Electrogenerated chemiluminescence from derivatives of aluminum quinolate and quinacridones: Cross-reactions with triarylamines lead to singlet emission through triplet-triplet annihilation pathways

Solution electrogenerated chemiluminescence (ECL) was evaluated for molecules of interest for organic light-emitting diodes (OLEDs), using high-frequency voltage pulses at a microelectrode. Radical cations of different energies were electrogenerated from a series of triarylamine hole-transport materials (x-TPD), in the presence of radical anions of a high electron affinity sulfonamide derivative of tris(8-hydroxyquinoline) aluminum (Al(qs)(3)), or a bis(isoamyl) derivative of quinacridone (DIQA). The resultant emission was from the excited singlet states Al-1(qs)(3)* Or (I)DIQA*, the same excited state produced in OLEDs based on these molecules. Zn solution, the majority of the reaction pairs had insufficient energy to populate Al-1(qs)(3)* or (I)DIQA* directly, but could form the triplet states Al-3(qs)(3)* or (3)DIQA*. The reaction order and the temporal response of the emission were consistent with subsequent formation of the excited singlet states via triplet-triplet annihilation (TTA). For reactions with a low excess Gibbs free energy to form the triplet state (Delta(T)G) the efficiency increased exponentially with an increase in driving force (increase in oxidation potential of x-TPD), then reached a plateau. At the maximum, the efficiencies for formation of Al-1(qs)(3)* or (I)DIQA* via the TTA route reach as high as a few percent. The computed energetics of these reactions suggest that similar light-producing electroluminescent reactions, proceeding via triplet formation, could also occur in condensed phase organic thin films.