Characterization of Emissive States for Structurally Precise Au25

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Article

Characterization of Emissive States for Structurally Precise Au (SCH) Monolayer-Protected Gold Nanoclusters Using Magnetophotoluminescence Spectroscopy 25

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Thomas D. Green, Patrick J. Herbert, Chongyue Yi, Chenjie Zeng, Stephen A. McGill, Rongchao Jin, and Kenneth L. Knappenberger J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05349 • Publication Date (Web): 14 Jul 2016 Downloaded from http://pubs.acs.org on July 18, 2016

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The Journal of Physical Chemistry

Characterization of Emissive States for Structurally Precise Au25(SC8H9)180 Monolayer-Protected Gold Nanoclusters Using Magnetophotoluminescence Spectroscopy Thomas D. Green, † Patrick J. Herbert, † Chongyue Yi, † Chenjie Zeng, ‡ Stephen McGill, § Rongchao Jin, ‡ and Kenneth L. Knappenberger, Jr.†* †

Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306-

3490 ‡

Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213

§

National High Magnetic Field Laboratory, Tallahassee, FL 32310

Abstract Electronic relaxation dynamics and near infrared emission of structurally-precise Au25(SC8H9)180 MPCs were studied using energy-resolved and time-resolved magneto-photoluminescence (MPL) spectroscopy. Measurements were carried out at sample temperatures spanning the range of 4.5 K to 20 K following 3.1 eV laser excitation. These measurements revealed two main PL peaks detected at 1.78 eV and 1.98 eV. The emissive states giving rise to these peaks were characterized using magnetic circular polarized photoluminescence (MCPL) spectra, which were

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obtained from energy-resolved PL collected at positive and negative field polarities. Analysis of MCPL magnetization data yielded a Lande g-factor of g=1.05±0.04 for the 1.98 eV peak and g=1.7±0.1for the 1.78 eV peak. The g-factor of the 1.78 eV peak suggested emission from a quartet state, which represents a high-spin configuration for this system. The time-resolved MPL data were fit to a biexpenoential decay function that included a stretch parameter. Arrhenius analysis of the 4.5 K field-dependent rate data identified an energy barrier of 0.66±0.04 meV, which was interpreted as the energy gap separating dark and bright fine structure components of the manifold of nanocluster emissive states. The temperature dependence of this energy barrier was attributed to thermal population of the upper state, reducing the effect of field-induced mixing. These data provide new insight into the optical properties of structurally precise, condensed-phase metal clusters.

1. Introduction

Structurally-precise, monolayer-protected gold clusters (MPCs) exhibit electronic and optical properties that are distinct from larger plasmon-supporting nanoparticles.1-15 For example, MPCs smaller than approximately 2.0 nm exhibit discrete extinction spectra and non-zero HOMO-LUMO electronic energy gaps .16-17 These energy gaps, which result from quantumconfinement effects, lead to unusual optical properties, including photoluminescence (PL), and cluster-specific nonlinear optical responses.2,

5, 12, 18-23

In combination, the inherent structural

specificity and optical properties position MPCs as an attractive platform for developing tailored photonic materials. The Au25(SR)18 cluster, where SR denotes an alkanethiol, has emerged as a prototype for understanding structure-dependent photonic properties of MPCs. Similar to other MPCs,


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Au25(SR)18 consists of three structural motifs: (i) thirteen-atom icosahedral gold core; (ii) six Au(I)2S3 “semi-ring” units with alternating Au(I)-S bonds that protect the gold core; and (iii) alkanethiol ligands and, in some cases, counter ions that facilitate solution-phase dispersion. Electronic structure calculations for the Au25(SR)18 MPC assign the HOMO and LUMO electronic states to superatomic orbitals delocalized over the Au135+ core.24-25 Whetten and coworkers attribute gold cluster luminescence to radiative recombination by metal-based electrons.26 Conversely, the PL from these systems has been observed to be sensitive to ligand states, suggesting that the passivating semi-ring units play an important role.20 This claim has been bolstered by ultrafast time-resolved spectroscopic studies, which indicate that energy transfer to the semi-ring states plays an important role in the relaxation dynamics of these systems.

