Work

Azaullazine-driven frontier orbital engineering enables nanosecond-lived singlet MLCT states in Fe(II) NHC complexes

Year: 2026

Type: preprint

Abstract: Iron(II) complexes are promising sustainable alternatives to noble-metal photosensitizers, yet their practical application is limited by ultrafast deactivation of metal-to-ligand charge-transfer (MLCT) states into low-lying metal-centred (MC) states. Here we report a Fe(II) complex displaying long-lived singlet MLCT excited states, with a fluorescence lifetime of 3.5 ± 0.5 ns and a quantum yield of 1.5%, representing record performances for iron-based chromophores. By introducing heteroleptic NHC–azaullazine ligands that combine high-energy π-donor and low-energy π* acceptor orbitals with suitable energy and symmetry matching to the Fe 3d manifold, we achieve controlled metal–ligand orbital mixing that induces both HOMO inversion and an unusual reordering of the lowest unoccupied molecular orbitals (LUMO inversion). This electronic structure enhances visible-light absorption and suppresses the detrimental ¹MLCT → ³MLCT → ³MC relaxation pathway. Ultrafast spectroscopy and spin–orbit coupling calculations reveal that intersystem crossing from the lowest-energy ¹MLCT states, i. e. excitation into the two lowest-lying acceptor orbitals, is selectively inhibited, thereby preventing population transfer into MC states. In contrast, conventional ultrafast deactivation occurs only for higher-lying ¹MLCT states involving excitation into higher-energy acceptor orbitals. These findings establish frontier-orbital symmetry control as a general strategy for stabilizing emissive charge-transfer singlet states in Earth-abundant transition-metal complexes.

Source: Research Square

DOI: 10.21203/rs.3.rs-9995902/v1