Synthesis, characterization and electroluminescence of two highly-twisted non-doped blue light-emitting materials

Two pyrene derivatives, substituted with 2-methylnaphthalene units on 1,3-position and 1,6-position of pyrene backbones, were designed and synthesized. DFT calculation confirmed that the two molecules were highly twisted and the dihedral angles between pyrene backbone and naphthalene unit were over 80°, being attributed to the steric hindrance of ortho-methyl group and the substitution position of pyrene itself. As a result, the intermolecular aggregation was greatly inhibited in the solid state, being beneficial for suppressing the fluorescence quenching. By analyzing the optical and thermal properties, it was found that the π-π conjugation extension could be adjusted and a balance for high fluorescent efficiency and avoiding quenching at the same time could be reached, which may guide the molecular design in the future. The electroluminescence properties of the non-doped devices were enhanced with the double hole-transporting layers by optimizing the energy level matching. The stable blue EL emission, with the Commission Internationaled’Eclairage (CIEx,y) color coordinates of (0.15, 0.13) and (0.15, 0.11) at 7, 8, 9 and 10 V respectively, was obtained.