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

Wavelength-Dependent Activation of Photoacids and Bases

Photoacids and bases allow remote control over pH in reaction solutions, which is of fundamental importance to an array of applications. Herein, we determine the wavelength-by-wavelength resolved photoreactivity of triarylsulfonium hexafluorophosphate salts as a representative photoacid generator and p-(benzoyl)benzyl triethylammonium tetraphenylborate as a photobase generator, constructing a wavelength-resolved photochemical action plot for each of the compounds. We monitor the pH change of the solution on-line within the cavity of the laser vial and demonstrate a marked mismatch between the absorption spectrum of the photoacid and base with the photochemical action plot, unveiling reactivity at very low absorptivities. Our findings are of critical importance for the use of photoacids and bases, unambiguously demonstrating that absorption is no guide to chemical reactivity with critical consequences for the wavelength employed in applications of photoacids and bases.

Visible Light [2 + 2] Cycloadditions of Thermoresponsive Dendronized Styryltriazines To Exhibit Tunable Microconfinement

Visible light-triggered photochemical reactions in aqueous media are highly valuable to tailor molecular structures and properties in an ecofriendly manner. Here we report visible light-induced catalyst-free [2 + 2] cycloadditions of thermoresponsive dendronized styryltriazines, which show tunable microconfinement to guest dyes in aqueous media. These dendronized styryltriazines are constituted of conjugated mono- or tristyryltriazines, which carry hydrophilic dendritic oligoethylene glycol (OEG) pendants. They underwent efficient [2 + 2] cycloadditions to form dendronized cyclobutane dimers or oligomers in water through irradiation with visible light of 400 nm, and their cycloaddition behavior was dominated by dendritic architectures and solvent conditions. Dendronization with dendritic OEGs also afforded them characteristic thermoresponsive properties with tunable phase transition temperatures in the range 36-65 °C, which can be further modulated through photocycloaddition of styryltriazine chromophores. Importantly, dendronized styryltriazines can form tunable microenvironments in aqueous media, which encapsulate hydrophobic solvatochromic Nile red to exhibit variable photophysical properties. The encapsulated guest dye can be simultaneously released through noninvasive visible light-induced [2 + 2] cycloaddition reactions.

Cyclodextrin 'Chaperones' Enable Quasi-Ideal Supramolecular Network Formation and Enhanced Photodimerization of Hydrophobic, Red-shifted Photoswitches in Water

Precision-engineered light-triggered hydrogels are important for a diversity of applications. However, fields such as biomaterials require wavelength outside the harsh UV regime to prevent photodamage, typically requiring chromophores with extended π-conjugation that suffer from poor water solubility. Herein, we demonstrate how cyclodextrins can be used as auxiliary agents to not only solubilize such chromophores, but even to preorganize them in a 2 : 2 host-guest inclusion complex to facilitate photodimerization. We apply our concept to styrylpyrene-end-functionalized star-shaped polyethylene glycols (sPEGs). We initially unravel details of the host-guest inclusion complex using spectroscopy and mass spectrometry to give clear evidence of a 2 : 2 complex formation. Subsequently, we show that the resultant supramolecularly linked hydrogels conform to theories of supramolecular quasi-ideal model networks, and derive details on their association dynamics using in-depth rheological measurements and kinetic models. By comparing sPEGs of different arm length, we further elucidate the model network topology and the accessible mechanical property space. The photo-mediated dimerization proceeds smoothly, allowing to transform the supramolecular model networks into covalent ones. We submit that our strategy opens avenues for executing hydrophobic photochemistry in aqueous environments with enhanced control over reactivity, hydrogel topology or programmable mechanical properties.

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.

Triplet-Triplet Energy Transfer: A Simple Strategy for an Efficient Visible Light-Induced Photoclick Reaction

Photoclick reactions combine the advantages offered by light-driven processes and classical click chemistry and have found applications ranging from surface functionalization, polymer conjugation, photo-crosslinking, and protein labeling. Despite these advances, the dependency of most of the photoclick reactions on UV light poses a severe obstacle for their general implementation, as this light can be absorbed by other molecules in the system resulting in their degradation or unwanted reactivity. However, the development of a simple and efficient system to achieve bathochromically shifted photoclick transformations remains challenging. Here, we introduce triplet-triplet energy transfer as a fast and selective way to enable visible light-induced photoclick reactions. Specifically, we show that 9,10-phenanthrenequinones (PQs) can efficiently react with electron-rich alkenes (ERAs) in the presence of a catalytic amount (as little as 5 mol %) of photosensitizers. The photocycloaddition reaction can be achieved under green (530 nm) or orange (590 nm) light irradiation, representing a bathochromic shift of over 100 nm as compared to the classical PQ-ERAs system. Furthermore, by combining appropriate reactants, we establish an orthogonal, blue and green light-induced photoclick reaction system in which the product distribution can be precisely controlled by the choice of the color of light.

