15-n-glyme-Au20(SC8H9)15 Monolayer- Protected ... - ACS Publications

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C: Physical Processes in Nanomaterials and Nanostructures 20

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Relaxation Dynamics of Electronically Coupled Au (SCH) n-glyme-Au (SCH) Monolayer-Protected Cluster Dimers 20

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Patrick J. Herbert, Chongyue Yi, Scott Compel, Christopher J. Ackerson, and Kenneth L. Knappenberger J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06144 • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018

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The Journal of Physical Chemistry

Relaxation Dynamics of Electronically Coupled Au20(SC8H9)15-n-glyme-Au20(SC8H9)15 MonolayerProtected Cluster Dimers Patrick J. Herbert,1‡ Chongyue Yi,2 ‡ W. Scott Compel,3 Christopher J. Ackerson,3 and Kenneth L. Knappenberger, Jr.1* 1

Department of Chemistry, The Pennsylvania State University, University Park, PA 16802

2

Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL, 32304

3

Department of Chemistry, Colorado State University, Fort Collins, CO, 80523

Abstract The electronic relaxation dynamics of photoexcited Au20(SC8H9)15-n-glyme and Au20(SC8H9)15-n-glyme-Au20(SC8H9)15 (n- = di-, tri-, and tetra-) dimers, where glyme refers to n-ethylene glycol dimethyl ether bridging molecules, were studied using pump-probe femtosecond time-resolved transient absorption spectroscopy (fsTAS). The utilization of nglyme molecular bridging linkers provided a method to prepare Au20(SC8H9)15-n-glymeAu20(SC8H9)15 dimers with control over inter-cluster spatial separation. A dimer-specific electronic absorption resonance was observed at 2.6 eV. Analysis of fsTAS differential spectra for dimer species revealed a pump-probe waiting time-dependent blue shift of the low energy excited-state absorption (ESA) feature, suggesting electronic relaxation into a dimer-specific

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excited-state. Single probe-energy differential signal amplitude analysis of the ESA feature yielded a distance-dependent growth component for the electronic relaxation dynamics with time constants of 130 ± 20 ps, 60 ± 8 ps, 36 ± 6 ps for the diglyme, triglyme and tetraglyme-bridged dimers, respectively. The increase in relaxation time was attributed to inter-cluster distancedependent stabilization of dimer electronic excited states. These results suggest a state-specific mechanism for inter-cluster electronic relaxation. Introduction Monolayer-protected clusters (MPCs) offer robust and highly tailorable platforms for photonic applications such as photocatalysis, bio-imaging, and nonlinear optical transduction.1-5 MPCs are structurally defined by 3 domains: 1) a core of metal atoms; 2) an inorganic layer of alternating metal-thiol (e.g. Au-S) semiring units; and 3) a passivating layer of organic ligands.6,7 Spectroscopic studies on MPCs have shown that each structural domain has influence over the global optical properties.8-10 Due to synthetic advances, each of these domains can be structurally tailored. This includes metal doping of the core and staple units, substitution of thiolate linkers for selenolate or tellurate, and complexation of a variety of organic ligands, to name a few.11-15 These structural modifications provide atomically precise routes to tailor MPCs for specific photonic applications and offer fundamental exploration for structure-function relationships at the nanoscale.16,17 While the influence of intracluster structural modifications is an active area of research, the utilization of MPCs as subunits for extended assemblies has emerged as a promising direction for metal nanocluster research.18 In larger metallic nanoparticles the local coupling between individual nanoparticles has been widely studied.19-23 At dimensions > 2 nm the optical properties of metal nanoparticles are dominated by phase-coherent, collective excitation of


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surface conduction band electrons.24 This collective excitation event is defined as the localized surface plasmon

resonances

(LSPR).

