Determining Quantitative Kinetics and the Structural Mechanism for Particle Growth in Porous Templates

Understanding the formation of nanoparticles and how they are influenced by a support is critically important for optimizing their activity. In situ pair distribution function (PDF) methods were used to probe the kinetics, mechanism, and energetics for Ag nanoparticle formation in a porous zeolite. The nanoscale structure detail and fast time resolution possible using the PDF method allows the separate processes of cation reduction, cluster formation, and nanoparticle growth to be distinguished, a multistep mechanism delineated, and rate constants and activation energies estimated for reduction and surface diffusion steps. Importantly, these insights are derived for the gas–solid phase reactions directly relevant to industrial processes

Probing the Anisotropic Distortion of Photoexcited Spin Crossover Complexes with Picosecond X‑ray Absorption Spectroscopy

For numerous spin crossover complexes, the anisotropic distortion of the first coordination shell around the transition metal center governs the dynamics of the high-spin/low-spin interconversion. However, this structural parameter remains elusive for samples that cannot be investigated with crystallography. The present work demonstrates how picosecond X-ray absorption spectroscopy is able to capture this specific deformation in the photoinduced high-spin state of solvated [Fe­(terpy)₂]²⁺, a complex which belongs to the prominent family of spin crossover building blocks with nonequivalent metal–ligand bonds. The correlated changes in Fe–N_Axial, Fe–N_Distal, and bite angle N_Distal–Fe–N_Axial extracted from the measurements are in very good agreement with those predicted by DFT calculations in <i>D</i>_2<i>d</i> symmetry. The outlined methodology is generally applicable to the characterization of ultrafast nuclear rearrangements around metal centers in photoactive molecular complexes and nanomaterials, including those that do not display long-range order

Toward Highlighting the Ultrafast Electron Transfer Dynamics at the Optically Dark Sites of Photocatalysts

Building a detailed understanding of the structure–function relationship is a crucial step in the optimization of molecular photocatalysts employed in water splitting schemes. The optically dark nature of their active sites usually prevents a complete mapping of the photoinduced dynamics. In this work, transient X-ray absorption spectroscopy highlights the electronic and geometric changes that affect such a center in a bimetallic model complex. Upon selective excitation of the ruthenium chromophore, the cobalt moiety is reduced through intramolecular electron transfer and undergoes a spin flip accompanied by an average bond elongation of 0.20 ± 0.03 Å. The analysis is supported by simulations based on density functional theory structures (B3LYP*/TZVP) and FEFF 9.0 multiple scattering calculations. More generally, these results exemplify the large potential of the technique for tracking elusive intermediates that impart unique functionalities in photochemical devices