Reassignment of the Near-Infrared Absorption of Dioxidized Rare-Earth Porphyrin Double-Decker Complexes: Identification of a Nearly Dark Lowest Excited State
Year: 2026
Type: preprint
Abstract: The intense near-infrared band (700–1250 nm) of dioxidized porphyrin double-decker complexes, characterized by a metal ion sandwiched between two porphyrinato ligands bearing two holes in total, has been assigned to the HOMO→LUMO transition for three decades. Herein, we demonstrate that this band does not correspond to the lowest excited state. In the spectra of [Y(oep) 2 ] + and [Tb(oep) 2 ] + (oep = octaethylporphyrinato), we observed a weak band near 1430 nm, with 2–3% of the absorbance of the intense band (at 812 nm and 830 nm) and a much smaller dependence on the metal. Time-dependent density functional theory calculations with an optimally tuned functional assigned the weak and intense bands to S 1 and S 3 , respectively, i.e., a configuration-interaction pair of the HOMO→LUMO and HOMO−1→LUMO excitations. As observed in crystal structures reported earlier, the two rings are twisted by 30–36°, which lowers the symmetry from D 4d (45°) to D 4 . The weak band S 1 is symmetry-forbidden in D 4d and owes its low intensity to this twist. The calculated slope of S 3 against the ionic radius was within 3% of the experimental value, whereas S 1 was nearly independent of the metal. Therefore, the lowest excited states of these strongly coupled π dimers are nearly dark.
Source: ChemRxiv
Authors: Wataru Imanaka, Ken-ichi Yamashita
Language: English
Subfield: Materials Chemistry
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7223296519