Photocatalytic Upcycling and Depolymerization of Vinyl Polymers

Photocatalytic upcycling and depolymerization of vinyl polymers have emerged as promising strategies to combat plastic pollution and promote a circular economy. This mini review critically summarizes current developments in the upcycling and degradation of vinyl polymers including polystyrene and poly(meth)acrylates. Of these material classes, polymethacrylates possess the unique possibility to undergo a photocatalytic depolymerization back to monomer under thermodynamically favourable conditions, thus presenting significant advantages over traditional thermal strategies. Our perspective on current formidable challenges and potential future directions are also discussed.

Photochemical Action Plots Reveal Red-shifted Wavelength-dependent Photoproduct Distributions

Photochemical action plots are a powerful tool for mapping photochemical reaction outcomes wavelength-by-wavelength. Typically, they map either the depletion of a reactant or the formation of a specific product as a function of wavelength. Herein, we exploit action plots to simultaneously map the formation of several photochemical products from a single chromophore. We demonstrate that the wavelength-resolved mapping of two reaction products formed during the irradiation of a chalcone species not only shows wavelength dependence - exhibiting the typical strong red-shift of the photochemical reactivity compared to the absorbance spectrum of the chromophore - but also a strong wavelength selectivity with remarkably different product distributions resulting from different irradiation wavelengths.

From Unregulated Networks to Designed Microstructures: Introducing Heterogeneity at Different Length Scales in Photopolymers for Additive Manufacturing

Photopolymers have been optimized as protective and decorative coating materials for decades. However, with the rise of additive manufacturing technologies, vat photopolymerization has unlocked the use of photopolymers for three-dimensional objects with new material requirements. Thus, the originally highly cross-linked, amorphous architecture of photopolymers cannot match the expectations for modern materials anymore, revealing the largely unanswered question of how diverse properties can be achieved in photopolymers. Herein, we review how microstructural features in soft matter materials should be designed and implemented to obtain high performance materials. We then translate these findings into chemical design suggestions for enhanced printable photopolymers. Based on this analysis, we have found microstructural heterogenization to be the most powerful tool to tune photopolymer performance. By combining the chemical toolbox for photopolymerization and the analytical toolbox for microstructural characterization, we examine current strategies for physical heterogenization (fillers, inkjet printing) and chemical heterogenization (semicrystalline polymers, block copolymers, interpenetrating networks, photopolymerization induced phase separation) of photopolymers and put them into a material scientific context to develop a roadmap for improving and diversifying photopolymers' performance.

Asymmetric Ion-Pairing Photoredox Catalysis for Stereoselective Cationic Polymerization under Light Control

By virtue of noninvasive regulations by light, photocontrolled polymerizations have attracted considerable attention for the precision synthesis of macromolecules. However, a cationic polymerization with simultaneous photocontrol and tacticity-regulation remains elusive so far. Herein, we introduce an asymmetric ion-pairing photoredox catalysis strategy that allows for the development of a stereoselective cationic polymerization with concurrent light regulation for the first time. By employing an ion pair catalyst (PC+/*A-) consisting of a photoredox active cation (PC+) and a sterically confined chiral anion (*A-) to deliver the stereochemical control, the cationic polymerization of vinyl ethers can be achieved with photocontrol and high isotactic selectivity (up to 91% m) at a remarkable low catalyst loading (50 ppm).

