Achieving blue luminescence of Alq3 through the pull-push effect of the electron-withdrawing and electron-donating substituents

A novel blue-emissive Alq3 derivative, tris-(4-methyl-6-fluoro-8-hydroxyquinoline) aluminum (6), has been designed and prepared. Because the strong electronegative F atom at 6-position lowers the HOMO significantly and the weak electron-donating methyl group at 4-position retains the drop of the LUMO, the band gap of 6 is broadened, and a blue-shift by 53 nm of the emission peak (λem) with substantially enhanced intensity is demonstrated when compared to that of Alq3. It is also found that 6 possesses good thermal stability and solubility in common solvents. Through theoretical simulations of the frontier orbitals, it is disclosed that 6's λem can be blue-shifted further if methyl is replaced with a stronger electron-donating substituent, such as methoxy or N,N′-dimethylamino group.