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.
Original language | English |
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Journal | Chemical Science |
Volume | 14 |
Issue number | 37 |
Pages (from-to) | 10129–10139 |
Number of pages | 11 |
ISSN | 2041-6520 |
DOIs | |
Publication status | Published - 2023 |
Bibliographical note
Funding Information:
We thank Sofia Essén for HRMS measurements and Göran Carlström for assistance with NMR measurements. The Swedish Strategic Research Foundation (SSF, EM16- 0067) and the Knut and Alice Wallenberg (KAW, 2018.0074) Foundation are gratefully acknowledged for their support. K. W. acknowledges support from the Swedish Research Council (VR, 2020-03207), the Swedish Energy Agency (Energimyndigheten, P48747-1), the LMK Foundation, and the Sten K Johnson Foundation. J. S. acknowledges support from the Royal Physiographic Society of Lund. R. L. is grateful to the Swedish Research Council for financial support (VR, 2020-05058). E. J. and M. D. gratefully acknowledge the support from the National Science Foundation (CHE-1554855) and the computing resources provided by North Carolina State University High Performance Computing Services Core Facility (RRID: SCR 022168).
Publisher Copyright:
© 2023 The Royal Society of Chemistry
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