Highly controllable ring-chain equilibrium in quadruply hydrogen bonded supramolecular polymers

Electron-rich dioxynaphthalene (DNP) group bridged bifunctional ureidopyrimidinone (UPy) derivatives (L1, L2, and L3) were synthesized, which could form small cyclic monomers, oligomers, or linear supramolecular polymers at certain concentration in solution, to achieve a highly controllable ring-chain equilibrium self-assembling supramolecular system. The ring-chain equilibrium of these supramolecular monomers constructed by different lengths of oligo(ethylene oxide) (oligoEO) chain as spacers were investigated by a combination of techniques, such as ¹H NMR, DOSY, single-crystal X-ray diffraction, and viscometry. The experiment results demonstrated that there exists intramolecular π-π stacking interaction between DNP group and intramolecularly dimerized UPy motif in the monomeric cyclic form of these supramolecular monomers, and the strength of this π-π stacking interaction directly depends on the length of the oligoEO chain. Furthermore, strong intramolecular π-π stacking interaction was found to promote self-assembly favorable for intramolecularly cyclic monomerization, leading to a great increase of critical polymerization concentration (CPC). Monomer L1a with the shortest length of oligoEO chain is present as an exclusive type of intramolecularly hydrogen-bonded assembly, namely the cyclic monomers, over a broad concentration range (1.6-500 mM) in solution. Single crystal structure of the cyclic monomer L1b, which is an analogue of L1a, was thoroughly studied. The CPC values of monomer L2 and L3 with longer oligoEO chain are ca. 70 and 23 mM, respectively. However, L2 and L3 could perform selective cyclization over the entire concentration range in solution after threading into the tetracationic cyclobis(paraquat-p-phenylene)cyclophane (CBPQT⁴⁺) driven by host-guest interaction, which provides another supramolecular strategy to control ring-chain equilibrium. The combined results may provide new insights into the ring-chain equilibrium and offer valuable information on the understanding of the correlation between supramolecular assistance and polymerizability.