Photo-Responsive Supramolecular Polymers Based on Host-Guest Interactions
The photoresponsive functional groups are introduced into supramolecular polymer systems via noncovalent interaction to obtain photoresponsive supramolecular polymers, which can combine the unique properties of supramolecular polymers with the advantages of photochemical reactions. This chapter summarizes the recent progress on photoresponsive supramolecular polymers based on different hosts, including cyclodextrin (CD), cucurbituril (CB), crown ether, calixarene, and pillararene, which dominate diverse systems with various functions like self-healing, donor–acceptor interactions, aggregation-induced emission, light-regulated assembly and dissociation, and so forth. Furthermore, prospects are made to pave a way for the development of photoresponsive supramolecular polymers.
Owing to their potential applications in the fields of optics, biology, and medicine, supramolecular materials are drawing more and more attention from researchers. Supramolecular polymers are constructed from monomer components, small supramolecular systems, or polymers that are repeatedly linked by such reversible noncovalent interactions as hydrogen bonds, π-π stacking, hydrophobic effects, host–guest interaction, and electrostatic interactions . Since the noncovalent interactions are reversible, supramolecular polymers tend to make differences to external factors, such as pH changes, chemical and electrochemical redox, light stimulation, temperature/concentration changes, enzyme stimuli, etc. and accompanied by assembly conformation or property changes.
The host–guest interaction is the driving force of the host–guest complex, which involves the synergy of a variety of noncovalent bonds, such as hydrophobic interactions, hydrogen bonds, ionic bonds, van der Waals forces, and electrostatic interactions. Since the discovery of cryptands and crown ethers by Lehn, Cram, and Pedersen, the host–guest system has greatly contributed to the development of supramolecular chemistry. As the name suggests, the host–guest system consists mainly of two components: the host and the guest, which form the supramolecular inclusion. The host molecule usually contains a cavity that specifically recognizes the guest molecule. The host–guest function has characteristic selectivity, because the subject has various restrictions on the object, such as size, shape, charge, and polarity. In the development of supramolecular chemistry, the control and regulation of molecular recognition based on host–guest interaction in supramolecular polymers has attracted a lot of attention . The mechanism of polymerization and the nature of the polymer are determined by the strength and nature of the host–guest units.
The host molecules involved in the preparation of supramolecular polymers based on host–guest systems are typically crown ethers, cyclodextrins, cucurbiturils, calixarenes, and column aromatics. And the guest molecules are generally organic compounds that can enter the bulky cavity. In the following, we will outline the development of photoresponsive supramolecular polymers in this field according to the classification of the host molecules.
Organic materials have many advantages such as good regulation, rich color, smart molecular design, low cost, and toxicity compared with inorganic materials. The combination of supramolecular chemistry to construct highly efficient luminescent materials not only simplifies the preparation process but also imparts good stimuli responsiveness and reversibility to these materials through noncovalent attachment. The noncovalently guided assembly–disassembly process can affect the aggregation and energy transfer between the fluorophores , thus regulate the luminescence behavior of the materials. Therefore, luminescent systems constructed concerning supramolecular systems are generally capable of responding to external stimuli such as pH, temperature, solvent polarity, light radiation, redox, etc. accompanied by reversible conformation and structural transformation. In recent years, materials with tunable luminescence properties have been demonstrated their potential applications in many fields, such as optoelectronic devices, fluorescence sensing, and imaging agents . Due to the introduction of many functional units , reports on supramolecular polymers will be further explored because of its luminescent properties.
Herein, we discussed an overview of the photoresponsive supramolecular systems induced by the host–guest interaction. The host–guest recognition is an important noncovalent bonding using hosts such as CD, CB, crown ether, calixarene, and pillararene to encapsulate the guests in the constructed supramolecular system. Furthermore, the guest unit undergoes a reversible structure or conformation difference under specific light stimulation, resulting in the corresponding changes of the supramolecular polymer. In this chapter, we introduce the photoresponsive supramolecular systems according to the different types of hosts and make summaries and outlooks on its functions and applications.
