Efficient photoluminescent complexes of 400–1800 nm wavelength emitting lanthanides containing organic sensitizers for optoelectronic devices

Abstract

An in situ solution processed reaction of a bidentate O,O′-chelating anionic hexafluoroacetylacetone (Hhfaa) and a bidentate N,N′-chelating 2,2′-bipyridine ligand with LnCl₃·6H₂O in the presence of a base afforded the UV-sensitised 400–1800 nm wavelength emitting lanthanide complexes, [Pr(hfaa)₃(bpy)₂] and [Ln(hfaa)₃(bpy)](Ln = Tb, Dy, Tm and Lu). The single-crystal analysis indicates that the Pr complex is ten-coordinate with a distorted bicapped square antiprism while the Dy complex is eight-coordinate with a distorted square antiprism geometry, and the bpy units, in the complexes, are involved in π–π stacking interactions and hydrogen bonding, respectively. The assembly of the hfaa⁻ (a low vibrational frequency ligand) and bpy ligand makes an efficient protective coordination environment (PrO₆N₄ or DyO₆N₂) around the Pr (red emission), Tb (green emission), Dy (yellow emission) and Tm (blue emission) ions which leads to high quantum yields and longer emission lifetimes. The quantum efficiency of the complexes is enhanced in the solid state. Furthermore, these volatile and luminescent complexes were used as emitting layers to fabricate red-, green- and yellow-light emitting devices and their electroluminescence performances were investigated. The best devices with the structure ITO/CuPc (20 nm)/[Pr(hfaa)₃(bpy)₂] or [Tb(hfaa)₃(bpy)] or [Dy(hfaa)₃(bpy)] (80 nm)/BCP (25 nm)/AlQ (30 nm)/LiF (1 nm)/Al (200 nm) exhibit a maximum brightness of 183, 1765 and 532 cd m⁻² with a current efficiency of 0.58, 3.6 and 0.76 cd A⁻¹, respectively, which indicates an improved EL performance over the devices based on Pr, Tb and Dy complexes in the literature.