Two water-soluble para-xylylene-connected 4,4′-bipyridinium (BIPY²⁺) polymers have been prepared. UV-Vis absorption, ¹H NMR spectroscopy, and cyclic voltammetry experiments support that in water the BIPY²⁺ units in the polymers form stable 1:1 charge-transfer complexes with tetrathiafulvalene (TTF) guests that bear two or four carboxylate groups. These charge-transfer complexes are stabilized by the donor–acceptor interaction between electron-rich TTF and electron-deficient BIPY²⁺ units and electrostatic attraction between the dicationic BIPY²⁺ units and the anionic carboxylate groups attached to the TTF core. On the basis of UV-Vis experiments, a lower limit to the apparent association constant of the TTF⋅BIPY²⁺ complexes of the mixtures, 1.8×10⁶ m⁻¹, has been estimated in water. Control experiments reveal substantially reduced binding ability of the neutral TTF di- and tetracarboxylic acids to the BIPY²⁺ molecules and polymers. Moreover, the stability of the charge-transfer complexes formed by the BIPY²⁺ units of the polymers are considerably higher than that of the complexes formed between two monomeric BIPY²⁺ controls and the dicarboxylate-TTF donor; this has been attributed to the mutually strengthened electron-deficient nature of the BIPY²⁺ units of the polymers due to the electron-withdrawing effect of the BIPY²⁺ units.

This Personal Account summarizes the development of the design of hydrogen-bonding-driven aromatic foldamers and their applications in supramolecular chemistry. This account begins with a short, non-exhaustive description of the background in the study of hydrogen-bonded aromatic foldamers. The construction of foldamers from discrete aromatic residues, including amide, urea, hydrazide and 1,2,3-triazole units, is then described. In the following parts, the applications of such compact aromatic oligomers in supramolecular chemistry, including the development of acyclic receptors, the construction of hybrid helical sequences through intramolecular stacking, the promotion of the formation of covalently and noncovalently bonded macrocycles, the tuning of the mechanical properties of copolymers, and the regulation of the shuttling behavior of [2]rotaxanes and the controlled release of dye from reverse vesicles formed from them, are described. In the final part, the binding-induced folding of conformationally flexible aromatic amide oligomers and their self-binding are described.

Two dynamic covalent polymers P1 and P2 were prepared by alternately linking electron-rich tetrathiafulvalene (TTF) and electron-deficient bipyridinium (BIPY²⁺) through hydrazone bonds. In acetonitrile, the polymers were induced by intramolecular donor–acceptor interactions to form pleated foldamers, which unfolded upon oxidation of the TTF units to the radical cation TTF^.+. Reduction of the BIPY²⁺ units to BIPY^.+ led to the formation of another kind of pleated secondary structures, which are stabilized by intramolecular dimerization of the BIPY^.+ units. The diradical dicationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT^2(.+)) could further force the folded structures to unfold by including the BIPY^.+ units of the polymers. Upon oxidation of the BIPY^.+ units of the cyclophane and polymers to BIPY²⁺, the first folded state was regenerated. Switching or conversion between the four conformational states was confirmed by UV/Vis spectroscopic experiments.

Two dynamic covalent polymers P1 and P2 were prepared by alternately linking electron-rich tetrathiafulvalene (TTF) and electron-deficient bipyridinium (BIPY²⁺) through hydrazone bonds. In acetonitrile, the polymers were induced by intramolecular donor–acceptor interactions to form pleated foldamers, which unfolded upon oxidation of the TTF units to the radical cation TTF^.+. Reduction of the BIPY²⁺ units to BIPY^.+ led to the formation of another kind of pleated secondary structures, which are stabilized by intramolecular dimerization of the BIPY^.+ units. The diradical dicationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT^2(.+)) could further force the folded structures to unfold by including the BIPY^.+ units of the polymers. Upon oxidation of the BIPY^.+ units of the cyclophane and polymers to BIPY²⁺, the first folded state was regenerated. Switching or conversion between the four conformational states was confirmed by UV/Vis spectroscopic experiments.

Herein, we report an efficient route for the asymmetric synthesis of β²-aminoxy acids as well as experimental and theoretical studies of conformations of peptides composed of β²-aminoxy acids. The nine-membered-ring intramolecular hydrogen bonds, namely, β NO turns, are generated between adjacent residues in those peptides, in accordance with our computational results. The presence of two consecutive homochiral β NO turns leads to the formation of β NO helical structures in solution, although both helical (composed of two β NO turns of the same handedness) and reverse-turn (composed of two β NO turns with opposite handedness) structures are of similar stability, as suggested by theoretical studies. Nevertheless, two slightly different conformations, with the same handedness, of β²-aminoxy monomers have been observed in the solid state and in solution according to our X-ray and 2D NOESY studies.