Synthesis, Properties And Applications Of Efficient And Novel Luminescent Multi-phenyl Methyl Radicals

In 1900,Moses Gomberg reported the first triphenylmethyl radical with relative stability,this was the first time that radicals had been proved to be a kind of chemical entities by scientists.Thus,the experimental study of radicals began.Through years of efforts by scientists,now,there have been many classes of organic radicals which are stable enough to be isolated and stored,such as carbon-center radicals,nitrogen-center radicals,oxygen-center radicals and so on.Organic radicals possess attractive optical,electrical and magnetic properties due to their unique electronic structures,and thus can be applied in many different fields,such as spintronics,non-liner optics,molecular magnet,electron paramagnetic resonance(EPR)imaging,organic field effect transistor(OFET),organic light-emitting diode(OLED),dynamical nuclear polarization(DNP)and so on.So far,the research on luminescent properties of radicals has attracted more and more attention.That is because radicals have one unpaired electron,both the ground state and the lowest excited state of radicals would be doublet,the transition between doublet states won't be spin-forbidden.Such unique luminescent properties enable radicals to circumvent the problem of utilization of triplet excitons effectively when applied in OLEDs,thus the theoretical upper limit of internal quantum efficiency(IQE)of OLEDs can be increased to 100%.Based on this,our group first successfully apply luminescent radicals in OLEDs in 2014,and through the device optimization in the following research,it is proved that the doublet exciton formation ratio can be close to 100% in radical-based OLEDs.This result is a major breakthrough in the field of OLED,which show the abilities of radicals to avoid the problem of the utilization of triplet excitons in traditional OLED devices,and reflects the important significance of the application of luminescent radicals in OLED.However,whether in the aspect of radical-based OLED with excellent performance or in the aspect of radical materials with extensive application prospect,there are still some important problems need to be solved.First,although radical-based OLEDs have shown excellent performance and a breakthrough compared to traditional fluorescent OLEDs,their IQEs are far from 100% which makes it doubtful whether IQE of radical-based OLEDs can truly achieve 100%.Second,although there are many classes of stable organic radicals,few of them possess room-temperature luminescent properties.So far,only derivatives of tris(2,4,6-trichlorophenyl)methyl radical(TTM)and perchlorotriphenylmethyl radical(PTM)possess room-temperature luminescent properties.This has greatly limited the further study on the luminescent properties and applications of organic luminescent radicals.Therefore,in this paper the following research work has been carried out around the above two important issues.1.Combined with experience of previous research,in order to get radical materials with excellent luminescent properties,two asymmetric donor-radical(D-R)type luminescent radicals,TTM-3NCz and TTM-3PCz,were designed and synthesized through introducing two different donors of carbazole derivatives into TTM.The introduction of the donors greatly improved the photoluminescence efficiency(PLQE)and stability of the two luminescent radicals.TTM-3NCz and TTM-3PCz showed deepred emission in different solutions,and the absolute PLQEs of their toluene solution were 49% and 46%,respectively.The fluorescence intensity of TTM-3NCz did not decrease significantly during the photostability measurements.At the same time,TTM-3NCz and TTM-3PCz doped thin films(3.0 wt%)in 4,4'-bis(9H-carbazole-9-yl)biphenyl(CBP)have absolute PLQEs of approximately 90% and 61% respectively.OLEDs which have emitting materials layers based on those doped films have maximum external quantum efficiencies(EQEs)of 27% and 17%.These results indicate that the OLED based on TTM-3NCz has an IQE of nearly 100%.This is the leading level of deep-red/near-infrared light-emitting diodes which have been reported so far,and also confirms that using radicals' doublet emission can indeed increase the theoretical upper limit of IQE of OLED to 100%.Through spectroscopy measurements and theoretical calculations,we also studied and analyzed the photophysical processes of luminescence of radicals.The results show that both of the radicals have obvious charge-transfer(CT)excited states,and the doublet emission comes from the transition of electrons from SOMO to HOMO.Subsequently,the formation process of the doublet excited state under the condition of electric excitation was discussed,and a reasonable hypothesis was proposed that electrons and holes are injected into the radicals' SOMO and HOMO respectively.2.In order to extend the classes of the luminescent radicals,we have successfully achieved the breakthrough on TTM skeleton through special molecular and synthesis route design,and obtained a stable organic radical with room-temperature luminescent properties,CzBTM.The pure solid sample of CzBTM do not have luminescent properties,but in solvents of different polarity,CzBTM show deep-red to near-infrared emission.Among them,its emission wavelength in the cyclohexane solvent was 697 nm,and the absolute PLQE of this solution was 2.0%.At the same time,compared with TTM,the thermal and photo stability of CzBTM radical were greatly improved through the introduction of carbazole moiety.CzBTM(5.0 wt%)doped film in CBP was used as the emitting materials layer to fabricate CzBTM radical based OLED.The device exhibited deep-red emission with a wavelength of 700 nm and a maximum EQE of 0.66%.By analyzing the performance of the device and considering that the absolute PLQE of the emitting materials layer is 5.0%,we calculated that the formation ratio of the doublet excitons in this device is about 44-66%.This result indicates that the problem of utilization of triplet excitons can also be circumvent in CzBTM radical based OLED.At present,CzBTM is the only luminescent radical that has been proved to be applied in OLED through vacuum evaporation except TTM derivatives,and it has a great potential for chemical modification.In the future,it is expected to develop a new class of stable luminescent biphenylmethyl radicals based on CzBTM.3.In the second part of the research work,we found that the radical CzBTM was finally generated directly through a one-step reaction,and the subsequent research results showed that the reaction could be effectively extended to a variety of Ncontaining heterocyclic rings similar to carbazole,and a variety of biphenylmethyl radicals similar to CzBTM were synthesized.Therefore,in order to explore the key process of radical formation in this reaction,we took the synthesis reaction of CzBTM as an example to carry out the study.The reaction process and main products of the reaction were analyzed by optimizing the reaction conditions and using EPR,mass spectrometry,nuclear magnetic resonance(NMR),crystal structure analysis and other methods.Finally,we successfully decomposed the seemingly one-step radical generation reaction into multiple processes,and successfully isolated the key substances related to radical formation,HCzBTM,and found the key step of radical formation,that is the oxidation of carbanion corresponding to the H-containing compound HCzBTM.