Spatiotemporal Photopatterning on Polycarbonate Surface through Visible Light Responsive Polymer Bound DASA Compounds

Robust Red-Absorbing Donor-Acceptor Stenhouse Adduct Photoswitches

Donor-Acceptor Stenhouse Adduct (DASA), a class of push-pull negative photochrome, has received large interest lately owing to its versatile synthesis, modularity and excellent photoswitching in solutions. From a technological perspective, it is imperative for this class of photoswitches to work robustly in solid state, e. g. thin films. We feature a molecular framework for the optimized design of DASAs by introducing a new thioindoline donor (D3) and assessing its performance against known 2nd generation indoline-based donors. The systematic structure-function investigations suggest that to achieve robust reversible photoswitching, a ground state with low charge separation is desired. DASAs with stronger electron donors and a larger charge separation in the ground state result in a low population of the photothermalstationary state (PTSS) and reduced photostability. The DASA with thioindoline donor (D3A3) seems to be a special case among the donor series as it causes a red shift (ca. 15 nm), however with less polarization of the ground state and marginally better photostability as compared to the unsubstituted 2-methyl indoline (D1A3). We also emphasize the consideration of the key additional factors that can modulate the red-light photoswitching properties of DASA chromophores in polymer thin films, which might not be dominant in homogenous solution state.

Enriched switching in a donor-acceptor Stenhouse adduct via reversible covalent bonding

We have utilized reversible covalent bonding to expand the accessible states of a molecular switch. Introducing a hydroxyl group onto the donor moiety of a donor-acceptor Stenhouse adduct (DASA) imparts an acidity response by forming an oxazolidine ring through intramolecular nucleophilic addition. Furthermore, we observed distinct color changes under cryogenic conditions, extending the thermal responsiveness beyond the cyclization equilibrium observed at elevated temperatures. These unique responses present promising prospects for diverse applications compared to traditional photoinduced binary isomerization.

Visible light-responsive materials: the (photo)chemistry and applications of donor-acceptor Stenhouse adducts in polymer science

Donor-acceptor Stenhouse adduct (DASA) photoswitches have gained a lot of attention since their discovery in 2014. Their negative photochromism, visible light absorbance, synthetic tunability, and the large property changes between their photoisomers make them attractive candidates over other commonly used photoswitches for use in materials with responsive or adaptive properties. The development of such materials and their translation into advanced technologies continues to widely impact forefront materials research, and DASAs have thus attracted considerable interest in the field of visible-light responsive molecular switches and dynamic materials. Despite this interest, there have been challenges in understanding their complex behavior in the context of both small molecule studies and materials. Moreover, incorporation of DASAs into polymers can be challenging due to their incompatibility with the conditions for most common polymerization techniques. In this review, therefore, we examine and critically discuss the recent developments and challenges in the field of DASA-containing polymers, aiming at providing a better understanding of the interplay between the properties of both constituents (matrix and photoswitch). The first part summarizes current understanding of DASA design and switching properties. The second section discusses strategies of incorporation of DASAs into polymers, properties of DASA-containing materials, and methods for studying switching of DASAs in materials. We also discuss emerging applications for DASA photoswitches in polymeric materials, ranging from light-responsive drug delivery systems, to photothermal actuators, sensors and photoswitchable surfaces. Last, we summarize the current challenges in the field and venture on the steps required to explore novel systems and expand both the functional properties and the application opportunities of DASA-containing polymers.

Meldrum's Acid Furfural Conjugate MAFC: A New Entry as Chromogenic Sensor for Specific Amine Identification

Bioactive amines are highly relevant for clinical and industrial application to ensure the metabolic status of a biological process. Apart from this, generally, amine identification is a key step in various bioorganic processes ranging from protein chemistry to biomaterial fabrication. However, many amines have a negative impact on the environment and the excess intake of amines can have tremendous adverse health effects. Thus, easy, fast, sensitive, and reliable sensing methods for amine identification are strongly searched for. In the past few years, Meldrum's acid furfural conjugate (MAFC) has been extensively explored as a starting material for the synthesis of photoswitchable donor-acceptor Stenhouse adducts (DASA). DASA formation hereby results from the rapid reaction of MAFC with primary and secondary amines, which has so far been demonstrated through numerous publications for different applications. The linear form of the MAFC-based DASA exhibits intense pink coloration due to its linear conjugated triene-2-ol conformation, which has inspired researchers to use this easy synthesizable molecule as an optical sensor for primary, secondary, and biogenic amines. Due to its new entry into amine identification, a collection of the literature exclusively on MAFC is demanded. In this mini review, we intend to present the state-of-the-art of MAFC as an optical molecular sensor in hopes to motivate researchers to find even more applications of MAFC-based sensors and methods that pave the way to their usage in medicinal applications.