Photoresponsive Supramolecular Polymers Based on Cyclodextrin (CDs)
CDs are a class of widely used supramolecular macrocyclic host molecules, which are cyclic oligomers composed of α-(1-4)-glycosidically linked glucopyranose units. In the structure of the CD, there is a hydrophilic surface and a hydrophobic cavity, and its hydrophobic cavity can form an inclusion in water with a series of organic molecules. Because of this unique advantage of CD, scientists have linked it to the phosphorescent emission of organic molecules. Reports on CD-induced room temperature phosphorescence (CD-RTP) were first published in the early 1980s . In 2011, we also reported a RTP addressing pseudorotaxane induced by the inclusion of β-CD and α-BrNp. After that, we constructed a supramolecular hydrogel system capable of rapidly self-healing and room temperature phosphorescence based on the action of β-CD and α-BrNp .
CD-Based Polymer Networks and Hydrogels
Xu et al. (2015)  reported a new kind of drug carrier constructed by supramolecular polymers. Due that the prolonged ultraviolet (UV) exposure is harmful to cells and UV light has limited tissue penetration ability, this system offers a solution using a magnetic field to aggregate microcapsules to an accurate area and then release the drug upon UV light. The grafted β-CD of CD-g-DexO and AD-PASP will be broken down in an acidic tumor environment, leading to the release of drugs to the certain location. This research of the drug carrier is expectedly designed as multimodal functional imaging probe for treatment of cancer.
CD-Based Polymer Self-Assemblies and Vesicles
Due to the excellent chemical stability and remarkable biocompatibility, nanodiamonds (NDs) have received widespread research attention by the biomedical field. The excellent water dispersibility of NDs has significant importance for biomedical applications. Therefore, surface modification of NDs with hydrophilic polymers has been extensively investigated over the past few decades. Zhang et al. (2018)  synthesized β-CD containing hyperbranched polymer functionalized ND (ND-β-CD-HPG) composites with high water dispersibility via supramolecular chemistry based on the host–guest interaction between β-CD and adamantine (Ad). The hydroxyl groups of NDs first reacted with 1, 1-adamantanecarbonyl chloride to obtain ND-Ad, which was further functionalized with β-CD containing hyperbranched polymers to form the final ND-β-CD-HPG composites. The successful preparation of ND-β-CD-HPG composites was confirmed by several characterization techniques. Furthermore, the loading and release of the anticancer agent doxorubicin hydrochloride (DOX) on ND-β-CD-HPG composites was also examined to explore its potential in drug delivery. When compared with traditional methods of surface modification of NDs, this method was convenient, fast, and efficient. We demonstrated that ND-β-CD-HPG composites have great water dispersibility, low toxicity, high drug-loading capacity, and controlled drug-release behavior. Based on these characteristics, ND-β-CD-HPG composites are expected to have high potential for biomedical applications.
Jiang et al. (2018)  prepared an oligo(ethylene glycol)-based amphiphilic star polymer containing fluorescent coumarin as end groups and dual tertiary amine as center. This polymer could self-assemble into vesicles in the aqueous solution. The crosslinking pattern in the hydrophobic membrane of the vesicles could form noncovalent crosslinking by adding γ-CD into the solution, and the formed 2/1 host–guest inclusion between γ-CD and coumarin groups led to a higher sensitivity and faster disassembly speed of the vesicles by injecting CO2; while after 365 nm light irradiation, the formed coumarin dimers acting as crosslinking point gave a more stable hydrophobic membrane and less sensitive to CO2. The work reported here gives a notable polymer system that the CO2-responsive behaviors can be easily tuned by controlling the crosslinking pattern, bearing a great promise in the areas including latexes, surfaces, sensors, carriers, and so on.
Zhang et al. (2014)  reported a fabrication of photoresponsive block-controllable supramolecular polymer which was constructed through host–guest interactions. This supramolecular polymer based on the assembly of two homopolymers according to the host–guest recognition between CDs and Ad/Azo moieties in aqueous solution is a kind of triblock polymer. Upon alternating irradiation of UV/visible light, this triblock polymer can reversibly transform into supramolecular diblock polymers with the change in morphology between the self-assembly and disassembly. In addition, this supramolecular polymer showed potential applications in stimuli-responsive drug delivery systems.
