Monodirectional Photocycle Drives Proton Translocation

Facile and Versatile Mechanochemical Synthesis of Indigoid Photoswitches

We demonstrate the application of mechanochemistry in the synthesis of indolone-based photoswitches (hemiindigos, hemithioindigos, and oxindoles) via Knoevenagel condensation reactions. Utilizing ball-milling and an organic base (piperidine) acting as catalyst and solvent for liquid assisted grinding (LAG) conditions, we achieve rapid, solvent-free transformations, obtaining a set of known and previously unreported photoswitches, including highly functional amino acid-based photoswitches, multichromophoric derivatives and photoswitchable cavitands based on resorcin[4]arenes. The reaction under mechanochemical conditions gives moderate-to-high yields and is highly stereoselective leading to Z-isomers of hemiindigos and hemithioindigos and E-isomers of oxindoles. For selected examples, reversible visible-light photoswiching properties have been demonstrated.

Synthesis and Evaluation of the Photochemical Properties of Heterocyclic Hemiindigos

This study reports a series of novel heterocyclic hemiindigos (Het-HI) synthesized via the condensation of indoxyl acetate with various heteroaromatic aldehydes. The influence of electron-rich and electron-poor heterocycles on the photochemical and photophysical properties of these compounds has been investigated. Our findings reveal that several Het-HIs exhibit noteworthy photoswitching behavior, including enhanced absorption at the visible region. Notably, certain derivatives respond efficiently to green and red light, achieving good conversions to the metastable E-isomer and displaying prolonged half-lives of up to 53 days in a polar solvent. The results highlight the potential of these photoswitches for applications in molecular devices and responsive materials.

Single-Wavelength Visible-Light-Induced Reversible Isomerization of Stiff-Stilbene under Dynamic Covalent Control

Formylation of stiff-stilbene enables isomerization by visible instead of UV light, as is monitored by UV-vis and 1H NMR spectroscopy. Further, it allows dynamic covalent imine formation, which gives rise to slightly blue-shifted absorbance. As a result, the irradiation wavelength that is used to promote E → Z isomerization for the formylated species, causes the opposite Z → E isomerization for the imine derivative. This feature can be used to produce either isomer with the same light source under control of imine formation and hydrolysis. Single crystal X-ray analysis and TD-DFT calculations provide structural and electronic insight.

Hemiphosphoindigos as a platform for chiroptical or water soluble photoswitching

Photoswitches are important molecular tools to precisely control the behavior of matter by using light irradiation. They have found application in virtually all applied chemical fields from chemical biology to material sciences. However, great challenges remain in advanced property design including tailored chiroptical responses or water solubility. Here, hemiphosphoindigo (HPI) photoswitches are presented as capable phosphorus-based photoswitches and a distinct addition to the established indigoid chromophore family. Phosphinate is embedded in the core indigoid chromophore and the resulting optimized photoswitches display high thermal stabilities, excellent fatigue resistance and high isomer enrichment. A series of planar, twisted and heterocyclic HPIs are investigated to probe design strategies for advantageous photophysical properties. The phosphinate provides a platform for easily accessible, water-soluble photoswitches, especially interesting for biological applications. Its chiral nature further allows light-induced modulation of chiroptical properties. HPIs therefore open up a distinct structural space for photoswitch generation and advanced light-responsive applications.

Switching Sides: Regiochemistry and Functionalization Dictate the Photoswitching Properties of Imines

Photoswitchable imines demonstrate light-dependent dynamic covalent chemistry and can function as molecular ratchets. However, the design of aryliminopyrazoles (AIPs) has been limited to N-pyrazole derivatives with ortho-pyrrolidine motifs. The impact of other functionalization patterns on the photoswitching properties remains unknown. Here, we present a systematic structure-property analysis and study how the photoswitching properties can be tuned through ortho- and para-functionalization of the phenyl ring in N-pyrazole and N-phenyl AIPs. This study establishes the first set of design rules for these AIP photoswitches and reports the most stable Z-isomer of an AIP to date, enabling its crystallization and resulting in the first reported crystal structure of a metastable Z-aldimine. Finally, we demonstrate that the AIPs are promising candidates for photoswitching in the condensed phase.

