Twisted dendrons for highly luminescent green emissive phosphorescent dendrimers†
A new class of twisted rigid biphenyl-based dendrons with t-butyl surface groups have been prepared and attached to a green emissive fac-tris(2-phenylpyridyl)iridium(III) core. Both singly (one dendron per ligand) and doubly (two dendrons per ligand) dendronised first generation dendrimers were prepared. In both cases the solution photoluminescence quantum yields (PLQYs) were over 10% higher than the equivalent dendrimers comprised of non-rigid biphenyl-dendrons and the same core, with the doubly dendronised compound having an almost quantitative PLQY. However, for both materials there was significant reduction in the PLQY in the solid-state indicating that while the t-butyl groups can provide a degree of solubility they do not shield the emissive core as well as surface groups such as 2-ethylhexyloxy, which have greater length. When the doubly dendronised material was blended with 4,4′,4′′-tris(carbazol-9-yl)triphenylamine the film PLQY approached that of the solution measurement and simple two-layer devices comprised of the emissive layer and an electron transport/hole-blocking layer of 1,3,5-tris(3-pyridyl-3-phenyl)benzene reached a maximum brightness of ≈10 000 cd m−2 and external quantum efficiency of 16.3%, corresponding to an out-coupling efficiency of around 20%.