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

Compartmentalizing Donor-Acceptor Stenhouse Adducts for Structure-Property Relationship Analysis

The development of photoswitches that absorb low energy light is of notable interest due to the growing demand for smart materials and therapeutics necessitating benign stimuli. Donor-acceptor Stenhouse adducts (DASAs) are molecular photoswitches that respond to light in the visible to near-infrared spectrum. As a result of their modular assembly, DASAs can be modified at the donor, acceptor, triene, and backbone heteroatom molecular compartments for the tuning of optical and photoswitching properties. This Perspective focuses on the electronic and steric contributions at each compartment and how they influence photophysical properties through the adjustment of the isomerization energetic landscape. An emphasis on current synthetic strategies and their limitations highlights opportunities for DASA architecture, and thus photophysical property expansion.

Visible-Light-Driven Solid-State Fluorescent Photoswitches for High-Level Information Encryption

Developing visible-light-driven fluorescent photoswitches in the solid state remains an enormous challenge in smart materials. Such photoswitches are obtained from salicylaldimines through excited-state intramolecular proton transfer (ESIPT) and subsequent cis-trans isomerization strategies. By incorporating a bulky naphthalimide fluorophore into a Schiff base, three photoswitches achieve dual-mode changes (both in color and fluorescence) in the solid state. In particular, the optimal one generates triple fluorescence changing from green, to yellow and finally to orange upon visible-light irradiation. This switching process is fully reversible and can be repeated at least 10 times without obvious attenuation, suggesting its good photo-fatigue resistance. Mechanism studies reveal that the naphthalimide group not only enables the tuning of multicolor with an additional emission, but also induces a folded structure, reducing molecular stacking and facilitating ESIPT and cis-trans isomerization. As such, photopatterning, ternary encoding and transient information recording and erasing are successfully developed. The present study provides a reliable strategy for visible-light-driven fluorescent photoswitches, showing implications for advanced information encryption materials.

Stenhouse Salts: Visible Light Photoswitches for Protic Environments

Designing photoswitches that have large structural changes, are visible-light responsive, and are compatible with water is a major challenge for moving toward applications in biological systems. Despite the potential for Stenhouse salts to be a water-compatible counterpart to the popular DASA photoswitches, there has not yet been any major investigation into their properties as a photoswitch. Here, we report a series of aniline-based Stenhouse salt (AnSten) photoswitches with electron donating and withdrawing groups. AnSten photoswitches reversibly switch from a visible light absorbing isomer to a visible light transparent isomer upon irradiation with green light. The dark equilibrium and switching kinetics are dependent on the electronics of the aniline. These molecules switch reversibly in common protic solvents including water and hydrogels. Stenhouse salts show significant potential as a visible light active, water compatible, negative photochromic, T-type photoswitch with a large structural change.

Merocyanines: Electronic Structure and Spectroscopy in Solutions, Solid State, and Gas Phase

Merocyanines, owing to their readily tunable electronic structure, are arguably the most versatile functional dyes, with ample opportunities for tailored design via variations of both the donor/acceptor (D/A) end groups and π-conjugated polymethine chain. A plethora of spectral properties, such as strong solvatochromism, high polarizability and hyperpolarizabilities, and sensitizing capacity, motivates extensive studies for their applications in light-converting materials for optoelectronics, nonlinear optics, optical storage, fluorescent probes, etc. Evidently, an understanding of the intrinsic structure-property relationships is a prerequisite for the successful design of functional dyes. For merocyanines, these regularities have been explored for over 70 years, but only in the past three decades have these studies expanded beyond the theory of their color and solvatochromism toward their electronic structure in the ground and excited states. This Review outlines the fundamental principles, essential for comprehension of the variable nature of merocyanines, with the main emphasis on understanding the impact of internal (chemical structure) and external (intermolecular interactions) factors on the electronic symmetry of the D-π-A chromophore. The research on the structure and properties of merocyanines in different media is reviewed in the context of interplay of the three virtual states: nonpolar polyene, ideal polymethine, and zwitterionic polyene.

