Ultrafast Relaxation Dynamics of Luminescent Rod-Shaped, Silver

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The Journal of Physical Chemistry

Ultrafast Relaxation Dynamics of Luminescent Rod-Shaped Silver Doped AgxAu25-x Clusters Meng Zhou,1 Juan Zhong2, Shuxin Wang,3 Qianjin Guo,1 Manzhou Zhu,3 Yong Pei,2* Andong Xia1* 1

Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of

Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing-100190, People’s Republic of China 2

Department of Chemistry Key Laboratory of Environmentally Friendly Chemistry and

Applications of Ministry of Education, Xiangtan University, Xiangtan, Hunan 411105, People’s Republic of China 3

Department of Chemistry and Center for Atomic Engineering of Advanced Materials, Anhui

University, Hefei, Anhui 230601, People’s Republic of China

Corresponding authors: [email protected];

[email protected]

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Abstract: The luminescent ligand protected metal clusters have attracted considerable attentions while the origin of the emission still remains elusive. As recently reported in our previous work, the rod-shaped Au25 cluster possesses a low photoluminescence quantum yield (QY=0.1%), whereas substituting silver atoms for central gold atom in the rod-shaped Au25 cluster can drastically enhance the photoluminescence with high quantum yield (QY=40.1%). To explore the enhancement mechanism of fluorescence, femtosecond transient absorption spectroscopy is performed to determine the electronic structure and ultrafast relaxation dynamics of the highly luminescent silver-doped AgxAu25-x cluster by comparing the excited state dynamics of doped and un-doped Au25 rod cluster, it is found that the excited state relaxation in AgxAu25-x, is proceeded with an ultrafast (~0.58 ps) internal conversion and a subsequent nuclear relaxation (~20.7 ps) followed by slow (7.4 µs) decay back to the ground state. Meanwhile, the observed nuclear relaxation is much faster in AgxAu25-x (~20.7 ps) compared to that in un-doped Au25 rod (~52 ps). We conclude that it is the central Ag atom which stabilizes the charges on LUMO orbital and enhances the rigidity of AgxAu25-x cluster that leads to strong fluorescence. Meanwhile, coherent oscillations around ~ 0.8 THz were observed in both clusters, indicating the symmetry preservation from Au cluster to Ag alloying Au clusters. The present results provide new insights for the structure-related excited state behaviors of luminescent ligand protected Ag alloying Au clusters.

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Introduction

Ligand protected metal nanoclusters (Au and Ag NCs) are emerging as promising new materials which have stimulated great research interests in both applications and fundamental sciences.1-6 When their sizes drop down to less than 2 nm, these nanoclusters begin to exhibit molecular behaviors such as large band gap with separating energy levels and fluorescence in the visible to near-infrared (NIR) region.7-9 With advantage of long fluorescence lifetime, large Stokes shift, low toxicity and good biocompatibility, fluorescent Au and Ag nanoclusters have been recognized as a new promising material for the application in sensing and imaging.10 However, metal nanoclusters suffer from low quantum yield, which largely limits their applications. Despite various strategies to enhance their luminescence, most of those thiolate as well as phosphine protected metal NCs are weakly luminescent.11-14 Since the size of metal nanoclusters is close to their Fermi wavelength (1ns), where ultrafast internal conversion (0.58 ps) and the subsequent structural relaxation (20.7 ps) have been identified. The strong fluorescence in Au12Ag13 rod is attributed to the stabilization of the charges on LUMO and the enhancement of rigidity due to the occupation of the Ag atom in the central position. Besides, coherent oscillations at 0.8 THz were observed in both clusters, suggesting that the breathing vibration mode remains regardless of the Ag alloying in rod shaped Au25 clusters.

Materials and Methods. Chemicals. Unless specified, reagents were purchased from Acros Organics or Sigma-Aldrich and used without further purification. Tetrachloroauric(III) acid (HAuCl4·3H2O, >99.99% metals basis), Sodium borohydride (>98%) was received from ACROS Organic. Toluene (HPLC grade, ≥99.9%, Aldrich) and Ethanol (HPLC grade, ≥99.9%) were sourced from Aldrich. Pure water ordered was from Wahaha Co LTD. Methylene chloride (HPLC grade, ≥99.9%). Phenethylthio silver was prepared as previously described. Synthesis of [AgxAu25-x(PPh3)10(RS)5Cl2]2+ (x=13-2). Details of the synthesis procedure has 6


