title: Spectroscopic, Structural, and Kinetic Investigation of the Ultrafast Spin Crossover in an Unusual Cobalt(II) Semiquinonate Radical Complex.
abstract: A comprehensive spectroscopic and structural investigation of [CoII(l-N4tBu2)(dbsq)][B(p-C6H4Cl)4] (1, l-N4tBu2=N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane, dbsq1-=3,5-di-tert-butylsemiquinonate), the first known octahedral complex with a low-spin (ls) CoIIsemiquinonate ground state, is reported. Above 200 K, solids as well as solutions of 1 exhibit thermally induced spin-crossover (SCO) from the ls to the high-spin (hs) CoIIsemiquinonate state instead of the frequently observed valence tautomerism from ls CoIIIcatecholate to hs CoIIsemiquinonate. DFT calculations demonstrate that the (closed shell) CoIIIcatecholate suffers from a triplet instability leading to the ls CoIIsemiquinonate ground state. The thorough temperature-dependent spectroscopic study of the SCO enables a photophysical investigation. Thus, by selective photoexcitation of the ls fraction of 1 in solution at room temperature, ultrafast conversion to the hs state is observed using femtosecond electronic and IR-vibrational (infrared) transient absorption spectroscopy. The kinetics of the photocycle is described by a stretched exponential with τ=3.3-3.6 ps and β=0.52-0.54, representing an upper limit for the hs-ls relaxation time. This is, to our knowledge, the fastest interconversion ever determined for a SCO complex, and is attributed to the special situation that in 1 a CoIIcomplex is coordinated to a π-radical ligand allowing very efficient coupling between the ls and hs spin states.
authors.affiliation: Department of Physics, University of Kaiserslautern, Erwin Schrödinger Str. 46, 67663, Kaiserslautern, Germany.
authors.forename: Fabian
authors.initials: F
authors.lastname: Rupp
authors.affiliation: Department of Physics, University of Kaiserslautern, Erwin Schrödinger Str. 46, 67663, Kaiserslautern, Germany.
authors.forename: Katharina
authors.initials: K
authors.lastname: Chevalier
authors.affiliation: Department of Chemistry, University of Kaiserslautern, Erwin Schrödinger Str. 52, 67663, Kaiserslautern, Germany.
authors.forename: Michèle
authors.initials: M
authors.lastname: Graf
authors.affiliation: Department of Chemistry, University of Kaiserslautern, Erwin Schrödinger Str. 52, 67663, Kaiserslautern, Germany.
authors.forename: Markus
authors.initials: M
authors.lastname: Schmitz
authors.affiliation: Department of Chemistry, University of Kaiserslautern, Erwin Schrödinger Str. 52, 67663, Kaiserslautern, Germany.
authors.forename: Harald
authors.initials: H
authors.lastname: Kelm
authors.affiliation: Department of Chemistry, University of Kaiserslautern, Erwin Schrödinger Str. 52, 67663, Kaiserslautern, Germany.
authors.forename: Anneken
authors.initials: A
authors.lastname: Grün
authors.affiliation: Department of Chemistry, University of Kaiserslautern, Erwin Schrödinger Str. 52, 67663, Kaiserslautern, Germany.
authors.forename: Manuel
authors.initials: M
authors.lastname: Zimmer
authors.affiliation: Department of Chemistry, University of Kaiserslautern, Erwin Schrödinger Str. 52, 67663, Kaiserslautern, Germany.
authors.forename: Markus
authors.initials: M
authors.lastname: Gerhards
authors.affiliation: Department of Chemistry, University of Kaiserslautern, Erwin Schrödinger Str. 52, 67663, Kaiserslautern, Germany.
authors.forename: Christoph
authors.initials: C
authors.lastname: van Wüllen
authors.affiliation: Department of Chemistry, University of Kaiserslautern, Erwin Schrödinger Str. 52, 67663, Kaiserslautern, Germany.
authors.forename: Hans-Jörg
authors.identifier: http://orcid.org/0000-0002-8652-2154
authors.initials: HJ
authors.lastname: Krüger
authors.affiliation: Department of Physics, University of Kaiserslautern, Erwin Schrödinger Str. 46, 67663, Kaiserslautern, Germany.
authors.forename: Rolf
authors.identifier: http://orcid.org/0000-0002-3629-8334
authors.initials: R
authors.lastname: Diller
keywords: cobalt dioxolene complexes; kinetics; metal radical complexes; spin crossover; time resolved spectroscopy