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.

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.

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.

Single Chain Nanoparticles in Catalysis.

Over the last six decades folded polymer chains-so-called Single Chain Nanoparticles (SCNPs)-have evolved from the mere concept of intramolecularly crosslinked polymer chains to tailored nanoreactors, underpinned by a plethora of techniques and chemistries to tailor and analyze their morphology and function. These monomolecular polymer entities hold critical promise in a wide range of applications. Herein, we highlight the exciting progress that has been made in the field of catalytically active SCNPs in recent years.

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.

Quantification of Synergistic Two-Color Covalent Bond Formation.

The emergence of highly wavelength resolved reactivity information for complex photochemical reaction processes allows the establishment of multi-color reaction modes. One particularly powerful mode is the synergistic two-color reaction, where two colors of light have to be present in the same volume element to either enable or enhance photochemical reactivity that leads to a specific photoproduct. Herein, we introduce a two-color synergistic photochemical reaction system based on a diaryl indenone epoxide (DIO) photoswitch and the cis-to-trans isomerization of a bridged ring-strained azobenzene (SA), which respond to ultraviolet (365 nm) and visible light (430 nm), respectively, with different rates, forming a well-defined heterocyclic photoadduct, DIOSA, that we structurally confirm via single crystal x-ray diffraction (SXRD). To quantitatively capture the effectiveness of the dual-color irradiation as a function of the reaction conditions such as light intensity and starting material ratio as a function of product yield, we introduce a parameter, the photochemical synergistic ratio. A reduced synergistic ratio - that extrapolates to conditions of infinitesimal conversions - allows to compare the efficiency of the synergistic photochemistry at varying reaction conditions.

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.

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.

Flexible Ink-Jet Printed Polymer Light-Emitting Diodes using a Self-Hosted Non-Conjugated TADF Polymer.

Thermally activated delayed fluorescent (TADF) emitters have become the leading emissive materials for highly efficient organic light-emitting diodes (OLEDs). The deposition of these materials in scalable and cost-effective ways is paramount when looking toward the future of OLED applications. Herein, a simple OLED with fully solution-processed organic layers is introduced, where the TADF emissive layer is ink-jet printed. The TADF polymer has electron and hole conductive side chains, simplifying the fabrication process by removing the need for additional host materials. The OLED has a peak emission of 502 nm and a maximum luminance of close to 9600 cd m-2 . The self-hosted TADF polymer is also demonstrated in a flexible OLED, reaching a maximum luminance of over 2000 cd m-2 . These results demonstrate the potential applications of this self-hosted TADF polymer in flexible ink-jet printed OLEDs and, therefore, for a more scalable fabrication process.

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.

Visible-Light-Induced Control over Reversible Single-Chain Nanoparticle Folding.

We introduce a class of single-chain nanoparticles (SCNPs) that respond to visible light (λmax =415 nm) with complete unfolding from their compact structure into linear chain analogues. The initial folding is achieved by a simple esterification reaction of the polymer backbone constituted of acrylic acid and polyethylene glycol carrying monomer units, introducing bimane moieties, which allow for the photochemical unfolding, reversing the ester-bond formation. The compaction and the light driven unfolding proceed cleanly and are readily followed by size exclusion chromatography (SEC) and diffusion ordered NMR spectroscopy (DOSY), monitoring the change in the hydrodynamic radius (RH ). Importantly, the folding reaction and the light-induced unfolding are reversible, supported by the high conversion of the photo cleavage. As the unfolding reaction occurs in aqueous systems, the system holds promise for controlling the unfolding of SCNPs in biological environments.

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.

Visible-Light-Reactive Single-Chain Nanoparticles.

We introduce a single-chain nanoparticle (SCNP) system capable of catalyzing the photooxidation of nonpolar alkenes up to three times more efficiently than an equivalent small-molecule photosensitizer at an identical concentration. Specifically, we construct a polymer chain constituted of poly(ethylene glycol) methyl ether methacrylate and glycidyl methacrylate which we compact via multifunctional thiol-epoxide ligation and functionalize with Rose Bengal (RB) in a one pot reaction, affording SCNPs with a hydrophilic shell and hydrophobic photocatalytic regions. Photooxidation of the internal alkene in oleic acid proceeds under green light. RB confined within the SCNP is three times more effective for nonpolar alkenes than free RB in solution, which we hypothesize is due to the spatial proximity of the photosensitizing units to the substrate in the hydrophobic region. Our approach demonstrates that SCNP based catalysts can afford enhanced photocatalysis via confinement effects in a homogeneous reaction environment.

Wavelength-Resolved PhotoATRP.

The careful mapping of photoinduced reversible-deactivation radical polymerizations (RDRP) is a prerequisite for their applications in soft matter materials design. Here, we probe the wavelength-dependent behavior of photochemically induced atom transfer radical polymerization (ATRP) using nanosecond pulsed-laser polymerization (PLP). The photochemical reactivities at identical photon fluxes of methyl acrylate in terms of conversion, number-average molecular weight, and dispersity of the resulting polymers are mapped against the absorption spectrum of the copper(II) catalyst in the range of 305-550 nm. We observe a red shift of the action spectrum relative to the absorption spectrum of the copper(II) catalyst. Both the number-average molecular weight and the dispersity show a wavelength dependence, while the molecular weight and conversion remain linearly correlated. The reported data allow the judicious selection of optimum wavelengths for photoATRP.

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.

Computational prediction of the molecular configuration of three-dimensional network polymers.

The three-dimensional arrangement of natural and synthetic network materials determines their application range. Control over the real-time incorporation of each building block and functional group is desired to regulate the macroscopic properties of the material from the molecular level onwards. Here we report an approach combining kinetic Monte Carlo and molecular dynamics simulations that chemically and physically predicts the interactions between building blocks in time and in space for the entire formation process of three-dimensional networks. This framework takes into account variations in inter- and intramolecular chemical reactivity, diffusivity, segmental compositions, branch/network point locations and defects. From the kinetic and three-dimensional structural information gathered, we construct structure-property relationships based on molecular descriptors such as pore size or dangling chain distribution and differentiate ideal from non-ideal structural elements. We validate such relationships by synthesizing organosilica, epoxy-amine and Diels-Alder networks with tailored properties and functions, further demonstrating the broad applicability of the platform.

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.

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.

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, λactivation =410-490 nm) and styrylpyrido[2,3-b]pyrazine (SPP, λactivation =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 (λactivation =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.

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.