New progress in aggregation-induced emission research

Abstract

The most fascinating aspect of scientific research is the constant deciphering of mysterious puzzles and the continuous transformation of our mode of thinking. While textbook knowledge is the foundation of human wisdom, some traditional concepts can later prove to be imperfect or even incorrect owing to limitations of the observations and understanding of the involved natural phenomena in particular periods of history. The once popular “geocentric theory”, for example, was totally wrong and hampered scientists’ exploration of the universe, and the “vitality of the doctrine” badly hindered advancement in organic chemistry research. Innovations are needed to break the restraints of such traditional concepts. The area of organic solids research focuses on investigations of molecular functional materials with peculiar optical, electronic and magnetic properties. In this area, a generally accepted concept is that chromophore aggregation always leads to luminescence quenching. This phenomenon of aggregation-caused quenching (ACQ) is termed the “concentration-quenching effect” in the textbook Molecular Fluorescence. Researchers are working on the development of organic light-emitting diodes (OLEDs), in which luminescent molecules are used as a solid film, one of the aggregation forms. Scientists have tried hard to suppress the aggregate formation of luminescent molecules through different approaches, with the aim of mitigating the ACQ effect and achieving OLEDs with high efficiencies. In 2001, Tang’s group discovered a phenomenon exactly opposite to the ACQ effect. The researchers found that luminogen aggregation played a constructive, instead of destructive, role in the light-emitting process; a series of silole molecules, such as hexaphenylsilole (HPS) and methylpentaphenylsilole (MPPS; Figure 1), were found to be nonluminescent in the solution state but emissive in the aggregated state with luminescent quantum yields three orders of