Synthesis, Structures And Optoelectronic Properties Of Carbazole Derivatives

In recent years,organic light-emitting diodes(OLEDs)has aroused widespread interest as a new generation of display and lighting technology,several related products have been gradually applied to the mobile phone,smart watches,television,lighting and other fields.The photoelectric functional materials based on conjugated small molecules have attracted much attention due to their simple synthesis and easy modification.With years of development,the emission colors of organic electroluminescent materials have been extended from deep blue to near infrared region.To date,the performance of deep blue OLEDs is still need to be improved compared with green and red devices.On the other hand,the traditional fluorescent materials encounter the low singlet excitons generation ratio of 25% in device.The advantage of phosphorescent materials is that they can utilize both singlet and triplet exciton states which can reach 100% internal quantum efficiency theoretically.While these materials exhibit high quantum efficiency,they require rare and expensive elements such as platinum and iridium.Therefore,the development of highly efficient blue fluorescent materials and alternatives of traditional phosphorescent materials are of great importantce for the practical use of OLEDs.The utilization of triplet excitons generated electrically in devices by the thermally activated delayed fluorescence(TADF)mechanism became an optional approach for traditional phosphorescent materials in OLEDs.TADF related studies opened up a novel way for harvesting triplet excitons in devices using pure organic molecules that are simple and relatively cheap for high performance OLEDs.In addition,phosphorescent materials with metal free are scarce but cheaper than the traditional ones,have also attracted much attention due to their potential applications in the PhOLEDs.Carbazole derivatives have unique thermal,electrochemical and photophysical properties,and they are also cheap and easily modified.In this regard,we designed and synthesized a series of carbazole based molecules by introducing different functional substituents.The relationship between the structures and properties of these materials was further studied systematically.Then we investigated their potential applications as deep blue fluorescent emitters,thermally activated delayed fluorescent and room temperature phosphorescent materials.1,In chapter ?,incorporating carbazole and imidazole as mild electron-donors and blue chromophores,and sulfone as an electron-acceptor with a ?-conjugation-breaking feature,three novel deep blue emitting materials(PIM-SO-Cz,PPI-SO-Cz and TPM-SO-Cz)with D-?-A structure were successfully synthesized.Then we systematically investigated the difference on the thermal,photophysical,electrochemical properties of these compounds with different substituents.All these molecules enjoy a highly twisted molecular conformation due to the sulfone,which is helpful to suppress close packing.More importantly,the sulfone,as linked to two freely rotatable benzene rings in this work,can also serve as a ?-conjugation breaker because of its tetrahedral electronic conformation which could effectively confine the ?-conjugation of the molecules.In the solid state,all these three compounds show deep blue emission(400-450 nm)with full width at half maximum(FWHM)less than 60 nm.Separation of HOMO and LUMO endows these compounds have bipolar charge transporting capability.PPI-SO-Cz exhibited the best thermal stability as well as highest fluorescence quantum yield(55%).These three compounds have potential applications in deep blue organic light emitting devices.Two novel carbazole derivatives(MCP-AN-PIM and MCP-AN-PPI)with anthracene moiety as the bridge were then designed and synthesised.Due to the repulsion between the hydrogen atoms of the anthracene ring and their adjacent hydrogen atoms on the phenyl groups,both molecules enjoy a highly twisted molecular conformation,which is helpful to suppress close packing and ?