title: Tracking excited-state charge and spin dynamics in iron coordination complexes.
abstract: Crucial to many light-driven processes in transition metal complexes is the absorption and dissipation of energy by 3d electrons. But a detailed understanding of such non-equilibrium excited-state dynamics and their interplay with structural changes is challenging: a multitude of excited states and possible transitions result in phenomena too complex to unravel when faced with the indirect sensitivity of optical spectroscopy to spin dynamics and the flux limitations of ultrafast X-ray sources. Such a situation exists for archetypal polypyridyl iron complexes, such as [Fe(2,2'-bipyridine)3](2+), where the excited-state charge and spin dynamics involved in the transition from a low- to a high-spin state (spin crossover) have long been a source of interest and controversy. Here we demonstrate that femtosecond resolution X-ray fluorescence spectroscopy, with its sensitivity to spin state, can elucidate the spin crossover dynamics of [Fe(2,2'-bipyridine)3](2+) on photoinduced metal-to-ligand charge transfer excitation. We are able to track the charge and spin dynamics, and establish the critical role of intermediate spin states in the crossover mechanism. We anticipate that these capabilities will make our method a valuable tool for mapping in unprecedented detail the fundamental electronic excited-state dynamics that underpin many useful light-triggered molecular phenomena involving 3d transition metal complexes.
authors.affiliation: PULSE Institute, SLAC National Accelerator Laboratory, Stanford University, Stanford, California 94305, USA.
authors.forename: Wenkai
authors.initials: W
authors.lastname: Zhang
authors.affiliation: LCLS, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA.
authors.forename: Roberto
authors.initials: R
authors.lastname: Alonso-Mori
authors.affiliation: LCLS, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA.
authors.forename: Uwe
authors.initials: U
authors.lastname: Bergmann
authors.affiliation: European XFEL, D-22761 Hamburg, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Bressler
authors.affiliation: LCLS, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA.
authors.forename: Matthieu
authors.initials: M
authors.lastname: Chollet
authors.affiliation: European XFEL, D-22761 Hamburg, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Galler
authors.affiliation: European XFEL, D-22761 Hamburg, Germany.
authors.forename: Wojciech
authors.initials: W
authors.lastname: Gawelda
authors.affiliation: Department of Chemistry, Stanford University, Stanford, California 94305, USA.
authors.forename: Ryan G
authors.initials: RG
authors.lastname: Hadt
authors.affiliation: 1] PULSE Institute, SLAC National Accelerator Laboratory, Stanford University, Stanford, California 94305, USA [2] Department of Chemistry, Stanford University, Stanford, California 94305, USA.
authors.forename: Robert W
authors.initials: RW
authors.lastname: Hartsock
authors.affiliation: Department of Chemistry, Stanford University, Stanford, California 94305, USA.
authors.forename: Thomas
authors.initials: T
authors.lastname: Kroll
authors.affiliation: 1] Centre for Molecular Movies, Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark [2] Centre for Molecular Movies, Department of Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark.
authors.forename: Kasper S
authors.initials: KS
authors.lastname: Kjær
authors.affiliation: 1] Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany [2] Deutsches Elektronen Synchrotron, Notkestraße 85, 22607 Hamburg, Germany.
authors.forename: Katharina
authors.initials: K
authors.lastname: Kubiček
authors.affiliation: LCLS, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA.
authors.forename: Henrik T
authors.initials: HT
authors.lastname: Lemke
authors.affiliation: 1] PULSE Institute, SLAC National Accelerator Laboratory, Stanford University, Stanford, California 94305, USA [2] Department of Chemistry, Stanford University, Stanford, California 94305, USA.
authors.forename: Huiyang W
authors.initials: HW
authors.lastname: Liang
authors.affiliation: 1] PULSE Institute, SLAC National Accelerator Laboratory, Stanford University, Stanford, California 94305, USA [2] Department of Chemistry, Stanford University, Stanford, California 94305, USA.
authors.forename: Drew A
authors.initials: DA
authors.lastname: Meyer
authors.affiliation: Centre for Molecular Movies, Department of Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark.
authors.forename: Martin M
authors.initials: MM
authors.lastname: Nielsen
authors.affiliation: PULSE Institute, SLAC National Accelerator Laboratory, Stanford University, Stanford, California 94305, USA.
authors.forename: Carola
authors.initials: C
authors.lastname: Purser
authors.affiliation: LCLS, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA.
authors.forename: Joseph S
authors.initials: JS
authors.lastname: Robinson
authors.affiliation: 1] Department of Chemistry, Stanford University, Stanford, California 94305, USA [2] SSRL, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA.
authors.forename: Edward I
authors.initials: EI
authors.lastname: Solomon
authors.affiliation: PULSE Institute, SLAC National Accelerator Laboratory, Stanford University, Stanford, California 94305, USA.
authors.forename: Zheng
authors.initials: Z
authors.lastname: Sun
authors.affiliation: SSRL, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA.
authors.forename: Dimosthenis
authors.initials: D
authors.lastname: Sokaras
authors.affiliation: Centre for Molecular Movies, Department of Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark.
authors.forename: Tim B
authors.initials: TB
authors.lastname: van Driel
authors.affiliation: Wigner Research Centre for Physics, Hungarian Academy of Sciences, H-1525 Budapest, Hungary.
authors.forename: György
authors.initials: G
authors.lastname: Vankó
authors.affiliation: SSRL, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA.
authors.forename: Tsu-Chien
authors.initials: TC
authors.lastname: Weng
authors.affiliation: LCLS, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA.
authors.forename: Diling
authors.initials: D
authors.lastname: Zhu
authors.affiliation: PULSE Institute, SLAC National Accelerator Laboratory, Stanford University, Stanford, California 94305, USA.
authors.forename: Kelly J
authors.initials: KJ
authors.lastname: Gaffney