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.

Careful examination of the X-ray structure of a ditopic hydrazide derivative 7 led to the concept that with malonyl groups as interhydrazide linkers hydrogen-bonding-mediated molecular duplex strands might be obtained. Complexation studies between 7, 8, and 9 confirmed this hypothesis. Two quadruple hydrogen-bonded heterodimers formed, in which spectator repulsive secondary electrostatic interaction was found to play an important role in determining the stability of the complexes. Extensive studies on 1-4 indicated that the hydrogen-bonding mode could persist in longer oligomeric hydrazide derivatives with chain extension from monomer to tetramer. Molecular duplex strands via two to fourteen interstrand hydrogen bonds were obtained. In addition to affecting the stability of the duplex strands, spectator repulsive secondary electrostatic interaction also played an important role in determining dynamic behavior of the duplex strands as exemplified by variable temperature (1)H NMR experiments. IR studies confirmed stronger hydrogen bonding in the longer oligomers. The assemblies of 1-4 on HOPG were also studied by STM technology. Molecular mechanical calculations further revealed double-helical structures for the longer oligomers. The results provide new opportunities for development of polymeric helical duplexes with well-defined structures.

The advent of topological semimetals enables the exploitation of symmetry-protected topological phenomena and quantized transport. Here, we present homogeneous rectifiers, converting high-frequency electromagnetic energy into direct current, based on low-energy Dirac fermions of topological semimetal-NiTe2, with state-of-the-art efficiency already in the first implementation. Explicitly, these devices display room-temperature photosensitivity as high as 251 mA W−1 at 0.3 THz in an unbiased mode, with a photocurrent anisotropy ratio of 22, originating from the interplay between the spin-polarized surface and bulk states. Device performances in terms of broadband operation, high dynamic range, as well as their high sensitivity, validate the immense potential and unique advantages associated to the control of nonequilibrium gapless topological states via built-in electric field, electromagnetic polarization and symmetry breaking in topological semimetals. These findings pave the way for the exploitation of topological phase of matter for high-frequency operations in polarization-sensitive sensing, communications and imaging.

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The hydrogen bonding interactions of the HNO dimer have been investigated using ab initio molecular orbital and density functional theory (DFT) with the 6-311++G(2d,2p) basis set. The natural bond orbital (NBO) analysis and atom in molecules (AIM) theory were applied to understand the nature of the interactions. The interrelationship between one N-H...O hydrogen bond and the other N-H...O hydrogen bond has been established by performing partial optimizations. The dimer is stabilized by the N-H...O hydrogen bonding interactions, which lead to the contractions of N-H bonds as well as the characteristic blue-shifts of the stretching vibrational frequencies nu(N-H). The NBO analysis shows that both rehybridization and electron density redistribution contribute to the large blue-shifts of the N-H stretching frequencies. A quantitative correlations of the intermolecular distance H...O (r(H...O)) with the parameters: rho at bond critical points (BCPs), s-characters of N atoms in N-H bonds, electron densities in the sigma*(N-H), the blue-shift degrees of nu(N-H) are presented. The relationship between the difference of rho (|Deltarho|) for the one hydrogen bond compared with the other one and the difference of interaction energy (DeltaE) are also illustrated. It indicates that for r(H...O) ranging from 2.05 to 2.3528 A, with increasing r(H...O), there is the descending tendency for one rho(H...O) and the ascending tendency for the other rho(H...O). r(H...O) ranging from 2.3528 to 2.85 A, there are descending tendencies for the two rho(H...O) with increasing r(H...O). On the potential energy surface of the dimer, the smaller the difference between one rho(H...O) and the other rho(H...O) is, the more stable the structure is. As r(H...O) increases, the blue-shift degrees of nu(N-H) decrease. The cooperative descending tendencies in s-characters of two N atoms with increasing r(H...O) contribute to the decreases in blue-shift degrees of nu(N-H). Ranging from 2.05 to 2.55 A, the increase of the electron density in one sigma*(N-H) with elongating r(H...O) weakens the blue-shift degrees of nu(N-H), simultaneously, the decrease of the electron density in the other sigma*(N-H) with elongating r(H...O) strengthens the blue-shift degrees of nu(N-H). Ranging from 2.55 to 2.85 A, the cooperative ascending tendencies of the electron densities in two sigma*(N-H) with increasing r(H...O) contribute to the decreases in blue-shift degrees of nu(N-H).