Yuan et al. (2017)  prepared a kind of functional (PCL-CD)16/Azo-PDMAEMA supramolecular aggregates with tunable morphologies based on the IC between the dendritic host polymer and the linear guest polymer. The morphologies of the aggregates could be adjusted by changing the molar ratio of (PCL-CD)16: Azo-PDMAEMA in the supramolecules. The supramolecular aggregates changed from nanorods to nanowires, and then to spherical micelles when the (PCL-CD)16:Azo-PDMAEMA molar ratio was changed from 1:1 to 1:8 and 1:16, respectively. Benefitting from the UV-response of β-CD/Azo IC, the supramolecular aggregates demonstrated UV responsive properties. Upon UV light irradiation, the morphologies of the aggregates became irregular and agglomeration occurred. Meanwhile, because of the thermoresponsive PDMAEMA, the supramolecular aggregates showed thermoresponsive properties. When the temperature was increased, the aggregates changed into smaller aggregates and then aggregated with each other upon further heating. Therefore, the supramolecular aggregates with tunable morphologies were UV- and thermoresponsive. These functional nanomaterials have potential applications in nanotechnology and biomedicine.
Photoresponsive Supramolecular Polymers Based on Cucurbituril (CBs)
However, due to the difficulty in the modification of CBs, supramolecular polymers based on CB, CB, and CB are rarely reported. The particularity of CB is that its larger cavity can entrap two specific guest molecules. Therefore, in the CB family, CB is the most promising host molecule for the construction of supramolecular polymers. However, limited by the poor water solubility (<1 mM) of CB, in most cases, dimerization and oligomerization cyclization dominate in diluted aqueous solutions compared to linear polymerization.
Adjustable Dynamic Photophysical Properties Based on CB
Supramolecular Polymer with Photoisomerism Based on CB
Scherman et al. (2017)  reported a light and chemical responsive supramolecular hyperbranched-like polymer (SHP) with 1:1:1 heteroternary complex between methyl viologen (MV), azobenzene, and CB, which could reversibly achieve the conversion between (Z)-B3–CB and (E)-B3–CB upon exposure to blue light or heat. SHP created an extended physically crosslinked supramolecular network which can be formed at the interface of a droplet, thus it could be further explored as surfactant in the future.
Zhang et al. (2018)  demonstrated the fabrication of water-soluble cucurbituril (CB)-mediated supramolecular polymers by connecting the fluorinated azobenzene (FAB)-containing monomers through host-enhanced heteroternary π-π stacking interactions. Benefiting from the unique visible-light-induced E → Z photoisomerization of the FAB photochromophores, the encapsulation behaviors between the CB macrocycle and the monomers can be regulated upon visible light irradiation, resulting in the depolymerization of such CB-mediated supramolecular polymers. This CB-based hybrid recognition motif was expected to be served as molecular trafficking system, which may find widespread applications such as drug delivery and release, photopharmacology, and so on. The use of visible light as external stimulus provided well remote spatiotemporal controllability, together with the advantages of mild aqueous conditions at room temperature. The development of such unique visible-light-regulated CB-mediated supramolecular polymers as well as the related hybrid host–guest systems was highly valuable in the construction of biocompatible smart materials.
Controllable Supramolecular Polymerization Based on CB
Photoresponsive Supramolecular Polymers Based on Crown Ether
A crown ether is a cyclic compound composed of a plurality of ether groups. As the first generation of synthetic macrocyclic bodies, crown ether marks the birth of supramolecular chemistry. The host–guest interaction between the crown ether and the guest molecule, usually the secondary ammonium salt and the paraquat derivative, is a secondary interaction that can be used to construct smart materials to simulate natural systems. In the past few decades, crown ethers like 18-crown-6 (18C6), diphenyl-24-crown-8 (DB24C8), double-form benzene-26-crown-8 (BMP26C8), and double-form benzene-32-crown-10 crown ethers (BMP32C10) are used to construct topologies such as molecular machines  and supramolecular polymers . In order to expand the applications of molecular recognition based on crown ethers, many researchers have begun to challenge the development of crown ethers, like preparing organic white light-emitting materials.
Networks and Hydrogels of Supramolecular Polymer Based on Crown Ether
Photoresponsive supramolecular polymer systems based on crown ethers are rarely reported in recent years. Huang et al. (2015)  reported novel supramolecular polymers by utilizing the host–guest interaction with dependent photoresponse and self-healing property. A conjugated polymer 1 and the guest component 2 were used for the fabrication of the polymer network which could be disassembled by the inducement of different signals. The conjugated polymer in its assembled state showed a weak fluorescence, while it was enhanced in the presence of potassium ion, chloride ion, pH increase, and heating.
