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

Wenkai Zhang, Kasper Skov Kjær, Roberto Alonso-Mori, Uwe Bergmann, Matthieu Chollet, Lisa A. Fredin, Ryan G. Hadt, Robert W. Hartsock, Tobias Harlang, Thomas Kroll, Katharina Kubiček, Henrik T. Lemke, Huiyang W. Liang, Yizhu Liu, Martin Meedom Nielsen, Petter Persson, Joseph S. Robinson, Edward I. Solomon, Zheng Sun, Dimosthenis SokarasTim Brandt van Driel, Tsu Chien Weng, Diling Zhu, Kenneth Wärnmark, Villy Sundström, Kelly J. Gaffney

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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.
Original languageEnglish
JournalChemical Science
Issue number1
Pages (from-to)515-523
Number of pages9
Publication statusPublished - 2017

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