“Unity is force” – Aesop. It is a common phenomenon that traditional chromophores exhibit high fluorescence in dilute solutions, yet luminescence is quenched at high concentrations or in the aggregate state, i.e. “aggregation-caused quenching” (ACQ). Tang reported the unusual photophysical observation that luminogens can exhibit weak or no fluorescence in solution, yet they are highly emissive in the aggregate or solid state; this is defined as aggregation-induced emission (AIE). The discovery of AIE helped solve the ACQ effect in traditional luminophores. Pyrene is an important polycyclic aromatic hydrocarbon (PAH), which exhibits very different photophysical behavior in solution versus the aggregate state, and the ACQ effect has played a dominant role in pyrene chemistry. The ACQ effect is harmful for some practical applications and is a challenge in organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells, for which the effect is more severe in the solid state. Thus, how to overcome the ACQ effect observed in pyrene chemistry still remains a challenge. In this review, we discuss how following basic AIE mechanisms such as the restriction of intramolecular motion (RIM), excited-state intramolecular proton transfer (ESIPT), and twisted intramolecular charge transfer (TICT), can transform pyrene-based ACQ luminogens to AIE luminogens with excellent optical properties. Furthermore, prospective applications of pyrene-based AIEgens are discussed, as is the potential for designing new organic functional materials.

This article describes a new schematic strategy for the preparation of pyrene-based aggregation-induced emission (AIE) luminogens, which show a potential application in smart materials. Utilising a restricted intramolecular rotation (RIR) mechanism, three non-typical pyrene-based luminogens 2–4 were synthesized and their AIE characteristics and mechanochromism (MC) were analysed in relation to their particle size distribution. The morphology of the synthesized 2–4 luminogens was investigated via scanning electron microscopy (SEM), transmission electron microscopy (TEM) and dynamic light scattering (DLS). The morphology of AIEgens 2 was changed from a gel-like species to small irregular patches and then to uniform, round-shaped nano-particle species through a crystal phase transition from the ordered packing structure to amorphous as the water fraction (f_w) was increased. The different size distribution and morphological changes of nano-particle species play a significant role in enhancing the emission intensity in the aggregate state. X-ray single crystal diffraction shows that both compounds 2 and 4 have a twisted conformation with a large dihedral angle between the pyrene core and the peripheral aromatic substituent group. The morphologies were further characterized by powder X-ray diffraction (PXRD) before and after grinding, which showed that the external force destroys the weak intermolecular interactions and leads to a change to a more planar molecular conformation. The minor change in packing pattern contributes to influencing the emissive intensity as well as emission colour. As a result, we observed more molecules aggregating, more nanoparticle homogeneity and more shine.