Controlling Isomerization of Photoswitches to Modulate 2D Logic-in-Memory Devices by Organic-Inorganic Interfacial Strategy

Logic-in-memory devices are a promising and powerful approach to realize data processing and storage driven by electrical bias. Here, an innovative strategy is reported to achieve the multistage photomodulation of 2D logic-in-memory devices, which is realized by controlling the photoisomerization of donor-acceptor Stenhouse adducts (DASAs) on the surface of graphene. Alkyl chains with various carbon spacer lengths (n = 1, 5, 11, and 17) are introduced onto DASAs to optimize the organic-inorganic interfaces: 1) Prolonging the carbon spacers weakens the intermolecular aggregation and promotes isomerization in the solid state. 2) Too long alkyl chains induce crystallization on the surface and hinder the photoisomerization. Density functional theory calculation indicates that the photoisomerization of DASAs on the graphene surface is thermodynamically promoted by increasing the carbon spacer lengths. The 2D logic-in-memory devices are fabricated by assembling DASAs onto the surface. Green light irradiation increases the drain-source current (I_ds ) of the devices, while heat triggers a reversed transfer. The multistage photomodulation is achieved by well-controlling the irradiation time and intensity. The strategy based on the dynamic control of 2D electronics by light integrates molecular programmability into the next generation of nanoelectronics.

Revisiting photocyclization of the donor-acceptor stenhouse adduct: missing pieces in the mechanistic jigsaw discovered

Donor-acceptor Stenhouse adducts (DASA) have recently emerged as a class of visible-light-induced photochromic molecular switches, but their photocyclization mechanism remains puzzling and incomplete. In this work, we carried out MS-CASPT2//SA-CASSCF calculations to reveal the complete mechanism of the dominant channels and possible side reactions. We found that a new thermal-then-photo isomerization channel, i.e., EEZ → EZZ → EZE, other than the commonly accepted EEZ → EEE → EZE channel, is dominant in the initial step. Besides, our calculations rationalized why the expected byproducts ZEZ and ZEE are unobserved and proposed a competitive stepwise channel for the final ring-closure step. The findings here redraw the mechanistic picture of the DASA reaction by better accounting for experimental observations, and more importantly, provide critical physical insight in understanding the interplay between thermal- and photo-induced processes widely present in photochemical synthesis and reactions.

Donor-Acceptor Stenhouse Adduct Displaying Reversible Photoswitching in Water and Neuronal Activity

The interest in the photochromism and functional applications of donor-acceptor Stenhouse adducts (DASAs) soared in recent years owing to their outstanding advantages and flexible design. However, their low solubility and irreversible conversion in aqueous solutions hampered exploring DASAs for biology and medicine. It is notably unknown whether the barbiturate electron acceptor group retains the pharmacological activity of drugs such as phenobarbital, which targets γ-aminobutyric acid (GABA)-type A receptors (GABA_ARs) in the brain. Here, we have developed the model compound DASA-barbital based on a scaffold of red-switching second-generation DASAs, and we demonstrate that it is active in GABA_ARs and alters the neuronal firing rate in a physiological medium at neutral pH. DASA-barbital can also be reversibly photoswitched in acidic aqueous solutions using cyclodextrin, an approved ingredient of drug formulations. These findings clarify the path toward the biological applications of DASAs and to exploit the versatility displayed in polymers and materials science.

MatriGrid® Based Biological Morphologies: Tools for 3D Cell Culturing

Recent trends in 3D cell culturing has placed organotypic tissue models at another level. Now, not only is the microenvironment at the cynosure of this research, but rather, microscopic geometrical parameters are also decisive for mimicking a tissue model. Over the years, technologies such as micromachining, 3D printing, and hydrogels are making the foundation of this field. However, mimicking the topography of a particular tissue-relevant substrate can be achieved relatively simply with so-called template or morphology transfer techniques. Over the last 15 years, in one such research venture, we have been investigating a micro thermoforming technique as a facile tool for generating bioinspired topographies. We call them MatriGrid^®s. In this research account, we summarize our learning outcome from this technique in terms of the influence of 3D micro morphologies on different cell cultures that we have tested in our laboratory. An integral part of this research is the evolution of unavoidable aspects such as possible label-free sensing and fluidic automatization. The development in the research field is also documented in this account.

Controlling the Isomerization of Photoresponsive Molecules through a Limiting Tautomerization Strategy

Controlling the multistage photoresponsivity remains a challenge, in part, due to the spontaneous tautomerization between isomers. Herein, we present a strategy to access three independent states (linear, cyclic keto, and cyclic enolate) of crown ether (CE)-substituted donor-acceptor Stenhouse adducts (DASAs) by limiting the tautomerization of the closed isomers. The linear-cyclic keto isomerization is reversibly triggered by treatment with metal ions (Na⁺ or K⁺) and CE, while the linear-cyclic enolate isomerization is induced by green light and heat. Density functional theory and molecular dynamics calculation results suggest that the steric effect and supramolecular interaction between the electron-donating and electron-withdrawing moieties play an important role in hindering the tautomerization between cyclic keto and cyclic enolate DASA-CE. The strategy to influence key steps in the photoswitching process inspires well-controlled multistage isomerization of photoresponsive molecules.