Conjugated Polymer Network and Its Disassembly Induced by Different Signals
Lee et al. (2016)  synthesized rotaxane-type hyperbranched polymers for the first time from A2B type semi-rotaxane monomers formed in situ via complexation of bis(m-phenylene)-32-crown-10 dimethanol and two paraquat x-n-alkylenecarboxylic acid derivatives with tris(p-t-butylphenyl)methylphenylalkylene stoppers. The molecular size increases upon formation of the hyperbranched polymers are confirmed by dynamic light scattering and by viscometry. As with covalent hyperbranched polymers, a number of potential applications exist; the unique mechanically linked character and the presence of uncomplexed host and guest moieties foreshadow the use of such systems for their responses to external stimuli with the added benefit of providing molecular recognition sites useful as delivery vehicles. Use of other host–guest motifs to form the semi-rotaxane A2B monomers is possible and complementary systems with higher binding constants will enable efficient syntheses of high molecular weight, mechanically linked hyperbranched polymers.
Liu et al. (2016)  successfully constructed a crown ether-based supramolecular gel with a three-dimensional network structure via a hierarchical induced assembly strategy. Therein, the coordination linkage of bis(terpyridyl)dibenzo-24-crown-8 (1) with Zn2+ produces the 1/Zn2+ supramolecular polymer as the primary assembly. Then, the anthryl-dibenzylammonium guest 2 was grafted to the 1/Zn2+ supramolecular polymer via the noncovalent association of dibenzylammonium moieties on guests with the dibenzo-24-crown-8 rings on supramolecular polymer. Furthermore, the supramolecular polymer with anthryl grafts underwent a photo-induced secondary assembly to produce the supramolecular gel, which could be reversibly disassembled by a thermoinduced dissociation of anthracene dimers. These photo/thermo-induced hierarchical assembly/disassembly behaviors will provide a potential way to construct degradable hierarchical supramolecular assemblies employing environment-friendly external stimuli, such as light and heat, as controlling method.
Photoresponsive Supramolecular Polymers Based on Calixarene
Calixarenes are macrocyclic molecules composed of 2- and 6-positions of methylene-linked phenolic units and are a class of widely studied organic supramolecular hosts. Because it is easy to be modified, its possibilities for development are unlimited. The encapsulation ability of the unmodified calixarene cavity is not as good as the crown ether, cyclodextrin, and cucurbituril mentioned above, so extensive chemical modification of calixarene is necessary to achieve sufficient inclusion.
Supramolecular Polymer with Photoisomerism Based on Calixarene
According to the host–guest recognition, Ma et al. (2014)  prepared a supramolecular polymer formed by sulfonated biphenylarene and dithiophene ethylene derivatives. In this supramolecular complex, the resulting polymer exhibits significant color changes and morphological changes at different wavelengths of light. In advance, it can achieve a variety of functions by combining different functional units, which is an important develop direction of supramolecular polymer.
Networks and Self-Assemblies of Supramolecular Polymer Based on Calixarene
Pappalardo et al. (2016)  synthesized a poly(p-phenyleneethynylene) polymer (PCF ), bearing two π-rich cone-like calixarene cavities (assembling cores) attached to a rigid p-phenyleneethynylene spacer by a Pd-catalyzed cross-coupling reaction. UV-vis absorption and fluorescence spectroscopies combined with dynamic light scattering measurements provided evidence for the self-assembly of PCF (homopolytopic host molecule) with a complementary C60 fulleropyrrolidine (C60-Pyr) guest in solution, in the construction of a supramolecular polymer network. Atomic force microscopy analysis of PCF/C60-Pyr highlighted the formation of a bicontinuous network consisting of a uniform distribution of prominent structures, within a polymeric background forming a biphasic structure.
Haino et al. (2014)  described a facile process for fabricating fullerene polymers driven by host–guest interactions between calixarene and C60. The formation and structures of the supramolecular polymers and networks were carefully examined using a variety of spectroscopic techniques. During the experiments, the dimensions of the supramolecular polymers were controlled by the rational design of the monomer components. Although each of the monomers was connected via noncovalent interactions, supramolecular polymers belong to the class of polymers in terms of macroscopic properties. The designability of supramolecular polymers can open up new possibilities for molecular organization in nanospace.
