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

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 II (ImP) 2 ] (ImP = bis(2,6-bis(3-methylimidazol-2-ylidene-1-yl)phenylene)) that combines carbene ligands with cyclometalation for...

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Bibliographic Details
Published in:	Chemical science (Cambridge) Vol. 14; no. 37; pp. 1129 - 1139
Main Authors:	Johnson, Catherine Ellen, Schwarz, Jesper, Deegbey, Mawuli, Prakash, Om, Sharma, Kumkum, Huang, Ping, Ericsson, Tore, Häggström, Lennart, Bendix, Jesper, Gupta, Arvind Kumar, Jakubikova, Elena, Wärnmark, Kenneth, Lomoth, Reiner
Format:	Journal Article
Language:	English
Published:	Cambridge Royal Society of Chemistry 27-09-2023 The Royal Society of Chemistry
Subjects:	Acetonitrile Carbenes Charge transfer Chemical Sciences Chemistry Field strength Fysikalisk kemi Iron Kemi Ligands Natural Sciences Naturvetenskap Physical Chemistry
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Summary:	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 II (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 3 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 ( 2 MLCT) state of its Fe( iii ) analogue [Fe III (ImP) 2 ] + , which led to a claim of dual (MLCT and LMCT) emission. Revisiting the photophysics of [Fe III (ImP) 2 ] + , we confirmed however that higher ( 2 MLCT) states of [Fe III (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 [Fe III (ImP) 2 ] + prepared by us only shows red emission from its lower 2 LMCT state (740 nm, 240 ps). The long-lived, higher energy emission previously reported for [Fe III (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 [Fe III (ImP) 2 ] + itself. Photoluminescence of the Fe III complex occurs only from its 2 LMCT state while its higher-energy but shorter-lived 2 MLCT state is non-luminescent. Blue emission arises instead from a photoproduct of the non-emitting Fe II complex.
Bibliography:	Electronic supplementary information (ESI) available. CCDC 2254083 For ESI and crystallographic data in CIF or other electronic format see DOI https://doi.org/10.1039/d3sc02806b 2254082 and ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 Shared authorship.
ISSN:	2041-6520 2041-6539 2041-6539
DOI:	10.1039/d3sc02806b