Abstract
In order to probe the structure of reaction intermediates of photochemical reactions a new setup for laser-initiated time-resolved X-ray absorption (XAS) measurements has been developed. With this approach the arrival time of each photon in respect to the laser pulse is measured and therefore full kinetic information is obtained. All X-rays that reach the detector are used to measure this kinetic information and therefore the detection efficiency of this method is high. The newly developed setup is optimized for time-resolved experiments in the microsecond range for samples with relatively low metal concentration (∼1mM). This setup has been applied to study a multicomponent photocatalytic system with a Co(dmgBF(2))(2) catalyst (dmg(2-) = dimethylglyoximato dianion), [Ru(bpy)(3)](2+) chromophore (bpy = 2,2'-bipyridine) and methyl viologen as the electron relay. On the basis of the analysis of hundreds of Co K-edge XAS spectra corresponding to different delay times after the laser excitation of the chromophore, the presence of a Co(i) intermediate is confirmed. The calculated X-ray transient signal for a model of Co(i) state with a 0.14 Å displacement of Co out of the dmg ligand plane and with the closest solvent molecule at a distance of 2.06 Å gives reasonable agreement with the experimental data.
Highlights
Development of catalysts for hydrogen evolution from water using sunlight is a challenging task of great practical importance.[1,2] Molecular catalysts based onPaper coordination complexes of 3d metals,[3,4] and especially cobalt,[5] have high potential for large-scale applications since they are earth-abundant in contrast to more robust platinum-based catalysts, that are more expensive and less abundant
In the present manuscript we report a pump–sequential-probes setup to study photocatalytic intermediates that have lifetimes in the microsecond range
This setup is realized at the SuperXAS beamline of the Swiss Light Source and described in the experimental setup section
Summary
Paper coordination complexes of 3d metals,[3,4] and especially cobalt,[5] have high potential for large-scale applications since they are earth-abundant in contrast to more robust platinum-based catalysts, that are more expensive and less abundant. In the present manuscript we report a pump–sequential-probes setup to study photocatalytic intermediates that have lifetimes in the microsecond range This setup is realized at the SuperXAS beamline of the Swiss Light Source and described in the experimental setup section. With this approach the arrival time of each photon in respect to the laser pulse is measured (method known as timetagged photon counting) and in contrast to currently available pump– probe setups, full kinetic information is obtained without any delay scan. In the application section we demonstrate how this method is used to identify the intermediate states of a Co(dmgBF2)[2] catalyst in the multicomponent system and to probe the structure of the Co(II) and Co(I) states of the catalyst in solution
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