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been reported elsewhere.25 Steady State Spectral Measurements. The steady state ultraviolet-visible (UV-vis) absorption was recorded on a U3010 (Hitachi) spectrometer. The steady state emission and excitation spectra were measured by using a F4600 (Hitachi) fluorescence spectrometer. Nanosecond Spectral Measurements. The lifetime of the emission and nanosecond transient absorption measurements were performed using a nanosecond flash photolysis setup Edinburgh LP920 spectrometer (Edinburgh Instruments Ltd.), combined with a Nd:YAG laser at 355 nm (Surelite II, Continuum Inc.). Ultrafast Transient Absorption Spectral Experiments. The femtosecond transient absorption spectra were measured at ~90 fs time-resolution using a home-built femtosecond broadband pump-probe setup, which has been described elsewhere.35 Details of the experiment are introduced in Supporting Information S1 section. Data Processing Spectral chirp corrections for the obtained transient absorption spectra were performed for group velocity dispersion of the probe beam before global fitting. The differential absorbance ∆A(t , λ ) was analyzed using the population dynamics modeling graphical interface program Glotaran and TIMP.36-37 Details of the data analysis can be seen in Supporting Information S1 section.

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Results and Discussions Steady state measurements and quantum chemical calculations. The linear absorption spectrum of Au25 rod gives prominent peak at 675 nm, relatively weak shoulders at 515 nm and 450 nm and a sharp peak at 415 nm, similar to that in previous investigations.19 Compared with Au25 rod, the Ag alloying Au12Ag13 exhibits remarkable spectral change, where two absorption bands at 430 nm and 370 nm were observed. Furthermore, the lowest absorption of Ag alloying Au12Ag13 is blue-shifted from 675 nm to 650 nm relative to that of Au25 rod as shown in Figure 1A. Since both HOMO and LUMO of Au25 rod originate from interaction of the two Au13 icosahedra, the peak at 675 nm is assigned to a unique electronic transition due to the dimeric structure while the absorption with higher energy were assigned to the electronic transition within the individual Au13 core.19, 38 As a result, the absorption around 650 nm in Au12Ag13 rod is also tentatively ascribed to a new electronic transition as a result of the dimeric structure of two vertex-sharing Au6Ag7 icosahedrons. Nevertheless, quantum chemical calculation is required to verify the origin of the electronic transitions. The emission spectra of both clusters were measured in ethanol with excitation of 675 nm for Au25 rod and 650 nm for Au12Ag13 rod (see Figure 1B). Au25 rod exhibited extremely weak emission at 850 nm while Au12Ag13 gave much stronger and broader emission band centered at 702 nm. Significant differences in spectral shape and emission intensity suggest that electronic structure and thus the excited state relaxation dynamics have been largely perturbed in Au12Ag13 due to the Ag alloying compared with that in Au25 rod.

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Figure 1. Steady state absorption (A) and emission (B) spectra of Au25 rod cluster and Au12Ag13 cluster dissolved in ethanol. The emission in Au25 rod is 200× scaling up for clarity. To model the electronic structure of Au12Ag13 rod, we have carried out the TD-DFT calculation as well as the population analysis of frontier orbitals for both [Au12Ag13(PPh3)10(SR)5Cl2]2+ and [Au25(PPh3)10(SR)5Cl2]2+ (R=C2H4Ph), on the basis of generalized gradient approximation with the Perdew-Burke-Ernzerhof (PBE) function.39-41 The triple-zeta polarized (TZP) basis set with inclusion of scalar relativistic effect via zeroth-order regular approximation (ZORA) implemented in the Amsterdam Density Functional (ADF) package are adopted.42 All the phenol rings in the clusters have been replaced by CH3 during the calculations for computing efficiency. The TD-DFT calculations evaluate lowest 300 singlet-to-singlet excitation energies for both clusters based on the optimized geometric structure. In present, up to ~3 eV transition energies are calculated (corresponding to excitation wavelength longer than 410 nm). As shown in Figure 2A, the theoretical absorption spectra of Au12Ag13 and Au25 rod clusters agree qualitatively well with the experimental spectra as shown in Figure 1A. We note that the GGA function generally underestimates the optical gap. Herein, a blue shift of theoretical optical absorption curve is 9



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observed for both clusters. Nevertheless, the major absorption peaks in experimental spectra are well reproduced by the TD-DFT/PBE calculations. Moreover, the theoretical results predict the same tendency that the first feature absorption peak of Au12Ag13 cluster is blue shifted in comparison to the un-doped Au25 cluster.

The further analysis of atomic orbital component in each Kohn-Sham (KS) molecular orbital and the electronic diagram of HOMO and LUMO are carried out. For Au12Ag13, the HOMO is mainly composed of atomic orbitals along the axis while LUMO is contributed mainly by 5s orbitals of the vertex sharing Ag atoms (see Figure 2B). Figure 2C shows the Kohn-Sham (KS) molecular orbitals (MO), energies and atomic orbital contributions. Furthermore, the main absorption peaks in Figure 2A were assigned to various excitation modes in Figure 2C (see black arrows). For both clusters, peak a is originated from LUMO←HOMO transition. In the electronic structure of Au25 rod, the HOMO and LUMO are mainly composed of electrons on Au 6s and 6p atomic orbitals. In Au12Ag13 rod, however, one may find that the LUMO orbital have significant contributions of the Ag 5s electrons (Figure 2B, C) while LUMO+n is contributed mainly by mixture of Au and Ag atoms. As a result, Ag doping at the central position enlarges the HOMO-LUMO gap and thus leads to the blue-shift of absorption and fluorescence spectra (see Figure 1). Moreover, we find that the occupied orbitals below HOMO-7 are contributed mainly from the component of atomic orbitals of Ag atoms and thiolate ligands (SR). The contributions of atomic orbitals of Au atoms are very small in KS orbitals below HOMO-7. Moreover, only when the waist side is fully occupied by Ag atoms are the clusters highly luminescent according

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to refs. 25 and

43

. These results indicate that the luminescence enhancement in Au12Ag13 rod is

due to the doped Ag atoms at the central position. All these theoretical results obtained here agree with the recently published theoretical investigation on Au12Ag13 rod cluster.43 To further reveal the excited state relaxation dynamics, femtosecond broadband transient absorption experiment was carried out as follows.

Figure 2. (A) Theoretical optical absorption of Au12Ag13 rod and Au25 rod; (B) Electronic density diagrams of HOMO and LUMO orbitals of Au12Ag13 rod; (C) Population analysis of KS orbitals of Au12Ag13 rod and Au25 rod. The Gauss broadening with a width at half-maximum of 0.1 eV is used to fit the optical curve based on the computed excitation energies by TD-DFT. Femtosecond and nanosecond transient absorption measurements. Comparative femtosecond transient absorption (TA) spectra of Au25 rod and Au12Ag13 rod with 11



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excitation of 400 nm at different time delays are shown in Figure 3. For Au25 rod, the TA spectra in the initial 1 ps are comprised of a broad positive excited state absorption (ESA) overlapped with several negative ground state bleaching (GSB) peaks, where the observed GSB peaks agree with the steady state absorption peaks (450 nm, 520 nm, 680 nm). Subsequently, the broad ESA between 475 nm and 600 nm decays rapidly (less than 1 ps) to give rise to two ESA peaks at 480 nm and 610 nm which exist until the end of the measurement (~1 ns). The femtosecond electronic TA spectra of rod Au25 here are consistent with that obtained by Sfeir and coworkers in 2011.19

For Au12Ag13, the excited state dynamics is similar with Au25 rod except for some differences: broad ESA between 500 nm and 650 nm overlapped with GSB at 430 nm, 500 nm and 650 nm decays quickly which is accompanied by rising of ESA around 450 nm and 740 nm. The ESA peaks at 450 and 740nm then remain constant until the end of the measurement (~1 ns). At time long delays (>3 ps), compared with Au25 rod, the ESA around 610 nm in Au25 rod is blue-shifted to 450 nm in Au12Ag13 rod which should arise from the electrons at LUMO contributed mainly by Ag 5s (see Figure 2C). As the electron is finally localized on silver atoms (LUMO), the excited state population should experience ultrafast internal conversion from LUMO+n to the low lying LUMO and subsequent relaxation to the ground state. Therefore, the excited state relaxation of Au12Ag13 rod follows a sequential kinetic model. To further differentiate the excited state dynamics of these two nanoclusters, global fitting procedure combined with singular value decomposition (SVD) is performed.

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Figure 3. Transient absorption spectra at different time delays in Au25 rod (A) and Au12Ag13 rod (B) (pump at 400 nm). Global fitting was performed with a sequential model to yield evolution-associated difference spectra (EADS) (see Figure 4A, B). In the global analysis, EADS is used to determine the minimum number of exponentials needed to fit correctly the data, which suggests that the population decays in a sequential pathway. Before the final relaxation model is proposed, EADS can be used to model the data effectively. Residuals of the fitting are shown in Figure S1 of the Supporting Information. In both clusters, three components (0.82 ps, 52 ps, >>1 ns for Au25 rod; and 0.58 ps, 20.7 ps, >>1 ns for Au12Ag13) are required to obtain the best fit. For Au25 rod, laser excitation at 400 nm populates the cluster into the higher excited states (LUMO+n←HOMO) and the first sub-picosecond component (0.82 ps) is ascribed to internal conversion (LUMO← LUMO+n). Besides the first sub-picosecond and the last long lived component which has been observed previously,19 an additional ~52 ps intermediate spectral component is needed to obtain the best fit. Since the second EADS is similar with the long lived EADS in spectral shape (see

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Figure 4A), the ~52 ps component is attributed to structural nuclear relaxation.

