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

• Published in: Inorganic chemistry 2022-11, Vol.61 (44), p.17515-17526
• Main Authors: Prakash, Om, Lindh, Linnea, Kaul, Nidhi, Rosemann, Nils W, Losada, Iria Bolaño, Johnson, Catherine, Chábera, Pavel, Ilic, Aleksandra, Schwarz, Jesper, Gupta, Arvind Kumar, Uhlig, Jens, Ericsson, Tore, Häggström, Lennart, Huang, Ping, Bendix, Jesper, Strand, Daniel, Yartsev, Arkady, Lomoth, Reiner, Persson, Petter, Wärnmark, Kenneth
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.2c02410

Fe(III) complexes with -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 -NHC complexes of this class: [Fe(brphtmeimb) ]PF (brphtmeimb = [(4-bromophenyl)tris(3-methylimidazol-2-ylidene)borate] , [Fe(meophtmeimb) ]PF (meophtmeimb = [(4-methoxyphenyl)tris(3-methylimidazol-2-ylidene)borate] , and [Fe(coohphtmeimb) ]PF (coohphtmeimb = [(4-carboxyphenyl)tris(3-methylimidazol-2-ylidene)borate] . These were derived from the parent complex [Fe(phtmeimb) ]PF (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) -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 ( LMCT) 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 ( GS), completely in line with the parent complex [Fe(phtmeimb) ]PF (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.