Two-Color Spatially Resolved Tuning of Polymer-Coated Metasurfaces

For the realization of truly reconfigurable metasurface technologies, dynamic spatial tuning of the metasurface resonance is required. Here we report the use of organic photoswitches as a means for the light-induced spatial tuning of metasurface resonances. Coating of a dielectric metasurface, hosting high-quality-factor resonances, with a spiropyran (SPA)-containing polymer enabled dynamic resonance tuning up to 4 times the resonance full-width at half-maximum with arbitrary spatial precision. A major benefit of employing photoswitches is the broad toolbox of chromophores available and the unique optical properties of each. In particular, SPA and azobenzene (AZO) photoswitches can both be switched with UV light but exhibit opposite refractive index changes. When applied to the metasurface, SPA induced a red shift in the metasurface resonance with a figure of merit of 97 RIU-1, while AZO caused a blue shift in the resonance with an even greater sensitivity of 100 RIU-1. Critically, SPA and AZO can be individually recovered with red and blue light, respectively. To exploit this advantage, we coated a dielectric metasurface with spatially offset SPA- and AZO-containing polymers to demonstrate wavelength-dependent, spatially resolved control over the metasurface resonance tuning.

Photochemical Action Plots Reveal the Fundamental Mismatch Between Absorptivity and Photochemical Reactivity

Over the last years, the authors' laboratory has employed monochromatic tuneable laser systems to reveal a fundamental mismatch between the absorptivity of a chromophore and its photochemical reactivity for the vast majority of covalent bond forming reactions as well as specific bond cleavage reactions. In the general chemistry community, however, the long-held assumption pervades that effective photochemical reactions are obtained in situations where there is strong overlap between the absorption spectrum and the excitation wavelength. The current Perspective illustrates that the absorption spectrum of a molecule only provides information about electronic excitations and remains entirely silent on other energy redistribution mechanisms that follow, which critically influence photochemical reactivity. Future avenues of enquiry on how action plots can be understood are proposed and the importance of action plots for tailoring photochemical applications with never-before-seen precision is explored.

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.

Two Photon Induced Pulsed Laser Polymerization with Near Infrared Light

We introduce two-photon (2P) pulsed laser polymerization (PLP) at 800 nm, demonstrating its working principle even through biological tissue. We show that 2P PLP is reliable in determining propagation rate coefficients on the example of the free radical polymerization of methyl methacrylate (MMA) at frequencies ranging from 10 to 100 Hz.

Near Infrared Light Induced Radical Polymerization in Water

We introduce a gold nanorod (AuNR) driven methodology to induce free radical polymerization in water with near infrared light (800 nm). The process exploits photothermal conversion in AuNR and subsequent heat transfer to a radical initiator (here azobisisobutyronitrile) for primary radical generation. A broad range of reaction conditions were investigated, demonstrating control over molecular weight and reaction conversion of dimethylacrylamide polymers, using nuclear magnetic resonance spectroscopy. We underpin our experimental data with finite element simulation of the spatio-temporal temperature profile surrounding the AuNR directly after femtosecond laser pulse excitation. Critically, we evidence that polymerization can be induced through biological tissues given the enhanced penetration depth of the near infrared light. We submit that the presented initiation mechanism in aqueous systems holds promise for radical polymerization in biological environments, including cells.

Regioisomerism in Symmetric Dimethyl Dialdehydes Dictates their Photochemical Reactivity

We herein report the first light-driven selective monoderivatization (desymmetrization) of two chemically equivalent carbonyl groups in a single chromophore. By comparing of four symmetric regioisomers, featuring two equivalent ortho-methylbenzaldehyde units, we identify dimethyltherephtalaldehydes (DMTAs) which can be activated in a dual wavelength-selective fashion. Under visible light and UV-light irradiation, DMTAs undergo two consecutive Diels-Alder reactions exhibiting near-quantitative endo-selectivity (>99%) and provide excellent yields (96-98%). The influence of the regioisomerism of the dialdehydes on their photochemical behavior is profound, evidenced by an in-depth investigation of their photochemical performance. We exemplify the capability of the photosystems via the synthesis of complex Diels-Alder adducts with various dienophiles, including alkynes.

