title: Spectral Signatures of Ultrafast Spin Crossover in Single Crystal [Fe(II)(bpy)3](PF6)2.
abstract: Solvated iron(II)-tris(bipyridine) ([Fe(II)(bpy)3](2+)) has been extensively studied with regard to the spin crossover (SCO) phenomenon. Herein, the ultrafast spin transition dynamics of single crystal [Fe(II)(bpy)3](PF6)2 was characterized for the first time using femtosecond transient absorption (TA) spectroscopy. The single crystal environment is of interest for experiments that probe the nuclear motions involved in the SCO transition, such as femtosecond X-ray and electron diffraction. We found that the TA at early times is very similar to what has been reported in solvated [Fe(II)(bpy)3](2+), whereas the later dynamics are perturbed in the crystal environment. The lifetime of the high-spin state is found to be much shorter (100 ps) than in solution due to chemical pressure exerted by the lattice. Oscillatory behavior was observed on both time scales. Our results show that single crystal [Fe(II)(bpy)3](PF6)2 serves as an excellent model system for localized molecular spin transitions.
authors.affiliation: Departments of Chemistry and Physics, University of Toronto, 80 St. George Street, Toronto, ON, M5S 3H6, Canada.|The Hamburg Centre for Ultrafast Imaging, Centre for Free Electron Laser Science, Max Planck Institute for the Structure and Dynamics of Matter, Bld. 99, Luruper Chaussee 149, 22761, Hamburg, Germany.
authors.forename: Ryan
authors.identifier: http://orcid.org/0000-0002-8407-6322
authors.initials: R
authors.lastname: Field
authors.affiliation: Departments of Chemistry and Physics, University of Toronto, 80 St. George Street, Toronto, ON, M5S 3H6, Canada.|The Hamburg Centre for Ultrafast Imaging, Centre for Free Electron Laser Science, Max Planck Institute for the Structure and Dynamics of Matter, Bld. 99, Luruper Chaussee 149, 22761, Hamburg, Germany.
authors.forename: Lai Chung
authors.identifier: http://orcid.org/0000-0002-7775-5933
authors.initials: LC
authors.lastname: Liu
authors.affiliation: European XFEL, Albert-Einstein-Ring 19, 22761, Hamburg, Germany.
authors.forename: Wojciech
authors.initials: W
authors.lastname: Gawelda
authors.affiliation: Departments of Chemistry and Physics, University of Toronto, 80 St. George Street, Toronto, ON, M5S 3H6, Canada.
authors.forename: Cheng
authors.initials: C
authors.lastname: Lu
authors.affiliation: Departments of Chemistry and Physics, University of Toronto, 80 St. George Street, Toronto, ON, M5S 3H6, Canada. dwayne.miller@mpsd.mpg.de.|The Hamburg Centre for Ultrafast Imaging, Centre for Free Electron Laser Science, Max Planck Institute for the Structure and Dynamics of Matter, Bld. 99, Luruper Chaussee 149, 22761, Hamburg, Germany. dwayne.miller@mpsd.mpg.de.
authors.forename: R J Dwayne
authors.initials: RJ
authors.lastname: Miller
keywords: charge transfer; electron-phonon coupling; iron; spin crossover; time-resolved spectroscopy