2, 18, 27-29

Indeed, recent research suggests that many factors including cluster oxidation

state, passivating ligand, and cluster size can all influence MPC emission energy and yield.30 Our temperature-dependent photoluminescence work indicates multiple vibrationally mediated relaxation channels influence the emission yields and rates of gold MPCs.31 The complexity of these data, which include detection of multiple MPC emission channels,29,31 indicate statespecific descriptions of nanocluster electronic relaxation dynamics will be necessary in order to fully appreciate the luminescent properties of sub-to-few nanometer metal domains. Recently, femtosecond time-resolved 2-D electronic spectroscopy has been used to isolate superatom-statespecific electronic relaxation following intraband excitation.32 Despite these recent advances in the understanding of MPC electronic relaxation dynamics, a detailed understanding of the PL mechanism for these clusters has not emerged. In this manuscript, energy- and time-resolved PL studies of Au25(PET)180, where PET represents phenyl ethanethiol (SC8H9), carried out at cryogenic temperatures and in the presence


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of high magnetic fields are presented in an effort to advance the current understanding of the photophysics and electronic relaxation dynamics of MPCs. These data provide evidence that MPC emission proceeds from multiple distinguishable electronic states, including high-spin superatomic configurations. This manifold of emissive states consists of radiatively bright and dark states separated by small (approximately 1 meV) energy gaps. Further, analysis of polarization-resolved magneto-photoluminescence (MPL) data indicate the Lande g-factors for the emissive states are consistent with expectations based on radiative recombination between the superatomic D and P states.

2. Materials and Methods

The synthetic protocol for Au25(PET)18 is reported in the literature.15,

33

Samples were

prepared for study by dispersing Au25(PET)18 clusters in a 25% solution of polystyrene in toluene. The Au25(PET)18 concentration was 650 µM. The solution was dropcast on a quartz coverslip and dried in a vacuum desiccator. Absorption spectra taken from the films showed no signs of nanocluster aggregation. Next, the substrate-supported sample was affixed to polarization film. The sample, polarization film, heater, and probe were all mounted to a cryogenically cooled oxygen-free copper block. The sample temperature was estimated to vary by less than one percent over the entire laser interaction volume. The magnetic field homogeneity over one centimeter was 0.1%, which provided uniform applied field strengths over the entire laser interaction region. PL data were recorded over the energy range of 1.35 eV to 2.15 eV. Owing to limitations imposed by the polarization optics, analysis of energy-resolved data was performed over the energy range of 1.55 eV to 2.15 eV.


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Electronic excitation was achieved using the 400 nm frequency doubled output of a 1 kHz regeneratively amplified Ti:Sapphire laser system producing laser pulses approximately 100 fs in duration centered at 800 nm. The 400 nm pulses were attenuated to 1 µJ/pulse and aligned into the sample space of the magnet. The resulting photoluminescence was collected through an optical fiber in transmission geometry. Long pass filters were used to isolate the PL signal from residual excitation light. Time-integrated energy-resolved measurements were carried out using a 0.75 m spectrometer (McPherson) coupled to a liquid nitrogen-cooled CCD detector (Princeton Instruments). Time-resolved measurements were performed using an avalanche photodiode (Quantique) coupled to a time-correlated single photon counting unit. (Becker & Hickl). The quantum efficiency of the avalanche photodiode is approximately 35% over most of the visible range, but decreases to less than 3% for photon energies less than 1.6 eV. Therefore, timedomain analysis was restricted primarily to PL emission energies exceeding 1.6 eV. The instrument response for the photon counting unit, under the experimental conditions, was 30 ns. In the presence of a magnetic field, degenerate states split according to their angular momentum projection along the field axis. Due to selection rules, the transitions from these fine structure states emit either right circularly polarized (RCP) or left circularly polarized (LCP) light. A polarization film consisting of a quarter-wave plate layered on top of a linear polarizer was used to selectively collect PL of one polarization state. When the RCP and LCP PL encounter the quarter-wave plate, they are transformed into plane-polarized light with orthogonal polarization axes. The linear polarizer is oriented such that only one of these polarization states is transmitted. Therefore, the PL that reaches the detector originated from a specific subset of fine structure states - either those shifted to lower energy than the zero-field energy or those shifted to higher energy. When the polarity of the magnetic field is reversed, the splitting of fine


