Directional Electron Transfer in Chromophore ... - ACS Publications

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Directional Electron Transfer in Chromophore-Labeled Quantum-Sized Au25 Clusters: Au25 as an Electron Donor Mary Sajini Devadas,† Kyuju Kwak,‡ Jung-Woo Park,‡ Ji-Hwan Choi,‡ Chul-Ho Jun,‡ Ekkehard Sinn,† Guda Ramakrishna,*,† and Dongil Lee*,‡ †

Department of Chemistry, Western Michigan University, Kalamazoo, Michigan 49008, and ‡Department of Chemistry, Yonsei University, Seoul 120-749, Korea

ABSTRACT Novel Au25(C6S)17PyS clusters (pyrene-functionalized Au25 clusters) showing interesting electrochemical and optical properties are synthesized and characterized. Significant fluorescence quenching is observed for pyrene attached to Au25 clusters, suggesting strong excited-state interactions. Time-resolved fluorescence upconversion and transient absorption measurements are utilized to understand the excited-state dynamics and possible interfacial electron- and energy-transfer pathways. Electrochemical investigations suggest the possibility of electron transfer from Au25 clusters to the attached pyrene. Fluorescence upconversion measurements have shown faster luminescence decay for the case of pyrene attached to Au25 clusters pointing toward ultrafast photoinduced electron/energy-transfer pathways. Femtosecond transient absorption measurements have revealed the presence of the anion radical of pyrene in the excited-state absorption, suggesting the directional electron transfer from Au25 clusters to pyrene. The rate of forward electron transfer from the Au25 cluster to pyrene is ultrafast (∼580 fs), as observed with femtosecond fluorescence upconversion and transient absorption. SECTION Nanoparticles and Nanostructures

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ighly stable quantum-sized monolayer-protected (MPCs) Au25 clusters are in the forefront of research in recent years.1-7 Atomically precise MPCs with exciting electrochemical, magnetic, optical, and catalytic properties5-25 are being synthesized and characterized. Quantum confinement, molecule-like electronic properties of Au25 MPCs and their functionalized counterparts are actively being explored for photoinduced electron- and energy-transfer properties which can lead to potential applications in solar energy conversion, catalysis, sensors, and other optical applications.26-30 The interfacial electron-transfer phenomena in molecule-nanoparticle hybrids have been extensively researched, especially for the molecule-semiconductor nanoparticle interface as it is the fundamental event in wellinvestigated dye-sensitized solar cells.31-33 Comparatively fewer investigations have been carried out on the moleculemetal nanoparticle interface. Many studies on the interface of molecule-metal nanoparticles have shown the evidence of energy transfer from the molecule to metal or vice versa.26-30,34-40 For example, pyrene-functionalized gold clusters were investigated by Montalti et al.,37 and the results show energy transfer from pyrene to the gold core together with interesting odd-even effects.38 A few studies have revealed that electron transfer is another viable deactivation process that occurs in the molecule-metal interface. In pyrene-attached gold nanoparticles, Kamat and co-workers34-36 reported that electron transfer takes place from

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photoexcited pyrene to the gold core. However, to the best of our knowledge, there are no reports of quantum-sized MPCs acting as electron donors, which can be exploited in the applications of catalysis, solar energy harvesting, and sensors. This report provides the first results supporting the directional electron flow in a pyrene-Au25 cluster system where the quantum-sized Au25 MPCs with a large energy gap25 act as electron donors. The present investigation is aimed at probing the interfacial electron-transfer pathways in Au25-pyrene clusters. The system chosen has, on average, only one dye molecule covalently bonded to the surface of Au25 nanocluster, so that intramolecular energy transfer and excimer formation would not interfere with the interfacial charge- or energy-transfer processes. Electrochemical and ultrafast time-resolved fluorescence and absorption investigations were carried out to probe the interfacial processes. Pyrene-labeled Au25 clusters were synthesized by the exchange of thiolated pyrene (PySH) onto hexanethiolate-capped Au25 clusters and characterized as Au25(C6S)17PyS using TEM, UV-vis, and voltammetric measurements. Details of syntheses6 and characterization are provided in the Supporting Information.

Received Date: March 25, 2010 Accepted Date: April 19, 2010 Published on Web Date: April 23, 2010

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DOI: 10.1021/jz100395p |J. Phys. Chem. Lett. 2010, 1, 1497–1503

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Figure 1. (A) Absorbance and (B) emission spectra of pyrene and pyrene-labeled Au25 clusters.