Triplet Upconversion under Ambient Conditions Enables Digital Light Processing 3D Printing

The rapid photochemical conversion of materials from liquid to solid (i.e., curing) has enabled the fabrication of modern plastics used in microelectronics, dentistry, and medicine. However, industrialized photocurables remain restricted to unimolecular bond homolysis reactions (Type I photoinitiations) that are driven by high-energy UV light. This narrow mechanistic scope both challenges the production of high-resolution objects and restricts the materials that can be produced using emergent manufacturing technologies (e.g., 3D printing). Herein we develop a photosystem based on triplet-triplet annihilation upconversion (TTA-UC) that efficiently drives a Type I photocuring process using green light at low power density (<10 mW/cm2) and in the presence of ambient oxygen. This system also exhibits a superlinear dependence of its cure depth on the light exposure intensity, which enhances spatial resolution. This enables for the first-time integration of TTA-UC in an inexpensive, rapid, and high-resolution manufacturing process, digital light processing (DLP) 3D printing. Moreover, relative to traditional Type I and Type II (photoredox) strategies, the present TTA-UC photoinitiation method results in improved cure depth confinement and resin shelf stability. This report provides a user-friendly avenue to utilize TTA-UC in ambient photochemical processes and paves the way toward fabrication of next-generation plastics with improved geometric precision and functionality.

Mechanistic Investigation, Wavelength-Dependent Reactivity, and Expanded Reactivity of N-Aryl Azacycle Photomediated Ring Contractions

Under mild blue-light irradiation, α-acylated saturated heterocycles undergo a photomediated one-atom ring contraction that extrudes a heteroatom from the cyclic core. However, for nitrogenous heterocycles, this powerful skeletal edit has been limited to substrates bearing electron-withdrawing substituents on nitrogen. Moreover, the mechanism and wavelength-dependent efficiency of this transformation have remained unclear. In this work, we increased the electron richness of nitrogen in saturated azacycles to improve light absorption and strengthen critical intramolecular hydrogen bonding while enabling the direct installation of the photoreactive handle. As a result, a broadly expanded substrate scope, including underexplored electron-rich substrates and previously unsuccessful heterocycles, has now been achieved. The significantly improved yields and diastereoselectivities have facilitated reaction rate, kinetic isotope effect (KIE), and quenching studies, in addition to the determination of quantum yields. Guided by these studies, we propose a revised ET/PT mechanism for the ring contraction, which is additionally corroborated by computational characterization of the lowest-energy excited states of α-acylated substrates through time-dependent DFT. The efficiency of the ring contraction at wavelengths longer than those strongly absorbed by the substrates was investigated through wavelength-dependent rate measurements, which revealed a red shift of the photochemical action plot relative to substrate absorbance. The elucidated mechanistic and photophysical details effectively rationalize empirical observations, including additive effects, that were previously poorly understood. Our findings not only demonstrate enhanced synthetic utility of the photomediated ring contraction and shed light on mechanistic details but may also offer valuable guidance for understanding wavelength-dependent reactivity for related photochemical systems.

Main-chain Macromolecular Hydrazone Photoswitches

Hydrazones-consisting of a dynamic imine bond and an acidic NH proton-have recently emerged as versatile photoswitches underpinned by their ability to form thermally bistable isomers, (Z) and (E), respectively. Herein, we introduce two photoresponsive homopolymers containing structurally different hydrazones as main-chain repeating units, synthesized via head-to-tail Acyclic Diene METathesis (ADMET) polymerization. Their key difference lies in the hydrazone design, specifically the location of the aliphatic arm connecting the rotor of the hydrazone photoswitch to the aliphatic polymer backbone. Critically, we demonstrate that their main photoresponsive property, i.e., their hydrodynamic volume, changes in opposite directions upon photoisomerization (λ=410 nm) in dilute solution. Further, the polymers-independent of the design of the individual hydrazone monomer-feature a photoswitchable glass transition temperature (Tg ) by close to 10 °C. The herein established design strategy allows to photochemically manipulate macromolecular properties by simple structural changes.

Photochemically Activated 3D Printing Inks: Current Status, Challenges, and Opportunities

3D printing with light is enabled by the photochemistry underpinning it. Without fine control over the ability to photochemically gate covalent bond formation by the light at a certain wavelength and intensity, advanced photoresists with functions spanning from on-demand degradability, adaptability, rapid printing speeds, and tailored functionality are impossible to design. Herein, recent advances in photoresist design for light-driven 3D printing applications are critically assessed, and an outlook of the outstanding challenges and opportunities is provided. This is achieved by classing the discussed photoresists in chemistries that function photoinitiator-free and those that require a photoinitiator to proceed. Such a taxonomy is based on the efficiency with which photons are able to generate covalent bonds, with each concept featuring distinct advantages and drawbacks.

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.