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structure states is inverted, effectively allowing the detection of PL from the other subset of states. Thus, the magnetic circularly polarized luminescence (MCPL) spectrum was constructed by taking the difference between the spectrally-resolved PL data collected at positive field polarity and negative field polarity. For example, the MCPL spectrum at 17.5 T represents the difference in the PL data collected at +17.5 T and –17.5 T. All magnetic-field-dependent PL results were reproducible over field cycling, and no signs of sample degradation were observed in either absorption or PL emission data. 3. Results and discussion

3.1 Energy-Resolved Measurements The 4.5 K PL spectrum of Au25(PET)18 at +17.5 T and –17.5 T is shown in figure 1.A. The maximum intensity was measured at approximately 1.6 eV emission energy, with two shoulders measured at energies greater than 1.7 eV. The “notch” in the spectrum at 1.71 eV was attributed to a known absorption peak of the optical fiber used to guide the signal photons from the excitation/collection volume to the spectrometer for detection.34 The high-energy portion (energies > 1.6 eV) of the spectrum was sensitive to both the magnitude and polarity of the applied magnetic field. This field sensitivity was observable owing to polarization-resolved detection of the PL. We note that the quarter-wave plate used for these experiments is not reliable for near infrared experiments. The low-energy portion of the PL emission will be examined in future research. In the presence of an applied magnetic field, electronic states undergo Zeeman splitting.


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Figure 1. A) The 4.5 K polarization-resolved PL spectrum of neutral Au25(PET)18 collected at +17.5 T (blue line) and –17.5 T (black line). The data reflect the sensitivity of the PL intensity to field polarity. The spectra were normalized at approximately 1.6 eV in order to emphasize the sensitivity of the high-energy portion of the emission spectrum to applied magnetic fields. B) General energy level diagram depicting Zeeman splitting of the emissive states in the presence of an applied field. The diagram assumes normal spin-orbit coupling for both excited and ground electronic states. Reversing the field polarity inverts the splitting of the fine structure states. C) MCPL spectra calculated from the difference in PL spectra at positive and negative field polarity for selected field magnitudes at 4.5 K sample temperature.


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Due to selection rules, the emission from these Zeeman-split fine structure states will be either right or left circularly polarized (RCP or LCP). Polarization optics provided the ability to isolate one of these polarization states for detection (figure 1.B, black arrows), while the other polarization state was extinguished (gray arrows). Reversing the polarity of the field has the effect of inverting the Zeeman splitting pattern of the fine structure states. Therefore, the collection of PL spectra with positive and negative field polarity using this detection scheme is equivalent to collecting RCP and LCP PL spectra. As such, the magnetic circularly polarized luminescence (MCPL) was constructed by taking the difference between the PL spectra collected at positive and negative field polarities (i.e. ∆I = Ircp – Ilcp = I+ – I–) (figure 1.C). The MCPL spectrum reflected the sensitivity of the PL spectrum to the field polarity and magnitude. Both the global PL and the individual component peaks that made up the spectrum were sensitive to the magnitude of the applied field; both increased in intensity as the field strength was increased from 2.5 T to 17.5 T. As the applied field strength was raised above 10 T, the differential MCPL intensity increase was reduced, which indicated the field-induced energy separation of the Zeeman-split emissive fine structure states was comparable the thermal energy (i.e. ∆E ≈ kBT). In order to describe the field-dependent PL with state specificity, the MCPL spectra obtained at each field strength were first fit to Gaussian peaks, which were then analyzed individually. The fit of the 17.5 T MCPL spectrum collected at 4.5 K is presented in Figure 2A. The best fit of the spectrum contained two positive amplitude Gaussian peaks centered at 1.78 eV and 1.98 eV, as well as one negative amplitude Gaussian peak centered at 1.58 eV. A small amplitude negative Gaussian peak was also included at 1.71 eV to account for the known instrumental artifact at that energy. The energies of the 1.58 eV and 1.78 eV peaks were in good agreement with peaks previously observed in temperature-dependent PL experiments (the 1.98


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eV peak was outside the energy range investigated in those experiments).31 The approximate 200 meV energy separation between PL peaks reported here is in good agreement with computational predictions for ligand field splitting of the superatom D states of the Au25(SR)18 MPC.24 These three signal peaks were used to fit MCPL spectra collected at a sample temperature of 15 K (figure 2b).

