High Energy Reverse Intersystem Crossing Process And Photoelectric Properties Of Hot Exciton Luminescent Materials

Organic light-emitting diodes?OLEDs?have gained widespread commercial applications,especially in the field of flat panel displays.The core part of OLEDs technology is organic electroluminescent?EL?materials.Due to the limitation of the spin statistics,the exciton utilization effiency?EUE?for traditional fluorescent materials is only 25%.Phosphorescent materials based on precious metal complexes can realize 100%EUE,as a result of the spin-orbit coupling?SOC?effect.However,phosphorescent materials still face problems such as high material cost and short lifetime of blue devices.Therefore,developing new-generation,low-cost and purely organic EL materials with high efficiency and good stablity,is beneficial to the sustainable development of the OLEDs industry.At present,fluorescent materials that can achieve high EUE mainly include triplet-triplet annihilation?TTA?materials,thermally activated delayed fluorescent?TADF?materials,hot exciton materials,and doublet radical materials.The hot exciton mechanism was first proposed by Yuguang Ma's research group internationally,and its core concept is to increase the EUE through the reverse intersystem crossing from the high energy triplet states?h RISC;T_n,n?2?to the singlet state?S_m,m?1?.After several years development,a series of high-efficiency hot exciton materials from deep-blue to near-infrared have been reported.However,there are still some basic problems that need to be further researched,such as the experimental determination of high-energy triplet energy levels,experimental evidence of the h RISC process and the determination of the rate constant,influence of SOC on h RISC process,development of high-efficiency hot exciton materials and the study of structure-properties relationship.This thesis focuses on these issues and carried out the following studies:1. Experimental evidence of high energy reverse intersystem crossing of hot exciton materials.The high-energy T₂ and T₃state energy levels of a phenantimidazole-anthracene?PPI-An?-based blue hot exciton material PAC was determined by nanosecond transient absorption spectroscopy.According to the measured T₂ state energy,ketone sensitizers with appropriate energy levels were selected to directly sensitize the T₂ state of PAC.Combined with the transient PL decay spectra,time-resolved fluorescence spectra,and the sensitized low-temperature delayed spectra,the h RISC process?T₂?S₁?of hot exciton mechanism is experimentally confirmed.Through device optimization,the PAC-based non-doped device showed pure-blue emission,with an external quantum efficiency?EQE?of 10.5%,an EUE of close to 100%,and good device stability.At the same time,through the magnetic field effect and transient EL spectroscopy study,it was confirmed that the high device efficiency and high EUE are mainly attributed to the hot exciton mechanism rather than the TTA effect.2. Influence of the connection mode between donor and acceptor on the photoelectric properties of the hot exciton materials.mPAC is an isomer of PAC.Compared to PAC,in which donor and acceptor are para-connected,the meta-linked m PAC breaks the conjugation.The excited state transferred form coexistence of local-excited?LE?and charge transfer?CT?in PAC to pure LE state in m PAC,resulting in blue-shifted spectrum.The crystal structure revealed that the molecular structure of m PAC is more distorted and the inter-molecular distance is increased,which results in poor carrier transmission channels and poorer transmission balance.m PAC based device showed deep-blue emission and improved color purity,with CIE coordinates of?0.16,0.09?.While the device efficiency is inferior to PAC,with an EQE of 6.76%.Taken together,the overall device performance of the para-connected blue hot exciton material is better.3. The effect of spin-orbit coupling on the excited state properties of hot exciton materials.Two novel hot exciton materials,PABP and PAIDO,were designed and synthesized by introducing ketone receptors with n-?*transition character on the PPI-An molecular skeleton.Theoretical calculations and photophysical tests revealed that the energy difference between T₂-T₃ states of the two materials is small,there exists non-adiabatic coupling between T₂/T₃.And the SOC between S₁-T₃ states is much larger than that between S₁-T₂,the h RISC process mainly occurs between T₂/T₃ and S₁.The experimentally tested h RISC rate is in the order of 10⁷ s^-1,which is 1-3 orders of magnitude higher than that of TADF materials.Material with larger SOC between singlet and high energy triplets showed faster h RISC rate important role in the excited state properties of hot exciton materials.4. Increasing the density of functional groups enhances the fluorescence efficiency of hot exciton materials.Two novel hot exciton materials,PADC and PCAC,were designed and synthesized,by increasing the number of carbazole groups of PAC.Due to the dense accumulation of functional groups,the rigidity of the molecule is increased,and the non-radiative energy loss resulting from vibration relaxation and internal conversion of the excited state is suppressed.The fluorescence efficiency both in solutions and films are increased.The PADC and PCAC based devices showed similar color purity with PAC,with CIE cooddinates of?0.16,0.13?and?0.16,0.12?,respectively.But the device efficiency is significantly improved compared with PAC.The maximum current efficiency are 15.74 cd A^-1and 14.58 cd A^-1,respectively;the maximum external quantum efficiency are 12.39%and11.26%,respectively.