Near-Infrared Light and Acid/Base Dual-Regulated Polymerization Utilizing Imidazole-Anion-Fused Perylene Diimides as Photocatalysts

This work presents the first example of acid/base-responsive and near-infrared (NIR)-absorbing photocatalysts based on imidazole-anion-fused perylene diimide chromophores. The photocatalysts were in situ generated by deprotonation of imidazole-fused perylene diimide under an alkaline environment. NIR (λ = 730 nm, 128 mW/cm2) photoinduced atom transfer radical polymerization (ATRP) was implemented, exhibiting high efficiency and excellent livingness under ppm level of photocatalysts (15 ppm relative to monomer) and Cu(II) complex (10 ppm relative to monomer) concentrations. The method showed capabilities to polymerize behind opaque barriers (i.e., paper and pig skin) and under aerobic condition. Notably, this work demonstrated a dual temporal control of polymerization by adding weak base/acid and switching NIR light on/off. The polymerization can even be halted by bubbling CO2 and was then fully recovered by adding triethylamine. The NIR photoATRP of acrylamide monomers in aqueous solution was also performed, which can be regulated by the change of pH.

Visible-Light Initiated Dispersion Photopolymerization of Styrene

Polystyrene (PS) particles were synthesized in ethanol/water mixture by dispersion polymerization using visible light irradiation, with either a N-heterocyclic carbene borane-based photoinitiating system (PIS) or a disulfide. With the full PIS and poly(ethylene glycol) methyl ether methacrylate (PEGMA) as stabilizer, the size distributions were broad and the amount of PEGMA had a strong impact on the experiment reproducibility. The addition of a base solved the problem, leading to faster polymerizations, narrower size distributions and larger particles. With the disulfide as sole PIS, bigger and narrowly distributed PS particles were again formed. Quantitative conversion was achieved in each system, with particle size ranging between 100 and 350 nm. The use of poly(N-vinylpyrrolidone) as stabilizer led to significantly larger particles, up to 1.2 μm, with narrow size distributions. The production of such large latex particles by photoinitiated polymerizations is unprecedented.

On-Demand Activation of Transesterification by Chemical Amplification in Dynamic Thiol-Ene Photopolymers

Chemical amplification is a well-established concept in photoresist technology, wherein one photochemical event leads to a cascade of follow-up reactions that facilitate a controlled change in the solubility of a polymer. Herein, we transfer this concept to dynamic polymer networks to liberate both catalyst and functional groups required for bond exchange reactions under UV irradiation. For this, we exploit a photochemically generated acid to catalyse a deprotection reaction of an acid-labile tert-butoxycarbonyl group, which is employed to mask the hydroxy groups of a vinyl monomer. At the same time, the released acid serves as a catalyst for thermo-activated transesterifications between the deprotected hydroxy and ester moieties. Introduced in an orthogonally cured (450 nm) thiol-click photopolymer, this approach allows for a spatio-temporally controlled activation of bond exchange reactions, which is crucial in light of the creep resistance versus reflow ability trade-off of dynamic polymer networks.

Temporal Regulation of PET-RAFT Controlled Radical Depolymerization

A photocatalytic RAFT-controlled radical depolymerization method is introduced for precisely conferring temporal control under visible light irradiation. By regulating the deactivation of the depropagating chains and suppressing thermal initiation, an excellent temporal control was enabled, exemplified by several consecutive "on" and "off" cycles. Minimal, if any, depolymerization could be observed during the dark periods while the polymer chain-ends could be efficiently re-activated and continue to depropagate upon re-exposure to light. Notably, favoring deactivation resulted in the gradual unzipping of polymer chains and a stepwise decrease in molecular weight over time. This synthetic approach constitutes a simple methodology to modulate temporal control during the chemical recycling of RAFT-synthesized polymers while offering invaluable mechanistic insights.

Colorful 3D Printing: A Critical Feasibility Analysis of Multi-Wavelength Additive Manufacturing

Employing two colors of light to 3D print objects holds potential for accessing advanced printing modes, such as the generation of multi-material objects from a single print. Thus, dual-wavelength-driven photoreactive systems (reactions that require or utilize two wavelengths) and their exploitation as chemo-technological solutions for additive manufacturing technologies have experienced considerable development over the last few years. Such systems saw an increase in printing speeds, a decrease in resolution thresholds, and─perhaps most importantly─the actual generation of multi-material objects. However, the pace at which such reactive systems are developed is moderate and varies significantly depending on the fashion in which the two colors of light are employed. Herein, we address for the first time the varying logic conjugations of light-activated chemical compounds in dual-wavelength photochemical processes in a systematic manner and consider their implications from a photochemical point of view. To date, four dual-wavelength reaction types have been reported, termed synergistic (λ1 AND λ2), antagonistic (reversed λ1 AND λ2), orthogonal (λ1 OR λ2), and─most recently─cooperative (λ1 AND λ2 or λ1 OR λ2). The progress of their implementation in additive manufacturing is assessed individually, and their concurrent and individual chemical challenges are identified. These challenges need to be addressed for future dual-wavelength photochemical systems to progress multi-wavelength additive manufacturing technologies beyond their current limitations.

