Because of the importance of novel macrocycles in supramolecular science, interest in the preparation of these substances has grown considerably. However, the discovery of a new class of macrocycles presents challenges because of the need for routes to further functionalization of these molecules and good host-guest complexation. Furthermore, useful macrocylic hosts must be easily synthesized in large quantities. With these issues in mind, the recently discovered pillararenes attracted our attention. These macrocycles contain hydroquinone units linked by methylene bridges at para positions. Although the composition of pillararenes is similar to that of calixarenes, they have different structural characteristics. One conformationally stable member of this family is pillar[5]arene, which consists of five hydroquinone units. The symmetrical pillar architecture and electron-donating cavities of these macrocycles are particularly intriguing and afford them with some special and interesting physical, chemical, and host-guest properties. Due to these features and their easy accessibility, pillararenes, especially pillar[5]arenes, have been actively studied and rapidly developed within the last 4 years. In this Account, we provide a comprehensive overview of pillararene chemistry, summarizing our results along with related studies from other researchers. We describe strategies for the synthesis, isomerization, and functionalization of pillararenes. We also discuss their macrocyclic cavity sizes, their host-guest properties, and their self-assembly into supramolecular polymers. The hydroxyl groups of the pillararenes can be modified at all positions or selectively on one or two positions. Through a variety of functionalizations, researchers have developed many pillararene derivatives that exhibit very interesting host-guest properties both in organic solvents and in aqueous media. Guest molecules include electron acceptors such as viologen derivatives and (bis)imidazolium cations and alkyl chain derivatives such as n-hexane, alkanediamines, n-octyltrimethyl ammonium, and neutral bis(imidazole) derivatives. These host-guest studies have led to the fabrication of (pseudo)rotaxanes or poly(pseudo)rotaxanes, supramolecular dimers or polymers, artificial transmembrane proton channels, fluorescent sensors, and other functional materials.

Introduction 7399 2. Synthesis of Rotaxanes Based on Various Macrocycles 7400 2.1. Crown Ethers 7401 2.1.1. Bis(m-phenylene)-32-crown-10 and Crown Ethers with Larger Sizes 7401 2.1.2. Dibenzo-24-crown-8 7402 2.1.3. Benzo-21-crown-7 7406 2.1.4. Crown Ether-Based Cryptands 7407 2.2. Cyclodextrins 7410 2.3. Cucurbiturils 7413 2.3.1. Cucurbit[6]uril 7413 2.3.2. Cucurbit[7]uril 7414 2.3.3. Cucurbit[8]uril 7415 2.4. Calixarenes 7416 2.4.1. Calix[4]arenes as Linkers or Stoppers 7416 2.4.2. Calix[5]arenes and Calix[6]arenes as Wheels 7416 2.4.3. Heterocalix[n]arenes or Calix[n]heteroarenes 7417 2.5. Pillararenes 7418 2.5.1.

The blue‐shifted and red‐shifted H‐bonds have been studied in complexes CH3CHO…HNO. At the MP2/6‐31G(d), MP2/6‐31+G(d,p) MP2/6‐311++G(d,p), B3LYP/6‐31G(d), B3LYP/6‐31+G(d,p) and B3LYP/6‐311++G(d,p) levels, the geometric structures and vibrational frequencies of complexes CH3CHO…HNO are calculated by both standard and CP‐corrected methods, respectively. Complex A exhibits simultaneously red‐shifted CH…O and blue‐shifted NH…O H‐bonds. Complex B possesses simultaneously two blue‐shifted H‐bonds: CH…O and NH…O. From NBO analysis, it becomes evident that the red‐shifted CH…O H‐bond can be explained on the basis of the two opposite effects: hyperconjugation and rehybridization. The blue‐shifted CH…O H‐bond is a result of conjunct CH bond strengthening effects of the hyperconjugation and the rehybridization due to existence of the significant electron density redistribution effect. For the blue‐shifted NH…O H‐bonds, the hyperconjugation is inhibited due to existence of the electron density redistribution effect. The large blue shift of the NH stretching frequency is observed because the rehybridization dominates the hyperconjugation. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

Two novel low-molecular-weight organogelators (LMOGs) 1 and 2 composed of an anthraquinone unit, a hydrazide group, and long alkyl chains were synthesized. They could form stable gels in wide tested solvents. Chloroalkanes and aromatic solvents tend to result in transparent gels, while alcohol and other solvents yield opaque gels. The FT-IR, PXRD, and (1)H NMR spectral studies revealed that hydrogen bonding and pi-pi interactions were the main driving forces for formation of the gels. Although the hydrazide unit and the anthraquinone group were connected by the sigma-bond, the chloroform gel of 1 could be changed into a red solution upon the addition of anion (F(-), AcO(-), and H(2)PO(4)(-)) due to the disruption of the intermolecular hydrogen-bondings. Moreover, the red color clearly faded at once and the solution regelated upon the addition of methanol. The results indicated that 1 and 2 as smart anion-responsive gel might provide the basis for the development of nonfluid systems for sensing anion with the naked eye.