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Photofunctionality of iron(III) N-heterocyclic carbenes and related d5 transition metal complexes

Chábera, Pavel LU ; Lindh, Linnea LU orcid ; Rosemann, Nils W. LU ; Prakash, Om LU ; Uhlig, Jens LU ; Yartsev, Arkady LU orcid ; Wärnmark, Kenneth LU ; Sundström, Villy LU and Persson, Petter LU (2021) In Coordination Chemistry Reviews 426.
Abstract

Despite a few reports of photoluminescent and strongly photo-oxidizing transition metal complexes with a d5 electronic configuration, the photophysics and photochemistry of this class of transition metal complexes have largely remained unexplored. Recent investigations of earth-abundant iron(III) N-heterocyclic carbene (NHC) complexes have demonstrated promising photophysical and photochemical properties associated with low-spin (doublet) ligand-to-metal charge transfer (2LMCT) excitations, including nanosecond photoluminescence (PL) and capabilities to drive both photo-oxidation and photo-reduction reactions. These encouraging results are at first sight surprising in light of the general scarcity of known... (More)

Despite a few reports of photoluminescent and strongly photo-oxidizing transition metal complexes with a d5 electronic configuration, the photophysics and photochemistry of this class of transition metal complexes have largely remained unexplored. Recent investigations of earth-abundant iron(III) N-heterocyclic carbene (NHC) complexes have demonstrated promising photophysical and photochemical properties associated with low-spin (doublet) ligand-to-metal charge transfer (2LMCT) excitations, including nanosecond photoluminescence (PL) and capabilities to drive both photo-oxidation and photo-reduction reactions. These encouraging results are at first sight surprising in light of the general scarcity of known photofunctional complexes of any transition metal complexes with a d5 electronic configuration, including 1st, 2nd and 3rd row transition metal complexes of Mn(II), Tc(II), Re(II), Fe(III), Ru(III) and Os(III). Here, we review the photophysical and photochemical properties of the new Fe(III) NHC complexes together with related d5 transition metal complexes as a basis for a broader understanding of the unorthodox photophysical and photochemical properties associated with this open-shell electronic configuration. This includes considerations of the role of charge and spin effects on the ground state electronic structure, as well as discussions of charge transfer (CT) and metal centered (MC) excited state properties. The special properties of 2LMCT excited states are emphasized as a key feature to understand the photophysics of many photofunctional d5 transition metal complexes. Further aspects of excited state dynamics with d5 light-harvesting complexes, including both intra- and inter-molecular charge transfer processes, are also discussed. Finally, some fundamental challenges and emerging opportunities for further development of photofunctional Fe(III) NHC and related LMCT/d5 complexes for light-harvesting, light-emitting, and photo(electro)chemical applications are outlined. This includes some general considerations of how the specific photochemical properties of the LMCT/d5 complexes provides an exciting opportunity to develop a unique niche within the diversity of photofunctional molecular systems alongside other types of organic and inorganic chromophores commonly used in the field of molecular photochemistry.

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author
; ; ; ; ; ; ; and
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
d, Iron, Ligand-to-metal charge transfer (LMCT), N-heterocyclic carbene, Photochemistry, Photoluminescence, Photophysics
in
Coordination Chemistry Reviews
volume
426
article number
213517
publisher
Elsevier
external identifiers
  • scopus:85090548032
ISSN
0010-8545
DOI
10.1016/j.ccr.2020.213517
language
English
LU publication?
yes
id
b33b1ff0-9b23-45b7-98b6-76888c2c167e
date added to LUP
2020-09-29 14:09:32
date last changed
2023-11-20 11:25:34
@article{b33b1ff0-9b23-45b7-98b6-76888c2c167e,
  abstract     = {{<p>Despite a few reports of photoluminescent and strongly photo-oxidizing transition metal complexes with a d<sup>5</sup> electronic configuration, the photophysics and photochemistry of this class of transition metal complexes have largely remained unexplored. Recent investigations of earth-abundant iron(III) N-heterocyclic carbene (NHC) complexes have demonstrated promising photophysical and photochemical properties associated with low-spin (doublet) ligand-to-metal charge transfer (<sup>2</sup>LMCT) excitations, including nanosecond photoluminescence (PL) and capabilities to drive both photo-oxidation and photo-reduction reactions. These encouraging results are at first sight surprising in light of the general scarcity of known photofunctional complexes of any transition metal complexes with a d<sup>5</sup> electronic configuration, including 1<sup>st</sup>, 2<sup>nd</sup> and 3<sup>rd</sup> row transition metal complexes of Mn(II), Tc(II), Re(II), Fe(III), Ru(III) and Os(III). Here, we review the photophysical and photochemical properties of the new Fe(III) NHC complexes together with related d<sup>5</sup> transition metal complexes as a basis for a broader understanding of the unorthodox photophysical and photochemical properties associated with this open-shell electronic configuration. This includes considerations of the role of charge and spin effects on the ground state electronic structure, as well as discussions of charge transfer (CT) and metal centered (MC) excited state properties. The special properties of <sup>2</sup>LMCT excited states are emphasized as a key feature to understand the photophysics of many photofunctional d<sup>5</sup> transition metal complexes. Further aspects of excited state dynamics with d<sup>5</sup> light-harvesting complexes, including both intra- and inter-molecular charge transfer processes, are also discussed. Finally, some fundamental challenges and emerging opportunities for further development of photofunctional Fe(III) NHC and related LMCT/d<sup>5</sup> complexes for light-harvesting, light-emitting, and photo(electro)chemical applications are outlined. This includes some general considerations of how the specific photochemical properties of the LMCT/d<sup>5</sup> complexes provides an exciting opportunity to develop a unique niche within the diversity of photofunctional molecular systems alongside other types of organic and inorganic chromophores commonly used in the field of molecular photochemistry.</p>}},
  author       = {{Chábera, Pavel and Lindh, Linnea and Rosemann, Nils W. and Prakash, Om and Uhlig, Jens and Yartsev, Arkady and Wärnmark, Kenneth and Sundström, Villy and Persson, Petter}},
  issn         = {{0010-8545}},
  keywords     = {{d; Iron; Ligand-to-metal charge transfer (LMCT); N-heterocyclic carbene; Photochemistry; Photoluminescence; Photophysics}},
  language     = {{eng}},
  month        = {{01}},
  publisher    = {{Elsevier}},
  series       = {{Coordination Chemistry Reviews}},
  title        = {{Photofunctionality of iron(III) N-heterocyclic carbenes and related d<sup>5</sup> transition metal complexes}},
  url          = {{http://dx.doi.org/10.1016/j.ccr.2020.213517}},
  doi          = {{10.1016/j.ccr.2020.213517}},
  volume       = {{426}},
  year         = {{2021}},
}