Experimental Researches On Donor-acceptor Type Fluorescent Molecules Constucted Of Benzo[c][1,2,5]thiadiazole And Naphtho[2,3-c][1,2,5]thiadiazole: Synthesis, Photoluminescent Properties And New Mechanism Of Electoluminescence

In rescent years, the development of organic light-emitting diodes(OLEDs) is very fast and OLEDs has shown great market potential in display and illumination. OLEDs owns a lot of advantages compares to traditional LED, such as high flexibility, high contrast ratio, wider visual angle and more friendly to environment. However, OLEDs’ high cost and complicated manufacturing progress confined its development which leading to its high production price and low market competitiveness. So, how to reduce material costs as well as simplify preparation progress become more and more important in OLEDs’ research. Some scientists come up with a few Donor-Acceptor type organic molecules which can provide high excition utilization and high EL performance. This kind of Donor-Acceptor type molecule mainly based on thermally activated delayed fluorescence(TADF) and triplet-triplet annealing(TTA) mechanism. Donor-Acceptor type organic molecules have relatively more balanced carrier injection and transportation ability, much easier to synthesise and control molecule structure. According to our group’s research, D-A type molecules which have large T2-T1 energy level difference could effectively reduce internal conversion rate, improve the reverse intersystem crossing(RISC) at high-lying states and harvest 100% excitons utilization.“Hot excition” mechanism is based on the reverse intersystem crossing(RISC) at VI high-lying states(Tn�' S1), which is very fast and difficult to be observed. We have taken a lot of fundamental research on the purpose of breaking the limitation on traditional photochemistry theory, to prove the “hot excition” mechanism, obtain high excition utilization and high EL performance material. According to the literarure research and our group’s work, benzo[c][1,2,5]thiadiazole(BZ) and naphtho[2,3-c][1,2,5]thiadiazole(NZ) were selected as the acceptor part in D-A molecules for their high PL efficience and large T2-T1 energy level to combine with different donor, synthesized through the different connections. We have synthesized fluorescent molecules of different properties which can harvest high excition and realize good EL performance. The detailed research progress is list as follow:1. Using single carbazole(Cz) as donor and single benzo[c][1,2,5]thiadiazole(BZ) as acceptor, four donor-acceptor(D-A) compounds were designed and synthesized through the different Cz-BZ connections. According to the photophysical measurement, DFT calculations, thermodynamic measurement and EL device fabrication, Cz P-BZP showed good thermodynamic stability, high PL efficience and good EL performance. The non-doped OLED of Cz P-BZP exhibited an excellent performance: a green emission with peak at 538 nm, a maximum external quantum efficiency(EQE, ηEQE) of 6.95%, a maximum current efficiency of 23.99 cd A-1. and This combined HLCT state and “hot exciton” strategy should be a practical way to design next-generation, low-cost, high-efficiency fluorescent OLED materials.2. By changing the proportion between carbazole and benzo[c][1,2,5]thiadiazole, three kinds of donor-acceptor compounds were obtained. The thermodynamic stability of this three compounds was improved compared with our former work and this three compounds were all of high PL effcience. DCz P-BZ showed good thermodynamic stability, high PL efficience and good EL performance.3. According to our previous work, we took carbazole(Cz) as donor, naphtho[2,3-c][1,2,5]thiadiazole(NZ) as acceptor which has larger T2-T1 energy level compares to benzo[c][1,2,5]thiadiazole. Two red organic flourescent compounds named Cz P-NZP and DCz P-NZ were designed and synthesized. DCz P-NZ expressed higher thermodynamic stability better EL performance. The non-doped EL device of DCz P-NZ possessed high efficiency stability, the maximum EQE was 1.77%, EL emission peaked at 640 nm and the maximal exciton utilization efficiency up to 63%. The doped EL device of DCz P-NZ was also fabricated, its maximum EQE was 3.03% and its emission peaked at 620 nm. The study could provide new ideas for the design of efficient red fluorescent molecules by emphasizing the full use of both singlet and triplet excitons.4. In order to further study “hot excition” and HLCT mechanism and expand their universality, we chose phenoxazine as donor, benzo[c][1,2,5]thiadiazole(BZ) and naphtho[2,3-c][1,2,5]thiadiazole(NZ) as acceptor, designed and synthesized four donor-acceptor(D-A) compounds through the different PXZ-BZ and PXZ-NZ connections. On the basis of the photophysical data and DFT calculations, we found the photophysical properties of D-A molecules were determined by the relative location of CT state and LE state in these D-A molecules. Under the specific D-A geometry, CT state and LE state have similar energy level, and could be mixed(PXZ-10-BZP and PXZ-10-NZP) or hybridized into a new state — HLCT state(PXZ-3-BZP and PXZ-3-NZP). The HLCT material(PXZ-3-BZP and PXZ-3-NZP) could achieve higher PL efficience and better EL performance. PXZ-3-BZP doped device exhibited the maximum EQE of 3.83%, emission peaked at 610 nm and the maximum brightness of 49374 cd m-2 which was among the brightest organic red fluorescent OLEDs reported so far. PXZ-3-NZP non-doped device exhibited the maximum EQE of 0.82%, which was among the highest results of undoped NIR fluorescent OLEDs reported so far.