Ultrafast Dynamics of Triplet Excitons in Alq3-Bridge-Pt(II)porphyrin Electroluminescent Materials

Excited-state dynamics are crucial for maximizing the performance of organic light-emitting diodes (OLEDs). Because electron−hole recombination yields singlet and triplet excited states in a 3:1 ratio, it is important to harvest the energy of triplets in light-emitting processes. Self-assembled multichromophore electroluminescent materials consisting of a trisquinolinolate aluminum(III) (Alq3) donor, fluorene-based conjugated oligomers as a bridge, and Pt(II)tetraphenylporphyrin as an acceptor and phosphorescent emitter are described. In these materials, the energy of singlet as well as triplet states is harvested and emitted as red phosphorescence from the porphyrin acceptor. Attention was devoted to the triplet exciton dynamics, which was investigated by ultrafast transient spectroscopy, and the observations are compared with phosphorescence in thin films and with electroluminescence from OLEDs. It was found that exothermicity of the forward Alq3-to-fluorene bridge triplet transfer appears to be a less stringent requirement for triplet transfer electroluminescence. In contradistinction, the energy alignment between the bridge and Pt(II)porphyrin emitter is of crucial importance. It is shown that the triplet exciton dynamics has a dominant effect on the electroluminescence properties of conjugated donor−bridge−acceptor materials. The triplet-energy transfer operates on an ultrafast time scale (k TTET = (4−6) × 1010 s-1) and requires careful energy alignment of the components (3ΔE D - B ≈ 3ΔE B - A ≥ 0.1 eV) to prevent endothermic energy transfer and severe quenching of the electroluminescence. To the best of our knowledge, this is the first time triplet dynamics was directly observed in donor−acceptor electroluminescent materials and direct connection to device efficiency was established.