Photochemical Action Plots Map Orthogonal Reactivity in Photochemical Release Systems

The wavelength-by-wavelength resolved photoreactivity of two photo-caged carboxylic acids, i. e. 7-(diethylamino)-coumarin- and 3-perylene-modified substrates, is investigated via photochemical action plots. The observed wavelength-dependent reactivity of the chromophores is contrasted with their absorption profile. The photochemical action plots reveal a remarkable mismatch between the maximum reactivity and the absorbance. Through the action plot data, the study is able to uncover photochemical reactivity maxima at longer and shorter wavelengths, where the molar absorptivity of the chromophores is strongly reduced. Finally, the laser experiments are translated to light emitting diode (LED) irradiation and show efficient visible-light-induced release in a near fully wavelength-orthogonal, sequence-independent fashion (λLED1 = 405 nm, λLED2 = 505 nm) with both chromophores in the same reaction solution. The herein pioneered wavelength orthogonal release systems open an avenue for releasing two different molecular cargos with visible light in a fully orthogonal fashion.

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.

Light-Triggered Click Chemistry

The merging of click chemistry with discrete photochemical processes has led to the creation of a new class of click reactions, collectively known as photoclick chemistry. These light-triggered click reactions allow the synthesis of diverse organic structures in a rapid and precise manner under mild conditions. Because light offers unparalleled spatiotemporal control over the generation of the reactive intermediates, photoclick chemistry has become an indispensable tool for a wide range of spatially addressable applications including surface functionalization, polymer conjugation and cross-linking, and biomolecular labeling in the native cellular environment. Over the past decade, a growing number of photoclick reactions have been developed, especially those based on the 1,3-dipolar cycloadditions and Diels-Alder reactions owing to their excellent reaction kinetics, selectivity, and biocompatibility. This review summarizes the recent advances in the development of photoclick reactions and their applications in chemical biology and materials science. A particular emphasis is placed on the historical contexts and mechanistic insights into each of the selected reactions. The in-depth discussion presented here should stimulate further development of the field, including the design of new photoactivation modalities, the continuous expansion of λ-orthogonal tandem photoclick chemistry, and the innovative use of these unique tools in bioconjugation and nanomaterial synthesis.

Laser Photodissociation Action Spectroscopy for the Wavelength-Dependent Evaluation of Photoligation Reactions

The nitrile imine-mediated tetrazole-ene cycloaddition is a widely used class of photoligation. Optimizing the reaction outcome requires detailed knowledge of the tetrazole photoactivation profile, which can only partially be ascertained from absorption spectroscopy, or otherwise involves laborious reaction monitoring in solution. Photodissociation action spectroscopy (PDAS) combines the advantages of optical spectroscopy and mass spectrometry in that only absorption events resulting in a mass change are recorded, thus revealing the desired wavelength dependence of product formation. Moreover, the sensitivity and selectivity afforded by the mass spectrometer enable reliable assessment of the photodissociation profile even on small amounts of crude material, thus accelerating the design and synthesis of next-generation substrates. Using this workflow, we demonstrate that the photodissociation onset for nitrile imine formation is red-shifted by ca. 50 nm with a novel N-ethylcarbazole derivative relative to a phenyl-substituted archetype. Benchmarked against solution-phase tunable laser experiments and supported by quantum chemical calculations, these discoveries demonstrate that PDAS is a powerful tool for rapidly screening the efficacy of new substrates in the quest toward efficient visible light-triggered ligation for biological applications.

Two colours of light drive PET–RAFT photoligation

By fusing the realms of photopolymerisation and photoligation, our contribution exploits two orthogonal wavelengths of visible light to readily synthesise and functionalise well defined polymers from a unique dual functionality RAFT agent.

