Design,Friedel Crafts Synthesis And Device Application Of Steric Hindrance Functionalized Fluorene Based Optoelectronic Materials

Optoelectronic material is the foundation of the whole optoelectronic industry.Organic optoelectronic materials act as one of the most core components in organic optoelectronic devices,and the relationship between their molecular structure and device performance is the central issue of fundamental science in organic electronics.It is a significant topic in this field to study their relationship so as to obtain molecular design strategies and develop excellent optoelectronic materials.It is an important strategy for molecular design to introduce the steric hindrance,which can change the spatial conformation and stacking mode of molecules,regulate the intramolecular or intermolecular interaction,effectively control the electronic energy level structure and aggregation behavior of materials,and thus affect the function and performance of optoelectronic devices.Therefore,steric hindrance strategy plays a crucial role in the field of organic optoelectronics.Diaryl fluorene functionalized sterically hindered optoelectronic materials are widely used in various kinds of organic optoelectronic devices,and hence it is particularly important to establish a fast,high efficient and green synthesis method based on diaryl fluorene building units.Since Friedel–Crafts reaction based on fluorenol has many advantages in the preparation of diarylfluorene semiconductor materials,such as no noble/heavy metal catalysis,mild reaction conditions and good compatibility of functional groups,we design and synthesis novel optoelectronic functional materials by steric hindrance strategy in this project,combined with Friedel-Crafts reaction based on fluorenol,and explore their applications in optoelectronic devices.In the first chapter,focus on the basic principle,development process and main progress of organic light emitting devices(OLEDs),among which the OLEDs of thermally activated delayed fluorescence(TADF)materials are known as the third generation organic light emitting materials.The triplet excitons can absorbed ambient heat energy,and turn into singlet ones through the process of reverse intersystem crossing(RISC)and electron turnover.Then the newly generated singlet excitons can achieve delayed fluorescence emission,and reach up to 100%exciton utilization without precious metals.The molecular design of TADF materials requires a small singlet-triplet energy difference(?E_ST)so as to ensure a large reverse intersystem transition rate(k _RISC)of triplet excitons,and a high singlet exciton radiative transition rate(6))in order for the high photoluminescence quantum yield(PLQY).When designing high efficient TADF molecules,we need to select appropriate electron donor/acceptor units with steric hindrance effect,matching with the corresponding bridging units,and optimize the overlap integrals of frontier molecular orbitals to achieve the smaller?E_ST as well as high fluorescence quantum yield.Diarylfluorene semiconductor materials have a wide range of applications in organic optoelectronic devices.we briefly introduce the exploration and establishment of Friedel-Crafts methodology based on fluorenol.In the second chapter,dendrimer is a kind of unique nanoscale macromolecular compounds,which combines the advantages of clear structure of small molecules and good film-forming and stability of polymers.It is described as the third category of organic optoelectronic materials besides organic small molecules and polymers.Based on an organic bottom-up synthesis approach,a series of soluble covalent dendrimer nano-frameworks with self-similar H-shaped conformations,G0.5–G2.5,were synthesized through the iterative Friedel Crafts reaction of fluorenol and Suzuki C–C cross-coupling reactions,which gives a new way to design and synthesize dendrimers.G0.5–G2.5had about 3.3–3.7 e V bandgaps and emitted blue fluorescence.The uniform layered nano-films of G2.5,self-assembled by an immersion-dip method,exhibited typical semiconductor properties by the C-AFM measurement.Importantly,G2.5 was found to be sensitive to dopants,and post-modification of G2.5 nano-film with 4-CPD diazonium salt can tune its electrical properties up to two orders of magnitude.Hence,it has been demonstrated that the?-conjugation-interrupted frameworks(CIFs)and their doped counterparts broaden the options for organic electronics.As a result of the H-shaped conformation and photosensitive properties of CIFs,we envisage their potential role as active components of memory devices and photodetectors in the future.Four kinds of spiro functionalized oligofluorene with different conjugation lengths were designed and synthesized by Huang's Friedel-Crafts reaction.All spiro-terfluorene derivatives display high photoluminescent quantum yield(PLQY)and radiative transition rate(K_r).clearly indicating that lateral spiro-substitution can effectively suppress aggregation-induced exciton quenching.In the third chapter,the D-A typed TADF molecule 4-DPFCz AIPh was designed and synthesized based on carbazole group as the electron donor unit and aromatic imide group as the electron acceptor unit,where a large stereo-rigid 9-phenylfluorene as the steric hindrance group was introduced into the carbazole group through Friedel-Crafts reaction.The separation of donor and acceptor units has been well realized,and accordingly the overlap integrals of HOMO and LUMO energy levels diminish.?E_ST goes down to 0.10 e V,the external quantum efficiency(EQE)of TADF device reaches19.6%,current efficiency(CE)is 57.1 cd/A,and the power efficiency(PE)is 51.8 lm/W,realizing the balance between low singlet-triplet energy difference(?E_ST)and high photoluminescence quantum yield(PLQY).We demonstrated that it is feasible to design and construct TADF molecules based on 9-phenyl fluorene as a large steric hindrance group.Furthermore,we systematically studied the effects of steric hindrance groups on the film forming,thermal stability,photophysical properties,electrochemical stability and electroluminescent properties of materials.In the fourth chapter,exciplex-type TADF materials have attracted much attention from researchers in virtue of easier access to smaller?E_ST.Spiro fluorene xanthene(SFX)was prepared by one-pot Friedel-Crafts method,and then strong electron-withdrawing triazine group was introduced into its active site through Suzuki reaction,constructing two electron acceptor materials(TRZSFX and DTRZSFX)based on SFX steric hindrance group.They doped with the electron donor material TCTA with a higher triplet energy level(T₁=2.84 e V),generating strong charge-transfer state and forming the exciplex TCTA:TRZSFX and TCTA:DTRZSFX.The strong charge transfer between electron donor and acceptor shortened the energy gap between triplet and singlet,and?E_ST of their exciplex films decreased to 0.03 e V and 0.06 e V,respectively.Smaller?E_ST makes the triplet excitons in exciplex system more prone to RISC process,so as to achieve high efficient exciplex-type TADF emission.At 300 K,the photoluminescence quantum yields(PLQY)of TCTA:TRZSFX and TCTA:DTRZSFX are 81%and 41%,respectively.In the OLED of TCTA:TRZSFX exciplex,high-efficiency TADF emission has been achieved,in which the maximum external quantum efficiency(EQE)comes up to 22.5%,the current efficiency(CE)is 79.6 cd/A,and the power efficiency(PE)is78.1 lm/W.The excellent performance of OLED devices is attributed to the strong electron acceptor ability.The triazine group introduces SFX with rigid skeleton structure as steric hindrance group,which provides a new way to further development of novel electron acceptor materials to achieve high-efficiency intermolecular D-A type thermally activated delayed fluorescent materials.