Donor-Acceptor Stenhouse Adduct-Polydimethylsiloxane-Conjugates for Enhanced Photoswitching in Bulk Polymers

Donor-acceptor Stenhouse adducts (DASAs) are a rapidly emerging class of visible light-activated photochromes and DASA-functionalized polymers hold great promise as biocompatible photoresponsive materials. However, the photoswitching performance of DASAs in solid polymer matrices is often low, particularly in polymeric materials below their glass transition temperature. To overcome this limitation, DASAs are conjugated to polydimethylsiloxanes which have a glass transition temperature far below room temperature and which can create a mobile molecular environment around the DASAs for achieving more solution-like photoswitching kinetics in bulk polymers. The dispersion of DASAs conjugated to such flexible oligomers into solid polymer matrices allows for more effective and tunable DASA photoswitching in stiff polymers, such as poly(methyl methacrylate), without requiring modifications of the matrix. The photoswitching of conjugates with varying polymer molecular weight, linker type and architecture is characterized via time-dependent UV-Vis spectroscopy in organic solvents and blended into polymethacrylate films. In addition, DASA-functionalized polydimethylsiloxane networks that are accessible by the same synthetic route provide an alternative solution for achieving fast and efficient DASA photoswitching in the bulk owing to their intrinsic softness and flexibility. These findings may contribute to the development of DASA-functionalized materials with better tunable, more effective, and more reversible modulation of their optical properties. This article is protected by copyright. All rights reserved.

Visible Light-Driven Molecular Switches and Motors: Recent Developments and Applications

Inspired by human vision, a diverse range of light-driven molecular switches and motors has been developed for fundamental understanding and application in material science and biology. Recently, the design and synthesis of visible light-driven molecular switches and motors have been actively pursued. This emerging trend is partly motivated to avoid the harmful effects of ultraviolet light, which was necessary to drive the classical molecular switches and motors at least in one direction, impeding their employment in biomedical and photopharmacology applications. Moreover, visible light-driven molecular switches and motors are demonstrated to enable benign optical materials for advanced photonic devices. Therefore, during the past several years, visible light-driven molecular switches based on azobenzene derivatives, diarylethenes, 1,2-dicyanodithienylethenes, hemithioindigo derivatives, iminothioindoxyls, donor-acceptor Stenhouse adducts, and overcrowded alkene based molecular motors have been judiciously designed, synthesized, and used in the development of functional materials and systems for a wide range of applications. In this Review, we present the recent developments toward the design of visible light-driven molecular switches and motors, with their applications in the fabrication of functional materials and systems in material science, bioscience, pharmacology, etc . The visible light-driven molecular switches and motors realized so far undoubtedly widen the scope of these interesting compounds for technological and biological applications. We hope this Review article could provide additional impetus and inspire further research interests for future exploration of visible light-driven advanced materials, systems, and devices.

pH-Dependent Selective Colorimetric Detection of Proline and Hydroxyproline with Meldrum’s Acid-Furfural Conjugate

Activated 2-furfural gives intense color formation when reacted with amines, due to a ring opening reaction cascade that furnishes a conjugated molecular system. Unique colorimetric characteristic of this reaction makes it an interesting candidate for developing chemosensors operating in visible range. Among many activated 2-furfural derivatives, Meldrum’s acid furfural conjugate (MAFC) recently gained significant interest as colorimetric chemosensor. MAFC has been explored as selective chemosensor for detecting amines in solution, secondary amines on polymer surfaces and even nitrogen rich amino acids (AA) in aqueous solution. In this work, the pH dependency of MAFC-AA reaction is explored. It was found that proline gives an exceptionally fast colored reaction at pH 11, whereas at other pHs, no naked eye color product formation was observed. The reaction sequence including ring opening reaction upon nucleophilic addition of cyclic amine of proline resulting in a conjugated triene was confirmed by NMR titrations. The highly pH dependent reaction can e.g., potentially be used to detect proline presence in biological samples. An even more intense color formation takes place in the reaction of natural proline derivative 4-hydroxyproline. The detection limit of proline and 4-hydroxyproline with MAFC solution was found to be 11 µM and 6 µM respectively.

