Main chain selective polymer degradation: controlled by the wavelength and assembly

The advent of reversible deactivation radical polymerization (RDRP) revolutionized polymer chemistry and paved the way for accessing synthetic polymers with controlled sequences based on vinylic monomers. An inherent limitation of vinylic polymers stems from their all-carbon backbone, which limits both function and degradability. Herein, we report a synthetic strategy utilizing radical ring-opening polymerization (rROP) of complementary photoreactive cyclic monomers in combination with RDRP to embed photoresponsive functionality into desired blocks of polyvinyl polymers. Exploiting different absorbances of photoreactive cyclic monomers, it becomes possible to degrade blocks selectively by irradiation with either UVB or UVA light. Translating such primary structures of polymer sequences into higher order assemblies, the hydrophobicity of the photodegradable monomers allowed for the formation of micelles in water. Upon exposure to light, the nondegradable blocks detached yielding a significant reduction in the micelle hydrodynamic diameter. As a result of the self-assembled micellar environment, telechelic oligomers with photoreactive termini (e.g., coumarin or styrylpyrene) resulting from the photodegradation of polymers in water underwent intermolecular photocycloaddition to photopolymerize, which usually only occurs efficiently at longer wavelengths and a much higher concentration of photoresponsive groups. The reported main chain polymer degradation is thus controlled by the irradiation wavelength and the assembly of the polymers.

Cyclodextrin 'Chaperones' Enable Quasi-Ideal Supramolecular Network Formation and Enhanced Photodimerization of Hydrophobic, Red-shifted Photoswitches in Water

Precision-engineered light-triggered hydrogels are important for a diversity of applications. However, fields such as biomaterials require wavelength outside the harsh UV regime to prevent photodamage, typically requiring chromophores with extended π-conjugation that suffer from poor water solubility. Herein, we demonstrate how cyclodextrins can be used as auxiliary agents to not only solubilize such chromophores, but even to preorganize them in a 2 : 2 host-guest inclusion complex to facilitate photodimerization. We apply our concept to styrylpyrene-end-functionalized star-shaped polyethylene glycols (sPEGs). We initially unravel details of the host-guest inclusion complex using spectroscopy and mass spectrometry to give clear evidence of a 2 : 2 complex formation. Subsequently, we show that the resultant supramolecularly linked hydrogels conform to theories of supramolecular quasi-ideal model networks, and derive details on their association dynamics using in-depth rheological measurements and kinetic models. By comparing sPEGs of different arm length, we further elucidate the model network topology and the accessible mechanical property space. The photo-mediated dimerization proceeds smoothly, allowing to transform the supramolecular model networks into covalent ones. We submit that our strategy opens avenues for executing hydrophobic photochemistry in aqueous environments with enhanced control over reactivity, hydrogel topology or programmable mechanical properties.

Synthesis of Tough and Fluorescent Self-Healing Elastomers by Scandium-Catalyzed Terpolymerization of Pyrenylethenylstyrene, Ethylene, and Anisylpropylene

The synthesis of fluorescent self-healing polymers by the incorporation of a fluorophore-containing olefin into a polyolefin backbone through catalyst-controlled multicomponent copolymerization is of fundamental interest and practical importance, but such an approach has remained unexplored to date. Herein, we report for the first time the synthesis of tough and fluorescent self-healing polymers by sequence-controlled terpolymerization of 4-[2-(1-pyrenyl)ethenyl]styrene (Pyr), ethylene (E), and anisylpropylene (AP) using a sterically demanding half-sandwich scandium catalyst. The resulting terpolymers consisted of relatively long alternating E-alt-AP sequences, isolated Pyr units, and short E-E blocks, which exhibited excellent tensile strength, remarkable self-healability, and high fluorescence quantum yield. The excellent mechanical and self-healing properties could be attributed to the nanophase separation of the crystalline E-E segments and the hard Pyr aggregates from a flexible E-alt-AP segment matrix, in which the Pyr units not only served as an efficient fluorophore but also played an important role in forming nanodomains and enhancing the polymer mobility. Furthermore, the styrenyl C═C bond of the Pyr unit in the terpolymers could undergo [2 + 2] cycloaddition under photoirradiation, which thus enabled the fabrication of a self-healable fluorescent two-dimensional image on a terpolymer film through photolithography. This work offers an unprecedented efficient protocol for the synthesis of a brand-new family of fluorescent self-healing materials, showcasing the high potential of catalyst-controlled sequence-regular copolymerization of different olefins for the creation of novel functional polymers.