-? intermolecular interactions,thereby producing deep-blue emission.On the other hand,good thermal stability benefits from an asymmetrically twisted molecular structure.These two compounds have high thermal decomposition temperatures(Td5: 510/520 oC),glass transition temperatures(Tg:183/215 oC)and melting points(Tm: 347/366 oC).Both carbazole derivatives in non-doped and doped OLEDs exhibited deep blue emission with the maximum external quantum efficiency(EQE)more than 3%.The extremely low driving voltages(~3 V)were also obtained for these fluorescent devices.In particular,the doped device based on MCP-AN-PIM exhibited high EQE of 5.3% with CIE coordinates of(0.154,0.053),which is very close to the CIE coordinates of the European Broadcasting Union(EBU)blue standard(0.15,0.06).2,In chapter ?,a novel electron transporting material,PIM-TRZ,was easily synthesized in two steps by integrating a benzimidazole moiety with a triazine unit.Single-carrier devices indicated that the electron transport ability of PIM-TRZ is similar to that of TmPyTz.TCTA and Tri-Cz were introduced as suitable electron donors for efficient exciplex-based OLED emitters by mixing with PIM-TRZ and TmPyTz.The absorption and photoluminescence spectra indicated the formation of new excited states of exciplexs in the mixed film.Besides,the exciplex system based on PIM-TRZ exhibits extremely small(35)EST(~0 eV).Transient fluorescence decay measurements were also carried out to confirm that TADF occurs in the exciplex emitters.In addition,all the exciplex based OLEDs show high performance behavior with low driving voltages and low efficiency roll-off.In particular,the exciplex OLED based on Tri-Cz/TmPyTz exhibited high EQE of 9.5% with a low driving voltage at 2.4 V,maximum current efficiency of 28.2 cd A-1 and maximum power efficiency of 29.2 lm W-1.3,In chapter ?,two novel naphthylamine moiety attaching carbazole derivatives,Cz-PNA and DCz-PNA,have been designed and synthesized.It is known that some naphthylamine derivatives are able to emit strong phosphorescence at low temperature while carbazole is able to emit detectable weak yellow persistent phosphorescence at room temperature,indicating the existence of weak spin-orbit coupling.All these two compounds exhibit blue fluorescence both in solid and dilute solution state at room temperature while they can emit strong yellowish-green phosphorescence as a frozen dilute solution.By doping Cz-PNA or its derivatives into a crystalline 4,4'-dibromobiphenyl(DBBP)matrix,efficient and long-lived organic RTP was obtained.The abundant intermolecular interactions in the packing structure build up a rigid and compact matrix of DBBP molecules that is helpful in suppressing molecular torsion and vibration motion and decrease the nonradiative decay of the triplet excited state.On the other hand,DBBP can serves as the diluent to avoid the emission quenching caused by the aggregation of phosphorescent chromophores.Furthermore,the external heavy-atom effect of DBBP molecules may enhance RTP by increasing ISC.The resulting materials showed strong and persistent RTP emission with a quantum efficiency of approximately 20% and a lifetime of a few to more than 100 milliseconds.The fluorescence/phosphorescence dual-emission feature may make this material system useful in white-light-emission and security-mark applications.4,In chapter V,the novel electron-accepting cyanopyridine group was attached to the 3 or 9-postion of carbazole moiety.Such direct linking(at 9-position)induces a large twist angle between donor(carbazole)and acceptor(cyanopyridine)segments,thus producing small overlap between HOMO and LUMO orbitals,which contributes to a small(35)EST,allows for efficient RISC.While linked with the carbazole at 3-position,PCzPyDCN exhibits a planar molecular conformation,which is not benefit for the separation of HOMO and LUMO orbitals,thus leading to a large(35)EST.Low temperature emission spectra confirmed that both Cz PyDCN and tBCzPyDCN have a small(35)EST about 0.2 eV while PCzPyDCN presents a large(35)EST as high as 0.46 eV.The rather large(35)EST is not benefit for the efficient up conversion.Transient fluorescence decay measurements were also carried out to confirm that TADF occurs in the CzPyDCN and tBCz PyDCN doped films with a lifetime in the order of ?s while PCzPyDCN doped film only exhibits a lifetime in the order of ns,indicating the fluorescence nature.Therefore,CzPyDCN and tBCzPyDCN can be used as TADF materials while PCzPyDCN can be used as a blue fluorescent material.In summary,we have designed and synthesized a series of novel organic optoelectronic molecules based on carbazole unit.The crystal structures,photophysical,thermal and electrochemical properties of these carbazole derivatives were then investigated systematically to explore their potential applications in the field of organic electroluminescence.In addition,the relationship between the structures and properties of these materials was further discussed according to theoretical calculations.Finally,we fabricated several deep blue fluorescent and exciplex-based OLED devices and actively explored metal-free organic room temperature phosphorescent materials.The research findings can provide basis and references for designing carbazole based functional organic materials.