Light- and mechanic field controlled dynamic soft matter materials

A photochemical reaction system that fuses photo- and mechanochemistry into one macromolecular design for light- and mechano-reversible modification of polymer endgroups is introduced.

Light-Induced Formation/Scission of C-N, C-O, and C-S Bonds Enables Switchable Stability/Degradability in Covalent Systems

The manipulation of covalent bonds could be directed toward degradable, recyclable, and sustainable materials. However, there is an intrinsic conflict between properties of stability and degradability. Here we report light-controlled formation/scission of three types of covalent bonds (C-N, C-O, and C-S) through photoswitching between equilibrium and nonequilibrium states of dynamic covalent systems, achieving dual benefits of photoaddressable stability and cleavability. The photocyclization of dithienylethene fused aldehyde ring-chain tautomers turns on the reactivity, incorporating/releasing amines, alcohols, and thiols reversibly with high efficiency, respectively. Upon photocycloreversion the system is shifted to kinetically locked out-of-equilibrium form, enabling remarkable robustness of covalent assemblies. Reaction coupling allows remote and directional control of a diverse range of equilibria and further broadens the scope. Through locking and unlocking covalent linkages with light when needed, the utility is demonstrated with capture/release of bioactive molecules, modification of surfaces, and creation of polymers exhibiting tailored stability and degradability/recyclability. The versatile toolbox for photoswitchable dynamic covalent reactions to toggle matters on and off should be appealing to many endeavors.

Shining Light on Cyclopentadienone-Norbornadiene Diels-Alder Adducts to Enable Photoinduced Click Chemistry with Cyclopentadiene

A new Diels-Alder (DA)-based photopatterning platform is presented, which exploits the irreversible, light-induced decarbonylation and subsequent cleavage of cyclopentadienone-norbornadiene (CPD-NBD) adducts. A series of CPD-NBD adducts have been prepared and systematically studied toward the use in a polymeric material photopatterning platform. By incorporating an optimized CPD-NBD adduct into polymer networks, it is demonstrated that cyclopentadiene may be unveiled upon 365 nm irradiation and subsequently clicked to a variety of maleimides with spatial control under mild reaction conditions and with fast kinetics. Unlike currently available photoinduced Diels-Alder reactions that rely on trapping transient, photocaged dienes, this platform introduces a persistent, yet highly reactive diene after irradiation, enabling the use of photosensitive species such as cyanine dyes to be patterned. To highlight the potential use of this platform in a variety of material applications, we demonstrate two proof-of-concepts: patterned conjugation of multiple dyes into a polyacrylate network and preprogrammed ligation of streptavidin into poly(ethylene glycol) hydrogels.

Potential use of the Diels-Alder reaction in biomedical and nanomedicine applications

The Diels-Alder reaction and its retro breakdown has garnered increasing research focus due to several of its advantageous properties including, atomic conservation, reversibility, and substituent retention. This is especially true in biomedical application and nanomedicine development which display a preference for rapid, efficient, and clean "click" chemistry reactions allowing for delivery of active ingredients and subsequent release upon temperature elevation. There are multiple variations on the Diels-Alder reaction based around substitution position and materials being coupled which can affect the temperature threshold for and rate of the retro reaction reversal. Hence, the Diels-Alder reaction offers a simple coupling reaction for active ingredients with tailorable release. In this review the incorporation of the Diels-Alder chemistries and linkers within the biomedical and nanomedicine field will be discussed, as well as its use in future potential technologies.

A photo–thermal dual-regulated latent monomer strategy for sequence control of polymers

A photo–thermal dual-regulated latent monomer was used for the synthesis of polymers with advanced sequence structures.

Hierarchical 3D Patterns with Dynamic Wrinkles Produced by a Photocontrolled Diels-Alder Reaction on the Surface

Three-dimensional (3D) reconfigurable patterns with dynamic morphologies enable the on-demand control of surface properties, such as optical, wetting, and adhesive properties, to achieve smart surfaces. Here, a simple yet general strategy is developed for fabricating 3D patterns with reversible wrinkles on the surface, in which a Diels-Alder (D-A) reaction in the top layer, which consists of a reversible cross-linked polymer network composed of a furan-containing copolymer (PSFB) and bismaleimide (BMI), can be spatially controlled by the photodimerization of BMI. When a photomask is used during irradiation with ultraviolet (UV) light, selective photodimerization of the maleimide leads to the diffusion of maleimide from the unexposed region to the exposed region, resulting in the generation of a diffused relief pattern. By controlling the reversible D-A reaction at different temperatures, orthogonal wrinkles can be sequentially and reversibly generated or erased in both the exposed and unexposed regions on the surface. Theoretical modeling with boundary effects reveals that the orientation of the wrinkle in the exposed region is perpendicular to the boundary, whereas the wrinkle in the unexposed region is parallel to the boundary. This strategy, based on a photocontrolled D-A reaction, provides an important and robust alternative for fabricating 3D patterned surfaces with dynamic topographies.