When

two

or more of

these nanoparticles

electromagnetically couple, interparticle resonances arise dependent on the separation, symmetry, surrounding dielectric, and plasmon mode quality factors of the constituent particles.25-27 These interparticle resonances can be tailored through structural and compositional modifications for applications in biosensing, electromagnetic energy transduction and for applied spectroscopies such as surface enhanced Raman (SERS).28-30 Similarly, assembly of MPCs can result in emergent electronic and optical properties through intercluster coupling.31 For few-tosub-nm MPC systems, the electronic structure is characterized by discrete electronic states.32-34 Therefore, descriptions for inter-cluster electronic interactions, which may differ from plasmonic assemblies, are needed. Due to the distinct structural motifs of MPCs, synthetic coupling of individual clusters has been achieved through several mechanisms. Jin et. al. showed direct bonding between 13 gold atom icosahedral cores to form ligand-protected rod-like Au25 and Au37 clusters.35 Single-crystal X-ray diffraction studies by Maran et. al. characterized extended 1D nanowires composed of neutral Au25(n-butanethiolate)18 constituents bound by an interlocking mechanism of the inorganic semiring units.36 Low temperature EPR measurements revealed a conversion

from

paramagnetic-to-diamagnetic

behavior

in

the

monomeric

Au25(n-

butanethiolate)18 cluster to the nanowire assembly. Goodson et. al. utilized a tethering method of replacing one (or more) of the glutathione ligand units of an Au25(glutathione)18 MPCs with a chromophoric (4,4-thiodibenzenethiol (TBT)) linker unit in the ligand shell to form oligomers of clusters.37 Large nonlinear conversion efficiencies resulted for oligomers. An important challenge is to understand how inter-cluster spatial separation influences the electronic and optical properties of assembled MPCs. Compel et. al. demonstrated that cluster


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spatial separation can be controlled through a molecular bridging linker.31 Dimers of the MPC Au20(SC8H9)15 were synthesized using diethylene glycol dimethyl ether (diglyme) as a molecular-bridge. This process generated an equilibrium mixture of Au20(SC8H9)15-diglyme monomer and Au20(SC8H9)15-diglyme-Au20(SC8H9)15 dimer. The monomer and dimer assignments were confirmed through a combination of chromatography, mass spectrometry and electron microscopy. A DFT model of the system suggests that diglyme binding is mediated through dative interactions of the terminal oxygen atoms of diglyme with gold atoms of the clusters.31 Isolation of the monomer and dimer species was achieved through silica gel size exclusion chromatography. The monomer and dimer systems could be easily distinguished by their visible electronic excitation spectra. Upon dimerization an absorption peak at 2.6-eV was observed that was absent in the monomer system. This emergent electronic transition was hypothesized to arise from intercluster coupling. Density functional theory (DFT) calculations on methyl-terminated Au20(SMe)15-diglyme monomer and Au20(SMe)15-diglyme-Au20(SMe)15 dimer supported this hypothesis.31 The dimer-specific absorption feature at 2.6-eV was attributed to an intercluster resonance. The relaxation dynamics of photoexcited cluster dimers was studied with femtosecond time-resolved transient absorption spectroscopy (fsTAS). Time-dependent analysis of differential transient signal amplitudes revealed a 120 ps relaxation channel that was unique to the dimer species; this component was not observed for the Au20(SC8H9)15-diglyme monomer. Additionally, polarization-dependent fsTAS studies provided evidence of spindependent dynamics in the dimer species, consistent with predictions from DFT calculations. Herein, the electronic relaxation dynamics for a series Au20(SC8H9)15-n-glymeAu(SC8H9)15 dimers, where n- is di-, tri- and tetra-, are reported and compared to the Au20(SC8H9)15-n-glyme monomer. Linear absorption spectra reveal a 2.6-eV absorption feature