Rubizhne Institute - A Birthplace of Photochromic Molecules

We introduce the community to the remarkable fact that two significant discoveries in the field of organic photoswitches are associated to the Rubizhne (Rubezhnoe) branch of the Research Institute of Organic Intermediates and Dyes during the last century. The institute in Rubizhne was a place where researchers of various nationalities carried out studies of organic dyes for printing and textiles. These efforts resulted in the discoveries of photoswitchable hemithioindigos by M. A. Mostoslavskii and peri-aryloxyquinones by Yu. E. Gerasimenko. Herein, based on the available literature, we reconstruct the circumstances surrounding these outstanding findings and highlight the unique role of the Rubizhne institute as a research center. Furthermore, we demonstrate the impact of the results of the Rubizhne researchers on the field of photoswitchable molecules.

Assembly of Dye Molecules in Covalent Organic Frameworks for Enhanced Colorimetric Biosensing

Colorimetric assays have been extensively investigated for biosensing applications due to their advantages of visual recognizability, ease of use, and low cost. However, advancing their development is a great challenge due to the inherent limitations of colorimetric dyes. Herein, we report a strategy to assemble dyes in covalent organic frameworks (COFs) to effectively reinforce the applicability of pH-responsive dyes in colorimetric bioassays. Experimental results reveal that three-dimensional COFs can promote the assembly of dyes through hydrogen bonding, resulting in the formation of a dye-supermolecule@COF assembly. Consequently, when sensitized at increased pH levels (e.g., hydroxyl ions), disruption of hydrogen bonds may trigger a rapid transition from their insoluble fixed state within the COFs into soluble, visibly detectable dye anions. This process can also be facilitated by increased hydrophilicity and elevated electrostatic repulsion between the dye anions and COFs, leading to the substantial release of chromogenic dye anions from the COF pores into the solution, thereby amplifying the colorimetric signal output. Therefore, by employing various synthesized dye-supermolecule@COFs as signal tags, we developed a colorimetric bioassay capable of accurately identifying breast cancer cell subtypes. This study not only highlights the effectiveness of dye-supermolecule@COFs in enhancing colorimetric biosensing but also underscores the potential of employing the COF-mediated dye assembly strategy for colorimetric assays.

Heterocyclic Hemipiperazines: Multistimuli-Responsive Switches and Sensors for Zinc or Cadmium Ions

Advance in the design of molecular photoswitches - adapters that convert light into changes at molecular level - opens up exciting possibilities in preparing smart polymers, drugs photoactivated inside humans, or light-fueled nanomachines that might in the future operate in our bloodstream. Hemipiperazines are recently reported biocompatible molecular photoswitches based on cyclic dipeptides. Here we report a multistimuli-responsive hemipiperazine-based switch that reacts on light, solvents, acidity, or metal ions. Its photoequilibration is controlled by the intramolecular hydrogen bonding pattern. The compound can be used as a mid-nanomolar photoswitchable fluorescent sensor for zinc and cadmium ions, applicable to monitor environmental pollution in real time.

Photoswitchable Imines Drive Dynamic Covalent Systems to Nonequilibrium Steady States

Coupling a photochemical reaction to a thermal exchange process can drive the latter to a nonequilibrium steady state (NESS) under photoirradiation. Typically, systems use separate motifs for photoresponse and equilibrium-related processes. Here, we show that photoswitchable imines can fulfill both roles simultaneously, autonomously driving a dynamic covalent system into a NESS under continuous light irradiation. We demonstrate this using transimination reactions, where E-to-Z photoisomerism generates a more kinetically labile species. At the NESS, energy is stored both in the metastable Z-isomer of the imine and in the system's nonequilibrium constitution; when the light is switched off, this stored energy is released as the system reverts to its equilibrium state. The system operates autonomously under continuous light irradiation and exhibits characteristics of a light-driven information ratchet. This is enabled by the dual-role of the imine linkage as both the photochromic and dynamic covalent bond. This work highlights the ability and application of these imines to drive systems to NESSs, thus offering a novel approach in the field of systems chemistry.