Photochemical Action Plots Reveal the Fundamental Mismatch Between Absorptivity and Photochemical Reactivity

Over the last years, the authors' laboratory has employed monochromatic tuneable laser systems to reveal a fundamental mismatch between the absorptivity of a chromophore and its photochemical reactivity for the vast majority of covalent bond forming reactions as well as specific bond cleavage reactions. In the general chemistry community, however, the long-held assumption pervades that effective photochemical reactions are obtained in situations where there is strong overlap between the absorption spectrum and the excitation wavelength. The current Perspective illustrates that the absorption spectrum of a molecule only provides information about electronic excitations and remains entirely silent on other energy redistribution mechanisms that follow, which critically influence photochemical reactivity. Future avenues of enquiry on how action plots can be understood are proposed and the importance of action plots for tailoring photochemical applications with never-before-seen precision is explored.

Peptide Self-Assembly Controlled Photoligation of Polymers

Highly efficient chemical ligations that operate in water under mild conditions are the foundation of bioorthogonal chemistry. However, the toolbox of suitable reactions is limited. Conventional approaches to expand this toolbox aim at altering the inherent reactivity of functional groups to design new reactions that meet the required benchmarks. Inspired by controlled reaction environments that enzymes provide, we report a fundamentally different approach that makes inefficient reactions highly efficient within defined local environments. Contrasting enzymatically catalyzed reactions, the reactivity controlling self-assembled environment is brought about by the ligation targets themselves─avoiding the use of a catalyst. Targeting [2 + 2] photocycloadditions, which are inefficient at low concentrations and readily quenched by oxygen, short β-sheet encoded peptide sequences are inserted between a hydrophobic photoreactive styrylpyrene unit and a hydrophilic polymer. In water, electrostatic repulsion of deprotonated amino acid residues governs the formation of small self-assembled structures, which enable a highly efficient photoligation of the polymer, reaching ∼90% ligation within 2 min (0.034 mM). Upon protonation at low pH, the self-assembly changes into 1D fibers, altering photophysical properties and shutting down the photocycloaddition reaction. Using the reversible morphology change, it is possible to switch the photoligation "ON" or "OFF" under constant irradiation simply by varying the pH. Importantly, in dimethylformamide, the photoligation reaction did not occur even at 10-fold higher concentrations (0.34 mM). The self-assembly into a specific architecture, encoded into the polymer ligation target, enables a highly efficient ligation that overcomes the concentration limitations and high oxygen sensitivity of [2 + 2] photocycloadditions.

Programmable DNA Interstrand Crosslinking by Alkene-Alkyne [2 + 2] Photocycloaddition

Covalent crosslinking of DNA strands provides a useful tool for medical, biochemical, and DNA nanotechnology applications. Here we present a light-induced interstrand DNA crosslinking reaction using the modified nucleoside 5-phenylethynyl-2'-deoxyuridine (^PhedU). The crosslinking ability of ^PhedU was programmed by base pairing and by metal ion interaction at the Watson-Crick base pairing site. Rotation to intrahelical positions was favored by hydrophobic stacking and enabled an unexpected photochemical alkene-alkyne [2 + 2] cycloaddition within the DNA duplex, resulting in efficient formation of a ^PhedU dimer after short irradiation times of a few seconds. A ^PhedU-dimer-containing DNA was shown to efficiently bind a helicase complex, but the covalent crosslink completely prevented DNA unwinding, suggesting possible applications in biochemistry or structural biology.

Expanding the 'aplysinospin cascade' through DNA-templated [2+2] photocycloaddition

Inspired by the unique ability of nucleic acids to template chemical transformations that are otherwise impossible in solution, we embarked on the generalisation of our DNA-templated [2+2] photo-induced homo- and heterodimerization of aplysinopsins. Our process ensures a straightforward access to cyclobutane containing natural products and analogues thereof. Most importantly, this conceptual biomimetic achievement presents interesting arguments to build a biosynthetic scenario.

DNA labelling in live cells via visible light-induced [2+2] photocycloaddition

We introduce a visible light-driven (λ_max = 451 nm) photo-chemical strategy for labelling of DNA in living HeLa cells via the [2+2] cycloaddition of a styrylquinoxaline moiety, which we incorporate into both the DNA and the fluorescent label. Our methodology offers advanced opportunities for the mild remote labelling of DNA in water while avoiding UV light activation.

