphere in dichloromethane. The

in toluene while sharp features were quite evident for Au25sphere (Figure 3B). The sharp features observed for Au25sphere in the wavelength region of 400 to 500 nm are ascribed to vibronic features. In addition, the absorption at ∼745 nm is attributed to Pz to lowest unoccupied molecular orbital transition, which was shown theoretically by Aikens56 and recently by Tlahuice-Flores et al.57 Analysis of the shift in the absorption energy as well as the oscillator strength of the lowenergy transitions of Au25-rod was carried out to obtain the electron−phonon coupling and average phonon energy associated with the coupling of the core-gold and shell-gold. νTo model the absorption maximum dependence on temperature, we used a modified version of the Bose−Einstein relationship that was developed by O’Donnell and Chen:58 ⎡ ⎤ ⎛ hν ⎞ ⎟ − 1⎥ E(T ) = E(0) − ⟨C⟩⟨hν⟩⎢coth⎜ ⎝ 2kT ⎠ ⎣ ⎦

(1)

where E(T) is the energy at particular temperature, ⟨C⟩ is the coupling constant, E(0) is the energy gap at 0 K, and ⟨hν⟩ is the average phonon mode responsible for electron−phonon interaction. The magnitude of [coth(⟨hν⟩/2kT) − 1] represents the effective number of available phonons. Figure 4 shows the corresponding plot of energy gap versus

Figure 2. Optical absorption spectra of Au25-rod in toluene and Au25sphere in dichloromethane.

electronic absorption spectrum of Au25-rod has two main features, one centered at 670 nm and another maximum at 415 nm along with other absorption peaks. The absorption features of Au25-rod in toluene matched well with the literature reports.48,49,52 It was theoretically shown that the absorption at 670 nm arises because of the coupling of two bi-icosahedrons while the absorption below 500 nm arises due to the individual Au13 icosahedrons.51 Although absorption of both Au25 clusters is roughly in the same wavelength region, their electronic transitions have different origins, owing to their different crystal structures. To gain further insights into the optical properties of Au25-rod clusters, temperature-dependent absorption and ultrafast luminescence measurements were carried out. 3.1. Temperature-Dependent Absorption Measurements. The absorption spectra at different temperatures for Au25-rod in toluene are shown in Figure 3A. As observed in our previous work on Au25-sphere,47 the absorption features of Au25-rod are also influenced by temperature as that of Au25sphere, but to a lesser extent. With a decrease in the temperature, there is a shift in the absorption maximum to higher energies and a slight increase in the oscillator strength. However, no new vibronic features were observed for Au25-rod

Figure 4. Temperature-dependent absorption maximum vs temperature for Au25-rod and Au25-sphere.

temperature for Au25-rod in toluene along with its fitting curve. Also, shown for comparison is the similar plot for Au25sphere. The fitting parameters obtained are provided in Table

Figure 3. Absorption spectra at different temperatures for (A) Au25-rod in toluene and (B) Au25-sphere in methylcyclohexane:methylcyclopentane (1:1 v/v). 23157

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Table 1. Average Phonon Energy and Coupling Constants Obtained for Au25-Rod, Au25-Sphere and Bi-Au38 from the Analysis of Temperature-Dependent Absorption Spectra sample

Eg (eV)

⟨hν⟩ (meV)

⟨C⟩

⟨S⟩

[Au25(PPh3)10(SC6)5Cl2]2+ (Au25-rod) [Au25(SC6)18]− (Au25-sphere) Au38(SC2Ph)24 (Au38-rod)

