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Rigid linear compounds G1 and G2, which contained two 4-phenylpyridinium (PhPy+) units, have been prepared to investigate their binding with cucurbit[8]uril (CB[8]). X-ray crystallographic structures revealed that in the solid state both compounds were included by CB[8], through antiparallel stacking, to form 2:2 quaternary complexes (G1)2@(CB[8])2 and (G2)2@(CB[8])2. For the former complex, CB[8] entrapped G1 by holding two heterodimers of its Py + and benzyl units, which were at opposite ends of the backbone. In contrast, for the first time, the second complex disclosed parallel stacking of two cationic Py + units of G2 in the cavity of CB[8] in the solid state, despite the generation of important electrostatic repulsion. Isothermal titrations in water afforded high apparent association constants of 4.36x 106 and 6.43 x 106 M-1 for 1:1 complexes G1@CB[8] and G2@CB[8], respectively, and 1H NMR spectroscopy experiments in D2O confirmed a similar stacking pattern to that observed in the solid state. A previous study and crystal structures of the 2:1 complexes formed between three new controls, G3-5, and CB[8] did not display such unusual stacking of the cationic Py+ unit; this may be attributed to the multivalency of the two CB[8] encapsulation interactions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]