Steric Hindrance, Electronic Communication, and Energy Transfer in the Photo- and Electroluminescence Processes of Aggregation-Induced Emission Luminogens

Methylpentaphenylsilole (MPPS) and tetraphenylethene (TPE) are two archetypal luminogens that show the novel effect of aggregation-induced emission (AIE). In this work, TPE moieties are attached to MPPS core as peripheral groups at 2,5- and 3,4-positions to generate new AIE luminogens. The resulting 2,5- and 3,4-BTPEMTPS adducts are weakly fluorescent in dilute solutions but become highly emissive as solid aggregates, verifying their AIE activities. Although weak in the absolute term, the emissions of the 2,5- and 3,4-BTPEMTPS luminogens are stronger than those of their parent forms of MPPS and TPE, due to the steric effect of the bulky TPE pedants on the intramolecular motions, especially the rotations of the phenyl rotors, in the BTPEMTPS systems. The emissions of 2,5-BTPEMTPS in the solution and aggregate states are more efficient and redder than those of 3,4-BTPEMTPS, revealing that the former is electronically more conjugated than the latter. Both the BTPEMTPS regioisomers are morphologically and thermally stable, showing high glass transition (Tg = 126 °C) and thermal-degradation temperatures (Td up to 400 °C). The AIE luminogens serve as excellent emitters in electroluminescence (EL) devices, with maximum luminance up to 12560 cd/m2. The EL deices using the blends of 2,5-BTPEMTPS and bis(tetraphenylethene) (BTPE) as emitting layers afford high current efficiencies (up to ∼7 cd/A) and external quantum efficiencies (up to ∼2.2%), thanks to the efficient energy transfer from the BTPE host to the 2,5-BTPEMTPS guest in the blending layer.