Synthesis Of Methyl Radicals And Study On Luminescence And Stability

Stable radicals,in which there is one or several unpaired electrons on them,have been widely investigated since 1990,due to their unique properties such aslow redoxpotential,narrow bandgap and paramagnetism.For the neutral free monoradical,the presence of one unpaired electron makes it different from the closed-shell organic molecules.Because there is only one electron in singly occupied molecular orbital(SOMO),the transition between SOMO and LUMO(lowest unoccupied molecular orbital)and/or between highest occupied molecular orbital(HOMO)and SOMO aretotally spin allowed.These makes the doublet emissionof the radical quite different fromthe singlet emission(fluorescence)and triplet emission(phosphorescence)of the closed shell molecules.Thus,the luminescent properties deserve massive study,not only from the view of science,but also it paves a new wayfor the OLED industry which is of great significances.However,at present,very few luminescentfree radicals havebeen reported,and most of themshowedlow luminescent efficiency and poor stability.Therefore,it shouldbe resolved thathow to improve the stability and luminescent efficiencyof free radicals.In general,conjugate effect can increase the stability and luminescence efficiency of organic emissive molecule.Thus,in this thesis,we enhance the conjugate effect of the radicals by connecting different donor and accepter units to them,in order to investigatethe stability and luminescent efficiency of radical through conjugation effect.Firstly,we synthesized two radicals,PTM-DPK and PTM-CPE,in which the PTM radical core was connected totwo electron withdrawing groups,DPK and CPE respectively.We studied the influence of the enhanced conjugation by connecting electron withdrawing groupson the stability and luminescent properties of the radical.The density functional theory(DFT)calculationsdemonstrated that the conjugation effect could be enhanced by connecting with poor electron moiety.The results of solvation effect showed that both of the two radicals are similar with PTM.The fluorescentspectra did not change in solvent of different polarities,indicating that the fluorescent emission was originated from the localized ?-?transition.comparing with PTM,the photoluminescent quantum yield(PLQY)of PTM-DPK,PTM-CPE has no obvious change,suggestingthat connecting poor-electron group to the radical core cannotenhance the luminescent properties of the radical,though it can enhance the conjugation effect.Through the photo-stability study,we found the stability of PTM-DPK and PTM-CPE was higher than that of PTM,whichcan be attributed to the conjugation effect.Theelectrochemical results demonstrated that the series of PTM radicals all possess high oxidation potential.It seems that the poor electron group had little influence onthe oxidation potential,which can be owing tothe steric hindrance effect from three phenyl and chlorine atoms of ortho-position.Secondly,we studiedthe influence of rich electron group on the PLQYs and stability of radicals.We synthesized three radicals,PTM-PSM,PTM-CzN,PTM-PDCz,in which the radical core PTM was connected tothree electron-donating groups with different electron donating ability.The DFT calculations revealed that the rich electron groupcannot enhancetheconjugation of the molecules.The solvation effect studies demonstrated that the luminescence of the radicals it originated from the charge transfer emission between the donor and the PTM radical core.DFT resultsindicated that the charge transfer transitions are from the HOMO of the rich electron group to the SOMO of the PTM radical core.Compared with that of PTM,the PLQYs of PTM-PSM,PTM-CzN,and PTM-PDCz are greatly improved.For example,the PLQYs of PTM-CzN was 0.54,whichis 34 times higher than that of PTM(0.016).In addition,the stability of the three radicals were also enhanced significantly.The above results show that connecting electron-donating group to the radical should be a useful strategy for developing radicals with high PLQYs and high stability.Interestingly,we achieved a intriguing electronic structure for PTM-PSM,PTM-CzN,PTM-PDCz,that is the SOMO-HOMO levels are conversed.The SOMO level keeps unchanged because of the space steric effect,so the HOMO(mostly from the donor group)level can be higher that the SOMO.The energy gap of SOMO and HOMO is different with different donors.For example,the SOMO of PTM-PSM is belowthe HOMO of it,the SOMO of PTM-CzN isbelow the HOMO-1 of it,and the SOMO of PTM-PDCz was below the HOMO-3.We found that the SOMO/HOMO energy gap bigger,the stability of radicals higher.Strictly speaking,the HOMO-SOMO conversionviolates the anti-aufbau principleand it is scarce in organic molecules.The studying on conversion of HOMO-SOMO energy level can deepen ourunderstanding of organic molecular.Furthermore,it can be afeasible strategy for designing radicals with high PLQYS and high stability.