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

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.

Dynamic effects on the nonlinear optical properties of donor acceptor stenhouse adducts: insights from combined MD + QM simulations

The second-order nonlinear optical (NLO) responses of a donor-acceptor stenhouse adduct (DASA) are investigated by using a computational approach combining molecular dynamics simulations and density functional theory (DFT) calculations. Specific force fields for the open and closed photoswitching forms are first parameterized and validated according to the Joyce protocol, in order to finely reproduce the geometrical features and potential energy surfaces of both isomers in chloroform solution. Then, DFT calculations are performed on structural snapshots extracted at regular time steps of the MD trajectories to address the influence of the thermalized conformational dynamics on the NLO responses related to hyper-Rayleigh scattering (HRS) experiments. We show that accounting for the structural dynamics largely enhances the HRS hyperpolarizability (βHRS) compared to DFT calculations considering solely equilibrium geometries, and greatly improves the agreement with experimental measurements. Furthermore, we show that the NLO responses of the NLO-active open form are correlated with the bond order alternation along the triene bridge connecting the donor and acceptor moieties, which is rationalized using simple essential state models.

Characteristics and long-term kinetics of an azobenzene derivative and a donor-acceptor Stenhouse adduct as orthogonal photoswitches

Light-triggered molecular switches are extensively researched for their applications in medicine, chemistry and material science and, if combined, particularly for their use in multifunctional smart materials, for which orthogonally, i.e. individually, addressable photoswitches are needed. In such a multifunctional mixture, the switching properties, efficiencies and the overall performance may be impaired by undesired mutual dependences of the photoswitches on each other. Within this study, we compare the performance of the pure photoswitches, namely an azobenzene derivative (Azo) and a donor-acceptor Stenhouse adduct (DASA), with the switching properties of their mixture using time-resolved temperature-dependent UV/VIS absorption spectroscopy, time-resolved IR absorption spectroscopy at room temperature and quantum mechanical calculations to determine effective cross sections, switching kinetics as well as activation energies of thermally induced steps. We find slightly improved effective cross sections, percentages of switched molecules and no increased activation barriers of the equimolar mixture compared to the single compounds. Thus, the studied mixture Azo + DASA is very promising for future applications in multifunctional smart materials.

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.

Tethered together: DASA design towards aqueous compatibility

Donor-acceptor Stenhouse adducts (DASAs) are an exciting class of photoswitches due to their facile tunability, visible light absorbance, and negative photochromism. While they have shown use in a variety of applications, to date all reported DASA derivatives have low equilibrium and/or poor photoswitching in polar protic solvents, which is vital for moving towards applications in biological systems. We demonstrate a strategy to introduce a substitution on the DASA triene that results in derivatives that are stable and have high dark equilibrium of the open form in polar protic solvents. Decreasing the charge separation of these new derivatives also allows for reversible switching in polar and protic solvents including THF : water mixtures.

Mechanistic Basis for Red Light Switching of Azonium Ions

Azonium ions formed by the protonation of tetra-ortho-methoxy-substituted aminoazobenzenes photoisomerize with red light under physiological conditions. This property makes them attractive as molecular tools for the photocontrol of physiological processes, for example, in photopharmacology. However, a mechanistic understanding of the photoisomerization process and subsequent thermal relaxation is necessary for the rational application of these compounds as well as for guiding the design of derivatives with improved properties. Using a combination of sub-ps/ns transient absorption measurements and quantum chemical calculations, we show that the absorption of a photon by the protonated E-H+ form of the photoswitch causes rapid (ps) isomerization to the protonated Z-H+ form, which can also absorb red light. Proton transfer to solvent then occurs on a microsecond time scale, leading to an equilibrium between Z and Z-H+ species, the position of which depends on the solution pH. Whereas thermal isomerization of the neutral Z form to the neutral E form is slow (∼0.001 s-1), thermal isomerization of Z-H+ to E-H+ is rapid (∼100 s-1), so the solution pH also governs the rate at which E/E-H+ concentrations are restored after a light pulse. This analysis provides the first complete mechanistic picture that explains the observed intricate photoswitching behavior of azonium ions at a range of pH values. It further suggests features of azonium ions that could be targeted for improvement to enhance the applicability of these compounds for the photocontrol of biomolecules.