For Au12Ag13, the first EADS has broad ESA decaying in 0.58 ps, which is very similar in lifetime and shape with the first EADS in Au25 rod. In the second and third EADS, however, the ESA around 610 nm disappears and the ESA at 470 nm is shifted to 450 nm compared with that in Au25 rod. According to the quantum chemical calculation described above (Figure 2B), LUMO+n orbitals are contributed by both Au and Ag atoms while the LUMO orbital is mainly composed by Ag atoms at the central vertex site. Therefore, the ultrafast internal conversion in Au12Ag13 leads to the electron localization to the vertex sharing Ag atom. Subsequently, excited state population experiences 20.7 ps nuclear relaxation and a very slow decay back to the ground state. The femtosecond TA measurements were repeated in methanol and no solvent dependent excited state behavior was observed (see Figure S2 A in the SI).

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Figure 4. Evolution associated difference spectra (EADS), concentration of each species, and the corresponding fitting kinetic traces at different wavelengths obtained from global fitting for Au25 rod (A , C, E), and Au12Ag13 rod (B, D, F) clusters. The green traces in C and D represent the 15



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instrument response.

Figure 5. (A) Transient absorption spectrum at 1.0 ns after excitation at 400 nm with 70 fs pulse (red) and at 180 ns after excitation at 355 nm with 8 ns pulse (blue). (B) Decay traces and the corresponding fit at 455 nm (green), 495 nm (red) and 705 nm (blue) of the nanosecond measurements. A global fit to the data gives a lifetime of 7.4 µs. To resolve the very long lived component in Au12Ag13, nanosecond transient absorption measurement with excitation of 355nm was then carried out using nanosecond laser flash photalysis (see Figure 5). It can be seen that the TA spectrum at 180 ns (blue) obtained by ns laser flash photalysis has almost the same spectral feature with that obtained at 1 ns (red) in the femtosecond TA measurement. Additionally, the transient absorption spectra decay shows monoexponential behavior back to the ground state, indicating that no intermediate state is formed during the last long decay. Global fit of the data gives a total lifetime of 7.4 µs for Au12Ag13 rod, which is 3 times longer than that for Au25 rod (2.37 µs).19 Nanosecond transient absorption of Au12Ag13 rod in methanol and n-propanol gave the same lifetime of 7.4 µs (see Figure S3 in SI), suggesting that the long lived state is insensitive to the surrounding solvents.

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Moreover, the long lived lifetime agrees with the lifetime of the luminescence around 720 nm (see Figure S4 in the SI), indicating that less non-radiative relaxations occurred. Using lifetime and quantum yield of the samples, the radiative and nonradiative decay rates have been calculated (details of the calculation can be seen in S6 section of Supporting Information). For Au25 rod, kr=0.42 ms-1, knr=0.42 µs-1, the excited state relaxation is dominated by nonradiative decay. While for Au12Ag13 rod, kr=54 ms-1, knr=81 ms-1, the radiative and nonradiative decay rates are rather close. The prominently longer excited lifetime with less nonradiative relaxation and larger energy gap in Au12Ag13 lead to much stronger and blue shifted fluorescence compared with Au25 rod. On the basis of the analysis discussed above, excited state relaxation scheme of Au25 and Au12Ag13 rod cluster is summarized in Scheme 2.

Scheme 2. Relaxation pathway for Au25 rod cluster (A) and Ag alloying Au12Ag13 cluster (B).

