Design And Synthesis Of Novel Red Thermally Activated Delayed Fluorescence Emitters For Organic Light-Emitting Diodes

Organic light-emitting diodes（OLEDs）are leading innovation in the display and solid-state lighting areas and greatly enhancing the visual experience in daily life,due to their unique advantages of compact and thin,high contrast,fast response,flexibility,foldability,etc.Thermally activated delayed fluorescent（TADF）materials can achieve100%exciton utilization without the adoption of noble metals,rendering them the best luminescent materials in OLED manufacture.There are currently two approaches to realizing the TADF feature:one is to employ intramolecular charge transfer between donor（D）and acceptor（A）units to design single-molecule TADF materials;the other is to employ intermolecular charge transfer between donor and acceptor molecules to construct two-component exciplex systems.However,among the TADF OLEDs fabricated based on the above two types of emitters,the development of red devices still lags behind the optimal blue and green devices.The non-radiative decay rate of excitons(k_nr)will be exponentially amplified with the decreased energy gap,which is the major limitation to the device efficiency of red TADF OLEDs.To address this issue,the exciton transition processes of TADF emitters were investigated thoroughly in this dissertation.For the single-molecule TADF materials and exciplex emitters that possess narrow energy gaps,the feasible strategies of promoting the fluorescent radiation rate（k _F）,increasing the rate of reverse intersystem crossing(k _RISC),and enhancing the rigidity to confine vibrational relaxation were proposed,respectively.Accordingly,a series of high-efficiency red TADF materials have been designed,and various novel red exciplex emitters have been constructed,which opened up new insights for the development of red TADF OLEDs.The specific research contents are as follows:1.A new strategy for introducing intramolecular hydrogen bonds between D and A units of red TADF emitters was proposed.The formation of intramolecular hydrogen bonds can effectively restrict the molecular conformation to facilitate the spatial overlap of the highest occupied molecular orbital（HOMO）and the lowest unoccupied molecular orbital（LUMO）,thereby greatly enhancing k _F.Comparing the three TADF emitters synthesized in this work,TPA-APm with intramolecular hydrogen bonds exhibited the fastest k _F of 6.1×10⁷ s^-1,the lowest k_nr of 0.64×10⁷ s^-1,and the highest photoluminescence quantum yield(Φ_PL)of 90.4%,respectively.Accordingly,the red TADF OLED employing TPA-APm as the emitter obtained a maximum EQE of 21.1%,which is twice and threefold than those of TPA-AP-and TPA-APy-based devices.These results confirmed that the introduction of intramolecular hydrogen bonds is a feasible strategy to increase k _F,and opened up a new path for the advancement of red TADF OLEDs.2.An innovative strategy for introducing quasi-degenerate orbital distribution in red TADF emitters was presented.The quasi-degenerate orbital distribution enables multiple excited states to be engaged in the reverse intersystem crossing process of triplet excitons,thus significantly increasing the rate of reverse intersystem crossing(k _RISC).In this context,four red TADF emitters were synthesized to systematically elaborate the promotion of k _RISC by quasi-degenerate orbital distribution.For the quasi-vertical conformation of the red TADF emitters,namely DPXZ-BPF and TPXZ-BPF,although fast k _RISCs of the order of 10⁶ s^-1 were already available due to the extremely small singlet-triplet energy gap(ΔE_ST),TPXZ-BPF still exhibited an improved k _RISC and the higher device efficiency with additional quasi-degenerate orbitals.Based on the same concept,a novel red TADF emitter,namely TTPA-DCPPm,was synthesized by inducing multiple quasi-degenerate orbitals together with intramolecular hydrogen bonds.Accordingly,TTPA-DCPPm exhibited a larger k _F of 2.2×10⁷ s^-1 along with a double increase in the k _RISC.Besides,TTPA-DCPPm enabled its OLEDs with an impressive EQE_max of 26.5%and deep-red emission peaked at 640 nm,which was among the state-of-the-art deep-red TADF OLEDs.3.An innovative bipolar material,namely 3Cz-o-TRz,with an elaborate D-Spacer-A conformation was developed to broaden the category of constituting molecules of the red exciplex emitters.In this molecule,the tercarbazole（3Cz）was employed as the secondary electron-donating group to promote HOMO delocalization;the triphenyltriazine（TRz）was employed as the planar electron-accepting group to enhance face-to-face interaction.Especially,a diphenyl ether was employed as a spacer group to inhibit the intramolecular conjugation between the 3Cz and TRz units.Accordingly,3Cz-o-TRz was supposed to preserve the intrinsic characteristics of 3Cz and TRz moieties in the single-molecule state,while it could facilely undergo intermolecular charge transfer in the aggregate state.It enables 3Cz-o-TRz to serve as both D components and A components,combining with three common hole-transporting materials and three common electron-transporting materials,respectively,to construct six novel exciplex emitters.And these exciplexes realized the modulation of the emission spectra with the peaks varied from 510 to 590 nm.Among them,3Cz-o-TRz as the D component and 3Cz-o-TRz as the A component achieved nearly identical performance in the exciplex OLEDs with maximum EQEs around 12%.These results confirmed that the D-Spacer-A type molecules would be an ideal candidate to expand the number of exciplex emitters.And the work in this chapter provided valuable experience for the subsequent construction of efficient red exciplex emitters.4.A series of innovative rigidity-enhancing strategies were proposed to improve the device efficiency of red exciplex emitters based on which,four novel acceptor materials with deep LUMO energy levels were designed and synthesized.Among them,the intramolecular vibrational relaxation of CNAI-TRz can be effectively confined by attaching a rigid steric hindrance group,and the 13AB:CNAI-TRz-based red exciplex OLED successfully exhibited a maximum EQE of 7.7%with red emission peaked at 604nm.On this basis,the nitrogen atoms with strong electronegativity at the periphery of TPA-2NP were employed to generate intermolecular hydrogen bonds in the red exciplex emitters,thereby enhancing the rigidity of the bimolecular system.Therefore,the k_nr of a novel red exciplex emitter,namely m PTBC:TPA-2NP,can be significantly restrained.Ultimately,the m PTBC:TPA-2NP-based red exciplex OLED successfully achieved an impressive EQE_max of 13.4%at the electroluminescence peak of 600 nm,which represents the state-of-the-art performance of current red exciplex OLEDs.And the work in this chapter can indicate the direction to develop high-performance red exciplex emitters in the future.