Advanced

Tracking the picosecond deactivation dynamics of a photoexcited iron carbene complex by time-resolved X-ray scattering

Leshchev, Denis; Harlang, Tobias C.B. LU ; Fredin, Lisa A. LU ; Khakhulin, Dmitry; Liu, Yizhu LU ; Biasin, Elisa; Laursen, Mads G.; Newby, Gemma E.; Haldrup, Kristoffer and Nielsen, Martin M., et al. (2018) In Chemical Science 9(2). p.405-414
Abstract

Recent years have seen the development of new iron-centered N-heterocyclic carbene (NHC) complexes for solar energy applications. Compared to typical ligand systems, the NHC ligands provide Fe complexes with longer-lived metal-to-ligand charge transfer (MLCT) states. This increased lifetime is ascribed to strong ligand field splitting provided by the NHC ligands that raises the energy levels of the metal centered (MC) states and therefore reduces the deactivation efficiency of MLCT states. Among currently known NHC systems, [Fe(btbip)2]2+ (btbip = 2,6-bis(3-tert-butyl-imidazol-1-ylidene)pyridine) is a unique complex as it exhibits a short-lived MC state with a lifetime on the scale of a few hundreds of picoseconds.... (More)

Recent years have seen the development of new iron-centered N-heterocyclic carbene (NHC) complexes for solar energy applications. Compared to typical ligand systems, the NHC ligands provide Fe complexes with longer-lived metal-to-ligand charge transfer (MLCT) states. This increased lifetime is ascribed to strong ligand field splitting provided by the NHC ligands that raises the energy levels of the metal centered (MC) states and therefore reduces the deactivation efficiency of MLCT states. Among currently known NHC systems, [Fe(btbip)2]2+ (btbip = 2,6-bis(3-tert-butyl-imidazol-1-ylidene)pyridine) is a unique complex as it exhibits a short-lived MC state with a lifetime on the scale of a few hundreds of picoseconds. Hence, this complex allows for a detailed investigation, using 100 ps X-ray pulses from a synchrotron, of strong ligand field effects on the intermediate MC state in an NHC complex. Here, we use time-resolved wide angle X-ray scattering (TRWAXS) aided by density functional theory (DFT) to investigate the molecular structure, energetics and lifetime of the high-energy MC state in the Fe-NHC complex [Fe(btbip)2]2+ after excitation to the MLCT manifold. We identify it as a 260 ps metal-centered quintet (5MC) state, and we refine the molecular structure of the excited-state complex verifying the DFT results. Using information about the hydrodynamic state of the solvent, we also determine, for the first time, the energy of the 5MC state as 0.75 ± 0.15 eV. Our results demonstrate that due to the increased ligand field strength caused by NHC ligands, upon transition from the ground state to the 5MC state, the metal to ligand bonds extend by unusually large values: by 0.29 Å in the axial and 0.21 Å in the equatorial direction. These results imply that the transition in the photochemical properties from typical Fe complexes to novel NHC compounds is manifested not only in the destabilization of the MC states, but also in structural distortion of these states.

(Less)
Please use this url to cite or link to this publication:
author
, et al. (More)
(Less)
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Chemical Science
volume
9
issue
2
pages
10 pages
publisher
Royal Society of Chemistry
external identifiers
  • scopus:85040123566
ISSN
2041-6520
DOI
language
English
LU publication?
yes
id
93f81012-9eb4-4beb-8afa-4b7ba8d3b83c
date added to LUP
2018-01-15 08:43:44
date last changed
2018-05-29 11:28:21
@article{93f81012-9eb4-4beb-8afa-4b7ba8d3b83c,
  abstract     = {<p>Recent years have seen the development of new iron-centered N-heterocyclic carbene (NHC) complexes for solar energy applications. Compared to typical ligand systems, the NHC ligands provide Fe complexes with longer-lived metal-to-ligand charge transfer (MLCT) states. This increased lifetime is ascribed to strong ligand field splitting provided by the NHC ligands that raises the energy levels of the metal centered (MC) states and therefore reduces the deactivation efficiency of MLCT states. Among currently known NHC systems, [Fe(btbip)<sub>2</sub>]<sup>2+</sup> (btbip = 2,6-bis(3-tert-butyl-imidazol-1-ylidene)pyridine) is a unique complex as it exhibits a short-lived MC state with a lifetime on the scale of a few hundreds of picoseconds. Hence, this complex allows for a detailed investigation, using 100 ps X-ray pulses from a synchrotron, of strong ligand field effects on the intermediate MC state in an NHC complex. Here, we use time-resolved wide angle X-ray scattering (TRWAXS) aided by density functional theory (DFT) to investigate the molecular structure, energetics and lifetime of the high-energy MC state in the Fe-NHC complex [Fe(btbip)<sub>2</sub>]<sup>2+</sup> after excitation to the MLCT manifold. We identify it as a 260 ps metal-centered quintet (<sup>5</sup>MC) state, and we refine the molecular structure of the excited-state complex verifying the DFT results. Using information about the hydrodynamic state of the solvent, we also determine, for the first time, the energy of the <sup>5</sup>MC state as 0.75 ± 0.15 eV. Our results demonstrate that due to the increased ligand field strength caused by NHC ligands, upon transition from the ground state to the <sup>5</sup>MC state, the metal to ligand bonds extend by unusually large values: by 0.29 Å in the axial and 0.21 Å in the equatorial direction. These results imply that the transition in the photochemical properties from typical Fe complexes to novel NHC compounds is manifested not only in the destabilization of the MC states, but also in structural distortion of these states.</p>},
  author       = {Leshchev, Denis and Harlang, Tobias C.B. and Fredin, Lisa A. and Khakhulin, Dmitry and Liu, Yizhu and Biasin, Elisa and Laursen, Mads G. and Newby, Gemma E. and Haldrup, Kristoffer and Nielsen, Martin M. and Wärnmark, Kenneth and Sundström, Villy and Persson, Petter and Kjær, Kasper S. and Wulff, Michael},
  issn         = {2041-6520},
  language     = {eng},
  number       = {2},
  pages        = {405--414},
  publisher    = {Royal Society of Chemistry},
  series       = {Chemical Science},
  title        = {Tracking the picosecond deactivation dynamics of a photoexcited iron carbene complex by time-resolved X-ray scattering},
  url          = {http://dx.doi.org/},
  volume       = {9},
  year         = {2018},
}