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Title: Ligand manipulation of charge transfer excited state relaxation and spin crossover in [Fe(2,2'-bipyridine) 2(CN) 2]

Abstract

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 short 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.

Authors:
 [1];  [2];  [3];  [4];  [3];  [5];  [6];  [7];  [3];  [8];  [3];  [6];  [9];  [10];  [3];  [11];  [3];  [12];  [13];  [3] more »;  [9];  [9];  [9];  [14] « less
  1. Stanford Univ., Menlo Park, CA (United States); Technical Univ. of Denmark, Lyngby (Denmark); Lund Univ., Lund (Sweden)
  2. Stanford Univ., Menlo Park, CA (United States); Beijing Normal Univ., Beijing (China)
  3. SLAC National Accelerator Lab., Menlo Park, CA (United States)
  4. Stanford Univ., Menlo Park, CA (United States)
  5. Stanford Univ., Stanford, CA (United States)
  6. Stanford Univ., Menlo Park, CA (United States); Stanford Univ., Stanford, CA (United States)
  7. Technical Univ. of Denmark, Lyngby (Denmark); Lund Univ., Lund (Sweden)
  8. Deutsches Elektronen-Synchrotron (DESY), Hamburg (Germany); Max Planck Institute for Biophysical Chemistry, Gottingen (Germany)
  9. Lund Univ., Lund (Sweden)
  10. Technical Univ. of Denmark, Lyngby (Denmark)
  11. Stanford Univ., Stanford, CA (United States); SLAC National Accelerator Lab., Menlo Park, CA (United States)
  12. Technical Univ. of Denmark, Lyngby (Denmark); SLAC National Accelerator Lab., Menlo Park, CA (United States)
  13. HPSTAR, Shanghai (China)
  14. Stanford Univ., Menlo Park, CA (United States); SLAC National Accelerator Lab., Menlo Park, CA (United States)
Publication Date:
Research Org.:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1362282
Grant/Contract Number:
AC02-76SF00515; AMOS program within the Chemical Sciences, Geosciences, and Biosciences Division of the Office of Ba
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Structural Dynamics
Additional Journal Information:
Journal Volume: 4; Journal Issue: 4; Journal ID: ISSN 2329-7778
Publisher:
American Crystallographic Association/AIP
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 36 MATERIALS SCIENCE; 75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY

Citation Formats

Kjaer, Kasper S., Zhang, Wenkai, Alonso-Mori, Roberto, Bergmann, Uwe, Chollet, Matthieu, Hadt, Ryan G., Hartsock, Robert W., Harlang, Tobias, Kroll, Thomas, Kubicek, Katharina, Lemke, Henrik T., Liang, Huiyang W., Liu, Yizhu, Nielsen, Martin M., Robinson, Joseph S., Solomon, Edward I., Sokaras, Dimosthenis, van Driel, Tim B., Weng, Tsu -Chien, Zhu, Diling, Persson, Petter, Warnmark, Kenneth, Sundstrom, Villy, and Gaffney, Kelly J.. Ligand manipulation of charge transfer excited state relaxation and spin crossover in [Fe(2,2'-bipyridine)2(CN)2]. United States: N. p., 2017. Web. doi:10.1063/1.4985017.
Kjaer, Kasper S., Zhang, Wenkai, Alonso-Mori, Roberto, Bergmann, Uwe, Chollet, Matthieu, Hadt, Ryan G., Hartsock, Robert W., Harlang, Tobias, Kroll, Thomas, Kubicek, Katharina, Lemke, Henrik T., Liang, Huiyang W., Liu, Yizhu, Nielsen, Martin M., Robinson, Joseph S., Solomon, Edward I., Sokaras, Dimosthenis, van Driel, Tim B., Weng, Tsu -Chien, Zhu, Diling, Persson, Petter, Warnmark, Kenneth, Sundstrom, Villy, & Gaffney, Kelly J.. Ligand manipulation of charge transfer excited state relaxation and spin crossover in [Fe(2,2'-bipyridine)2(CN)2]. United States. doi:10.1063/1.4985017.
Kjaer, Kasper S., Zhang, Wenkai, Alonso-Mori, Roberto, Bergmann, Uwe, Chollet, Matthieu, Hadt, Ryan G., Hartsock, Robert W., Harlang, Tobias, Kroll, Thomas, Kubicek, Katharina, Lemke, Henrik T., Liang, Huiyang W., Liu, Yizhu, Nielsen, Martin M., Robinson, Joseph S., Solomon, Edward I., Sokaras, Dimosthenis, van Driel, Tim B., Weng, Tsu -Chien, Zhu, Diling, Persson, Petter, Warnmark, Kenneth, Sundstrom, Villy, and Gaffney, Kelly J.. 2017. "Ligand manipulation of charge transfer excited state relaxation and spin crossover in [Fe(2,2'-bipyridine)2(CN)2]". United States. doi:10.1063/1.4985017. https://www.osti.gov/servlets/purl/1362282.
@article{osti_1362282,
title = {Ligand manipulation of charge transfer excited state relaxation and spin crossover in [Fe(2,2'-bipyridine)2(CN)2]},
author = {Kjaer, Kasper S. and Zhang, Wenkai and Alonso-Mori, Roberto and Bergmann, Uwe and Chollet, Matthieu 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 Robinson, Joseph S. and Solomon, Edward I. and Sokaras, Dimosthenis and van Driel, Tim B. and Weng, Tsu -Chien and Zhu, Diling and Persson, Petter and Warnmark, Kenneth and Sundstrom, Villy and Gaffney, Kelly J.},
abstractNote = {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 short 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 3d transition metal complexes.},
doi = {10.1063/1.4985017},
journal = {Structural Dynamics},
number = 4,
volume = 4,
place = {United States},
year = 2017,
month = 7
}

