Synthesis And Study On Electroluminescence Of Blue Phosphorescent Materials And D-A Type Fluorescent Materials

Organic light-emitting diodes(OLEDs)have the advantages of low cost,wide viewing angle,high contrast,low energy consumption and high efficiency.OLED has been considered as the new generation of display technology in replacement of traditional display technologies(such as liquid crystal displays).Organic electroluminescent materials exert tremendous influence on the performance of OLED.Among all the RGB materials,the luminous efficiency of blue materials is generally inferior to those of green and red materials.Hence the development of blue light-emitting materials is the key point for the commercialization of OLEDs.Here,a series of blue phosphorescent tris-cyclometalated iridium complexes and blue fluorescent materials with donor-acceptor structure were designed and synthesized.The photophysical,electrochemical,thermal and electroluminescent properties of these materials were studied.Material types and specific works are as follows:(1)Blue tris-cyclometalated iridium complexes:A cyclometalating ligand named 1-benzyl-4-(pyridin-3-yl)-1H-1,2,3-triazole(pt)was prepared by Click reaction.A series of tris-cyclometalated iridium complexes,M1-M3,were synthesized employing pt as the second cyclometalating ligand.Meanwhile,complexes M4-M6 with 2',6'-difluoro-2,3'-bipyridine(dfbpy)or 2-(2,4-difluorophenyl)pyridine(dfppy)as the second ligand were synthesized as reference molecules to study the influence of the second cyclometalating ligand on the emission properties of iridium complexes.M1,M3,M4 and M6 were used as the doped emitters to fabricate phosphorescent OLEDs(Ph-OLEDs),and sky-blue electroluminescences were obtained for all devices.A maximum external quantum efficiency(?ext)of 10.7%and CIE coordinates of(0.14,0.30)were achieved for M1 based device.And the EL spectra of M1 or M3 based device showed improved color purity with lower intensity of long-wavelength shoulder peak and narrowed full width at half maxima(FWHM)than M2 or M6,indicating that the incorporation of pt as the second cyclometalating ligand is in favor of improving blue color purity for the corresponding iridium complexes.(2)Blue fluorescent materials incorporating benzoylpyridine as acceptor:3,5-dibenzoylpyridine(PDC)and 3-benzoylpyridine(PC)were selected as the electron acceptors(A)to design D-A-D type materials,PDC-3-Cz,PDC-TPA,PDC-tBuDPA and D-A type materials,PC-3-Cz,PC-TPA,PC-tBuDPA,by connecting with electron donating groups(D),N-phenyl carbazole,triphenylamine and di(4-tert-butylphenyl)amine.The transient photo luminescence(PL)decay curves of the doped films of the resultant blue fluorophors in mCP host revealed that PDC-tBuDPA possesses thermally activated delayed fluorescence(TADF)with a lifetime of 0.17 ?s.Vacuum deposited OLEDs incorporating these materials as dopants all showed intense blue emission.And PDC based D-A-D type compounds showed better device performance than their PC analogues.Among this group of blue fluorophors,PDC-tBuDPA exhibited the best device performance with maximum r7ext of 15.4%,while PDC-3-Cz showed blue emission with CIE coordinates of(0.15,0.15)and maximum ?ext of 11.8%.(3)Blue fluorescent materials incorporating benzophenone(BP)as acceptor:Benzophenone was selected as acceptor(A)to design and prepare D-A-D type fluorophor BP-DDPA and D-A type BP-SDPA by connecting with electron donating group di(4-tert-butylphenyl)amine.The transient PL decay curve of BP-DDPA doped film showed a long lifetime of 3.44 ms,while BP-SDPA doped film showed a short lifetime of 6.31 ns,suggesting that BP-DDPA is a TADF material,while BP-SDPA a normal fluorescent molecule.The blue OLED with BP-DDPA as doped emitter exhibited the maximum efficiencies of 49.2 cd A'1 and 26.5%,which are the highest values among TADF materials containing benzophenone moiety reported so tar.(4)Acridone based compounds as deep-blue fluorescent materials and hosts for phosphorescent and TADF OLEDs:Acridone(AC)was selected as acceptor to design D-A type compounds AC-Ph-Cz,AC-Ph-tBuCz,AC-Ph-mCz,AC-3-Cz,AC-Py-Cz and AC-Py-tBuCz by connecting with electron donating groups carbazole and 3,6-di-tert-butylcarbazole.The spatial repulsion of the H atom on the 4-site and 5-site of acridone endows these molecules with dihedral angle of 90,which caused these materials with rigid and twisted structure.The emission spectra of these materials in toluene solution showed small Stokes shifts and narrow FWHMs.OLEDs based on these materials exhibited inferior efficiencies and deep-blue electroluminescence with CIE coordinates of(0.16,0.04-0.06),which are quite close to the National Television System Committee(NTSC)standard blue emission with CIE coordinates of(0.14,0.08).In addition,these materials were employed as host materials for phosphorescent material lr(ppy)3 and TADF material 4CzIPN to fabricated Ph-OLEDs and TADF-OLEDs.All the doped OLEDs exhibited small efficiency roll-off at high brightness.For example,Ph-OLED based on AC-Py-Cz showed maximum efficiencies of 89.8 lm W-1 and 25.2%.At high brightness of 1000 cd m-2 and 10000 cd m-2,r,,,t of the OLED maintained at 24.8%and 21.2%,and dropped by 1.6%and 15.9%in comparison with the maximum value,respectively.