| Doublet emission materials are a unique kind of open shell molecular systems.There is an unpaired single electron in the outermost orbital of the molecule,and the ground state and the first excited state electronic structures are doublet.Electron transfers from the first-excited state to the ground state in a radiative way,and this type of fluorescence is called doublet fluorescence.The existence of single electron determines that doublet emission materials possess unique optical,electronic,and magnetic properties,which makes them ideal multifunctional materials.In terms of luminescence form,the theoretical internal quantum efficiency(IQE)of doublet emission materials can reach up to 100%because the electron transition is not limited by the spin condition,which indicates that they possess advantages in organic electroluminescent devices(OLED).For normal closed-shell luminescent molecules,singlet excitons and triplet excitons can be generated in a ratio of 1:3under the electroexcitation.According to spin statistics,only singlet excitons can generate radiative transitions,while triplet excitons deactivate in a non-radiative way.Therefore,the maximum theoretical IQE of the device is only 25%.To solve this problem,several schemes have been developed,including phosphorescent materials,triplet annihilation luminescence(TTA)materials,thermal activation delayed fluorescence(TADF)materials,and local hybrid charge transfer luminescence(HLCT)materials,etc.However,these materials are still concentrated in the closed shell molecular system and always face the problem of utilizing non-emission triplet excitons.Different from the above material system,in 2015,our research group reported the first OLED device based on doublet emission materials,bypassing the problem of triplet excitons harvest in the closed-shell molecular system,providing a new way to realize high-efficiency OLED devices.Up to now,the research on doublet emission materials and OLED devices are still in the initial stage,and there still exist many problems.First,the doublet emission material systems are still rare,which mainly include stable luminescent radicals,rare earth metal cerium(Ce3+)complex,and transition metal complexes with low spin d5electronic structure.Early research works on doublet emission OLED are mainly centered on triphenylmethyl stable luminescent radical(TTM)systems,but the species of TTM radicals are relatively few.It is of great importance to improve the OLEDs performance by introducing functional groups to TTM to improve the photophysical properties.Secondly,transition metal complexes with low spin d5electronic structure,such as luminescent Fe3+complexes,theoretically has metal-ligand charge transfer(MLCT)doublet emission,but relevant OLED have not been reported.Considering that iron is the richest transition metal element in the earth crust,verifying its feasibility in OLED application will greatly reduce the cost of OLED devices,which is of great significance.In addition,although the doublet emission molecules are ideal multifunctional materials,while relevant applications are still narrow.Here,we consider that the luminesce of TTM radicals mainly locate in the red region,which can be used in bioluminescence imaging.Besides,it has been reported that triphenylmethyl radicals can promote the generation of reactive oxygen species(ROS)under light excitation,which is expected to achieve photodynamic therapy and fluorescence imaging at the same time.Therefore,TTM radicals are anticipated to play a key role in the field of life science theoretically.However,as organic molecules,the non-water-soluble characteristic limit their application in the biological direction,it is urgent to resolve this problem.In view of the above problems,we carried out the following works:1.First,the TTM-DACz luminescent radical was successfully synthesized by introducing a carbazole derivative,a weak electron donor 1,5-diazacarbazole(DACz)to the TTM radical.A series of photophysical performance tests were carried out.The EPR test showed that TTM-DACz has typical carbon radical resonance absorption signal,and the UV-Vis absorption spectrum also showed two typical characteristic absorption peaks of TTM series radicals.TTM-DACz exhibits orange-red light emission in dichloromethane solution,and the fluorescence peak is located at 605 nm,which is red-shifted by 37 nm compared with the emission of TTM in dichloromethane solution.Its photoluminescence efficiency in dichloromethane solution is 57.0%,which is nearly 30 times higher than that of TTM.The solvation test results of fluorescence emission and fluorescence lifetime showed that the luminescence of TTM-DACz belongs to CT state luminescence,while TTM belongs to localized state luminescence.We analyzed its electronic excitation process by time-dependent density functional theory(TD-DFT),which further confirms this conclusion.The results of photostability test showed that after the introduction of DACz group,the photostability of TTM radical was increased about 4 times.The results of thermogravimetric analysis show that TTM-DACz has good thermal stability and meets the prerequisites for vacuum thermal evaporation.We confirmed the frontier molecular orbital energy leveles of TTM-DACz through cyclic voltammetry tests combined with the optical band gap.Finally,we selected TPB as the precursor to prepare OLED devices with different doping concentrations.By optimizing the device structure,the EQE of the TTM-DACz-doped OLED device can reach up to 10.6%,and the doublet exciton utilization rate was nearly 62%,which exceeds the theoretical upper limit of 25%exciton utilization of traditional closed-shell molecules.This work provides some help for the design and synthesis of TTM series radicals with better physiochemical properties and further improving the performance of OLED devices.2.To develop novel doublet emission materials for OLED application,we synthesized the iron complex[Fe(phtmeimb)2]PF6which possess a doublet metal-to-ligand charge transfer(2MLCT)fluorescence.And we test the thermostability and electrochemical stability,TGA result showed that when the mass loss reaches to 5%,the thermal decomposition temperature of[Fe(phtmeimb)2]PF6is343°C,and there is no need to worry about material deterioration during thermal evaporation.Cyclic voltammetry(CV)tests showed that[Fe(phtmeimb)2]PF6has good electrochemical stability,and the CV curve remains unchanged after multiple scans.Then we prepared the OLED devices by doping[Fe(phtmeimb)2]PF6into CBP matrix and the electroluminescence peak located at 600 nm.By optimizing the device structure,the maximum brightness of the device exceeded 3000cd/m2,the CIE chromaticity coordinates locate at(0.58,0.40),and the maximum EQE was 0.67%.After calculation,IQE of the optimized device was close to 100%.The experimental results showed that the transition metal material system with 2MLCT doublet emission is feasible for the preparation of high-efficiency OLED devices.This work provides substantial help for subsequent development of high-efficiency doublet emission transition metal complexes and their application in OLED.3.In order to further expand the application of doublet emission materials,we selected TTM radical as the core,by introducing three imidazole groups in its periphery and further reacting with benzyl bromide,we successfully synthesized water-soluble radical TTM-3IMB,open the door for its application in biology.TTM-3IMB exhibits orange-red emission in aqueous solution,the fluorescence peak is located at 580 nm,the photoluminescence quantum yield in aqueous solution is2.0%,and the fluorescence lifetime is 33 ns.In order to tune the photophysical properties of TTM-3IMB,we constructed the corresponding supramolecular self-assembly system by introducing cucurbituril[7](CB[7])and cucurbituril[8](CB[8]),and studied the photophysical properties before and after self-assembly progress.After TTM-3IMB was combined with CB[7]and CB[8],the absorption spectrum and emission spectrum did not change significantly,but the fluorescence lifetime was prolonged,and the fluorescence quantum yield was improved to varying degrees.In addition,we found that the combination with CB[8]can improve the photostability of TTM-3IMB,while CB[7]does not have this effect.The results of biochemical experiments showed that the cytotoxicity of TTM-3IMB is weak,which is suitable for cell imaging ability and can be used for mitochondrial organelle localization.The combination of TTM-3IMB and CB[7]can promote the rapid generation of reactive oxygen species in cells under light excitation,which means that TTM-3IMB and CB[7]supramolecular assembly has the ability for photodynamic therapy,while the pure radical system requires a higher concentration to achieve the same effect.This work provides a novel approach on how to design and synthesize water-soluble luminescent radicals,and lays a foundation for their applications in cell fluorescence imaging and photodynamic therapy. |
PDF Full Text Request