A Photoreactor-Interfaced Mass Spectrometer: An Online Platform to Monitor Photochemical Reactions

An experimental platform is reported that allows for the online characterization of photochemical reactions by coupling a continuous flow photoreactor, equipped with LED light irradiation and a dual-tipped ESI source, directly to a mass spectrometer with electrospray ionization. The capabilities of this platform are demonstrated with two classes of photoreactions: (1) the photopolymerization of methyl methacrylate and (2) photocatalyzed alkyne insertion into a 1,2,3-benzotriazinone. The online technique provides rapid information to inform the underlying photochemical mechanism and evaluate the overall photochemistry.

Simultaneously recorded photochemical action plots reveal orthogonal reactivity

We map the photochemical reactivity of two chromophores-a pyrene-chalcone and a methylene blue protected amine-from a one-pot reaction mixture based on their dynamic absorptivity changes upon light exposure, constructing a dual action plot. We employ the action plot data to determine a pathlength-independent λ-orthogonality window, allowing the orthogonal folding of distinct polymer chains into single chain nano-particles (SCNPs) from the same reaction mixture.

Photocatalytic ATRP Depolymerization: Temporal Control at Low ppm of Catalyst Concentration

A photocatalytic ATRP depolymerization is introduced that significantly suppresses the reaction temperature from 170 to 100 °C while enabling temporal regulation. In the presence of low-toxicity iron-based catalysts and under visible light irradiation, near-quantitative monomer recovery could be achieved (up to 90%), albeit with minimal temporal control. By employing ppm concentrations of either FeCl2 or FeCl3, the depolymerization during the dark periods could be completely eliminated, thus enabling temporal control and the possibility to modulate the rate by simply turning the light "on" and "off". Notably, our approach allowed preservation of the end-group fidelity throughout the reaction, could be carried out at high polymer loadings (up to 2M), and was compatible with various polymers and light sources. This methodology provides a facile, environmentally friendly, and temporally regulated route to chemically recycle ATRP-synthesized polymers, thus opening the door for further opportunities.

The Power of Action Plots: Unveiling Reaction Selectivity of Light-Stabilized Dynamic Covalent Chemistry

Exploiting the optimum wavelength of reactivity for efficient photochemical reactions has been well-established based on the development of photochemical action plots. We herein demonstrate the power of such action plots by a remarkable example of the wavelength-resolved photochemistry of two triazolinedione (TAD) substrates, i.e., aliphatic and aromatic substituted, that exhibit near identical absorption spectra yet possess vastly disparate photoreactivity. We present our findings in carefully recorded action plots, from which reaction selectivity is identified. The profound difference in photoreactivity is exploited by designing a 'hybrid' bisfunctional TAD molecule, enabling the formation of a dual-gated reaction manifold that demonstrates the exceptional and site-selective (photo)chemical behavior of both TAD substrates within a single small molecule.

Cooperative Network Formation via Two-Colour Light-Activated λ-Orthogonal Chromophores

Independently addressing photoreactive sites within one molecule with two colours of light is a formidable challenge. Here, we combine two sequence independent λ-orthogonal chromophores in one heterotelechelic dilinker molecule, to exploit their disparate reactivity utilizing the same reaction partner, a maleimide-containing polymer. We demonstrate that polymer network formation only proceeds if two colours of light are employed. Upon single colour irradiation, linker-decorated post-functionalized polymers are generated at either wavelength and in either sequence. Network formation, however, is only achieved by sequential or simultaneous two colour irradiation. The herein introduced photoreactive system demonstrates the power of wavelength orthogonal chemistry in macromolecular synthesis.

