Ligand- and Solvent-Dependent Electronic Relaxation Dynamics of

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Ligand- and Solvent-Dependent Electronic Relaxation Dynamics of Au (SR) Monolayer-Protected Clusters 25

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Chongyue Yi, Hongjun Zheng, Patrick J. Herbert, Yuxiang Chen, Rongchao Jin, and Kenneth L. Knappenberger J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09347 • Publication Date (Web): 20 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017

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The Journal of Physical Chemistry

Ligand- and Solvent-Dependent Electronic Relaxation Dynamics of Au25(SR)18- Monolayer-Protected Clusters Chongyue Yi,a Hongjun Zheng,b Patrick J. Herbert,b Yuxiang Chen,c Rongchao Jin,c and Kenneth L. Knappenberger, Jr.b* a

Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32310 Department of Chemistry, The Pennsylvania State University, University Park, PA 16802 c Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213 b

ABSTRACT: Electronic relaxation dynamics of Au25(PET)18-1 and Au25(PET*)18-1 monolayer-protected clusters (MPCs) were examined using femtosecond time-resolved transient absorption spectroscopy (fsTA). The use of two different excitation wavelengths (400 nm and 800 nm) allowed for quantification of state-resolved and ligand-dependent carrier dynamics for gold MPCs. Specifically, one-photon 400-nm (3.1 eV) and two-photon 800-nm (1.55 eV) interband excitation promoted electrons from the MPC ligand band into gold superatom D states. Following rapid internal conversion, carriers generated by interband excitation exhibited picosecond relaxation dynamics that depended upon both ligand structure and the dielectric of the dispersing medium. These solvent- and ligand-dependent effects were attributed to charge-transfer processes mediated by the manifold of ligand-based states. In contrast, one-photon intraband (gold sp-sp) excitation by 800-nm light resulted in solvent- and ligand-independent relaxation dynamics. The observed solvent independences of these data were attributed to internal relaxatin via Superatom P and D states localized to the MPC core. Effectively, these core-based transitions were screened from dielectric influences of the dispersing medium by the MPC gold-thiolate protecting units. Additionally, a low frequency (2.4 THz) modulation of TA signal amplitude was detected following intraband excitation. The 2.4 THz mode was consistent with Au-Au expansion in the MPC core. Based on these data, we conclude that intraband relaxation among the MPC Superatom states is mediated by low-frequency vibrations of the gold core. Structurespecific and state-resolved descriptions of MPC electron dynamics are necessary for integration of metal clusters as functional components in photonic materials.

1. Introduction Ultrasmall (< 2 nm), quantum-confined, metal clusters hold great potential for light-harvesting and energy-conversion applications including photocatalysis1-5, optical sensing6-8, medical therapy,9-10 and as catalytic sites for chemical reactions11-13. The electronic properties of these systems, including near infrared (NIR) photoluminescence (PL) emission, motivate many of these applications.14-17 In contrast to the volumescalable electronic and optical properties of larger metal nanoparticles, metal clusters exhibit size-independent effects that are determined by domain structure. Therefore, structurefunction correlations are critical for developing predictive descriptions of metal cluster functional properties. Monolayerprotected metal clusters (MPCs) can be synthesized with atomic-level precision.18-21 One of the most widely studied MPC systems is the Au25(SR)18 cluster, where SR represents an alkanethiol.22 Through X-ray crystallographic studies the cluster’s structure has been determined; a central thirteengold-atom icosahedron is protected by an organometallic layer, which is further encapsulated by alkane thiols.23-25 Advances in synthesis and alkane-thiol ligand exchange permit isolation and dispersion in both organic and aqueous phases.26-27 In order to describe the electronic structure of Au25(SR)18, a superatom model has been proposed, being adapted from gasphase experiments on bare metal clusters.28-30 On the basis of this model, the valence electrons of the metal atoms are treated as being delocalized over a spherical potential constrained to

