Research Of Highly Efficient NIR TADF Materials Based On Strong Acceptor Units

OLED means organic light-emitting diode,which has the advantages of fast response rate,wide operating temperature range and thin thickness.Any electroluminescent device forms two types of excitons,roughly 25%singlet excitons and 75%triplet excitons.We expect to capture as many excitons as possible to generate photons and achieve high efficiency.Traditional fluorescent materials can only utilize 25%singlet excitons,while phosphorescent materials could achieve 100%utilization of excitons through spin-orbit coupling of heavy metals.Thermally activated delayed fluorescent(TADF)materials can achieve 100%utilization of excitons by narrowing the singlet-triplet energy splitting(ΔEST)of molecules.Currently,significant breakthroughs have been made in TADF materials based on blue-light,green-light and red-light materials.But the development of near-infrared TADF materials with emission wavelengths over 680 nm was very slow.In recent years,NIR OLED have attracted widespread attention due to their potential applications value in solar cells,bioimaging,information storage and phototherapy equipment.The twisted configuration of the donor(D)and acceptor(A)units in TADF molecules will lead to a spatial separation of the highest occupied molecular orbital(HOMO)and the lowest unoccupied molecular orbital(LUMO),resulting in a small ΔEST.However,the reporting of NIR OLEDs based on TADF materials can hardly be seen because the limited conjugation between donor and acceptor units in TADF molecules obstructs near-infrared emission.In this paper,a series of efficient NIR TADF materials were designed and synthesized by increasing the molecular aspect ratio to improve the level of molecular horizontal dipole orientation and introducing electron-donating groups on the acceptor unit to induce intramolecular charge transfer(ICT).1.In chapter two,we designed and discussed two Y-shaped NIR TADF materials,named Py-TPA and Py-CN-TPA,which have two pyridine units connected to positions 2 and 9 of the phenanthroline to increase the molecular aspect ratio.Increasing the molecular aspect ratio can improve the horizontal dipole orientation in organic lightemitting diodes,therefore the Y-shaped structure may be advantageous for light coupling efficiency.In addition,multiple sub-acceptor units such as cyano,thiophene and pyridine were used in Py-TPA and Py-CN-TPA,which can significantly lower the LUMO energy level and narrow the bandgap.The donor triphenylamine is located directly above the cyano group to form a twisted D-A structure,which can to some extent limit the rotation and vibration of the triphenylamine.Based on Py-TPA,unprecedented performance was achieved in NIR OLEDs,with an EQE of 13.2%at 761 nm and CIE coordinates of(0.70,0.27).This work provides useful insights for designing effective NIR TADF materials by combining rigid and electron-accepting extension frameworks into slender molecules.2.In Chapter 3,we designed and discussed two TADF molecules containing benzothiophene,which named BTP-TPA and BTP-CN-TPA.Benzothiophene is an electron-donating group,we introduced the donor into the acceptor skeleton to induce intramolecular charge transfer.We chose the frequently-used triphenylamine as the donor and added the tert-butyl group to increase the solubility of the molecule.The first synthesized molecule,BTP-TPA,was barely satisfactory.On this basis,we added two cyano groups to enhance the electron-withdrawing ability and lower the LUMO energy level.BTP-CN-TPA shifted to the near-infrared region successfully and achieved an outstanding EQE.In doping devices based on CBP host,BTP-CN-TPA reached EQEs of 18.3%,16.8%,15.2%and 12.7%at doping concentrations of 3%,5%,7%and 9%,respectively,with corresponding emission wavelengths of 678 nm,694 nm,710 nm and 724 nm.In addition to the typical strategy of enhancing the acceptor strength,this work introduces donor groups into the acceptor skeleton to enhance the ICT effect of molecules and adds cyano groups to regulate LUMO energy level,providing an effective idea for designing effective NIR TADF materials.