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Charge Transfer Dynamics Between Photoexcited CdS Nanorods and Mononuclear Ru Water-Oxidation Catalysts Huan-Wei Tseng, Molly B. Wilker, Niels H. Damrauer, and Gordana Dukovic J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja400178g • Publication Date (Web): 13 Feb 2013 Downloaded from http://pubs.acs.org on February 17, 2013
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Charge Transfer Dynamics Between Photoexcited CdS Nanorods and Mononuclear Ru Water-Oxidation Catalysts Huan-‐Wei Tseng,‡ Molly B. Wilker,‡ Niels H. Damrauer,* and Gordana Dukovic* Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, Colorado 80309, USA Supporting Information Placeholder ABSTRACT: We describe the charge transfer interactions between photoexcited CdS nanorods and a mononuclear + water oxidation catalyst derived from the [Ru(bpy)(tpy)Cl] parent structure. Upon excitation, hole transfer from CdS 2+ 3+ oxidizes the catalyst (Ru →Ru ) on a 100 ps – 1 ns timescale. This is followed by 10 – 100 ns electron transfer (ET) that 3+ reduces the Ru center. The relatively slow ET dynamics may provide opportunities for accumulation of the multiple holes at the catalyst, which is necessary for water oxidation.
Using solar photons to drive fuel-‐generating reactions, such as splitting water into H2 and O2, will allow for storage 1 of solar energy necessary for on-‐demand availability. In-‐ spired by natural photosynthesis, our interest is in exploring artificial systems that feature light absorbers directly coupled 2 to redox catalysts. Colloidal semiconductor nanocrystals are attractive light harvesters because they have tunable absorp-‐ 5 7 -‐1 -‐1 tion spectra and high molar absorptivities (10 -‐10 M cm ). + Their coupling with H reduction catalysts has been recently 2,3 4,5 reviewed and new studies continue to be reported. The resulting hybrid structures are capable of light-‐driven H2 generation with the use of sacrificial electron donors. Water oxidation, the other half reaction of water splitting, is a mechanistically complicated process involving the transfer of multiple electrons and protons and the formation of an O-‐O 6 bond. Over the last three decades, there has been significant progress in the discovery of ruthenium complexes that cata-‐ 7-‐13 lyze water oxidation. Consistent with the complexity of this reaction, the molecular catalysts operate at turnover frequencies (TOFs) that are considerably lower than TOFs + 2,7 for H reduction. For this reason, successful delivery of photoexcited holes to the catalyst is particularly critical. Understanding the competition between charge transfer and photophysical carrier deactivation pathways, such as elec-‐ tron-‐hole recombination, is of paramount importance for the design of nanocrystal-‐based water-‐splitting systems. Herein, we describe the charge transfer dynamics between photoexcited CdS nanorods (NRs) and a mononuclear water-‐ oxidation catalyst [Ru(deeb)(tpy)Cl](PF6) (deeb = diethyl 2,2’-‐bipyridine-‐4,4’-‐dicarboxylate, tpy = 2,2’:6’,2”-‐terpyridine) in methanol. We refer to the catalyst as complex 1 (Fig 1b). The interaction between the two species results in concen-‐ tration-‐dependent quenching of CdS NR photoluminescence
(PL) and has a marked impact on CdS excited state dynam-‐ ics, as measured by transient absorption (TA) spectroscopy. We find that there are two distinct charge transfer steps in the hybrid nanocrystal-‐catalyst system: hole transfer (HT) followed by electron transfer (ET), both from the photoexcit-‐ ed CdS NR to complex 1, with the overall result being elec-‐ tron-‐hole recombination at the metal center. The HT occurs on the timescale of 100 ps – 1 ns, while the subsequent ET occurs in 10-‐100 ns. The relatively slow rate of recombination exposes opportunities for diverting the photoexcited CdS electrons via auxiliary electron transfer processes.
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Figure 3. (a) TA decay kinetics at 470 nm for CdS NRs in the presence and absence of 1 (λpump = 400 nm). The results point to electron transfer that has an onset that is delayed by 250 ps. (b) Proposed charge transfer steps between photoexcited CdS and 1. (c) TA decay kinetics at 470 nm for CdS NRs alone, and in the presence of: complex 1, the hole scavenger ascorbate (Asc), the electron acceptor methylene blue (MB), and both 1 and Asc. As described in the text, these kinetic traces are consistent with the 3 charge transfer steps shown in (b).
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