1.90 ± 0.01 1.893 ± 0.003 1.675 ± 0.001

20 ± 10 43 ± 6 54 ± 7

0.9 ± 0.1 3.6 ± 0.4 1.8 ± 0.3

0.2 ± 0.1 2.1 ± 0.2 3.1 ± 0.2

⎡ ⎛ ⟨hν⟩ ⎞⎤ ⎟⎥ f ≈ f0 exp⎢ −⟨S⟩coth⎜ ⎝ 2kT ⎠⎦ ⎣

1. It can be observed from Table 1 that the phonon energy obtained for Au25-rod (20 ± 10 meV) is significantly smaller than that of the Au25-sphere (43 ± 6 meV) suggesting weaker involvement of shell phonons. The energy of 160 ± 80 cm−1 is closer to the Au−Au and Au−S−Au vibrational energies.59,60 From this analysis, the involvement of shell phonons on the absorption spectrum of Au25-rod was found to be very small. On the other hand, for Au25-sphere, the shift in the absorption energy was coupled with a phonon energy of 340 cm−1 which is closer to the vibrational energies of −S−Au−S−Au−S−,58 pointing to the active involvement of shell-gold phonons. In addition to the absorption maximum shift, there is also a slight increase in the absorption intensity for Au25-rod with a decrease in temperature. The oscillator strengths of the lowest energy transitions are obtained from the electronic absorption spectrum by using the expression fabs ≈ 4.3 × 10−9∫ ε(v)̅ dv,̅ and the normalized oscillator strength is plotted as a function of temperature (shown in Figure 5). The main idea behind this

(2)

where ⟨S⟩ is a Hugh−Rhyns factor which describes the coupling strength between the exciton and average phonon mode ⟨hν⟩,29 and f 0 is the oscillator strength at 0 K. The obtained fits are presented in Figure 5. We have used the same average phonon mode that was obtained from the analysis of the change in absorption maximum with the temperature. The ⟨S⟩ was determined to be 0.2 ± 0.1 for Au25-rod, which is significantly smaller than for Au25-sphere (2.1) and Au38-rod clusters (3.1). This result shows that the coupling of the exciton and phonon is extremely small, and it suffices to say that the interaction of the core-gold and shell-gold is negligible in Au25rod. This explanation is valid, as the crystal structure of Au25rod also shows no associated staple motifs except Au−SR−Au bridged linkages. If we think of shell-gold for Au25-rod, it can be probably related to 5 Au-SR-Au motifs whose contribution is significantly smaller when compared to 6 −SR−Au−SR−Au− SR− staple motifs of Au25-sphere and 6 −SR−Au−SR−Au− SR− and 3 −SR−Au−SR− staple motifs for Au38-rod. The temperature-dependent absorption measurements of Au25-rod validate the hypothesis that the core-gold and shell-gold interactions play a major role in the electronic interactions of Au MPCs and are probably the reason behind greater catalytic effect of Au25-sphere clusters over Au25-rod. Further ultrafast luminescence measurements were carried out to confirm the interactions of core-gold and shell-gold. 3.2. Ultrafast Luminescence Measurements: Core− Shell Interactions. In an earlier investigation, our research group has shown the presence of weak but fast decaying visible luminescence that is quite unique to quantum sized gold clusters different from gold nanoparticles with surface plasmon resonance.45 In addition, Au25-sphere clusters with specific core-gold and shell-gold have shown distinctive wavelengthdependent growth and decay kinetics which is absent in larger sized gold clusters. This phenomenon was ascribed to relaxation of core-gold to shell-gold through a manifold of intermediate shell-gold electronic states. To improve the understanding of core-gold and shell-gold interactions, femtosecond luminescence measurements were carried out for Au25rod and Au38-rod after excitation at 400 nm. It should be mentioned that the visible luminescence quantum efficiency of the investigated clusters (both Au25-rod and Au38-rod) is very low (10−8), and thus, we have used longer collection times to get sufficient luminescence counts. Shown in Figure 6A are the luminescence kinetic traces of Au25-rod in toluene at different monitoring wavelengths from 480 to 580 nm. All the luminescence decay traces of Au25-rod were fit with an instrument response limited rise and single exponential decay. The luminescence decay was slower than the instrument response and has shown small changes with different emission wavelengths (Figure 6C). Shown in Figure 6B are the luminescence decay traces of Au38-rod in toluene at different emission wavelengths from 480 to 580 nm after excitation at

