Three-Dimensional Fully pi-Conjugated Macrocycles: When Truly 3D-Aromatic and when 2D-Aromatic-in-3D?

Recently, several fully pi-conjugated macrocycles with strongly puckered or cage-type structures have been synthesized and found to exhibit aromatic character according to both experiments and computations. Herein, we examine their electronic structures and put them in relation to truly 3D-aromatic molecules (e.g., closo-boranes and certain charged fullerenes) as well as 2D-aromatic polycyclic aromatic hydrocarbons. We use qualitative theory combined with quantum chemical calculations, and find that the macrocycles explored thus far should be described as 2D-aromatic with three-dimensional structures (abbr. 2D-aromatic-in-3D) instead of truly 3D-aromatic. Besides fulfilling the 6n + 2 pi-electron rule, 3D-aromatic molecules with highly symmetric structures (e.g., Td and Oh) have a number of molecular orbital (MO) levels that are (at least) triply degenerate. At lower symmetries, the triple (or higher) orbital degeneracies should be kept in approximate sense. This last criterion is not fulfilled by macrocyclic cage molecules that are 2D-aromatic-in-3D. Their aromaticity results from a fulfillment of Hückel’s 4n + 2 rule for each individual macrocyclic path, yet, their pi-electron counts are coincidentally 6n + 2 numbers for macrocycles with three tethers of equal lengths. We instead link the 3D-macrocyclic molecules explored earlier to naphthalene, motivating their description as 2D-aromatics albeit with 3D structures. It is notable that macrocyclic cages which are 2D-aromatic-in-3D can be aromatic also when the tethers are of different lengths, i.e., when their pi-electron counts differ from 6n + 2. Finally, we identify tetrahedral and cubic pi-conjugated molecules that fulfill the 6n + 2 rule and which exhibit significant electron delocalization. Yet, their properties are similar to those of analogous compounds with electron counts that differ from 6n + 2. Thus, despite that these tetrahedral and cubic molecules show substantial pi-electron delocalization they should not be classified as true 3D-aromatics.

Advances in Chirality Sensing with Macrocyclic Molecules

The construction of chemical sensors that can distinguish molecular chirality has attracted increasing attention in recent years due to the significance of chiral organic molecules and the importance of detecting their absolute configuration and chiroptical purity. The supramolecular chirality sensing strategy has shown promising potential due to its advantages of high throughput, sensitivity, and fast chirality detection. This review focuses on chirality sensors based on macrocyclic compounds. Macrocyclic chirality sensors usually have inherent complexing ability towards certain chiral guests, which combined with the signal output components, could offer many unique advantages/properties compared to traditional chiral sensors. Chirality sensing based on macrocyclic sensors has shown rapid progress in recent years. This review summarizes recent advances in chirality sensing based on both achiral and chiral macrocyclic compounds, especially newly emerged macrocyclic molecules.

The Effects of the Solvents on the Macrocyclic Structures: From Rigid Pillararene to Flexible Crown Ether

The differences in the macrocyclic structures lead to different flexibilities, and yet the effect of solvents on the conformations is not clear so far. In this work, the conformations of four representational macrocyclic molecules (pillar[5]arene, p-tert-butyl calix[6]arene, benzylic amide macrocycle and dibenzo-18-crown-6) in three solvents with distinct polarity have been studied by all-atom molecular dynamics simulations. The structural features of the macrocycles in the solvents indicate that the conformations are related to the polarity of the solvents and the formation of hydrogen bonds. For the pillar[5]arene, the benzylic amide macrocycle and the dibenzo-18-crown-6, that cannot form intramolecular hydrogen bonds, the polarity of solvents is the major contributing factor in the conformations. The formation of intramolecular hydrogen bonds, in contrast, determinates the conformations of the calix[6]arene. Furthermore, the slight fluctuations of the structures will result in tremendous change of the intramolecular hydrogen bonds of the macrocycles and the intermolecular hydrogen bonds between the macrocycles and the solvents. The current theoretical studies that serve as a basis for the macrocyclic chemistry are valuable for the efficient structural design of the macrocyclic molecules.

Design, synthesis and applications of responsive macrocycles

Inspired by the lock and key principle, the development of supramolecular macrocyclic chemistry has promoted the prosperous growth of host-guest chemistry. The updated induced-fit and conformation selection model spurred the emerging research on responsive macrocycles (RMs). This review introduces RMs, covering their design, synthesis and applications. It gives readers insight into the dynamic control of macrocyclic molecules and the exploration of materials with desired functions.

Encapsulation of the vitamins D3 and E in cucurbit[7]uril: A computational investigation

In this work, molecular dynamics simulation (MD) was used to study the encapsulation of fat-soluble vitamins D3 (vD3) and E (a-TOC) into cucubit[7]uril (CB[7]) in an aqueous solution. Cucurbiturils is a class of macrocyclic molecules largely used as carrier and controlled release agent in order to improve the solubility and chemoprotective of drugs. Along 50 ns of MD trajectory, the vitamins formed a stable complex with CB[7] without significantly altering its structure. Moreover, the second solvation shell of the CB[7] was not disrupted by the inclusion of the vitamins. The solvation enthalpy was ~ –173.0 kcal/mol for both complexes and –177.6 kcal/mol for the isolated CB[7], suggesting that the vitamin@CB[7] complexes are soluble in water. The binding free energy indicates that CB[7] can act as carrier agent for these vitamins, with values of –17.54 and –23.76 kcal/mol for vD3@CB[7] and a-TOC@CB[7], respectively. Finally, herein we highlight that CB[7] can be a new host to be used for vitamin delivery in biological systems.

Vapochromism of Organic Crystals Based on Macrocyclic Compounds and Inclusion Complexes

Vapochromic materials, which change color and luminescence when exposed to specific vapors and gases, have attracted considerable attention in recent years owing to their potential applications in a wide range of fields such as chemical sensors and environmental monitors. Although the mechanism of vapochromism is still unclear, several studies have elucidated it from the viewpoint of crystal engineering. In this mini-review, we investigate recent advances in the vapochromism of organic crystals. Among them, macrocyclic molecules and inclusion complexes, which have apparent host–guest interactions with analyte molecules (specific vapors and gases), are described. When the host compound is properly designed, its cavity size and symmetry change in response to guest molecules, influencing the optical properties by changing the molecular inclusion and recognition abilities. This information highlights the importance of structure–property relationships resulting from the molecular recognition at the solid–vapor interface.

Recent Development of Supramolecular Sensors Constructed by Hybridization of Organic Macrocycles with Nanomaterials

Macrocyclic compounds have attracted tremendous attention for their superior performance in supramolecular recognition, catalysis, and host-guest interaction. With these admirable properties, macrocyclic compounds were used as modifiers for enhancing the sensitivity and selectivity of electrodes and optical sensors. The classic macrocyclic compounds, including crown ethers, cyclodextrins, calixarenes, cucurbiturils, and pillararenes, were employed as receptors for electrochemical and optical sensors to develop new analytical methods with the wilder detection range, lower detection limit, and better tolerance of interference. Macrocyclic molecules functionalized with nanomaterials, the small entities with dimensions in the nanoscale, realized the versatility and diversification of the nano-hybrid materials, which improved the capabilities of recognition and response with the combining characteristics of two components. Herein, this review focused on the development in the research field of hybridization of organic macrocycles with nanoparticles and their applications for chemosensors, aiming at both existing researchers in the field and who would like to enter into the research.