Design And Synthesis Of Novel Pure Organic Light-Emitting Materials And Their Electroluminescent Properties

Organic light-emitting diodes(OLEDs)have attracted considerable attention owing to their potential applications in both of new generation full-color flat-panel displays and solid-state lighting.During the past three decades,numerous efforts have been devoted to developing new OLED emitters,and great success has been achieved for red and green light-emitting materials.However,in terms of the blue ones,which are extremely critical to commercial applications of OLEDs at present,most of the reported emitters showed unimpressive external quantum efficiency in comparison with their green and red counterparts,or displayed inadequate sky-blue emission.Developing new efficient blue TADF materials still remains a significant challenge.Therefore,the current thesis focuses on the blue emitters,in partular,the design,synthesis and their performances of novel pure organic blue light-emitting molecules for converntional fluorescence and thermally activated delayed fluorescence.In Chapter 2,Two couple of novel naphtho[1,2-d]imidazole derivatives were developed as deep blue light-emitting materials for organic light-emitting diodes(OLEDs).The 1H-naphtho[1,2-d]imidazole(NI-2)based compounds exhibit significantly superior performance than the 3H-naphtho[1,2-d]imidazole(NI-1)analogues in single-layer devices owing to their much higher electron-injection ability directly from the cathode compared with their isomeric counterparts,giving a groundbreaking EQE(external quantum efficiency)of 4.37% and a very low turn-on voltage of 2.7 V,and this is hitherto the best performance of the non-doped single-layer fluorescent OLEDs.While for the multi-layer devices consisting of both hole-and electron-transporting layers,identically excellent performances with EQE values of 4.12-6.08% and deep blue light emission with Commission Internationale de l’Eclairage(CIE)y values filled in 0.077-0.115 were obtained for both isomers due to the improved carrier injection and confinement within the emissive layer.In addition,they showed significantly more desirable blue color purity than the analogous molecules based on benzimidazole or phenanthro[9,10-d] imidazole segments.In Chapter 3,two blue thermally activated delayed fluorescence molecules based on bis(phenylsulfonyl)benzene isomers with very small singlet-triplet splitting energy were designed and synthesized by combining 3,6-di-tert-butylcarbazole with 1,4-bis(phenylsulfonyl)benzene and 1,3-bis(phenylsulfonyl)benzene.Compared with the parent DTC-DPS with a ΔEST of 0.32 eV,new developed DTC-pBPSB and DTC-mBPSB show smaller ΔEST values of 0.05 eV and 0.24 e V,respectively.Given the more pronounced bathochromic shift in emission wavelength of DTC-pBPSB(λmax: 461 nm)compared with DTC-mBPSB(λmax: 434 nm),it can be deduced that the effective conjugation length is longer in the former.The smaller ΔEST value of the former is probably due to its lower intramolecular charge transition state and hence a smaller gap with respect to the lowest localized triplet state.Therefore,the connection mode of the emitter molecule strongly affects its photophysical properties.The use of DTC-p BPSB and DTC-mBPSB translates to sky-blue(0.18,0.19)and deep-blue(0.15,0.08)devices with EQEs of 11.7% and 5.5%,respectively.In Chapter 4,by introducing an acceptor which contains double sulfonyl groups and a twisted biphenyl core,an efficient blue thermally activated delayed fluorescence(TADF)emitter,ACR-BPSBP,was successfully developed.Comparative study and analysis are carried out on it with an analogous molecule,ACR-pBPSB,which also imports two sulfonyl groups as acceptors and exactly same donor units but shows stretched shape and displays green emission.Results of quantum chemistry computation estimate the introduction of such a twisted backbone is beneficial to increase energies of both singlet and triplet states.As a contrast to its counterpart,the newly developed blue emission compound ACR-BPSBP with contorted structure enables a desirable emission with a peak wavelength of 460 nm and superior photoluminescence quantum yield up to 82%.Moreover,remarkable external quantum efficiency over 24% and Commission International de l’Eclairage(CIE)coordinates with total(x+y)value < 0.4 were achieved from the electroluminescent device based on the blue emitter,while this value was almost twice to that of the green emission device based on its analog.In Chapter 5,An applicable strategy which intentionally point at designing efficient small molecule TADF emitters with anisotropic features is demonstrated,in consideration of high internal quantumefficiency(ηint)can be obtained via TADF process.By designing emitters as stick-like molecules,a series of novel compounds which are variable in particular chemical structure were developed.These elongated compounds are inclined to flat lie in vacuum deposited thin solid films and parallel their transition dipole moments in major degree.Their transition dipole moments are horizontally oriented with very high degrees of 86-93% in thin film,and their emission spectra cover the region from greenish blue to pure blue.Owing to simultaneous benefit from effective exciton utilization and raised intrinsic out-coupling factor,remarkable external quantum efficiencies near 21% for pure blue device,as well as that over 35% and close to 30% for greenish blue and sky blue devices were respectively achieved.A compatible strategy on devising high-performance emitters for organic electroluminescence is advocated therein.