Figure 2. A) Best fit of 17.5 T MCPL spectra collected at 4.5 K contained three signal peaks centered at 1.58, 1.78 and 1.98 eV. B) Fit of 17.5 T MCPL spectrum collected at 15 K using the same three signal peaks. At 15 K the peaks were blueshifted slightly.

The energy of the peaks was shifted to higher energy with respect to the 4.5 K data. This effect was consistent with temperature-dependent PL experiments previously reported, which showed a temperature-dependent blue shift in the PL peak energies for increasing sample temperature up to 40 K.31 At 15 K, the MCPL intensity of the 1.78 eV and 1.98 eV was reduced by 75% and 60%, respectively, while the intensity of the 1.58 eV peak increased four-fold compared to the 4.5 K data. The decrease in intensity of the high-energy peaks with increasing sample temperature was suggestive of a Faraday C-term for generating the magnetic-fielddependent optical response.35 If the fine structure of the emitting state reaches a thermal quasi-


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equilibrium before radiative relaxation, the increase in temperature causes increased population in the upper fine structure state, leading to a decrease in the intensity of the difference spectrum. The presence of a Faraday C-term is consistent with expectations for the Au25(SC8H9)180 cluster, which is known to be paramagnetic; the C-term typically outweighs other Faraday terms for paramagnetic species. The observed increase in intensity of the 1.58 eV peak at the higher sample temperature indicated possible contributions from additional Faraday terms governed the field-dependent response of the cluster. The MCPL magnetization curves for the 1.78 eV and 1.98 eV peaks are depicted in figure 3. The 1.58 eV peak intensity was too weak at 4.5 K to reliably analyze. For the 1.78 eV peak, the peak intensity at 4.5 K increased with field up to approximately µBB/2kBT = 0.75, above which the effect began to saturate. At a sample temperature of 15 K, the data did not exhibit this saturation behavior. This was also observed in the data for the 1.98 eV peak, although the onset of saturation occurred at a higher field than for the 1.78 eV peak. This could have been the result of a larger Lande g-factor for the 1.78 eV peak, which would lead to more rapid saturation in the magnetization curve. In order to quantify the Lande g-factor, which is a representation of the total angular momentum of the emitting state, the MCPL intensity data, ∆I, were fit to the equation36

∆ = tanh

(1)

where Asat is the saturation limit, µB is the Bohr magneton, B is the applied field, kB is the Boltzmann constant, and T is the sample temperature. The fit yielded a value of g = 1.7 ± 0.1 for the 1.78 eV peak and g = 1.05 ± 0.04 for the 1.98 eV peak. We note that the Au25(SC8H9)180 cluster is a seven-electron open-shell superatom P system, and therefore S = ½, 3/2, leading to


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doublet and quartet terms, were expected. While an unambiguous assignment of the term symbols of the emitting states based on these g-factors will require further computational treatment of the states involved, g = 1.7 is inconsistent with what would be predicted for a doublet term. The measured g value is, however consistent with the theoretical g-factor for a quartet term. This observation is consistent with results of previous femtosecond transient absorption experiments, where an excited state decay process with a lifetime of several hundred picoseconds was attributed to intersystem crossing (ISC) to form a high-spin excited state.18

Figure 3. Magnetization curves for the A) 1.78 eV and B) 1.98 eV MCPL peaks at 4.5 K (black circles) and 15 K (red circles). The data were fit to the equation described in the text (solid lines).

