Ferrous and ferric complexes with cyclometalating N-heterocyclic carbene ligands: a case of dual emission revisited
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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.
Originalsprog | Engelsk |
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Tidsskrift | Chemical Science |
Vol/bind | 14 |
Udgave nummer | 37 |
Sider (fra-til) | 10129–10139 |
Antal sider | 11 |
ISSN | 2041-6520 |
DOI | |
Status | Udgivet - 2023 |
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© 2023 The Royal Society of Chemistry
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