Construction of a Rod-Coil Supramolecular Copolymer through CB[8]-encapsulation-enhanced Donor-Acceptor Interaction

Compared to traditional polymers, supramolecular polymers have quite limited structural diversity. In this context, to develop supramolecular polymers with novel backbone structures is highly desired, since this extends not only the diversity of supramolecular polymers, but also might bring in new functions and applications. In this contribution, a rigid rod-like supramolecular monomer, in which electron-deficient viologen units are incorporated at both ends of its skeleton, was designed and synthesized. Its co-assembly with an electron-rich flexible monomer leads to the formation of a rod-coil supramolecular copolymer, driven by cucurbit[8]uril (CB[8])-encapsulation-enhanced donor-acceptor interaction between naphthol segments of the flexible monomer and viologen units of the rigid monomer. The as-prepared supramolecular polymer was systematically characterized by H-1-NMR analysis, UV-Vis spectroscopy, diffusion-ordered NMR spectroscopy (DOSY), dynamic light scattering (DLS) and transmission electron microscopy (TEM). While the H-1-NMR result indicated host-guest complexation through the encapsulation of a viologen unit and a naphthol segment in the cavity of one CB[8], the existence of donor-acceptor interaction between the two guest molecules was clearly evidenced by UV-Vis spectrum which exhibited a notable charge-transfer absorption in visible region. The formation of large size supramolecular entities in water was revealed by DOSY and DLS studies. And their linearly polymeric backbones were directly observed by TEM. Based on these experimental results, the polymeric structures of the assembled material were confirmed. Furthermore, DLS and TEM investigations also revealed that the supramolecular polymer adopted an extended linear conformation at low concentration, which further curled into aggregated morphology at high concentration. This might be attributed to the tendency of the supramolecular polymer to decrease its surface energy with increasing concentration. The supramolecular copolymer reported in this paper represents a new type of water-soluble supramolecular polymer. Its concentration-dependent morphology changes are quite interesting, which might shed light on the exploration of unique features and new applications for the supramolecular polymers.