Synthesis, Properties Of Dimeric 1,3,4-Oxadiazoles Derivatives With Flexible Linkers And End-Substituted Alkoxy Groups And Their Poly (Phenylenevinylene) Copolymers

Organic light-emitting diodes based on electroluminescence ofconjugated polymers and molecular materials have attracted greatattention during the past decade because of their potential applicationsin a new generation of display and lighting technologies.A lot of newOLED materials have been synthesized as its rapid advances. Manywidely-used emitters, such as derivatives of poly(phenylenevinylene)and poly(fluorene), are predominantly hole-transporting (p-dopable)materials, i.e. they have low electron affinities. In a device structurethis creates an imbalance of electron injection (from the low work-function cathode) and hole injection (from the high work-function anode).Hole injection predominates and charge recombination occurs near thepolymer/cathode interface which results in a lowering of EL efficiencydue to quenching of excitons by the metal electrode. In this regard,bilayer or multilayer structures with high performance electon transporting materials (ETMs) could significantly improve the emissiveefficiency.1,3,4-oxadiazole derivatives are the most widely studied classesof electron injection and/or hole-blocking materials, mainly because oftheir electron deficiency, high photoluminescence quantum yield, andgood thermal and chemical stabilities.In past years, branched or starb-urst oxadiazole molecules with higher Tgs (125-222 °C) and improvedthermal stability have been investigated as ETMs. Although these mat-erials processible to uniform thin films by vacuum evaporation, theycan not be applied in single layer device easily because of their poorsolubility. In this thesis, we focus on the synthsis and properties of anew bis-oxadiazole system (OXDs) with flexible linkers and end-substituted alkoxy groups which are expected to possess not only highglass transition temperatures but also good solubilities in organicsolvent.Among the structure properties the series of materials designedby us have include the following: i) the main structure is a dimericeoxadiazole linked with phenyl ring. ii) the two oxadiazole rings andthe three phenyl ring directely connected with them form a bulkyconjugated system. iii) the whole conjugated system joints on thep-methoxyl phenyl with propyloxy and hexyloxy group. There areflexiblity and rigidity both in the configuration. The purpose of ourdesign is that the target molecule could possess the noncrystallinenature of rigid ring, the conjugacy of aromatic multi-ring and goodsolubility of long flexible alkoxy chain. In addition, we want toinvestigate how the aromatic ring, which joints on the dimericeoxadiazole with the long alkoxy chain, effect on the propertiesoxadiazole.The purity and chemical structures of the OXDs wereconfirmed by melting point, 1HNMR, elemental analysis and FT-IR.The result compounds are all soluble in the organic solvent such aschloroform etc.We systemically researched the spectroscopy property,electrochemical behavior and energy level structure, thermodynamicbehavior and congregative structure.The structure and propertiesrelationship were discussioned through researching the effect of theflexible linker and the ended-substituents on these properties.In solidfilm the optical spetroscopies indicate that the intermolecular interaction can be stronger when the the length of the linkers increases andweaker when that of end-substitute decreased. In solution severalOXDs can form the intramolecular exciplex (IE), while end-substituted alkoxy groups restrain the IE construction as they could block themolecular folding. The conclusion of the electro -chemical research isthat the OXDs compounds possessed good electron transport abilityand lower LUMO, HOMO energy levels which suggests excellentelectron injection and hole blocking abilities. When the end-substituted alkoxy groups is absence or methoxy group, the related compoun dsdemostrate stable amorphous characteristics cooled quickly, in whichthe Tgs of 3OXD0 and 6OXD0 are 150°C and 148°C respec tivly, aswell as Tgs of 3OXD1 and 6OXD1 both exceed 180°C.We fabricated OLED with of 5% OXDs blended into MEH-PPVas the emissive layer in order to examine the electron transport abilityof OXDs. In the same applied voltage the emitting efficiency havebeen improved 23 folds .This result demonstrates that oxadiazolemoiety can improve the electron transport ability and block holesefficiently. The compounds can form stable amorphous glass either inthe process of spin coating fabricating devices, since the solvent isfavorable for volatilization and the course of the moleculer rapidlyprecipitating is the same as that of the rapidly cooling. Thus effiencyof their devices were improved.Following the combination of electron transport, hole trasportand electroluminescence in one multifunctional materials, 3OXD4 and3OXD12 were copolymerized with 1,4-Dibromo-2-methyloxy-5-(2-ethylhexyl-oxy)xylene respectively, resulted in copolymersOXD-PPV-4C and OXD-PPV-12C, in which the ratio of two unites is5:1. The copolymers have the improved thermal stability and solubility and the number-average molecular weight of the copolymers wasdetermined to be 105 magnitude by GPC.The results show that there is no interaction between theoxadiazol pendants and the main chain in ground state and the sidechain cannot effect the position and the shape of emitting peak of mainchain but the incomplete energy transfer from side chain to the mainchain exists. The OLEDs were fabricated with the copolymer as theemitting layer. Because the side chain electron obital overlies mainchain so that the intrachain transport is predominant , which couldresult in a reduction of trapped electrons during the interchaintransport, the electron transports of copolymers are improved. Theefficiencies of devices are increased by more than two magnitude asthat of MEH-PPV.There are p-type conjugated backbones (donor cable) bearingdirectly grafted n-type groups oxadiazole moieties (acceptor-cable) incopolymer structure, which could be considered as double-cablepolymer cried up in the organic photovoltaic device. Under monochromatic light (487nm) illumination where oxadiazole moity has noabsorption, the open circuit voltage and the short circuit currentdensity of OXD-PPV-4C are 1.5 and 1.4 folds as those of MEH-PPVrespectively in device ITO/PEDOT/polymer/Al.Under white lightillumination at intensity of 100 mW/cm2 the power conversionefficiencies are significantly increased than those of MEH-PPV. Thisimproved performance is determined by the chemical structure ofcopolymer and excellent electron transport ability of oxadiazolependant.