Nonlinear Optical Responses of Photoswitchable Donor-Acceptor Stenhouse Adducts

This work combines hyper-Rayleigh scattering (HRS) experiments performed in the NIR range (1.30 and 1.60 μm) and quantum chemical calculations to provide a comprehensive description of the second harmonic generation (SHG) responses of donor-acceptor Stenhouse adducts (DASAs). Representative derivatives of the three generations of DASAs, which differ by the nature of their electron-donating and withdrawing moieties and also include clickable species, have been synthesized and their photoswitching behavior fully characterized. The HRS measurements allow us to establish relationships between the magnitude of the SHG response of open forms and the nature of the donor and acceptor groups. The largest SHG responses are obtained for derivatives incorporating either a barbituric acid or an indanedione acceptor unit, while N-methylaniline appears as the most efficient donor group. The calculations support well the experimental data and show that high hyperpolarizabilities are associated to low excitation energies and large extent of the photoinduced intramolecular charge transfer, which enhances the dipole moment variation between the ground and first dipole-allowed electronic excited state. In addition, a complete investigation of the photoswitching kinetics of DASAs in chloroform solution shows important differences, highlighting in particular the role of the donor group on the photoswitching efficiency.

Enhanced Sampling Aided Design of Molecular Photoswitches

Advances in the evolving field of atomistic simulations promise important insights for the design and fundamental understanding of novel molecular photoswitches. Here, we use state-of-the-art enhanced simulation techniques to unravel the complex, multistep chemistry of donor-acceptor Stenhouse adducts (DASAs). Our reaction discovery workflow consists of enhanced sampling for efficient chemical space exploration, refinement of newly observed pathways with more accurate ab initio electronic structure calculations, and structural modifications to introduce design principles within future generations of DASAs. We showcase our discovery workflow by not only recovering the full photoswitching mechanism of DASA but also predicting a plethora of new plausible thermal pathways and suggesting a way for their experimental validation. Furthermore, we illustrate the tunability of these newly discovered reactions, leading to a potential avenue for controlling DASA dynamics through multiple external stimuli. Overall, these insights could offer alternative routes to increase the efficiency and control of DASA's photoswitching mechanism, providing new elements to design more complex light-responsive materials.

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.

Acidochromism of donor-acceptor Stenhouse adducts in organic solvent

First generation DASA derivatives can be reversibly isomerized from the coloured, open form to the colourless, closed isomer upon protonation, thus behaving as acidochromic compounds in halogenated organic solvent.

Stepping Out of the Blue: From Visible to Near-IR Triggered Photoswitches

Light offers unique opportunities for controlling the activity of materials and biosystems with high spatiotemporal resolution. Molecular photoswitches are chromophores that undergo reversible isomerization between different states upon irradiation with light, allowing a convenient means to control their influence over the system of interest. However, a significant limitation of classical photoswitches is the requirement to initiate the switching in one or both directions using deleterious UV light with poor tissue penetration. Red-shifted photoswitches are hence in high demand and have attracted keen recent research interest. In this Review, we highlight recent progress towards the development of visible- and NIR-activated photoswitches characterized by distinct photochromic reaction mechanisms. We hope to inspire further endeavors in this field, allowing the full potential of these tools in biotechnology and materials chemistry applications to be realized.

Tandem Thio-Michael Addition/Remote Lactone Activation of 5-Hydroxymethylfurfural-Derived δ-Lactone-Fused Cyclopentenones

The creation of structurally diverse chemical entities from fairly simple biorefinery products remains a challenge. In this work 5-hydroxymethylfurfural (HMF) was identified as a key synthon for preparing highly complex cyclopentenones (CP) via tandem 1,4-addition/elimination/remote lactone activation to external O- and N-nucleophiles in δ-lactone-fused-CPs hotspots. This scaffold was also reactive enough to be incorporated into model cysteine-peptides in low concentrations, paving the way to a potential translation generating complexity in the synthesis of small peptides. The new enones also exhibited activity against intraerythrocytic Plasmodium falciparum (IC50 =1.32 μm).

Mechanism of the Aza-Piancatelli Reaction: Scope and Limitations of Furan Substitution in Donor-Acceptor Stenhouse Adduct Synthesis

The aza-Piancatelli reaction has been widely used to synthesize donor-acceptor Stenhouse adducts (DASAs), a new class of molecular photoswitches with unique properties. However, the substitution pattern of furan cores has been limited to position 3, as 3,4-disubstituted furans remain unreactive. Herein, we explore the aza-Piancatelli reaction mechanism using density functional theory (DFT) calculations to understand the influence of the different substituents on the reactivity. We found that all the reaction pathways are kinetically accessible, but the driving force of the reaction is lost in disubstituted furans due to the loss of conjugation in the DASA products. Finally, a simple model is proposed to guide the design of synthetic routes using this reaction.

