Theoretical Investigation For Carbazole And Spirosilabifluorene Derivatives As OLED Materials

With the aim to explore the nature of OLED materials, calculations on the ground statewere carried out using ab initio method and density functional theory (DFT). The energies ofelectronic transitions have been obtained by TD-DFT/B3LYP calculations performed on theoptimized geometries. The excited geometries were optimized by ab initio CIS. Based on theexcited geometries, the emission spectra are investigated. Significant progress has been made in utilization of electrophosphorescence device whichis a host material doped with the phosphorescent dye to improve the external quantumefficiency. The host material for such phosphorescent or triplet emitters should not only matchthe HOMO and LUMO level with the Fermi levels of the electrode or the levels ofneighboring layers, but also the triplet energy of the host should be higher than that of theguest to prevent triplet energy transfer from the guest to host. A prominent class of materialsthat fulfills the above mentioned boundary conditions is the class of carbazoles. In order toinvestigate the effects of the substituent for six carbazole derivatives on the electronicproperties and triplet emission spectra, ab initio HF, CIS and TD-DFT methods are used tooptimize the ground and excited triplet states and to predict the optical properties forinterested molecules. The calculated results suggest that tuning the HOMO level withoutinfluencing the triplet energy for the dimers by different substituent is ascribed to the identicaldistribution pattern of FMOs for excited states and their similar energy gap.The results of such computational study on these carbazole compounds agree well withexperiments data. It is very helpful to improve the thermal and spectral stabilities by introducing thespirobifluorene units into the electroluminescent molecules. The molecules containing silolering have been current interest in the optical and electronic properties. We analyzed groundand excited state properties of 9,9'-spiro-9-silabifluorene (SSF) derivatives to explore theeffect of adding substituents on the optical spectral properties of this class of asymmetricallysubstituted SSFs. According to our results, it was found that the ending substitution can notsignificantly affect the absorption and emission spectra.