Bipolar Heteroleptic Green Iridium Dendrimers Containing Oligocarbazole and Oxadiazole Dendrons for Bright and Efficient Nondoped Electrophosphorescent Devices

Abstract

Bipolar heteroleptic green light-emitting iridium (Ir) dendrimers G(OXD) and G(DOXD) have been designed and synthesized under mild conditions in high yields, in which the first C^N and second O^O ligands are functionalized with oligocarbazole- and oxadiazole-based dendrons, respectively. To avoid affecting the optical properties of the emissive iridium core, all the functional moieties are attached to the ligands through a flexible spacer. Compared with the unipolar dendrimer G(acac), dendrimers G(OXD) and G(DOXD) exhibit the close emission maxima of 511-512 nm and photoluminescence quantum yield of 0.39-0.40 in a solution of toluene. Moreover, on going from G(acac) to G(OXD) and G(DOXD), we have found that the introduction of oxadiazole fragments decreases the lowest unoccupied molecular orbital (LUMO) energy levels to facilitate the electron injection and electron transporting, while their highest occupied molecular orbital (HOMO) energy levels remain unchanged. This means that, we can individually tune the HOMO and LUMO energy levels based on the heteroleptic structure to ensure the relative independence between the hole and electron in the emitting layer (EML), which is a favorable feature for bipolar optoelectronic materials. As a result, a bilayer nondoped electrophosphorescent device with G(DOXD) as the EML gives a maximum luminous efficiency of 25.5 cd A(-1) (η(ext): 7.4%) and a brightness of 33,880 cd m(-2). In comparison to G(acac) (17.2 cd A(-1), 17,680 cd m(-2)), both the efficiency and brightness are improved by about 1.5 and 2 times, respectively. These state-of-the-art performances indicate the potential of these bipolar heteroleptic iridium dendrimers as solution-processible emitting materials for nondoped device applications.