A multi-stage single photochrome system for controlled photoswitching responses

The ability of molecular photoswitches to convert on/off responses into large macroscale property change is fundamental to light-responsive materials. However, moving beyond simple binary responses necessitates the introduction of new elements that control the chemistry of the photoswitching process at the molecular scale. To achieve this goal, we designed, synthesized and developed a single photochrome, based on a modified donor-acceptor Stenhouse adduct (DASA), capable of independently addressing multiple molecular states. The multi-stage photoswitch enables complex switching phenomena. To demonstrate this, we show spatial control of the transformation of a three-stage photoswitch by tuning the population of intermediates along the multi-step reaction pathway of the DASAs without interfering with either the first or final stage. This allows for a photonic three-stage logic gate where the secondary wavelength solely negates the input of the primary wavelength. These results provide a new strategy to move beyond traditional on/off binary photochromic systems and enable the design of future molecular logic systems.

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.

Improving the kinetics and dark equilibrium of donor-acceptor Stenhouse adduct by triene backbone design

DFT calculations were used to find an optimal substitution site on the triene backbone of a donor-acceptor Stenhouse adduct photoswitch to tune the equillibrium and switching kinetics of DASA without modifying the donor and acceptor groups. Using this approach we demonstrate a new means to tuning DASA based photoswitches by increasing the energy of the closed form relative to the open form. To highlight the potential of this approach a new DASA derivative bearing a methyl substituent on the 5-position of the triene was synthesized and the effect of this substitution was studied using ¹H NMR spectroscopy, time-dependent UV-Vis and solvatochromic analysis. The new DASA derivative shows a higher dark equillibrium, favoring the open form, and drastically faster thermal recovery than the unsubstituted derivative with the same donor and acceptor.

A visible-light-gated donor–acceptor Stenhouse adduct chemosensor: synthesis, photochromism and naked-eye colorimetric/fluorometric sensing of Al3+ and Zn2+

A visible-light-gated donor–acceptor Stenhouse adduct chemosensor is designed for the colorimetric/fluorometric sensing of Al3+ and Zn2+.

In Silico Discovery of Multistep Chemistry Initiated by a Conical Intersection: The Challenging Case of Donor-Acceptor Stenhouse Adducts

Detailed mechanistic understanding of multistep chemical reactions triggered by internal conversion via a conical intersection is a challenging task that emphasizes limitations in theoretical and experimental techniques. We present a discovery-based, hypothesis-free computational approach based on first-principles molecular dynamics to discover and refine the switching mechanism of donor-acceptor Stenhouse adducts (DASAs). We simulate the photochemical experiment in silico, following the "hot" ground state dynamics for 10 ps after photoexcitation. Using state-of-the-art graphical processing units-enabled electronic structure calculations we performed in total ∼2 ns of nonadiabatic ab initio molecular dynamics discovering (a) critical intermediates that are involved in the open-to-closed transformation, (b) several competing pathways which lower the overall switching yield, and (c) key elements for future design strategies. Our dynamics describe the natural evolution of both the nuclear and electronic degrees of freedom that govern the interconversion between DASA ground-state intermediates, exposing significant elements for future design strategies of molecular switches.

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.

Influence of Polarity Change and Photophysical Effects on Photosurfactant-Driven Wetting

Photosurfactants have shown considerable promise for enabling stimuli-responsive control of the properties and motion of fluid interfaces. Recently, a number of photoswitch chemistries have emerged to tailor the photoresponsive properties of photosurfactants. However, systematic studies investigating how photoresponsive surfactant behavior depends on the photochemical and photophysical properties of the switch remain scarce. In this work, we develop synthetic schemes and surfactant designs to produce a well-controlled library of photosurfactants to comparatively assess the behavior of photoswitch chemistry on interfacial behavior. We employ photoinduced spreading of droplets at fluid interfaces as a model for such studies. We show that although photosurfactant response is largely guided by expected trends with changes in polarity of the photoswitch, interfacial behavior also depends nontrivially and sometimes counter-intuitively on the kinetics and mechanisms of photoswitching, particularly at the interface of two solvents, as well as on complex interactions with other surfactants. Understanding these complexities enables the design of new photosurfactant systems and their optimization toward responsive functions including triggered spreading, dewetting, and destabilization of droplets on solid and fluid surfaces.

