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Iron N-Heterocyclic Carbene Photoactive Complexes with Rigid Phenylethynyl Substituents as Ligand π-System Extensions

Persson, Samuel LU ; Koninti, Raj Kumar ; Barakat, Mariam ; Mishra, Abhishek LU ; Lindgren, Fredrik ; Ericsson, Tore ; Häggström, Lennart ; Lidin, Sven LU ; Gonzalez, Ana LU and Jakubikova, Elena , et al. (2025) In Inorganic Chemistry 64(24). p.12120-12131
Abstract

The design of iron complexes with long-lived charge transfer states suitable for applications as photosensitizers remains a formidable challenge. Here, we investigated the effect of an extended ligand π-system on the ground- and excited-state properties of iron(II) complexes with N-heterocyclic carbene (NHC) ligands. For this purpose, three iron complexes based on the established [Fe(II)(pbmi)2]2+ motif (pbmi = (1,1′-(pyridine-2,6-diyl)bis(3-methylimidazole-2-ylidene))) have been modified with phenylethynyl moieties attached to the pyridine part of the ligand. In general, the introduction of the phenylethynyl units served to red shift the main absorption band, as well as to increase the extinction coefficient of... (More)

The design of iron complexes with long-lived charge transfer states suitable for applications as photosensitizers remains a formidable challenge. Here, we investigated the effect of an extended ligand π-system on the ground- and excited-state properties of iron(II) complexes with N-heterocyclic carbene (NHC) ligands. For this purpose, three iron complexes based on the established [Fe(II)(pbmi)2]2+ motif (pbmi = (1,1′-(pyridine-2,6-diyl)bis(3-methylimidazole-2-ylidene))) have been modified with phenylethynyl moieties attached to the pyridine part of the ligand. In general, the introduction of the phenylethynyl units served to red shift the main absorption band, as well as to increase the extinction coefficient of the same, compared to the parent complex. The lowered MLCT energies are in line with the electrochemical data that revealed substantially easier reduction of the phenylethynyl-modified ligands, while the potentials of the Fe(III/II) couple are only moderately increased. Only minor modifications of the electronic effect intrinsic to the phenylethynyl moieties could be implemented with bromide and dimethylamino substituents on the phenylene units. As a result, all three complexes experience similar stabilization of their 3MLCT states, about 0.3 eV compared to the parent complex, and feature transient absorption data in line with ES dynamics that are dominated by a moderately long-lived (∼17 ps) 3MLCT state. These values exceed the 3MLCT lifetimes reported for the parent complex (up to 9 ps) and resemble the results for carboxylic acid and imidazolinium derivatives with comparable 3MLCT energies and lifetimes.

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organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Inorganic Chemistry
volume
64
issue
24
pages
12 pages
publisher
The American Chemical Society (ACS)
external identifiers
  • pmid:40489787
  • scopus:105007909343
ISSN
0020-1669
DOI
10.1021/acs.inorgchem.5c01461
language
English
LU publication?
yes
additional info
Publisher Copyright: © 2025 The Authors. Published by American Chemical Society.
id
b2b064d5-a7d9-4e05-be6c-a86a8c22942e
date added to LUP
2025-12-17 09:49:59
date last changed
2025-12-18 03:00:16
@article{b2b064d5-a7d9-4e05-be6c-a86a8c22942e,
  abstract     = {{<p>The design of iron complexes with long-lived charge transfer states suitable for applications as photosensitizers remains a formidable challenge. Here, we investigated the effect of an extended ligand π-system on the ground- and excited-state properties of iron(II) complexes with N-heterocyclic carbene (NHC) ligands. For this purpose, three iron complexes based on the established [Fe(II)(pbmi)<sub>2</sub>]<sup>2+</sup> motif (pbmi = (1,1′-(pyridine-2,6-diyl)bis(3-methylimidazole-2-ylidene))) have been modified with phenylethynyl moieties attached to the pyridine part of the ligand. In general, the introduction of the phenylethynyl units served to red shift the main absorption band, as well as to increase the extinction coefficient of the same, compared to the parent complex. The lowered MLCT energies are in line with the electrochemical data that revealed substantially easier reduction of the phenylethynyl-modified ligands, while the potentials of the Fe(III/II) couple are only moderately increased. Only minor modifications of the electronic effect intrinsic to the phenylethynyl moieties could be implemented with bromide and dimethylamino substituents on the phenylene units. As a result, all three complexes experience similar stabilization of their <sup>3</sup>MLCT states, about 0.3 eV compared to the parent complex, and feature transient absorption data in line with ES dynamics that are dominated by a moderately long-lived (∼17 ps) <sup>3</sup>MLCT state. These values exceed the <sup>3</sup>MLCT lifetimes reported for the parent complex (up to 9 ps) and resemble the results for carboxylic acid and imidazolinium derivatives with comparable <sup>3</sup>MLCT energies and lifetimes.</p>}},
  author       = {{Persson, Samuel and Koninti, Raj Kumar and Barakat, Mariam and Mishra, Abhishek and Lindgren, Fredrik and Ericsson, Tore and Häggström, Lennart and Lidin, Sven and Gonzalez, Ana and Jakubikova, Elena and Lomoth, Reiner and Wärnmark, Kenneth}},
  issn         = {{0020-1669}},
  language     = {{eng}},
  month        = {{06}},
  number       = {{24}},
  pages        = {{12120--12131}},
  publisher    = {{The American Chemical Society (ACS)}},
  series       = {{Inorganic Chemistry}},
  title        = {{Iron N-Heterocyclic Carbene Photoactive Complexes with Rigid Phenylethynyl Substituents as Ligand π-System Extensions}},
  url          = {{http://dx.doi.org/10.1021/acs.inorgchem.5c01461}},
  doi          = {{10.1021/acs.inorgchem.5c01461}},
  volume       = {{64}},
  year         = {{2025}},
}