Crowding for Confinement: Reversible Isomerization of First-Generation Donor-Acceptor Stenhouse Adduct Derivatives in Water Modulated by Thermoresponsive Dendritic Macromolecules

Mimicking nature, the reversible isomerization of hydrophobic dyes in aqueous solutions is appealing for bio-applications. Here, we report on the reversible isomerization of first-generation solvatochromic donor-acceptor Stenhouse adducts (DASAs) in water within dendritic matrices, realized either through the dendronization of DASAs or the incorporation of DASA pendants into dendronized copolymers. These dendritic macromolecules contain three-fold dendritic oligoethylene glycols (OEGs), which afford the macromolecules water-solubility and unprecedented thermoresponsive behavior. The thermoresponsive behavior of both dendronized DASAs and dendronized copolymers is dominated by the peripherals of dendritic OEGs. However, the hydrophilicity of the acceptor from DASA moieties also play a role in mediating their thermal phase transitions, and more importantly, tailor the hydrophobic interactions between dendritic OEGs and DASA moieties. Intriguingly, dendritic topologies contribute confinement to encapsulate the DASA moieties through crowding effects, and cooperative interactions from the crowded dendritic OEGs modulate the DASA moieties with different isomerization in aqueous media. The thermally induced collapse of dendritic OEGs, accompanied by the aggregation of dendritic macromolecules, leads to the formation of hydrophobic domains, which exert enhanced crowding effects to efficiently encapsulate the DASA moieties. Compared to the low molar mass of dendronized DASAs, thermally collapsed dendronized copolymers can efficiently retard the hydration of DASA pendants through cooperation between neighboring dendritic OEGs and afford the DASA pendants with better confined microenvironments to mediate their isomerization recovery by up to 90% from a cyclic charged (hydrophilic) state into a noncharged (hydrophobic) linear state in water. This dendritic confinement exhibits excellent fatigue resistance after several cycles of alternating photo-irradiation and thermal annealing at elevated temperatures.

A DASA displaying highly efficient and rapid reversible isomerization within sustainable nano/micro capsules: one step closer to sustainability

Donor-acceptor Stenhouse adducts (DASAs), derived from bio-based furfural, demonstrate reversible isomerization when exposed to light and heat, positioning them as attractive candidates for sustainable smart materials. However, achieving efficient and rapid isomerization in high bio-content solid-state matrices, especially under mild conditions, remains a significant hurdle due to restricted molecular mobility and limited matrix options. To address this, we developed a novel solid matrix in the form of sustainable nano/micro capsules, which boast the highest bio-content reported to date (57%). Composed of polymethylmethacrylate (PMMA) and a lauric-stearic acid eutectic mixture (L-SEM), these capsules facilitate highly efficient and rapid reversible isomerization of a third-generation DASA (DASA-1). Remarkably, the system achieves 84% forward and 90% reverse isomerization under mild temperatures, significantly enhancing the material's photo-switching capabilities. This advancement not only addresses the critical challenge of isomerization within high bio-content solid matrices but also opens broader possibilities for the application of bio-based DASAs in environmentally friendly technologies, such as color-rich rewritable papers. By innovating in the design of sustainable smart materials, this work has the potential to extend the utility of DASAs across various scientific fields, contributing to the global shift towards a low-carbon, environmentally sustainable society.

Photoswitchable Carbamazepine Analogs for Non-Invasive Neuroinhibition In Vivo

A problem of systemic pharmacotherapy is off-target activity, which causes adverse effects. Outstanding examples include neuroinhibitory medications like antiseizure drugs, which are used against epilepsy and neuropathic pain but cause systemic side effects. There is a need of drugs that inhibit nerve signals locally and on-demand without affecting other regions of the body. Photopharmacology aims to address this problem with light-activated drugs and localized illumination in the target organ. Here, we have developed photoswitchable derivatives of the widely prescribed antiseizure drug carbamazepine. For that purpose, we expanded our method of ortho azologization of tricyclic drugs to meta/para and to N-bridged diazocine. Our results validate the concept of ortho cryptoazologs (uniquely exemplified by Carbazopine-1) and bring to light Carbadiazocine (8), which can be photoswitched between 400-590 nm light (using violet LEDs and halogen lamps) and shows good drug-likeness and predicted safety. Both compounds display photoswitchable activity in vitro and in translucent zebrafish larvae. Carbadiazocine (8) also offers in vivo analgesic efficacy (mechanical and thermal stimuli) in a rat model of neuropathic pain and a simple and compelling treatment demonstration with non-invasive illumination.