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.

Sequence-independent activation of photocycloadditions using two colours of light

We exploit two reactive chromophores to establish sequence-independent photochemical activation, employing ortho-methyl benzaldehyde (oMBA) and N,N-(dimethylamino)pyrene aryl tetrazole (APAT) with N-(2-hydroxy)ethyl maleimide (NHEM), without any additives. Critically, the order of the irradiation sequence is irrelevant, as the shorter wavelength does not activate the higher wavelength activated species. Therefore, full sequence-independent λ-orthogonality is achieved through differences in both the reaction quantum yields (Φ r,oMBA and Φ r,APAT) and wavelength-dependent reactivity profiles of the employed chromophores.

Action Plots in Action: In-Depth Insights into Photochemical Reactivity

Predicting wavelength-dependent photochemical reactivity is challenging. Herein, we revive the well-established tool of measuring action spectra and adapt the technique to map wavelength-resolved covalent bond formation and cleavage in what we term "photochemical action plots". Underpinned by tunable lasers, which allow excitation of molecules with near-perfect wavelength precision, the photoinduced reactivity of several reaction classes have been mapped in detail. These include photoinduced cycloadditions and bond formation based on photochemically generated o-quinodimethanes and 1,3-dipoles such as nitrile imines as well as radical photoinitiator cleavage. Organized by reaction class, these data demonstrate that UV/vis spectra fail to act as a predictor for photochemical reactivity at a given wavelength in most of the examined reactions, with the photochemical reactivity being strongly red shifted in comparison to the absorption spectrum. We provide an encompassing perspective of the power of photochemical action plots for bond-forming reactions and their emerging applications in the design of wavelength-selective photoresists and photoresponsive soft-matter materials.

Two Sides of the Same Coin: Light as a Tool to Control and Map Microsphere Design

Herein, we establish the effect of intensity and wavelength on the size of microparticles formed via precipitation polymerization, employing photocrosslinkable prepolymers. Simultaneous measurement of backscattered laser irradiation enabled real-time tracking of particle growth and provides the ability to vary the LED intensity (λ_max = 415 nm) during various stages of particle growth. Critically, particle diameters can be controlled between 200 and 700 nm by varying the LED power from 73 to 0.63 mW, respectively. High intensities during the nucleation phase-spanning only the initial seconds-were found to dictate the particle diameter, irrespective of the energy used during the growth phase. Finally, a bathochromic shift was observed between the wavelength generating the highest rate of particle formation and the absorbance maxima of the photoactive group. We submit that these findings are broadly applicable in the continuously developing field of photoinitiated synthesis of polymer particles.

Chemiluminescent self-reported unfolding of single-chain nanoparticles

We demonstrate the light-induced, crosslinker mediated collapse of linear polymer chains into single-chain nanoparticles (SCNPs) capable of self-reporting their unfolding. The crosslinker entails a phenyloxalate motif allowing for the targeted degradation of the SCNPs via addition of hydrogen peroxide that triggers chemiluminescence (CL). The time-dependant CL emission can serve as a guide to follow the time dependent unfolding of the SCNPs, allowing for a qualitative assessment of the underlying mechanism.

Wavelength-Gated Photochemical Synthesis of Phenalene Diimides

Herein, we pioneer a wavelength‐gated synthesis route to phenalene diimides. Consecutive Diels–Alder reactions of methylisophthalaldehydes and maleimides afford hexahydro‐phenalene‐1,6‐diol diimides via 5‐formyl‐hexahydro‐benzo[f]isoindoles as the intermediate. Both photoreactions are efficient (82–99 % yield) and exhibit excellent diastereoselectivity (62–98 % d.r.). The wavelength‐gated nature of the stepwise reaction enables the modular construction of phenalene diimide scaffolds by choice of substrate and wavelength. Importantly, this synthetic methodology opens a facile avenue to a new class of persistent phenalenyl diimide neutral radicals, constituting a versatile route to spin‐active molecules.

The bright and the dark side of the sphere: light-stabilized microparticles

We introduce degradable microparticles, synthesized from prepolymers in a precipitation-like polymerization. The narrow disperse particles are stabilized with continuous irradiation of green light and can be spontaneously degraded in the dark.

Green light LED activated ligation of a scalable, versatile chalcone chromophore

Herein we present a photoreactive chalcone moiety that can be synthesized at a scale of several grams with ease, and can efficiently undergo a [2 + 2] photocycloaddition with light close to 500 nm as determined by an action plot.

Mapping Photochemical Reactivity Profiles on Surfaces

Herein, we introduce a comprehensive methodology to map the reactivity of photochemical systems on surfaces. The reactivity of photoreactive groups in solution often departs from their corresponding solution absorption spectra. On surfaces, the relationship between the surface absorption spectra and reactivity remains unexplored. Thus, herein, the reactivity of an o-methylbenzaldehyde and a tetrazole, as ligation partners for maleimide functionalized polymers, was investigated when the reactive moieties are tethered to a surface. The ligation reaction of tetrazole functionalized surfaces was found to proceed rapidly leading to high grafting densities, while o-methylbenzaldehyde functionalized substrates required longer irradiation times and resulted in lower surface coverage at the same wavelength (330 nm). Critically, wavelength resolved reactivity profiles were found to closely match the surface absorption spectra, contrary to previously reported red shifts in solution for the same chromophores.

It's in the Fine Print: Erasable Three-Dimensional Laser-Printed Micro- and Nanostructures

3D printing, on all scales, is currently a vibrant topic in scientific and industrial research as it has enormous potential to radically change manufacturing. Owing to the inherent nature of the manufacturing process, 3D printed structures may require additional material to structurally support complex features. Such support material must be removed after printing-sometimes termed subtractive manufacturing-without adversely affecting the remaining structure. An elegant solution is the use of photoresists containing labile bonds that allow for controlled cleavage with specific triggers. Herein, we explore state-of-the-art cleavable photoresists for 3D direct laser writing, as well as their potential to combine additive and subtractive manufacturing in a hybrid technology. We discuss photoresist design, feature resolution, cleavage properties, and current limitations of selected examples. Furthermore, we share our perspective on possible labile bonds, and their corresponding cleavage trigger, which we believe will have a critical impact on future applications and expand the toolbox of available cleavable photoresists.

It's a Trap: Thiol-Michael Chemistry on a DASA Photoswitch

Donor-acceptor Stenhouse adducts (DASA) are popular photoswitches capable of toggling between two isomers depending on the light and temperature of the system. The cyclized polar form is accessed by visible-light irradiation, whereas the linear nonpolar form is recovered in the dark. Upon the formation of the cyclized form, the DASA contains a double bond featuring a β-carbon prone to nucleophilic attack. Here, an isomer selective thiol-Michael reaction between the cyclized DASA and a base-activated thiol is introduced. The thiol-Michael addition was carried out with an alkyl (1-butanethiol) and an aromatic thiol (p-bromothiophenol) as reaction partners, both in the presence of a base. Under optimized conditions, the reaction proceeds preferentially in the presence of light and base. The current study demonstrates that DASAs can be selectively trapped in their cyclized state.

Chemiluminescent read-out of para-fluoro-thiol reaction events

We exploit the fluoride that is released via the para-fluoro-thiol reaction (PFTR) to cleave silyl ethers, turning the PFTR into an effective self-reporting chemiluminescence (CL) probe.

Multi-angle fluorometer technique for the determination of absorption and scattering coefficients of subwavelength nanoparticles

A thorough analysis of the resonance light scattering (RLS) technique for quantitative scattering measurements of subwavelength nanoparticles is reported. The systematic error associated with using a measurement at a single angle to represent all of the scattered light is investigated. In-depth analysis of the reference material was performed to identify and minimize the error associated with the reference material. Semiconductor ZnO nanobullets and spherical Au nanoparticles of various sizes were used to verify the approach. A simple and inexpensive modification to standard fluorometers is demonstrated using a glass prism allowing scattering measurements in the slightly forward and backwards directions. This allows quantification of the systematic error associated with RLS which is consistently overlooked.