Design,Synthesis And Optoelectronic Properties Of Phenanthroimidazole Derivatives

Organic light-emitting diode(OLED),known as the "Dream Display",has been regarded as a promising technology for flat-panel displays due to its self-luminescence,wide viewing angle,low power consumption,fast response time,thin thickness and flexibility.Compared with fluorescent materials,phosphorescent materials have attracted considerable attention due to the achievement of 100% internal quantum efficiency.To date,green and red phosphorescent materials have made significant progress and can meet the demand of industrialization.However,blue phosphorescent materials usually exhibit short life and serious efficiency roll-off at high brightness.In addition,color purity,efficiency and stability of blue phosphorescent materials are poor,which limits their large-scale application in industry.Compared with blue phosphorescent materials,blue fluorescent materials have attracted great interest because of easy molecule design,high device stability and long device life.Furthermore,blue-emitting materials cannot only reduce power consumption,but also serve as energy-transfer donors for long-wavelength emitters to achieve full-color displays and white light.Therefore,the study of high-performance blue materials is the key to achieve full-color display.In general,phosphorescent materials need to be dispersed into a suitable host material to avoid aggregation quenching or triplet-triplet annihilation,and improve the luminous efficiency.Therefore,host materials play an important role for the performance of phosphorescent devices and production cost.In conclusion,the development of highly efficient blue fluorescent materials and phosphorescent host materials is important to promote OLED industrialization.Phenanthro[9,10-d]imidazole(PI)derivates have received more and more attention due to simple synthesis,easy modification,wide band gap,high fluorescence quantum yield,good thermal stability and balanced carrier injection/transportion property.In this thesis,PI as a basic building block is modified at the C2 position of imidazole,a series of novel optoelectronic materials containing PI group have been designed and synthesized.Their thermal properties,electrochemical properties,photophysical properties,crystal structures and carrier transport properties were systematically studied.We also investigated the relationship between molecular structures and properties,and fabricated a series of organic electroluminescence devices.These results provide an efficient strategy for the development of novel PI derivates.The content of this thesis includes the following four parts:1.In chapter II,a series of anthracene-containing PI derivatives have been synthesized by introducing anthracene derivatives into the C2 position of PI.The results indicate that the introduction of anthracene derivatives endows these compounds good thermal stability,high fluorescence quantum efficiency,and the HOMO level to match with the transport layer which can reduce the interface barrier.The single-carrier devices show that these compounds have good bipolar transport characteristics.The non-doped OLEDs using DPAA-PPI as emitting layers showed the maximum power efficiencies(PE)of 13.9 lm W-1,corresponding to the maximum external quantum efficiencies(EQE)of 5.2%.The DPAA-PPI-based OLED exhibited an extremely low turn-on voltage of 2.4 V and its luminescence reached 100,1000,10000 and over 100000 cd m-2 under driving voltages of 2.8,3.6,5.1 and 7.5 V,respectively.2.In chapter III,we designed and synthesized three bisphenanthroimidazole derivates with donor-acceptor(D-A)structures because PI can behave as respectively as a mild donor or acceptor.The theoretical calculations show that the HOMOs and LUMOs of these compounds display adequate separation features,which benefits the hole-and electron-transport.The strong deep-blue emission and good bipolar carrier transport characteristics of these compounds ensure that the OLEDs with three compounds as non-dopant emitting layers display an extremely high EQE of 8.1% with deep-blue CIE(0.15,0.07).Furthermore,high triplet energy levels and good carrier transport properties allow them to be used as hosts to fabricate green,yellow and red phosphorescent OLEDs.High performance green,yellow and red phosphorescent OLEDs with the maximum EQE of 18.0%,19.8% and 18.0%,respectively,have been successfully achieved based on these compounds and phosphors.3.In chapter IV,we have designed and synthesized two blue-emitting compounds,mTPE-PPI and mTPE-DPI,with the combination of a triphenylethylene(TPE)moiety and one or two PI moiety.The TPE unit is adopted because it is a typical group with multiple rotatable single-bonds for realizing AIE properties.The PI unit is a wide-bandgap fluorophore with good carrier transport property.The twisted molecular conformations of present AIE systems endow these compounds with bright blue emission in the solid state.In addition,these compounds are capable of transporting both electrons and holes,and their HOMO and LUMO levels match well with the transport layers for carrier injections.These features enable them to serve as excellent blue-emitting materials in OLEDs.A non-doped OLED based on mTPE-PPI displayed deep-blue light with the CIE coordinate of(0.15,0.09)and a maximum EQE of 2.30%.An analogous device with mTPE-DPI as the emitting layer emitted blue light(CIE: 0.15,0.14)with a high EQE of 3.69% and low efficiency roll-off.Further improvement of device performance has been achieved by the doping technique.The doped device of mTPE-DPI was able to realize deep-blue emission(CIE: 0.15,0.10)matching well the blue standard,meanwhile reaching a maximum EQE of 5.52% that represents the highest value reported for deep-blue OLEDs based on AIE compounds.4.In chapter V,two PI-based host materials BP-DPPI and BP-DTPI composed of two PPI or 1,4,5-triphenyl-1H-imidazole(TPI)groups bridged with the 2,2?-positions of a biphenyl linker have been successfully designed and synthesized.The ortho-ortho linkage endows the compounds highly twisted molecular conformations,which endows it stable morphology and sufficiently high ET.The single-carrier devices indicate that BP-DPPI has good bipolar transport character,while BP-DTPI shows hole-dominated transport property.More importantly,high device performances with the maximum PE of 92.7 lm W-1 and EQE of 26.6% were achieved for the BP-DPPI hosted green emitter of Ir(ppy)3.In addition,the BP-DPPI hosted green phosphorescence device also achieved a low efficiency roll-off.