Energy Level Regulation Of Aggregation-induced Emission Luminogens And Their Applications In Optoelectronic Devices And Theranostic

Most of the traditional organic luminescent dyes have large-plane conjugated aromatic structure,which emits light strongly in a dilute solution or single molecule state,but weak or even disappeared fluorescence in a high concentration solution or in aggregation state?nanoparticles,micelles,solid film or powder?,so it was called aggregation caused fluorescence quenching?ACQ?effect.In flexible displays and biological applications based on water environment,luminescent materials mainly exist in the form of aggregation,so it is very urgent to overcome such ACQ effect.The concept of aggregation-induced emission?AIE?provides a good strategy for solving traditional ACQ,which limits the non-radiative transition channel of the excited state energy in the aggregation state,makes the energy mainly emit fluorescence in the form of radiative transition.Therefore,the rise of the AIE has opened another door for the development of display lighting and disease diagnosis and treatment technologies.At present,the main processes of luminescence that can be utilized are mainly photoluminescence?PL?and electroluminescence?EL?,the difference between the processes and the exciton generation mechanisms lead to significant differences in its application scenarios.As one of electroluminescence applications,organic light emitting diode?OLEDs?have many advantages such as high contrast,low power consumption,fast response speed,ultra-thin feature,unlimited viewing angle,and flexible display preparation.It has extremely important application prospects and economic benefits in the future display and lighting fields.The mechanism of electroluminescence is mainly that the holes generated by the anode and the electrons generated by the cathode move under the action of the electric field,and are injected into the hole transport layer and the electron transport layer respectively.The excitons are generated once holes and electrons recombine in the emitting layer?contain 25%radiated singlet excitons and 75%non-radiated triplet excitons according to statistics?,which excites luminescent molecules to obtain ultraviolet,visible and infrared emission.However,the formation of triplet excitons in electroluminescence is mainly produced in the direct form of injection and recombination.Radiation transitions theoretically originate from the relaxation process of singlet excitons,leading to the conversion of electroluminescent triplet excitons into radiated singlet excitons is one of the key factors for determining the efficiency of the device.From this perspective,there are differences in the generation,migration and conversion of photoluminescence and electroluminescence excitons.In photoluminescence,it is generally believed that dyes can be excited to a high-energy singlet excited state after absorbing protons?in accordance with the principle of electron spin?,part of it returns to the ground state in the form of radiation relaxation and releases photons,and another part of excitons can translate into triplet excitons by the intersystem crossing?ISC?.Due to the long life and high energy of these triplet excitons,they can exist in the system for a relatively long time and be dissipated in various forms.Their utilization has always been the focus of research.For example,the use of long lifetime characteristic of excitons to achieve effective resolution of charges at the interface of organic photovoltaic cells,the use of activating excited state energy and active substances to improve the activity of the catalyst,and the use of its own fluorescence energy level difference to prepare phosphorescent materials?room temperature phosphorescence,low-temperature phosphorescence?and devices?anti-counterfeiting,white light,etc.?,use it to interact with oxygen in the triplet ground state to generate active species and accelerate oxidation.In summary,no matter in photoluminescence or electroluminescence,the effective regulation of excitons is very important to improve the performance of luminescent materials in different scenarios,and this excitonic regulation is ultimately in the design of the chemical structure to achieve its effective control of the excited state energy level distribution.In short,the energy level barrier between signlet and triplet state should be regulated to activate or passivate the ISC or RISC channel to achieve S?T or T?S exciton conversion by researching structure-property relationship of a specific building block.Based on the above analysis,start from the functional application orientation,this paper selects anthracene-like derivatives as the key research object according to essential difference of the exciton generation pathway in the EL and PL process,and discusses its structure-property relationship,at the same time,it deeply analyzes the energy level distribution law,so as to realize its efficient application in the field of optoelectronic devices and fluorescent diagnostic probes through the continuous optimization of substitution groups.On the one hand,in molecular design,we take the AIE-active feature transformation as the main direction,and introduce or amplify the effect of fluorescence enhancement in the aggregation state through the introduction of rotor groups;On the other hand,it focuses on the basic influence of structure on the singlet and triplet states of excitons during their generation and transformation,so that the target molecule can be designed more effectively according to functional needs.The thesis can be summarized into two main parts:electroluminescence and photoluminescence.In the first part,OLED requires to select fluorophores with highly efficient photoluminescence quantum efficiency?PLQY?and exciton utilization efficiency?EUE?as the light-emitting layer.It needs to consider the large conjugate plane to provide highly efficient PLQY.The construction of the CT state provides a certain dipole of the molecule to reduce the barrier of charge injection and transmission,and provides more effective RISC channel for excitons such as TADF and HLCT.The specific research results are as follows:In the second chapter,we chose the anthracene-liked building block with nitrogen heteroatom——Acridone?ADO?as the research object.On the one hand,it was expected that enhancing the proportion of the energy dissipation in the radiation transition process by increasing the rigidity of the molecular skeleton structure plane.On the other hand,it was hoped that the process of electron inversion on the carbonyl group would be used to enhance the efficiency of triplet excitons conversion to singlet states.The ACQ effect of ADO was effectively suppressed by introducing