the nuclei of the core. While also accounting for the oxidation state and electron-withdrawing nature of the ligands, the electrons are added into “superatomic” orbitals, analogous to the Aufbau building-up principle of atomic systems. Electron configurations of closed-shell superatomic orbitals result in stable systems. For Au25(SR)18-1 MPCs, it follows that there is a total of twenty-five 6s electrons contributed by gold atoms, eighteen electron-withdrawing ligands and a charge state of -1, giving a total of eight valence electrons. This leads to a closedshell superatomic P-orbital configuration, 1S21P6. While this model has proven effective in predicting cluster stability, it also serves as the basis for computationally modelling the electronic structure and dynamics. In this case, using the superatom description of Au25(SR)18 clusters, low-energy optical absorption features (less than 2.5 eV excitation energy) correspond to intraband (sp) transitions from superatom P to D electronic states (HOMO-LUMO), which are localized to the thirteen-gold-atom core.23 In contrast, higher energy interband (d→sp) electronic transitions (greater than 2.5 eV) are attributed to excitation from states localized to the surface thiolates and Au-SR units.18, 24, 30-32 Although signiﬁcant progress has been made on synthetic methods, structural characterization, and theoretical calculation, further understanding of the optical properties and electronic relaxation mechanisms, including the origin of photoluminescence for photo-excited metal clusters is needed.

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Several papers have been devoted to describing the spectrally broad photoluminescence emission of Au25(SR)18, which spans visible and NIR emission energies.33-35 Previously, Jin and co-workers found that NIR emission was significantly modified by the electron donating capabilities of the ligand shell, and a mechanism for charge-transfer relaxation from the ligand shell to the metal core was proposed.36 Furthermore, Green et al used temperature-dependent photoluminescence analysis to determine that these emissive pathways were coupled to two vibrational modes, 3.1 and 5.3 THz. These values were attributed to Au-Au phonon modes in the core and Au-S stretching modes of the semiring units, respectively.37 The lower-frequency phonon mode has also been observed in several ultrafast transient absorption studies.33, 38-39 Based on timeresolved magneto-photoluminescence data, Green et al attribute a portion of the visible emission as originating from states localized to the icosahedral core.40 The experimental observation of state-dependent, vibrationally-coupled emission pathways put great demand for more fully understanding carrier electronic relaxation dynamics in these systems.41-44 In order to understand electronic relaxation of MPC hot carriers, several groups have used time-resolved pump-probe measurements using visible and/or near infrared probing.33, 3839, 45-47 Most recently, our group employed 2-D femtosecond time-resolved electronic spectroscopy (2DES) using few-cycle pulses to study electron and hole dynamics of the Au25(SC8H9)18-1 superatomic P and D states. Based on the 2DES data, it was determined that the lifetime of hot electron and hole relaxation in the superatom D or P states were 200 and 300 fs, respectively.48 Taken together with ultrafast and PL measurements, these studies indicate that both the superatom states of the metal core, as well as states localized to the passivating ligands, play important roles in the radiative and nonradiative carriers relaxation dynamics. In this manuscript, we report and compare the excited-state relaxation dynamics for [Au25(SR)18]-1 clusters following either interband (400 nm, 3.1 eV) or intraband (800 nm, 1.55 eV) excitation by using femtosecond time-resolved transient absorption measurements. We detail the influence of dispersing solvent and passivating ligands on the relaxation rates of Au25(SR)18- MPCs following both interband and intraband excitation. The two ligands investigated in this report are phenylethanethiol (PET) and 2-phenylethanethiol (PET*). The data collected indicate that interband excitation results in electronic relaxation dynamics that are particularly sensitive to the dispersing dielectric. When the electron-hole pair is created between Superatomic orbitals by using intraband excitation, charge carrier relaxation dynamics are assisted by lattice phonons, and are insensitive to the changes in the local dielectric. By monitoring these excited-state dynamics in a state-resolved manner, an excitation energy-dependent description of the two relaxation mechanisms is obtained. In summary, this study provides compelling evidence for both nonadiabatic phononassisted and charge transfer assisted contributions to Au25(SR)18-1 electronic energy relaxation dynamics. The remainder of this manuscript is organized as follows: experimental methods are detailed in Section 2; Results and Discussions are provided in Section 3, including descriptions of steady-state and transient optical spectral assignments in 3a, relaxation dynamics of interband-excited clusters in 3b, and

