Charge Transported And Optoelectronic Properties Of Stilbene/Fluorene Hybrid Derivatives: A Density Functional Theory Study

Since the reports by C.W.Tang,π-conjugated organic molecules have been attracted much attention as light emitters in organic light-emitting diodes(OLEDs) because of their potential application in large-area flat panel displays.A series of organic molecules or oligomers based on the fluorine,carbazole,quinoline and pyrrole derivatives have been synthesized.Accordingly,a better understanding of charge transported,photophysical and optoelectronic properties of various electroluminescent(EL) organic materials was required by means of theoretical calculation.Balance of charge carrier(electron and hole) injection and transport is crucial to obtain highly effective OLEDs.However,it is reported that most of the electroluminescentπ-conjugated organic molecules tend to be p-type semiconductors that are in favor of hole transport rather than electron transport which give rise to the waste of holes.Such unbalanced charge transport is one of the main reasons for the poor quantum efficiency.As is known,The 3,7-Bis(N,N-diphenylamino)-5, 5-spirofluorenyl-5H-dibbenzo[a,d]cyclo heptene(la) and 3,7-Bis[N,N-di(4-methoxyphenyl)amino] -5,5-spirofluorenyl-5H-dibenzo[a,d]cycloheptene(1b) have been experimentally synthesized as the sky-blue fluorescent OLED materials which bear high EL brightness and working efficiencies[12].For the sake of further balancing the charge transport properties and improving quantum efficiency,in this paper,the 3,7-Bis(N,N-carbazyl)-5,5-spirofluorenyl-5H-dibenzo[a,d]cycloheptene(2a) and 3,7-Bis[N,N-(3,6-dimethoxy) carbazyl]-5,5-spirofluorenyl-5 H-dibenzo[a,d]cyclo heptene(2b),which substitute carbazyl for diphenylamino unit,were designed theoretically,and then an in-depth theoretical research was performed to check up the optoelectronic properties of this series of stilbene/fluorene hybrid derivatives 1a,1b, 2a and 2b,including absorption and emission spectra,ionization potential(IP), electron affinity(EA),hole extraction potential(HEP),electron extraction potential (EEP),reorganization energy(λ),optical band gap(Eg),highest occupied molecular orbital(HOMO) and lowest unoccupied molecular orbital(LUMO).Our theoretical results may be helpful for further OLED design.Density functional theory(DFT),configuration interaction with single excitation (CIS) and time-dependent DFT(TD-DFT) were used to characterize the structures, charge transported properties and spectral information of a series of stilbene/fluorene hybrids.The calculated results of 3,7-Bis(N,N-carbazyl)-5,5-spirofluorenyl-5H-dibenzo [a,d]cycloheptene(2a) and 3,7-Bis[N,N-(3,6-dimethoxy)carbazyl]-5,5-spiro fluorenyl-5H-dibenzo[a,d]cycloheptene(2b) exhibit comparatively higher electron affinities corresponding with lower LUMO energies,which implies that they are more advantaged in electron transport than their counterparts as a result of substituting carbazyl for diarylamino units.As a conclusion,2b was conjectured as a promising OLED material owing to its balanceable charge transport properties that the difference betweenλ_hole andλ_electron for 2b is merely 0.08 eV.The UV-vis absorption and emission spectra were also investigated by TD-DFT calculations in order to understand their low-lying electronic transition character.Well agreement between theoretical calculated and experimental results has been achieved.