Tuning the photoreactivity of photocycloaddition by halochromism

Harnessing the power of light for chemical transformation is a long-standing goal in organic synthesis, materials fabrication and engineering. Amongst all photochemical reactions, [2 + 2] photocycloadditions are inarguably the most important and most frequently used. These photoreactions have green characteristics by enabling new bond formation in a single step procedure under light irradiation, without the need for heat or chemical catalysis. More recently, substantial progress has been made in red-shifting the activation wavelength of photocycloadditions in response to research trends moving towards green and sustainable processes, and advanced applications in biological environments. In the past 5 years, our team has further expanded the toolbox of photocycloaddition reactions that can be triggered by visible light. In our exploration of photochemical reactivity, we found that reactivity is often red-shifted compared to the substrate’s absorption spectrum. Our efforts have resulted in red-shifted photochemical reactions, providing some of the lowest energy – and catalyst-free – photo-activated [2 + 2] cycloadditions (up to 550 nm). More recently, we introduced an additional level of control over such finely wavelength gated reactions by altering the pH of the reaction environment, thus exploiting halochromic effects to enhance or impede the photoreactivity of red-shifted [2 + 2] photocycloaddition reactions. In this account, we discuss the current state of halochromically regulated photochemical reactions and their potential in soft matter materials on selected examples

Visible‐Light‐Degradable 3D Microstructures in Aqueous Environments

The additive manufacturing technique direct laser writing (DLW), also known as two-photon laser lithography, is becoming increasingly established as a technique capable of fabricating functional 3D microstructures. Recently, there has been an increasing effort to impart microstructures fabricated using DLW with advanced functionalities by introducing responsive chemical entities into the underpinning photoresists. Herein, a novel photoresist based on the photochemistry of the bimane group is introduced that can be degraded upon exposure to very mild conditions, requiring only water and visible light (λmax = 415–435 nm) irradiation. The degradation of the microstructures is tracked and quantified using AFM measurements of their height. The influence of the writing parameters as well as the degradation conditions is investigated, unambiguously evidencing effective visible light degradation in aqueous environments. Finally, the utility of the photodegradable resist system is demonstrated by incorporating it into multimaterial 3D microstructures, serving as a model for future applications

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\u27s 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

Visible‐Light‐Induced Control over Reversible Single‐Chain Nanoparticle Folding

We introduce a class of single-chain nanoparticles (SCNPs) that respond to visible light (λ$_{max}$=415 nm) with complete unfolding from their compact structure into linear chain analogues. The initial folding is achieved by a simple esterification reaction of the polymer backbone constituted of acrylic acid and polyethylene glycol carrying monomer units, introducing bimane moieties, which allow for the photochemical unfolding, reversing the ester-bond formation. The compaction and the light driven unfolding proceed cleanly and are readily followed by size exclusion chromatography (SEC) and diffusion ordered NMR spectroscopy (DOSY), monitoring the change in the hydrodynamic radius (R$_H$). Importantly, the folding reaction and the light-induced unfolding are reversible, supported by the high conversion of the photo cleavage. As the unfolding reaction occurs in aqueous systems, the system holds promise for controlling the unfolding of SCNPs in biological environments

Two-colour light activated covalent bond formation

We introduce a photochemical bond forming system, where two colours of light are required to trigger covalent bond formation. Specifically, we exploit a visible light cis/trans isomerization of chlorinated azobenzene, which can only undergo reaction with a photochemically generated ketene in its cis state. Detailed photophysical mapping of the reaction efficiencies at a wide range of monochromatic wavelengths revealed the optimum irradiation conditions. Subsequent small molecule and polymer ligation experiments illustrated that only the application of both colours of light affords the reaction product. We further extend the functionality to a photo reversible ketene moiety and translate the concept into material science. The presented reaction system holds promise to be employed as a two-colour resist

Visible light-induced switching of soft matter materials properties based on thioindigo photoswitches

Thioindigos are visible light responsive photoswitches with excellent spatial control over the conformational change between their trans- and cis- isomers. However, they possess limited solubility in all conventional organic solvents and polymers, hindering their application in soft matter materials. Herein, we introduce a strategy for the covalent insertion of thioindigo units into polymer main chains, enabling thioindigos to function within crosslinked polymeric hydrogels. We overcome their solubility issue by developing a thioindigo bismethacrylate linker able to undergo radical initiated thiol-ene reaction for step-growth polymerization, generating indigo-containing polymers. The optimal wavelength for the reversible trans-/cis- isomerisation of thioindigo was elucidated by constructing a detailed photochemical action plot of their switching efficiencies at a wide range of monochromatic wavelengths. Critically, indigo-containing polymers display significant photoswitching of the materials’ optical and physical properties in organic solvents and water. Furthermore, the photoswitching of thioindigo within crosslinked structures enables visible light induced modulation of the hydrogel stiffness. Both the thioindigo-containing hydrogels and photoswitching processes are non-toxic to cells, thus offering opportunities for advanced applications in soft matter materials and biology-related research

Break Up to Make Up: Utilization of Photocleavable Groups in Biolabeling of Hydrogel Scaffolds

Understanding cell behavior within the extracellular matrix is essential for the implementation of biomaterials in medical applications. Conjugation of bioactive molecules to hydrogel scaffolds imparts the materials with biological cues necessary for controlling cell behavior such as adhesion, growth and migration. Photo-induced ligation is a powerful tool to precisely control when and where such bioactivity can be initiated. Over the past decade, advances in light-regulated technology have emerged to address the stringent demands for photochemical ligation in biological environments, including high sensitivity, fast reaction kinetics, longer wavelength absorption and site-selectivity. Here, the utilization of contemporary photolabile groups for light-mediated bioconjugation of hydrogels are highlighted. In this approach, reactive groups such as thiol, amine and aldehyde are masked with photolabile moieties, which can be photo-released to participate in click reactions, such as Michael addition and oxime ligation. New chromophores with long wavelength visible light absorption are also examined for potential employment in light-induced bioconjugation.</p