Pyrene-based aggregation-induced emission luminogens (AIEgen): structure correlated with particle size distribution and mechanochromism

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 (fw) 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.