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Manipulating charge transfer excited state relaxation and spin crossover in iron coordination complexes with ligand substitution

Zhang, Wenkai and Kjær, Kasper S. and Alonso-Mori, Roberto and Bergmann, Uwe and Chollet, Matthieu and Fredin, Lisa A. and Hadt, Ryan G. and Hartsock, Robert W. and Harlang, Tobias and Kroll, Thomas and Kubiček, Katharina and Lemke, Henrik T. and Liang, Huiyang W. and Liu, Yizhu and Nielsen, Martin M. and Persson, Petter and Robinson, Joseph S. and Solomon, Edward I. and Sun, Zheng and Sokaras, Dimosthenis and van Driel, Tim B. and Weng, Tsu-Chien and Zhu, Diling and Wärnmark, Kenneth and Sundström, Villy and Gaffney, Kelly J. (2017) Manipulating charge transfer excited state relaxation and spin crossover in iron coordination complexes with ligand substitution. Chemical Science, 8 (1). pp. 515-523. ISSN 2041-6520. PMCID PMC5341207. doi:10.1039/c6sc03070j.

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Developing light-harvesting and photocatalytic molecules made with iron could provide a cost effective, scalable, and environmentally benign path for solar energy conversion. To date these developments have been limited by the sub-picosecond metal-to-ligand charge transfer (MLCT) electronic excited state lifetime of iron based complexes due to spin crossover - the extremely fast intersystem crossing and internal conversion to high spin metal-centered excited states. We revitalize a 30 year old synthetic strategy for extending the MLCT excited state lifetimes of iron complexes by making mixed ligand iron complexes with four cyanide (CN) ligands and one 2,2'-bipyridine (bpy) ligand. This enables MLCT excited state and metal-centered excited state energies to be manipulated with partial independence and provides a path to suppressing spin crossover. We have combined X-ray Free-Electron Laser (XFEL) Kβ hard X-ray fluorescence spectroscopy with femtosecond time-resolved UV-visible absorption spectroscopy to characterize the electronic excited state dynamics initiated by MLCT excitation of [Fe(CN)_4(bpy)]^(2-). The two experimental techniques are highly complementary; the time-resolved UV-visible measurement probes allowed electronic transitions between valence states making it sensitive to ligand-centered electronic states such as MLCT states, whereas the Kβ fluorescence spectroscopy provides a sensitive measure of changes in the Fe spin state characteristic of metal-centered excited states. We conclude that the MLCT excited state of [Fe(CN)_4(bpy)]^)2-) decays with roughly a 20 ps lifetime without undergoing spin crossover, exceeding the MLCT excited state lifetime of [Fe(2,2'-bipyridine)_3]^(2+) by more than two orders of magnitude.

Item Type:Article
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URLURL TypeDescription CentralArticle Information
Bergmann, Uwe0000-0001-5639-166X
Fredin, Lisa A.0000-0002-4091-0899
Hadt, Ryan G.0000-0001-6026-1358
Persson, Petter0000-0001-7600-3230
Solomon, Edward I.0000-0003-0291-3199
Wärnmark, Kenneth0000-0002-9022-3165
Additional Information:© 2017 The Author(s). This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. The article was received on 12 Jul 2016, accepted on 24 Aug 2016 and first published on 25 Aug 2016. Experiments were carried out at LCLS and SSRL, National User Facilities operated for DOE, OBES by Stanford University. WZ, RWH, HWL, ZS, and KJG acknowledge support from the AMOS program within the Chemical Sciences, Geosciences, and Biosciences Division of the Office of Basic Energy Sciences, Office of Science, U. S. Department of Energy. EIS acknowledges support from the NSF CHE-0948211. RGH acknowledges a Gerhard Casper Stanford Graduate Fellowship and the Achievements Rewards for College Scientists (ARCS) Foundation. TK acknowledges the German Research Foundation (DFG), grant KR3611/2-1. KSK, MMN, and TBvD acknowledge support from the Danish National Research Foundation and from DANSCATT. KSK gratefully acknowledge the support of the Carlsberg Foundation and the Danish Council for Independent Research. YL, TH, KW, LF, PP, and VS acknowledge support from the Crafoord Foundation, the Swedish Research Council (VR), the Knut and Alice Wallenberg (KAW) Foundation, the European Research Council (ERC, 226136-VISCHEM) and the Swedish Energy Agency. KK thanks the Volkswagen Foundation for support under the Peter Paul Ewald fellowship program (Az.: I/85832). PP acknowledges support from the Swedish National Supercomputing Centre and the Lund University Intensive Computation Application Research Center supercomputing facilities.
Funding AgencyGrant Number
Department of Energy (DOE)UNSPECIFIED
Stanford UniversityUNSPECIFIED
Deutsche Forschungsgemeinschaft (DFG)KR3611/2-1
Danish National Research FoundationUNSPECIFIED
Carlsberg FoundationUNSPECIFIED
Danish Council for Independent ResearchUNSPECIFIED
Crafoord FoundationUNSPECIFIED
Swedish Research CouncilUNSPECIFIED
Knut and Alice Wallenberg FoundationUNSPECIFIED
European Research Council (ERC)226136-VISCHEM
Swedish Energy AgencyUNSPECIFIED
Volkswagen StiftungI/85832
Swedish National Supercomputing CentreUNSPECIFIED
Lund UniversityUNSPECIFIED
Issue or Number:1
PubMed Central ID:PMC5341207
Record Number:CaltechAUTHORS:20180612-103705875
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:87000
Deposited By: George Porter
Deposited On:12 Jun 2018 18:05
Last Modified:15 Nov 2021 20:44

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