Theoretical investigation of tetra-substituted pyrenes for organic light emitting diodes

Pyrene derivatives have been developed to blue fluorescent light emitting materials for organic light emitting diodes (OLEDs). Luminescence efficiency of pyrene molecule is very low, however pyrenes substituted with large group can give a bright blue fluorescence. Substituted pyrenes were investigated to increase luminescence efficiency and color purity. In this paper, we studied 1,3,6,8-tetra-substituted pyrenes theoretically. The investigation was studied by ab initio method. Density functional theory (DFT) utilizing the B3LYP functional with the 6-31G(d) basis set was used for energy calculation and geometry optimization of the ground electronic state of those compounds. The geometry optimization of the first excited electronic states was determined using configuration interaction with single excitations (CIS) method. Energies of vertical and adiabatic transition were calculated using time-dependent density functional theory (TD-DFT) with B3LYP/6-31G(d) calculation. The investigation on 1,3,6,8-tetra-substituted pyrenes with the effect of the aromatic substituents showed that the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) was decreased as the conjugation length was increased. This trend was also appeared when the nitrogen atom was substituted with carbon atom in the aromatic substituents. Also the substituted pyrenes with large moieties were slightly distorted from the planar structure. Thus, the luminescent efficiencies of tetra-substituted pyrenes were increased more than that of the pyrene molecule. In this paper, t(dpe)py molecule was proposed as a new efficient blue emitter for OLED from calculated result.