Scaffold-Hopping Strategies in Aurone Optimization: A Comprehensive Review of Synthetic Procedures and Biological Activities of Nitrogen and Sulfur Analogues

Aurones, particular polyphenolic compounds belonging to the class of minor flavonoids and overlooked for a long time, have gained significative attention in medicinal chemistry in recent years. Indeed, considering their unique and outstanding biological properties, they stand out as an intriguing reservoir of new potential lead compounds in the drug discovery context. Nevertheless, several physicochemical, pharmacokinetic, and pharmacodynamic (P3) issues hinder their progression in more advanced phases of the drug discovery pipeline, making lead optimization campaigns necessary. In this context, scaffold hopping has proven to be a valuable approach in the optimization of natural products. This review provides a comprehensive and updated picture of the scaffold-hopping approaches directed at the optimization of natural and synthetic aurones. In the literature analysis, a particular focus is given to nitrogen and sulfur analogues. For each class presented, general synthetic procedures are summarized, highlighting the key advantages and potential issues. Furthermore, the biological activities of the most representative scaffold-hopped compounds are presented, emphasizing the improvements achieved and the potential for further optimization compared to the aurone class.

Cu2O-catalyzed cascade phosphinylation/cyclization of 2'-aminochalcones for the synthesis of hemi-indigo derivatives

A Cu2O-catalyzed cascade phosphinylation/cyclization reaction of 2'-aminochalcones and diphenylphosphine oxides to produce hemi-indigo derivatives has been developed. This strategy facilitates the sequential formation of a C-P bonds and a C-N bond in a single reaction step. Notably, the approach features one-pot operation, an earth-abundant copper catalyst, readily available starting materials, a broad substrate scope and high compatibility with functional groups, providing 33 compounds in acceptable yields.

Photoswitchable imines: aryliminopyrazoles quantitatively convert to long-lived Z-isomers with visible light

Arylimines offer promise in dynamic-covalent materials due to their recyclability and ease of synthesis. However, their light-triggered E/Z isomerism has received little attention. This is attributed to challenges that include low thermal stability of their metastable state (<60 s at 20 °C), incomplete photoswitching (<50% to the metastable state), and the need for UV light (≤365 nm). We overcome these limitations with a novel class of imine photoswitch, the aryliminopyrazoles (AIPs). These AIPs can be switched using visible light (470 nm), attain photostationary states with over 95% of the Z-isomer, exhibit great resistance to fatigue, and have thermal half-lives up to 19.2 hours at room temperature. Additionally, they display T-type and negative photochromism under visible light irradiation-a useful property. The photochromic properties, quantitative assembly and accessibility of precursors set these photoswitches apart from their azo-based analogues. These findings open avenues for next-generation photoresponsive dynamic-covalent materials driven solely by these new photochromic linkages and further exploration of photocontrolled dynamic combinatorial chemistry.

Reversible Switching Based on Truly Intramolecular Long-Range Proton Transfer─Turning the Theoretical Concept into Experimental Reality

Herein, we demonstrate a working prototype of a conjugated proton crane, a reversible tautomeric switching molecule in which truly intramolecular long-range proton transfer occurs in solution at room temperature. The system consists of a benzothiazole rotor attached to a 7-hydroxy quinoline stator. According to the experimental and theoretical results, the OH proton is delivered under irradiation to the quinolyl nitrogen atom through a series of consecutive proton transfer and twisting steps. The use of a rigid rotor prevents undesired side processes that decrease the switching performance in previously described proton cranes and provides an unprecedented switching efficiency and fatigue resistance. The newly designed system confirms the theoretical concept for the application of proton transfer-initiated intramolecular twisting as the switching mechanism, developed more than 10 years ago, and provides unique insights for the further development of tautomeric molecular switches and motors, molecular logic gates, and new molecular-level energy storage systems.