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

Zhang, Wenkai; Kjær, Kasper S. LU ; Alonso-Mori, Roberto; Bergmann, Uwe; Chollet, Matthieu; Fredin, Lisa A. LU ; Hadt, Ryan G.; Hartsock, Robert W.; Harlang, Tobias LU and Kroll, Thomas, et al. (2016) In Chemical Science 8(1). p.515-523
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... (More)

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.

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published
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Chemical Science
volume
8
issue
1
pages
9 pages
publisher
Royal Society of Chemistry
external identifiers
  • scopus:85007271060
  • wos:000391454500060
ISSN
2041-6520
DOI
10.1039/C6SC03070J
language
English
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yes
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511685fc-e41b-440b-874c-062a8269daf3
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2017-02-24 07:11:53
date last changed
2017-10-01 05:31:08
@article{511685fc-e41b-440b-874c-062a8269daf3,
  abstract     = {<p>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<sup>-;</sup>) 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)<sub>4</sub>(bpy)]<sup>2-</sup>. 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)<sub>4</sub>(bpy)]<sup>2-</sup> decays with roughly a 20 ps lifetime without undergoing spin crossover, exceeding the MLCT excited state lifetime of [Fe(2,2′-bipyridine)<sub>3</sub>]<sup>2+</sup> by more than two orders of magnitude.</p>},
  author       = {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.},
  issn         = {2041-6520},
  language     = {eng},
  number       = {1},
  pages        = {515--523},
  publisher    = {Royal Society of Chemistry},
  series       = {Chemical Science},
  title        = {Manipulating charge transfer excited state relaxation and spin crossover in iron coordination complexes with ligand substitution},
  url          = {http://dx.doi.org/10.1039/C6SC03070J},
  volume       = {8},
  year         = {2016},
}