Photophysical Integrity of the Iron(III) Scorpionate Framework in Iron(III)-NHC Complexes with Long-Lived 2LMCT Excited States

Om Prakash; Linnea Lindh; Nidhi Kaul; Nils W Rosemann; Iria Bolaño Losada; Catherine Johnson; Pavel Chábera; Aleksandra Ilic; Jesper Schwarz; Arvind Kumar Gupta; Jens Uhlig; Tore Ericsson; Lennart Häggström; Ping Huang; Jesper Bendix; Daniel Strand; Arkady Yartsev; Reiner Lomoth; Petter Persson; Kenneth Wärnmark

doi:10.1021/acs.inorgchem.2c02410

Photophysical Integrity of the Iron(III) Scorpionate Framework in Iron(III)-NHC Complexes with Long-Lived ²LMCT Excited States

Inorg Chem. 2022 Nov 7;61(44):17515-17526. doi: 10.1021/acs.inorgchem.2c02410. Epub 2022 Oct 24.

Authors

Om Prakash¹, Linnea Lindh^{2

3}, Nidhi Kaul⁴, Nils W Rosemann², Iria Bolaño Losada³, Catherine Johnson⁴, Pavel Chábera², Aleksandra Ilic¹, Jesper Schwarz¹, Arvind Kumar Gupta¹, Jens Uhlig², Tore Ericsson⁵, Lennart Häggström⁵, Ping Huang⁴, Jesper Bendix⁶, Daniel Strand¹, Arkady Yartsev², Reiner Lomoth⁴, Petter Persson³, Kenneth Wärnmark¹

Affiliations

¹ Centre for Analysis and Synthesis, Department of Chemistry, Lund University, Box 124, SE-22100Lund, Sweden.
² Chemical Physics Division, Department of Chemistry, Lund University, Box 124, SE-22100Lund, Sweden.
³ Theoretical Chemistry Division, Department of Chemistry, Lund University, Box 124, SE-22100Lund, Sweden.
⁴ Department of Chemistry - Ångström Laboratory, Uppsala University, Box 523, SE-75120Uppsala, Sweden.
⁵ Department of Physics - Ångström Laboratory, Uppsala University, Box 523, SE-75120Uppsala, Sweden.
⁶ Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100Copenhagen, Denmark.

Abstract

Fe(III) complexes with N-heterocyclic carbene (NHC) ligands belong to the rare examples of Earth-abundant transition metal complexes with long-lived luminescent charge-transfer excited states that enable applications as photosensitizers for charge separation reactions. We report three new hexa-NHC complexes of this class: [Fe(brphtmeimb)₂]PF₆ (brphtmeimb = [(4-bromophenyl)tris(3-methylimidazol-2-ylidene)borate]^-, [Fe(meophtmeimb)₂]PF₆ (meophtmeimb = [(4-methoxyphenyl)tris(3-methylimidazol-2-ylidene)borate]^-, and [Fe(coohphtmeimb)₂]PF₆ (coohphtmeimb = [(4-carboxyphenyl)tris(3-methylimidazol-2-ylidene)borate]^-. These were derived from the parent complex [Fe(phtmeimb)₂]PF₆ (phtmeimb = [phenyltris(3-methylimidazol-2-ylidene)borate]^- by modification with electron-withdrawing and electron-donating substituents, respectively, at the 4-phenyl position of the ligand framework. All three Fe(III) hexa-NHC complexes were characterized by NMR spectroscopy, high-resolution mass spectroscopy, elemental analysis, single crystal X-ray diffraction analysis, electrochemistry, Mößbauer spectroscopy, electronic spectroscopy, magnetic susceptibility measurements, and quantum chemical calculations. Their ligand-to-metal charge-transfer (²LMCT) excited states feature nanosecond lifetimes (1.6-1.7 ns) and sizable emission quantum yields (1.7-1.9%) through spin-allowed transition to the doublet ground state (²GS), completely in line with the parent complex [Fe(phtmeimb)₂]PF₆ (2.0 ns and 2.1%). The integrity of the favorable excited state characteristics upon substitution of the ligand framework demonstrates the robustness of the scorpionate motif that tolerates modifications in the 4-phenyl position for applications such as the attachment in molecular or hybrid assemblies.