Quest for Luminescent Low-Spin Chromium(I) Complexes with Doublet Metal-to-Ligand Charge-Transfer Excited States

Abstract

Two paramagnetic Cr(I) complexes with homoleptic fac-bis(triarylisocyanide) ligands were prepared by chemical oxidation of the Cr(0) precursors and emission spectra with peak maximum at respective 568 and 525 nm have been recorded with THF solutions of these two low-spin d⁵ metal complexes at 298 K. Structural, spectroscopic, and theoretical studies revealed that the emission of the Cr(I) complexes could be tentatively assigned to be spin-allowed ²MLCT excited states. Femtosecond-resolved transient absorption spectra confirmed that the lifetimes of the excited states were in the tens of picoseconds, which could be explained by the fast nonradiative decay of ²MLCT state through low-lying quartet metal-centered (⁴MC) states. The Cr(I) complexes with luminescent ²MLCT excited states were distinct from the previously reported d⁵ Fe(III) complexes with luminescent ²LMCT excited states, enriching the diversity of emissive excited states of earth-abundant 3d transition metal complexes.