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.