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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 enablesmore » 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.« 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 transition metal complexes [Ru(dmb){sub 3}]{sup 2+}, where dmb is 4,4{prime}-dimethyl-2,2{prime}-bipyridine and dpb is 4,4{prime}-diphenyl-2,2{prime}-bipyridine, have been studied by femtosecond visible electronic absorption spectroscopy. Spectroelectrochemical measurements in conjunction with nanosecond time-resolved absorption spectroscopy allow for the assignment of various features in the excited-state differential absorption spectra as both ligand-based {pi}* {l{underscore}arrow} {pi}* and ligand-to-metal charge transfer (LMCT) in nature. A unique absorptive feature centered at {approximately} 530 nm in [Ru(dpb){sub 3}]{sup 2+} was identified as an optical marker for the thermalized (and hence fully intraligand delocalized) excited state. Single wavelength and full spectrum transient absorption data were obtained on bothmore » molecules in CH{sub 3}CN solution at room temperature following metal-to-ligand charge transfer (MLCT) excitation at 400 nm. Data on [Ru(dmb){sub 3}]{sup 2+} at 532 nm, a region of net excited-state absorption, revealed biphasic decay kinetics ({approximately}120 fs and 5 ps) attributed to a combination of {sup 1}MLCT {r{underscore}arrow} {sup 3}MLCT intersystem crossing and vibrational cooling dynamics. Dynamics for [Ru(dpb){sub 3}]{sup 2+} under identical conditions revealed biphasic rise times in the region of the ligand-based {pi}* {l{underscore}arrow} {pi}* absorption at {lambda}{sub probe} = 532 nm. Although the origin of the gas component ({approximately}200 fs) is not yet clear, the ca. 2 ps rise is assigned to rotation of the peripheral aryl ring and thus corresponds to the time scale for intraligand electron delocalization.« less
  • Metal-to-ligand charge-transfer (MLCT) absorption bands for the complexes Ru(bpy)/sub 3//sup 2 +/, Os(bpy)/sub 3//sup 2 +/, Os(bpy)/sub 2/(py)/sub 2//sup 2 +/, Os(bpy)/sub 2/(CH/sub 3/CN)/sub 2//sup 2 +/, and Os(bpy)/sub 2/(1,2-(Ph/sub 2/P)/sub 2/C/sub 6/H/sub 4/)/sup 2 +/ (bpy is 2,2'-bipyridiene;py is pyridine) are solvent dependent. The dependence can be interpreted with use of dielectric continuum theory but the D/sub 3/ ions Ru(bpy)/sub 3//sup 2 +/ and Os(bpy)/sub 3//sup 2 +/ only if in the excited state the excited electron is localized on a single ligand rather than delocalized over all three. 37 references, 4 figures, 2 tables.