Molecular photoswitches in aqueous environments

Molecular photoswitches enable dynamic control of processes with high spatiotemporal precision, using light as external stimulus, and hence are ideal tools for different research areas spanning from chemical biology to smart materials. Photoswitches are typically organic molecules that feature extended aromatic systems to make them responsive to (visible) light. However, this renders them inherently lipophilic, while water-solubility is of crucial importance to apply photoswitchable organic molecules in biological systems, like in the rapidly emerging field of photopharmacology. Several strategies for solubilizing organic molecules in water are known, but there are not yet clear rules for applying them to photoswitchable molecules. Importantly, rendering photoswitches water-soluble has a serious impact on both their photophysical and biological properties, which must be taken into consideration when designing new systems. Altogether, these aspects pose considerable challenges for successfully applying molecular photoswitches in aqueous systems, and in particular in biologically relevant media. In this review, we focus on fully water-soluble photoswitches, such as those used in biological environments, in both in vitro and in vivo studies. We discuss the design principles and prospects for water-soluble photoswitches to inspire and enable their future applications.

β-Amyloid Peptides Manipulate Switching Behaviors of Donor-Acceptor Stenhouse Adducts

Molecular switching plays a critical role in biological and displaying systems. Donor-acceptor Stenhouse adducts (DASAs) is a newly re-discovered series of switchable photochromes, and light is the most used approach to control its switching behavior. In this report, we speculated that hydrophobic binding pockets of biologically relevant peptides/proteins could be harnessed to alter its switching behavior without the assistance of light. We designed and synthesized a DASA compound SHA-2, and we demonstrated that the Aβ40 species could stabilize SHA-2 in the linear conformation and decrease the rate of molecular switching via fluorescence spectral studies. Moreover, molecular dynamics simulation revealed that SHA-2 could bind to the hydrophobic fragment of the peptide and resulted in substantial changes in the tertiary structure of Aβ40 monomer. This structural change is likely to impede the aggregation of Aβ40, as evidenced by the results from thioflavin T fluorescence and ProteoStat aggregation detection experiments. We believe that our study opens a new window to alter the switching behavior of DASA via DASA-peptide/protein interactions.

Electrochemical Switching of First-Generation Donor-Acceptor Stenhouse Adducts (DASAs): An Alternative Stimulus for Triene Cyclisation

Donor-acceptor Stenhouse adducts (DASAs) are a photo-switch class that undergoes triene cyclisation in response to visible light. Herein, electrochemical oxidation is demonstrated as an effective alternative stimulus for the triene cyclisation commonly associated with photo-switching.

Encapsulation and Stabilization of a Donor-Acceptor Stenhouse Adduct Isomer in Water Inside the Blue Box: A Combined Experimental and Theoretical Approach

We synthesized two types of donor-acceptor Stenhouse adducts (DASAs), a new type of photochromic molecules showing dual color in two different isomeric forms in solution phase, using Meldrum acid (DASA-Mel) and barbituric acid (DASA-Bar), along with a naphthalimide derivative to obtain interesting fluorescence properties. DASA-Mel was found to have fast photochromic conversion in comparison to DASA-Bar, evident from ultraviolet-visible (UV-vis) and fluorescence spectroscopic studies. The colored form of DASA-Mel was encapsulated inside the water-soluble Stoddart's blue box and became soluble in water much faster than DASA-Bar. Interestingly, the competitive encapsulation experiment showed that DASA-Mel was selectively encapsulated inside the blue box in water whereas DASA-Bar was mostly separated out from the solution after centrifugation, and this phenomenon was confirmed by ¹H and DOSY NMR and mass spectroscopies. Moreover, we found through density functional theory (DFT) optimization that the open form of DASA-Mel was more stable during the encapsulation reaction in a water medium in comparison to DASA-Bar. The calculated binding energies of encapsulated DASA-Mel and DASA-Bar are -10.2 and -9.9 kcal/mol, respectively, clearly showing that the former is more stable by 0.3 kcal. Consequently, the organic macrocycle selectively separating one kind of DASA from a mixture by encapsulation in water is reported for the first time with experimental and theoretical support in the literature.

Designing of Rewritable Paper by Hydrochromic Donor-Acceptor Stenhouse Adducts

Rewritable paper is meaningful to the recyclable and sustainable utilization of environmental resources and thus has been extensively investigated for several decades. In this work, we demonstrated an efficient and convenient strategy to fabricate rewritable paper based on reversible hydrochromism of donor-acceptor Stenhouse adducts (DASAs). The kinetics and efficiency of isomerization could be well-controlled by adjusting the surrounding temperature and humidity. Monocolored rewritable paper was prepared by coating cyclic DASA·xH₂O on the paper surface. Writing, printing, stamping and patterning were realized on the rewritable paper. The information could be controllably erased by treatment in a humid atmosphere. More importantly, the rewritable paper was upgraded to multicolored by combination of two DASA materials. The color of chirography was switched by controlling the writing speed.