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intraband-excited systems in 3c; Conclusions are given in Section 4. 2. Experimental methods Synthesis: The molecular structures of the PET and PET* ligands used in this work are shown in Scheme 1. The Au25(PET)18-1 (counterion: tetraoctylammonium, TOA+) was prepared by a previously reported approach.21 The Au25(PET*)18-1 (counterion: TOA+) also followed a reported approach.49

Scheme 1. Structures of PET (left) and PET* (right). Absorption and photoluminescence Studies: Powdered samples of Au25(PET)18-1 and Au25(PET*)18-1 were dispersed in toluene, acetone and tetrahydrofuran (THF). Absorption spectra were collected at room temperature using a Lambda 950 UV/VIS spectrometer (PerkinElmer). Similarly, roomtemperature photoluminescence (PL) spectra were collected using a Fluormax-4 spectrofluorometer (Horiba) following 3.1 eV excitation. Additionally, PL spectra was collected for both Au25(PET)18-1 and Au25(PET*)18-1 at 4.5 K. Here, samples were dispersed into a toluene/polystyrene matrix and dried onto a glass coverslip. The samples were then inserted under vacuum into the bore of a cryostat. Excitation of the samples was achieved using the frequency-doubled output of a 1 kHzregeneratively amplified Ti:Sapphire system centered at 1.55 eV. Resulting PL emission was collected using a fiber optical cable and sent to a spectrometer (McPherson) for energyresolved spectral analysis. Femtosecond Time-Resolved Transient Absorption: Femtosecond transient absorption measurements were carried out using a 1 kHz-regeneratively amplified Ti:Sapphire system generating 800 µJ pulses centered at 1.55 eV. The amplified pulses were characterized by frequency-resolved optical gating (FROG) pulse diagnostics. The majority of the laser output (96%) was attenuated and directed to the sample as the excitation pump source. In the case of 3.1 eV pumping, the laser was first frequency-doubled using a β-Barium Borate (BBO) crystal. The remaining laser output was focused onto one of two sapphire crystals with thicknesses of 0.3 and 1.2 cm to generate spectrally broad probe pulses in the visible and near-IR, respectively. The pump-probe time delay was controlled by using a retroreflecting mirror on an adjustable translational stage. Both pump and probe laser pulses were focused and overlapped onto the sample which was contained in a 2 mm pathlength cell. For studies done in the visible (2.75 eV to 1.46 ev), probe pulses were spectrally dispersed onto a silicon diode array detector. In the case of the NIR studies (1.38 eV to 0.83 eV), the probe was detected using an InGaAs array sensor. Differential extinction of the probe was measured for several time delays between the pump and probe laser pulses by mechanically chopping the pump pulse at 500 Hz. The resultant transient absorption of the probe light was reported here as differential absorption. Data analysis methods: In order to determine the temporal evolution of transient absorption spectra, global regression analysis was used. Global regression analysis allowed for deconvolution of individual components that contributed to high-

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ly congested time-resolved spectra. The singular value decomposition (SVD) method50 was employed using a program written in-house to convert the raw 3-D data matrix into principle kinetics and spectra of the individual components. The SVD method produced a linearly independent set of eigenvectors, which stored spectral and kinetic information. In particular, the principle kinetics eigenvectors were especially useful for discerning the number of unique electronic energy relaxation pathways. In this way, the principle kinetics eigenvectors could be used to characterize the time dependence of the transient spectra. The principle kinetics were ﬁt globally to the sum of multiple exponential decay functions /

