Synthesis And Optoelectronic Properties Of Hinder-Functionalized Fluorene's Aeceptors

With the development of modern technology,the business status of organic light emitting diode?Organic Light Emitting Diode,OLED?has gradually been valued by panel companies,especially for the research of organic light emitting materials which has gradually become the focus.In many luminescent materials,fluorene-based molecules are widely used in the preparation fieldof light-emitting devices for their advantages such as blue light emission,easy modification,and high photoluminescence quantum efficiency.And how to achieve high-efficiency organic light-emitting diodes has been studied in depth.The steric hindrance formed by constructing the perpendicular connection mode with the 2,7 and 9 positions of the fluorene group can effectively inhibit the interaction between molecules to improve the luminous efficiency of the material.The introduction of steric hindrance can improve the photophysical properties such as thermal and morphological stability,photoluminescence quantum efficiency,horizontal dipole orientation,and transient lifetime,Therefore,fluorene-based molecules play an important role in improving the light-emitting performance of the device.In order to achieve high-performance luminescence,Exciplexes are often used as the light-emitting layer of the device,because they usually have a quite small?EST,and are prone to have reverse intersystem crossing,which can use singlet and triplet Exciton luminescence efficiently,that is,the exciplex delayed fluorescence process?Exciplex-Thermally Activated Delayed Fluorescence,Exciplex-TADF?.However,there are still few studies on exciplex-type luminescent materials and devices containing C?sp³?sterically hindered fluorene-based acceptor materials,and the relationship between the optoelectronic properties of related materials and the structure of the acceptor materials remains to be studied in-depthly.Therefore,on the basis of the research group's preliminary studies on the optoelectronic properties of fluorene-based acceptor materials,we carried out the synthesis and studied the optoelectronic properties of sterically hindered functional fluorene-based acceptor materials.We used 9-phenylfluorene as the backbone,introduced cyano acceptor units at its 2,7 positions,and introduced carbazole donor units at its 9 position?carbazole units consist of an aromatic moiety with a high dissociation energy and an unstable C?sp³?bond with a low dissociation energy,which has better stability than acridine or phenoxazine?,designed and synthesized a series of fluorene-based acceptor optoelectronic materials.The specific work is as follows:1.The single-cyano fluorene-based acceptor materials 2CNSFX,2CNPFCz,2CN were successfully synthesized by using one-pot method,Grignard reaction,Friedel-Crafts alkylation,and cyanation reaction.The 2CN material exhibited spectral red-shift characteristics relative to 2CNSFX and2CNPFCz.The single-cyano fluorene-based acceptor materials 2CNSFX,2CNPFCz,2CN and host material TAPC were blended and doped at the rate of 1:1 to form exciplex emission.This exciplex was applied to organic electroluminescent devices,and a series of electroluminescent properties were characterized.The electroluminescence spectrum showed that the three-peak emission formed white light,and the CIE coordinates of 2CNPFCz-TAPC and 2CN-TAPC were close to the standard white light?0.33,0.33?,the maximum current efficiency and power efficiency of 2CNSFX,2CNPFCz,2CN were 6.22 cd A^-1/6.51 lm W^-1,16.80 cd A^-1/18.85 lm W^-1,15.15 cd A^-1/16.99 lm W^-1,the maximum external quantum efficiency was 3.25%,8.09%,6.83%.2.Based on the previous research on the optoelectronic properties of the single cyano hindered fluorene-based materials,a one-pot method,Grignard reaction,Friedel-Crafts alkylation,and cyanation reaction were used to successfully synthesize the dicyanofluorene-based acceptor material DCNSFX,DCNPFCz,DCN,Tri CN,TCN.These acceptor materials were doped with host material TAPC at the rate of 1:1 to form thin films.The UV absorption-fluorescence emission spectrum?room temperature transient lifetime and other photophysical characterizations showed that the doped films formed exciplex emission.The experimental results of the OLED device showed that the highest current efficiency/power efficiency and the maximum external quantum efficiency of the exciplex-based device based on the acceptor material DCN were 83.58 cd A^-1/93.74 lm W^-1/25.76%,which achieved the advanced level in the field of luminous efficiency of exciplex-TADF devices.3.Based on the previous research on the optoelectronic properties of the dicyanofluorene-based acceptor materials,this chapter focused on the luminescence properties of the four materials DCNPFCz,DCN,Tri CN,and TCN,and discovered the piezochromatic luminescence properties.By testing the photoluminescence spectra of materials in different polar solvents,the four molecules showed that as the polarity of the solvent increased,the PL spectrum would have a red-shift,ranging from 150-200 nm.The powders of the four materials were being ground,and the luminescence colors before and after grinding were compared.It was found that only DCNPFCz and DCN molecules exhibited the piezochromatic luminescence phenomenon,and the piezochromatic luminescence properties were reversible.Then the reasons why the materials exhibited piezochromic luminescence were explored.By introducing different steric hindrance and the number of cyano groups on the C?sp³?fluorenyl unit,the optoelectronic properties of exciplexes formed by doping C?sp³?acceptor materials containing fluorenyl units with classical host materials were studied.By means of quantum chemistry calculations,the effect of the steric hindrance of the acceptor material on the photophysical properties of the exciplex and the electroluminescence properties of the device was analyzed,which provided a material basis for the future design the development of new acceptor materials and the preparation of high performance light emitting devices.