Photoresponsive Supramolecular Polymers Based on Pillararene
Cross-Linked Polymer Networks Based on Pillararene
Based on the reversible [4 + 4] cycloaddition reaction of anthracene and the host–guest binding between columnarene and imidazole, Yang et al. (2013)  designed a supramolecular polymer with light/heat double stimulus response SP7. Short-time heating (1 min) of the supramolecular polymer solution will only lead to decomposition of the host and guest complexes. However, prolonged heating (1 day) and recooling will result in the depolymerization of anthracene dimer units. The depolymerization of the unit, which forms an oligomer, can be restored to its original supramolecular state by cooling or light irradiation, but it is to be noted that the transition time of the polymer to the monomer is too long to achieve effective light control.
Supramolecular Polymer with Photoisomerism Based on Pillararene
The construction of supramolecular systems with azobenzene as bridging agent has recently set off a boom. Yu et al. (2014)  represented a linear supramolecular polymer and a supramolecular polymer network, and both of them were based on the host–guest interactions with photoresponsive characteristics. The linear supramolecular polymer was fabricated from self-assembly of an azobenzene moiety acting as a molecular bridge, pillararene dimer and a bisammonium salt. The supramolecular polymer network was constructed by the secondary ammonium salt which exhibited the recognition of molecular motif. The presence of the host–guest complexation made it possible of the reversible transitions between polymer–oligomer and polymer–monomer upon UV irradiation and PH changes.
Supramolecular Polymer with Two Guest Monomers Based on Pillararene
Yang et al. (2018)  presented the successful fabrication of a new family of AIE cross-linked supramolecular polymer through hierarchical self-assembly involving coordination and host–guest interaction. From a newly synthesized dipyridyl donor G1 containing a TPE scaffold and two nitrile units, a rhomboidal metallacycle G2 with four nitrile units was smoothly obtained. A pillararene dimer H was also successfully obtained to play as a linker. Subsequently, driven by the host–guest interaction between rhomboidal metallacycle G2 and pillararene dimer H, a new family of AIE cross-linked supramolecular polymer was obtained. Interestingly, such fluorescent supramolecular polymer exhibited dual-stimuli responsive behaviors to halide and competitive guest. Thus this research enriches the family of supramolecular polymer with multiple functionalities, which may find potential applications in the fields of molecular sensors and biological imaging in the future.
The overview we discussed here is surrounded with the photoresponsive supramolecular polymer systems constructed via the host–guest interaction, using hosts including CD, CB, crown ether, calixarene, and pillararene, to encapsulate the guests in the constructed supramolecular polymer systems. Furthermore, guest units undergo different reversible structures or conformations under specific light stimulation, resulting in the corresponding changes of the supramolecular polymer. As far as we can see, photoresponsive supramolecular polymer is one of the hotspots of supramolecular chemistry in recent years. Since the photoresponsive supramolecular polymer combines the properties of supramolecular polymer, photoisomerization unit, and light absorption unit, it has extra advantages like long-range responsiveness, low toxicity, and so on. The changing of specific light not only can achieve the reversible regulation of supramolecular polymer structure and its assembly morphology but also will effectively expand its applications in tissue engineering, drug carrier, and chemical conversion. At the same time, it has important academic significance to seek simple and effective means to achieve the transformations between supramolecular polymer and covalent polymer . According to the recent published research, the wavelength of most incident lights are still concentrated in the ultraviolet and visible areas, therefore the ability to introduce near infrared components into the polymer will greatly expand its application in biochemistry. Nowadays, self-assembly supramolecular assembled by thermal equilibrium has been progressively approaching the kinetic-driven system, which has proved more potential value in the development of supramolecular chemistry. In addition, the challenge to be solved is finding more effective and universal methods to precisely dominate the supramolecular polymers, such as the control of the size and dimension of supramolecular polymer .
We gratefully acknowledge the financial support from the National Natural Science Foundation of China (NSFC) (21722603, 21871083, and 21476075), Project supported by Shanghai Municipal Science and Technology Major Project (Grant No.2018SHZDZX03), the Innovation Program of Shanghai Municipal Education Commission (2017-01-07-00-02-E00010), and the Fundamental Research Funds for the Central Universities.
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