Here, G(t) is a Gaussian function that deconvoluted the instrument response function (IRF) to the Gaussian pump and probe laser pulses, i is the total number of components, Ai is the amplitude coeffcient of the ith component, t is the pump− probe time delay, and τi is the relaxation time constant of the ith component. In addition, single peak fitting at selected probe detection wavelengths was used to extract probe-energy-resolved kinetics. Here, the transient absorption signal amplitude was temporally monitored at select wavelengths. The resulting timeresolved transient data were fit using an in-house program that uses an iterative least-squares approach. The best fits were obtained using one of the two equations:

same extinction amplitudes at the 3.1 eV excitation feature. Here, a four-fold enhancement in integrated emission intensity was observed, as well as a blue shift in energy from 1.54 eV to 1.59 eV when the ligand is changed from PET* to PET, while maintaining the dielectric medium (toluene). This agrees well with the ligand-to-metal charge transfer (LMCT) mechanism reported by Wu et al.36 The PL intensity also increased with increasing solvent polarity. Additionally, it was observed that not only was the PL emission intensity ligand dependent, but the energy gap between the emissive states was affected per low temperature PL measurement (Fig SI-1). At low temperature (4.5 K), two broad but distinguishable peaks (1.59 eV and 1.75 eV) for both clusters were observed. The chiral ligand (PET*) passivated Au25(SR)18-1 clusters exhibited a red shift of the low-energy peak and lower intensity of high-energy peak. The 1.75-eV PL peak did not exhibit a ligand-dependent energy shift. This result suggested the local environment and passivating ligand could modify the electronic recombination transition that involved the surface ligand states.

where, G(t) is a Gaussian function which deconvolutes the instrument response function to the Gaussian pump and probe laser pulses, An is the amplitude coefficient of the nth component, τn is the time constant of the nth component, and Ainf is a non-decaying amplitude that accounts for long-lived transient signals that exceeded the experimental range. 3.

Result and Discussion 3.a: Steady-state and Transient Spectral Assignments The linear absorbance spectra of PET and PET* protected Au25(SR)18-1 clusters are shown in Figure 1a. For both samples, the extinction features are nearly identical, suggesting the crystal structures are not affected by surface ligand modification in this case.51-52 The electronic structure and optical properties of Au25(SR)18-1 clusters have been well described by superatom orbital theory.23 The first electronic transition appears at 1.8 eV, and corresponds to an intraband transition from the Au-based superatomic orbital P to D (or HOMO to LUMO+1). Due to the similarity of ligand structure, the highenergy absorption spectrum (> 2.5 eV) of Au25(PET)18-1 and Au25(PET*)18-1, which arise from transitions associated with ligand-based states, are qualitatively similar. Figure 1b portrays the photoluminescence spectra of both clusters dispersed in different solvents. Solutions were prepared to maintain the

Figure 1. (a) Linear absorption of Au25(PET)18-1 and Au25(PET*)18-1 dispersed in toluene, (b) Photoluminescence spectra of Au25(PET)18-1 and Au25(PET*)18-1 in different solvents: Au25(PET)18-1 in acetone (black), THF (blue), toluene (red) and Au25(PET*)18-1 in toluene (green).

In order to understand these ligand- and solventdependent effects, we examined the excited state dynamics for both MPCs using femtosecond time-resolved transient absorption spectroscopy. Fig 2a and 2b portray the transient extinction spectra of Au25(PET)18-1 clusters at different pump-probe time delays following 3.1 eV or 1.55 eV excitation, respectively. These data show several noticeable features in the transient spectra: a ground-state bleach at approximately 1.85 eV and two distinguishable excited state absorption (ESA) peaks with maximum intensity at 2.1 and 2.4 eV. Additionally, within 2 ps, the formation of a third ESA peak at 2.6 eV, as well as an apparent blue shift of the ground-state bleach, was observed. Interestingly, the profile of the transient spectra displayed two different time-dependent behaviors for pumping at 3.1 eV or 1.55 eV. At short times (< 1 ps), the 3.1 eV pumped spectra displayed only two ESA features with a third higher-energy component that built up at longer pump-probe delay times. For 1.55 eV pumping, all three ESA features were apparent instantaneously. This observation suggested that the different optical

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responses from 3.1 eV and 1.55 eV pumping resulted from hot charge carriers created within different excited state manifolds. According to the electronic state configuration and superatom theory, the interband transition (ligand to sp-band) was excited after absorption of a 3.1 eV photon by the cluster. This resulted in hot electron promotion into the band of spstates while a hot hole was created in ligand states. The measured ESA data agreed with this model. As the hot electron internally converted from the sp-band through the manifold of superatomic D states, the intensity of the two lowest energy ESA features decreased. It was not until electronic relaxation to the LUMO that the highest energy ESA transition became spectroscopically accessible, and the corresponding 2.6 eV ESA feature was observed. In contrast, 1.55 eV pumping resulted in excitation from superatomic P orbitals directly to the superatomic D manifold. As a result, ESA intensity of the 2.4 eV and 2.1 eV peaks was more gradual with time, and the 2.6 eV component was observed instantaneously upon excitation.

