Self-Assembled Pure Covalent Tubes Exhibiting Circularly Polarized Luminescence.

Here, we successfully synthesized four structurally analogous, self-assembled chiral molecular tubes with relatively high yields. This achievement involved the condensation of six equivalents of enantiomerically pure trans-cyclohexane-1,2-diamine (trans-CHDA) and three equivalents of the corresponding tetraformyl precursor. Each precursor was equipped with a luminescent linker terminated by two m-phthalaldehyde units. Even though these tetraformyl precursors are barely soluble in almost all organic solvents, the molecular tubes are highly soluble in nonpolar solvents such as chloroform, allowing us to fully characterize them in solution. The stereo-chirality of the chiral bisamino building blocks endows the frameworks of molecular tubes with planar chirality. As a consequence, all of these molecular tubes exhibit circularly polarized luminescence (CPL) with relatively large dissymmetry values |glum| up to 7×10-3, providing an efficient method for synthesizing CPL-active materials.

Self-Assembly of an Unlikely Occurring Quadrangular Tube by Modulating Intramolecular Forces.

One of the central focuses in self-assembly is precisely controlling the self-assembly pathway so that the target molecules can be produced exclusively. Trans-1,2-cyclohexanediamine contains two amino units that form a 60° angle when projected on a plane. This angle naturally favors the formation of triangular products in most cases when trans-1,2-cyclohexanediamine is used as a bisamino building block in the synthesis of macrocycles and tubes. Here, we synthesized a slightly bent tetraformyl precursor bearing a central dibenzothiophene moiety, whose 3,7-positions are functionalized with two m-phthalaldehyde units. We observed that combining this tetraformyl building block with trans-1,2-cyclohexanediamine yielded a quadrangular tube when the concentrations of the precursors were relatively high. Both experimental measurements and theoretical calculations indicate that the formation of this unlikely occurring quadrangular product was driven by the intramolecular C-H···π interactions between the dibenzothiophene building blocks within the tube framework. This driving force, however, was disturbed in the triangular tube, a smaller counterpart whose formation was considered previously much more thermodynamically favored. These results improved our fundamental understanding on how to create those products whose syntheses are considered difficult or impossible, by modulating the intramolecular driving forces.

A Self-Assembled Cage Binding Iodide Anions over Other Halide Ions in Water.

A triangular-prism shaped cage was self-assembled by the formation of a dynamic covalent bond, namely a hydrazone, in acidic aqueous solution. The hexacationic host bears a number of relatively acidic protons pointing inside the cage cavity, which is able to accommodate an iodide anion selectively in water over other halide anions such as F- , Cl- , and Br- , which are more hydrated. As a comparison, a macrocycle analogue bearing fewer positive charges and fewer hydrogen bond donors shows no anion binding ability. As inferred from the single-crystal structure and theoretical calculations, the ability of the cage to selectively recognize iodide anions results from the combination of electrostatic forces, C-H⋅⋅⋅I- hydrogen bonds, and the hydrophobic effect.

A Self-Assembled Cage for Wide-Scope Chiral Recognition in Water.

Herein, we report the self-assembly of an anionic homochiral octahedral cage by condensing six Ga3+ cations and four trisacylhydrazone ligands. The robust nature of the hydrazone bond renders the cage stable in water, where it can take advantage of the hydrophobic effect for host-guest recognition. In addition to the internal binding site, namely, the inner cavity, the octahedral cage possesses four "windows", each of which represents an external binding site allowing peripheral complexation. These internal and external binding sites endow the cage with the capability to bind a broad range of guests whose sizes could either be smaller than or exceed the volume of the cage's inner cavity. Upon accommodation of a chiral guest, one of the two cage enantiomers becomes more favored than the other, producing circular-dichroism (CD) signals. The CD signal intensity of the cage is observed to be proportional to the ee value of the chiral guest, allowing a quantitative determination of the latter.

Self-Assembly of a Purely Covalent Cage with Homochirality by Imine Formation in Water.

Self-assembly of host molecules in aqueous media via metal-ligand coordination is well developed. However, the preparation of purely covalent counterparts in water has remained a formidable task. An anionic tetrahedron cage was successfully self-assembled in a [4+4] manner by condensing a trisamine and a trisformyl in water. Even although each individual imine bond is rather labile and apt to hydrolyze in water, the tetrahedron is remarkably stable or inert due to multivalence. The tetrahedral cages, as well as its neutral counterparts dissolved in organic solvent, have homochirality, namely that their four propeller-shaped trisformyl residues adopt the same rotational conformation. The cage is able to take advantage of hydrophobic effect to accommodate a variety of guest molecules in water. When a chiral guest was recognized, the formation of one enantiomer of the cage became more favored relative to the other. As a consequence, the cage could be produced in an enantioselective manner. The tetrahedron is able to maintain its chirality after removal of the chiral guest-probably on account of the cooperative occurrence of intramolecular forces that restrict the intramolecular flipping of phenyl units in the cage framework.

Self-Assembled Triangular Prismatic Cages with Kinetic Inertness.

Two triangular prismatic cages were synthesized by combining a trishydrazide and two bisformyl precursors in strongly acidic water, where the dynamic nature of hydrazone was turned ON. An anionic guest was used as the template to drive the cage formation. Performing counterion exchange removed both the template and the Brønsted acid. The removal of the latter afforded the cages' kinetic inertness by turning OFF the reversibility of hydrazone. The cages can thus be used for recognizing various guests in water without observable degradation, driven by the hydrophobic effect. Upon being accommodated within the cage cavities, an anthracene derivative was protected from UV-stimulated oxidation, which would occur otherwise in the bulk solution without the protection from the host.

A trefoil knot self-templated through imination in water.

The preparation of topologically nontrivial molecules is often assisted by covalent, supramolecular or coordinative templates that provide spatial pre-organization for all components. Herein, we report a trefoil knot that can be self-assembled efficiently in water without involving additional templates. The direct condensation of three equivalents of a tetraformyl precursor and six equivalents of a chiral diamine produces successfully a [3 + 6] trefoil knot whose intrinsic handedness is dictated by the stereochemical configuration of the diamine linkers. Contrary to the conventional wisdom that imine condensation is not amenable to use in water, the multivalent cooperativity between all the imine bonds within the framework makes this trefoil knot robust in the aqueous environment. Furthermore, the presence of water is proven to be essential for the trefoil knot formation. A topologically trivial macrocycle composed of two tetraformyl and four diamino building blocks is obtained when a similar reaction is performed in organic media, indicating that hydrophobic effect is a major driving force behind the scene.