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that is specific to dimer formation. From femtosecond time-resolved transient absorption measurements, differential absorption spectra are monitored for pump-probe waiting times up to three nanoseconds. The electronic relaxation-dynamics are quantified through global regression and time-dependent single-probe energy amplitude (SPA) analyses. Results from these analyses identify an excited-state relaxation pathway into a dimer-specific electronic excited-state. It is concluded that the intercluster separation, determined by the bridging linker, has the effect of controlling the relative energy of the dimer-specific excited state. Material and Methods Synthetic Method: A 250 mL Erlenmeyer flask was charged with 2-phenylethanethiol (48 mL, 100 mM, 4.8 mmol, 3 equiv.) in tetrahydrofuran and HAuCl4•3H2O (16 mL, 100 mM, 1.6 mmol, 1 equiv.) in n-glyme solution, where n- is di-, tri- and tetra-, was added to the reaction flask. The reaction was stirred at room temperature for 3 h or until the cloudy yellow solution turned milky white. Five min prior to the end of the 3 h, a suspension of NaBH4 in n-glyme (8 mL, 50 mM, 0.8 mmol, 0.5 equiv.) was sonicated at room temperature for 5 min. 120 mL of n-glyme was added to the reaction vessel, followed by dropwise addition of 8 mL of the NaBH4 suspension over the course of 1 min. The reaction appears yellow/orange, indicating the formation of both Au20(SC8H9)15-n-glyme monomer and dimer. The reaction is allowed to stir at room temperature for an additional hour. Precipitated byproducts were filtered out using a Büchner funnel with medium frit, and the remaining orange solution was transferred to a 1-L fleaker. The reaction was quenched via the addition of methanol to 1 L, and the quenched solution was filtered out using a fine frit Büchner funnel. The product was collected in CHCl3 into a round bottom flask and methanol was added until the product precipitated, which was then dried through rotary


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evaporation. Thin layer chromatography was run using 9:11 hexanes:chloroform to characterize the product. Linear Absorption Spectroscopy. Samples of Au20(SC8H9)15-n-glyme and Au20(SC8H9)15-nglyme-A20(SC8H9)15 were dispersed in tetrahydrofuran (THF) forming yellow and orange solutions, respectively. Sample solutions were transferred to a 2 mm path length quartz cuvette. Linear absorption spectra were collected at room temperature using a Lambda 950 UV/vis spectrometer (PerkinElmer). Femtosecond Time-Resolved Transient Absorption Spectroscopy. Details of the fsTAS experiment setup have been described previously.38 Briefly, 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. For 3.1 eV pumping, the laser was first frequency-doubled using a β-barium borate (BBO) crystal. The remaining laser output was focused onto a sapphire crystal with thickness of 0.2 cm to generate spectrally broad probe pulses in the visible. 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 colloidal sample which was contained in a 2 mm path length cell. Pump pulse energies were attenuated from 800 – 1400 nJ using a variable neutral density filter. A magnetic stir bar was used to circulate the colloidal solution in order to prevent photodegradation of the sample. Probe pulses were then spectrally dispersed onto a silicon diode array detector. Differential extinction of the probe was measured for several time delays between


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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. Time-dependent transient differential spectra were analyzed with global regression analysis. Global regression analysis enabled the time-correlated deconvolution of individual components that contributed to highly congested differential spectra. The singular value decomposition (SVD) method39 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. (1)

S (t ) = G (t ) ⊗ ∑ Ai e − t /τ i i

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 coefficient of the ith component; t is the pump-probe time delay; and τi is the relaxation time constant of the ith component. In addition to global analysis, time-dependent transient signal amplitudes were monitored at select probe-detection energies in order to analyze charge-carrier relaxation dynamics. The resulting time-resolved transient data were fit using an in-house program that uses an iterative least-squares approach. The best fits were obtained using the equation.


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(2)

−t

−t

τ1

τ2

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S (t ) = G (t ) ⊗ [ A1e + A2 e + Ainf ]

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; and Ainf is a nondecaying amplitude that accounts for long-lived transient signals that exceeded the experimental range.

Results

Figure 1. (a) Normalized absorption spectra of Au20(SC8H9)15-diglyme monomer and Au20(SC8H9)15-n-glyme-Au20(SC8H9)15 MPC systems. (b) First derivative of absorption spectra. Peak energy position for dimer species is indistinguishable (inset).