Programming Photodegradability into Vinylic Polymers via Radical Ring-Opening Polymerization

Incorporation of photolabile moieties into the polymer backbone holds promise to remotely-control polymer degradation. However, suitable synthetic avenues are limited, especially for radical polymerizations. Here we report a strategy to program photodegradability into vinylic polymers by exploiting the wavelength selectivity of photocycloadditions for radical ring-opening polymerization (rROP). Irradiation of coumarin terminated allylic sulfides with UVA light initiated intramolecular [2+2] photocycloaddition producing cyclic macromonomers. Subsequent RAFT-mediated rROP with methyl acrylate yielded copolymers that inherited the photoreactivity of the cyclic parent monomer. Irradiation with UVB initiated efficient photocycloreversion of the coumarin dimers, causing polymer degradation within minutes under UVB light or days under sunlight exposure. Our synthetic strategy may pave the way to insert photolabile linkages into vinylic polymers, tuning degradation for specific wavelengths.

Fluorescence turn-on by photoligation - bright opportunities for soft matter materials

Photochemical ligation has become an indispensable tool for applications that require spatially addressable functionalisation, both in biology and materials science. Interestingly, a number of photochemical ligations result in fluorescent products, enabling a self-reporting function that provides almost instantaneous visual feedback of the reaction's progress and efficiency. Perhaps no other chemical reaction system allows control in space and time to the same extent, while concomitantly providing inherent feedback with regard to reaction success and location. While photoactivable fluorescent properties have been widely used in biology for imaging purposes, the expansion of the array of photochemical reactions has further enabled its utility in soft matter materials. Herein, we concisely summarise the key developments of fluorogenic-forming photoligation systems and their emerging applications in both biology and materials science. We further summarise the current challenges and future opportunities of exploiting fluorescent self-reporting reactions in a wide array of chemical disciplines.

[2+2] Photocycloaddition in a bichromophoric dyad: photochemical concerted forward reaction following Woodward-Hoffmann rules and photoinduced stepwise reverse reaction of the ring opening via predissociation

A novel biphotochromic dyad with styrylbenzo[f]quinoline photochromes was designed and synthesized to study the [2+2] photocycloaddition (PCA) reaction leading to cyclobutane with two benzo[f]quinoline (BQ) substituents and the reverse four-membered ring opening reaction. In the dyad, PCA occurs in a concerted manner according to the Woodward-Hoffmann rules in the ππ* excited state after excitation of the whole conjugated π-system comprising the ethylene group. Nanosecond time-resolved emission spectroscopy indicated formation of an excimer as a possible intermediate of the PCA reaction. The reverse reaction of photoinduced cyclobutane ring opening is assumed to proceed stepwise according to the predissociation mechanism: after excitation of the BQ substituent, energy transfer (ET) occurs from the bonding ππ* term localized on the BQ substituent to the dissociative πσ* term localized on cyclobutane; the efficiency of such a process was measured for the first time. For the first time, it is suggested that the predissociation mechanism is common to the ring-opening reaction of any cyclobutane with an unsaturated substituent where the π-system of the substituent rather than the σ-system of cyclobutane is excited under light irradiation.

Hybridization-specific chemical reactions to create interstrand crosslinking and threaded structures of nucleic acids

The interstrand crosslinking and threaded structures of nucleic acids have high potential in oligonucleotide therapeutics, chemical biology, and nanotechnology. For example, properly designed crosslinking structures provide high activity and nuclease resistance for anti-miRNAs. The noncovalent labeling and modification by the threaded structures are useful as new chemical biology tools. Photoreversible crosslinking creates smart materials, such as reversible photoresponsive gels and DNA origami objects. This review introduces the creation of interstrand crosslinking and threaded structures, such as catenanes and rotaxanes, based on hybridization-specific chemical reactions and their functions and perspectives.

Wavelength Orthogonal Photodynamic Networks

The ability of light to remotely control the properties of soft matter materials in a dynamic fashion has fascinated material scientists and photochemists for decades. However, only recently has our ability to map photochemical reactivity in a finely wavelength resolved fashion allowed for different colors of light to independently control the material properties of polymer networks with high precision, driven by monochromatic irradiation enabling orthogonal reaction control. The current concept article highlights the progress in visible light‐induced photochemistry and explores how it has enabled the design of polymer networks with dynamically adjustable properties. We will explore current applications ranging from dynamic hydrogel design to the light‐driven adaptation of 3D printed structures on the macro‐ and micro‐scale. While the alternation of mechanical properties via remote control is largely reality for soft matter materials, we herein propose the next frontiers for adaptive properties, including remote switching between conductive and non‐conductive properties, hydrophobic and hydrophilic surfaces, fluorescent or non‐fluorescent, and cell adhesive vs. cell repellent properties.

Wavelength-Orthogonal Stiffening of Hydrogel Networks with Visible Light

Herein, we introduce the wavelength-orthogonal crosslinking of hydrogel networks using two red-shifted chromophores, i.e. acrylpyerene (AP, λactivation =410-490 nm) and styrylpyrido[2,3-b]pyrazine (SPP, λactivation =400-550 nm), able to undergo [2+2] photocycloaddition in the visible-light regime. The photoreactivity of the SPP moiety is pH-dependent, whereby an acidic environment inhibits the cycloaddition. By employing a spiropyran-based photoacid generator with suitable absorption wavelength, we are able to restrict the activation wavelength of the SPP moiety to the green light region (λactivation =520-550 nm), enabling wavelength-orthogonal activation of the AP group. Our wavelength-orthogonal photochemical system was successfully applied in the design of hydrogels whose stiffness can be tuned independently by either green or blue light.

Orange-Light-Induced Photochemistry Gated by pH and Confined Environments

We introduce a new photochemically active compound, i.e., pyridinepyrene (PyPy), entailing a pH-active moiety that effects a significant halochromic shift into orange-light (λ = 590 nm) activatable photoreactivity while concomitantly exerting control over its reaction pathways. With blue light (λ = 450 nm) in neutral to basic pH, a [2 + 2] photocycloaddition can be triggered to form a cyclobutene ring in a reversible fashion. If the pH is decreased to acidic conditions, resulting in a halochromic absorption shift, photocycloaddition on the small-molecule level is blocked due to repulsive interactions and exclusive trans-cis isomerization is observed. Through implementation of PyPy into the confined environment of a single-chain nanoparticle (SCNP) design, one can overcome the repulsive forces and exploit the halochromic shift for orange light (λ = 590 nm)-induced cycloaddition and formation of macromolecular three-dimensional (3D) architectures.

Wavelength-Selective Photocycloadditions of Styryl-Anthracene

Light-tunable covalent chemistry is highly urgent in the fields of chemistry, biology and material especially for the smart materials and surface, due to the spatiotemporal control and feasible operation. Here, we report a new type of wavelength-selective photo-cycloaddition of styryl-anthracene carboxylic acid (SACA). Upon the irradiation of 450 nm visible light or 365 nm UV light, SACA can undergo [2+2] or [2+4] photocycloaddition, respectively. Furthermore, the [2+2] photocycloaddition induced by vis-light of 450 nm is reversible and can be disrupted by 365 nm UV light to form dimer-24 which cannot be photo-cleavable. Owing to the feasibility and spatiotemporal characteristics of UV-Vis light-controlled photocycloaddition, the SACA possesses potential applications in various areas such as self-assembly, dynamic wrinkle and fluorescence patterns, which is also explored and exhibited in this work. This article is protected by copyright. All rights reserved.

Rapid Alkene-Alkene Photo-Cross-Linking on the Base-Flipping-Out Field in Duplex DNA

Specific chemical reactions by enzymes acting on a nucleobase are realized by flipping the target base out of the helix. Similarly, artificial oligodeoxynucleotides (ODNs) can also induce the base flipping and a specific chemical reaction. We now report an easily prepared and unique structure-providing photo-cross-linking reaction by taking advantage of the base-flipping-out field formed by alkene-type base-flipping-inducing artificial bases. Two 3-arylethenyl-5-methyl-2-pyridone nucleosides with the Ph or An group were synthesized and incorporated into the ODNs. We found that the two Ph derivatives provided the cross-linked product in a high yield only by a 10 s photoirradiation when their alkenes overlap each other in the duplex DNA. The highly efficient reaction enabled forming a cross-linked product even when using the duplex with a low T_m value.

