The Application Of Tricyclic Fused Ring Type Acceptor Units In Phosphorescent Host Materials

Over the past three decades,organic light-emitting diode (OLED) has not only reached great breakthroughs in efficiency and lifetime,but also made steady progress in commercialization.OLED with high efficiency and long lifetime depends on the development of new emitters and optimization of device structure.The light-emitting materials has grown with three generations,which are fluorescent materials,phosphorescent materials and thermal activated delayed fluorescent (TADF) materials,respectively.The theoretically 100% internal quantum efficiency (IQE) of phosphors and TADF materials has made them the preferred choice of achieving highly efficient OLED.Compared with phosphorescent materials,TADF emitters are restricted in dominating the commercialized market due to the problems of operational duration and spectrum stability.For high-performance phosphorescent OLED (PHOLED),host-guest system is usually employed by doping the luminescent guest material into a certain host material in light-emitting layer (EML).This technique is helpful in avoiding concentration quenching effect and suppressing the luminescence quenching phenomenon caused by triplet excitons in EML effectively.Thus,the development of novel host materials has become a research hotspot.Donor-acceptor (D-A) type materials possess both hole-transport and electron-transport abilities which can improve OLED characteristics.Tricyclic fused ring compound is a class of significant organic compounds,and it provides numerous moieties in designing OLED materials.It is widely used in donors,while relatively few in acceptors for OLED materials.Herein,this work is focusing on the application of tricyclic fused ring type electron-acceptor units in PHOLED,and discussing the relationship between the molecular structure and optoelectronic characteristics in three chapters.It is aimed to explore the potential of these compounds in OLED applications and furnish available approaches for designing materials.1.In chapter 2,fluorenone was taken as an electron-acceptor unit for the application of OLED host materials.N-phenyl-carbazole donor unit was brought into the four mono-substituted derivative sites (1-,2-,3-,and 4-position) of fluorenone,respectively,to obtain four host materials with D-A structure.The four regioisomers were named as compound 1,2,3,and 4.The effects of different substitutions of fluorenone were investigated on the thermal stability,photophysical properties,electrochemical behavior,charge-transport behavior and electroluminescent properties.Compound 1 based device exhibited the relatively optimal electroluminescence performance.The result indicates that the 1-position substitution of fluorenone can bring some newfangled properties to the material,and open up a more extensive application prospect of fluorenone.2.In chapter 3,phenanthridone with a lactam structure was built with the introduction of a nitrogen atom at the carbonyl position of fluorenone moiety,and it was applied as the acceptor unit in OLED host materials.Substituted or unsubstituted triphenylamine donor unit was linked to the phenanthridone to afford a series of host materials,TPA,TPAtBu and TPAOMe.It was manifested that the lowest unoccupied orbital (LUMO) energy level and triplet energy (ET) of phenanthridone were enhanced compared to fluorenone.In addition,different donor units had a few impacts on fundamental properties of the materials,while they all showed acceptable device performance.The red PHOLED device based on TPAOMe had the most favorable performance,and all three devices displayed a low efficiency roll-off.The results reveal the possibility of phenanthridone as an acceptor unit for OLED materials and expand the scope of its further applications.3.In chapter 4,a novel tricyclic fused ring compound with diacyl diamine structure was constructed by introducing an additional nitrogen atom on the other side of carbonyl group of phenanthridone fragment,namely 6H-dibenzo[d,f][1,3]diazepin-6-one unit.Two host materials 7-TPA and 7-TBu were given by connecting substituted or unsubstituted triphenylamine donor units with the 6H-dibenzo[d,f][1,3]diazepin-6-one segment.The introduction of nitrogen atom had further improved the LUMO energy level and ET of 6H-dibenzo[d,f][1,3]diazepin-6-one compared to phenanthridone.This brings new inspiration for people to design materials with high ET.It appeared on the basic properties of 7-TPA and 7-TBu that different donor units dramatically influenced the thermal property,photophysical properties,electrochemical behavior and charge-transport behavior of the materials.Both 7-TPA and 7-TBu hosted red PHOLEDs presented great device performance and low efficiency roll-offs,which suggests that 6H-dibenzo[d,f][1,3]diazepin-6-one is a promising candidate for OLED materials.