Design,Synthesis And Electroluminescence Properties Of Blue To Near-infrared Organic Luminescent Materials Based On Pyrene

As one of the most competitive candidates for next-generation display and illumination technology,OLEDs(Organic Light Emitting Devices)have many novel features such as thinness,flexibility,and high contrast ratio.Especially,the flexibility can bring infinite possibilities to future products.At present,OLED technology has made great progress in the field of commercialization.Many companies have launched a variety of practical products.With further development,it is believed that OLED will gradually enter our daily life.However,the technology of OLED still faces several key challenges,such as prolonging the life of the device,improving the color purity,and developing high-efficiency and cheap luminescent materials.Solving these problems is of great significance to promote the development of OLEDs.Starting from the development of new luminescent materials can fundamentally solve the above problems,and can greatly simplify the design of complex fabrication processes and driving circuits.Pyrene and its derivatives have been widely used in the OLED field as blue light-emitting materials due to their advantages such as easy modification,thermal stability,and high ?PL(Photoluminescence Quantum Yield).But they are mainly concentrated in the traditional fluorescence field.How to realize the effective utilization of triplet excitons and expand region of emitting light has become important direction to explore its potential as luminescent material.Therefore,a series of high-performance organic luminescence materials emitting light from blue to near-infrared has been designed and synthesized through suitable molecular design strategy.The influence of molecular structure and aggregation on properties of materials is also explored.These results provide reasonable strategy to further achieve high-performance emitters.The main contents are as follows:In Chapter 2,Two blue emitting molecules were constructed by attaching two donor units,phenylcarbazole and indolocarbazole,to the pyrene core.By expanding the molecular orbitals from pyrene to the attached donor,the emission spectra of these emitters bathochromic shift compared with pyrene.The device based on 3tBuCz-2iPrPy achieved a maximum EQE(External Quantum Efficiency)of 4.25% with an EL peak at 432 nm and the CIE(Commission Internationale de L'Eclairage)coordinates of(0.17,0.11).The 3t Bu ICz-2i Pr Py based device displayed a maximum EQE of 6.38% with an electroluminescence peak at 453 nm and the CIE coordinates of(0.15,0.10).Benefiting from the more rigid structure of indolocarbazole,the device based on 3t Bu ICz-2i Pr Py achieved a bluer color coordinate with a redder electroluminescence peak.In addition,the devices based on two molecules exhibited low efficiency roll-off,maintaining EQE value of 4.11% and 6.17% at a brightness of 1000 cd m-2.It is obvious that the EQE value of both devices breaks through the traditional limit of fluorescence materials.Further investigations exhibit that TTA(triplet-triplet annihilation)process and enhanced light outcoupling efficiency may play key roles for this performance.In Chapter 3,PDCN unit with strong electron-accepting ability was synthesized based on oxidation and cyclization reaction of pyrene.Then a pair of yellow-green emitting materials was obtained by attaching the PDCN moiety to the different positions of phenylcarbazole core.Benefiting from the large dihedral angle between the donor and acceptor of two molecules,the efficient separation of the frontier molecular orbitals is achieved.Among them,the 3Cz-Ph-PDCN molecule has a more twisted structure,which reduces the orbital overlap,resulting in smaller single-triplet energy split and ?PL.Further theoretical analysis exhibits that the PDCN group is likely to assist the reverse intersystem crossing process,which contributes to the TADF(Thermally Activated Delayed Fluorescence)property.The device based on 3Cz-Ph-PDCN achieves a maximum EQE of 11.81% at the CIE coordinates of(0.46,0.53),while the device of 4Cz-Ph-PDCN with larger ?PL exhibits higher maximum EQE of 15.47% at the CIE coordinates of(0.45,0.54).This is the first reported yellow-green TADF material based on pyrene.In Chapter 4,the author further extended the conjugation of the PDCN acceptor core in the previous chapter to construct a PPDCN unit with stronger electron-accepting ability.Two pairs of isomers have been synthesized by attaching electron-donor 9,9-diphenyl-9,10-dihydroacridine(DPAC)moiety to the different positions of two kinds of acceptor cores(PDCN and PPDCN).By regulating the substitution position of donor and ?-conjugation space of acceptor,the emission colors of these emitters can be easily shifted from orange to deep red colors(599-726 nm).Interestingly,the ?PLs of transisomers T-DA-1 and T-DA-2(78 and 89%)are remarkably higher than those of their corresponding cis-isomers C-DA-1 and C-DA-2(12 and 14%).Further investigations including computational simulations and photophysical measurements demonstrated that donor-acceptor arrangement patterns play a key role in luminescent properties.The twisted structure of cis-isomer weakens the overlapping of vibrational wave functions and makes radiative transition more difficult to take place.As a result,a red TADFOLED based on T-DA-2 displayed a maximum EQE of 26.26% at 640 nm.Notably,at a brightness of 100 cd m-2,the EQE value of T-DA-2-based device still remained at an extremely high level of 23.95%,representing the highest value for reported red TADFOLEDs at the same brightness during the same period.In Chapter 5,the author used the PPDCN acceptor with strong electron-accepting ability obtained in the previous chapter as the core,and nitrogen atom of triphenylamine donor with stronger electron-donating ability was attached to different positions of PPDCN to obtain a pair of isomers TPA-PPDCN and TPA-TPPDCN.The luminescence of compound can further red-shift to deep red and near-infrared region.The optimized geometry of two molecules show large dihedral angle between the donor and acceptor,result in efficient spatial charge separation of the HOMO and LUMO and strong intramolecular charge transfer emission properties.Benefiting from the rigid planar framework and two electron deficient CN groups of the PPDCN acceptor,the TPA-PPDCN exhibits excellent thermal stability,small singlet-triplet energy split and strong DR/NIR emission with the ?PLs of 73-87% in doped thin films.More importantly,highly efficient DR and NIR OLEDs with emission peaks at 664 and 692 nm and the maximum external quantum efficiencies of 20.2% and 16.4% have been achieved using TPA-PPDCN,which respectively represent the highest device performance among the reported DR/NIR TADF OLEDs during the same period.