Molecular orbital study on the ground and excited states of methyl substituted tris(8-hydroxyquinoline) aluminum(III)

We have calculated the absorption and emission energies for methyl substituted tris(8-hydroxyquinoline)aluminum (III), Almq₃, molecules at the ZINDO, CIS, and TD-DFT levels of theory. The excited-state geometries were optimized at the ab initio CIS level. The TD-DFT method provides the most reliable results for the absorption (S₀ → S1) and emission (S₁ → S0) transition energies, provided hybrid functionals are used. Moreover, the TD-DFT calculations reliably estimate the changes of absorption and emission λ_max values upon methyl substitution, with errors of 0.7% and 1.4%, respectively. The Stokes shifts calculated at the TD-DFT level agree very well with the experimental data.