Macrocyclic Bipolar Blue-emitting Host Material Containing Diphenylphosphine Oxide:Synthesis And Optoelectronic Properties

With the introduction of triplet excitons in Organic Light-emitting Diodes (OLED), a 100% internal quantum yield can be theoretically achieved. As singlet and triplet excitons form in the ratio 1:3, emission from the triplet excitons via phosphorescent host materials would be more efficient than those from conventional fluorenscent materials. The hole transporting and electron transporting groups which introduce to the host material would improve the charge transporting ability in OLED. However, the utilization of conventional "unipolar" electron-or hole-transporting host materials may risk a narrow recombination zone, hence increasing the possibility of triplet-triplet annihilation and consequentially leading to low device efficiency. More recently, the design of host materials has shifted to bipolar molecules.In this article, a novel macrocyclic oligomer (F-tPA-dPPO)3 was designed for use as bipolar host material. In the product, three triphenylamine-diphosphine oxide groups are connected to the three core fluorenes through the C-9 centers to form the marcocyclic. The key step for synthesizing the product is Friedel-Crafts reation. The C-9 of fluorene is easy to form a carbocation which is a strong alkylation agent for this F-C progress. The structures of the purified macrocyclic was identified by NMR and MS. This highly rigid molecular includes four important features:(ⅰ) excellent solubility; (ⅱ) non-conjugated and highly rigid structures suppress undesired chain interaction; (ⅲ) easily doped capability due to hollow space structure; (ⅳ) higher thermal stability and morphological stability. The degradation temperature and glass-transition temperature of (F-tPA-dPPO)3 is 430 C and 247℃, which was determined by TGA and DSC analysis. Melting and crystalline peaks were not observed, which shows high morphological stability of the target molecule.The Phosphorescent Organic Light-emitting device (PhOLED) of (F-tPA-dPPO)3 doped with FIrpic was prepared by using a spin-coating process. Two types of device were tested. They are ITO/PEDOT:PSS(25 nm)/FIrpic(15%):PO/TPBI (35 nm)/Ca(20 nm)/Ag (Device A) and ITO/PEDOT:PSS(25 nm)/FIrpic(15%):PO/TmPyPB(35 nm)/Ca(20 nm)/Ag (Device B). According to the testing result, a better device performance has been achieved by using TPBI as the blocking layer. The device has a maximum current efficiency of 19.37 cd A-1, a maximum luminance of 11500 cd m-2 and a power efficiency of 8.58 lm W"1 for a blue OLED.While the imbalance of charge carrier transportation partially results in efficiency roll-off. Hence, a pyridine group instead of the phenyl group in the tPA would significantly improve the electron transporting. However, a linear polymer was obtained in the last step, no macrocyclic products were observed. Therefore, the hole transporting group carbonzole was introduced to the macrocyclic which is a novel biolar host material. The structure was identified by the NMR and MS spectrum. The thermal properties, photophysical properties, electrochemical properties and PhOLED are ongoing.