Copillar[5]arene Appended Pyrene Schiff Base: Photophysics, Aggregation Induced Emission and Picric Acid Recognition

Herein, we report the synthesis of copillar[5]arene-based pyrene Schiff base 1 and its characterization by using 1H, 13C NMR, FT-IR and mass spectrometry. UV-vis absorption, steady-state fluorescence and time-resolved fluorescence are done to elucidate the photophysical behaviors of 1. To understand the electronic structure of 1, density functional theory (DFT) calculations are performed. Owing to the presence of pyrene via a Schiff base linkage, compound 1 exhibits aggregation-induced emission (AIE) characteristics. It shows aggregation in aqueous THF and DMF. The aggregation behavior is successfully demonstrated by steady-state fluorescence, dynamic light scattering (DLS) and time-correlated single-photon counting (TCSPC) experiments. Experimental findings reveal that hydrophobic effect is the driving force in the formation of aggregates. As application, the aggregated state of 1 in aqueous THF fluorimetrically recognizes picric acid (PA) selectively over a series of nitro- and nonnitroaromatics with a detection limit of 1.62×10-7 M. The emission of the aggregated state is fully quenched upon interaction with PA.

Glyoxylic acid monohydrate promoted reductive addition of sodium sulfinates to pillar[4]arene[1]quinone

An efficient synthesis of sulfonate esters through reductive addition of sodium sulfinates to pillar[4]arene[1]quinone has been established (15 examples). Compared to the arylsulfonylation of p-quinone with sodium arylsulfinates under other acidic conditions, this work affords the hydroquinone-type 4-O-sulfonyl derivatives by using glyoxylic acid monohydrate as a promoter. The protocol features mild reaction conditions and high selectivity and is an alternative protocol for the O-sulfonylation of pillar[4]arene[1]hydroquinone.

Ortho-Functionalization of Pillar[4]arene[1]benzoquinone Monoxime via Selective 1,4-Addition of Grignard Reagents

Ortho-functionalization of pillar[n]arenes has been a formidable challenge, partially due to the fragility of their macrocyclic skeletons. In this concise report, we describe a facile synthetic method for monoarylation/alkylation at the position ortho to the oxime functionality in pillar[4]arene[1]benzoquinone monoxime (1) via addition of Grignard reagents. The described method enables the creation of various mono-ortho-alkyl/aryl-substituted pillar[5]arene derivatives that were previously inaccessible.

Pillar[5]arenes decorated with six-membered-ring aromatics at all the substitution positions

Macrocyclic molecules have characteristic properties different from linear ones, such as high symmetry and guest-inclusion ability. To bring drastic changes to these properties, direct introduction of many substituents is a challenging but effective tool. Herein, we attain direct installation of ten six-membered-ring aromatic π-units into both rims of a pillar[5]arene. In contrast to previous pillar[n]arenes with less hindered five-membered-ring units, which showed conformational complexity and crushed crystal structures, the per-phenyl-substituted pillar[5]arene has a cylinder-shaped crystal structure with a dichloromethane inside the cavity and is obtained as a single pair of D 5-symmetric enantiomers. The average dihedral angles between the core and peripheral benzene rings sharply increase from 38° to 66°. These differences indicate the importance of local steric repulsion on both rims for determining the structures and properties of macrocycles.

Per-Arylation of Pillar[n]arenes: An Effective Tool to Modify the Properties of Macrocycles

Installation of various substituents is a reliable and versatile way to alter the properties of macrocyclic molecules, but high-yield and controlled methods are not always available especially for multifold reactions. Herein, we report 10- and 12-fold introduction of aryl substituents onto both rims of cylinder-shaped pillar[n]arenes, which usually have alkoxy substituents slanting to the cylinder axes. Although alkoxy pillar[5]arenes exist as D₅-symmetric enantiomeric pairs, arylated pillar[5]arenes provide crushed single-crystal structures and stereoisomerism including C₂-symmetric conformations depending on the aryl groups. Pillar[n]arenes with 2-benzofuranyl groups display bright fluorescence with quantum yields of 88-90% and no host-guest complexation with electron-deficient molecules in solution due to large deviation from alkoxy compounds. A benzofuran-appended pillar[6]arene instead captures small gaseous molecules in the solid state, probably owing to outside spaces surrounded by aromatic rings.

ortho-Functionalization of Pillar[5]arene: An Approach to Mono-ortho-Alkyl/Aryl-Substituted A1/A2-Dihydroxypillar[5]arene

Despite the fact that the rim and lateral functionalizations of pillar[n]arenes have been well explored, ortho-functionalization has rarely been realized. In this work, we report a facile method of introducing a single functionality ortho to the hydroxyl group in A1/A2-dihydroxypillar[5]arene via a Grignard addition to pillar[4]arene[1]quinone followed by a dienone-phenol rearrangement. The described ortho-alkylation/arylation method allowed formation of various mono ortho-alkyl/aryl-substituted A1/A2-dihydroxypillar[5]arenes previously difficult to obtain.

Functional supramolecular gels based on pillar[n]arene macrocycles

Supramolecular gels constructed from low-molecular-weight gelators via noncovalent interactions have received increasing attention. The rapid development of stimuli-responsive supramolecular gels with attractive properties is highly desirable to meet the ever-growing demand of materials science and chemistry. The inherent reversible and dynamic nature of noncovalent interactions in supramolecular gels endows the materials with sensing, processing, and actuating functions in response to specific environmental changes and offers them great potential in flexible biomaterials and intelligent devices. In particular, pillar[n]arenes with symmetrical pillar-shaped architectures have been recognized as an emerging class of synthetic macrocycles after crown ethers, cyclodextrins, calixarenes, and cucurbiturils, and proven to be excellent candidates for the fabrication of functional supramolecular gels due to their many advantages including facile synthesis, diverse functionalization, and appealing host-guest properties. This review provides a comprehensive overview of recent progress in supramolecular gels involving pillar[n]arenes and their derivatives as synthetic macrocyclic arenes, from the viewpoints of the synthetic approach, controllable assembly, stimuli-responsiveness, and functions. Perspectives of this burgeoning field of research are also given at the end.