Blue Fluorescence Materials Derived From Electron Acceptor Unit Of Benzonitrile Based On Twisting D-π-A Structure

Till now,organic phosphorescence materials based on rare earth metal complex have been widely utilized in organic light emitting diode（OLED）production,owing to their high exciton utilizing efficiency（η_s）.However,the costs of organic phosphorescent materials are expensive due to high cost of rare earth metal,which induce in high cost of OLED products.Hence,the research and development on organic fluorescence materials with highη_s have been hot point recently.In this work,blue organic fluorescence materials based on twisting D-π-A structure were designed and prepared in order to improveη_s and fluorescence quantum(η_pl),which can be used in blue-light OLED.Furthermore,the relationship between molecular spatial configuration and material properties were analyzed and discussed in detail.The main research work as following:1.The molecule conformations of 9,9′-spirobi[9H-fluorene]（SBF）and spiro[fluorene-9,9′-xanthene]（SFX）are three-dimensional twisting structure.It can avoidπ-πstacking of molecules and formation of excimer effectively,which leads to improvement in luminous efficiency.Additional,the twisting structure can also improve thermal stability.Accordingly,the blue fluorescence materials based on twisting D-π-A structure were designed and synthesized,named as SBF（PFCP）₄ and SFX（PFCP）₄,in which SBF and SFX acted as electron donor（D）unit,respectively.And,benzonitrile（CP）served as electron acceptor（A）unit,which was linked with D unit（i.e.SBF,SFX）byπ-conjugated bridge unit of 9,9-dioctylfluorene.By spectral analysis,it can be found the photoluminescence（PL）spectra of SBF（PFCP）₄ and SFX（PFCP）₄ in dilute solution exhibit the emission peaks located at 411 nm and 413 nm,and the PL spectra of SBF（PFCP）₄ and SFX（PFCP）₄ in film exhibit the emission peaks located at 424 nm and 422 nm,which all show obvious blue emission.The SBF（PFCP）₄ and SFX（PFCP）₄ express good thermal stability.The absoluteη_pl of SBF（PFCP）₄ is 88.6%in the film state.With increasing solvent polarity,the emission peaks in PL spectra of SBF（PFCP）₄and SFX（PFCP）₄ show no red shift,which means no solvation effect.It is indicated that the excited states of SBF（PFCP）₄ and SFX（PFCP）₄ possess not remarkable charge transfer（CT）characteristic.2.As an effective and universal electron donor group,carbazole（Cz）with rigid molecular structure exhibits such properties as strong electron donor ability,high thermal stability,and multiple chemical modification sites.Hence,a series of blue fluorescence materials with twisting D-π-A structure were designed and synthesized,in which Cz served as D unit and linked with A unit of CP byπ-conjugated bridge unit of 9,9-dioctylfluoren.By adding number of A units of Cz and adjusting linking sites between D unit and A unit,blue fluorescence materials with three different molecular spatial configuration were acquired,which are 2,7-Cz（PFCP）₂,3,6-Cz（PFCP）₂ and Cz（PFCP）₄,respectively.The PL spectra of 2,7-Cz（PFCP）₂,3,6-Cz（PFCP）₂ and Cz（PFCP）₄ in dilute solution all exhibit emission peaks located at around 417 nm,which show obvious blue emission.From low polarity hexane to high polarity acetonitrile,the emission peaks of 2,7-Cz（PFCP）₂,3,6-Cz（PFCP）₂ and Cz（PFCP）₄ in PL spectra show large redshift of 43 nm,112nm and 90 nm with increasing solvent polarity,which exhibit remarkable solvation effects.Among them,the 3,6-Cz（PFCP）₂ show the strongest solvation effects,suggesting the most remarkable CT characteristic of the excite state of3,6-Cz（PFCP）₂.However,the emission peaks of 2,7-Cz（PFCP）₂ is less affected by the solvent polarity owing to its higher conjugation degree,leading to enhance LE characteristic of the excite state of 2,7-Cz（PFCP）₂.It results in highη_pl（95.7%）of 2,7-Cz（PFCP）₂ in film.Furthermore,the blue-light OLED with 2,7-Cz（PFCP）₂and 3,6-Cz（PFCP）₂ as emissive materials were fabricated,which exhibit excellent device performance.The blue-light OLED with 3,6-Cz（PFCP）₂ as emissive material exhibits the emission peak in electroluminescence spectrum located at432 nm,CIE Coordinate of（0.156,0.069）,higher maximum EQE of 6.68%.Ultimately,theη_s of 3,6-Cz（PFCP）₂ was estimated to be 37%,respectively,which are higher than the upper limit ofη_s（25%）for traditional fluorescence materials.3.Triphenylamine（TPA）is a strong electron donor unit with twisting molecular structure,which can frustrate the intramolecular stacking effectively.Therefore,a series of blue fluorescence materials with twisting D-π-A structure were designed and prepared,in which TPA served as D unit and conjugated with A unit of CP by theπ-conjugated bridge unit of 9,9-dioctylfluoren.By adding the number of A units of CP,blue fluorescence materials with three different molecular spatial configuration were acquired,which are TPA（PFCP）,TPA（PFCP）₂ and TPA（PFCP）₃,respectively.The PL spectra of TPA（PFCP）,TPA（PFCP）₂ and TPA（PFCP）₃ in dilute solution exhibit the emission peaks located at 433 nm,435 nm and 437 nm,respectively.During measurement of solvation effect,it can be seen that the emission peaks of TPA（PFCP）,TPA（PFCP）₂ and TPA（PFCP）₃ in PL spectra show large redshift of 130 nm with increasing solvent polarity,which exhibit remarkable solvation effects.It is suggested that the excited states of all three materials express CT characteristic,which is corresponding to intramolecular CT from TPA to CP.The blue-light OLED with TPA（PFCP）as emissive material exhibits the emission peak in electroluminescence spectrum located at 448 nm,higher maximum EQE of 5.83%.Ultimately,theη_s of TPA（PFCP）was estimated to be 30%,respectively,which are higher than the upper limit ofη_s（25%）for traditional fluorescence materials.