Construction and Properties of Donor-Acceptor Stenhouse Adducts on Gold Surfaces

The construction of a donor-acceptor Stenhouse adduct molecular layer on a gold surface is presented. To avoid the incompatibility of the thiol surface-binding group with the donor-acceptor polyene structure of the switch, an interfacial reaction approach was followed. Poly(dopamine)-supported gold nanoparticles on quartz slides were chosen as substrates, which was expected to facilitate both the interfacial reaction and the switching process by providing favorable steric conditions due to the curved particle surface. The reaction between the surface-bound donor half and the CF3-isoxazolone-based acceptor half was proved to be successful by X-ray photoelectron spectroscopy (XPS). However, UV-vis measurements suggested that a closed, cyclopentenone-containing structure of the switch formed on the surface irreversibly. Analysis of the wetting behavior of the surface revealed spontaneous water spreading that could be associated with conformational changes of the closed isomer.

A simple strategy to overcome concentration dependence of photoswitching properties in donor-acceptor Stenhouse adducts

Photoswitchable donor-acceptor Stenhouse adducts (DASAs) have been reported to exhibit an undesirable concentration dependence, where photoswitching is greatly inhibited with increasing photochrome concentration. Here we show that the use of piperazine-based donor moieties eliminates this concentration dependence and results in complete, rapid and reversible photoswitching behaviour for first generation DASAs, even in chlorinated solvents. Structural data and computational studies reveal proton transfer during isomerisation to the terminal amine rather than the donor amine. The improvement in photoswitching efficiency is attributed to resultant differences in supramolecular association.

Photophysical Properties of Donor-Acceptor Stenhouse Adducts and Their Inclusion Complexes with Cyclodextrins and Cucurbit[7]uril

Donor-acceptor Stenhouse adducts (DASAs) are a novel class of solvatochromic photoswitches with increasing importance in photochemistry. Known for their reversibility between open triene and closed cyclized states, these push-pull molecules are applicable in a suite of light-controlled applications. Recent works have sought to understand the DASA photoswitching mechanism and reactive state, as DASAs are vulnerable to irreversible "dark switching" in polar protic solvents. Despite the utility of fluorescence spectroscopy for providing information regarding the electronic structure of organic compounds and gaining mechanistic insight, there have been few studies of DASA fluorescence. Herein, we characterize various photophysical properties of two common DASAs based on Meldrum's acid and dimethylbarbituric acid by fluorescence spectroscopy. This approach is applied in tandem with complexation by cyclodextrins and cucurbiturils to reveal the zwitterionic charge separation of these photoswitches in aqueous solution and the protective nature of supramolecular complexation against degradative dark switching. DASA-M, for example, was found to form a weak host-guest inclusion complex with (2-hydroxypropyl)-γ-cyclodextrin, with a binding constant K = 60 M-1, but a very strong inclusion complex with cucurbit[7]uril, with K = 27,000 M-1. This complexation within the host cavity was found to increase the half-life of both DASAs in aqueous solution, indicating the significant and potentially useful stabilization of these DASAs by host encapsulation.

An Ab Initio Computational Study of Electronic and Structural Factors in the Isomerization of Donor-Acceptor Stenhouse Adducts

In this work, the photochemically and thermally induced isomerization of multiple donor-acceptor Stenhouse adducts (DASAs) of the first, second, and third generation is studied by means of state-of-the-art ab initio electronic structure methods leading to new insight into multiple facets of the reaction mechanism. Importantly, prior to any studies of the reaction mechanism, a set of test calculations demonstrate the suitability of the applied ADC(2) and CC2 methods in the present context. An important aspect in this regard is the availability of electronic energies and gradients under implicit consideration of solvent effects. On the basis of calculated reaction energies and barriers as well as a thorough analysis of the wave function compositions, interesting features of the reaction mechanism are deduced. For example, the closed form of second- and third-generation DASAs can be significantly stabilized by π - π interactions between the donor and acceptor termini when certain structural requirements are fulfilled. The central point of this work concerns the delicate balance between neutral and zwitterionic resonance structures that governs the relative barrier height for the crucial C₂-C₃ and C₃-C₄ bond rotations. Finally, a set of calculations on yet unreported derivatives highlights how this balance and hence the barrier heights can be tuned through variation of the donor-acceptor strength as well as the solvent polarity.

