The Deep-red Fluorescence Materials Derived From Quinoxaline-2,3-dicarbonitrile As Electron Acceptor Group

Red emitting materials exhibit important applications in the fields of optoelectronics,biological imaging,fluorescent probes and etc.,and organic red fluorescent materials have become a research hotspot due to the advantages of flexible molecular structure,easy control and low price.However,organic red fluorescent materials with high luminous efficiency and deep red emission are lacking.The important reason is that deep red emitting materials have narrow band gap,which induces in decreasing luminous efficiency owing to enhancement of the vibrational wave function and non-radiative transition of the excited state.In order to solve this problem,different materials with donor-acceptor(D-A)structure were designed and synthesized by adjusting the electron donor(D)unit.Herein,the electron acceptor unit(A)of quinoxaline-2,3-dicarbonitrile(QxDC)was designed and synthesized by introducing two cyano groups into the quinoxaline unit,which induces in strong electron accepting ability for lowering the lowest unoccupied molecular orbital(LUMO)energy level of materials.Hence,the QxDC possesses the huge potential for designing and synthesizing the deep-red fluorescence materials.In this thesis,based on the A group of QxDC,the deep-red fluorescence materials with twisted D-A configuration were designed and synthesized,in which the phenyl carbazole and triphenylamine(TPA)with different structures and electron donating abilities were selected as the electron donor(D)groups.Then,the relationship between the molecular structure and the luminescence characteristics of materials was studied.The specific research content of this paper is as follows:(1)The red fluorescence materials with twisted D-A configuration of were designed and synthesized,in which phenyl carbazole and QxDC serve as D unit and A unit.The two deep-red fluorescence materials with different molecule structures were acquired by varying connection site between the carbazole in phenyl carbazole group and the benzene ring,which were titled as 4-Cz-Ph-QxDC and 3-Cz-Ph-QxDC,respectively.In photoluminescence(PL)spectra,4-Cz-Ph-QxDC film and 3-Cz-Ph-QxDC film exhibit red emission peaks located at620 nm and 595 nm with high photoluminescent quantum yield(?_PL)of 29.20%and 14.10%,respectively.According to theoretical calculations and solvatochromic effect,4-Cz-Ph-QxDC and 3-Cz-Ph-QxDC exhibit charge transfer(CT)state characteristics during emission process.Then,the red-light OLEDs with 4-Cz-Ph-QxDC and 3-Cz-Ph-QxDC as the emitting layer were fabricated.Among them,the device performance of the OLED with3-Cz-Ph-QxDC were better,i.e.the emission peak in electroluminescence(EL)spectrum located at 600 nm,the CIE coordinates of(0.55,0.44)and the maximum external quantum efficiency(EQE)of 4.04%.According to the work in this part,it can be seen that using QxDC as the A group to construct D-A structural molecules is beneficial to reduce the LUMO energy level and narrow the luminescence band gap of the molecule,achieving red light emission.Meanwhile,Materials with phenylcarbazole severed as the D group realize red emitting devices with high luminous efficiency,but the emission wavelength is not long enough.(2)In order to increase the emission wavelength and achieve deep-red fluorescence emission,QxDC was served as A unit,and TPA unit with stronger electron donating ability was served as D group.The deep-red fluorescence materials of 6-TPA-QxDC and5-TPA-QxDC were designed and synthesized,which exhibit twisted D-A configuration.In PL spectra,6-TPA-QxDC film and 5-TPA-QxDC film exhibit deep-red emission peaks at755 nm and 808 nm with?_PL of 5.95%and 2.19%,respectively.According to theoretical calculations and solvatochromic effect,6-TPA-QxDC and 5-TPA-QxDC exhibit CT state characteristics during emission process.The deep-red OLED with 6-TPA-QxDC and5-TPA-QxDC as emitting layer were fabricated.Among them,OLED with 6-TPA-QxDC as the emitting layer realized better performance,for example,the emission peak in EL spectrum located at 676 nm,the CIE coordinates of(0.66,0.33)and the maximum EQE of1.13%.According to the work in this part,it can be seen that enhancing the electron donating ability of D group can further strength the CT state characteristics during emission process of red-fluorescence material,which is conducive to further redshift of emission peak of red fluorescence material.(3)In previous parts of the work,deep-red fluorescent emission was achieved,but the EQE of the device was rather lower.Based on this,in order to further improve the luminous efficiency of deep-red fluorescence material,QxDC was served as A,TPA was served as D,and o-QxDC-DTPA with the symmetric twisted D-A-D structure was designed and synthesized.The o-QxDC-DTPA film shows deep-red emission with emission peak located at 729 nm in PL spectrum with?_PL of 1.35%.According to theoretical calculations and solvatochromic effect,o-QxDC-DTPA exhibits CT state characteristics during emission process.The deep-red OLEDs with o-QxDC-DTPA as the emitting layer were fabricated.And,the OLED realizes excellent performance,the emission peak in EL spectrum located at680 nm,the CIE coordinates of(0.70,0.30)and the maximum EQE of 5.21%.So,it can be seen that the introduction of two D groups in deep-red fluorescence molecule is conducive to improve the luminous efficiency;at the same time,compared to twisted D-A structure,the symmetric twisted D-A-D structure can effectively improve the problem of the imbalance of the dipole moment in molecule,and obtain high luminous efficiency.