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 Softening of Hydrogel Networks.

Photoresponsive hydrogels hold key potential in advanced biomedical applications including tissue engineering, regenerative medicine, and drug delivery, as well as intricately engineered functions such as biosensing, soft robotics, and bioelectronics. Herein, the wavelength-dependent degradation of bio-orthogonal poly(ethylene glycol) hydrogels is reported, using three selective activation levels. Specifically, three chromophores are exploited, that is, ortho-nitrobenzene, dimethyl aminobenzene, and bimane, each absorbing light at different wavelengths. By examining their photochemical action plots, the wavelength-dependent reactivity of the photocleavable moieties is determined. The wavelength-selective addressability of individual photoreactive units is subsequently translated into hydrogel design, enabling wavelength-dependent cleavage of the hydrogel networks on-demand. Critically, this platform technology allows for the fabrication of various hydrogels, whose mechanical properties can be fine-tuned using different colors of light to reach a predefined value, according to the chromophore ratios used. The softening is shown to influence the spreading of pre-osteoblastic cells adhering to the gels as a demonstration of their potential utility. Furthermore, the materials and photodegradation processes are non-toxic to cells, making this platform attractive for biomaterials engineering.

A Self-Catalyzed Visible Light Driven Thiol Ligation.

We introduce a highly efficient ligation system based on a visible light-induced rearrangement affording a thiophenol which rapidly undergoes thiol-Michael additions. Unlike conventional light-triggered thiol-ene/yne systems, which rely on the use of photocaged bases/nucleophiles, (organo)-photo catalysts, or radical photoinitiators, our system provides a light-induced reaction in the absence of any additives. The ligation is self-catalyzed via the pyridine mediated deprotonation of the photochemically generated thiophenol. Subsequently, the thiol-Michael reaction between the thiophenol anion and electron deficient alkynes/alkenes proceeds additive-free. Hereby, the underlying photoinduced rearrangement of o-thiopyrinidylbenzaldehyde (oTPyB) generating the free thiol is described for the first time. We studied the influence of various reactions conditions as well as solvents and substrates. We exemplify our findings in a polymer end group modification and obtained macromolecules with excellent end group fidelity.

Reversible Diels-Alder and Michael Addition Reactions Enable the Facile Postsynthetic Modification of Metal-Organic Frameworks.

Functionalization of metal-organic frameworks (MOFs) is critical in exploring their structural and chemical diversity for numerous potential applications. Herein, we report multiple approaches for the tandem postsynthetic modification (PSM) of various MOFs derived from Zr(IV), Al(III), and Zn(II). Our current work is based on our efforts to develop a wide range of MOF platforms with a dynamic functional nature that can be chemically switched via thermally triggered reversible Diels-Alder (DA) and hetero-Diels-Alder (HDA) ligations. Furan-tagged MOFs (furan-UiO-66-Zr) were conjugated with maleimide groups bearing dienophiles to prepare MOFs with a chemically switchable nature. As HDA pairs, phosphoryl dithioester-based moieties and cyclopentadiene (Cp)-grafted MOF (Cp-MIL-53-Al) were utilized to demonstrate the cleavage and rebonding of the linkages as a function of temperature. In addition to these strategies, the Michael addition reaction was also applied for the tandem PSM of IRMOF-3-Zn. Maleimide groups were postsynthetically introduced in the MOF lattice, which were further ligated with cysteine-based biomolecules via the thiol-maleimide Michael addition reaction. On the basis of the versatility of the herein presented chemistry, we expect that these approaches will help in designing a variety of sophisticated functional MOF materials addressing diverse applications.

Electrochemical Stimulation of Water-Oil Interfaces by Nonionic-Cationic Block Copolymer Systems.

