Near-unity thermally activated delayed fluorescence efficiency in three- and four-coordinate Au(i) complexes with diphosphine ligands

Abstract

The synthesis and photoluminescence properties of three-coordinate Au(I) complexes with rigid diphosphine ligands L_Me {1,2-bis[bis(2-methylphenyl)phosphino]benzene}, L_Et {1,2-bis[bis(2-ethylphenyl)phosphino]benzene}, and L_iPr {1,2-bis[bis(2-isopropylphenyl)phosphino]benzene} are investigated. The L_Me and L_Et ligands afford two types of complexes: dinuclear complexes [μ-L_Me(AuCl)₂] (1d) and [μ-L_Et(AuCl)₂] (2d) with an Au(I)–Au(I) bond and mononuclear three-coordinate Au(I) complexes L_MeAuCl (1) and L_EtAuCl (2). On the other hand, the bulkiest ligand, L_iPr, affords three-coordinate Au(I) complexes, L_iPrAuCl (3) and L_iPrAuI (4), but no dinuclear complexes. X-ray analysis suggests that both 3 and 4 possess a highly distorted trigonal planar geometry. Moreover, luminescence data reveal that at room temperature, 3 and 4 exhibit yellow-green thermally activated delayed fluorescence in the crystalline state with maximum emission wavelengths at 558 and 549 nm, respectively. The emission yields are close to unity. Quantum chemical calculations suggest that the emission of 4 originates from the (σ + X) → π* excited state that possesses strong intraligand charge-transfer character. The luminescent properties of four-coordinate Au(I) complex (5) possessing a tetrahedral geometry are discussed on the basis of the emission spectra and decay times measured in a temperature range of 309–77 K.