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.

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.

Increased antibacterial properties of indoline-derived phenolic Mannich bases

The search for antibacterial agents for the combat of nosocomial infections is a timely problem, as antibiotic-resistant bacteria continue to thrive. The effect of indoline substituents on the antibacterial properties of aminoalkylphenols was studied, leading to the development of a library of compounds with minimum inhibitory concentrations (MICs) as low as 1.18 μM. Two novel aminoalkylphenols were identified as particularly promising, after MIC and minimum bactericidal concentrations (MBC) determination against a panel of reference strain Gram-positive bacteria, and further confirmed against 40 clinical isolates (Staphylococcus aureus, Staphylococcus epidermidis, Enterococcus faecalis, Enterococcus faecium, and Listeria monocytogenes). The same two aminoalkylphenols displayed low toxicity against two in vivo models (Artemia salina brine shrimp and Saccharomyces cerevisiae). The in vitro cytotoxicity evaluation (on human keratinocytes and human embryonic lung fibroblast cell lines) of the same compounds was also carried out. They demonstrated a particularly toxic effect on the fibroblast cell lines, with IC₅₀ in the 1.7-5.1 μM range, thus narrowing their clinical use. The desired increase in the antibacterial properties of the aminoalkylphenols, particularly indoline-derived phenolic Mannich bases, was reached by introducing an additional nitro group in the indolinyl substituent or by the replacement of a methyl by a bioisosteric trifluoromethyl substituent in the benzyl group introduced through use of boronic acids in the Petasis borono-Mannich reaction. Notably, the introduction of an additional nitro moiety did not confer added toxicity to the aminoalkylphenols.

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.

π-Bridge Substitution in DASAs: The Subtle Equilibrium between Photochemical Improvements and Thermal Control*

Donor-acceptor Stenhouse adducts (DASAs) are playing an outstanding role as innovative and versatile photoswitches. Until now, all the efforts have been spent on modifying the donor and acceptor moieties to modulate the absorption energy and improve the cyclization and reversion kinetics. However, there is a strong dependence on specific structural modifications and a lack of predictive behavior, mostly owing to the complex photoswitching mechanism. Here, by means of a combined experimental and theoretical study, the effect of chemical modification of the π-bridge linking the donor and acceptor moieties is systematically explored, revealing the significant impact on the absorption, photocyclization, and relative stability of the open form. In particular, a position along the π-bridge is found to be the most suited to redshift the absorption while preserving the cyclization. However, thermal back-reaction to the initial isomer is blocked. These effects are explained in terms of an increased acceptor capability offered by the π-bridge substituent that can be modulated. This strategy opens the path toward derivatives with infra-red absorption and a potential anchoring point for further functionalization.

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.

Carbon Spacer Strategy: Control the Photoswitching Behavior of Donor-Acceptor Stenhouse Adducts

Understanding the relationship between chemical structure and photoswitching property of donor-acceptor Stenhouse adducts (DASAs) is necessary for developments and applications of the novel photoresponsive molecule. In the current work, we demonstrated a close relationship between the length of carbon spacer and photoswitching property of DASAs. A series of DASAs with barbituric acid substituted electron-withdrawing part and N-methylaniline substituted electron-donating part were synthesized. With shortening the carbon spacer between the phenyl and amine groups in the electron-donating part, the efficiency and rate of the light-induced linear-to-cyclic isomerization are improved in all the test solvents. The molecular energy variation during the isomerization process was investigated by density functional theory calculation to further understand the mechanism. This work provides a reliable carbon spacer strategy to control the photoswitching behavior of DASAs using chemical methods.

Redesigning donor-acceptor Stenhouse adduct photoswitches through a joint experimental and computational study

Many studies have recently explored a new class of reversible photoswitching compounds named Donor-Acceptor Stenhouse Adducts (DASAs). Upon light irradiation, these systems evolve from a coloured open-chain to a colourless closed-ring form, while the thermal back-reaction occurs at room temperature. In order to fulfill the requirements for different applications, new molecules with specific properties need to be designed. For instance, shifting the activation wavelength towards the red part of the visible spectrum is of relevance to biological applications. By using accurate computational calculations, we have designed new DASAs and predicted some of their photophysical properties. Starting from well-studied donor and acceptor parts, we have shown that small chemical modifications can lead to substantial changes in both photophysical and photoswitching properties of the resulting DASAs. Furthermore, we have also analysed how these substitutions impact the electronic structure of the systems. Finally, some pertinent candidates have been successfully synthesized and their photoswitching properties have been characterized experimentally.