The observed excited state relaxation path of Au12Ag13 is similar to that of Au25 rod, which is

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in contrast to the excited state behavior of Au25(SR)16 clusters.44 For Au25(SR)16 and other thiolate protected Au clusters, the crystal structure is composed of icosahedral gold core protected by S-Au-S-Au-S motif ligands. These Au clusters have their electron density localized on the outer ligands and excess energy could therefore dissipate into the environment easily. The luminescence is thus originated from a manifold of Au-ligand surface states and the emission lifetime is relatively short. For Au25 rod, the electronic excitation is localized on the central Au core and nearly no electrons were relaxed through the surface states, which give rise to its low luminescence quantum yield and solvent independence. Similar to that of Au25 rod, Au12Ag13 rod has no electron density on surface ligands (see Figure 2B). However, when the number of doped Ag atoms in AgxAu25-x reaches 13, crystal structure shows that silver atoms occupy the central waist sides of the biicosahedral Au25 structure (see scheme 1.). The HOMO and LUMO orbitals reside the waist position so that the doping silver strongly modifies the HOMO-LUMO electronic structure. A very recent theoretical investigation reported that when Ag atoms are substituting Au atoms, Ag atom at the central position attracts more positive charge than Au atom,40 which stabilizes the LUMO and enlarges the HOMO-LUMO gap, where the more positive charge after central Au was replaced by Ag leads to the significantly faster nuclear relaxation in Au12Ag13 rod cluster (20.7 ps) compared with that in Au25 rod (52 ps). Furthermore, the rigidity may have been enhanced due to doping of the central Ag atom, which largely increases the fluorescence quantum yield. Ag doping has led to the elongation of the cluster along the vertex axis by ~0.5 Å (the distance between central atom and icosahedral centers has been increased),43 which proved the rigidity enhancement. The analysis mentioned above 18


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indicates that atomic and positional precise HOMO-LUMO engineering could be utilized to modify the electronic structure of ligand protected Au clusters and thus enhancing their luminescence rationally.

Coherent oscillations. Impulsive laser excitation can result in coherent acoustic lattice vibration in both metal nanoparticles and nanoclusters.26,

45-47

When femtosecond laser pulse is short enough, the

acoustic vibration of Au nanoclusters will be observed as coherent oscillations in TA kinetic traces.48 Coherent oscillations have been observed experimentally in ligand protected nanoclusters such as Au20 (with thiolate and phosphine ligands),35, 49 Au25 (both spherical and rod-shaped),19, 44 Au144 and Au309.50 After a careful examination of the kinetic traces, we observed coherent oscillations in both Au25 and Au12Ag13 clusters at almost all wavelengths (see Figure 6). The Fourier transformations on the data gave an oscillation frequency of 26 cm-1 (0.78 THz) in Au12Ag13 and 28 cm-1 (0.84 THz) in Au25 rod (see Figure S5 in Supporting Information) and no higher vibration frequency oscillations were observed here. The oscillations around 0.8 THz agree with the results obtained by Sfeir and coworkers.19 Based on previous investigations, the coherent oscillations of Au nanocluster can be well explained by displacive excitation mechanism, which requires the excitation of breathing vibrations modes without changing the symmetry.51 From the coherent oscillations observed here, it is suggested although Ag alloying gives rise to dramatic differences in electronic structure between rod-shaped Au12Ag13 and Au25 clusters, the low frequency breathing vibration mode still remains the same in Au12Ag13 19



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compared with that in rod Au25.

Figure 6. Coherent oscillations obtained in these two clusters. (A) Selected transient kinetics in rod Au25. (B) Selected transient kinetics in rod Au12Ag13.

Conclusion. In summary, a comparative investigation on excited state dynamics of silver doped and undoped rod-shaped Au25 clusters has been performed. The TA spectral dynamics of silver doped [AgxAu25-x(PPh3)10(SC2H4Ph)5Cl2]2+ (x≤13) clusters can be described by an ultrafast internal conversion (0.58 ps) and subsequent nuclear relaxation (20.8 ps) followed by a very slow electron relaxation (7.4 µs) back to the ground state, where the nuclear relaxation in Ag doped cluster (20.7 ps) is remarkably faster than that in un-doped Au25 rod cluster (52 ps). It is the central Ag atom which stabilizes the charges on LUMO orbital and enhances the rigidity of AgxAu25-x cluster that leads to its strong fluorescence. Moreover, coherent oscillations around 0.8 THz observed in both clusters indicates that the symmetry was remained after Ag doping on the rod shaped Au25 clusters despite the great perturbation on the electronic structure. These results

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are of value to further understanding and rational design of the luminescent silver alloying gold nanoclusters. Acknowledgement This work was supported by the 973 Program (2013CB834604), NSFCs (21173235, 21127003, 21333012, 21373176, 21422305 and 21373232), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12020200), and Scientific Research Fund of Hunan Provincial Education Department (13A100). Supporting Information Available: The details of transient absorption spectroscopy, data analysis, femtosecond results in methanol, nanosecond results in methanol and propanol as well as the oscillatory frequency analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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TOC Graphic

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Ultrafast Relaxation Dynamics of Luminescent Rod-Shaped, Silver

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