TY - JOUR

T1 - Ferrous and ferric complexes with cyclometalating N-heterocyclic carbene ligands

T2 - a case of dual emission revisited

AU - Johnson, Catherine Ellen

AU - Schwarz, Jesper

AU - Deegbey, Mawuli

AU - Prakash, Om

AU - Sharma, Kumkum

AU - Huang, Ping

AU - Ericsson, Tore

AU - Häggström, Lennart

AU - Bendix, Jesper

AU - Gupta, Arvind Kumar

AU - Jakubikova, Elena

AU - Wärnmark, Kenneth

AU - Lomoth, Reiner

N1 - 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

PY - 2023

Y1 - 2023

N2 - 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.

AB - 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.

U2 - 10.1039/d3sc02806b

DO - 10.1039/d3sc02806b

M3 - Journal article

C2 - 37772113

AN - SCOPUS:85171463916

VL - 14

SP - 10129

EP - 10139

JO - Chemical Science

JF - Chemical Science

SN - 2041-6520

IS - 37

ER -