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 molecul es of interest for organic light-emitting diodes (OLEDs), using high-freque ncy voltage pulses at a microelectrode. Radical cations of different energi es were electrogenerated from a series of triarylamine hole-transport mater ials (x-TPD), in the presence of radical anions of a high electron affinity sulfonamide derivative of tris(8-hydroxyquinoline) aluminum (Al(qs)(3)), o r 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 exci ted state produced in OLEDs based on these molecules. Zn solution, the majo rity 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 c onsistent with subsequent formation of the excited singlet states via tripl et-triplet annihilation (TTA). For reactions with a low excess Gibbs free e nergy to form the triplet state (Delta(T)G) the efficiency increased expone ntially with an increase in driving force (increase in oxidation potential of x-TPD), then reached a plateau. At the maximum, the efficiencies for for mation 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 l ight-producing electroluminescent reactions, proceeding via triplet formati on, could also occur in condensed phase organic thin films.