Photochemical modifications for DNA/RNA oligonucleotides

Light-triggered chemical reactions can provide excellent tools to investigate the fundamental mechanisms important in biology. Light is easily applicable and orthogonal to most cellular events, and its dose and locality can be controlled in tissues and cells. Light-induced conversion of photochemical groups installed on small molecules, proteins, and oligonucleotides can alter their functional states and thus the ensuing biological events. Recently, photochemical control of DNA/RNA structure and function has garnered attention thanks to the rapidly expanding photochemistry used in diverse biological applications. Photoconvertible groups can be incorporated in the backbone, ribose, and nucleobase of an oligonucleotide to undergo various irreversible and reversible light-induced reactions such as cleavage, crosslinking, isomerization, and intramolecular cyclization reactions. In this review, we gather a list of photoconvertible groups used in oligonucleotides and summarize their reaction characteristics, impacts on DNA/RNA thermal stability and structure, as well as their biological applications.

Light manipulation for fabrication of hydrogels and their biological applications

The development of biocompatible materials with desired functions is essential for tissue engineering and biomedical applications. Hydrogels prepared from these materials represent an important class of soft matter for mimicking extracellular environments. In particular, dynamic hydrogels with responsiveness to environments are quite appealing because they can match the dynamics of biological processes. Among the external stimuli that can trigger responsive hydrogels, light is considered as a clean stimulus with high spatiotemporal resolution, complete bioorthogonality, and fine tunability regarding its wavelength and intensity. Therefore, photoresponsiveness has been broadly encoded in hydrogels for biological applications. Moreover, light can be used to initiate gelation during the fabrication of biocompatible hydrogels. Here, we present a critical review of light manipulation tools for the fabrication of hydrogels and for the regulation of physicochemical properties and functions of photoresponsive hydrogels. The materials, photo-initiated chemical reactions, and new prospects for light-induced gelation are introduced in the former part, while mechanisms to render hydrogels photoresponsive and their biological applications are discussed in the latter part. Subsequently, the challenges and potential research directions in this area are discussed, followed by a brief conclusion. STATEMENT OF SIGNIFICANCE: Hydrogels play a vital role in the field of biomaterials owing to their water retention ability and biocompatibility. However, static hydrogels cannot meet the dynamic requirements of the biomedical field. As a stimulus with high spatiotemporal resolution, light is an ideal tool for both the fabrication and operation of hydrogels. In this review, light-induced hydrogelation and photoresponsive hydrogels are discussed in detail, and new prospects and emerging biological applications are described. To inspire more research studies in this promising area, the challenges and possible solutions are also presented.

Action Plots in Action: In-Depth Insights into Photochemical Reactivity

Predicting wavelength-dependent photochemical reactivity is challenging. Herein, we revive the well-established tool of measuring action spectra and adapt the technique to map wavelength-resolved covalent bond formation and cleavage in what we term "photochemical action plots". Underpinned by tunable lasers, which allow excitation of molecules with near-perfect wavelength precision, the photoinduced reactivity of several reaction classes have been mapped in detail. These include photoinduced cycloadditions and bond formation based on photochemically generated o-quinodimethanes and 1,3-dipoles such as nitrile imines as well as radical photoinitiator cleavage. Organized by reaction class, these data demonstrate that UV/vis spectra fail to act as a predictor for photochemical reactivity at a given wavelength in most of the examined reactions, with the photochemical reactivity being strongly red shifted in comparison to the absorption spectrum. We provide an encompassing perspective of the power of photochemical action plots for bond-forming reactions and their emerging applications in the design of wavelength-selective photoresists and photoresponsive soft-matter materials.

Visible-Light-Mediated Modification and Manipulation of Biomacromolecules

Chemically modified biomacromolecules-i.e., proteins, nucleic acids, glycans, and lipids-have become crucial tools in chemical biology. They are extensively used not only to elucidate cellular processes but also in industrial applications, particularly in the context of biopharmaceuticals. In order to enable maximum scope for optimization, it is pivotal to have a diverse array of biomacromolecule modification methods at one's disposal. Chemistry has driven many significant advances in this area, and especially recently, numerous novel visible-light-induced photochemical approaches have emerged. In these reactions, light serves as an external source of energy, enabling access to highly reactive intermediates under exceedingly mild conditions and with exquisite spatiotemporal control. While UV-induced transformations on biomacromolecules date back decades, visible light has the unmistakable advantage of being considerably more biocompatible, and a spectrum of visible-light-driven methods is now available, chiefly for proteins and nucleic acids. This review will discuss modifications of native functional groups (FGs), including functionalization, labeling, and cross-linking techniques as well as the utility of oxidative degradation mediated by photochemically generated reactive oxygen species. Furthermore, transformations at non-native, bioorthogonal FGs on biomacromolecules will be addressed, including photoclick chemistry and DNA-encoded library synthesis as well as methods that allow manipulation of the activity of a biomacromolecule.

