Synthesis And Characterization Of Blue Organic Small Molecule Luminescent Materials Based On Difluorene Structure With Different Electron-withdrawing Groups

The emergence of new smartphone with folding screen has attracted more and more attention to the new generation of display technology centered on OLEDs.Compared to the excellent performance of red and green light devices,pure blue devices still have potential for improvement in balance of efficiency and stability.In the process of researching high-efficiency organic light-emitting materials,scientists have proposed molecular design methods for donor-acceptor?D-A?structures.Connecting the electron donor and acceptor through a covalent bond can effectively improve the bipolar characteristics of the material so that it can balance carriers transport better.In addition,by changing the connection position and number of donors and acceptors,the properties of the stability and energy gap structure of the luminescent material can be effectively adjusted.Fluorene is a wide-bandgap planar molecule with medium electron-donating ability and has many chemically modified sites,of which the special one is the carbon atom at position 9.Affected by steric hindrance,the mutual repulsion between the substituents at position 9 will distort the molecular structure so that the interaction between the fluorene groups will be reduced in the aggregated state.Inspired by this,in this paper,the difluorene formed by the connection of two arylfluorenes is used as the electron-donating group,and different acceptors are introduced at the9th position.Several small organic molecules with D-A-D or D-A structure was synthesized to explore the influence of the molecular structure and electron-withdrawing properties of the acceptor groups on the luminescent properties and stability of the compound.The first part uses 9,9,9'-tris?4-butoxyphenyl?-9H-9'H-2,2'-difluorene?DF?as the electron donor.By introducing diphenylsulfone and triphenylphosphine as the electron acceptor with a bridging effect,two small molecule blue compounds of?DF?₂SO₂ and?DF?₂PO with D-A-D structure were synthesized to study the influence of electron-withdrawing groups on the photoelectric properties of the material.The T_d of compounds?DF?₂SO₂,?DF?₂PO and DF are375?,429?and 383?,and the T_g are 66?,73?and 75?.Due to the longer molecular chain,the T_g of?DF?₂SO₂and?DF?₂PO is lower than DF.In terms of photophysical properties,?DF?₂SO₂,?DF?₂PO and DF all exhibit wide-bandgap deep blue light emission of over 3.3 e V in toluene solution,with emission wavelengths of 394/375 nm,394/375 nm and 390/371 nm.The singlet energy levels of?DF?₂SO₂ and?DF?₂PO are similar to DF,but the introduction of electron-withdrawing groups makes their triplet energy level become lower,making the?E_ST of?DF?₂SO₂ and?DF?₂PO is much higher than 0.3 e V,which greatly reduces the possibility of TADF.The absolute photoluminescence quantum yields of?DF?₂SO₂ and?DF?₂PO solid powders are20.83%and 10.03%,far lower than DF's 59.46%;their front-line molecular orbital energy levels are also deeper than DF,and their HOMO/LUMO energy level reached-5.75/-2.40 e V,-5.78/-2.42e V.The electroluminescence devices were prepared by solution spin coating,it was found that the OLED devices based on m CP host had the best guest blue light emission.The electroluminescence chromaticity coordinates of all devices based on m CP:PVK host are located in the blue region,and the luminous chromaticity coordinates of the DF-based device are?0.19,0.10?,which is the closest to the deep blue region,and its maximum external quantum efficiency is 1.45%,higher than?DF?₂SO₂?0.87%?and?DF?₂PO?0.25%?,which is consistent with the difference in solid-state quantum efficiency between them.In the second part,the DF group was still used as the electron donor.In order to study the influence of the electron-withdrawing ability of the acceptor and the conjugated structure on the molecular luminescence characteristics,2,4,6-triphenyl-1,3,5-triazine,4-bromobenzophenone and triphenylphosphine oxide were selected as the electron-withdrawing acceptors to synthesize three D-A structure compounds DFTRZ,DFCO and DFPO.The T_g of the compounds DFTRZ,DFCO and DFPO are 135?,89?and 97?,higher than the compounds of D-A-D structure because of the shortened molecular segment.In addition,DFTRZ with the strongest rigidity receptor showed the best thermal stability,its T_d reached 436?,far exceeding DFCO and DFPO.In terms of photophysics,the three compounds are still dominated by the local emission of DF in toluene solutions,but DFTRZ exhibits intramolecular charge transfer characteristics.The S₁ and T₁ energy levels of the three compounds are 3.31/2.76 e V,3.32/2.43 e V and 3.31/2.82 e V.The absolute luminescence quantum yields of DFTRZ,DFCO,and DFPO solid powders are 14.35%,1.13%and 7.57%.In these three compounds,the rigid acceptor increases the molecular conjugate structure and enhances the luminescence,while the DFCO molecule has the lowest quantum yield due to the quenching effect of the carbonyl functional group on the exciton.In the third part,a single fluorene unit in DF was used as the electron donor,and2,4,6-triphenyl-1,3,5-triazine was used as the acceptor.In order to ensure the structure of difluorene,compound 2FTRZ was synthesized with D-A structure for a dimer form.Like the previous compounds,in toluene solution,2FTRZ is dominated by the deep blue light emission of difluorene structure in the dimer,with an emission peak wavelength of 394/376 nm.2FTRZ exhibits the same intramolecular charge transfer characteristics as DFTRZ.However,which is different with DFTRZ is that 2FTRZ exhibited a certain degree of aggregation-induced luminescence enhancement properties in a mixed solution of water and tetrahydrofuran.2FTRZ is still a transient fluorescence emission,but the absolute luminescence quantum yield of its solid powder reaches 18.82%,which is higher than that of compound DFTRZ.The obvious improvement indicates that the increase in the number of planar acceptor triazines is conducive to the enhancement of the overall conjugated structure of the molecule and improves the luminous efficiency of the molecule.