Shown in Figure 1A are the optical absorption spectra of PySH, Au25(C6S)18, and Au25(C6S)17PyS. As seen in the figure, the optical properties of Au25 clusters and PySH are unaltered in the hybrid PyS-Au25 cluster system. This suggests that pyrene and Au25 do not have strong ground-state interactions that can alter the absorption spectrum of either pyrene or Au25. Interestingly, the fluorescence of PySH is quenched dramatically in the PyS-Au25 hybrid, indicating stronger excited-state interactions (Figure 1B). The fluorescence quantum yield of PySH was determined to be 0.54, and this decreased to 0.008 in the hybrid system, indicating strong electronic interactions which can be due to either electron- or energy-transfer processes. Care has been taken while measuring the fluorescence quantum yield of Au25(C6S)17PyS, where the absorption by gold clusters is factored into the calculation and the error bars for measured quantum yields are about 15%. Square wave voltammetry of Au25(C6S)18 and Au25(C6S)17PyS was carried out to determine whether the addition of the dye molecule alters the redox behavior of Au25 clusters. As seen in Figure 2, the oxidation potentials of the Au25 clusters are substantially shifted to more positive values; for example, the first oxidation potential (ox1) of the Au25 cluster is shifted from -0.71 to -0.48 V (versus Fc/Fcþ) when PyS is attached. The reduction peak of Au25(C6S)17PyS at 20 ps (7%)). The average lifetime of the pyrene-Au25 cluster is reduced when compared to free PySH and is consistent with the fluorescence quantum yield measurements. The presence of such ultrafast decay can be due to either interfacial electronor energy-transfer processes. The origin of ultrafast decay of the pyrene-Au25 cluster is further probed with femtosecond transient absorption measurements. Comparative transient absorption measurements are carried out on PySH, Au25(C6S)18, and Au25(C6S)17PyS dissolved in DCM after excitation at 345 nm. Shown in parts A and B of Figure 4 are the transient absorption spectra at different time delays and kinetics at representative wavelengths for PySH. Immediately after excitation at 345 nm (at a 100 fs time delay, Figure 4A), an excited-state absorption spectrum (ESA) centered at around 580 nm is observed, which decays very rapidly to give rise to the ESA centered at around 530 nm, which in turn decays with a long lifetime. ESA at 580 nm is ascribed to the S2 f Sn singlet-singlet absorption, while that of 527 nm is attributed to the S1f Sn absorption of PySH. Similar S2 and S1 states were observed in the transient absorption of pyrene derivatives.42 Kinetics at 580 and

Figure 3. Fluorescence decay of pyrene and pyrene-labeled Au25 clusters monitored at 380 nm after excitation at 280 nm.

Figure 4. (A)Transient absorption spectra at different time delays from 100 fs to 25 ps of PySH in DCM. (B) Kinetic decay traces at two ESA peaks, 580 and 510 nm.


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Figure 5. (A) Transient absorption spectra at different time delays from 200 fs to 25 ps of Au25(C6S)18. (B) Kinetic decay traces at two ESA peaks at 565 and 530 nm.

Figure 6. Transient absorption spectra at different time delays (A) from 100 to 550 fs and (B) from 600 fs to 20 ps of Au25(C6S)17PyS in DCM after excitation at 345 nm. (C) Kinetic decay traces of two ESA peaks at 660 and 480 nm and (D) species-associated spectra obtained from global fit analysis with predominantly three lifetime components.

ESA features of Au25(C6S)17PyS at short and intermediate time delays. There is a growth of ESA at very short time scales up to 600 fs giving rise to an ESA with a maximum at around 503 nm while the same feature decays as the time delay is increased to 20 ps. It is clear from the transient absorption features that the ESA features of pyrene-labeled Au25 clusters are completely altered from those of free pyrene and free Au25 clusters. It is quite interesting to note that the excitation at 345 nm excites both Au25 clusters and the pyrene chromophore with roughly 50/50 probability. However, the transient features do not show any feature corresponding to Au25


clusters which can be ascribed to lower excited-state extinction coefficients for Au25 clusters when compared to pyrene and its radical's extinction coefficients. Very small negative absorption at around 570 nm for the hybrid system can be ascribed to the bleach of Au25(C6S)17PyS (Figure 6A). The results suggest that additional excited-state reactions are taking place in the hybrid the pyrene-Au25 system, which can be due to either electron- or energy-transfer processes. Energy transfer is ruled out since the ESA of the pyrene-Au25 is completely different from that of the bare Au25 or pyrene itself. Electron transfer from pyrene to Au25 is also ruled out as

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Figure 7. (A) Normalized ESA spectra of PySH, Au25(C6S)18, and Au25(C6S)17PyS at a time delay of 1 ps after excitation at 345 nm. (B) Schematic of photoinduced directional electron transfer from quantum-sized Au25 clusters to pyrene.

have suggested energy-transfer mechanisms26-30 from the molecule-gold nanocluster, which has been ruled out here. The transient absorption analysis of Au25(C6S)17PyS has shown an ESA spectrum with a maximum at ∼503 nm. The maximum at 503 nm matches very well with that of the anion radical of pyrene, as reported in the literature.42,47 Thus, transient absorption results provide direct evidence of directional electron transfer from Au25 to pyrene, giving rise to the anion radical of the dye molecule. Our results explicitly show that Au25 clusters can act as electron donors, opening new avenues for potential applications in solar energy harvesting, catalysis, and sensors. Finally, it is interesting to know why Au25 acts as an electron donor in the Au25(C6S)18PyS cluster system. For photoexcited pyrene-Au25 clusters, the electron transfer can occur via (i) electron transfer from Au25 to the excited pyrene or (ii) electron transfer from the excited pyrene to Au25. Pathway (ii) has been reported for a pyrene-functionalized gold nanoparticle system.34-36 To examine the thermodynamic feasibility of these pathways, we have estimated the free energy of electron transfer by using a simplified Rehm-Weller equation48 ΔGET ¼ e½E D=D þ - E A=A - - E 00 þ wp ð1Þ