Evolution of the Selection Methods of DNA-Encoded Chemical Libraries

In the past two decades, a DNA-encoded chemical library (DEL or DECL) has emerged and has become a major technology platform for ligand discovery in drug discovery as well as in chemical biology research. Although based on a simple concept, i.e., encoding each compound with a unique DNA tag in a combinatorial chemical library, DEL has been proven to be a powerful tool for interrogating biological targets by accessing vast chemical space at a fraction of the cost of traditional high-throughput screening (HTS). Moreover, the recent technological advances and rapid developments of DEL-compatible reactions have greatly enhanced the chemical diversity of DELs. Today, DELs have been adopted by nearly all major pharmaceutical companies and are also gaining momentum in academia. However, this field is heavily biased toward library encoding and synthesis, and an underexplored aspect of DEL research is the selection methods. Generally, DEL selection is considered to be a massive binding assay conducted over an immobilized protein to identify the physical binders using the typical bind-wash-elute procedure. In recent years, we and other research groups have developed new approaches that can perform DEL selections in the solution phase, which has enabled the selection against complex biological targets beyond purified proteins. On the one hand, these methods have significantly widened the target scope of DELs; on the other hand, they have enabled the functional and potentially phenotypic assays of DELs beyond simple binding. An overview of these methods is provided in this Account.Our laboratory has been using DNA-programmed affinity labeling (DPAL) as the main strategy to develop new DEL selection methods. DPAL is based on DNA-templated synthesis; by using a known ligand to guide the target binding, DPAL is able to specifically establish a stable linkage between the target protein and the ligand. The DNA tag of the target-ligand conjugates serves as a programmable handle for protein characterization or hit compound decoding in the case of DEL selections. DPAL also takes advantage of the fast reaction kinetics of photo-cross-linking to achieve high labeling specificity and fidelity, especially in the selection of DNA-encoded dynamic libraries (DEDLs). DPAL has enabled DEL selections not only in buffer and cell lysates but also with complex biological systems, such as large protein complexes and live cells. Moreover, this strategy has also been employed in other biological applications, such as site-specific protein labeling, protein detection, protein profiling, and target identification. In the Account, we describe these methods, highlight their underlying principles, and conclude with perspectives of the development of the DEL technology.

Perylene-Cy3 FRET System to Analyze Photoactive DNA Structures

We report a new Förster resonance energy transfer (FRET) system for structural analyses of DNA duplexes using perylene and Cy3 as donor and acceptor, respectively, linked at the termini of a DNA duplex via D-threoninol. Experimentally obtained FRET efficiencies were in good agreement with theoretical values calculated based on canonical B-form DNA. Due to the relatively long Förster radius, this system can be used to analyze large DNA structures, and duplexes containing photo-reactive molecules can be analyzed since perylene can be excited with visible light. The system was used to analyze a DNA duplex containing stilbene, demonstrating that in the region of the stilbene cluster the duplex adopts a ladder-like structure rather than helical one. Upon photodimerization between stilbene residues, FRET efficiencies indicated the reaction does not disturb DNA duplex. This FRET system will be useful for analysis of photoreactions of nucleobases as well as a wide range of nucleic acid structures.

Visible Light Photochemical Reactions for Nucleic Acid-Based Technologies

The expanding scope of chemical reactions applied to nucleic acids has diversified the design of nucleic acid-based technologies that are essential to medicinal chemistry and chemical biology. Among chemical reactions, visible light photochemical reaction is considered a promising tool that can be used for the manipulations of nucleic acids owing to its advantages, such as mild reaction conditions and ease of the reaction process. Of late, inspired by the development of visible light-absorbing molecules and photocatalysts, visible light-driven photochemical reactions have been used to conduct various molecular manipulations, such as the cleavage or ligation of nucleic acids and other molecules as well as the synthesis of functional molecules. In this review, we describe the recent developments (from 2010) in visible light photochemical reactions involving nucleic acids and their applications in the design of nucleic acid-based technologies including DNA photocleaving, DNA photoligation, nucleic acid sensors, the release of functional molecules, and DNA-encoded libraries.

