Yiliu Liu

Supramolecular Polymerization Promoted and Controlled through Self‐Sorting

A new method in which supramolecular polymerization is promoted and controlled through self-sorting is reported. The bifunctional monomer containing p-phenylene and naphthalene moieties was prepared. Supramolecular polymerization is promoted by selective recognition between the p-phenylene group and cucurbit[7]uril (CB[7]), and 2:1 complexation of the naphthalene groups with cucurbit[8]uril (CB[8]). The process can be controlled by tuning the CB[7] content. This development will enrich the field of supramolecular polymers with important advances towards the realization of molecular-weight and structural control.

Supramolecular photosensitizers with enhanced antibacterial efficiency.

Photosensitizers, a key component in photodynamic therapy (PDT), are compounds that can transfer the energy of light to surrounding oxygen, thereby producing highly reactive oxygen species, for example singlet oxygen (O2), to destroy diseased tissues or microorganisms. From a practical application point of view, readily accessible, highly fluorescent photosensitizers with strong absorbance at long wavelengths and high singlet oxygen quantum yields are highly desirable. In particular, porphyrins and their derivatives are popularly employed as one class of important photosensitizers owing to their excellent photophysical properties. They have very intense absorption bands in the visible region and high singlet oxygen quantum yield because of their large p-conjugated aromatic domains. However, the porphyrins easily form aggregates based on hydrophobic p–p interactions in aqueous medium, especially at high local concentrations induced by uptake and accumulation processes inside cells or microorganisms. The aggregation can produce a severe selfquenching effect of the excited state, leading to quenched fluorescence and greatly reducing the ability for O2 generation, and therefore lowering the efficiency for phototherapy. To address this issue, it has been common practice to introduce space-demanding hydrophilic substituents to the parent porphyrins, for example, segregate porphyrins into the focal core of hydrophilic dendrimers. In doing so, the quenching effect can be suppressed, which leads to an appreciable improvement of the photocytotoxicity. However, such covalent practice often involves time-consuming tedious chemical synthesis and purification processes, thus raising the costs of preparation. In addition, organic solvents and toxic reagents used in chemical synthesis may be incorporated into photosensitizers and reduce their biocompatibility. Cucurbit[n]uril (CB[n]), a family of barrel-shaped macrocyclic hosts, have been developed into an interesting research area, because of their rich host–guest chemistry. The CB[n] molecules possess a hydrophilic exterior and hydrophobic cavities. Because of the existence of the hydrophobic cavity, CB[n] has been widely used, for example, to encapsulate and solubilize dyes and to enhance weak supramolecular interactions. Generally, compared with other hosts, such as cyclodextrins and calixarenes, the binding constant of CB[n] with its guests is much larger, especially to the cationic species, driven by a combination of ion–dipole interactions, hydrogen bonds, and the hydrophobic effect. Herein, the large molecular volume and hydrophilic exterior of CB[n] molecules have encouraged us to explore the possibility of using CB[n] as bulky “noncovalent building blocks” to weaken the close stacking of porphyrins, thus suppressing the self-quenching of the excited states and improving the antibacterial efficiencies even upon aggregation. In addition, the rich host–guest chemistry of CB[n] can enable the bulky substituents to be noncovalently attached to the porphyrins, which is environmentally friendly and can greatly decrease the required steps of chemical synthesis. For this purpose, a new kind of supramolecular photosensitizer has been designed as shown in Figure 1. Porphyrins are readily modified with four positive charges (TPOR), so as to efficiently adsorb onto the negative charged surface of bacteria. The other building block, CB[7], is selected as the bulky hydrophilic heads of the supramolecular photosensitizers. The strong host–guest interaction between CB[7] and naphthalene–methylpyridinium moiety on TPOR is used as the driving force for the construction of the supramolecular photosensitizers. For the construction of the desired supramolecular photosensitizers, CB[7] was added to the aqueous solutions of TPOR in a molar ratio of TPOR:CB[7] = 1:4. Different methods were employed to confirm the formation of the desired supramolecular photosensitizers. Firstly, isothermal titration calorimetry (ITC) was carried out to provide information about the binding ability of CB[7] with TPOR. The obtained titration isotherm is shown in Figure 2 a. The binding stoichiometry between TPOR and CB[7] is calculated to be 1:4, indicating that the desired supramolecular photosensitizers shown in Figure 1 have been obtained. By fitting the data, the binding constant of the naphthalene–methylpyridinium subgroup with CB[7] is calculated to be K = 6.6 10m , indicating that the driving force is quite strong and efficient interactions can take place for the noncovalent construction of the TPOR/(CB[7])4 supramolecular photosensitizers. Secondly, the formation of the supramolecular photosensitizers is confirmed by dynamic light scattering [*] K. Liu, Y. L. Liu, Y. X. Yao, Prof. Z. Q. Wang, Prof. X. Zhang Key Lab of Organic Optoelectronics & Molecular Engineering Department of Chemistry, Tsinghua University Beijing 100084 (China) E-mail: xi@mail.tsinghua.edu.cn

Cucurbit[8]uril-based supramolecular polymers.