Metal-free C-C bond formation via coupling of nitrile imines and boronic acids

The challenges of developing sustainable methods of carbon-carbon bond formation remains a topic of considerable importance in synthetic chemistry. Capitalizing on the highly reactive nature of the nitrile imine 1,3-dipole, we have developed a novel metal-free coupling of this species with aryl boronic acids. Photochemical generation of a nitrile imine intermediate and trapping with a palette of boronic acids enabled rapid and facile access to a broad library of more than 25 hydrazone derivatives in up to 92% yield, forming a carbon-carbon bond in a metal free fashion. This represents the first reported example of direct reaction between boronic acids and a 1,3-dipole.

Self-reporting visible light-induced polymer chain collapse

We introduce a facile photoinduced self-reporting crosslinking methodology for the compaction of polymer chains in highly diluted solution.

Spatially-Resolved Multiple Metallopolymer Surfaces by Photolithography

A tetrazole-based photoligation protocol for the spatially-resolved encoding of various defined metallopolymers onto solid surfaces is introduced. By using this approach, fabrication of bi- and trifunctional metallopolymer surfaces with different metal combinations were achieved. Specifically, α-ω-functional copolymers containing bipyridine as well as triphenylphosphine ligands were synthesized through reversible addition-fragmentation chain transfer (RAFT) polymerization, and subsequently metal loaded to afford metallopolymers of the widely-used metals gold, palladium, and platinum. Spatially-resolved surface attachment was achieved by means of a nitrile imine-mediated tetrazole-ene cycloaddition (NITEC) based photoligation protocol, exploiting tethered tetrazoles and metallopolymers equipped with a maleimide chain terminus. Metallopolymer coated surfaces with three different metals were prepared and characterized by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and spatially-resolved X-ray photoelectron spectroscopy (XPS) mapping, supporting the preserved chemical composition of the surface-bound metallopolymers. The established photochemical technology platform for arbitrary spatially-resolved metallopolymer surface designs enables the patterning of multiple metallopolymers onto solid substrates. This allows for the assembly of designer metallopolymer substrates.

Photochemically Driven Polymeric Network Formation: Synthesis and Applications

Polymeric networks have been intensely investigated and a large number of applications have been found in areas ranging from biomedicine to materials science. Network fabrication via light-induced reactions is a particularly powerful tool, since light provides ready access to temporal and spatial control, opening an array of synthetic access routes for structuring the network geometry as well as functionality. Herein, the most recent light-induced modular reactions and their use in the formation of precision polymeric networks are collated. The synthetic strategies including photoinduced thiol-based reactions, Diels-Alder systems, and photogenerated reactive dipoles, as well as photodimerizations, are discussed in detail. Importantly, applications of the fabricated networks via the aforementioned reactions are highlighted with selected examples. Concomitantly, we provide future directions for the field, emphasizing the most critically required advances.

Degradable fluorescent single-chain nanoparticles based on metathesis polymers

We introduce the facile synthesis of fluorescent single-chain nanoparticles (SCNPs) based on chain-shattering acyclic diene metathesis (ADMET) polymers featuring self-immolative azobenzene motifs.

Site-Selective Disulfide Modification of Proteins: Expanding Diversity beyond the Proteome

The synthetic transformation of polypeptides with molecular accuracy holds great promise for providing functional and structural diversity beyond the proteome. Consequently, the last decade has seen an exponential growth of site-directed chemistry to install additional features into peptides and proteins even inside living cells. The disulfide rebridging strategy has emerged as a powerful tool for site-selective modifications since most proteins contain disulfide bonds. In this Review, we present the chemical design, advantages and limitations of the disulfide rebridging reagents, while summarizing their relevance for synthetic customization of functional protein bioconjugates, as well as the resultant impact and advancement for biomedical applications.

Light-driven nitrile imine-mediated tetrazole-ene cycloaddition as a versatile platform for fullerene conjugation

An efficient methodology for modular fullerene functionalization via the photo-induced nitrile imine-mediated tetrazole-ene cycloaddition (NITEC) is introduced. The versatility and platform character of the method is illustrated by the light-driven reaction of fullerenes with small molecule, polymeric and surface-immobilized tetrazoles. The efficient fullerene conjugation is evidenced via mass spectrometric techniques.