Visible Light-Responsive Drug Delivery Nanoparticle via Donor-Acceptor Stenhouse Adducts (DASA)

Stimuli-responsive drug release from a nanocarrier triggered by light enables the control of the amount of drug locally. Here, block copolymer micelles based on poly(ethylene glycol) methyl ether methacrylate (PEGMEMA) as the hydrophilic block and a polymer with pendant donor-acceptor Stenhouse adducts (DASA) are used as a means to trigger the release of drugs under green light. The micelles are loaded with ellipticine to yield light-responsive nanoparticles with sizes of around 35 nm according to transmission electron microscopy (TEM) analysis. Two micelles with a drug loading content of 4.75 and 7.4 wt% are prepared, but the micelle with the higher drug loading content leads to substantial protein adsorption. The release of ellipticine from the micelle, which is monitored using the polarity-sensitive fluorescence of ellipticine, can be switched on by light and off by thermal recovery of DASA in the dark. The micelles are readily taken up by Michigan Cancer Foundation-7 breast cancer cells. Subsequent light irradiation leads to enhanced drug release inside the cell as seen by the enhanced fluorescence.

Invisible Inks for Secrecy and Anticounterfeiting: From Single to Double-encryption by Hydrochromic Molecules

Secret information recorded by traditional single-encrypted invisible inks is easily cracked because the inks can switch only between "NONE" and "TRUTH". Developing double-encrypted systems makes the information reversibly switchable between "FALSE" and "TRUTH", which is helpful to ensure the safety of the secret information during transport. Here, we prepared heat-developed invisible inks by hydrochromic molecules donor-acceptor Stenhouse adducts (DASAs) and oxazolidines (OXs) and promoted the invisible inks from single to double encryption. DASAs coordinate with water molecules and form stable colorless cyclic DASA·xH₂O molecules, which lose coordinated water molecules after heating and switch to colored linear DASAs. In contrast, OXs are colored with water and are colorless after heating. Single-encrypted secrecy was realized by DASA invisible inks. The information is invisible under the encrypted state and becomes bright purple after heating. Vapor treating re-encrypted the information in ∼5 min. Furthermore, the single-encryption was promoted to double-encryption by a DASA/OX invisible inks system. Heating and vapor treating switch the information between the "FALSE" and "TRUTH" reversibly. The DASA/OX invisible ink system is applied for secrecy of texts, graphic images, and quick response (QR) codes.

Chemistry of photoswitching molecules in the confined nanospace of aqueous molecular vessels

This Frontier article highlights the photoswitching behaviour of azobenzenes, spiropyrans and DASA molecules inside the nano cavity of self-assembled coordination molecular vessels in aqueous medium and in the solid state.

Inducing molecular isomerization assisted by water

Light is not the only stimulus that can induce linear-to-cyclic isomerization of donor-acceptor Stenhouse adducts (DASAs). Here we demonstrate the water-induced linear-to-cyclic isomerization of DASAs. The mechanism of the water-induced linear-to-cyclic isomerization of DASAs is investigated by density functional theory (DFT) calculations. Water molecules coordinate with DASAs and stabilize the intermediates and cyclic isomers, which favors cyclization thermodynamically. Moreover, the linear-to-cyclic isomerization is reversible. Heating removes the coordinated H2O molecules, which further triggers cyclic-to-linear isomerization. DASAs have been applied in information hiding/displaying and color switching under water vapor and heating control.

Photoactive Polyoxometalate/DASA Covalent Hybrids for Photopolymerization in the Visible Range

The synthesis of TBA-DASA-POM-DASA, the first photoactive covalent hybrid polyoxometalate (POM) incorporating a donor-acceptor Stenhouse adduct (DASA) reverse photochrome, is presented. It has been evidenced that in solution the equilibrium between the colorless cyclopentenone and the highly colored triene conformers is strongly dependent not only on the nature of the solvent but also the countercations, allowing to tune its optical properties. This complex has been further associated to photochromic spironaphtoxazine cations, resulting in a material which can be activated by two distinct optical stimuli. Moreover, when combined with N-methyldiethanolamine, TBA-DASA-POM-DASA constitutes a performing photoinitiating system for polyethylene glycol diacrylate polymerization and under visible light irradiation, a promising result in a domain scarcely developed in POM chemistry.

Multiresponsive Donor-Acceptor Stenhouse Adduct: Opportunities Arise from a Diamine Donor

A new donor-acceptor Stenhouse adduct based on a N,N,N'-trimethylethylenediamine donor has been reported. An unprecedented isomer has been isolated, and rich conversions between three isomers have been achieved upon visible-light irradiation or base/acid stimuli. The drastic color change associated with structural conversion has been utilized to selectively sense volatile primary amines as well as high-charged hard Lewis acids (Sc³⁺, Ti⁴⁺, Cr³⁺, and Al³⁺).

Comparing photoswitching of acrylate or methacrylate polymers conjugated with donor–acceptor Stenhouse adducts

Polymers with donor–acceptor Stenhouse adduct (DASA) groups were synthesized using RAFT methods to evaluate the effect of polymer length (20 vs. 100 DP units) and backbone rigidity (acrylate and methacrylate blocks).