Steady-State and Transient Spectral Assignment:


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Figure 1a shows the normalized linear absorption for the Au20(SC8H9)15-diglyme monomer and Au20(SC8H9)15-n-glyme-Au20(SC8H9)15 clusters dispersed in THF. As previously reported, upon dimerization, a prominent absorption feature is observed at approximately 2.6-eV that is absent in the monomer.31 This 2.6-eV absorption peak is observed for all linker units. The differences between Au20(SC8H9)15-n-glyme-Au20(SC8H9)15 and Au20(SC8H9)15-diglyme are further demonstrated in a plot of the first derivative of absorption versus energy (Figure 1b). To address the possibility of ligand substitution effects, the linear absorption spectra were recorded for all monomeric Au20(SC8H9)15-n-g systems. The distinct 2.6-eV absorption peak was not observed for any of the three n-glyme substituted monomeric species; dimerization was necessary to induce the 2.6-eV resonance (SI figure 1). This supports the hypothesis that the 2.6eV absorption feature arises from inter-cluster electronic coupling and is not the result of ligandsubstitution effects. No detectable dimerization effects were observed in the higher energy portion of the absorption spectrum (>2.7-eV). This observation suggests the >2.7-eV electronic transitions for the dimers were indistinguishable from those states localized to the Au20(SC8H9)15-n-glyme monomer subunits. Another important observation based on the Figure 1 data is that although the prominent 2.6-eV absorption feature is distinct to dimers, any distancedependent influences on inter-cluster electronic excitation and optical properties is beyond the resolution of the linear, steady-state absorption measurements. For example, differences in the excitation energy of the inter-cluster resonance that gave rise to the approximate 2.6-eV transition were likely less than the 400-meV peak width determined through single-Gaussian fitting. In order to further examine the inter-cluster distance dependence of Au20(SC8H9)15-nglyme-Au20(SC8H9)15 electronic properties, excited-state carrier relaxation dynamics were studied using fsTAS.


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Figure 2. a) Transient absorption spectra of Au20(SC8H9)15-diglyme monomer (red) and Au20(SC8H9)15-diglyme-Au20(SC8H9)15 dimer (blue) recorded at 1 ps after excitation. b) Timedependent differential transient absorption spectra for the Au20(SC8H9)15-diglyme-Au20(SC8H9)15 dimer at early waiting times (< 2ps). c) Linker-dependent transient absorption spectra monitored at 1 ps waiting times. d) Mean ground state bleach energies for Au20(SC8H9)15-n-diglymeAu20(SC8H9)15 samples. The error bars reflect the peak resolution (σ) obtained from singlecomponent Gaussian fitting. Femtosecond time-resolved transient absorption spectroscopy has proven to be a powerful tool for dissecting the electron and nuclear dynamics for MPCs.40-44 Figure 2a shows the transient absorption spectra for the monomer (red) and diglyme-linked dimer (blue) upon 3.1 eV excitation at 1 ps pump-probe waiting time delay. Based on the linear absorption spectra, 3.1


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eV excitation events are expected to occur between states localized to the individual Au20(SC8H9)15- subunits. Any observed difference between the monomer and dimer timedependent transient spectra are expected to reflect distinguished electronic properties of the two species. For both monomer and diglyme-linked dimer systems a broad excited state absorption (ESA) feature is observed. A prominent ground state bleach (GSB) is observed centered at approximately 2.6 eV for the dimer – this bleach is not detected in the monomer transient spectrum (figure 2a). The approximate peak energy of 2.6 eV is in agreement with the intercluster resonance determined from the dimer linear absorption spectrum. Excitation from the 3.1 eV pump pulse leads to a depletion of electrons in the ground state. As a result, a negative differential absorption feature is expected at detection energies corresponding to linear absorption transitions. Figure 2b shows the time-dependent differential spectra for the diglymebridged dimer species at early waiting times (

15-n-glyme-Au20(SC8H9)15 Monolayer- Protected ... - ACS Publications

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