Imidazole Based Electroluminescent Materials For Light Emitting Devices: Design, Synthesis, Characterization And Application

Development of novel organic electroluminescent(EL)materials has become one of the most extensively studied topics in material science and engineering owing to its multiple applications in optoelectronic devices.A significant progress has been made in the field of organic light emitting diodes(OLEDs).Since Tang and VanSlyke constructed EL devices using organic materials as emitters,development of efficient and stable EL materials have flourished tremendously.Among different types of devices based on architecture,non-doped devices are of special significance due to low cost and easy fabrication process.Many efforts have been done to explore different types of non-doped blue fluorescent emitters to enhance their EL performances.Imidazole based compounds have proven to be a bright prospect in the development of high performance materials for optoelectronic applications.Owing to their bipolar nature,imidazole based compounds are used as hole transport,electron transport as well as emissive materials.We have designed and synthesized high performance deep blue imidazole and phenanthroimidazole derivatives for efficient non-doped OLEDs.Different moieties,such as triphenyl amine,carbazole and pyrene were attached to phenyl group of imidazole and phenanthroimidazole to increase electron donating ability,the triplet energy level and photoluminescence quantum yield.The TPAPyPI based device exhibited low turn-on voltages(as low as 3.2 V),high current efficiency(CE,4.24 cd/A),power efficiency(PE,3.67 lm/W),external quantum efficiency(EQE,4.66%),comparatively small efficiency roll-off at higher current densities,and stable deep blue emissions with Commission International de L'Eclairage coordinates(CIEy)of 0.13.Also,we have explored some novel multifunctional materials based on imidazole(I)and triphenyl amine(TPA)units containing tetraphenylethene(TPE)moiety for OLED devices.All the compounds have shown great potential to synthesize efficient blue emitters.Two materials BPTPETPAI and 2TPETPAI neither exhibited typical aggregation caused quenching(ACQ)behavior nor typical aggregation induced emission(AIE)phenomenon,whereas TPETPAI and TPETPAPI showed typical ACQ effect.BPTPETPAI and 2TPETPAI emitted blueish green light,whereas TPETPAI and TPETPAPI emitted deep blue light.Non-doped devices achieved excellent performance among the other non-doped devices reported so far.OLEDs displayed blueish emission and EQEs as high as 3.25%(6.80 cd/A,4.35 lm/W),with small efficiency roll-off.Similarly,another main issue related to blue fluorescent materials is the reduction of intensity of fluorescence on aggregation(solid state).To solve this problem,Tang's group discovered a new phenomenon which is known as aggregation-induced emission(AIE).In this case,intensity of light is increased in the aggregated form instead of decreasing the intensity of emission.Therefore,we have explored simple structured AIE emitters for OLED devices.All the compounds have shown great potential to synthesize efficient AIE emitters.All materials showed AIE phenomenon.Non-doped devices achieved excellent performance among the other non-doped devices reported so far.OLEDs displayed blueish green emission and CE as high as 6.97 cd/A with small efficiency roll-off.These data illustrate that these electroluminescent materials based on highly emissive central core like imidazole have significant potential to be employed as emissive layers in OLEDs.