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

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
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  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:
Grant/Contract Number:
AC02-76SF00515; CHE-0948211; 226136-VISCHEM; KR3611/2-1
Accepted Manuscript
Journal Name:
Chemical Science
Additional Journal Information:
Journal Name: Chemical Science; Journal ID: ISSN 2041-6520
Royal Society of Chemistry
Research Org:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org:
Country of Publication:
United States
OSTI Identifier: