Microspheres from light-a sustainable materials platform

Driven by the demand for highly specialized polymeric materials via milder, safer, and sustainable processes, we herein introduce a powerful, purely light driven platform for microsphere synthesis – including facile synthesis by sunlight. Our light-induced step-growth precipitation polymerization produces monodisperse particles (0.4–2.4 μm) at ambient temperature without any initiator, surfactant, additive or heating, constituting an unconventional approach compared to the classically thermally driven synthesis of particles. The microspheres are formed via the Diels-Alder cycloaddition of a photoactive monomer (2-methylisophthaldialdehyde, MIA) and a suitable electron deficient dienophile (bismaleimide). The particles are stable in the dry state as well as in solution and their surface can be further functionalized to produce fluorescent particles or alter their hydrophilicity. The simplicity and versatility of our approach introduces a fresh opportunity for particle synthesis, opening access to a yet unknown material class.

Photopolymerization provides a safe and mild fabrication pathway towards polymeric particles but the implementation of photochemistry from solution to dispersed media to produce particles is far from trivial. Here, the authors demonstrate an additive-free step-growth photopolymerization with sunlight, exploiting the photoinduced Diels-Alder to fabricate micrometer sized polymeric particles.

Predicting wavelength-dependent photochemical reactivity and selectivity

Predicting the conversion and selectivity of a photochemical experiment is a conceptually different challenge compared to thermally induced reactivity. Photochemical transformations do not currently have the same level of generalized analytical treatment due to the nature of light interaction with a photoreactive substrate. Herein, we bridge this critical gap by introducing a framework for the quantitative prediction of the time-dependent progress of photoreactions via common LEDs. A wavelength and concentration dependent reaction quantum yield map of a model photoligation, i.e., the reaction of thioether o-methylbenzaldehydes via o-quinodimethanes with N-ethylmaleimide, is initially determined with a tunable laser system. Combined with experimental parameters, the data are employed to predict LED-light induced conversion through a wavelength-resolved numerical simulation. The model is validated with experiments at varied wavelengths. Importantly, a second algorithm allows the assessment of competing photoreactions and enables the facile design of λ-orthogonal ligation systems based on substituted o-methylbenzaldehydes.

Functional Materials for Two-Photon Polymerization in Microfabrication

Direct laser writing methods based on two-photon polymerization (2PP) are powerful tools for the on-demand printing of precise and complex 3D architectures at the micro and nanometer scale. While much progress was made to increase the resolution and the feature size throughout the years, by carefully designing a material, one can confer specific functional properties to the printed structures thus making them appealing for peculiar and novel applications. This Review summarizes the state-of-the-art of functional resins and photoresists used in 2PP, discussing both the range of material functions available and the methods used to prepare them, highlighting advantages and disadvantages of different classes of materials in achieving certain properties.

100th Anniversary of Macromolecular Science Viewpoint: Photochemical Reaction Orthogonality in Modern Macromolecular Science

The ability to perform multiple chemical reactions independently (orthogonally) in a single reaction vessel can allow simplified reaction protocols for intricate chemical syntheses. Light is an especially advantageous external stimuli to enact such orthogonal chemical reactions due to its independence with other stimuli, instantaneous spatiotemporal control, and material penetrability. The potential to combine orthogonal chemistry and polymerization is also very appealing, as these systems may open the door for polymeric materials to find applications in emerging and high-tech fields, including biotechnology, microelectronics, sensors, energy, and others. We highlight the use of light in orthogonal polymerization protocols, particularly for living and controlled polymerization, and explore potential future directions and challenges for this technology.

Light-Controlled Orthogonal Covalent Bond Formation at Two Different Wavelengths

We report light-induced reactions in a two-chromophore system capable of sequence-independent λ-orthogonal reactivity relying solely on the choice of wavelength and solvent. In a solution of water and acetonitrile, LED irradiation at λ_max =285 nm leads to full conversion of 2,5-diphenyltetrazoles with N-ethylmaleimide to the pyrazoline ligation products. Simultaneously present o-methylbenzaldehyde thioethers are retained. Conversely, LED irradiation at λ_max =382 nm is used to induce ligation of the o-methylbenzaldehydes in acetonitrile with N-ethylmaleimide via o-quinodimethanes, while 2,5-diphenyltetrazoles also present are retained. This unprecedented photochemical selectivity is achieved through control of the number and wavelength of incident photons as well as favorable optical properties and quantum yields of the reactants in their environment.

Seeing the Light: Advancing Materials Chemistry through Photopolymerization

The application of photochemistry to polymer and material science has led to the development of complex yet efficient systems for polymerization, polymer post-functionalization, and advanced materials production. Using light to activate chemical reaction pathways in these systems not only leads to exquisite control over reaction dynamics, but also allows complex synthetic protocols to be easily achieved. Compared to polymerization systems mediated by thermal, chemical, or electrochemical means, photoinduced polymerization systems can potentially offer more versatile methods for macromolecular synthesis. We highlight the utility of light as an energy source for mediating photopolymerization, and present some promising examples of systems which are advancing materials production through their exploitation of photochemistry.

