Supramolecular Self-assembly Based On Quadruple Hydrogen Bonding

Hydrogen bonding has been playing an important role in supramolecular chemistry due to its directional and reversible properties. Inspired by the base pairs in DNA, scientists have tried to synthesize hydrogen bonding arrays with more binding sites to realize enhanced association constant. In 1997, Meijer and coworkers from Netherlands have successfully synthesized the ureidopyrimidinone (UPy) motif for the first time, which can self-assemble into dimers via quadruple hydrogen bonding. Since then, UPy has attracted much attention due to its high association constant (Kass> 107 M-1 in chloroform) and synthetic accessibility. In this thesis, we have constructed a series of complex supramolecular systems based on UPy, and their self-assembly mechanism and functionalization were thoroughly studied. This work consists three parts as follows:Part 1, we synthesized a series of dynamic [2]catenanes interlocked by quadruple hydrogen bonding. The dynamic catenation process was due to the selective cyclization of bifunctional UPy molecules over a wide concentration range with the assistance of the cyclobis(paraquat-p-phenylene) (CBPQT4+) macrocycle. The self-assembled [2]catenanes were stabilized by a combination of different non-covalent interactions comprising quadruple hydrogen bonding and host-guest interactions. By taking the advantage of hierarchical self-assembly, all of the isolated yields of these catenane are exceed 90%. In addition, we investigate the dynamic properties of such kind of non-covalent interlocked catenanes by employing the ring-exchange experiment. This type of non-covalent bonded catenane is the first one interlocked by multiple hydrogen bonding interactions instead of metal-ligand interactions.Part 2, 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 Lla 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 Lla, was thoroughly studied. The CPC values of monomer L2 and L3 with longer oligoEO chain are ca.70 and 23 mM, respectively. 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.Part 3, a new kind of dynamic supramolecular polymers have been constructed by orthogonal self-assembly. We synthesized two different kinds of monomers:the heteroditopic building block H1, which bears one B21C7 moiety and one UPy unit, and the homoditopic building block Gl, which is comprised of two symmetrical dialkylammonium groups. Linear supramolecular polymers were achieved, in which a supramolecular dimer of H1 was initially obtained followed by polymerization upon addition of Gl. More importantly, it has been further demonstrated that the disassembly/assembly of supramolecular polymers in solution can be reversibly and orthogonally switched by addition/removal of K+ without interfering with UPy dimerization.In summary, several complex supramolecular systems based on ureidopyrimidinone including dynamic catenane, supramolecular polymerization mechanism, and supramolecular polymers by orthogonal self-assembly have been studied. The results we obtained can provide further insights into molecular devices and smart materials in future.