Shedding Light on the Nature of Excited States in a Hydrogen Generating Supramolecular RuPt Catalyst by Ultrafast X-Ray Spectroscopy

Annemarie Huijser^a, Qing Pan^a, David van Duinen^a, Mads G. Laursen^b, Amal El Nahas^c, Pavel Chabera^c, Qingyu Kong^d, Xiaoyi Zhang^d, Kristoffer Haldrup^b, Wesley R. Browne^e, Grigory Smolentsev^f, Jens Uhlig^c

^aUniversity of Twente, MESA+ Institute for Nanotechnology, Netherlands

^bTechnical University of Denmark, Department of Physics, Fysikvej, 312, Kongens Lyngby, Denmark

^cLund University, Department of Chemical Physics, Getingevägen 60, Lund, 22241, Sweden

^dX-ray Sciences Division, Argonne National Laboratory, United States

^eUniversity of Groningen, The Netherlands, Nijenborgh, 4, Groningen, Netherlands

^fPaul Scherrer Institute, OLGA/113, Villigen PSI, 5232, Switzerland

Materials for Sustainable Development Conference (MATSUS)
Proceedings of nanoGe Fall Meeting 2018 (NFM18)

S1 Solar Fuel 18

Torremolinos, Spain, 2018 October 22nd - 26th

Organizers: Shannon Boettcher and Kevin Sivula

Oral, Annemarie Huijser, presentation 093
DOI: https://doi.org/10.29363/nanoge.nfm.2018.093
Publication date: 6th July 2018

Organometallic complexes with reactive metal centers are promising candidates for photocatalysis. To improve their performance, insight into the nature of excited states and control thereof is crucial. Ultrafast x-ray spectroscopy is a powerful method to achieve mechanistic insight into processes at catalytic metal moieties and is particularly useful to detect light-induced changes in oxidation state and coordination geometry. It can also be applied to probe the essential charge transfer step to the catalyst in real-time, including the potential involvement of atomic rearrangements.
We recently developed a series of new Ru-metal photocatalysts, of which the RuPt derivative showed a H2 turnover number of 80 after 6 h of irradiation at 470 nm. The photodynamics of RuPt studied by femtosecond transient absorption are highly complex and differ significantly from the RuPd analogue [1], indicating an important role of the catalytic moiety and motivating ultrafast x-ray absorption studies performed at the Advanced Photon Source at Argonne. This work particularly focuses at light-induced redox processes at the Pt moiety and the timescales thereof. Another question to be answered involves the local structure of the catalytic moiety and possible temporal or permanent changes.
The earliest differential absorption spectrum at 30 ps indicates that the Pt moiety is already reduced at this timescale. The signal intensity partially decays on a timescale of ca. 930 ps. The observation of a differential absorption signal at timescales far beyond indicates branching. The spectra at late timescales can be modelled by a hexa-coordinated PtIV (or possibly PtIII) species. This intermediate species may be formed by oxidative addition of iodine, is long-lived (>10 microseconds) and ultimately recovers into the original ground state structure. Hence, activation of the catalyst by a single photon may lead to withdrawal of two electrons. This mechanism presents a new paradigm for H2 generation by supramolecular photocatalysts.

[1] Q. Pan, F. Mecozzi, J.P. Korterik, J.G. Vos, W.R. Browne, A. Huijser, "The Critical Role Played by the Catalytic Moiety in the Early-Time Photodynamics of Hydrogen-Generating Bimetallic Photocatalysts", ChemPhysChem 17, 2654-2659 (2016).

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