Synthesis and Electroluminescence Properties of Delayed Fluorescence Materials Based on Dibenzopyridoquinoxaline
Objective To obtain novel and efficient thermally activated delayed fluorescence(TADF)materials,BPQPXZ and BPQTPA are synthesized using dibenzopyridoquinoxaline(BPQ)as acceptor(A)and triphenylamine(TPA)and phenoxazine(PXZ)as donors(D).The results show that the two materials have typical delayed fluorescence characteristics,a smaller energy gap(ΔEST)between singlet and triplet states,and a larger oscillator strength(f).The device based on BPQPXZ combined with a strong acceptor and a strong donor achieves deep-red emission with λEL at 660 nm.However,due to the influence of the energy-gap law,the external quantum efficiency(EQE)is only 1.0％.BPQTPA combined with a strong acceptor and a weak donor has a larger fluorescence quantum yield(82.7％)because of the weaker rigidity of TPA than that of PXZ.As a result,the donor and acceptor of BPQTPA have less distortion,more orbital overlap,and largerf.At the same time,the intramolecular charge transfer effect of BPQTPA is weakened,and the electron-donating ability of TPA is weaker than that of PXZ.BPQTPA exhibits a blue-shifted emission compared with BPQPXZ.Therefore,the device based on BPQTPA exhibits yellow emission with λEL at 555 nm.Compared with BPQPXZ,the turn-on voltage of BPQTPA is reduced to 2.8 V;the maximum current efficiency and power efficiency are increased by 32-fold and 36-fold,respectively,and the EQE is increased by 6-fold to 7.0％.Methods In this study,BPQPXZ and BPQTPA materials are synthesized using the Suzuki reaction and Buchwald-Hartwig reaction.The photophysical properties,electrochemical properties,thermal properties,and device performance of the two materials are investigated.Comparative analysis is conducted on the luminescent properties of two materials.Results and Discussions The structures of two materials,BPQPXZ and BPQTPA,are verified by 1H nuclear magnetic resonance(1H NMR)spectroscopy and high-resolution mass spectrometry(HRMS).BPQPXZ exhibits deep-red emission with λPL at 655 nm,and BPQTPA exhibits yellow emission with λPL at 585 nm(Fig.3).Compared with BPQPXZ,BPQTPA exhibits blue-shifted emission because of weaker electron-donating ability of TP A than PXZ.Similarly,the rigidity of TPA is weaker than that of PXZ,resulting in a greater degree of overlap between the HOMO and LUMO of BPQTPA,a higher oscillator strength,and a larger fluorescence quantum yield(82.7％)for BPQTPA,which is consistent with the density functional theory simulation results(Fig.2).As shown in the transient PL decay spectra(Fig.4),the delay component is observed,and such phenomena are typical behaviors of TADF.As shown in the cyclic voltammogram(Fig.5),the HOMO levels of BPQTPA and BPQPXZ are-5.38 eV and-5.25 eV,respectively.The calculated LUMO levels are-3.00 eV and-3.27 eV for BPQTPA and BPQPXZ,respectively.BPQTPA shows better thermal stability with a higher decomposition temperature(Td,with 5％weight loss)of 492.6 ℃ than BPQPXZ(Td=439.2 ℃).The higher thermal stability of BPQTPA can be ascribed to its better planarity than that of BPQPXZ.These devices based on BPQTPA and BPQPXZ achieve good performance(Fig.7).The device based on BPQTPA exhibits much higher EQE(7.0％)than the device based on BPQPXZ(EQE is 1.0％),especially.Conclusions In this study,BPQTPA and BPQPXZ materials are designed and synthesized using BPQ with a highly rigid conjugated planar structure as an acceptor and TPA and PXZ as donors.The results show that two materials have typical delayed fluorescence characteristics.BPQTPA and BPQPXZ achieve good orbital separation between HOMO and LUMO,as well as a certain degree of orbital overlap,resulting in a smaller ΔEST and a larger oscillator strength.The device based on BPQPXZ combined with a strong acceptor and a strong donor achieves deep-red emission with λEL at 660 nm.However,due to the influence of energy-gap law,non-radiative decay is serious,with an EQE of only 1.0％,as well as low current and power efficiency.The device based on BPQTPA combined with a strong acceptor and a weak donor is less rigid than that based on BPQPXZ,making the degree of donor and acceptor distortion of BPQTPA less than BPQPXZ,and the degree of overlap between HOMO and LUMO orbitals of BPQTPA increases,so oscillator strength of BPQTPA is 2.2 times that of BPQPXZ.As a result,BPQTPA has a higher PLQY(82.7％).Meanwhile,due to the much weaker electron-donating ability of TPA than PXZ,the intramolecular charge transfer effect of BPQTPA is weakened,resulting in a significant blue-shift in both photoluminescence and electroluminescence peaks.The device based on BPQTPA exhibits yellow emission with λEL at 555 nm.Compared with BPQPXZ,the turn-on voltage of the device based on BPQTPA is reduced to 2.8 V,and the current efficiency and power efficiency are significantly improved by 32-fold and 36-fold,respectively.The EQE is increased by 6-fold to 7.0％.In particular,we investigate the effects of reasonable combinations of donor and acceptor on the photophysical and electroluminescent properties of materials through structure-activity relationships,and the study is of certain reference significance for the research on long-wavelength TADF materials.
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