Upgrading the Toolbox: Two-Photon Absorption Induced Cleavage of Coumarin Dimers for Light-Based 4D Printing

The use of light for shaping and changing matter is of high relevance in polymer and material science. Herein, a photopolymer method is presented, which comprises the combination of 3D photo-printing at 405 nm light and subsequent modification under two-photon absorption (TPA) conditions at 532 nm light, adding the fourth dimension. The TPA-triggered cycloreversion reaction of an intramolecular coumarin dimer (ICD) structure occurs within the absorbing material. The 3D-printable matrix does not show any degradation under the TPA conditions. With the presented photochemical tool of TPA processes inside absorbing 3D photo-printable matrices, new possibilities for post-printing modification, e. g. for smart materials, are added.

Molecular Photoswitching of Main-Chain α-Bisimines in Solid-State Polymers

Photoisomerization of chromophores usually shows significantly less efficiency in solid polymers than in solution as strong intermolecular interactions lock their conformation. Herein, we establish the impact of macromolecular architecture on the isomerization efficiency of main-chain-incorporated chromophores (i.e., α-bisimine) in both solution and the solid state. We demonstrate that branched architectures deliver the highest isomerization efficiency for the main-chain chromophore in the solid state─remarkably as high as 70% compared to solution. The macromolecular design principles established herein for efficient solid-state photoisomerization can serve as a blueprint for enhancing the solid-state isomerization efficiency for other polymer systems, such as those based on azobenzenes.

Manipulation of an electron by photoirradiation in the electron-catalyzed cross-coupling reaction

An electron has recently been shown to catalyze the cross-coupling reaction of organometallic compounds with aryl halides. In terms of green and sustainable chemistry, the electron catalysis is much more desirable than the inevitably used transition metal catalysis but a high temperature of more than 100°C is required to achieve it. Here, we disclose that visible light photoirradiation accelerates the electron-catalyzed reaction of arylzinc reagents with aryl halides with the aid of a photoredox catalysis. Photoexcitation of a photoredox catalyst and an anion radical intermediate respectively affects the supply and transfer of the electron catalyst, promoting the cross-coupling reaction to proceed at room temperature. The supply of the electron catalyst by the photoredox catalysis makes the scope of aryl halides wider.

Operando surface optical nanometrology reveals diazonium salts' visible photografting mechanism

High resolution and quantitative surface modification through photografting is a highly desirable strategy towards the preparation of smart surfaces, enabling chemical functions to be precisely located onto specific regions of inert surfaces. Although promising, the mechanisms leading to direct (without the use of any additive) photoactivation of diazonium salts using visible wavelengths are poorly understood, precluding the generalization of popular diazonium-based electrografting strategies into high resolution photografting ones. In this paper, we employ quantitative phase imaging as a nanometrology tool for evaluating the local grafting rate with diffraction-limited resolution and nanometric precision. By carefully measuring the surface modification kinetics under a range of different conditions, we reveal the reaction mechanism while evaluating the influence of key parameters, such as the power density, the radical precursor concentration and the presence of side reactions.

Visible Light Degradable Acridine-Containing Polyurethanes in an Aqueous Environment

Light degradable polymers hold significant promise in a wide range of applications including the fabrication of optically recyclable materials, responsive coatings and adhesives, and controlled drug delivery. Here, we report the synthesis of polyurethanes that can be degraded under irradiation of visible light (≤450 nm) from commercial LED (3-15 W) light sources. The photolysis occurs in an aqueous environment via photocleavage of an acridine moiety incorporated within the backbone of the polymer chains. Analysis of the quantum yield as a function of wavelength reveals highly efficient photoreactivity at up to 440 nm activation, which is red-shifted compared to the UV-vis absorbance of the chromophore. The potential of our chemical system in biomaterials is demonstrated by the fabrication of an in situ forming hydrogel that can be degraded by visible light.

Skeletal diversification by C-C cleavage to access bicyclic frameworks from a common tricyclooctane intermediate

Herein, the diversification of tricyclo[3.2.1.03,6]octane scaffolds to afford diverse bicyclic scaffolds is described. The strained tricyclooctanes are prepared in two steps featuring a blue light-mediated [2+2] cycloaddition. Strategies for the cleavage of this scaffold were then explored resulting in the selective syntheses of the bicyclo[3.1.1]heptane, bicyclo[3.2.1]octane, and bicyclo[3.2.0]heptane cores. These findings may guide future studies of C-C cleavage reactions in strained carbon frameworks and their application in complex molecule synthesis.

