Design,Synthesis And Electroluminescence Of Blue Thermally Activated Delayed Fluorescence Emitters

The thermally activated delayed fluorescence（TADF）type luminescent material,which was constructed by all organic blocks,can make the lowest triplet excited state（T₁）excitons convert to the lowest excited singlet（S₁）state through the reverse intersystem crossing（RISC）process for light emission,achieving almost 100%exciton utilization efficiency.However,blue TADF materials still encounter some common problems including relatively large singlet-triplet energy gaps(ΔE_ST),the low RISC rate constants(K_RISC),and unsatisfied color purity.This thesis focuses on structure modification of the traditional donors and acceptors for the construction of novel blue TADF materials with improved performance.Due to the relatively weak electron-donating ability,carbazole（Cz）is the most suitable donor for constructing blue TADF emitters.However,limited by the weak intramolecular-charge transfer（ICT）and largeΔE_ST that was caused by less twisted configuration due to the five-member structure of the Cz ring,many emitters with Cz as the donor are difficult to achieve TADF properties.Accordingly,Cz derivatives with a bulky substituent at 1-site have been invented,which are expected to enhance ICT and separate frontier orbitals to minimizeΔE_ST and finally realize TADF.But due to the electron-donating feature of these 1-site substituents,the emission of the TADF emitters inevitably red-shift.In this thesis,two new donors 3,6-di-tert-butyl-1-（pyridin-3-yl）-9H-carbazole（Py Bu Cz）and 3,6-di-tert-butyl-9H-carbazole-1-carbonitrile（CNBu Cz）were designed with electron-withdrawing pyridyl（Py）or cyano（CN）incorporated at 1-site of Cz ring and TADF molecules were constructed.The substituent at 1-site is more than a steric hindrance group to facilitate the separation of frontier molecular orbital,but an electron-withdrawing group to weaken the ICT and make the emission blue shift.The exclusive charge transfer triplet excited state（³CT_D）of donor Py Bu Cz between carbazole and pyridyl units was proved to act as the media state to facilitate the multi-channel RISC process,leading to improved K_RISC.As a result,Py Bu Cz-TRZ and Py Bu Cz-Me TRZ realized pure blue emission in the doped OLEDs with an external quantum efficiency of 15.3%.However,a too strong electron-withdrawing group such as CN at 1-site of Cz ring（CNBu Cz）will make TADF disappear due to too weak ICT.This is the first report to introduce an electron-withdrawing group into the 1-site of Cz as a donor to construct TADF molecules,which would be universal to design more blue TADF materials.9,9-dimethyl-9H-thioxanthene-10,10-dioxide（DMSFDO）was used as acceptor,in combination with the above pyridyl or CN substituted Cz donor,to design two ultraviolet emitters Py Bu Cz-DMSFDO and CNBu Cz-DMSFDO.However,their ICT may be too weak to produce TADF properties.Therefore,on the base of DMSFDO,the new acceptors,N-cyano-diphenyl sulfoxide imine（SFCN）and N-cyano-9,9-dimethyl-9H-diphenyl sulfoxide imine（DMSFCN）,were designed to produce a stronger ICT and enhance the TADF.A series of TADF emitters were constructed by connecting SFCN or DMSFCN to the donors 3,6-di-tert-butyl-9H-carbazole（Bu Cz）,3,6-Diphenyl-9H-carbazole（Ph Cz）and 9,9-Dimethyl-9,10-dihydroacridine（DMAC）,respectively.The emission of the high-energy charge transfer states can be detected in the photoluminescence spectra of DMAC-SFCN and DMAC-DMSFCN at room temperature,especially at low temperature.The possible luminescence mechanisms are verified by theoretical calculation and spectroscopic experiments.The RISC process and the more effective high-lying reverse intersystem crossing（h RISC）process can both occur in the molecules whose vibrational coupling effects are suppressed.This will be conducive to obtaining more obvious TADF properties,higher color purities and efficiencies.