3.2 Time-Resolved Measurements In order to provide a deeper understanding of the electronic relaxation mechanisms of Au25(PET)18, magnetic field-dependent, time-resolved PL measurements were carried out. The time-resolved data were obtained by detecting the PL signal over the entire visible PL band, and extending to 1.6 eV. The resulting data are given in figure 4.A. At 4.5 K the data exhibited strong field dependence, decaying at a faster rate as the field strength was increased from 0 to


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17.5 T. The data were fit to a biexponential decay function. A stretch parameter was required for the longer lifetime component to accurately fit the experimental data. Fitting of the 4.5 K data at zero field yielded τ1 = 51.3 ± 0.7 µs and τ2 = 158 ± 8 µs. Both τ1 and τ2 were found to decrease as the field strength was increased. At +17.5 T the lifetimes were determined to be τ1 = 27.8 ± 0.4 µs and τ1 = 71 ± 1 µs. At temperatures greater than 4.5 K the influence of the magnetic field on the lifetimes was reduced (Figure 4.B). At 10 K the lifetimes were determined to be τ1 = 45.4 ± 0.5 µs and

Figure 4. A) Time-resolved PL data obtained at 4.5 K at selected applied field strengths. As the field is increased, the PL decay rate increased. B) Time-resolved PL data collected at 0 T (red lines) and +17.5 T (black lines) at selected temperatures. At temperatures greater than 4.5 K, the PL decay rate was less sensitive to the applied field.

τ2 = 84 ± 1 µs at zero field and τ1 = 19.5 ± 0.5 µs and τ2 = 52 ± 1 µs at 17.5 T. At 20 K, the time constants at zero field and 17.5 T were found to be the same within experimental error, τ1 = 17.6 ± 0.3 µs and τ2 = 45 ± 1 µs. The stretch parameter ranged from 0.76 to 0.82 over the field and temperature range investigated here. Two observations were made based on these data: 1) The zero-field PL decay rate increased as the temperature was increased from 4.5 K to 20 K, and 2)


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the sensitivity of the PL decay rate to the magnetic field was dependent on the sample temperature. At 4.5 K the time-resolved PL signal collected at +17.5 T decayed markedly more rapidly than the signal collected at zero field. At 10 K this effect was significantly reduced, and by 20 K no resolvable difference in the time-resolved PL data was observed. This behavior was indicative of a very small energy barrier between the emitting states.37 Due to their approximately spherical morphology, the clusters were isotropically oriented in the polymer film, which allowed quantification of both the Lande g factor through magnetic-field-induced Zeeman-type splitting and the energy separation between fine structure states via field-induced state mixing; the latter is exploited to quantify the energy separation of emissive fine structure states. Table 1. Au25(PET)180 Energy Barriers Obtained from Arrhenius Analysis Temperature (K)

Ea from k1 (meV)

Ea from k2 (meV)

4.5

0.61 ± 0.13

0.66 ± 0.04

10

0.52 ± 0.09

0.44 ± 0.07

12

0.20 ± 0.03

0.25 ± 0.03

In order to quantify the energy barrier responsible for the field-dependent dynamics, an Arrhenius analysis was performed using the PL time constants obtained from fitting the timeresolved data (figure 5). At each temperature the available energy was calculated by accounting for both the thermal energy (kBT) as well as the energy of the magnetic field (µBB). The sum of these energies was converted to an effective sample temperature, which was used for the


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Arrhenius analysis. For the 4.5-K data, only data collected at magnetic fields greater than 2.5 T were included in the analysis, as the data corresponding to 2.5 T and zero field did not follow an Arrhenius type behavior. The results of the Arrhenius analysis are summarized in Table 1. Both rates yielded very similar energy barriers that decreased with increasing temperature. For two emitting states separated by a small energy gap (∆EBD), the field can mix the lowest energy excited states with more energetic, but nearly degenerate, ones. At sufficiently low sample temperatures such that kT < ∆EBD, this field-induced mixing has the effect of relaxing radiative spectroscopic selection rules, allowing the so-called “dark states” to emit without coupling to vibrational modes.


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Figure 5. Arrhenius analysis of the slower PL decay rate data collected at 4.5 K (black circles), 10 K (red circles) and 12 K (blue circles). The inset shows the same analysis for the faster PL decay rate data. Both analyses yielded comparable energy barriers.

