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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.. 2016. "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 = 2016,
month = 8
}

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  • Here, we have used femtosecond resolution UV-visible and Kβ x-ray emission spectroscopy to characterize the electronic excited state dynamics of [Fe(bpy) 2(CN) 2], where bpy=2,2'-bipyridine, initiated by metal-to-ligand charge transfer (MLCT) excitation. The excited-state absorption in the transient UV-visible spectra, associated with the 2,2'-bipyridine radical anion, provides a robust marker for the MLCT excited state, while the transient Kβ x-ray emission spectra provide a clear measure of intermediate and high spin metal-centered excited states. From these measurements, we conclude that the MLCT state of [Fe(bpy) 2(CN) 2] undergoes ultrafast spin crossover to a metal-centered quintet excited state through a shortmore » lived metal-centered triplet transient species. These measurements of [Fe(bpy) 2(CN) 2] complement prior measurement performed on [Fe(bpy) 3] 2+ and [Fe(bpy)(CN) 4] 2– in dimethylsulfoxide solution and help complete the chemical series [Fe(bpy) N(CN) 6–2N] 2N-4, where N = 1–3. The measurements confirm that simple ligand modifications can significantly change the relaxation pathways and excited state lifetimes and support the further investigation of light harvesting and photocatalytic applications of 3 d transition metal complexes.« less
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  • The lifetimes of the ligand-to-metal charge-transfer (LMCT) excited states produced by 527-nm excitation of M(phen)/sub 3//sup 3 +/ and M(bpy)/sub 3//sup 3 +/ (M = Fe or Os (iron or osmium); phen = 1,10-phenanthroline; bpy = 2,2'-bipyridine) are reported at temperatures between 10 and 295 K and are compared to the low-temperature lifetimes of the metal-to-ligand charge-transfer (MLCT) excited states of the corresponding M(phen)/sub 3//sup 2 +/ and M(bpy)/sub 3//sup 2 +/ complexes. The observations that the MLCT states of the Osmium(II) complexes are 10/sup 5/-10/sup 6/ times longer-lived than the LMCT states of the Os(III) complexes (microseconds vs. picoseconds)more » and that the lifetimes of both types of charge-transfer states are increased less than a factor of 3 upon deuteration are rationalized in terms of radiationless-decay theory. The dominant active modes for both types of charge-transfer states appear to be intermediate-frequency (1300 to 1600 cm/sup -1/) vibrations and it is proposed that the shorter lifetimes of the Os(III) complexes are mainly a consequence of their smaller energy gaps. 5 figures, 3 tables.« less
  • In contrast to Ru(bpy)/sub 3//sup 2 +/ and Os(bpy)/sub 3//sup 2 +/ (bpy = 2,2'-bipyridine) the lowest excited state of Fe(bpy)/sub 3//sup 2 +/ is LF (ligand field) rather than MLCT (metal-to-ligand charge transfer) in character. Replacement of a bpy by two stronger field CN/sup -/ ligands to give Fe(bpy)/sub 2/(CN)/sub 2/ does not alter this ordering. Here the authors report observations that show that in water this ordering is retained for Fe(bpy)(CN)/sub 4//sup 2 -/ even with its stronger ligand field but that the extraordinary solvent sensitivity of this complex can be used to access the MLCT state: themore » MLCT state is observed in the weak acceptor solvent acetone.« less
  • The absorption spectra of Ru{sup II}(NH{sub 3}){sub 5}L and Ru{sup III}(NH{sub 3}){sub 5}L (L is an aromatic N-heterocycle or nitrile) complexes in 50:50 glycerol-water glasses at 77 K (D{sub s} = 3.9) are a function of the applied field in the 10{sup 6}-10{sup 7} V/m range. Analysis of the spectra in terms of the Liptay equations yields ground-excited state dipole-moment differences ranging from 4 to 37 D, depending upon the nature of L. The measured dipole moment differences, particularly those for the MLCT transitions, are much smaller than the values estimated from a simple consideration of the electron-transfer distances. Themore » discrepancy between the observed and naive dipole moment estimates arises mainly from the multielectron nature of the response to excitation. Good agreement is obtained with the predictions of a model which includes refinement of the effective electron-transfer distance, the shift in the valence electron distribution in the excited state, and the effects of electron delocalization ({pi}-backbonding for Ru(II) and {pi}-bonding for the Ru(III) complexes). Other contributions, namely the dipole moment induced by the NH{sub 3} ligands and by the surrounding solvent molecules, are also considered. 55 refs., 11 figs., 4 tabs.« less