Blue Fluorescent Materials With High Exciton Utilizing Efficiency Derived From N-(4-aminophehyl) Carbazole

Recently,Organic light emitting diode(OLED)has gained rapid development.Various technologies based on OLED gradually become industrialized from the laboratory and integrate into people′s everyday life.The widely utilization of organic phosphorescent materials in OLED can harvest both singlet and triplet excitons,which can extremely improve device efficiency and lower energy consumption.In organic phosphorescent materials,heavy metal ions(i.e.Ir Pt,Ru and so on)commonly incorporate into the organic aromatic frameworks for acquiring phosphorescence luminance with high efficiency.As we all know,the storage of heavy metals in the earth is very scarce,which induces in high cost of organic phosphorescent materials.In addition,heavy metals are toxic and not environment-friendly.The above disadvantages impose restriction on future development of organic phosphorescent materials in mass industrial production.Therefore,it is necessary to research efficient fluorescent materials with high exciton utilizing efficiency.Here,we aim to design and synthesize highly efficient fluorescent materials with exciton utilizing efficiency more than 25%.We further studied the excited state properties and explored the luminescent mechanism,so as to provide ideas for designing efficient fluorescent materials.Specific research and work are expressed as the following:1.Using N-(4-aminophenyl)carbazole(Cz PAF)as eclectron donor(D)moiety,trifluoromethylphenyl(TFMP)as electron acceptor(A)moiety,and 9,9-dioctyl-fluorene as the long π-conjugation unit,we designed and synthesized blue fluorescent material of Cz PAF-TFMP with A-π-D-π-A structure.Cz PAF-TFMP exhibits good thermal stability with thermal decomposition temperature(Td)of 405 °C.Cz PAF-TFMP shows remarkable solvatochromic effect with a large bathochromic shift of 79 nm in different polar solvents,suggesting obvious charge transfer(CT)character.Study based on Lippert-Mataga equation and transient PL decay measurements prove that Cz PAF-TFMP possesses hybridized local and charge transfer(HLCT)state.There is a certain degree of overlap and separation between the HOMO and LUMO of Cz PAF-TFMP.Optimized geometry of Cz PAF-TFMP shows twisting configuration,for example,the dihedral angles of 52° between D moiety and fluorene ring,40° between A moiety and fluorene ring,79° between benzene and carbazole.Quantum efficiency of Cz PAF-TFMP in film state is remained to be 36.8%,due to its twisting configuration.OLED devices A1-A3 were fabricated with Cz PAF-TFMP as emissive material.The device performance of Device A2 with doping emission layer is the best,which emits deep-blue light VII with emission peak at 436 nm,Commission International de L′Eclairage(CIE)coordinates of(0.155,0.066)and full width at half maximum(FWHM)of only 47 nm.Additionally,the Device A2 exhibits high EQE of 5.4% and high exciton utilizing efficiency of 73.4%.2.In order to investigate the effect of electron acceptor moiety on the photophysical properties of materials and device performance,pyridine(PD)was chosen as A moiety.The Cz PAF-PD with A-π-D-π-A structure was synthesized.The Td and glass transition temperature(Tg)are 414 °C and 104 °C,respectively.The bathochromic shift(86 nm)of Cz PAF-PD in different polar solvents is larger than that of Cz PAF-TFMP,suggesting stronger CT character.Cz PAF-PD possesses HLCT state,which has been identified by Lippert-Mataga equation and transient PL decay measurements.Comparing with Cz PAF-TFMP,Cz PAF-PD shows large spatial separation between HOMO and LUMO,and less twisting configuration.The dihedral angles between D moiety and fluorene ring,PD moiety and fluorene ring,benzene and carbazole are decreased to 42°,37°,58°,respectively.Excimer emission is observed in Cz PAF-PD film.Due to the intermolecular interaction between PD moiety,quantum efficiency of Cz PAF-PD film is reduced to 32.5%.We fabricated OLED devices B1-B3 based on Cz PAF-PD.The device performance of Device B2 with doping emission layer is excellent.Comparing with Device A2,the emission peak of Device B2 red-shifts to 444 nm.The CIE coordinates are(0.158,0.099)and FWHM is 50 nm.Device B2 exhibits EQE of 5.9% and exciton utilizing efficiency of 90.8%,which are all higher than those of Device A2.3.To further modulate the emissive behavior of material and improve device efficiency,cyanobenzene(CP)moiety with strong electron withdrawing ability was selected to synthesize Cz PAF-CP,which shows good thermal stability with Td of 311 °C and Tg of 96 °C.The strong electron withdrawing ability of CP moiety endows Cz PAF-CP with large bathochromic shift of 115 nm in different polar solvents,indicating remarkable CT character.According to further investigation based on Lippert-Mataga equation and transient PL decay measurements,Cz PAF-CP possesses HLCT state.Strong electron withdrawing ability of CP moiety leads to big spatial separation between HOMO and LUMO.Cz PAF-CP reveals twisting A-π-D-π-A structure.The dihedral angles between D moiety and fluorene ring,cyanobenzene and fluorene ring,benzene and carbazole are 47°,40°,67°,respectively.High quantum efficiency of 44.8% is achieved in Cz PAF-CP film.Based on Cz PAF-CP,OLED devices C1-C3 were fabricated.The EL performance of Device C2 with doping emission layer is the best among devices C1-C3.Comparing with Device A2 and Device B2,the emission peak of Device C2 further red-shifts to 452 nm,the EQE of 6.3% is the highest,and the corresponding exciton utilizing efficiency is 71.6%.