Synthesis And Optoelectronic Properties Of Thermally Activated Delayed Fluorescence Emitters And Host Materials Based On Nitrogen Heterocyclic Units

At present,the luminescent materials utilized in organic light-emitting diodes?OLEDs?are mainly composed of traditional fluorescent materials,organic phosphorescent materials and thermal active delay fluorescent?TADF?materials.Among them,TADF materials become a research hotspot in recent years which have advantages of not including rare metals and low cost while realizing 100%exciton utilization,thanks to the reverse intersystem crossing of T₁ excitons forming S₁ excitons by thermal activation.In this thesis,we concentrate on molecular design,being committed to the research and development of novel and efficient TADF materials as well as their host materials,lucubrating energy level regulation,photophysical properties of materials and device performances.In chapter 2,four novel sky-blue and green luminescent molecules were designed and synthesized employing nitrogen heterocycle-pyrazine?PRZ?with extremely high molecular symmetry as electron withdrawing group,among which exist a contrast of structure and orientation.The manner of linking between the groups and CT strength are the critical factors that affect the photophysical properties of the luminescent materials under photic and electric driving.For m-PRZ-Ph-PXZ,it breaks the conjugation of the whole molecule by meta-linking,eventuating in the hypochromatic emission and the reduction of?E_ST,which triggers faster k_RISC.Consequently,the efficiency roll-off of the device under high brightness was minimized.While strengthening the conjugation by para-linking and introducing phenoxazine unit with strong electron-donating ability for p-PRZ-Ph-PXZ,which can improve the planarity between the acceptor and the benzene ring,as well as increase horizontal orientation and PLQY simultaneously,are capable of improving the overall performance notably with regard to the device.However,efficient RISC process is restricted in virtue of the relatively large?E_STT despite increasing the horizontal orientation of the material,generating obvious efficiency roll-off.Correspondingly,the sky-blue electroluminescence device based on m-PRZ-Ph-Ac could achieve maximum EQE of 12.8%,and green electroluminescence devices based on p-PRZ-Ph-PXZ and m-PRZ-Ph-PXZ were capable of raising the efficiency to 26.0%and15.5%,which exerted the excellent performances.In chapter 3,pure-blue emitter TRZ-TC2 as well as host material PMD-TC2 of yellow and green TADF devices was developed employing nitrogen heterocycle-pyrimidine?PMD?and triazine?TRZ?with strong chemical stability as electron withdrawing group,and tert-butyl carbazole as electron donating group,which possess relatively high T₁ energy,superior electrochemical stability and suppressed effciency roll-off at high brightness in devices.In particular,the maximum EQE of 25.2%and CIE coordinates of?0.15,0.17?were achieved in pure-blue electroluminescence device based on TRZ-TC2 as the dopant,along with suppressed effciency roll-off and relatively narrow full width at half maximum of the EL spectrum?70 nm?,which was suitable for practical application in blue devices.In addition,with respect to the devices of 4CZIPN and tri-PXZ-TRZ,taking advantages of faster radiative rate constant and excellent electrochemical stability of PMD-TC2 as host,the maximum EQEs of about 20%were achieved in devices,which not only manifested the performance levels for TADF emitters,but also exhibited extremely low effciency roll-off at high brightness.This work can offer some valid strategies toward molecular design of highly efficient host and guest materials with high triplet energy and outstanding stability.