The Effects Of Introducing Halogen Atoms To Biphenylmethyl Radical On Photostability,Photophysical And Electroluminescent Properties

For a long time,radicals,as a special species,were easy to be attacked by oxygen and water due to their unpaired electron in the molecular structure.And because of this,it is impossible to connect radicals to the word‘stability'.This concept was not broken until the report of triphenylmethyl radical in 1900.Unlike radicals researchers studied before,the radical center of triphenylmethyl radical is surrounded by three phenyl rings,and the molecular configuration is twisted.Since then,a lot of‘persistent radicals'which were similar with triphenylmethyl radical,together with the‘stable radicals'which could be obtained separated and were stable at room temperature were designed and synthesized.By the way,theses radicals are all organic radicals.In recent years,stable emissive organic radicals applied in organic light-emitting didoes?OLEDs?have obtained much attention.Due to the unpaired electron,organic luminescent radicals at ground the excited states are both doublet,the excition transition to the ground state is totally spin-allowed.Consequentaly,if treat the emissive organic radicals as luminescent layer of OLED,not only the internal quantum efficiency?IQE?could achieve the theoretical maximum value 100%,but also the utilization of triplet excitons could be avoided.So far,however,the species and numbers of reported organic luminescent radials are very limited and confined to the methyl radical which the carbon atom is the radical center.One of the reasons which leads to the rare emissive radical is that how molecular structure could affect the photostability and photophysical properties of radicals were not studied adequately.Recently,our group has reported a novel biphenylmethyl radical?N-carbazolyl?-bis?2,4,6-trichlorophenyl?methyl?CzBTM?.CzBTM is a type of electron donor-accept luminescent radical.When excited from ground state to the excited state,it underdoes an intramolecular charge-transfer process from carbazole moiety to the radical center.Till now,only one derivative of CzBTM has been reported,and how molecular structure could affect the photostability and photophysical properties of it were not studied enough.Based on this problem,this paper has down the following works:Based on the latest reported luminescent radical CzBTM,three new radicals?3,6-difluoro-N-carbazoly?-bis?2,4,6-trichlorophenyl?methyl?F₂CzBTM?,?3,6-dichloro-N-carbazoly?-bis?2,4,6-trichlorophenyl?methyl?Cl₂CzBTM?and?3,6-dibromo-N-carbazoly?-bis?2,4,6-trichlorophenyl?methyl?Br₂CzBTM?were synthesized and characterized.After a series of research,we found that by introducing Cl and Br both of which electronegativity are stronger than H into CzBTM,the electron donating ability of substituted carbazole moieties is decreased,leading to the increase of energy gap of Cl₂CzBTM and Br₂CzBTM,then the blue-shift of fluorescence spectra and increase of photoluminescence quantum yield?PLQY?in cyclohexane solution.By introducing F to CzBTM,the torsion angle between carbazole unit and the radical center has a minimum value with respect to other three radicals.This result makes F₂CzBTM undergoes an easier charge transfer process from the ground state to the excited state in spite of the weakest electron donating ability of F substituted carbazole unit.Thus,F₂CzBTM has a same energy gap with CzBTM,then the same fluorescence spectrum.Meanwhile,there is strong vibrational coupling between carbazole moiety and radical center in F₂CzBTM owing to the minimum value of torsion angle between this two unit,leading to an obviously increase of non-radiative transition rate,then the decreased PLQY in cyclohexane solution.In addition,two phenyl rings in F₂CzBTM twisted in a relatively large extent,the steric shielding effect becomes stronger compared to other three radical,leading to a greatly promotion of photostability of F₂CzBTM.And the OLED using Cl₂CzBTM and Br₂CzBTM as emitters were fabricated.The electroluminescence spectra of them are blue-shifted comparing with that of CzBTM,and the maximum external quantum yield?EQE?of the devices are also enhanced.Based on these results,it can be verified that the changed molecular structure which caused by introducing the halogen atoms into CzBTM,could affect the photostability,photophysical and electroluminescent properties of the biphenylmethyl radical in some extent.And it could be concluded that the electronegativity of the outside group and the molecular configuration of electron donor-accept luminescent radical affect its photophysical property at the same time.