Pillararenes as Versatile Building Blocks for Fluorescent Materials

Fluorescent materials have received more and more attention in the past few decades because of their great potentials in the fields of luminescent devices, sensing, data storage, bioimaging, and other optical applications. Fluorescent materials comprising organic molecules are of broad interest owing to their highly tunable emission. Initial studies on organic fluorescent materials were mainly focused on the design and covalent modification of fluorophores in order to improve their photophysical properties at the molecular level, whereas in recent decades, many studies have revealed that the intermolecular or intramolecular noncovalent interactions also play a crucial role in luminescence. For example, the modulation of noncovalent interactions in aggregates and self-assemblies was proven to be capable of adjusting aggregation-induced emission (AIE)-active fluorophores by the restriction of intramolecular motions (RIM). In addition, in the crystalline state, intermolecular noncovalent interactions are able to promote phosphorescence by decreasing nonradiative decays. Introducing supramolecular macrocycles into organic fluorescent materials is an intriguing prospect because multiple noncovalent interactions are incorporated. On one hand, the photophysical properties of fluorophores can be changed upon inclusion within the macrocycles, providing unforeseen luminescence. On the other hand, the dynamic and reversible features of host-guest recognition endow the materials with controllability and stimuli-responsiveness, which is beneficial to the fabrication of smart materials. Among numerous supramolecular macrocycles, pillararenes are promising candidates that can be included in fluorescent materials. The advantages of pillararenes are their easy functionalization and planar chirality. After modification with proper substituents, pillararene derivatives possess high solubility and stability in both organic and aqueous media, and reversible guest binding remains. Such features make pillararenes versatile hosts in different environments. Additionally, pillararenes are planar chiral, and the interconversion between enantiomers can be adjusted with different-sized substituents and external stimuli, which are favorable to the construction of chiroptical materials. In this Account, we summarize research progress in the field of pillararene-based luminescent materials, which mainly includes the contributions made by our group. Using pillararenes as building blocks can facilitate the fabrication of high-performance fluorescent materials, in solution or the solid state, with different functions and mechanisms. Therefore, we categorize pillararene-based luminescent materials as those in solution or in the solid state. The applications and the advantages of pillararenes are discussed in detail. For example, in the solid state, pillararene-based host-guest complexation is capable of minimizing the aggregation-caused quenching (ACQ) of fluorophores. This broadens the application of fluorophores in crystalline materials. In solution, the host-guest complexes of pillararenes and fluorophores can self-assemble into well-defined nanostructures, which not only adjust the photophysical properties but also enable functions such as bioimaging. The remaining challenges and future perspectives are outlined at the end. It is expected that this Account will inspire new researchers in different fields and offer new opportunities for the construction of novel luminescent materials with pillararenes and other macrocycles.

Automated summary

Fluorescent materials, especially those comprising organic molecules, have been extensively studied due to their tunable emission properties. Noncovalent interactions, both intermolecular and intramolecular, play a crucial role in luminescence. Supramolecular macrocycles, such as pillararenes, have shown promise in modulating fluorescent properties and offering controllability and stimuli-responsiveness. Pillararenes are attractive building blocks with easy functionalization and planar chirality.