Noncovalent Assembly of Functional Groups on Calix[4]Arene Molecular Boxes

Abstract

Calix[4]arenes diametrically substituted at the upper rim with two melamine units spontaneously form welldefined box-like assemblies in the presence of two equivalents of 5,5-diethylbarbituric acid. These assemblies, consisting of nine different components, are held together by 36 hydrogen bonds and are stable in apolar solvents at concentrations of up to 1 0 -4 M . This paper reports the first X-ray crystal structure, and the MALDI TOF mass spectra together with the complete 'H NMR spectroscopic characterization of these hydrogen-bonded assemblies. The crystal structure clearly shows that the assemblies are stereogenic, as a result of the antiparallel orientation of the two rosette motifs. Furthermore, the synthesis of twelve new l ,3-bis(melamine)calix[4]arenes carrying different numbers and types of functionalities at the upper rim is described. Detailed 'HNMR spectroscopic studies on the as-

Chiral exciton coupling of merocyanine dyes within a well defined hydrogen-bonded assembly

“…Our assignment of M and P helicity is based on a clockwise (P) or counterclockwise (M) orientation of the two melamine units of 1 in assembly 1 3 ⅐(BAR) 6 . Because both dimelamine 1 and BAR are achiral, assembly 1 3 ⅐(BAR) 6 is formed as a racemic mixture of P and M enantiomers and therefore is optically inactive (18). However, the supramolecular arrangement of the BAR chromophores can be controlled in a unidirectional manner by using chiral building blocks (21).…”

Induction of Liquid Crystallinity by Self‐Assembled Molecular Boxes

“…The thermal stability of the liquid-crystalline phase is remarkable, considering the dimensions (1.2 nm in height; 3.3 nm in width) and the noncovalent nature (12 DAD·ADA hydrogen bonds; D = donor, A = acceptor) of the double-rosette assembly. [19] In contrast, the DSC thermogram of 1 exhibits only one peak. This peak is attributed to a transition between a crystalline phase and an isotropic phase on the basis of POM, which reveals the spherulite texture of the crystalline phase.…”

“…This observation indicates that the side-chains of the cyanuric acid components do not interfere with the self-organization and packing, probably because the dimensions of the doublerosette scaffold are much larger than those of the side chains. [19] Herein, we have demonstrated that the self-assembled double rosettes 1 3 ·(DEB) 6 and 1 3 ·(BuCYA) are able to form liquid crystals. The strength of the hydrogen bonds within the double-rosette assemblies and the spatial disposition of the side chains of the barbituric and cyanuric acid derivatives are crucial factors for the stability of the mesophase.…”

“…The mesogenic double-rosette assemblies 1 3 ·(DEB) 6 (DEB = 5,5-diethylbarbituric acid) and 1 3 ·(BuCYA) 6 (BuCYA = N-butylcyanuric acid) form spontaneously upon mixing 1 with 2 equiv of either DEB or BuCYA in an apolar solvent ( Figure 1). [19] The assemblies, which are held together by 36 hydrogen bonds, are thermodynamically stable, even at a concentration of 5 mm. [20] The calix [4]arene dimelamine 1 was functionalized with octadecyl chains to promote the self-organization of the double-rosette assemblies into a liquid-crystalline phase.…”

“…The signals assigned to the imide protons (a, b) of DEB and BuCYA at d NH = 15-14 ppm are diagnostic of the formation of the assemblies; upon hydrogen-bond formation, the signals of these protons undergo a strong downfield shift (in free DEB, d NH = 8.40 ppm; in free BuCYA, d NH = 11.2 ppm). [19] The thermal behavior of the assembly and of the isolated component 1 was determined by polarized optical microscopy (POM) and differential scanning calorimetry (DSC; Table 1). The DSC thermograms of the assemblies each exhibit two peaks associated with phase transitions.…”