Theoretical Study Of The Emission Mechanism Of Organic Conjugated Radical Molecules

Recently,the radical molecules were suggested as the new type of emitters in organic light emitting diodes while their emission mechanism is not clear.In particular,the internal quantum efficiency of the emissive doublet excitons can achieve 100% in the experimental study while the theoretical prediction is no more than 50%.Herein,Two different conjugated organic radicals have been studied to solve these problems mentioned above.First of all,time dependent density functional theory calculations are performed to unravel the emission mechanism of(4-Ncarbazolyl-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)methyl(TTM-1Cz).Our study indicates the electroluminescent of TTM-1Cz is arisen from the first excited state that is generated by injecting one hole into highest doubly occupied molecule orbital(HDMO)and one electron into the singly occupied molecule orbital(SOMO).Natural transition orbital analysis indicates that D1 is a typical charge transfer state.Based on the disclosed emission mechanism,the puzzle of 100% formation ratio of the emissive doublet exciton in the experimental study are revealed.Moreover,the molecular design demonstrates that simply decoration of the TTM-1Cz with electron-withdrawing groups could tune the HDMO-SOMO gap and extend the emission light to yellow and green region while the chemical stability of derivatives should not be affected.Then,time dependent density functional theory calculations are performed to(Ncarbazolyl)bis(2,4,6-tirchlorophenyl)-methyl radical(CzBTM).Our study indicates the ground state structures of CzBTM in different environment are similar.Molecular orbital analysis indicates that the first,twelfth and thirteenth excited states are local excitation state,the third excited state is a typical charge transfer state and the ninth excited state is a mixed excitation state.