Multi Stimuli Responsive Dual Aggregation-Induced Emission and Photochromic Behavior of a Tetraphenyl Substituted Triphenylamine Derivative and its Application as Anti-counterfeiting Agent

A multi-stimuli responsive tetraphenyl substituted tripehnylamine-based aggregation induced emissive (AIE) material coupled with spiropyran was prepared. Owing to the presence of AIE and photochromic moiety, the molecule exhibits emissive aggregates, photochromism, and acidochromism. The multiple stimuli sensitive behavior of the molecule was explored for anti-counterfeiting behavior on TLC plate and commercial banknotes. The fluorogenic and photogenic response under UV and visible light established the potential of the candidate as a new generation encryption material.

Photoinduced π-Bond breakage causing dynamic closing-opening shell transition of Z-type Diphenylmaleonitriles molecules

Organic molecules with dynamic covalent-bonding characteristics have attracted much attention for their important role in constructing stimulus-responsive smart materials. However, it is difficult to realize sensitive and reversible covalent bond cleavage/formation through external stimuli in the aggregated state of molecules. Herein, a series of 2,3-diphenylmaleonitriles (DPMNs) with photoinduced π-bond cleavage properties have been designed and synthesized to construct the dynamic covalent bond materials. The cis-form 2,3-diphenylmaleonitriles (Z-DPMNs) exhibit significant photochromism in both solid and solution states under ultraviolet light and visible light. The photochromism stems from the photoinduced π-bond splitting of Z-DPMNs, resulting in a transition from the closed-shell to open-shell structure. Moreover, the twisted structure and molecular stacking of Z-DPMNs, the push-pull electron effect of substituents, and the external factors including temperature and solvent polarity have important effects on the dynamic conversion of π-bonds. Based on the sensitive and reversible optical performance transformation, Z-DPMNs can be applied as safety ink in anti-counterfeiting, information encryption and storage systems. This work not only provides an approach for constructing dynamic covalent bonds but also greatly enriches stimulus-responsive materials.

Development and characterization of amino donor-acceptor Stenhouse adducts

Donor-acceptor Stenhouse adducts (DASAs) are molecular photoswitches spurring wide interest because of their dynamic photophysical properties, complex photoswitching mechanism, and diverse applications. Despite breakthroughs in modularity for the donor, acceptor, and triene compartments, the backbone heteroatom remains static due to synthetic challenges. We provide a predictive tool and sought-after strategy to vary the heteroatom, introduce amino DASA photoswitches, and analyze backbone heteroatom effects on photophysical properties. Amino DASA synthesis is enabled by aza-Piancatelli rearrangements on pyrrole substrates, imparting an aromaticity-breaking rearrangement that capitalizes on nitrogen's additional bonding orbital and the inductive properties of sulfonyl groups. Amino DASA structure is confirmed by single crystal X-ray diffraction, the photochromic properties are characterized, and the photoswitch isomerization is investigated. Overall, the discovered pyrrole rearrangement enables the study of the DASA backbone heteroatom compartment and furthers our insight into the structure-property relationship of this complex photoswitch.

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.

Aurones: Unexplored Visible-Light Photoswitches for Aqueous Medium

The development of synthetically accessible photoswitches showing an efficient performance in aqueous medium has recently become an urgent task due to the rapid progress of photopharmacology and novel biomedical applications. In response to this challenge, in this work, aurone derivatives are introduced as a novel class of efficient visible-light photoswitches for aqueous medium. In general, aurones exhibit superior performance in water, including significantly higher quantum yields, compared with other indigoid photoswitches (hemithioindigo and hemiindigo). Especially remarkable are the half-lives of the photoinduced E-isomers of aurones in water, reaching up to 7 years. Further modification of the aurone scaffold with substituents that increase water solubility does not affect most of the photoswitching characteristics and even improves some them. The highly advantageous property profile of the aurone photoswitches make them a perfect novel platform for the design of light-controllable systems in the areas requiring photoswitching in aqueous medium.