rotor-type electron donors such as triphenylamine?TPA?and carbazole?Cz?.At the same time,the introduction of benzocyano group in the nitrogen atom of ADO constructed a high-level CT state to weaken the binding energy of excitons in the EL process,and realized the capture of high-energy level T excitons as well as the effective adjustment of EUE in the EL process.Unfortunately,the emission color and solid-state luminous efficiency of ADO derivatives did not possess advantages because the internal conversion process of T excitons is not effectively suppressed.We proof that ADO was not suitable to serve as a model primitive for energy level regulation.In the third chapter,we selected naphtho[2,3-c][1,2,5]thiadiazole?NZ?acceptor with anthracene-like transition dipole crossover feature and"hot excitons"characteristics as the object,which was hoped to strengthen the CT effect of substitution direction and effective electron conjugation to achieve the balance of PLQY and exciton conversion.The introduction of TPA unit on one side of NZ endowed the compound?TNZ?significant AIE performance,and on the other side the introduction of multifunctional planar element phenanthroimidazole?PPI?to improve the optical absorption cross-section and carrier transport performance,and adding different tert-butyl groups to regulate single molecule CT state and intermolecular interaction.Compared with ADO derivatives,the TNZPPI structure not only significantly improved the solid-state luminous efficiency,but also exhibited the expected"baffle effect"in the?T₂T₁ energy level difference,thereby suppressing the internal conversion competition process of high energy T excitons,and realized a near-infrared device with high solid-state fluorescence and high excitons utilization.It also proved that TNZ is more suitable as the core of energy level regulation research.In chapter 4,in order to further clarify the essential rules of the structure change and energy level distribution of multifunctional molecular planar substituents introduced in TNZ,we selected different fusing rings as substituent and systematically compared the change rule of energy level distribution?especially?T₂T₁?.We not only obtained a deep red AIEgen with a high solid-state luminous efficiency?36%?,realized an undoped saturated red light device with the EUE of 63%and an external quantum efficiency of 2.63%with low efficiency roll-off,and summarized three elements to construct the"hot excitons"channel efficiently:large?T₂T₁,small?E_st and high SOC.In chapter 5,based on the understanding of the above works,we introduced an electron-rich non-planar unit——tetraphenylethylene?TPE?in the TNZ block.On the one hand,the intramolecular electronic effect and PLQYs of the material were enhanced.On the other hand,the spatial configuration of TPE was used to adjust its aggregation state.The solid-state PLQY of TNZTPE in the deep red region(?_em>650nm)was increased to 50%,and the efficiency of its non-doped OLED device could reach 3.58%,which is the highest value in the non-doped standard saturated red light.In chapter 6,in order to prove that the possible mechanism of"molecular cross dipole strategy"might be the key to construct the triplet level with"baffle effect"?suppressing the conversion of T₂?T₁?,we replaced the NZ motif to the Naphtho[2,3-d][1,2,3]triazole?NTZ?block,which was expected to make the conversion of T₂?S₁ more efficient.The energy splitting between T₂ and S₁ of the two derivatives of TNTZ obtained by TPA modification was reduced to 0.1 e V,while?T₂T₁ was as high as 1.01 e V,the EUE could be as high as 94%,and the external quantum efficiency could also be 8.0%without optimization.The above results basically proved the feasibility of our speculation,and also provided a new idea for the design of the“hot excitons”building block.The second part,the significant increase of?T₂T₁ will inevitably lead to the inhibition of the internal conversion of T exictons,the generation of the T₂ excitons can directly depend on the carrier recombination in electroluminescence,while in photoluminescence it needed to be derived from S₁ or S₂ conversion,then it means that for the photogenerated excitons in the material system with large?T₂T₁:either adopted the S₁ radiation transition to emit light,which was conducive to improving the luminous efficiency;or stay at the high-energy triplet state?T₂,etc.?,thus provided a greater possibility for the electron transfer process?photochemical reaction?to occur.The specific research results are as follows:In chapter 7,based on the T₂T₁ distribution characteristics of TNZ,we further strengthened the intramolecular CT effect to weaken the excited state exciton binding energy and improved the conversion efficiency between singlet and triplet states,and through electron-rich donors and anion reducibility to enhance such electron transfer process,thus obtaining a class of free radical ROS with strong oxidability.As photosensitizers,the PDT efficiency was significantly improved,which was significantly better than the common traditional PDT treatment that relies on singlet oxygen at the cellular and animal levels.In chapter 8,based on the understanding of the large?T₂T₁ construction rule and its potential photoluminescence applications,we constructed phenazine moiety with greater spatial crossover characteristics and electron-donating capability as research object.Preliminary research showed that the photophysical processes of phenazine-based derivatives were very special,and their excited state composition and energy level distribution were relatively affected by substituent.Combining theoretical calculation and spectral comparison of these derivatives,its AIE/AEE mechanism may not only include RIR mechanism.Furthermore,based on its illuminated fluorescent imaging mechanism,selective imaging of intracellular lipid droplets has achieved good results.In summary,this doctoral thesis focuses on the energy-level distribution of luminescent materials based on different application orientations,starting from the different excitons generation pathways and conversion processes in the EL and PL processes.Based on the adjustment of skeleton structure and subsituents for anthracene-like derivates,it is preferable to produce a material system with both luminous efficiency and controllable excitons.Not only enriched the structural system of such materials?NTZ,PZ?,but also initially obtained the energy level adjustment rule for materials in the electroluminescence and photoluminescence applications?cross dipole-large T₂T₁?.The development of these new AIE structures and the understanding of the regulation of energy level distribution have important reference significance for the subsequent function-oriented material design and development.