Sub-picosecond fluorescence dynamics of organic light-emitting diode tris(8-hydroxyquinoline) metal complexes

For a few complexes belonging to a new class of organic light-emitting diodes (OLED), namely Al(III)– and Ga(III)–tris-8-hydroxyquinoline (Alq3 and Gaq3), femtosecond fluorescence up-conversion measurements are reported. The fluorescence decays contain picosecond components when the complexes are dissolved in liquid solution. The fast components are not found in the time-resolved fluorescence of the complexes doped in an inert host crystal. The results in liquid solution are characteristic of a dynamic Stokes shift due to the solvation in a ligand-localized excited state. Furthermore, from the temporal behavior of the total fluorescence intensity it is concluded that, concomitant with solvation, adiabatic electron transfer takes place.