This has the effect of making radiative relaxation from the “dark” state more efficient, leading to a shorter lifetime. As temperature is increased, the thermal energy becomes great enough to begin to populate the upper state, allowing the bright state to emit even when no field is applied. Therefore, when the field mixes the states, the effect of the field on the lifetime is less pronounced.

Once the temperature is increased so that kT > ∆EBD, the lifetime becomes

independent of applied field. The field-dependent data presented here imply that electronvibration (including phonon) coupling is an important factor that mediates radiative recombination from electronically excited MPCs. This finding is consistent with our previously published vibrational mode analysis that implicated Au-Au and Au-S stretching for mediating the radiative relaxation.31 Taken together, the field-dependent state mixing and g-factor determination point to the importance of vibrationally mediated radiative recombination by cluster-core-localized superatom electronic states in the mechanism of Au25(PET)180 PL. Future research is needed to examine the influence of passivating ligands on the properties of the emissive states in order to understand their role in MPC radiative relaxation. 4. Conclusion In summary, energy- and time-resolved magnetophotoluminescence measurements were used to understand the PL properties of the Au25(SC8H9)180 monolayer-protected cluster. From the MCPL data, two peaks centered at 1.98 eV and 1.78 eV were identified with g-factors of 1.05 ± 0.04 and 1.7 ± 0.1, respectively. The latter g-factor was found to be consistent with emission from a quartet state, although additional computational work is required for a definitive


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assignment of the electronic structure of the emissive states. These findings support our previous femtosecond transient spectroscopy results that revealed a vibrationally mediated picosecond intersystem crossing process that preceded emission from a high-spin state, which occurred on the microsecond time scale. An Arrhenius analysis of the field-dependent PL lifetime data indicated field-induced mixing of dark and bright states, which led to the observation of shorter lifetimes at 17.5 T than the lifetime at zero field. The field-dependent lifetimes were attributed to the reduced dependence on vibrationally (or phonon)-assisted PL emission from dark states at temperatures greater than 4.5 K. Taken together, the data presented in the current study underscore the importance of electronic spin conversion and vibrationally/phonon assisted dynamics for determining the optical properties of structurally precise MPCs. These parameters will be important considerations for achieving structural control over the optical properties of cluster-based photonic materials. AUTHOR INFORMATION Corresponding Author *[email protected] Phone: +1-850-645-8617 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by an award from the National Science Foundation to K. L. K., under grant number CHE-1150249 and a grant from the Air Force Office of Scientific Research under award

REFERENCES

1.

Bongiorno, A.; Landman, U. Water-Enhanced Catalysis of Co Oxidation on Free and Supported Gold Nanoclusters. Phys. Rev. Lett. 2005, 95, 106102.

2.

Devadas, M. S.; Kim, J.; Sinn, E.; Lee, D.; Goodson, T.; Ramakrishna, G. Unique Ultrafast Visible Luminescence in Monolayer-Protected Au25 Clusters. J. Phys. Chem. C 2010, 114, 22417-22423.

3.

Huang, T.; Murray, R. W. Visible Luminescence of Water-Soluble Monolayer-Protected Gold Clusters. J. Phys. Chem. B 2001, 105, 12498-12502.

4.

Kogo, A.; Sakai, N.; Tatsuma, T. Photocatalysis of Au25-Modified Tio2 Under Visible and near Infrared Light. Electrochem. Commun. 2010, 12, 996-999.

5.

Link, S.; Beeby, A.; FitzGerald, S.; El-Sayed, M. A.; Schaaff, T. G.; Whetten, R. L. Visible to Infrared Luminescence from a 28-Atom Gold Cluster. J. Phys. Chem. B 2002, 106, 3410-3415.

6.

Link, S.; El-Sayed, M. A.; Gregory Schaaff, T.; Whetten, R. L. Transition from Nanoparticle to Molecular Behavior: A Femtosecond Transient Absorption Study of a Size-Selected 28 Atom Gold Cluster. Chem. Phys. Lett. 2002, 356, 240-246.

7.

