Bipyridinium Polymers That Dock Tetrathiafulvalene Guests in Water Driven by Donor-Acceptor and Ion Pair Interactions

Two water-soluble para-xylylene-connected 4,4-bipyridinium (BIPY2+) polymers have been prepared. UV-Vis absorption, (HNMR)-H-1 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 TTFBIPY2+ complexes of the mixtures, 1.8x10(6)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.