X-ray Transient Absorption and Picosecond IR Spectroscopy of Fulvalene(tetracarbonyl)diruthenium on Photoexcitation

11Nsciescopu

Ultrafast dynamics of two copper bis-phenanthroline complexes measured by x-ray transient absorption spectroscopy:Paper

Ultrafast structural dynamics of the metal to ligand charge transfer (MLCT) states of two copper bis-phenanthroline complexes were captured by using x-ray transient absorption (XTA) spectroscopy at the Linac Coherent Light Source and further described by theoretical calculations. These complexes have the general formula [Cu(I)(R)_2]+, where R = 2,9-dimethyl-1,10-phenanthroline (dmp) and 2,9-diphenyl-1,10-phenanthroline disulfonic acid disodium salt (dpps). [Cu(I)(dmp)_2]+ has methyl groups at the 2,9 positions of phenanthroline (phen) and adopts a pseudo-tetrahedral geometry. In contrast, [Cu(I)(dpps)_2]+ possesses two bulky phenyl-sulfonate groups attached to each phen ligand that force the molecule to adopt a flattened tetrahedral geometry in the ground state. Previously, optical transient absorption (OTA) and synchrotron based XTA experiments with 100 ps time resolution have been employed to study the relationship between structural distortions and excited state relaxation pathways in the two complexes. However, the dynamics of the MLCT transition during the first few picoseconds after excitation in these complexes remained unclear because of limitations in element specificity in OTA and in the time resolution of synchrotron sources in XTA. In this experiment, the local coordination geometry and oxidation state of copper were probed with a temporal resolution of ~300 fs. Unexpectedly, the depletion of the Cu(I) signal due to the MLCT transition was found to be non-impulsive in the case of [Cu(I)(dpps)_2]+ with a time constant of 0.6 ps, while the Cu(I) depletion in [Cu(I)(dmp)_2]+ was instantaneous within the 300 fs instrument response time. The slower Cu(I) depletion kinetics in [Cu(I)(dpps)_2]+, previously unobserved in femtosecond OTA experiments, is likely due to intramolecular motions on the sub-picosecond time scale that could alter the localization of the transferred electron in the phen ligands

Characterization of an amorphous iridium water-oxidation catalyst electrodeposited from organometallic precursors

Upon electrochemical oxidation of the precursor complexes [Cp*Ir(H2O)3]SO4 (1) or [(Cp*Ir)2(OH)3]OH (2) (Cp* = pentamethylcyclopentadienyl), a blue layer of amorphous iridium oxide containing a carbon admixture (BL) is deposited onto the anode. The solid-state, amorphous iridium oxide material that is formed from the molecular precursors is significantly more active for water-oxidation catalysis than crystalline IrO2 and functions as a remarkably robust catalyst, capable of catalyzing water oxidation without deactivation or significant corrosion for at least 70 h. Elemental analysis reveals that BL contains carbon that is derived from the Cp* ligand ( 3c 3% by mass after prolonged electrolysis). Because the electrodeposition of precursors 1 or 2 gives a highly active catalyst material, and electrochemical oxidation of other iridium complexes seems not to result in immediate conversion to iridium oxide materials, we investigate here the nature of the deposited material. The steps leading to the formation of BL and its structure have been investigated by a combination of spectroscopic and theoretical methods. IR spectroscopy shows that the carbon content of BL, while containing some C-H bonds intact at short times, is composed primarily of components with C=O fragments at longer times. X-ray absorption and X-ray absorption fine structure show that, on average, the six ligands to iridium in BL are likely oxygen atoms, consistent with formation of iridium oxide under the oxidizing conditions. High-energy X-ray scattering (HEXS) and pair distribution function (PDF) analysis (obtained ex situ on powder samples) show that BL is largely free of the molecular precursors and is composed of small, <7 \uc5, iridium oxide domains. Density functional theory (DFT) modeling of the X-ray data suggests a limited set of final components in BL; ketomalonate has been chosen as a model fragment because it gives a good fit to the HEXS-PDF data and is a potential decomposition product of Cp*. \ua9 2013 American Chemical Society