Multiresponsive Azobenzene End-Cap for Self-Immolative Polymers

Small-Molecule-based Supramolecular Plastics Mediated by Liquid-Liquid Phase Separation

Plastics are one of the most widely used polymeric materials. However, they are often undegradable and non-recyclable due to the very stable covalent bonds of macromolecules, causing environmental pollution and health problems. Here, we report that liquid-liquid phase separation (LLPS) could drive the formation of robust, stable, and sustainable plastics using small molecules. The LLPS process could sequester and concentrate solutes, strengthen the non-covalent association between molecules and produce a bulk material whose property was highly related to the encapsulated water amounts. It was a robust plastic with a remarkable Young's modulus of 139.5 MPa when the water content was low while became adhesive and could instantly self-heal with more absorbed water. Finally, responsiveness enabled the material to be highly recyclable. This work allowed us to understand the LLPS at the molecular level and demonstrated that LLPS is a promising approach to exploring eco-friendly supramolecular plastics that are potential substitutes for conventional polymers.

Recent advances in self-immolative linkers and their applications in polymeric reporting systems

Interest in self-immolative chemistry has grown over the past decade with more research groups harnessing the versatility to control the release of a compound from a larger chemical entity, given...

Advances and opportunities in the exciting world of azobenzenes

Azobenzenes are archetypal molecules that have a central role in fundamental and applied research. Over the course of almost two centuries, the area of azobenzenes has witnessed great achievements; azobenzenes have evolved from simple dyes to 'little engines' and have become ubiquitous in many aspects of our lives, ranging from textiles, cosmetics, food and medicine to energy and photonics. Despite their long history, azobenzenes continue to arouse academic interest, while being intensively produced for industrial purposes, owing to their rich chemistry, versatile and straightforward design, robust photoswitching process and biodegradability. The development of azobenzenes has stimulated the production of new coloured and light-responsive materials with various applications, and their use continues to expand towards new high-tech applications. In this Review, we highlight the latest achievements in the synthesis of red-light-responsive azobenzenes and the emerging application areas of photopharmacology, photoswitchable adhesives and biodegradable materials for drug delivery. We show how the synthetic versatility and adaptive properties of azobenzenes continue to inspire new research directions, with limits imposed only by one's imagination.

Hypersensitive azobenzenes: facile synthesis of clickable and cleavable azo linkers with tunable and high reducibility

The aim of this work is to show that by increasing the number of donor substituents in a donor/acceptor system, the sensitivity of the azobenzene linkage towards a reductive cleavage reaction can be enhanced to unprecedented high levels. For instance, in a triple-donor system, less than a second constitutes the half-life of the azo (N[double bond, length as m-dash]N) bond. Synthetic access to such redox active scaffolds is highly practical and requires only 1-2 synthetic steps. The fundamental molecular design is also adaptable. This is demonstrated through scaffold functionalization by azide, tetraethylene glycol, and biotin groups. The availability of the azide group is shown in a copper-free 'click' reaction suitable in context with protein conjugation and proteomics application. Finally, the clean nature of the scission process is demonstrated with the help of liquid chromatography coupled with mass analysis. This work, therefore, describes development of cleavable azobenzene linkers that can be accessed with synthetic ease, can be multiply functionalized, and show a clean and rapid response to mild reducing conditions.

Depolymerization of Trityl End-Capped Poly(Ethyl Glyoxylate): Potential Applications in Smart Packaging

The temperature-dependent depolymerization of self-immolative poly(ethyl glyoxylate) (PEtG) capped with triphenylmethyl (trityl) groups is studied and its potential application for smart packaging is explored. PEtGs with four different trityl end-caps are prepared and found to undergo depolymerization to volatile products from the solid state at different rates depending on temperature and the electron-donating substituents on the trityl aromatic rings. Through the incorporation of hydrophobic dyes including Nile red and IR-780, the depolymerization is visualized as a color change of the dye as it changes from a dispersed to aggregated state. The ability of this platform to provide information on thermal history through an easily readable signal makes it promising in smart packaging applications for sensitive products such a food and other cargo that is susceptible to degradation.