Figure 2. Transient absorption spectra of Au25(PET)18-1 at different delay time under 3.1 eV (a) and 1.55 eV (b) laser excitation. Transient absorption kinetic traces probed at (c) 2.36 eV (visible) and (d) 1.23 eV (NIR) under different types of excitation.

To confirm the excitation mechanisms, pump powerdependent studies were carried out for both excitation energies (3.10 eV and 1.55 eV). The excitation photon order was quantified by determining the transient differential-spectra integrated area for several pump powers. The photon order could be determined in this manner because the integrated intensity, I, depends on the pump power, P, as I = σPN, where σ is the absorption cross section at the pump energy and N is the photon order. Therefore, the slope of the log of the integrated intensity plotted against the log of the laser power gave the photon order. As shown in Fig S2, the low-pump-power region for both 3.10 eV and 1.55 eV excitation yielded N = 1, which indicated the MPCs were excited by a single-photon absorption process. In the case of 1.55 eV excitation, pump powers larger than 50 mW yielded N = 2, which indicated MPCs were excited by a two-photon absorption process for these higher laser fluences. Nonlinear MPC excitation is also reported by Ramakrishna and co-workers.53 Further evidence of twophoton absorption at high pump power was obtained by monitoring the time-dependent transient signal amplitudes of the 2.4 eV ESA feature under 3.1 eV and 1.55 eV excitation with-

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in the two photon-excitation regime. As shown in Figure 2c, the relaxation dynamics of the MPCs showed similar decay rates for one-photon 3.10 eV and two-photon 1.55 eV excitation. However, using linear excitation, a rapid decay was observed following 3.10 eV excitation as compared to 1.55 eV. This observation was more apparent when near infrared probe photons were used to examine relaxation dynamics (Figure 2d). The Figure-2d data exhibit a prompt ESA signal with an amplitude plateau following 1.55 eV excitation; the data exhibited a gradual signal decay extending beyond our temporal experimental range (3 ns). The data resulting from the combination of 3.1 eV excitation and near infrared probing exhibited a first-order growth (0.42 ps) of the ESA signal amplitude, followed by a long-lived signal plateau that was similar to that obtained following 1.55 eV pumping. This result was in agreement with the 300 fs time constant observed for d band hole relaxation in our previous 2DES data.48 3.b: Interband excitation: Charge transfer assisted relaxation Although the transient absorption spectra resulting from 3.10 eV interband excitation were heavily dominated by longlived ESA signals, several important distinguishing features were observed on the few-hundred femtosecond time scale. For example, an apparent time-dependent spectral blue shift of transient bleach measured at 1.85 eV was clearly observable (Figure 2a). The bleach component was fit to a Gaussian function, and the average energy of the signal maximal intensity was plotted versus pump-probe time delay in Figure 3a. The energy blue shift was fit to a first-order time constant of 0.73 ± 0.37 ps. This sub-picosecond blue shift has been thoroughly explored by using two-dimensional electronic spectroscopy (2DES).48 The 2-D data show that intense ESA amplitudes overwhelm the bleach signal during the first few hundredfemtosecond time scale.48 Here, we show further evidence of the spectral overlapping of the ESA and bleach features. By fitting the ESA decay at 1.95 eV detection energy in Figure 3b, which is nearly degenerate to the bleach maximum, a similar time constant (0.70 ps) was also obtained. The timedependent transient signals were further quantified using global regression analysis, which is a powerful tool for deconvolution of congested spectra.54 Figure 3c portrays the decomposition of energy-resolved spectra of Au25(PET)18-1 clusters by using global analysis and singular value decomposition (SVD). A total of three components were retrieved. Component-1 depicted a broad, positive ESA feature with a time constant of 0.69 ± 0.17 ps. Component-2 had a longer time constant, 4.92 ± 0.78 ps, and displayed a positive amplitude at higher energies, but also included a negative bleach feature around 1.80 eV. Finally, component-3 exhibited the longest time constant of the three components (>100 ps) and included a prominent bleach at 1.87 eV. The 0.7 ps ESA component-1 was consistently observed in all experiments, independent of solvent or ligand. We attribute the ligand-independent nature of the component-one signals, which had positive amplitude in the region of the transient bleach and was in good agreement with the time-dependence of the observed spectral blue shift, to rapid electron relaxation within the manifold of superatom D states.45 As excitation occurred here between core-localized states, a solvent-independent trend in dynamics was consistent with expectations. This result is consistent with our previous 2DES study that concluded the rapid decay of ESA cross-peak