Supramolecular polymers, whose building blocks are noncovalently connected, have attracted much attention over the last few decades. The noncovalent nature of these polymeric systems endows them with great potential for applications in the preparation of dynamic materials. Cucurbituril (CB)-based host-guest systems, which have strong binding affinity and good selectivity, have received extensive recent attention. Among the CB family, is unique for its ability to bind two guests in its cavity and form strong ternary complexes. Such -based host-guest systems have been widely used in the construction of supramolecular architechtures. In this Focus Review, following a brief description of the host-guest interactions in -based systems, we summarize the current state of play in the fabrication of -based supramolecular polymers, which mainly include small-molecule-based supramolecular polymers and polymer-based supramolecular polymers, as a good example of the marriage between the supramolecular chemistry of cucurbiturils and polymer science.

Host-Enhanced π–π Interaction for Water-Soluble Supramolecular Polymerization

Host-enhanced π-π interaction based on anthracene derivatives and cucurbit[8]uril can be used as the driving force for constructing water-soluble supramolecular polymers. For this purpose, two anthracene moieties were encapsulated into one cucurbit[8]uril cavity, forming a ternary complex. After encapsulation in the host, the distance between the two anthracene moieties was shortened, and the π-π interaction between them was enhanced significantly. To realize supramolecular polymerization, a bifunctional monomer consisting of two anthracene moieties and a short linker in between was carefully designed. Cyclization was avoided in this way. Thus, host-enhanced π-π interaction can function as a new driving force for supramolecular polymerization.

Water-soluble supramolecular hyperbranched polymers based on host-enhanced π–π interaction

Host-enhanced π–π interaction is employed as the driving force to construct supramolecular hyperbranched polymers. A three-arm monomer which possesses one naphthalene moiety in each arm has been designed and synthesised. Supramolecular hyperbranched polymers can be formed spontaneously by mixing the monomer and cucurbit[8]uril in 2:3 ratio.

Supramolecular Polymerization at Low Monomer Concentrations: Enhancing Intermolecular Interactions and Suppressing Cyclization by Rational Molecular Design

We present the construction of long-chain water-soluble supramolecular polymers at low monomer concentrations. Naphthalene-based host-enhanced π-π interactions, which possess high binding constants, were used as the driving force of supramolecular polymerization. A monomer, DNDAB, with a rigid, bulky 1,4-diazabicyclo[2.2.2]octane-1,4-diium linker was designed. The design of the monomer structure strongly influenced the efficiency of the supramolecular polymerization. The rigid, bulky linker in DNDAB effectively eliminates cyclization, promoting the formation of long-chain supramolecular polymers at low monomer concentrations. In contrast, a reference monomer containing a flexible linker (DNPDN) only forms oligomers owing to cyclization.

Mimicking biological structured surfaces by phase-separation micromolding.

In this letter, we present a very convenient and efficient technique of direct replication of biological structures via a two-step phase-separation micromolding process (PSmicroM). Our study has demonstrated that PSmicroM can be used to replicate the surface structure of a lotus leaf. On one hand, the micro/nanostructures of the lotus leaf are well replicated after a two-step PSmicroM. On the other hand, the replicated artificial lotus leaf shows good superhydrophobicity, similar to that of the natural lotus leaf. In addition, we have also applied the same technique to replicate a rice leaf and have confirmed that replicated artificial rice leaves can exhibit not only a very similar structure of the natural rice leaf but also surface anisotropic wetting. It is greatly anticipated that this PSmicroM can be extended to mimic many other biostructures, therefore opening new avenues for surface molecular engineering.

Biostructure-like Surfaces with Thermally Responsive Wettability Prepared by Temperature-Induced Phase Separation Micromolding

We present a very efficient and convenient approach to obtain smart biosurfaces by directly replicating biological surface structures. It is realized by a two-step replication process combining regular replica molding and temperature-induced phase separation micromolding (PSmicroM). The negative replicas of biological surface structures using poly(dimethylsiloxane) as the replication material are durable molds for further replication. The positive replicas of biological surface structures are obtained by the second step replication using PSmicroM of poly(N-isopropylacrylamide) aqueous solution, which can be easily carried out just by adjusting temperature. With cold water as good solvent and hot water as nonsolvent, an environmentally friendly PSmicroM process is successfully achieved, and organic solvents for PSmicroM are completely avoided. Our study has demonstrated that the micro- and nanostructures of the lotus leaf and rice leaf can be well replicated using this two-step replication process, and the replicated artificial lotus leaf and rice leaf using poly(N-isopropylacrylamide) exhibit good thermally responsive wettability.

Porphyrin-containing hyperbranched supramolecular polymers: enhancing 1O2-generation efficiency by supramolecular polymerization

Hyperbranched supramolecular polymers were obtained by mixing a naphthyl-substituted porphyrin derivative and cucurbit[8]uril in aqueous solution, which was driven by host–guest interactions. The formation of a supramolecular polymeric structure can cause disruption of the porphyrin aggregation, thus leading to enhancement of their 1O2-generation efficiency.

Water-soluble supramolecular polymers fabricated through specific interactions between cucurbit[8]uril and a tripeptide of Phe-Gly-Gly

A bifunctional monomer FGG-PEG8-GGF bearing two Phe-Gly-Gly residues and an octaethylene glycol linker has been designed and synthesized. Water-soluble supramolecular polymers can form spontaneously by mixing the monomer and cucurbit[8]uril (CB[8]) in a 1 : 1 ratio through specific interactions between CB[8] and the Phe-Gly-Gly residues.