Design,Synthesis And Study On Photophysical Properties Of High-efficiency Triphenylmethyl Radicals And Their Application On OLED

Organic radicals,known as the intermediates of chemical reaction and metabolism,have long attracted tremendous attentions in the study of organic reaction mechanism,initiating and catalyzing of polymerization reaction,biochemistry,etc.And the organic radicals are impressed with high reactivity,instability,reacting easily with oxygen or water,etc.However,since the first report of relatively stable neutral organic racial,triphenylmethyl radical,more and more“persistent radical”and even“stable radical”,which can be stored several years at the ambient condition without degradation,are obtained.The unique single electron of the stable neutral radical confers it the potential application of organic magnetism,spintronics,polarization optics and so on.Recently,organic radicals also find their applications in the field of optoelectronics and devices,such as organic field effect transistor?OFET?,carriers transport,organic light emitting diode?OLED?,etc.The previous works of our group found that for luminescent neutral organic radicals which only have one single electron,there is just one single electron for both the ground and excited states when they are electric excited.So the ground and excited states are both doublet,which means the doublet excitons can all return to the ground state by deexcitation.In other words,the theoretical maximum internal quantum efficiency?IQE?of OLEDs which utilize luminescent radicals as emitting layer can reach as high as 100%.This special doublet-emission mechanism is distinguished from the traditional methods for improving the upper limit of IQE of OLEDs,such as phosphorescence,thermal active delay fluorescence?TADF?and triplet-triplet annihilation?TTA?,whose attentions focus on the utilization of triplet excitons.And this mechanism provides a new and simple route to achieve high efficiency OLEDs.However,there are still several serious problems for the application of luminescent organic radicals in OLEDs.The greatest challenge is the rare species of luminescent radicals.Although certain amounts of stable neutral organic radicals are reported,such as nitroxides,hydrazyl radicals and aminyl radicals,hardly any of them shows luminescence at the ambient condition.As far as we know,triaryl methyl radiclas are the only neutral organic radical species which are both stable and luminescent at the room temperature.As a result,improving fluorescence efficiencies of the limited variety of radicals is the most immediate and simplest solution in this case.Perchlorotriphenylmethyl?PTM?and tris?2,4,6-trichlorophenyl?-methyl?TTM?are the most extensively researched and easily chemical modificated radicals in the triarylmethyl radicals.Therefore this paper will do the following works concentrating on this two radical species:1.By Suzuki coupling reactions,electron-donating triphenylamine?TPA?and carbazole derivatives?PCz&3PCz?were incorporated to the PTM and TTM radicals to fabricate three substituted donor-acceptor?D-A?type PTM radicals,PTM-PCz,PTM-3PCz and PTM-TPA,as well as two D-A type substituted TTM radicals,TTM-PCz and TTM-3PCz.As a result,we found the incorporation of electron-donation groups observably enhanced the photoluminescent quantum efficiencies?PLQEs?as well as the photostabilities of the PTM and TTM radicals in nonpolar solvents.It should be noted that PTM-3PCz possessed the highest PLQE of56.6%with the deep-red emission of 679 nm in the cyclohexane solution.In order to explore the substituent effect on the PLQEs of triphenyl-methyl?TPM?radicals,theoretical chemical calculation and optic measurement were executed and the photophysical parameters were analyzed detailedly.Through comparison of the parameters of the substituted radcials,it is confirmed that increased rigidity of substituent groups and smaller dihedral angle between the D-A system are the key factors to realize high luminescent efficiency radicals.This work provides a novel strategy to design and synthesize high efficiency D-A type organic radicals.2.In order to obtain deep-red to near-infrared?NIR?emitted organic radicals,bicarbazoles?BiCz?and tricarbazoles?TCz?were incorporated to the TTM radical to fabricate three substituted TTM radicals,TTM-1BiCz,TTM-2BiCz and TTM-TCz.The incorporation of electron-donation groups also observably enhanced the PLQEs as well as the photostabilities of the TTM radicals in nonpolar solvents.The results showed whether incorporation of multiple electron-donating groups?TTM-2BiCz?or enhancing the electron-donating abilities of the substituted groups?TTM-TCz?,their emission wavelengths were all red-shifted than TTM-1BiCz.According to the“energy gap”law,the longer emission wavelength means a narrower energy gap between the ground state and excited state,which will lead to more available routes for non-radiative transition by intersystem crossing and result in a lower PLQE.However,the experimental results showed that although the PLQE of TTM-2BiCz?0.11?was smaller than that of TTM-1BiCz?0.17?,the PLQE of TTM-TCz?0.26?was higher than the latter.In other words,TTM-TCz achieved bathochromic-shift of the emission along with higher efficiency.To explore the reasons of this phenomenon,we also compared the theoretical chemical calculations and photophysical parameters of the related radicals.And we found the key factor was that the non-radiative transition rate(k_nr)of TTM-TCz was only about half of the value of TTM-BiCz,which is caused by the greater rotation restriction of the outer carbazole groups in TTM-TCz.Consequently,incorporation of substituent groups with strong electron donating ability as well as restricted the rotation of outer groups is the important factor to realize NIR-emitted radicals with high luminescent efficiencies.3.Except for the higher PLQEs,the application of organic radicals in OLED need to face another key challenge:the emission wavelength coverage of radical is narrower than normal fluorescent organic materials.Up to now,the electroluminescent?EL?colors of the reported radical based OLEDs are limited between orange-red?600 nm?to deep-red?710 nm?,Due to the restriction of molecular designing and structure of TPM radicals,it seems very hard to realize the blue-shifted emission along with high stability.On the contrary,the NIR emitted radicals seems more attractive.In the previous works,the more chlorines of PTM radicals or the incorporation of high molecular weight multi-carbazole groups all hider their application on evaporation.Therefore,we incorporated small and strong electron-donating groups,TPA,to the TTM radical to fabricate a thermally stable NIR emitted radical TTM-TPA.The radical was doped into the 4,4-bis?carbazol-9-yl?biphenyl?CBP?as emitted layer.EQEs of the radical based NIR OLED reached 2.6%with maximum EL wavelength of 810 nm.It is a remarkable fact that the EQE of TTM-TPA based OLED is almost among all the NIR fluorescent OLED with maximum EL wavelength over780 nm.So this work also provides new routes to design and fabricate high-efficiency NIR OLEDs.