Surface with Reversible Green-Light-Switched Wettability by Donor-Acceptor Stenhouse Adducts

In this report, we designed surfaces with reversible green-light-switched wettability via donor-acceptor Stenhouse adducts (DASAs). Photoresponsive micro/nanoparticles were prepared by coating the surfaces of silica micro/nanoparticles with polydopamine and then postmodifying with DASA molecules. Then, the particles were immobilized on a glass substrate surfaces either with double-sided adhesive tape or cross-linking poly(dimethylsiloxane). Silica micro/nanoparticles with various diameters (0.2, 2.5, and 85 μm) were used to fabricate the photoresponsive surface. Green light irradiation switches the hydrophobic linear DASA to a hydrophilic cyclic isomer, which further increases the wettability and contact angle hysteresis on the surface. On the other hand, heating (100 °C) induces the cyclic-to-linear isomerization of DASA molecules and switches the surface back to hydrophobic. The wettability of the DASA-modified surface is reversible under alternate green light irradiation and heating.

Structure-function relationships of donor-acceptor Stenhouse adduct photochromic switches

The first in-depth, systematic study of the photoswitching properties of Donor-Acceptor Stenhouse Adducts (DASAs) is reported. Barbituric acid derived DASAs functionalised with 14 different amines ranging from dimethylamine to 4-methoxy-N-methylaniline were structurally characterised in solution using 1H and 13C NMR spectroscopy and, in eight cases, in the solid state by single crystal X-ray diffraction. The distribution of coloured and colourless isomers in the dark, their photostationary states under irradiation, apparent thermal half-lives, and fatigue resistance are systematically compared. A simple kinetic model is used to characterise photoswitching behaviour and reveals that minor structural modifications can significantly improve the photoswitching properties of DASA photochromes. These modifications result in excellent photoswitching properties for '1st generation' DASAs in chloroform, including exceptional fatigue resistance, opening the door for these photochromic molecules to find widespread applications.

Second-order nonlinear optical properties of Stenhouse photoswitches: insights from density functional theory

We report the first investigation of the second-order nonlinear optical (NLO) properties of donor-acceptor Stenhouse adducts (DASAs), an emerging class of colored photochromes that undergo photoswitching with visible light to a colorless form. By using time-dependent density functional theory, we provide insights into the relationships linking the nature of the chemical substituents to the amplitude and contrasts of the NLO response. Solvent and frequency dispersion effects are also analyzed. The calculations predict that DASAs behave as high contrast NLO switches, a finding that extends their potential applications to photo-responsive NLO materials and devices.

3D Microcontact Printing for Combined Chemical and Topographical Patterning on Porous Cell Culture Membrane

Micrometer-scale biochemical or topographical patterning is commonly used to guide the cell attachment and growth, but the ability to combine these patterns into an integrated surface with defined chemical and geometrical characteristics still remains a technical challenge. Here, we present a technical solution for simultaneous construction of 3D morphologies, in the form of channels, on porous membranes along with precise transfer of extracellular matrix proteins into the channels to create patterns with geometrically restricting features. By combining the advantages of microthermoforming and microcontact printing, this technique offers a unique patterning process that provides spatiotemporal control over morphological and chemical feature in a single step. By use of our 3D-microcontact printing (3DμCP), determined microstructures like channels with different depths and widths even with more complex patterns can be fabricated. Collagen, fibronectin, and laminin were successfully transferred inside the predesigned geometries, and the validity of the process was confirmed by antibody staining. Cells cultivated on 3DμCP patterned polycarbonate membrane have shown selective adhesion and growth. This technique offers a novel tool for creating freeform combinatorial patterning on the thermoformable surface.

Donor-acceptor Stenhouse adduct-grafted polycarbonate surfaces: selectivity of the reaction for secondary amine on surface

Donor-acceptor Stenhouse adducts (DASAs) are gaining attention from organic and material chemists due to their visible light-stimulated photochromic properties. In this report, we present a facile method for grafting coloured triene on polycarbonate surface, without involving any pre-treatments like plasma activation, etc. The chemoselectivity of carbonate with a primary amine and Meldrum's activated furan (MAF) with polymer bound secondary amine has been exploited to graft photoswitchable DASA on the polymer surface. Primary, secondary and tertiary amine-functionalized polycarbonate surfaces have been prepared to evaluate the reactivity of amine with MAF.

Visible Light-Controlled Inversion of Pickering Emulsions Stabilized by Functional Silica Microspheres

A new class of donor-acceptor Stenhouse adduct (DASA)-functionalized silica microspheres (SMs) is designed and described to formulate Pickering emulsions with inversion property and large polarity change upon visible light irradiation. By tuning the hydrophilicity of the functional SM particles with visible light, these Pickering emulsions can easily perform inversion from water-in-oil to oil-in-water. The inversion performance of the emulsions is ascribed to DASA photoisomerization from an extended, hydrophobic, and intensely purple-colored triene to a compact, zwitterionic, and colorless cyclopentenone upon irradiation with visible light. This unique inversion behavior has been applied to control encapsulation and the release of fluorescein sodium salt.