Mustalahti, S.; Myllyperkiö, P.; Malola, S.; Lahtinen, T.; Salorinne, K.; Koivisto, J.; Häkkinen, H.; Pettersson, M. Molecule-Like Photodynamics of Au102(Pmba)44 Nanocluster. ACS Nano 2015, 9, 2328-2335.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

8.

Page 18 of 23

Negishi, Y.; Takasugi, Y.; Sato, S.; Yao, H.; Kimura, K.; Tsukuda, T. Magic-Numbered Aun Clusters Protected by Glutathione Monolayers (N = 18, 21, 25, 28, 32, 39): Isolation and Spectroscopic Characterization. J. Amer. Chem. Soc. 2004, 126, 6518-6519.

9.

Polavarapu, L.; Manna, M.; Xu, Q.-H. Biocompatible Glutathione Capped Gold Clusters as One- and Two-Photon Excitation Fluorescence Contrast Agents for Live Cells Imaging. Nanoscale 2011, 3, 429-434.

10. Ramakrishna, G.; Varnavski, O.; Kim, J.; Lee, D.; Goodson, T. Quantum-Sized Gold Clusters as Efficient Two-Photon Absorbers. J. Amer. Chem. Soc. 2008, 130, 5032-5033. 11. Varnavski, O.; Ramakrishna, G.; Kim, J.; Lee, D.; Goodson, T. Optically Excited Acoustic Vibrations in Quantum-Sized Monolayer-Protected Gold Clusters. ACS Nano 2010, 4, 3406-3412. 12. Wijngaarden, J. T. v.; Toikkanen, O.; Liljeroth, P.; Quinn, B. M.; Meijerink, A. Temperature-Dependent Emission of Monolayer-Protected Au38 Clusters. J. Phys. Chem. C 2010, 114, 16025-16028. 13. Yoon, B.; Häkkinen, H.; Landman, U. Interaction of O2 with Gold Clusters: Molecular and Dissociative Adsorption. J. Phys. Chem. A 2003, 107, 4066-4071. 14. Zhu, M.; Aikens, C. M.; Hollander, F. J.; Schatz, G. C.; Jin, R. Correlating the Crystal Structure of a Thiol-Protected Au25 Cluster and Optical Properties. J. Amer. Chem. Soc. 2008, 130, 5883-5885. 15. Zhu, M.; Eckenhoff, W. T.; Pintauer, T.; Jin, R. Conversion of Anionic [Au25(Sch2ch2ph)18]− Cluster to Charge Neutral Cluster Via Air Oxidation. J. Phys. Chem. C 2008, 112, 14221-14224. 16. Jin, R., Quantum Sized, Thiolate-Protected Gold Nanoclusters. Nanoscale 2010, 2, 343362.


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Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17. Yi, C.; Zheng, H.; Tvedte, L. M.; Ackerson, C. J.; Knappenberger, K. L. Nanometals: Identifying the Onset of Metallic Relaxation Dynamics in Monolayer-Protected Gold Clusters Using Femtosecond Spectroscopy. J. Phys. Chem. C 2015, 119, 6307-6313. 18. Green, T. D.; Knappenberger, K. L., Jr. Relaxation Dynamics of Au25l18 Nanoclusters Studied by Femtosecond Time-Resolved near Infrared Transient Absorption Spectroscopy. Nanoscale 2012, 4, 4111-4118. 19. Wang, G.; Huang, T.; Murray, R. W.; Menard, L.; Nuzzo, R. G. Near-Ir Luminescence of Monolayer-Protected Metal Clusters. J. Amer. Chem. Soc. 2005, 127, 812-813. 20. Wu, Z.; Jin, R. On the Ligand’s Role in the Fluorescence of Gold Nanoclusters. Nano Lett. 2010, 10, 2568-2573. 21. Zheng, J.; Petty, J. T.; Dickson, R. M. High Quantum Yield Blue Emission from WaterSoluble Au8 Nanodots. J. Amer. Chem. Soc. 2003, 125, 7780-7781. 22. Zheng, J.; Zhou, C.; Yu, M.; Liu, J. Different Sized Luminescent Gold Nanoparticles. Nanoscale 2012, 4, 4073-4083. 23. Knoppe, S.; Häkkinen, H.; Verbiest, T. Nonlinear Optical Properties of ThiolateProtected Gold Clusters: A Theoretical Survey of the First Hyperpolarizabilities. J. Phys. Chem. C 2015, 119, 27676-27682. 24. Aikens, C. M. Geometric and Electronic Structure of Au25(Sphx)18− (X = H, F, Cl, Br, Ch3, and Och3). J. Phys. Chem. Lett. 2010, 1, 2594-2599. 25. Aikens, C. M. Electronic Structure of Ligand-Passivated Gold and Silver Nanoclusters. J. Phys. Chem. Lett. 2011, 2, 99-104. 26. Bigioni, T. P.; Whetten, R. L.; Dag, Ö., Near-Infrared Luminescence from Small Gold Nanocrystals. J. Phys. Chem. B 2000, 104, 6983-6986.