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amplitude is due to internal conversion from LUMO+n to LUMO superatomic orbitals after interband excitation. It is believed that this blue shift is independent of stimulated emission signals. Typically, stimulated emission exhibits a timedependent red shift of negative-amplitude transient signal. Further insight into state-resolved dynamics can be obtained when NIR probe photons were used for detection. Because the NIR photons are less energetic than the Au25(SC8H9)18HOMO-LUMO energy gap, this pump-probe detection scheme generates transient ESA peaks that can be monitored in the time domain to track dynamics of the superatom P and D states.45 The NIR transient absorption spectra are shown in Figure S3. A rapid, 0.43 ± 0.15 ps, growth component was observed following interband (3.1 eV) excitation (Figure 2d, blue). In contrast, this growth component is not observed when hot carriers were created in the metal core via interband excitation (Figure 2d, red). The observed growth component is attributed to the internal conversion of holes in ligand states.

0.60 ps. The component-2’s lifetime of 4-5 ps was also reported in previous work.33, 39 This component was assigned to electron relaxation within the semiring units. Here, we assert that reorganization of solvent dipoles around photo-excited clusters could have the effect of lowering and energy stabilizing the charge-transfer states. Higher dielectric solvent molecules could give rise to accelerated CT relaxation rates, as was reflected by the shorter component-2 time constants obtained when MPCs were dispersed in acetone.

Figure 4. (a) Component-2 relaxation rate constant plotted vs. solvent dielectric constant; (b) amplitude ratio of component-3 and 1 plotted vs. solvent dielectric constant.

Figure 3. (a) Blue shift of bleach center energy at different delay time. (b) ESA kinetics at 1.95 eV. (c) Global analysis result of Au25(PET)18-1 in THF. (d) Principle kinetics of Au25(PET)18-1 in toluene (blue), THF (red) and acetone (green) excited by 3.1 eV pulses.

In contrast to component-1’s results, both components two and three data were found to depend on the surrounding dielectric medium. The transient absorption spectra of Au25(SR)18 clusters dispersed in different solvents are shown in Figure S4. Component-2 (red in Figure 3c) was initially ascribed to ligand-band transitions, as it exhibited marked solvent dependence, which is a common feature of excited dynamics involving charge transfer states.55-56 A charge transfer mechanism has been proposed for MPCs in which electronic energy relaxation occurs through states residing in the ligand band.42, 57-59 To understand the origin of this solvent-dependent component, femtosecond transient absorption measurements of Au25(SR)181 clusters were performed in three different solvents (toluene, THF and acetone). The principle kinetics obtained from SVD of Au25(PET)18-1 in different solvents are shown in Figure 3d. Upon increasing the solvent dielectric constant, the relaxation time constant of component 2 extracted from principle kinetics decreased for both passivating ligands (Figure 4a): as the dielectric constant of solvent increased from 2.4 to 21, the component-2 time constant decreased from 4.90 ± 0.80 to 2.90 ±