Variable interfacial tension could be desirable for many applications. Beyond classical stimuli like temperature, we introduce an electrochemical approach employing polymers. Hence, aqueous solutions of the nonionic-cationic block copolymer poly(ethylene oxide)114-b-poly{[2-(methacryloyloxy)ethyl]diisopropylmethylammonium chloride}171 (i.e., PEO114-b-PDPAEMA171 with a quaternized poly(diisopropylaminoethyl methacrylate) block) were investigated by emerging drop measurements and dynamic light scattering, analyzing the PEO114-b-qPDPAEMA171 impact on the interfacial tension between water and n-decane and its micellar formation in the aqueous bulk phase. Potassium hexacyanoferrates (HCFs) were used as electroactive complexants for the charged block, which convert the bishydrophilic copolymer into amphiphilic species. Interestingly, ferricyanides ([Fe(CN)6]3-) act as stronger complexants than ferrocyanides ([Fe(CN)6]4-), leading to an insoluble qPDPAEMA block in the presence of ferricyanides. Hence, bulk micellization was demonstrated by light scattering. Due to their addressability, in situ redox experiments were performed to trace the interfacial tension under electrochemical control, directly utilizing a drop shape analyzer. Here, the open-circuit potential (OCP) was changed by electrolysis to vary the ratio between ferricyanides and ferrocyanides in the aqueous solution. While a chemical oxidation/reduction is feasible, also an electrochemical oxidation leads to a significant change in the interfacial tension properties. In contrast, a corresponding electrochemical reduction showed only a slight response after converting ferricyanides to ferrocyanides. Atomic force microscopy (AFM) images of the liquid/liquid interface transferred to a solid substrate showed particles that are in accordance with the diameter from light scattering experiments of the bulk phase. In conclusion, the present results could be an important step toward economic switching of interfaces suitable, e.g., for emulsion breakage.

Light-induced Ligation of o-Quinodimethanes with Gated Fluorescence Self-reporting.

We introduce a highly efficient photoligation system, affording a pro-fluorescent Diels-Alder product that, on demand, converts into an intensively fluorescent naphthalene via E1 elimination in the presence of catalytic amounts of acid. The Diels-Alder reaction of the photocaged diene (o-quinodimethane ether or thioether) with electron-deficient alkynes is induced by UV or visible light. In contrast to previously reported ligation techniques directly leading to fluorescent products, the fluorescence is turned on after the photoligation. Thus, the light absorption of the fluorophore does not undermine the photoligation via competitive absorption, and as a result, photobleaching or side reactions of the fluorophore are not observed. Critically, the gated generation of a fluorescent product allows for fluorometric determination of the conversion. We employ a simple synthesis strategy for heterobifunctional electron-deficient alkynes allowing for facile functionalization of payload molecules.

Light-fueled dynamic covalent crosslinking of single polymer chains in non-equilibrium states.

While polymer synthesis proceeds predominantly towards the thermodynamic minimum, living systems operate on the reverse principle - consuming fuel to maintain a non-equilibrium state. Herein, we report the controlled formation of 3D macromolecular architectures based on light-fueled covalent non-equilibrium chemistry. In the presence of green light (525 nm) and a bivalent triazolinedione (TAD) crosslinker, naphthalene-containing polymers can be folded into single chain nanoparticles (SCNPs). At ambient temperature, the cycloaddition product of TAD with naphthalene reverts and the SCNP unfolds into its linear parent polymer. The reported SCNP is the first example of a reversible light triggered folding of single polymer chains and can readily be repeated for several cycles. The folded state of the SCNP can either be preserved through a constant supply of light fuel, kinetic trapping or through a chemical modification that makes the folded state thermodynamically favored. Whereas small molecule bivalent TAD/naphthalene cycloaddition products largely degraded after 3 days in solution, even in the presence of fuel, the SCNP entities were found to remain intact, thereby indicating the light-fueled stabilization of the SCNP to be an inherent feature of the confined macromolecular environment.

Adaptable and Reprogrammable Surfaces.

Establishing control over chemical reactions on interfaces is a key challenge in contemporary surface and materials science, in particular when introducing well-defined functionalities in a reversible fashion. Reprogrammable, adaptable and functional interfaces require sophisticated chemistries to precisely equip them with specific functionalities having tailored properties. In the last decade, reversible chemistries-both covalent and noncovalent-have paved the way to precision functionalize 2 or 3D structures that provide both spatial and temporal control. A critical literature assessment reveals that methodologies for writing and erasing substrates exist, yet are still far from reaching their full potential. It is thus critical to assess the current status and to identify avenues to overcome the existing limitations. Herein, the current state-of-the-art in the field of reversible chemistry on surfaces is surveyed, while concomitantly identifying the challenges-not only synthetic but also in current surface characterization methods. The potential within reversible chemistry on surfaces to function as true writeable memories devices is identified, and the latest developments in readout technologies are discussed. Finally, we explore how spatial and temporal control over reversible, light-induced chemistries has the potential to drive the future of functional interface design, especially when combined with powerful laser lithographic applications.

Engineering Nitroxide Functional Surfaces Using Bioinspired Adhesion.

We pioneer a versatile surface modification strategy based on mussel-inspired oxidative catecholamine polymerization for the design of nitroxide-containing thin polymer films. A 3,4-dihydroxy-l-phenylalanine (l-DOPA) monomer equipped with a 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-derived oxidation-labile hydroxylamine functional group is employed as a universal coating agent to generate polymer scaffolds with persistent radical character. Various types of materials including silicon, titanium, ceramic alumina, and inert poly(tetrafluoroethylene) (PTFE) were successfully coated with poly(DOPA-TEMPO) thin films in a one-step dip-coating procedure under aerobic, slightly alkaline (pH 8.5) conditions. Steadily growing polymer films (∼1.1 nm h-1) were monitored by ellipsometry, and their thicknesses were critically compared with those obtained from atomic force microscopic cross-sectional profiles. The heterogeneous composition of surface-adherent nitroxide scaffolds examined by X-ray photoelectron spectroscopy was correlated to that examined by in-solution polymer analysis via high-resolution electrospray ionization mass spectrometry, revealing oligomeric structures with up to six repeating units, mainly composed of covalently linked dihydroxyindole along the polymer backbone. Critically, the reversible redox-active character of the nitroxide-containing polymer scaffolds was investigated by cyclic voltammetric measurements, revealing a convenient and facile access route to electrochemically active nitroxide polymer coatings with potential application in electronic devices such as organic radical batteries.

Wavelength-Gated Dynamic Covalent Chemistry.

Chemical reactions are classically controlled by the judicious choice of functional groups as well as external factors such as temperature and catalysts. However, the use of light-induced reactions not only offers precise temporal and spatial control, but critically allows highly specific reaction channels to be selectively addressed through wavelength and intensity, thereby enabling targeted covalent bonds to be made and broken. Photoreversible cycloadditions are the most promising candidates to seize the outlined potential upon selective cyclization and cycloreversion, but are today still far from fulfilling these expectations. The current Minireview critically explores the current challenges in the application of photoreversible cycloadditions and discusses the steps necessary to realize their potential in molecular biology, biomimetic systems, 3D laser lithographic processes, and advanced soft matter materials with reprogrammable and self-healing properties.

Light-Induced Step-Growth Polymerization of AB-Type Photo-Monomers at Ambient Temperature.

We introduce two AB-type monomers able to undergo a facile catalyst-free photoinduced polycycloaddition of photocaged dienes, enabling rapid Diels-Alder ligations under UV-irradiation (λmax = 350 nm) at ambient temperature, closely adhering to Carother's equation established by a careful kinetic study (17800 g mol-1 < Mw < 24700 g mol-1). The resulting macromolecules were in-depth analyzed via size exclusion chromatography (SEC) and nuclear magnetic resonance (NMR) spectroscopy. Additionally, SEC hyphenated to high resolution-electrospray ionization-mass spectrometry (HR-ESI-MS) enabled the careful mapping of the end group structure of the generated polymers. Furthermore, we demonstrate that both monomer systems can be readily copolymerized. The study thus demonstrates that Diels-Alder ligation resting upon photocaged dienes is a powerful tool for accessing step-growth polymers.

Chemically reprogrammable metal organic frameworks (MOFs) based on Diels-Alder chemistry.

We present the reversible Diels-Alder functionalization of metal organic frameworks (MOFs). Cyclopentadiene (Cp) functional MOFs are ligated with dienophiles to fabricate functional MOFs with a reprogrammable chemical nature. Our strategy thus constitutes an unprecedented concept for chemically dynamic MOFs able to be recoded.

A facile route to segmented copolymers by fusing ambient temperature step-growth and RAFT polymerization.

We introduce the facile synthesis of segmented copolymers by a catalyst-free Diels-Alder (DA) reaction at ambient temperature via step-growth and subsequent reversible addition fragmentation chain transfer (RAFT) polymerization. High molecular weight step-growth polymers are readily obtained (Mw = 40 000 g mol-1), featuring trithiocarbonate moieties in their chain, which allow monomer insertion via RAFT polymerization yielding high molecular weight species.

Fluorescent Glyco Single-Chain Nanoparticle-Decorated Nanodiamonds.

We introduce the light-induced collapse of single glycopolymer chains in water generating fluorescent glyco single-chain nanoparticles (SCNPs) and their subsequent functionalization onto nanodiamonds. The glycopolymer precursors are prepared by polymerizing an acetylated mannose-based methacrylate monomer followed by a deprotection and postpolymerization functionalization step, introducing profluorescent photoactive tetrazole groups and furan-protected maleimide moieties. Subsequent UV irradiation in highly diluted aqueous solution triggers intramolecular tetrazole-mediated cycloadditions, yielding glyco SCNPs featuring fluorescence as well as lectin binding properties. The obtained SCNPs are coated onto nanodiamonds by adsorption, and the obtained hybrid nanoparticles are in depth characterized in terms of size, functionality, and bioactivity. Different coating densities are achieved by altering the SCNP concentration. The prepared nanoparticles are nontoxic in mouse RAW 264.7 macrophages. Furthermore, the fluorescence of the SCNPs can be exploited to image the SCNP-coated nanodiamonds in macrophage cells via confocal fluorescence microscopy.

Spatially resolved photochemical coding of reversibly anchored cysteine-rich domains.

We present a novel methodology to generate recodable surfaces using cysteine-rich domains (CRD) via a combination of photolithography and reversible covalently peptide-driven disulfide formation. Therefore, two 21mer CRD peptide derivatives were synthesized, one bearing an electron deficient fumarate group for immobilization via nitrile imine-ene mediated cycloaddition (NITEC) to a tetrazole-functional surface. Secondly, a bromine moiety is introduced to the CRD for analytic labelling purposes to detect surface encoding. The photolithography is conducted by selectively passivating the surface with a polyethylene glycol (PEG)-fumarate via NITEC using a photomask in a dotted pattern. Consecutively, the CRD-fumarate is immobilized via NITEC adjacent to the PEG-functional areas to the unaffected tetrazole covered surface layer. Subsequently, the CRD-bromide is covalently linked to the CRD-fumarate by forming disulfide bonds under mild reoxidative conditions in a buffer solution. The CRD-bromide is released from the surface upon reduction to recover the prior state of the surface without the bromine marker. The analysis of the CRD precursors is based on electrospray ionization mass spectrometry (ESI-MS). The surface analytics were carried out via time-of-flight secondary ion mass spectrometry (ToF-SIMS), unambiguously verifying the successful immobilization as well as coding and decoding of the CRD-bromide on the surface based on dynamically reversible disulfide bond formation.

Ultrathin Monomolecular Films and Robust Assemblies Based on Cyclic Catechols.

We introduce a newly designed catechol-based compound and its application for the preparation of homogeneous monomolecular layers as well as for robust assemblies on various substrates. The precisely defined cyclic catechol material (CyCat) was prepared from ortho-dimethoxybenzene in a phenolic resin-like synthesis and subsequent deprotection, featuring molecules with up to 32 catechol units. The CyCat's chemical structure was carefully assessed via matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF), proton nuclear magnetic resonance (1H NMR), diffusion ordered spectroscopy (2D DOSY) and high resolution electrospray ionization mass spectrometry (ESI MS) experiments. The formation of colloidal aggregates of the CyCat material in alkaline solution was followed by dynamic light scattering (DLS) and further verified by dropcasting CyCat from solution on highly oriented pyrolytic graphite (HOPG), which was examined by Kelvin probe force microscopy (KPFM). The adsorption behavior of the CyCat to form monomolecular layers was investigated in real time by surface plasmon resonance (SPR). Formation of these thin CyCat layers (1.6-2.1 nm) on Au, SiO2 and TiO2 substrates was corroborated by spectroscopic ellipsometry (SE) and X-ray photoelectron spectroscopy (XPS). The prepared coating perfectly reflects the surface structure of the underlying substrate and does not exhibit CyCat colloidal aggregates as verified by atomic force microscopy (AFM). The functional nature of the prepared catechol monolayers was evidenced by reaction with 4-bromophenethylamine and bis(3-aminopropyl)-terminated poly(ethylene oxide) (PEO). Multilayer assemblies were prepared by a simple procedure of iterative immersion in solutions of CyCat and a multifunctional amine on Au, SiO2 and TiO2 substrates forming thicker coatings (up to 12 nm). Postmodification with small organic molecules was performed to covalently attach trifluoroacetyl, tetrazole and 2-bromo-2-methylpropanoyl moieties to the amine groups of the multilayer assembly coating. Furthermore, the versatility of the novel multilayer coating was underpinned by "grafting-to" of phenacyl sulfide-terminated PEO and "grafting-from" of poly(methyl methacrylate) via surface-initiated atom transfer radical polymerization (ATRP).