Postsynthetic Addition of Ligand Struts in Metal-Organic Frameworks: Effect of Syn/Anti Addition on Framework Structures with Distinct Topologies

For the parent metal-organic framework (MOF) of UiO-type 1, postsynthetic bromination (anti addition) was readily carried out on the stilbene-ligand struts by directly adding excess bromine, whereas successful hydrogenation (syn addition) can only be achieved by slowing the reaction rate; otherwise, the crystalline structure of 1 will be irreversibly damaged. Meanwhile, Kagomé-like MOF 2 can be smoothly modified by both postsynthetic bromination and hydrogenation. This study of the structural conversion not only represents the first example that adopts postsynthetic hydrogenation for modifying MOFs but also reveals various factors such as the reaction manner of syn/anti, framework topology, and reaction rate that can influence postsynthetic modification on the ligand struts of MOFs.

Molecular characteristics of a fluorescent chemosensor for the recognition of ferric ion based on photoresponsive azobenzene derivative

Metal ion recognition is of great significance in biological and environmental detection. So far, there is very few research related to the ferric ion sensing based on photoresponsive azobenzene derivatives. In this work, we report a highly selective fluorescent "turn-off" sensor for Fe³⁺ ions and the molecular sensing characteristics based on an azobenzene derivative, N-(3,4,5-octanoxyphenyl)-N'-4-[(4-hydroxyphenyl)azophenyl]1,3,4-oxadiazole (AOB-t8). The binding association constant was determined to be 6.07×10³M^-1 in ethanol and the stoichiometry ratio of 2:2 was obtained from Job's plot and MS spectra. The AOB-t8 might be likely to form the dimer structure through the chelation of ferric ion with the azobenzene moiety. Meanwhile, it was found that the photoisomerization property of AOB-t8 was regulated by the binding with Fe³⁺. With the chelation of Fe³⁺, the regulated molecular rigidity and the perturbed of electronic state and molecular geometry was suggested to be responsible for the accelerated isomerization of AOB-t8 to UV irradiation and the increased fluorescence lifetime of both trans- and cis-AOB-t8-Fe(III). Moreover, the reversible sensing of AOB-t8 was successfully observed by releasing the iron ion from AOB-t8-Fe(III) with the addition of citric acid.

Poly(ester amide)s with pendant azobenzenes: multi-responsive self-immolative moieties for modulating polymer assemblies

Polymer nanoassemblies containing pendant azobenzenes in their cores were prepared. Light-induced trans–cis isomerization of the azobenzenes increased the polarity of the assembly core, while reduction led to assembly degradation.

Green chain-shattering polymers based on a self-immolative azobenzene motif

A chain-shattering polymer system consisting of nontoxic, partially renewable resource-based monomersviaacyclic diene metathesis (ADMET) chemistry is introduced.

Novel cyclic azobenzene-containing vesicles: photo/reductant responsiveness and potential applications in colon disease treatment

The vesicles formed by an amphiphilic copolymer with cyclic azobenzene pendants revealed higher drug loading content and better photo/reductant responsiveness than an analogue of the amphiphilic copolymer with linear azobenzene units.

Rapid, On-Command Debonding of Stimuli-Responsive Cross-Linked Adhesives by Continuous, Sequential Quinone Methide Elimination Reactions

Adhesives that selectively debond from a surface by stimuli-induced head-to-tail continuous depolymerization of poly(benzyl ether) macro-cross-linkers within a poly(norbornene) matrix are described. Continuous head-to-tail depolymerization provides faster rates of response than can be achieved using a small-molecule cross-linker, as well as responses to lower stimulus concentrations. Shear-stress values for glass held together by the adhesive reach 0.51±0.10 MPa, whereas signal-induced depolymerization via quinone methide intermediates reduces the shear stress values to 0.05±0.02 MPa. Changing the length of the macro-cross-linkers alters the time required for debonding, and thus enables the programmed sequential release of specific layers in a glass composite material.