Ultrafast Excited State Relaxation of a Metalloporphyrin Revealed by Femtosecond X-ray Absorption Spectroscopy

Journal Article · Wed Jul 06 00:00:00 EDT 2016 · Journal of the American Chemical Society

DOI:https://doi.org/10.1021/jacs.6b02176· OSTI ID:1390845

Shelby, Megan L. ^[1]; Lestrange, Patrick J. ^[2]; Jackson, Nicholas E. ^[3]; Haldrup, Kristoffer ^[4]; Mara, Michael W. ^[1]; Stickrath, Andrew B. ^[5]; Zhu, Diling ^[6]; Lemke, Henrik T. ^[6]; Chollet, Matthieu ^[6]; Hoffman, Brian M. ^[3]; Li, Xiaosong ^[2]; Chen, Lin X. ^[1]

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1. Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States; Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
2. Department of Chemistry, University of Washington, Seattle, Washington 98195, United States
3. Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
4. Physics Department, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark
5. Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States
6. LCLS, SLAC National Laboratory, Menlo Park, California 94025, United States

Photoexcited Nickel(II) tetramesitylporphyrin (NiTMP), like many open-shell metalloporphyrins, relaxes rapidly through multiple electronic states following an initial porphyrin-based excitation, some involving metal centered electronic configuration changes that could be harnessed catalytically before excited state relaxation. While a NiTMP excited state present at 100 ps was previously identified by X-ray transient absorption (XTA) spectroscopy at a synchrotron source as a relaxed (d,d) state, the lowest energy excited state (J. Am. Chem. Soc., 2007, 129, 9616 and Chem. Sci., 2010, 1, 642), structural dynamics before thermalization were not resolved due to the similar to 100 ps duration of the available X-ray probe pulse. Using the femtosecond (fs) X-ray pulses of the Linac Coherent Light Source (LCLS), the Ni center electronic configuration from the initial excited state to the relaxed (d,d) state has been obtained via ultrafast Ni K-edge XANES (X-ray absorption near edge structure) on a time scale from hundreds of femtoseconds to 100 ps. This enabled the identification of a short-lived Ni(I) species aided by time-dependent density functional theory (TDDFT) methods. Computed electronic and nuclear structure for critical excited electronic states in the relaxation pathway characterize the dependence of the complex's geometry on the electron occupation of the 3d orbitals. Calculated XANES transitions for these excited states assign a short-lived transient signal to the spectroscopic signature of the Ni(I) species, resulting from intramolecular charge transfer on a time scale that has eluded previous synchrotron studies. These combined results enable us to examine the excited state structural dynamics of NiTMP prior to thermal relaxation and to capture intermediates of potential photocatalytic significance.

• Research Organization: Argonne National Lab. (ANL), Argonne, IL (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• DOE Contract Number: AC02-06CH11357
• OSTI ID: 1390845
• Journal Information: Journal of the American Chemical Society, Vol. 138, Issue 28; ISSN 0002-7863
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English