Spatially resolved coding of λ-orthogonal hydrogels by laser lithography

A λ-orthogonal reaction system is introduced, where visible light induced radical thiol-ene and UV light induced NITEC (Nitrile-Imine mediated Tetrazole-Ene Conjugation) ligations are consecutively employed to fabricate and functionalize PEG-based hydrogels. The fluorescent pyrazoline cycloadducts from the NITEC reaction are exploited to visualize the written structures within the hydrogels as well as to attach RGD containing functional groups to promote spatially resolved cell attachment on the hydrogel surface.

Installing lactone chain termini during photoinduced polymerization

We exploit the Thorpe–Ingold effect as a spontaneous end group transformation method during photo-induced polymerization of methacrylates using the functional (2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropio-phenone) species as radical photoinitiator.

Thermoresponsive hybrid double-crosslinked networks using magnetic iron oxide nanoparticles as crossing points

Preparation and characterization of stimuli-sensitive hybrid double-crosslinked hydrogels based on iron oxide nanoparticles as the nano-crosslinkers and a difuran-functionalized PEO as the diene partner for the thermoreversible Diels–Alder reaction.

Stability of Diels–Alder photoadducts in macromolecules

The study investigates the thermal stability of ligation points resulting from photochemically induced Diels–Alder reactions within soft matter materials on the molecular level.

3D Laser Micro- and Nanoprinting: Challenges for Chemistry

3D printing is a powerful emerging technology for the tailored fabrication of advanced functional materials. This Review summarizes the state-of-the art with regard to 3D laser micro- and nanoprinting and explores the chemical challenges limiting its full exploitation: from the development of advanced functional materials for applications in cell biology and electronics to the chemical barriers that need to be overcome to enable fast writing velocities with resolution below the diffraction limit. We further explore chemical means to enable direct laser writing of multiple materials in one resist by highly wavelength selective (λ-orthogonal) photochemical processes. Finally, chemical processes to construct adaptive 3D written structures that are able to respond to external stimuli, such as light, heat, pH value, or specific molecules, are highlighted, and advanced concepts for degradable scaffolds are explored.

Wavelength-Selective Sequential Polymer Network Formation Controlled with a Two-Color Responsive Initiation System

We report a wavelength-selective polymerization process controlled by visible/UV light, whereby a base is generated for anion-mediated thiol-Michael polymerization reaction upon exposure at one wavelength (400-500 nm), while radicals are subsequently generated for a second stage radical polymerization at a second, independent wavelength (365 nm). Dual wavelength, light controlled sequential polymerization not only provides a relatively soft intermediate polymer that facilitates optimum processing and modification under visible light exposure but also enables a highly cross-linked, rigid final material after the UV-induced second stage radical polymerization. A photobase generator, NPPOC-TMG, and a photo-radical initiator, Irgacure 2959, were selected as the appropriate initiator pair for sequential thiol-Michael polymerization and acrylate homopolymerization. FT-IR and rheological tests were utilized to monitor the dual cure photo-polymerization process, and mechanical performance of the polymer was characterized at each distinct stage by dynamic mechanical analysis (DMA). By demonstrating complete light control in another sequential polymerization system (thiol-Michael and thiol-ene hybrid polymerization), this initiator pair exhibits great potential to regulate many other coupled anion and radical hybrid polymerizations in both a sequential and controllable manner.

Molecular Switch for Sub-Diffraction Laser Lithography by Photoenol Intermediate-State Cis-Trans Isomerization

Recent developments in stimulated-emission depletion (STED) microscopy have led to a step change in the achievable resolution and allowed breaking the diffraction limit by large factors. The core principle is based on a reversible molecular switch, allowing for light-triggered activation and deactivation in combination with a laser focus that incorporates a point or line of zero intensity. In the past years, the concept has been transferred from microscopy to maskless laser lithography, namely direct laser writing (DLW), in order to overcome the diffraction limit for optical lithography. Herein, we propose and experimentally introduce a system that realizes such a molecular switch for lithography. Specifically, the population of intermediate-state photoenol isomers of α-methyl benzaldehydes generated by two-photon absorption at 700 nm fundamental wavelength can be reversibly depleted by simultaneous irradiation at 440 nm, suppressing the subsequent Diels-Alder cycloaddition reaction which constitutes the chemical core of the writing process. We demonstrate the potential of the proposed mechanism for STED-inspired DLW by covalently functionalizing the surface of glass substrates via the photoenol-driven STED-inspired process exploiting reversible photoenol activation with a polymerization initiator. Subsequently, macromolecules are grown from the functionalized areas and the spatially coded glass slides are characterized by atomic-force microscopy. Our approach allows lines with a full-width-at-half-maximum of down to 60 nm and line gratings with a lateral resolution of 100 nm to be written, both surpassing the diffraction limit.

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.

Direct light-induced (co-)grafting of photoactive polymers to graphitic nanodiamonds

We report the light-driven grafting and controlled simultaneous co-grafting of various functional polymers to graphitic nanodiamonds (grNDs).

Pyreneacyl sulfides as a visible light-induced versatile ligation platform

We report a visible light responsive moiety capable of generating highly reactive thioaldehydes.

Star polymer synthesis via λ-orthogonal photochemistry

λ-Orthogonal photo-induced ligation in two directions is introduced via a modular, light driven selective star shaped polymer formation.

Recent development of unimolecular micelles as functional materials and applications

Unimolecular micelles have high functionalities, encapsulation capabilities and site specific confinement abilities in various applications.