Blocked Phosphates as Photolatent Catalysts for Dynamic Photopolymer Networks

While latent catalysts are a well-established strategy for initiating and controlling the rate of polymerization reactions, their use in dynamic polymer networks is still in its infancy. The ideal latent catalyst should be thermally stable and release a highly active species in response to an external trigger. Here, we have synthesized a temperature resistant (>200 °C) organic phosphate with a photolabile o-nitrobenzyl protecting group that can be cleaved by UV light. Introduced in a visible light curable thiol-click photopolymer, the sequence-dependent λ-orthogonality of the curing and cleavage enables an efficient network formation at 451 nm, without premature release of the catalyst. Once cured, irradiation at 372 nm spatiotemporally activates the phosphate, which catalyzes transesterifications at elevated temperature. The formed catalyst has no effect on the thermal stability of the polymeric network and allows the activation of bond exchange reactions in selected domains of printed 3D objects.

Organocatalytic [2 + 2] Photopolymerization under Visible Light: Accessing Sustainable Polymers from Cinnamic Acids

Herein, the successful development of a metal-free, solution [2 + 2] photopolymerization of natural cinnamic acid-derived bisolefinic monomers is reported, which is enabled by a strategy based on direct triplet state access via energy transfer catalysis. 2,2'-Methoxythioxanthone has been identified as an effective organic photocatalyst for the [2 + 2] photopolymerization in solution, which can be excited by visible light and activate the biscinnamate monomers via triplet energy transfer. This method features its metal-free conditions, visible light utilization, solution polymerization, and abundant biomass-based feedstock, as well as processable polymer products, which is different from the rigid, insoluble products obtained from solid-state photopolymerization. This solution polymerization method also shows a good compatibility to monomer structures; cinnamic acid-derived bisolefinic monomers with different linkers, including diamine, natural diol, and bisphenol, can all readily undergo [2 + 2] photopolymerization, and be transformed into colorless, sustainable polymers.

A practical perspective for chromatic orthogonality for implementing in photolithography

Theoretically, it is more challenging to anticipate the conversion and selectivity of a photochemical experiment compared to thermally generated reactivity. This is due to the interaction of light with a photoreactive substrate. Photochemical reactions do not yet receive the same level of broad analytical study. Here, we close this research gap by presenting a methodology for statistically forecasting the time-dependent progression of photoreactions using widely available LEDs. This study uses NiS/ZnO in perovskite (MAPbI₃) solar cells as an additive (5 volume %). The effect of monolithic perovskite solar cells (mPSCs) on forecasting the wavelength of LEDs has been carefully investigated using various characterization methods, including X-ray diffraction (XRD) and Transmission electron microscopy (TEM). The photocatalytic activity was analyzed by measuring the voltage produced. Various factors like selectivity, stability and sensitivity were also examined. This work provides a new perspective to validate NiS/ZnO photocatalysts for predicting the wavelength of different light sources and to apply in photolithography.

Linear Cyclobutane-Containing Polymer Synthesis via [2 + 2] Photopolymerization in an Unconfined Environment under Visible Light

The [2 + 2] photopolymerization of diolefinic monomers is an appealing approach for the construction of polymeric materials. Herein, we demonstrate that the establishment of an effective donor-acceptor conjugation by introducing electron-donating alkoxy groups at appropriate positions of the benzene ring could activate p-phenylenediacrylate (PDA), thus enabling the development of the first solution [2 + 2] photopolymerization of such monomers under the irradiation of visible light. Variation on the alkoxy groups and the ester parts could allow access to a series of linear cyclobutane-containing polymer products with high molecular weight (up to 140 kDa) and good solubility in common solvents. Further, temporal control and postpolymerization modification with preinstalled pendant C═C bonds via thiol-ene click reaction are also demonstrated with this [2 + 2] photopolymerization system.

Characterising different molecular landscapes in dynamic covalent networks

Dynamic covalent networks present a unique opportunity to exert molecular-level control on macroscopic material properties, by linking their thermal behaviour to the thermodynamics and kinetics of the underlying chemistry. Yet, existing methods do not allow for the extraction and analysis of the influence of local differences in chemical reactivity caused by available reactants, catalysts, or additives. In this context, we present a rheological paradigm that allows us to correlate the composition of a reactive polymer segment to a faster or slower rate of network rearrangement. We discovered that a generalised Maxwell model could separate and quantify the dynamic behaviour of each type of reactive segment individually, which was crucial to fully comprehend the mechanics of the final material. More specifically, Eyring and Van 't Hoff analysis were used to relate possible bond catalysis and dissociation to structural changes by combining statistical modelling with rheology measurements. As a result, precise viscosity changes could be measured, allowing for accurate comparison of various dynamic covalent network materials, including vitrimers and dissociative networks. The herein reported method therefore facilitated the successful analysis of virtually any type of rate-enhancing effect and will allow for the design of functional and fast (re)processable materials, as well as improve our ability to predict and engineer their properties for future applications.