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.

From Visible to Near-Infrared Light-Triggered Photochromism: Negative Photochromism

Photochromic compounds, whose key molecular properties can be effectively modulated by light irradiation, have attracted significant attention for their potential applications in various research fields. The restriction of photoisomerization coloration induced by ultraviolet light limits their applications in the biomedical field and some other fields. Negative photochromism, wherein a relatively stable colored isomer transforms to a colorless metastable isomer under low-energy light irradiation, offers advantages in applications within materials science and life science. This review provides a summary of negatively photochromic compounds based on different molecular skeletons. Their corresponding design strategies and photochromic properties are presented to provide practical guidelines for future investigations. Negatively photochromic compounds can effectively expand the range of photochromic switches for future applications, offering unique properties such as responsiveness to visible to near-infrared light.

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.

Light-Based Anti-Biofilm and Antibacterial Strategies

Biofilm formation and antimicrobial resistance pose significant challenges not only in clinical settings (i.e., implant-associated infections, endocarditis, and urinary tract infections) but also in industrial settings and in the environment, where the spreading of antibiotic-resistant bacteria is on the rise. Indeed, developing effective strategies to prevent biofilm formation and treat infections will be one of the major global challenges in the next few years. As traditional pharmacological treatments are becoming inadequate to curb this problem, a constant commitment to the exploration of novel therapeutic strategies is necessary. Light-triggered therapies have emerged as promising alternatives to traditional approaches due to their non-invasive nature, precise spatial and temporal control, and potential multifunctional properties. Here, we provide a comprehensive overview of the different biofilm formation stages and the molecular mechanism of biofilm disruption, with a major focus on the quorum sensing machinery. Moreover, we highlight the principal guidelines for the development of light-responsive materials and photosensitive compounds. The synergistic effects of combining light-triggered therapies with conventional treatments are also discussed. Through elegant molecular and material design solutions, remarkable results have been achieved in the fight against biofilm formation and antibacterial resistance. However, further research and development in this field are essential to optimize therapeutic strategies and translate them into clinical and industrial applications, ultimately addressing the global challenges posed by biofilm and antimicrobial resistance.

A Red-Light-Responsive DASA-Polymer with High Water Stability for Controlled Release

Photoresponsive polymers hold vast potential in the realm of drug delivery. Currently, most photoresponsive polymers use ultraviolet (UV) light as the excitation source. However, the limited penetration ability of UV light within biological tissues serves as a significant hindrance to their practical applications. Given the strong penetration ability of red light in biological tissues, the design and preparation of a novel red-light-responsive polymer with high water stability, incorporating the reversible photoswitching compound and donor-acceptor Stenhouse adducts (DASA) for controlled drug release is demonstrated. In aqueous solutions, this polymer exhibits self-assembly into micellar nanovectors (~33 nm hydrodynamic diameter), facilitating the encapsulation of the hydrophobic model drug Nile red (NR) within the micellar core. Upon irradiation by a 660 nm LED light source, photons are absorbed by DASA, leading to the disruption of the hydrophilic-hydrophobic balance of the nanovector and thereby resulting in the release of NR. This newly designed nanovector incorporates red light as a responsive switch, successfully avoiding the problems of photodamage and limited penetration of UV light within biological tissues, thereby further promoting the practical applications of photoresponsive polymer nanomedicines.

Small-molecule photoswitches for fluorescence bioimaging: engineering and applications

Fluorescence microscopy has revolutionised our understanding of biological systems, enabling the visualisation of biomolecular structures and dynamics in complex systems. The possibility to reversibly control the optical or biochemical properties of fluorophores can unlock advanced applications ranging from super-resolution microscopy to the design of multi-stimuli responsive and functional biosensors. In this Highlight, we review recent progress in small-molecule photoswitches applied to biological imaging with an emphasis on molecular engineering strategies and promising applications, while underlining the main challenges in their design and implementation.