Highly Efficient Stable Luminescent Carbon Radicals and Their Multi-Functionalization
Author:A Li Mu A Bo Du Re He Man Alim Abdurahma
Stable organic luminescent radicals are a special class of compounds in the family of radicals,which have many special properties,such as optical,electrical,magnetic and so on,and belong to the category of multi-functional materials.They have broad application prospects in the fields of organic magnetism,spintronics,organic optoelectronics,fluorescent probes and chemical sensing.In recent years,organic lightemitting devices(OLEDs)using luminescent radicals as emission layers have attracted much attention because they effectively avoid the use of triplet excitons.The emission of radical comes from the radiation decay of doublet exciton.Due to the absence of spin forbidden transition for doublet exciton,the upper limit of the internal quantum efficiency(IQE)of radical based OLEDs can theoretically reach up to 100%.Recently,the external quantum efficiency(EQE)of a deep red OLED device using a radical as the light-emitting layer has reached 27%,which is the highest reported value in the deep red OLEDs.In addition to achieving good results in the field of OLEDs,luminescent radicals have also shown broad application potentials in other research fields such as fluorescent probes,magnetoluminescence and circularly polarized luminescence,etc.Up to now,there are many types of stable radicals that have been developed,but a few of them are emissive.Triarylmethyl-based carbon radicals are currently the only stable radical species capable of achieving room temperature luminescence.However,there are still some problems remained in this kind of luminescent radicals.First,the emission color of luminescent radicals is mainly concentrated in the orange,deep red and near-infrared regions,lacking the pure red emission,which limits its commercial application in the field of red OLEDs.Secondly,most of the luminescent radicals have low luminescence efficiency.In addition,the aggregation-caused quenching(ACQ)phenomenon of luminescent radicals is severe,leading to their non-luminescence in the solid state.This problem limits radicals’ applications in high-tech fields such as fluorescent sensors,bioimaging and tracing biological activity.Therefore,this paper has carried out the following aspects around the above issues:1.We designed and synthesized a stable luminescent biphenylmethyl radical PyIDBTM using β-carboline as weak donor.PyID-BTM has a pure red emission of 664 nm in cyclohexane solution,which is 33 nm blue-shifted compared with CzBTM,a biphenylmethyl radical carbazole as donor.It is noteworthy that the photoluminescence efficiency(?)of PyID-BTM is 19.5%,which is nearly ten times higher than that of CzBTM(2%).It was found that the preferable luminescence properties of PyID-BTM were attributed to the higher molar extinction coefficient and the slower internal conversion resulting from a β-carboline substituent instead of a carbazole moiety.We also compared the internal conversion rates of the two luminescent radicals for the first time using MOMAP(Molecular Materials Property Prediction Package)program.PyID-BTM has good paramagnetic,thermal,electrochemical and photostability.In addition,PyID-BTM has good proton responsive properties,which allows its potential application in pH sensing and metal ion detection.We doped PyID-BTM into CBP as luminescent layer,and realized a pure red OLED device with the maximum EQE of 2.8%,and the formation ratio of doublet excitons of the device was up to 70%.This study provides a new way to design multifunctional and efficient luminescent biphenylmethyl radicals.2.In order to adjust the luminescence color while improving the free radical luminescence efficiency,we chose four carboline groups(α,β,γ and δ-carboline)with weak electron donating properties,to modify the tris(2,4,6-trichlorophenyl)methyl radical(TTM)obtained four monocarboline-substituted TTM-based radicals αPyIDTTM,βPyID-TTM,γPyID-TTM,δPyID-TTM and two kinds of bicarboline-substituted TTM-based radicals 2αPyID-TTM and 2δPyID-TTM.These six radicals exhibit pure red emission at 611-645 nm in chloroform solution,which is 45-77 nm red-shifted compared with TTM(566 nm)and 44-76 nm blue-shifted compared with TTM-1Cz(687 nm).The photoluminescence efficiencies of the six radicals in chloroform solution were: αPyID-TTM(91%),βPyID-TTM(89%),γPyID-TTM(32%),δPyID-TTM(99%),2αPyID-TTM(81%)and 2δPyID-TTM(89%)were 12 to 38 times TTM(? = 2.6%)and 6 to 20 times TTM-1Cz(? = 5%),respectively.It is found that the radiation transition rates of these six radicals are much larger than their non-radiative transition rates,which is exactly the opposite of TTM and TTM-1Cz.With the increase of solvent polarity,the emission wavelengths of the six radicals all have significant red-shift,but the displacement is only about half of TTM-1Cz,and the luminescence efficiency of the former is less affected by solvent polarity,and has been maintained at a higher level.We systematically studied the luminescence efficiency,fluorescence lifetime,radiation and non-radiation transition rates and Lippert-Mataga solvation model of six carboline-substituted TTM radicals in various solvents,compared with TTM and TTM-1Cz.And we concluded the following two conclusions:(1)The excited state of carboline substituted TTM radicals is a hybrid state formed by local(LE)state and charge transfer(CT)state.(2)Nitrogen atom on the carboline may accelerate the relaxation of molecules,thereby decreasing the rate of non-radiative transition while increasing the rate of radiation transition of the radical molecules.These six carboline-substituted TTM-based radicals have good proton responsive properties and are expected to be used in chemical sensing,optical recording and other fields.In addition,they also have good thermal,electrochemical and photostability.The OLED devices based on four monocarboline-substituted TTM radicals achieved pure red emission with CIE coordinates of αPyID-TTM(0.66,0.34),βPyID-TTM(0.67,0.32),γPyID-TTM(0.61,0.37)and δPyID-TTM(0.67,0.33).Among them,the βPyIDTTM based device achieved the maximum EQE of 12.2%,which is a high level in pure red OLED devices.3.In addition to improving the luminous efficiency of radicals and regulating the color of emission,how to solve the ACQ phenomenon of radicals also has important research significance.At present,solving the ACQ issue of luminescent radicals by chemical modification has not yet been achieved.Hence,we demonstrated the design strategy for luminescent radical polymers,in which the polymer backbone(polystyrene,PS)is decorated with randomly distributed pendants of luminescent radicals(CzTTM),and firstly synthesized the solid-state luminescent radical polymer PS-CzTTM.PSCzTTM exhibits deep red emission in cyclohexane solution and spin-coated film,with photoluminescence efficiencies of 37.5% and 24.4%,respectively.PS-CzTTM also has good paramagnetic,thermal and photostability.In addition to this,the non-doped device of PS-CzTTM prepared by spin coating method showed deep red emission with a maximum EQE of 0.95%.Solid-state-luminescent radical polymers not only have potential applications in the field of OLED,but also as a new solid luminescent material,it is expected to be applied in the fields of fluorescence sensing,bioimaging and tracing biological activity and other fields.It provides a meaningful exploration to promote the multi-functionalization of luminescent radicals.