Fluorescent Covalently Cross-Linked Cellulose Networks via Light-Induced Ligation

A facile light-induced procedure for the covalent cross-linking of cellulose at ambient conditions employing the nitrile imine mediated tetrazole-ene cycloaddition (NITEC) reaction is presented. Cellulose-tetrazoles with 2 degrees of substitution (0.14 and 0.23) were synthesized in a solution-based transesterification procedure in an ionic liquid. Two bismaleimides with either a trioxatridecane or a dithiodipropionyl backbone were used as cross-linkers to form fluorescent, covalently cross-linked cellulose networks and films, which were characterized by UV/vis spectroscopy, fluorescence spectroscopy, DSC, and TGA. The films showed a broad emission band from 500-700 nm and were thermally stable up to 200 °C. Using the bismaleimide with a disulfide moiety as the cross-linker, reductive degradation of the films can be induced. Finally, cellulose-tetrazole was cross-linked in a spatially resolved fashion, providing a strategy for the shaping of films based on renewable resources.

Catalyst free visible light induced cycloaddition as an avenue for polymer ligation

The current study introduces a tetrazole species able to perform a rapid, visible light induced nitrile imine-mediated tetrazole-ene cycloaddition (NITEC).

Classical photopolymerization kinetics, exceptional gelation, and improved diffraction efficiency and driving voltage in scaffolding morphological H-PDLCs afforded using a photoinitibitor

Ketyl radical inhibition results in prolonged gelation and an enhanced diffraction efficiency, allowing for the facile fabrication of colored 3D images.

A disulfide intercalator toolbox for the site-directed modification of polypeptides

A disulfide intercalator toolbox was developed for site-specific attachment of a broad variety of functional groups to proteins or peptides under mild, physiological conditions. The peptide hormone somatostatin (SST) served as model compound for intercalation into the available disulfide functionalization schemes starting from the intercalator or the reactive SST precursor before or after bioconjugation. A tetrazole-SST derivative was obtained that undergoes photoinduced cycloaddition in mammalian cells, which was monitored by live-cell imaging.

Photochemical Design of Functional Fluorescent Single-Chain Nanoparticles

We report the facile ambient temperature generation of size tunable and well-defined (pro)fluorescent single-chain nanoparticles (SCNPs) via the photoinduced nitrile imine intramolecular cross-ligation of linear precursor polymers, constituting a platform technology as novel imaging agents. A set of three linear precursor polymers (M_n ≈ 14000 g mol^-1, Đ ≈ 1.25) was synthesized via nitroxide-mediated statistical copolymerization of styrene and 4-(chloromethyl)styrene (CMS), followed by a postpolymerization modification of the resulting copolymer installing protected maleimide (PG-Mal) as well as tetrazole (Tet) moieties. The tetrazole content (% Tet) along the lateral polymer chains was varied between 12 and 24% in order to preselect not only the size of the corresponding SCNPs, but also their fluorescence and reactive properties. Finally, the applicability of the profluorescent SCNPs for fluorescence labeling was demonstrated utilizing residual surface expressed Tet moieties on the SCNPs surface in a reaction with maleimide functional polymeric microspheres. The (pro)fluorescent single-chain nanoparticles were in-depth characterized by ¹H NMR spectroscopy, dynamic light scattering (DLS), size exclusion chromatography (SEC), and atomic force microscopy (AFM), as well as UV/vis and fluorescence spectroscopy.

Fluorescent polymers from non-fluorescent photoreactive monomers

A facile, fast and ambient-temperature avenue towards highly fluorescent polymers is introduced via polymerizing non-fluorescent photoreactive monomers based on light-induced NITEC chemistry, providing a platform technology for fluorescent polymers. The resulting polypyrazolines were analyzed in depth.