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.

Two-colour light activated covalent bond formation

We introduce a photochemical bond forming system, where two colours of light are required to trigger covalent bond formation. Specifically, we exploit a visible light cis/trans isomerization of chlorinated azobenzene, which can only undergo reaction with a photochemically generated ketene in its cis state. Detailed photophysical mapping of the reaction efficiencies at a wide range of monochromatic wavelengths revealed the optimum irradiation conditions. Subsequent small molecule and polymer ligation experiments illustrated that only the application of both colours of light affords the reaction product. We further extend the functionality to a photo reversible ketene moiety and translate the concept into material science. The presented reaction system holds promise to be employed as a two-colour resist.

Dual-wavelength photochemical systems open up new avenues for novel lithographic techniques but currently only few wavelength-orthogonal photoreactive compounds undergoing reversible photoreaction are known. Here, the authors exploit cis/trans photoisomerization of azobenzenes and demonstrate photoligation of the cis state with a photochemically generated ketene.

Wavelength Orthogonal Photodynamic Networks

The ability of light to remotely control the properties of soft matter materials in a dynamic fashion has fascinated material scientists and photochemists for decades. However, only recently has our ability to map photochemical reactivity in a finely wavelength resolved fashion allowed for different colors of light to independently control the material properties of polymer networks with high precision, driven by monochromatic irradiation enabling orthogonal reaction control. The current concept article highlights the progress in visible light‐induced photochemistry and explores how it has enabled the design of polymer networks with dynamically adjustable properties. We will explore current applications ranging from dynamic hydrogel design to the light‐driven adaptation of 3D printed structures on the macro‐ and micro‐scale. While the alternation of mechanical properties via remote control is largely reality for soft matter materials, we herein propose the next frontiers for adaptive properties, including remote switching between conductive and non‐conductive properties, hydrophobic and hydrophilic surfaces, fluorescent or non‐fluorescent, and cell adhesive vs. cell repellent properties.

Photostationary State in Dynamic Covalent Networks

We explore a cross-linked polymer network based on a visible light photodynamic [2 + 2] cycloaddition driven by styrylpyrene chemistry. Based on a polymer backbone with pendent styrylpyrene units, the network can be formed by using λ = 450 nm irradiation. Upon irradiation with λ = 340 nm, a photostationary state is generated within the network with ∼17% of the styrylpyrene units open compared to close to 2% in the visible light cured state. The limited fraction of open [2 + 2] couples is caused by their proximity and is in sharp contrast to solution experiments on the photoreactive moiety. Thus, the polymer network retains its mechanical properties even at the photostationary point. We hypothesize that the application of an additional stimulus could serve as a second gate for inducing network disintegration by spacing the [2 + 2] units during ultraviolet irradiation.

Wavelength-Orthogonal Stiffening of Hydrogel Networks with Visible Light

Herein, we introduce the wavelength‐orthogonal crosslinking of hydrogel networks using two red‐shifted chromophores, i.e. acrylpyerene (AP, λ_activation=410–490 nm) and styrylpyrido[2,3‐b]pyrazine (SPP, λ_activation=400–550 nm), able to undergo [2+2] photocycloaddition in the visible‐light regime. The photoreactivity of the SPP moiety is pH‐dependent, whereby an acidic environment inhibits the cycloaddition. By employing a spiropyran‐based photoacid generator with suitable absorption wavelength, we are able to restrict the activation wavelength of the SPP moiety to the green light region (λ_activation=520–550 nm), enabling wavelength‐orthogonal activation of the AP group. Our wavelength‐orthogonal photochemical system was successfully applied in the design of hydrogels whose stiffness can be tuned independently by either green or blue light.

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.

Green Light Enabled Staudinger-Bertozzi Ligation

We introduce a visible light-induced Staudinger-Bertozzi ligation via photo-uncaging of a triphenylphosphine moiety with a photolabile coumarin derivative. Our action plot study examines the conversion as the function of wavelength,...