Design,Synthesis And Photoelectric Properties Of Boron-based Blue Thermally Activated Delayed Fluorescence Materials With Narrow Emission

Organic light-emitting diodes（OLEDs）have been widely used in new-type displays owing to their self-luminescence,lightweight and rollable characteristics.To satisfy the requirement of wide color gamut and high color purity for the new-generation of ultra-high-definition displays without using unique optical technology,the emission spectrum of organic luminescent materials are required with narrow full width at half-maximum（FWHM）.Boron-based thermally activated delayed fluorescence（TADF）emitters based on multiple resonance effect（MR）exhibit narrow emission due to their rigid molecular skeleton and unique frontier molecular orbital distribution characteristics,showing good application advantages in ultra-high-definition display with high color purity and wide color gamut.However,the boron-based TADF emitters with narrow emission still face many problems in blue regions at present,such as lack of deep-blue materials,serious luminescence quenching caused by molecular rigid plane,unclear influence factor of borylation site in material synthesis,and a single type of deep-blue TADF sensitizer used to improve device performance,etc.This paper carries out corresponding research on the above issues.The specific research contents are as follows:（1）A design strategy of"weak donor substitution"was proposed for deep-blue boron/oxygen-based TADF emitters with narrow emission.The introduction of weak electron donor groups carbazole and 5H-benzofuro[3,2-c]carbazole with fusion feature in the molecule suppressed the intramolecular charge transfer,which endowed both TDBA-Cz and TDBA-BFCz with deep-blue and narrow emission.It was found compared with carbazole,the introduction of fusion donor group on the boron/oxygen-based multiple resonance core was beneficial to enhance thermal stability,reduce the singlet-triplet energy gap(ΔE_ST),enhance the reverse-system transmutation rate(k _RISC),and improve the photoluminescence quantum yield（PLQY）of the emitter.As result,the devices based on TDBA-BFCz with fusion donor showed better performance.The corresponding doped and nondoped devices achieved maximum external quantum efficiency(EQE_max)of 4.2%and4.1%,CIE coordinates of（0.159,0.032）and（0.156,0.045）,and FWHM of 40 nm and 44nm,respectively.（2）Three blue MR-TADF emitters were designed and synthesized by introducing the inert substituents with different rigidity or steric hindrance to the periphery of the boron/nitrogen-based multiple resonance core.The introduction of rigid 9,9-dimethylfluorene and large steric hindrance tert-butylbiphenyl increased the PLQY of the MR-TADF emitter.But,the 9,9-dimethylfluorene group caused the stacking of molecular structures,resulting in a serious loss of device efficiency and poor color purity under a high doping concentration of emitter.In contrast,the tert-butylbiphenyl group inhibited luminescence quenching caused by intermolecular interactions,inhibited device efficiency loss,and improved color purity under a high doping concentration of emitter.As result,t Ph3Cz-BN with tert-butylbiphenyl group achieved the best device performance,the EQE_max,CIE coordinates,and FWHM of the device were 21.8%,（0.126,0.108）,and 28 nm,respectively.（3）A“periphery cladding”strategy was proposed to design deep-blue MR-TADF emitter.By cladding a different number of tert-butyl units at the periphery of the MR-TADF emitter,the intermolecular interactions and aggregation-induced emission quenching were suppressed.Among emitters,3tPAB with full periphery cladding showed the most significant suppression.It also maintained excellent color purity at high doping concentrations and exhibited the highest device efficiency.The device based on 3tPAB achieved EQE_max of 19.3%,CIE coordinates of（0.141,0.076）,and FWHM of 26 nm,respectively.（4）Two boron/nitrogen-based blue MR-TADF emitters（tDPAC-BN and tDMAC-BN）were designed and synthesized based on one-shot electrophilic C-H borylation reaction.The influencing factors of the electrophilic borylation site and photoelectric properties of new emitters were explored.The nuclear magnetic resonance data demonstrated different cyclization structures for two emitters.Theoretical calculations found the electrophilic borylation site was affected by HOMO electron cloud density and steric hindrance effect,which can be controlled by varying the steric hindrance effect and electron-donating ability of the intermediate substituent.The TADF-sensitized device based on new emitters tDPAC-BN and tDMAC-BN achieved EQE_max of 21.6%and 22.3%,FWHM of 28 nm and 34 nm,respectively.Moreover,compared with non-sensitized devices,TADF-sensitized devices showed enhanced efficiency and alleviative efficiency roll-off.（5）A strategy employing a deep-blue MR-TADF material as a sensitizer was proposed to improve the performance of blue devices with high color purity.Specifically,MR-TADF emitter 3tPAB synthesized in（3）was selected as a sensitizer for blue traditional fluorescence emitter（DtPAPy）and a new synthesized MR-TADF emitter（PhDMAC-BN）.After the addition of the sensitizer in devices,MR-TADF-sensitized fluorescent（TSF）and MR-TADF-sensitized MR-TADF（TST）devices presented EQE_max of 14.4%and 33.9%,respectively.Compared with the device without sensitizer,the efficiency was increased approximately by 2.4 and 1.25 times,respectively.The CIE coordinates and FWHM of the TST device were（0.108,0.223）and 35 nm,respectively.