Construction And Photoelectric Properties Of Different Hybrid Local And Charge-Transfer Excited States

At present,consumers’demand for high-quality electronic products is increasing,it is significant to develop new organic light emitting diode（OLED）technology,which is beneficial for breaking through foreign technical barriers and improving the domestic industrial upgrading.Since the OLED was invented in 1987,the OLED materials have experienced many technological advances:in the early days,the OLED materials can only use 25%singlet excitons according to the spin-statistics rule.Then the heavy metal complexes,which can significantly enhance the spin-orbit coupling（SOC）by the heavy-atom effect were reported to break the transition barrier between singlet excitons and triplet excitons,improving the exciton utilizing efficiency（EUE）.However,the heavy metal elements are expensive and harmful to the environment.And blue light materials of heavy metal complexes are very scarce.Therefore,the utilization of triplet excitons with low-cost pure organic materials has become the focus of current research.These new luminescence mechanisms include:thermally-activated delayed fluorescence（TADF）,triplet-triplet annihilation（TTA）and Hot Exciton materials.Except for the utilization of triplet excitons,the high-performance blue electroluminescent devices are also considered as the focus of researchers.And the blue electroluminescent devices have been restricted by the bottleneck of efficiency,color purity and stability.In essence,the excited state determines the luminescent properties of materials.Therefore we need to adjust the excited states and design new luminescence mechanisms which can realize new generation pure organic materials with low-cost and high efficiency.The hybridized local and charge-transfer（HLCT）state is mainly researched in this paper.The HLCT state emphasizes the hybridization of locally-emissive（LE）component and charge-transfer（CT）component in excited state,which possesses the advantages of high photoluminescence quantum yield（PLQY）and high EUE.Generally speaking,the LE state can lead to high PLQY and high energy level of excited state,which is favorable to generate high efficiency blue-shifted emission,but the strong LE state also limits the increasing of EUE and the EUE is less than 25%;on the contrary the CT state can promote the EUE because of small singlet-triplet splitting(ΔE_ST),but it causes reduced PLQY and red-shifted emission,which is harmful to the construction of blue light materials.The HLCT state formed by the hybrid of LE state and CT state combines the advantages of two states,which can realize high PLQY and EUE in high efficiency blue electroluminescent device.In addition,the HLCT emitters display no-delayed lifetime,which can avoid efficiency roll-off at high brightness and promote the stability of device and benefit the preparation of non-doped devices with simplified processing steps.In this paper,three new material systems with different excited state characteristics are designed and synthesized.Based on quantum chemical calculation and photophysical properties,we comprehensively study the design and regulation of CT and LE state in forming HLCT state,so as to achieve the purpose of preparing high-efficiency blue light devices.The specific research contents are as follows:1.Phenanthroimidazole is a common group that can be used to construct the HLCT state.Based on triphenylamine-phenanthroimidazole backbone,the DRZ-TPM and TRZ-TPM are designed and synthetized.Because of strong donor（D）triphenylamine and strong acceptor（A）triazine,the DRZ-TPM generates typical CT state,leading to green-emitting device.By changing the distance between donor and acceptor,we weaken the CT component of the excited state,forming TRZ-TPM with CT-dominated HLCT state.As a result,we successfully realize the sky-blue electroluminescence by the doped device of TRZ-TPM.In addition,compared with doped film,TRZ-TPM displays obvious red-shift emission in non-doped state which is caused by the exciplex among the same TRZ-TPM molecules according to the experimental characterization.Therefore,we need to alleviate the negative effect of exciplex on its light color by manufacturing doped devices.2.Based on the work of the previous chapter,we believe that the strong CT component of HLCT will cause the red-shift of electroluminescence and adversely affect the design of blue light materials.Furthermore,we reduce the CT component and increase the LE component by changing the connection position and electron donors,constructing three molecules:N-SBPMCN,N-ABPMCN and N-TBPMCN.Among them,the N-SBPMCN displays typical LE features,while the CT component is excessively suppressed,preventing N-SBPMCN from forming HLCT state.As a result,the N-SBPMCN demonstrates deep blue emission with lower EUE.Thus N-TBPMCN and N-ABPMCN introduce CT component through different methods:the former increases the component of the intramolecular CT state to form the intramolecular HLCT state,while the latter forms the intermolecular HLCT state by using the CT component of the excimer in non-doped state,realizing the EUE of two molecules successfully break the limit of the spin-statistics rule.However,the performance of both non-doped devices needs to be improved:non-doped device of N-TBPMCN demonstrates broadened electroluminescence spectrum because of the incomplete hybridization of CT and LE,and the non-doped device of N-ABPMCN demonstrates obvious red-shift of light color due to the presence of excimer.In addition,we also characterize the phenomenon of N-ABPMCN stimulus response,which deepens the understanding of excimer.3.Next,we design a molecular system with"linear"D-π-A structure to promote the effective hybridization of LE and CT states and improve the performance of non-doped devices.We select 9,9’-dimethyl-9H-fluorene asπbridge group and design three"linear"D-π-A molecules with different donors:PXZ-FR-DRZ,DMAC-FR-DRZ and CZ-FR-DRZ.The proportion of CT component in the excited states of three molecules is regulated by changing the electron acceptors with different intensities and different twist angles.The PXZ-FR-DRZ possesses the largest twist angle and strongest electron donor（phenoxazine,PXZ）,which forms an obvious CT excited state and induces TADF emission of PXZ-FR-DRZ.Because of the TADF,the PXZ-FR-DRZ realizes satisfied external quantum efficiency（EQE）(EQE_max=11.5%).However,PXZ-FR-DRZ also shows its shortcomings:electroluminescence of PXZ-FR-DRZ displays obvious red-shifted,and non-doped device of PXZ-FR-DRZ demonstrates reduced EQE.DMAC-FR-DRZ possesses a relative weak electron donor and a reduced twist angle,which alleviates the red-shift of electroluminescence to a certain extent.However,the CT component of DMAC-FR-DRZ is still strong which is unable to realize the complete hybridization of CT component and LE component.Thus we further design CZ-FR-DRZ with weaker electron donor and smaller twist angle,and finally realize the quasi-equivalent HLCT state of CZ-FR-DRZ.Attributed to the equivalent HLCT state,the doped device（5 wt%）of CZ-FR-DRZ achieves an EQE_max of 3.1%at 420 nm,and the EQE_max of non-doped device is increased to 4.6%with low efficiency roll-off,which indicates that CZ-FR-DRZ is more suitable for the blue non-doped devices with excellent comprehensive performance（high efficiency,color purity and stability）,providing a new idea for the development and industrialization of blue OLEDs.