The (photo)chemistry of Stenhouse photoswitches: guiding principles and system design

Molecular photoswitches comprise chromophores that can be interconverted reversibly with light between two states with different photochemical and physicochemical properties. This feature renders them useful for diverse applications, ranging from materials science, biology (specifically photopharmacology) to supramolecular chemistry. With new and more challenging systems to control, especially extending towards biomedical applications, using visible or near-infrared light for photoswitch activation becomes vital. Donor-acceptor Stenhouse adducts are a novel class of visible light-responsive negative photochromes that provide a possible answer to current limitations of other photoswitch classes in the visible and NIR window. Their rapid development since their discovery in 2014, together with first successful examples of applications, demonstrate both their potential and areas where improvements are needed. A better understanding of DASA characteristics and its photoswitching mechanism has revealed that they are in fact a subset of a more general structural class of photochromes, namely Stenhouse photoswitches. This tutorial review aims at providing an introduction and practical guide on DASAs: it focuses on their structure and synthesis, provides fundamental insights for understanding their photoswitching behavior and demonstrates guiding principles for tailoring these switches for given applications.

Photoswitching an Isolated Donor-Acceptor Stenhouse Adduct

Donor-acceptor Stenhouse adducts (DASAs) are a new class of photoswitching molecules with excellent fatigue resistance and synthetic tunability. Here, tandem ion mobility mass spectrometry coupled with laser excitation is used to characterize the photocyclization reaction of isolated, charge-tagged DASA molecules over the 450-580 nm range. The experimental maximum response at 530 nm agrees with multireference perturbation theory calculations for the S₁ ← S₀ transition maximum at 533 nm. Photocyclization in the gas phase involves absorption of at least two photons; the first photon induces Z-E isomerization from the linear isomer to metastable intermediate isomers, while the second photon drives another E-Z isomerization and 4π-electrocyclization reaction. Cyclization is thermally reversible in the gas phase with collisional excitation.

Shedding Light on the Photoisomerization Pathway of Donor-Acceptor Stenhouse Adducts

Donor-acceptor Stenhouse adducts (DASAs) are negative photochromes that hold great promise for a variety of applications. Key to optimizing their switching properties is a detailed understanding of the photoswitching mechanism, which, as yet, is absent. Here we characterize the actinic step of DASA-photoswitching and its key intermediate, which was studied using a combination of ultrafast visible and IR pump-probe spectroscopies and TD-DFT calculations. Comparison of the time-resolved IR spectra with DFT computations allowed to unambiguously identify the structure of the intermediate, confirming that light absorption induces a sequential reaction path in which a Z-E photoisomerization of C2-C3 is followed by a rotation around C3-C4 and a subsequent thermal cyclization step. First and second-generation DASAs share a common photoisomerization mechanism in chlorinated solvents with notable differences in kinetics and lifetimes of the excited states. The photogenerated intermediate of the second-generation DASA was photo-accumulated at low temperature and probed with time-resolved spectroscopy, demonstrating the photoreversibility of the isomerization process. Taken together, these results provide a detailed picture of the DASA isomerization pathway on a molecular level.

Level of Theory and Solvent Effects on DASA Absorption Properties Prediction: Comparing TD-DFT, CASPT2 and NEVPT2

Donor-acceptor Stenhouse adducts (DASAs) are a very recent class of organic photoswitches that combine excellent properties, such as color and polarity change, a large structural modification, and excellent fatigue resistance. Despite their potential applications in different fields, very few studies have focused on rationalizing their electronic structure properties. Here, by means of different state-of-the-art theoretical methods, including solvent and vibrational effects, we show that while time dependent-density functional theory (TD-DFT) can qualitatively describe DASAs' excited states, multiconfigurational quantum chemistry methods along with dynamic electron correlation (CASPT2, NEVPT2) are required for a quantitative agreement with the experiment. This finding is reasoned based on the different charge transfer characteristics observed. Moreover, the TD-DFT computed two-photon absorption properties are reported and suggested to red-shift the absorption band, as required for biological applications.

Visible Light-Responsive DASA-Polymer Conjugates

A modular synthesis of Donor-Acceptor Stenhouse Adduct (DASA) polymer conjugates is described. Pentafluorophenyl-ester chemistry is employed to incorporate aromatic amines into acrylate and methacrylate copolymers, which are subsequently coupled with activated furans to generate polymers bearing a range of DASA units in a modular manner. The effect of polymer glass transition temperature on switching kinetics is studied, showing dramatic rate enhancements in going from a glassy to a rubbery matrix. Moreover, tuning the DASA absorption profile allows for selective switching, as demonstrated by ternary photopatterning, with potential applications in rewriteable data storage.