The global analysis results not only provided strong evidence in support of charge transfer-assisted relaxation dynamics, but also suggested the mechanism of near infrared PL emission for Au25(SR)18 clusters. It is proposed that near infrared emission occurs during electronic relaxation through a manifold of ligand-to-nanocluster CT states. We examined the solvent influence on electronic relaxation dynamics by comparison of the relative transient signal amplitude coefficients.60 Here, we assume that solvent- and ligand-independent ESA (A1) amplitude is proportional to the total population of photoexcited clusters and the integrated area of the bleach feature in component-3 represents the population that is available for radiative recombination. Component-3 is composed of both a long-lived negative amplitude bleach and positive amplitude absorption signals. Taken together, the relative population of molecules which are involved in radiative recombination can be illustrated by the A3(bleach)/A1 ratio, which is shown in Figure 4b. This analysis has been used previously to elucidate the relaxation mechanism is SO(SO2)n+ gas-phase clusters.60 With increasing solvent polarity, the population of radiative species increased. Combination of PL and transient absorption studies support the charge transfer assisted relaxation mechanism: when CT is stabilized by the surrounding environment, the population of excited charge carriers (A3(bleach)/A1) relaxing through radiative channels is greater, giving rise to stronger PL intensity. Radiative recombination occurs between electrons at the core-based superatomic LUMO and the hole residing in the ligand band after interband excitation. This solvent dependent relaxation behavior is then attributed to the hole relaxation, which matches the model proposed by the temperature-dependent PL study.32, 40 This ligand-dependent result is

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also in agreement with the NIR PL study: low PL intensity of PET* was due to less favorable radiative relaxation through charge transfer states. Taken together, the data support a charge-transfer assisted relaxation mechanism resulting in electron-hole pair annihilation after creation by interband 3.1 eV photon excitation. 3.c: Intraband excitation: Phonon-assisted relaxation mechanism

Figure 6. (a) Transient absorption traces of Au25(PET)18-1 probed at 1.98 eV and 1.35 eV following 1.55 eV excitation while dispersed in Toluene. (b) Residual transient data of Au25(SR)18-1 passivated with different ligands and dispersed in different solvents recorded at 1.98 eV.

Figure 5. (a) Transient absorption spectra of Au25(PET)18-1 dispersed in different solvents at 1 ps time delay following 1.55 eV excitation. (b) Principle kinetics Au25(PET)18-1 in different solvents following 1.55 eV excitation.

As described in section 3.a, one-photon 1.55 eV intraband excitation results in the generation of an electron-hole pair in the HOMO-LUMO manifold that carriers are spatially localized to the Au13 metal core. Dynamics of bound electron-hole pair and state-to-state nonadiabatic process were examined by monitoring the ESA features generated in the visible range (Figure 5). The progression of ESA peaks between 2 eV to 2.6 eV resulted from electronic probing of excited electrons from the LUMO manifold to higher energy excited states. Figure 5 shows the spectrally resolved (a) and time-dependent (b) transient signal amplitudes for Au25(PET)18- dispersed in different solvents. Due to the nature of intraband excitation as being core-based, it is expected that the spectral and temporal response to the dielectric change should be minimal. Therefore, the solvent independence of the Figure-5 data is consistent with expectations.

Additionally, a prominent oscillation was present in the transient absorption decay trace (Figure 6). It is important to note that these time-dependent amplitude oscillations were only detectable at select probe energies. In Figure 6a, the residual transient signals obtained from Au25(PET)18-1 probed at 1.35 eV and 1.98 eV are shown. Fitting the residual data using a single-frequency damped-oscillator function yielded a 2.4 THz (80 cm-1) frequency. This oscillation frequency was also clearly observed in fast Fourier transform (FFT) analysis (Fig S6). This frequency agreed well with theoretical predictions for low-frequency Au-Au vibrations following intraband HOMO-LUMO excitation.24 The oscillatory features at both probe energies are damped within a few amplitude recurrences. By overlaying the transient residuals obtained from these two different probe energies, following time-zero correction, a π/2 phase shift in the time-dependent signals was clearly observed (Figure 6a). This phase shift is attributed to the timedependent excited-state electron population of the potential energy surfaces, resulting in dynamic Franck-Condon overlap. The observed oscillations are due to the time dependence of the vibrational superposition of states within the Franck−Condon region of the electronic transition. As a result of electronic dynamics, the mean frequency of the vibrational wavepacket is time dependent. This time dependence is reflected in the π/2 phase shift observed when the probe wavelength is tuned from resonant excitation from the low-energy to the high-energy portion of the electronic potential surface. An important observation from this analysis is the consistency of the 2.4 THz oscillation, independent of solvents and ligands. This solvent- and ligand-independent 2.4 THz coherent vibration (Figure 6b) supports assignment to the Au-Au stretching mode of the metal icosahedral core of clusters.61-63

