Abstract
This report will describe our recent studies of transition metal complex structural dynamics on the fs and ps time scales using an X-ray free electron laser source, Linac Coherent Light Source (LCLS). Ultrafast XANES spectra at the Ni K-edge of nickel(ii) tetramesitylporphyrin (NiTMP) were measured for optically excited states at a timescale from 100 fs to 50 ps, providing insight into its sub-ps electronic and structural relaxation processes. Importantly, a transient reduced state Ni(i) (π, 3dx2-y2) electronic state is captured through the interpretation of a short-lived excited state absorption on the low-energy shoulder of the edge, which is aided by the computation of X-ray transitions for postulated excited electronic states. The observed and computed inner shell to valence orbital transition energies demonstrate and quantify the influence of the electronic configuration on specific metal orbital energies. A strong influence of the valence orbital occupation on the inner shell orbital energies indicates that one should not use the transition energy from 1s to other orbitals to draw conclusions about the d-orbital energies. For photocatalysis, a transient electronic configuration could influence d-orbital energies up to a few eV and any attempt to steer the reaction pathway should account for this to ensure that external energies can be used optimally in driving desirable processes. NiTMP structural evolution and the influence of the porphyrin macrocycle conformation on relaxation kinetics can be likewise inferred from this study.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 639-658 |
| Number of pages | 20 |
| Journal | Faraday Discussions |
| Volume | 194 |
| DOIs | |
| State | Published - 2016 |
Funding
We acknowledge support for this work from the Solar Energy Photochemistry program (experimental work) and Ultrafast Initiative (theoretical work) of the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, through Argonne National Laboratory under Contract No. DE-AC02-06CH11357 and MLS is supported by the National Institute of Health, under Contract No. R01-GM115761 (LXC) and R01-HL63203 (BMH). Use of the Linac Coherent Light Source (LCLS), SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. Computations on modeled spectra were facilitated through the use of advanced computational, storage, and networking infrastructure provided by the Hyak supercomputer system at the University of Washington, funded by the Student Technology Fee. PJL is also grateful for support by the State of Washington through the University of Washington Clean Energy Institute. MLS also thanks the National Institute of General Medical Sciences of NIH for support through the Molecular Biophysics training grant administered by Northwestern University (5T32 GM008382). KH gratefully acknowledges support from DANSCATT and from the Villum and Carlsberg Foundations. The authors would like to thank Tim Brandt Van Driel for invaluable assistance with the phase cavity timing correction by providing a means to calibrate the phase cavity data.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry