Light-Sensitive Alkoxyamines as Versatile Spatially- and Temporally- Controlled Precursors of Alkyl Radicals and Nitroxides

Spin-Labeled Riboswitch Synthesized from a Protected TPA Phosphoramidite Building Block

The nitroxide TPA (2,2,5,5-tetramethyl-pyrrolin-1-oxyl-3-acetylene) is an excellent spin label for EPR studies of RNA. Previous synthetic methods, however, are complicated and require special equipment. Herein, we describe a uridine derived phosphoramidite with a photocaged TPA unit attached. The light sensitive 2-nitrobenzyloxymethyl group can be removed in high yield by short irradiation at 365 nm. Based on this approach, a doubly spin-labeled 27mer neomycin sensing riboswitch was synthesized and studied by PELDOR. The overall thermal stability of the fold is not much reduced by TPA. In-line probing nevertheless detected changes in local mobility.

Dynamic porous organic polymers with tuneable crosslinking degree and porosity

Porous organic polymers (POPs) show enormous potential for applications in separation, organic electronics, and biomedicine due to the combination of high porosity, high stability, and ease of functionalisation. However, POPs are usually insoluble and amorphous materials making it very challenging to obtain structural information. Additionally, important parameters such as the exact molecular structure or the crosslinking degree are largely unknown, despite their importance for the final properties of the system. In this work, we introduced the reversible multi-fold nitroxide exchange reaction to the synthesis of POPs to tune and at the same time follow the crosslinking degree in porous polymer materials. We synthesised three different POPs based on the combination of linear, trigonal, and tetrahedral alkoxyamines with a tetrahedral nitroxide. We could show that modulating the equilibrium in the nitroxide exchange reaction, by adding or removing one nitroxide species, leads to changes in the crosslinking degree. Being able to modulate the crosslinking degree in POPs allowed us to investigate both the influence of the crosslinking degree and the structure of the molecular components on the porosity. The crosslinking degree of the frameworks was characterised using EPR spectroscopy and the porosity was determined using argon gas adsorption measurements. To guide the design of POPs for desired applications, our study reveals that multiple factors need to be considered such as the structure of the molecular building blocks, the synthetic conditions, and the crosslinking degree.

We synthesised three different POPs via a nitroxide exchange reaction and modulated their crosslinking degree. That allowed us to investigate the influence of the crosslinking degree and the structure of the molecular components on the porosity.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Laser Direct Writing of Arbitrary Complex Polymer Microstructures by Nitroxide-Mediated Photopolymerization

In this paper, we demonstrate the possibility of generating arbitrary polymer microstructures covalently linked to a first polymer layer by laser direct writing. At the molecular scale, the process relies on nitroxide-mediated photopolymerization triggered by a light-sensitive alkoxyamine. In addition to the proof of concept and examples of achievable structures, including multichemistry patterns and 3D structures, this paper aims at investigating the physicochemical phenomena involved under such conditions. In particular, the parameters influencing the repolymerization process are considered, and special attention is paid to the study of the impact of oxygen on the spatial control of the polymerization. Such a work opens many possibilities toward the fabrication of on-demand high-resolution (multi)functional polymer micro and nanostructures.

Design and Photo-Induced Dynamics of Radical-Chromophore Adducts with One- or Two-Atom Separation: Toward Potential Probes for High Field Optical DNP Experiments

Breaking the maximum enhancement barrier of 660 at room temperature in a conventional dynamic nuclear polarization (DNP) experiment has the immense potential of practical applications. Optical DNP experiments with radical-chromophore (RC) adducts, which harnesses hyperpolarized radicals, instead of thermalized radicals, offers a powerful way to achieve this. Typical DNP and NMR experiments, however, are carried out at high magnetic fields of about 5-10 T, whereas the large electron spin hyperpolarization (ESP) demonstrated in the RC adducts so far are at a much low field of 0.3 T. Thus, in order to realize a successful optical DNP experiment, it is imperative to ask whether the RC adducts, which are currently available, can achieve a large ESP even at high fields. The present work poses this question and shows that the current RC adducts would not generate a large ESP at high fields unless the separation between the chromophore and nitroxyl moiety is reduced to less than four bonds. Two serious bottlenecks in this direction are the near impossibility of synthesizing such RC adducts using the common nitroxyl radicals and the absence of any photophysical studies on RC adducts with such short spacer groups. In this regard, the present work exploits the spin trapping methodology to synthesize one- and two-atom separated naphthalene-nitroxyl RC adducts. Good yields and excellent stability of the adducts have been demonstrated. Furthermore, the present work presents their detailed photophysical and photochemical studies by transient optical and time-resolved EPR studies. On the basis of the present results, a potential RC adduct is proposed for the high field optical DNP experiments. Finally, the prospect of exploiting the large EPR signal enhancement due to ESP in the field of spin trapping studies has been discussed.

Role of External Field in Polymerization: Mechanism and Kinetics

The past decades have witnessed an increasing interest in developing advanced polymerization techniques subjected to external fields. Various physical modulations, such as temperature, light, electricity, magnetic field, ultrasound, and microwave irradiation, are noninvasive means, having superb but distinct abilities to regulate polymerizations in terms of process intensification and spatial and temporal controls. Gas as an emerging regulator plays a distinctive role in controlling polymerization and resembles a physical regulator in some cases. This review provides a systematic overview of seven types of external-field-regulated polymerizations, ranging from chain-growth to step-growth polymerization. A detailed account of the relevant mechanism and kinetics is provided to better understand the role of each external field in polymerization. In addition, given the crucial role of modeling and simulation in mechanisms and kinetics investigation, an overview of model construction and typical numerical methods used in this field as well as highlights of the interaction between experiment and simulation toward kinetics in the existing systems are given. At the end, limitations and future perspectives for this field are critically discussed. This state-of-the-art research progress not only provides the fundamental principles underlying external-field-regulated polymerizations but also stimulates new development of advanced polymerization methods.

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.

Photochemical generation of the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical from caged nitroxides by near-infrared two-photon irradiation and its cytocidal effect on lung cancer cells

Novel caged nitroxides (nitroxide donors) with near-infrared two-photon (TP) responsive character, 2,2,6,6-tetramethyl-1-(1-(2-(4-nitrophenyl)benzofuran-6-yl)ethoxy)piperidine (2a) and its regioisomer 2b, were designed and synthesized. The one-photon (OP) (365 ± 10 nm) and TP (710–760 nm) triggered release (i.e., uncaging) of the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical under air atmosphere were discovered. The quantum yields for the release of the TEMPO radical were 2.5% (2a) and 0.8% (2b) in benzene at ≈1% conversion of 2, and 13.1% (2a) and 12.8% (2b) in DMSO at ≈1% conversion of 2. The TP uncaging efficiencies were determined to be 1.1 GM at 740 nm for 2a and 0.22 GM at 730 nm for 2b in benzene. The cytocidal effect of compound 2a on lung cancer cells under photolysis conditions was also assessed to test the efficacy as anticancer agents. In a medium containing 100 μg mL⁻¹ of 2a exposed to light, the number of living cells decreased significantly compared to the unexposed counterparts (65.8% vs 85.5%).

Chemical modifications of imidazole-containing alkoxyamines increase C-ON bond homolysis rate: Effects on their cytotoxic properties in glioblastoma cells

Previously, we described alkoxyamines bearing a pyridine ring as new pro-drugs with low molecular weights and theranostic activity. Upon chemical stimulus, alkoxyamines undergo homolysis and release free radicals, which can, reportedly, enhance magnetic resonance imaging and trigger cancer cell death. In the present study, we describe the synthesis and the anti-cancer activity of sixteen novel alkoxyamines that contain an imidazole ring. Activation of the homolysis was conducted by protonation and/or methylation. These new molecules displayed cytotoxic activities towards human glioblastoma cell lines, including the U251-MG cells that are highly resistant to the conventional chemotherapeutic agent Temozolomide. We further showed that the biological activities of the alkoxyamines were not only related to their half-life times of homolysis. We lastly identified the alkoxyamine (RS/SR)-4a, with both a high antitumour activity and favourable logD_7.4 and pK_a values, which make it a robust candidate for blood-brain barrier penetrating therapeutics against brain neoplasia.

Visible-light unmasking of heterocyclic quinone methide radicals from alkoxyamines

Controllable room-temperature visible-light activated nitroxide (TEMPO)-release from benzimidazolequinone alkoxyamines and bis-alkoxyamines.

Design, fabrication and anti-aging behavior of a multifunctional inorganic–organic hybrid stabilizer derived from co-intercalated layered double hydroxides for polypropylene

The concomitant presence of HALS/DBHP co-intercalated within the LDH structure strongly improved the anti-aging performance of PP, which opens the pathway for potential future research of high-performance PP associated with hybrid fillers.

Dynamic Nitroxide Functional Materials

A substrate-independent and versatile coating platform for (spatially resolved) surface functionalization, based on nitroxide radical coupling (NRC) reactions and the formation of thermo-labile alkoxyamine functional groups, was introduced. Nitroxide-decorated poly(glycidyl methacrylate) (PGMA) microspheres, obtained through bioinspired copolymer surface deposition using dopamine and a nitroxide functional dopamine derivative as monomers, were conjugated with small functional groups in a rewritable process. Reversible coding of the nitroxide functional microspheres by NRC and decoding through thermal alkoxyamine fission were monitored and characterized by electron paramagnetic resonance (EPR) spectroscopy and X-ray photoelectron spectroscopy (XPS). In addition, this nitroxide coating system was exploited in "grafting-to" polymer surface ligations of poly(methyl methacrylate) (PMMA) and poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA) in spatially confined areas. Polymer strands terminated with an Irgacure 2959 (2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone) photoinitiator were obtained through chain-transfer polymerization, and subsequently coupled to nitroxide-immobilized poly(dopamine) (PDA)-coated silicon substrates by using rapid photoclick NRC reactions. Light-driven polymer surface coding was visualized by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and XPS imaging.