Search

Transition metal coordination compounds have a rich photochemistry and are interesting candidates as both light harvesters (photosensitizers) and catalysts in photocatalytic systems. Knowledge of electronic and molecular structure of excited states of photosensitizers and intermediates of catalysts is a key topic for rational design of systems for artificial photosynthesis. We describe recent advances in the field of time-resolved X-ray absorption spectroscopy that provide information on local structure around metal centers, their orbital structure and oxidation state, and thereby insights into the mechanisms of their photochemical reactions. Photosensitizers with metal centers, multicomponent molecular catalytic systems, and supramolecular model sensitizer-catalysts with two metal centers are used as examples to demonstrate the possibilities of the technique. We overview different experimental methods that can be used to investigate intermediates with lifetimes in the range from hundreds of picoseconds to hundreds of microseconds. Theoretical methods to extract the structural and electronic information from X-ray absorption near edge structure spectroscopy (XANES) are also discussed. [ABSTRACT FROM AUTHOR]

Rational development of efficient photocatalytic systems for hydrogen production requires understanding the catalytic mechanism and detailed information about the structure of intermediates in the catalytic cycle. We demonstrate how time-resolved X-ray absorption spectroscopy in the microsecond time range can be used to identify such intermediates and to determine their local geometric structure. This method was used to obtain the solution structure of the CoI intermediate of cobaloxime, which is a non-noble metal catalyst for solar hydrogen production from water. Distances between cobalt and the nearest ligands including two solvent molecules and displacement of the cobalt atom out of plane formed by the planar ligands have been determined. Combining in situ X-ray absorption and UV/Vis data, we demonstrate how slight modification of the catalyst structure can lead to the formation of a catalytically inactive CoI state under similar conditions. Possible deactivation mechanisms are discussed. [ABSTRACT FROM AUTHOR]

Rational development of efficient photocatalytic systems for hydrogen production requires understanding the catalytic mechanism and detailed information about the structure of intermediates in the catalytic cycle. We demonstrate how time-resolved X-ray absorption spectroscopy in the microsecond time range can be used to identify such intermediates and to determine their local geometric structure. This method was used to obtain the solution structure of the Co(I) intermediate of cobaloxime, which is a non-noble metal catalyst for solar hydrogen production from water. Distances between cobalt and the nearest ligands including two solvent molecules and displacement of the cobalt atom out of plane formed by the planar ligands have been determined. Combining in situ X-ray absorption and UV/Vis data, we demonstrate how slight modification of the catalyst structure can lead to the formation of a catalytically inactive Co(I) state under similar conditions. Possible deactivation mechanisms are discussed., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

This study uses transient X-ray absorption (XA) spectroscopy and time-dependent density functional theory (TD-DFT) to directly visualize the charge density around the metal atom and the surrounding ligands following an ultrafast metal-to-ligand charge-transfer (MLCT) process in the widely used Ru(II) solar cell dye, Ru(dcbpy)2(NCS)2 (termed N3). We measure the Ru L-edge XA spectra of the singlet ground ((1)A1) and the transient triplet ((3)MLCT) excited state of N3(4-) and perform TD-DFT calculations of 2p core-level excitations, which identify a unique spectral signature of the electron density on the NCS ligands. We find that the Ru 2p, Ru eg, and NCS π* orbitals are stabilized by 2.0, 1.0, and 0.6 eV, respectively, in the transient (3)MLCT state of the dye. These results highlight the role of the NCS ligands in governing the oxidation state of the Ru center.