Figure 5. Comparison of the normalized oscillator strength vs temperature for Au25-rod, Au25-sphere and Au38-rod.

exercise is to compare the oscillator strength changes between Au25-rod and Au25-sphere and also that of Au38-rod. Note from Figure 5 that the oscillator strength increase in Au25-rod is only close to 40% while for Au25-sphere it is as high as 250% and close to 180% is observed for Au38-rod. The results present a major difference between the absorption properties of Au25 clusters with different shapes. For most organic and inorganic materials, the oscillator strength of an electronic transition does not change with decrease in temperature. The solvent density or change in the refractive index can account for a maximum of 10% increase in the oscillator strength. However, there is an increase in oscillator strength close to 40% for Au25-rod, which is not as high as Au25-sphere, but significant enough. Thus, we used a model where the exciton−phonon interactions can be used to account for the changes in oscillator strengths.61,62 The temperature-dependent oscillator strengths (f) of the electronic transitions can be modeled using the Debye−Waller expression61,63 that correlates the oscillator strength with exciton−phonon interactions. 23158

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Figure 6. Luminescence kinetic decay traces of (A) Au25-rod and (B) Au38-rod. Absence of wavelength dependence is observed in the case of Au25rod and indicates no core−shell transitions. (C) Growth and decay components obtained from the fitting of fluorescence kinetic decay traces of the investigated clusters.

400 nm. Similar to the data obtained on Au25-sphere,45 Au38rod has also shown wavelength-dependent growth and decay components confirming the relaxation of core-gold to shell-gold dominating its decay. Figure 6C shows the comparison of the growth and decay components of Au25-rod, Au25-sphere and Au38-rod. Note that the growth component is instrument response limited for Au25-rod while the wavelength dependence of growth and decay is evident for Au38-rod. The results indicate that, for Au25-rod, the excited state decay is dominated by vibrational cooling and indicates the lack of shell-gold electronic states mediating the relaxation.

comparison to Au25-sphere. In other words, the shell-gold atoms in the −SR−Au−SR−Au−SR− are implied to be important in catalysis. The results are further corroborated by femtosecond luminescence decay traces where Au25-rod has shown wavelength independent decay dynamics while that of Au25-sphere and Au38-rod show specific wavelength-dependent growth and decay dynamics indicating a cascade relaxation from core-gold to shell-gold. The obtained insight into the electronic and phonon properties of Au clusters is expected to promote the fundamental understanding of the structure−property relationships as well as the catalytic reactivity.

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4. CONCLUSIONS The temperature dependence of optical absorption and femtosecond luminescence dynamics of Au25-rod clusters are reported and compared with Au25-sphere and Au38-rod clusters. The optical absorption spectral features became sharper and the lowest energy maximum is shifted to higher energies for Au25rod similar to what was observed for Au25-sphere and Au38-rod. However, the electron−phonon coupling frequency is found to be significantly smaller for Au25-rod when compared to Au25sphere and Au38-rod suggesting negligible involvement of the shell-gold phonons. The oscillator strength increase is also minimal for Au25-rod in comparison to other clusters, which again points to the limited interaction of shell-gold phonons with core-gold excitons. The result is in accordance with the crystal structure of Au25-rod that comprises negligible shellgold. The difference in the core-gold and shell-gold might be the reason behind the low catalytic efficiency of Au25-rod in

ASSOCIATED CONTENT

* Supporting Information S

ESI-MS data for Au25-rod and Au25-sphere. 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 We would like to thank Prof. Rongchao Jin, Carnegie Mellon University, for the gift of Au38-rod clusters and also for valuable discussions and constructive suggestions. We acknowledge the support of U.S Army Research Office under Grant No. W911NF-09-C-0135. G.R. acknowledges the support of 23159

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Western Michigan University-FRACAA and WMU start-up funds for the project.

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