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Page 20 of 23

27. Miller, S. A.; Womick, J. M.; Parker, J. F.; Murray, R. W.; Moran, A. M. Femtosecond Relaxation Dynamics of Au25l18− Monolayer-Protected Clusters. J. Phys. Chem. C 2009, 113, 9440-9444. 28. Qian, H.; Y. Sfeir, M.; Jin, R., Ultrafast Relaxation Dynamics of [Au25(Sr)18]Q Nanoclusters: Effects of Charge State. J. Phys. Chem. C 2010, 114, 19935-19940. 29. Wen, X.; Yu, P.; Toh, Y. R.; Tang, J. Structure-Correlated Dual Fluorescent Bands in BSA-Protected Au25 Nanoclusters. J. Phys. Chem. C 2012, 21, 11830-11836. 30. Zheng, Z.; Zhou, Z.; Yu, M.; Liu, J. Different Sized Luminescent Gold Nanoparticles. Nanoscale 2012, 21, 4073-4083. 31. Green, T. D.; Yi, C.; Zeng, C.; Jin, R.; McGill, S.; Knappenberger, K. L., Jr., Temperature-Dependent Photoluminescence of Structurally-Precise Quantum-Confined Au25(Sc8h9)18 and Au38(Sc12h25)24 Metal Nanoparticles. J. Phys. Chem. A 2014, 118, 10611-10621. 32.

Stoll, T.; Sgro, E.; Jarrett, J. W.; Rehault, J.; Oriana, A.; Sala, L.; Branchi, F.; Cerullo, G.; Knappenberger, K. L. Jr. Superatom State-Resolved Dynamics of the Au25(SC8H9)18- Cluster from Two-Dimensional Electronic Spectroscopy. J. Amer. Chem. Soc. 2016, 138, 1788-1791.

33. Zhu, M.; Lanni, E.; Garg, N.; Bier, M. E.; Jin, R. Kinetically Controlled, High-Yield Synthesis of Au25 Clusters. J. Amer. Chem. Soc. 2008, 130, 1138-1139. 34. The Book on the Technologies of Polymicro. http://issuu.com/molx/docs/polymicrotechnologies. (accessed April 21st, 2016) 35. Phiepho, S., B.; Schatz, P., N., Croup Theory in Spectroscopy with Applications to Magnetic Circular Dichroism; John Wiley and Sons: New York, 1983. 36. Neese, F.; Solomon, E. I., Mcd C-Term Signs, Saturation Behavior, and Determination of Band Polarizationsin Randomly Oriented Systems with Spin S ≥ 1/2. Applications to S = 1/2 and S = 5/2. Inorg. Chem. 1999, 38, 1847-1865.


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37. Knowles, K. E.; Nelson, H. D.; Kilburn, T. B.; Gamelin, D. R., Singlet–Triplet Splittings in the Luminescent Excited States of Colloidal Cu+:Cdse, Cu+:Inp, and Cuins2 Nanocrystals: Charge-Transfer Configurations and Self-Trapped Excitons. J. Amer. Chem. Soc. 2015, 137, 13138-13147.


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Table of content


Characterization of Emissive States for Structurally Precise Au25

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