the cation radical of the pyrene has the maximum absorption from 425 to 450 nm36 while the transients observed in the present system has a maximum at 503 nm. It is interesting to note that the present ESA with a maximum at 503 nm clearly matches the absorption spectrum of the pyrene anion radical observed earlier with pulse radiolysis and transient absorption measurements.42,47 Thus, present dynamics and spectral features are ascribed to electron transfer from Au25 clusters to the excited pyrene. Growth of the anion radical and decay of the ESA gives the charge separation and charge recombination rate constants. Figure 6C shows the growth of ESA at 480 nm, while decay is observed in the longer wavelength regions of 660 nm. As the kinetics is quite complex, global fit analysis of all of the kinetics at different wavelength regions is performed with single-value decomposition (Figure 6D). Interesting spectral features are obtained from this analysis. There are three main features in the dynamics, the first being a charge separation time constant of 580 fs, which matches with the ESA of the pyrene singlet state with a maximum of 530 nm.41 The amplitude spectra associated with the 8.2 ps time constant has a maximum absorption of 503 nm (Figure 6D), matched well with the absorption spectrum of the pyrene anion radical,42,47 which can be ascribed to the charge recombination The negative amplitude observed at around 570 nm (Figure 6D) is attributed to the recombination of the charged species to give rise to the long-lived ESA which resembles that of the Au25 ESA or the triplet states of pyrene. These results confirm that the forward electron transfer from Au25 clusters to pyrene is at around 580 fs while the recombination is multiexponential with a major contribution from the 8.2 ps time constant. Comparative transient absorption spectral analyses will give a better picture of the photoinduced electron-transfer pathways. Figure 7A shows the ESA spectra of Au25(C6S)18, PySH and Au25(C6S)17PyS at a time delay of 1 ps after excitation at 345 nm. It is evident from Figure 7A that the ESA spectra at 1 ps for the Au25(C6S)18 and PySH are entirely different from that of the Au25(C6S)17PyS. The fact that the transient absorption spectrum of the hybrid pyrene-Au25 does not match either Au25 or PySH rules out the possibility of energy transfer being the excitation deactivation pathway. Several literature reports of molecule-gold clusters


where ED/Dþ and E A/A- represent the oxidation and reduction potentials of the donor and acceptor, E00 is the excitation energy of the photoexcited species, and wp the Coulombic interaction term. Generally, in polar media, the magnitude of the Coulombic term is small48 and is neglected here. The free energies of electron transfer estimated49 for pathways (i) and (ii) are -1.37 and -0.16 eV, respectively. The vast difference in the driving force for electron transfer between pathways (i) and (ii) can explain the directional electron transfer observed here. In addition, with excitation at 345 nm, both Au25 and pyrene are photoexcited, and the electron transfer quenching can occur via (iii) electron transfer from the excited Au25 to pyrene. The driving force estimated for this scenario is, however, 0.73 eV, which is thermodynamically unfavorable. Thus, as depicted in Figure 7B, the electron-transfer quenching in pyrene-Au25 clusters occurs via electron transfer from the HOMO of Au25 to the HOMO of photoexcited pyrene. These thermodynamic considerations form the basis for the observed

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directional electron transfer. The large HOMO-LUMO gap in the quantum-sized Au25 clusters facilitates the directional electron transfer from Au25 to excited pyrene. In summary, precisely formulated Au25(C6S)17PyS clusters have been synthesized with one pyrene species attached to each Au25 cluster, and its electrochemical and optical properties have been investigated. It is unambiguously shown that Au25 clusters can work as electron donors with electrochemical and optical measurements. Ultrafast electron transfer from the Au25 cluster to pyrene was demonstrated with fluorescence upconversion and transient measurements. The interesting electron-transfer ability of Au25 clusters can be utilized in making better sensors, catalysts, and light harvesting systems.

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SUPPORTING INFORMATION AVAILABLE Synthesis and experimental details are furnished. This material is available free of charge via the Internet at http://pubs.acs.org. (12)

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: rama. [email protected] (G.R.); [email protected] (D.L.).

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ACKNOWLEDGMENT We acknowledge the support from U.S Army Grant No. W911QY-07-1-0003. G.R. acknowledges the support of the Western Michigan University Faculty Research and Creative Activities Support Fund (FRACASF). The work at Yonsei University was supported by the World Class University (R32-2008000-10217-0) and the Priority Research Centers (2009-0093823) programs through the National Research Foundation of Korea and the Yonsei University Research Fund. We acknowledge the help of Prof. Theodore Goodson (University of Michigan, Ann Arbor) in transient absorption measurements.

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