Modular functionalization and hydrogel formation via red-shifted and self-reporting [2+2] cycloadditions

We introduce a modular photodynamic covalent crosslinker, named qStyPy, with an increased water-solubility that undergoes [2+2] cycloadditions upon irradiation with 470 nm and directly self-reports on its cycloadduct formation.

Green light triggered [2+2] cycloaddition of halochromic styrylquinoxaline-controlling photoreactivity by pH

Photochemical reactions are a powerful tool in (bio)materials design due to the spatial and temporal control light can provide. To extend their applications in biological setting, the use of low-energy, long wavelength light with high penetration propertiesis required. Further regulation of the photochemical process by additional stimuli, such as pH, will open the door for construction of highly regulated systems in nanotechnology- and biology-driven applications. Here we report the green light induced [2+2] cycloaddition of a halochromic system based on a styrylquinoxaline moiety, which allows for its photo-reactivity to be switched on and off by adjusting the pH of the system. Critically, the [2+2] photocycloaddition can be activated by green light (λ up to 550 nm), which is the longest wavelength employed to date in catalyst-free photocycloadditions in solution. Importantly, the pH-dependence of the photo-reactivity was mapped by constant photon action plots. The action plots further indicate that the choice of solvent strongly impacts the system's photo-reactivity. Indeed, higher conversion and longer activation wavelengths were observed in water compared to acetonitrile under identical reaction conditions. The wider applicability of the system was demonstrated in the crosslinking of an 8-arm PEG to form hydrogels (ca. 1 cm in thickness) with a range of mechanical properties and pH responsiveness, highlighting the potential of the system in materials science.

Synthesis and [2+2]-photodimerisation of monothiomaleimide functionalised linear and brush-like polymers

[2+2]-Photodimerisation of monothiomaleimides has been demonstrated on functionalised linear and brush-like polymers. In water/acetonitrile (95 : 5) mixtures the rate of reaction is accelerated significantly by irradiation of the thiomaleimide end group (λmax = 350 nm) with UV light, reaching full conversion within 10 minutes.

Photocycloreversions within single polymer chains

Reversible photocycloadditions hold great potential to control formation and fissure of bonds. However, the wavelength selective addressability of cycloaddition and -reversion was found to be lost in the confined environment of single polymer chains.

Photocycloadditions in disparate chemical environments

We elucidate the wavelength dependence of a photocycloaddition by accessing action plots dependent on the reactivity relative to the number of absorbed photons and establish the effect of concentration and solvent on the reactivity.

Photoregulation between small DNAs and reversible photochromic molecules

Oligonucleotides are widely used biological materials in the fields of biomedicine, nanotechnology, and materials science. Due to the demands for the photoregulation of DNA activities, scientists are placing more and more research interest in the interactions between reversible photochromic molecules and DNAs. Photochromic molecules can work as switches for regulating the DNAs' behavior under light irradiation; meanwhile, DNAs also exert influence over the photochromic molecules. The photochromic molecules can be attached to DNAs either by covalent bonds or by noncovalent forces, which results in different regulative functions. Azobenzenes, spiropyrans, diarylethenes, and stilbene-like compounds are important photochromic molecules working as photoswitches. By summarizing their interactions with oligonucleotides, this review intends to facilitate the relevant research on oligonucleotides/photochromic molecules in the biological and medicinal fields and in materials science.

Fully independent photochemical reactivity in one molecule

We introduce a chemically λ-orthogonal bichromophore triggered simply by different colours of light, enabling two different photochemical reactions in one molecule. Uniquely, the short wavelength (λ = 314 nm) does not trigger the red-shifted reaction system (λ = 416 nm), opening possibilities for the light controlled gating of specific molecular sites independent of wavelength.

