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

Abstract

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 statemore » characteristic of metal-centered excited states. Here, 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.« less

Authors:
 [1];  [2];  [1];  [1];  [1];  [3];  [4];  [5];  [3];  [5];  [6];  [1];  [1];  [3];  [7];  [3];  [1];  [5];  [1];  [1] more »;  [7];  [1];  [1];  [3];  [3];  [1] « less
  1. SLAC National Accelerator Lab., Menlo Park, CA (United States)
  2. SLAC National Accelerator Lab., Menlo Park, CA (United States); Lund Univ., Lund (Sweden); Technical Univ. of Denmark, Lyngby (Denmark)
  3. Lund Univ., Lund (Sweden)
  4. Stanford Univ., Stanford, CA (United States)
  5. SLAC National Accelerator Lab., Menlo Park, CA (United States); Stanford Univ., Stanford, CA (United States)
  6. Max Planck Institute for Biophysical Chemistry, Gottingen (Germany)
  7. Technical Univ. of Denmark, Lyngby (Denmark)
Publication Date:
Research Org.:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1335920
Grant/Contract Number:
AC02-76SF00515; CHE-0948211; 226136-VISCHEM; KR3611/2-1
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Chemical Science
Additional Journal Information:
Journal Name: Chemical Science; Journal ID: ISSN 2041-6520
Publisher:
Royal Society of Chemistry
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Zhang, Wenkai, Kjaer, Kasper S., Alonso-Mori, Roberto, Bergmann, Uwe, Chollet, Matthieu, Fredin, Lisa A., Hadt, Ryan G., Hartsock, Robert W., Harlang, Tobias, Kroll, Thomas, Kubicek, Katharina, Lemke, Henrik T., Liang, Huiyang W., Liu, Yizhu, Nielsen, Martin M., Persson, Petter, Robinson, Joseph S., Solomon, Edward I., Sun, Zheng, Sokaras, Dimosthenis, van Driel, Tim B., Weng, Tsu -Chien, Zhu, Diling, Warnmark, Kenneth, Sundstrom, Villy, and Gaffney, Kelly J. Manipulating charge transfer excited state relaxation and spin crossover in iron coordination complexes with ligand substitution. United States: N. p., 2016. Web. doi:10.1039/C6SC03070J.
Zhang, Wenkai, Kjaer, Kasper S., Alonso-Mori, Roberto, Bergmann, Uwe, Chollet, Matthieu, Fredin, Lisa A., Hadt, Ryan G., Hartsock, Robert W., Harlang, Tobias, Kroll, Thomas, Kubicek, Katharina, Lemke, Henrik T., Liang, Huiyang W., Liu, Yizhu, Nielsen, Martin M., Persson, Petter, Robinson, Joseph S., Solomon, Edward I., Sun, Zheng, Sokaras, Dimosthenis, van Driel, Tim B., Weng, Tsu -Chien, Zhu, Diling, Warnmark, Kenneth, Sundstrom, Villy, & Gaffney, Kelly J. Manipulating charge transfer excited state relaxation and spin crossover in iron coordination complexes with ligand substitution. United States. doi:10.1039/C6SC03070J.
Zhang, Wenkai, Kjaer, Kasper S., Alonso-Mori, Roberto, Bergmann, Uwe, Chollet, Matthieu, Fredin, Lisa A., Hadt, Ryan G., Hartsock, Robert W., Harlang, Tobias, Kroll, Thomas, Kubicek, Katharina, Lemke, Henrik T., Liang, Huiyang W., Liu, Yizhu, Nielsen, Martin M., Persson, Petter, Robinson, Joseph S., Solomon, Edward I., Sun, Zheng, Sokaras, Dimosthenis, van Driel, Tim B., Weng, Tsu -Chien, Zhu, Diling, Warnmark, Kenneth, Sundstrom, Villy, and Gaffney, Kelly J. Thu . "Manipulating charge transfer excited state relaxation and spin crossover in iron coordination complexes with ligand substitution". United States. doi:10.1039/C6SC03070J. https://www.osti.gov/servlets/purl/1335920.
@article{osti_1335920,
title = {Manipulating charge transfer excited state relaxation and spin crossover in iron coordination complexes with ligand substitution},
author = {Zhang, Wenkai and Kjaer, 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 Kubicek, 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 Warnmark, Kenneth and Sundstrom, Villy and Gaffney, Kelly J.},
abstractNote = {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. Here, 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.},
doi = {10.1039/C6SC03070J},
journal = {Chemical Science},
number = ,
volume = ,
place = {United States},
year = {Thu Aug 25 00:00:00 EDT 2016},
month = {Thu Aug 25 00:00:00 EDT 2016}
}

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