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Ferrous and ferric complexes with cyclometalating N-heterocyclic carbene ligands : a case of dual emission revisited

Johnson, Catherine Ellen ; Schwarz, Jesper LU ; Deegbey, Mawuli ; Prakash, Om LU ; Sharma, Kumkum LU ; Huang, Ping ; Ericsson, Tore ; Häggström, Lennart ; Bendix, Jesper and Gupta, Arvind Kumar LU , et al. (2023) In Chemical Science 14(37). p.10129-10139
Abstract

Iron N-heterocyclic carbene (FeNHC) complexes with long-lived charge transfer states are emerging as a promising class of photoactive materials. We have synthesized [FeII(ImP)2] (ImP = bis(2,6-bis(3-methylimidazol-2-ylidene-1-yl)phenylene)) that combines carbene ligands with cyclometalation for additionally improved ligand field strength. The 9 ps lifetime of its 3MLCT (metal-to-ligand charge transfer) state however reveals no benefit from cyclometalation compared to Fe(ii) complexes with NHC/pyridine or pure NHC ligand sets. In acetonitrile solution, the Fe(ii) complex forms a photoproduct that features emission characteristics (450 nm, 5.1 ns) that were previously attributed to a higher... (More)

Iron N-heterocyclic carbene (FeNHC) complexes with long-lived charge transfer states are emerging as a promising class of photoactive materials. We have synthesized [FeII(ImP)2] (ImP = bis(2,6-bis(3-methylimidazol-2-ylidene-1-yl)phenylene)) that combines carbene ligands with cyclometalation for additionally improved ligand field strength. The 9 ps lifetime of its 3MLCT (metal-to-ligand charge transfer) state however reveals no benefit from cyclometalation compared to Fe(ii) complexes with NHC/pyridine or pure NHC ligand sets. In acetonitrile solution, the Fe(ii) complex forms a photoproduct that features emission characteristics (450 nm, 5.1 ns) that were previously attributed to a higher (2MLCT) state of its Fe(iii) analogue [FeIII(ImP)2]+, which led to a claim of dual (MLCT and LMCT) emission. Revisiting the photophysics of [FeIII(ImP)2]+, we confirmed however that higher (2MLCT) states of [FeIII(ImP)2]+ are short-lived (<10 ps) and therefore, in contrast to the previous interpretation, cannot give rise to emission on the nanosecond timescale. Accordingly, pristine [FeIII(ImP)2]+ prepared by us only shows red emission from its lower 2LMCT state (740 nm, 240 ps). The long-lived, higher energy emission previously reported for [FeIII(ImP)2]+ is instead attributed to an impurity, most probably a photoproduct of the Fe(ii) precursor. The previously reported emission quenching on the nanosecond time scale hence does not support any excited state reactivity of [FeIII(ImP)2]+ itself.

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organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Chemical Science
volume
14
issue
37
pages
11 pages
publisher
Royal Society of Chemistry
external identifiers
  • pmid:37772113
  • scopus:85171463916
ISSN
2041-6520
DOI
10.1039/d3sc02806b
language
English
LU publication?
yes
id
670484e8-78a7-44d6-9b05-9b17561dbfba
date added to LUP
2023-12-28 10:18:25
date last changed
2024-04-26 18:29:00
@article{670484e8-78a7-44d6-9b05-9b17561dbfba,
  abstract     = {{<p>Iron N-heterocyclic carbene (FeNHC) complexes with long-lived charge transfer states are emerging as a promising class of photoactive materials. We have synthesized [Fe<sup>II</sup>(ImP)<sub>2</sub>] (ImP = bis(2,6-bis(3-methylimidazol-2-ylidene-1-yl)phenylene)) that combines carbene ligands with cyclometalation for additionally improved ligand field strength. The 9 ps lifetime of its <sup>3</sup>MLCT (metal-to-ligand charge transfer) state however reveals no benefit from cyclometalation compared to Fe(ii) complexes with NHC/pyridine or pure NHC ligand sets. In acetonitrile solution, the Fe(ii) complex forms a photoproduct that features emission characteristics (450 nm, 5.1 ns) that were previously attributed to a higher (<sup>2</sup>MLCT) state of its Fe(iii) analogue [Fe<sup>III</sup>(ImP)<sub>2</sub>]<sup>+</sup>, which led to a claim of dual (MLCT and LMCT) emission. Revisiting the photophysics of [Fe<sup>III</sup>(ImP)<sub>2</sub>]<sup>+</sup>, we confirmed however that higher (<sup>2</sup>MLCT) states of [Fe<sup>III</sup>(ImP)<sub>2</sub>]<sup>+</sup> are short-lived (&lt;10 ps) and therefore, in contrast to the previous interpretation, cannot give rise to emission on the nanosecond timescale. Accordingly, pristine [Fe<sup>III</sup>(ImP)<sub>2</sub>]<sup>+</sup> prepared by us only shows red emission from its lower <sup>2</sup>LMCT state (740 nm, 240 ps). The long-lived, higher energy emission previously reported for [Fe<sup>III</sup>(ImP)<sub>2</sub>]<sup>+</sup> is instead attributed to an impurity, most probably a photoproduct of the Fe(ii) precursor. The previously reported emission quenching on the nanosecond time scale hence does not support any excited state reactivity of [Fe<sup>III</sup>(ImP)<sub>2</sub>]<sup>+</sup> itself.</p>}},
  author       = {{Johnson, Catherine Ellen and Schwarz, Jesper and Deegbey, Mawuli and Prakash, Om and Sharma, Kumkum and Huang, Ping and Ericsson, Tore and Häggström, Lennart and Bendix, Jesper and Gupta, Arvind Kumar and Jakubikova, Elena and Wärnmark, Kenneth and Lomoth, Reiner}},
  issn         = {{2041-6520}},
  language     = {{eng}},
  month        = {{08}},
  number       = {{37}},
  pages        = {{10129--10139}},
  publisher    = {{Royal Society of Chemistry}},
  series       = {{Chemical Science}},
  title        = {{Ferrous and ferric complexes with cyclometalating N-heterocyclic carbene ligands : a case of dual emission revisited}},
  url          = {{http://dx.doi.org/10.1039/d3sc02806b}},
  doi          = {{10.1039/d3sc02806b}},
  volume       = {{14}},
  year         = {{2023}},
}