Design and Applications of Photoresponsive Hydrogels

Hydrogels are the most relevant biochemical scaffold due to their tunable properties, inherent biocompatibility, and similarity with tissue and cell environments. Over the past decade, hydrogels have developed from static materials to "smart" responsive materials adapting to various stimuli, such as pH, temperature, chemical, electrical, or light. Light stimulation is particularly interesting for many applications because of the capability of contact-free remote manipulation of biomaterial properties and inherent spatial and temporal control. Moreover, light can be finely adjusted in its intrinsic properties, such as wavelength and intensity (i.e., the energy of an individual photon as well as the number of photons over time). Water is almost transparent for light in the photochemically relevant range (NIR-UV), thus hydrogels are well-suited scaffolds for light-responsive functionality. Hydrogels' chemical and physical variety combined with light responsiveness makes photoresponsive hydrogels ideal candidates for applications in several fields, ranging from biomaterials, medicine to soft robotics. Herein, the progress and new developments in the field of light-responsive hydrogels are elaborated by first introducing the relevant photochemistries before discussing selected applications in detail.

Alkyne-Alkyne Photo-cross-linking on the Flipping-out Field

The base flip-inducing nucleic acids are expected to create a specific field for various chemical reactions. We now report a novel type of base-flip-inducing oligodeoxynucleotide and photo-cross-linking reaction. Two 3-arylethynyl-5-methyl-2-pyridone nucleosides, Ph and An, were synthesized, and their properties were investigated. The alkyne-alkyne photo-cross-linking rapidly proceeded by taking advantage of the base-flipping-out field where two alkynes overlap each other. This photo-cross-linking would be a new candidate to form cross-linked DNAs.

Tunable Orthogonal Reversible Covalent (TORC) Bonds: Dynamic Chemical Control over Molecular Assembly

Dynamic assembly of macromolecules in biological systems is one of the fundamental processes that facilitates life. Although such assembly most commonly uses noncovalent interactions, a set of dynamic reactions involving reversible covalent bonding is actively being exploited for the design of functional materials, bottom-up assembly, and molecular machines. This Minireview highlights recent implementations and advancements in the area of tunable orthogonal reversible covalent (TORC) bonds for these purposes, and provides an outlook for their expansion, including the development of synthetically encoded polynucleotide mimics.

Wavelength-Selective Coupling and Decoupling of Polymer Chains via Reversible [2 + 2] Photocycloaddition of Styrylpyrene for Construction of Cytocompatible Photodynamic Hydrogels

Reversible photocycloaddition reactions have previously been employed in chemical cross-linking for the preparation of biomaterial scaffolds. However, the processes require activation by high-energy UV light, rendering them unsuitable for modification in biological environments. Here we demonstrate that the [2 + 2] photocycloaddition of styrylpyrene can be activated by visible light at λ = 400-500 nm, enabling rapid and effective conjugation and cross-linking of poly(ethylene glycol) (PEG) in water and under mild irradiation conditions (I = 20 mW cm^-2). Notably, the reversion of the cycloaddition can be triggered by low-energy UV light at λ = 340 nm, which allows for efficient cleavage of the dimer adduct. Using this wavelength-gated reversible photochemical reaction we are able to prepare PEG hydrogels and modulate their mechanical properties in a bidirectional manner. We also demonstrate healing of the fractured hydrogel by external light triggers. Furthermore, we show that human mesenchymal stem cells can be encapsulated within the gels with high viability post encapsulation. This photochemical approach is therefore anticipated to be highly useful in studies of cell mechanotransduction, with relevance to disease progression and tissue regeneration.

Recent developments in reversible photoregulation of oligonucleotide structure and function

There is a growing interest in the photoregulation of biological functions, due to the high level of spatiotemporal precision achievable with light. Additionally, light is non-invasive and waste-free. In particular, the photoregulation of oligonucleotide structure and function is a rapidly developing study field with relevance to biological, physical and material sciences. Molecular photoswitches have been incorporated in oligonucleotides for 20 years, and the field has currently grown beyond fundamental studies on photochemistry of the switches and DNA duplex stability, and is moving towards applications in chemical biology, nanotechnology and material science. Moreover, the currently emerging field of photopharmacology indicates the relevance of photocontrol in future medicine. In recent years, a large number of publications has appeared on photoregulation of DNA and RNA structure and function. New strategies are evaluated and novel, exciting applications are shown. In this comprehensive review, the key strategies for photoswitch inclusion in oligonucleotides are presented and illustrated with recent examples. Additionally the applications that have emerged in recent years are discussed, including gene regulation, drug delivery and materials design. Finally, we identify the challenges that the field currently faces and look forward to future applications.