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4. Conclusions Here, we have described the electronic relaxation pathways of PET and PET* monolayer protected Au25(SR)18-1 clusters photo-excited by interband and intraband excitation processes. Based on these results, we propose a model (Figure 7) to account for electronic relaxation dynamics following both interband (d to sp) and intraband (sp) excitation. As shown in Figure 7, interband excitation using 3.1 eV photon energy promotes an electron from the ligand band to high-energy states in the sp band, generating a hole in the ligand motif. Based on experimental data, we observed electron internal conversion from the sp-band to the LUMO on a time scale of 700 fs. This pathway was elucidated from the solvent independence of component-1 data. Simultaneously, the hole non-radiatively relaxes through the ligand band in approximately 4 ps, which was deduced from experiments using NIR probe photons. This is supported through our observation of component-2 of the three-step relaxation process. The ligand- and dielectricdependent nature of component-2 implicated carrier relaxation through charge transfer (CT) states. Component-3 is characterized as the radiative exciton recombination of the electron occupied LUMO and hole on the microsecond time scale. This mechanism is in contrast to the observed excited-state kinetics following intraband excitation. In the case of intraband pumping, there is insufficient excitation energy to promote an electron from the ligand band. Instead, the transition occurs from the superatomic-P HOMO and HOMO-n to the superatomic-D LUMO+n manifold localized in the icosahedral Au13 core. This conclusion is supported by several observables: 1) The dynamics are solvent-independent; 2) the absence of a growth in the NIR probed transient amplitude decay trace relative to interband excitation; and 3) the observed oscillation in the transient signal trace which agrees in frequency with a 2.4 THz Au-Au phonon mode. The latter observation also supports the hypothesis that this relaxation pathway is phonon assisted. Previous temperature-dependent PL studies on the emissive pathways of the Au25(PET)18 cluster have shown that the electronic-vibrational coupling in these systems is significant.38 While, the magnitude of this coupling has been quantified, the time scale of the PL experiments (nanoseconds to microseconds) is too long to ascribe the ultrafast internal relaxation events. In addition, magnetic field-dependent studies have proven capable of resolving and characterizing these vibrationally-coupled emissive states.41 Here, we have temporally resolved the excited state relaxation dynamics and attribute vibrational processes induced by intraband excitation. In future research, the combination of temperature- and fielddependent transient spectroscopy is expected to provide significant new insight into state-resolved electronic relaxation dynamics for these clusters. Also, in light of the solventdependent results reported in this article, experiments should be carried out to understand the relaxation dynamics of MPC assemblies and in other condensed phase. This information will be crucial for predictive MPC use in applications that feature metal clusters as functional components. These stateresolved examinations are essential for establishing structurefunction photonic correlations for structurally precise monolayer-protected clusters.

Figure 7. Relative energy-level diagram depicting proposed cluster electronic relaxation following interband and intraband excitation.

ASSOCIATED CONTENT Supporting Information. Photoluminescence of gold cluster at low temperature, Excitation power dependence, NIR transient absorption spectra, Transient absorption of PET and PET* protected Au25(SR)18-1 clusters, transient kinetic trace under intraband excitation and global analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT KLK acknowledges financial support from an award from the National Science Foundation, (CHE-1507550). RJ acknowledges the financial support from the Air Force Office of Scientific Research under AFOSR (FA9550-15-1-0154).

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Ligand- and Solvent-Dependent Electronic Relaxation Dynamics of

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