Quadruple Switching of Pleated Foldamers of Tetrathiafulvalene-Bipyridinium Alternating Dynamic Covalent Polymers

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 center dot+. Reduction of the BIPY2+ units to BIPY center dot+ led to the formation of another kind of pleated secondary structures, which are stabilized by intramolecular dimerization of the BIPY center dot+ units. The diradical dicationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT(2(center dot+))) could further force the folded structures to unfold by including the BIPY center dot+ units of the polymers. Upon oxidation of the BIPY center dot+ 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.