Taking phototherapeutics from concept to clinical launch

More than four decades have passed since the first example of a light-activated (caged) compound was described. In the intervening years, a large number of light-responsive derivatives have been reported, several of which have found utility under a variety of in vitro conditions using cells and tissues. Light-triggered bioactivity furnishes spatial and temporal control, and offers the possibility of precision dosing and orthogonal communication with different biomolecules. These inherent attributes of light have been advocated as advantageous for the delivery and/or activation of drugs at diseased sites for a variety of indications. However, the tissue penetrance of light is profoundly wavelength-dependent. Only recently have phototherapeutics that are photoresponsive in the optical window of tissue (600-900 nm) been described. This Review highlights these recent discoveries, along with their limitations and clinical opportunities. In addition, we describe preliminary in vivo studies of prospective phototherapeutics, with an emphasis on the path that remains to be navigated in order to translate light-activated drugs into clinically useful therapeutics. Finally, the unique attributes of phototherapeutics is highlighted by discussing several potential disease applications.

General Principles for the Design of Visible-Light-Responsive Photoswitches: Tetra-ortho-Chloro-Azobenzenes

Molecular photoswitches enable reversible external control of biological systems, nanomachines, and smart materials. Their development is driven by the need for low energy (green-red-NIR) light switching, to allow non-invasive operation with deep tissue penetration. The lack of clear design principles for the adaptation and optimization of such systems limits further applications. Here we provide a design rulebook for tetra-ortho-chloroazobenzenes, an emerging class of visible-light-responsive photochromes, by elucidating the role that substituents play in defining their key characteristics: absorption spectra, band overlap, photoswitching efficiencies, and half-lives of the unstable cis isomers. This is achieved through joint photochemical and theoretical analyses of a representative library of molecules featuring substituents of varying electronic nature. A set of guidelines is presented that enables tuning of properties to the desired application through informed photochrome engineering.

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.

Putting Photomechanical Switches to Work: An Ab Initio Multiple Spawning Study of Donor-Acceptor Stenhouse Adducts

Photomechanical switches are light sensitive molecules capable of transducing the energy of a photon into mechanical work via photodynamics. In this Letter, we present the first atomistic investigation of the photodynamics of a novel class of photochromes called donor-acceptor Stenhouse adducts (DASA) using state-of-the-art ab initio multiple spawning interfaced with state-averaged complete active-space self-consistent field theory. Understanding the Z/E photoisomerization mechanism in DASAs at the molecular level is crucial in designing new derivatives with improved photoswitching capabilities. Our dynamics simulations show that the actinic step consists of competing nonradiative relaxation pathways that collectively contribute to DASAs' low (21% in toluene) photoisomerization quantum yield. Furthermore, we highlight the important role the intramolecular hydrogen bond plays in the selectivity of photoisomerization in DASAs, identifying it as a possible structural element to tune DASA properties. Our fully ab initio simulations reveal the key degrees of freedom involved in the actinic step, paving the way for the rational design of new generations of DASAs with improved quantum yield and efficiency.

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.

Reversible Photoswitching of Isolated Ionic Hemiindigos with Visible Light

Indigoid chromophores have emerged as versatile molecular photoswitches, offering efficient reversible photoisomerization upon exposure to visible light. Here we report synthesis of a new class of permanently charged hemiindigos (HIs) and characterization of photochemical properties in gas phase and solution. Gas-phase studies, which involve exposing mobility-selected ions in a tandem ion mobility mass spectrometer to tunable wavelength laser radiation, demonstrate that the isolated HI ions are photochromic and can be reversibly photoswitched between Z and E isomers. The Z and E isomers have distinct photoisomerization response spectra with maxima separated by 40-80 nm, consistent with theoretical predictions for their absorption spectra. Solvation of the HI molecules in acetonitrile displaces the absorption bands to lower energy. Together, gas-phase action spectroscopy and solution NMR and UV/Vis absorption spectroscopy represent a powerful approach for studying the intrinsic photochemical properties of HI molecular switches.

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

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

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.