Study On Bipolar Deep-blue Phenanthromidazole Derivatives

In the past30years, organic light-emitting devices (OLED) have attracted more and moreattention due to their potential application in full-color panel displays since the first fabricatedOLED in1987. This technology was praised as fantastic display in future. However, comparedwith successful emitters of red and green, the blue one has been in puzzled because of itscontradiction between wide band gap and balanced charge injection and transports. Recently,this problem has been solved by a kind of compounds with “bipolar” properties in somedegree, which made great progress in the field of material design and device application.Comparing to the traditional bipolar materials with double functional group,phenanthro[9,10-d]imidazole (PI) not only showed the potential bipolar characteristic in singlefunctional group, but also exhibited facial synthetic progress, low-cost raw materials, excellentefficiency and simple modified, which made it become new generation star molecule inbipolar blue-light emitters.4,4’-bis(1-phenyl-phenanthro[9,10-d]imidazole-2-yl)biphenyl(BPPI) was reported in ourprevious work, and was chosen as model compound for PI-based compounds in this thesis. Forbalancing its carriers injection/transports ability and improving color purity in blue area, someefficient methods are introduced in design and preparation, such as inserting functional group,changing connection position and structure. By collecting the structure-properties relationshipof phenanthro[9,10-d]imidazole, some valuable information is accumulated to the design ofdeep-blue emitters. Some results were shown as follows.(1) To balance carriers injection/transports ability in BPPI, carbazole, as a better hole injectionand transporting group, was inserted in phenanthro[9,10-d]imidazole. The link methods ofmeta-and para-position were optimization, and mCz-BPPI and pCz-BPPI were prepared.Compared with BPPI, their injection barrier are decreased obviously based onelectrochemical data, and the fluorescence peaks have a certain blue-shift.(2) By changing the biphenyl center of BPPI, the four compounds, named L-BPPI, Z-BPPI,pmT-BPPI and mmT-BPPI, were prepared. There is obvious blue-shift in fluorescencepeaks. For decreasing electron injection barrier in further, the cyano group is inserted inT-BPPI. (3) N-BPPI was prepared through tuning linked position from C2of BPPI to N1-position, andexhibited excellent blue and bipolar property. Introducing the theory of molecular hybridexcited state analysis, N1-substituted one show a weak CT state and enhance RISC process,which is benefit to realize improving the PL efficiency and excition conversation efficiency,and the undoped blue OLED give the increased efficiency obviously.