Mechanically Gated Degradable Polymers

Degradable polymers are desirable for the replacement of conventional organic polymers that persist in the environment, but they often suffer from the unintentional scission of the degradable functionalities on the polymer backbone, which diminishes polymer properties during storage and regular use. Herein, we report a strategy that combats unintended degradation in polymers by combining two common degradation stimuli-mechanical and acid triggers-in an "AND gate" fashion. A cyclobutane (CB) mechanophore is used as a mechanical gate to regulate an acid-sensitive ketal that has been widely employed in acid degradable polymers. This gated ketal is further incorporated into the polymer backbone. In the presence of an acid trigger alone, the pristine polymer retains its backbone integrity, and delivering high mechanical forces alone by ultrasonication degrades the polymer to an apparent limiting molecular weight of 28 kDa. The sequential treatment of ultrasonication followed by acid, however, leads to a further 11-fold decrease in molecular weight to 2.5 kDa. Experimental and computational evidence further indicate that the ungated ketal possesses mechanical strength that is commensurate with the conventional polymer backbones. Single molecule force spectroscopy (SMFS) reveals that the force necessary to activate the CB molecular gate on the time scale of 100 ms is approximately 2 nN.

Bifunctional Molecular Photoswitches Based on Overcrowded Alkenes for Dynamic Control of Catalytic Activity in Michael Addition Reactions

The emerging field of artificial photoswitchable catalysis has recently shown striking examples of functional light-responsive systems allowing for dynamic control of activity and selectivity in organocatalysis and metal-catalysed transformations. While our group has already disclosed systems featuring first generation molecular motors as the switchable central core, a design based on second generation molecular motors is lacking. Here, the syntheses of two bifunctionalised molecular switches based on a photoresponsive tetrasubstituted alkene core are reported. They feature a thiourea substituent as hydrogen-donor moiety in the upper half and a basic dimethylamine group in the lower half. This combination of functional groups offers the possibility for application of these molecules in photoswitchable catalytic processes. The light-responsive central cores were synthesized by a Barton-Kellogg coupling of the prefunctionalized upper and lower halves. Derivatization using Buchwald-Hartwig amination and subsequent introduction of the thiourea substituent afforded the target compounds. Control of catalytic activity in the Michael addition reaction between (E)-3-bromo-β-nitrostyrene and 2,4-pentanedione is achieved upon irradiation of stable-(E) and stable-(Z) isomers of the bifunctional catalyst 1. Both isomers display a decrease in catalytic activity upon irradiation to the metastable state, providing systems with the potential to be applied as ON/OFF catalytic photoswitches.

Conditional repair by locally switching the thermal healing capability of dynamic covalent polymers with light

Healable materials could play an important role in reducing the environmental footprint of our modern technological society through extending the life cycles of consumer products and constructions. However, as most healing processes are carried out by heat alone, the ability to heal damage generally kills the parent material's thermal and mechanical properties. Here we present a dynamic covalent polymer network whose thermal healing ability can be switched 'on' and 'off' on demand by light, thereby providing local control over repair while retaining the advantageous macroscopic properties of static polymer networks. We employ a photoswitchable furan-based crosslinker, which reacts with short and mobile maleimide-substituted poly(lauryl methacrylate) chains forming strong covalent bonds while simultaneously allowing the reversible, spatiotemporally resolved control over thermally induced de- and re-crosslinking. We reason that our system can be adapted to more complex materials and has the potential to impact applications in responsive coatings, photolithography and microfabrication.

Diversity-Oriented Approaches to Polycyclics and Bioinspired Molecules via the Diels-Alder Strategy: Green Chemistry, Synthetic Economy, and Beyond

We describe diverse approaches to various dienes and their utilization in the Diels-Alder reaction to produce a variety of polycycles. The dienes covered here are prepared by simple alkylation reaction or via the Claisen rearrangement or by enyne metathesis of alkyne or enyne building blocks. Here, we have also included the Diels-Alder chemistry of dendralenes, a higher analog of cross-conjugated dienes. The present article is inclusive of o-xylylene derivatives that are generated in situ starting with benzosultine or benzosulfone derivatives. The Diels-Alder reaction of these dienes with various dienophiles gave diverse polycyclic systems and biologically important targets.

From slow to fast--the user controls the rate of the release of molecules from masked forms using a photoswitch and different types of light

Exposure to UV light generates a ring-closed isomer of a diarylethene, which undergoes very slow bond breaking and release even after the light is turned off. The rate of release is increased by exposing the isomer to UV and/or visible light.

Photomodulable phosphines incorporating diarylethene moieties

Incorporation of the dithienylethene moiety as a substituent provides new ‘switchable’ phosphine ligands whose electronic properties are reversibly modulated by light.

Immobilisation of photoswitchable diarylcyclohexenes synthesised via cobalt-mediated Diels-Alder reaction

Functionalised photoswitches--photochromic dithienylcyclohexenes--were prepared in two steps by a cobalt-mediated Diels-Alder reaction of internal alkynes with the isoprenylpinacolboronic ester. The three-component one-pot reaction sequence provides the photochromic dithienylcyclohexenes in up to 67% overall yield.

Optically healable polymers

Polymers that can easily be repaired after being damaged are attractive as this characteristic can improve the reliability, functionality, and lifetime of many products. In the last decade, researchers have thus developed new approaches to create stimuli-responsive polymer systems, which have the ability to autonomously heal or can be repaired upon exposure to an external stimulus. This review summarizes the current knowledge of optically healable or photo-healable polymers. The use of light as a stimulus for healing offers several attractive features, including the ability to deliver the stimulus locally, which opens up the possibility of healing the material under load, as well as the ability to tailor the wavelength of light to selectively address a specific component of the material, e.g. only the damaged parts. So far, two main classes of optically healable polymers have been explored, which are structurally dynamic polymers and mechanically activated reactive systems.