Highly sensitized near-infrared luminescence in Ir–Ln heteronuclear coordination polymers via light-harvesting antenna of Ir(iii) unit

Abstract

Four novel iridium–lanthanide heteronuclear coordination polymers {Ln[Ir(ppy)₂(dcbpy)]₂(OH)}·H₂O (Ir–Ln; Ln = Gd, Yb, Er, Nd, ppy = 2-phenylpyridine, dcbpy = 2,2′-bipyridine-4,4′-dicarboxylate) were successfully obtained by hydrothermal reactions based on a Ir unit (Ir(ppy)₂(Hdcbpy), L–H). In these complexes, the Ir unit acts as a good light-harvesting antenna showing high visible light absorption via triplet metal-to-ligand charge transfer (³MLCT), and effectively sensitizes Ln(III)-based near-infrared (NIR) luminescence by d → f energy transfer from the Ir unit to the Ln³⁺ ions. Those coordination polymers display the characteristic NIR emission of Ln(III) ions (Ln = Yb, Er, Nd) when being excited by 500 nm visible light, while the characteristic ³MLCT luminescence of the Ir unit was almost quenched. The Ir–Nd polymer exhibited the maximum quenching of the ³MLCT luminescence of the Ir unit because the Nd(III) ion is the most effective energy-acceptor with a high density of low-lying f–f states. The coordination polymer structures of Ir–Ln reduce the effect of high-energy oscillators (O–H, C–H) and further increase the Ln(III)-based emission lifetimes and quantum yields. The successfully acquired highly sensitized NIR luminescent Ir–Ln coordination polymers pave a new approach to the design and synthesis of NIR luminescent materials.