Two-in-One: λ-Orthogonal Photochemistry on a Radical Photoinitiating System.

An alkyne functional radical photoinitiator, 2-(4-(2-hydroxy-2-methylpropanoyl)phenoxy)ethyl hex-5-ynoate, and evidence that both reactive moieties - the alkyne and the photoinitiator terminus - can be independently addressed with light of disparate wavelength (λ-orthogonality) are introduced. The alkyne functionality is subjected to a visible light (420 nm) induced copper-catalyzed Huisgen reaction, which is employed for the selective functionalization of the initiator with a poly(ethylene glycol) (PEG) chain. This reaction proceeds completely λ-orthogonal in the presence of the UV-reactive photoinitiating moiety. Conversely, it is demonstrated that the alkyne functionality of the photoinitiator is quantitatively orthogonal to UV irradiation emitted by the pulsing action of an excimer laser (351 nm, pulsed-laser polymerization, PLP) and the generated radical species. In turn, the PEGylated initiator can readily be employed as a macrophotoinitiator during PLP. The introduced λ-orthogonally addressable dual functional initiator can be used in a wide range of applications, including surface lithography and post-synthetic modification of photocured materials.

Dynamic covalent single chain nanoparticles based on hetero Diels-Alder chemistry.

We introduce the fully reversible folding of single chain nanoparticles (SCNPs) based on covalent hetero Diels-Alder (HDA) chemistry. A cyclopentadiene (Cp)-protected cyanodithioester (CDTE) monomer is designed and copolymerized with methyl methacrylate (MMA) via RAFT polymerization. The polymer chains are folded and subsequently unfolded by exploiting the reversible nature of the HDA reaction.

Just Add Salt: A Mass Spectrometric Analysis Method for Imaging Anion-Exchanged Poly(Ionic Liquid)s.

We report the first mass spectrometric analysis of poly(ionic liquid)s (PILs) containing weakly coordinating anions introduced by a fast, simple, and quantitative postmodification method on the example of the hydrophilic, well-defined poly(vinylbenzylpyridinium chloride) p([VBPy]Cl) species, analyzed with an in-source collision induced dissociation-Orbitrap mass spectrometry (MS) protocol. Using the MS approach allows for the precise structural elucidation of ion-exchanged p([VBPy]Cl) utilizing AgX (X = NO3- , CF3 CO2- , BF4- ) salts. The anion exchange is shown to be quantitative - without observing residual chlorinated PIL - on rapid time scales, using only filtration as a standard procedure during sample preparation. In addition, the influence of weakly coordinating anions on the ionization behavior of PILs is studied in detail.

Universal mass spectrometric analysis of poly(ionic liquid)s.

We introduce a universal high resolution mass spectrometric method for the analysis of poly(ionic liquid)s (PILs), which belong to the most challenging polyelectrolytes from an analytical perspective, by fusing high resolution collision-induced dissociation (CID)-Orbitrap mass spectrometry (MS) with supercharging agents as well as quadrupole time-of-flight (QToF) MS. The study includes a wide array of hydrophilic halide-containing PILs, which were analyzed in negative mode. The influence of the core structures (based on imidazolium, triazolium, ammonium, phosphonium and pyridinium moieties), and variable styrene-, acrylate- and vinyl-type IL polymers on the ionization behavior is mapped in detail. Variable end group functionalities were introduced via functional chain transfer agents (CTA) in reversible addition-fragmentation chain transfer (RAFT) polymerization to study their behavior during the MS analysis. Furthermore, the demanding class of vinylimidazolium halide IL polymers was investigated. The current contribution thus introduces a new analytical technology platform for an entire polymer class.