Nondegenerate π-Donor/π-Acceptor [2]Catenanes Containing Proton-Ionizable 1H-1,2,4-Triazole Subunits: Synthesis and Spontaneous Resolution

Two-ring chirality generated by the alignment of two achiral phenylacetylene macrocycles

When two achiral rings are bound mechanically, a chiral source is generated in the assembly. The chiroptical properties could be modulated according to the relative occupation of each ring in the assembly. In fact, we have found that two isomeric assemblies (1 and 2) show unique properties in each assembly with two achiral rings of phenylacetylene macrocycle (PAM). When considering the difference in the chiroptical properties of these two isomeric assemblies (6PAM × 2), no comparison was available based on no activity of the achiral component element itself (6PAM). In this work, we synthesized a two-ring chiral analog (4) by the ring-fusion of two 6PAMs to an 11PAM, and examined the chiroptical properties of 4, since the single helix was imparted as a chiral source. By comparison of the chiroptical properties (molar circular dichroism and molar optical rotation) of 1 and 2 to those of 4, we demonstrated that the disparity was related to the alignment of the two achiral rings.

The Story of the Little Blue Box: A Tribute to Siegfried Hünig

The tetracationic cyclophane, cyclobis(paraquat-p-phenylene), also known as the little blue box, constitutes a modular receptor that has facilitated the discovery of many host-guest complexes and mechanically interlocked molecules during the past 35 years. Its versatility in binding small π-donors in its tetracationic state, as well as forming trisradical tricationic complexes with viologen radical cations in its doubly reduced bisradical dicationic state, renders it valuable for the construction of various stimuli-responsive materials. Since the first reports in 1988, the little blue box has been featured in over 500 publications in the literature. All this research activity would not have been possible without the seminal contributions carried out by Siegfried Hünig, who not only pioneered the syntheses of viologen-containing cyclophanes, but also revealed their rich redox chemistry in addition to their ability to undergo intramolecular π-dimerization. This Review describes how his pioneering research led to the design and synthesis of the little blue box, and how this redox-active host evolved into the key component of molecular shuttles, switches, and machines.

Temperature-Dependent Emission and Turn-Off Fluorescence Sensing of Hazardous "Quat" Herbicides in Water by a Zn-MOF Based on a Semi-Rigid Dibenzochrysene Tetraacetic Acid Linker

A zinc metal-organic framework, i.e., Zn-MOF (Zn-DBC), with ca. 27% solvent-accessible void volume was synthesized from a rationally designed tetraacid based on sterically insulated dibenzo[g,p]chrysene core; the latter inherently features concave shapes. Due to rigidification of the fluorophore in the MOF, Zn-DBC exhibits a respectable fluorescence quantum yield of ca. 30% in the solid state. The fluorescent and water-stable Zn-DBC MOF was found to display intriguing temperature-dependent emission behavior with an activation barrier of 1.06 kcal/mol for radiationless deactivation from the singlet-excited state. It is shown that the Zn-MOF can be employed as an efficient sensory material for detection of hazardous "quat" dicationic herbicides in water by diffusion-limited "turn-off" fluorescence. Due to confinement of the cationic guest analytes within the pores of the MOF, the fluorescence quenching via excited-state charge transfer mechanism is shown to depend on the molecular size of the analyte in addition to the redox potentials. Remarkably, Zn-DBC permits sensing of DQ, a well-known toxic "quat" herbicide, with a detection limit as low as 2.8 ppm in water. The unique structural attributes of the Zn-MOF for highly efficient fluorescence sensing of toxic herbicides in water are thus exemplified for the first time.

Chiral diversification through the assembly of achiral phenylacetylene macrocycles with a two-fold bridge

We demonstrate so-called "chiral diversification", which is a design strategy to create multiple chiral molecules through the assembly and double-bridging of achiral components. We used phenylacetylene macrocycles (PAMs) as an achiral element. In a molecule, two achiral rings of [6]PAM are stacked one above the other, or bound to each other mechanically. As an alternative, a single enlarged ring of [12]PAM was also assumed to be a doubled form of [6]PAM. In any case, one or two ring(s) are doubly-bridged by covalent bonds to exert chirality. Through intramolecular two-bond formation, these multiple chiral molecules were obtained as a set of products in one reaction. The dynamic chirality generated in molecules with either two helically-stacked rings of [6]PAM or a single helically-folded ring of [12]PAM was characterized by induced Cotton effects with the aid of an external chiral source. Thus, a chiral structure based on [12]PAM could be demonstrated as the first success. Alternatively, enantiomeric separation was achieved for molecules with two interlocked rings of [6]PAM to show remarkable chiroptical properties.

A systematic study of thermochromic aromatic donor-acceptor materials

Molar mixtures (1:1) of electron-rich dialkoxynapthalene (Dan) and electron-deficient 1,4,5,8-napthalenetetracarboxylic diimide (Ndi) derivatives form highly tunable, columnar mesophases with a dark red color due to a charge transfer absorbance derived from alternating face-centered stacking. Certain Dan-Ndi mixtures undergo a dramatic color change from dark red to an almost colorless material upon crystallizing from the mesophase. Macroscopic morphology of the solid is not changed during this process. In order to investigate the origins of this interesting thermochromic behavior, Dan and Ndi side chains were systematically altered and their 1:1 mixtures were studied. We have previously speculated that the presence or absence of steric interactions due to side chain branching on the aromatic units controlled the level of color change associated with crystallization. Results from the present study further refine this conclusion including a key crystal structure that provides a structural rationale for the observed results.

2,2'-Dimethyl-4,4'-bipyridine

In the crystal structure of the title compound, C(12)H(12)N(2), the mol-ecule is twisted around the central C-C bond, with a dihedral angle of 8.32 (5)° between the mean planes of the pyridyl rings. The crystal structure is stabilized by arene stacking inter-actions, with a distance of 3.81 (1) Å between the ring centroids.