Effects of π-extension on pyrrole hemithioindigo photoswitches

Computational Rational Design of Bridgehead Nitrogen Heterocyclic Azobenzene Photoswitches

Azobenzenes are proven to be one of the most successful molecular photoswitches applied across different fields such as organic chemistry, material science, cosmetics, and pharmaceuticals. Such a widespread implementation is possible because of their photochromic properties contingent upon the substitution pattern and aryl-core nature. In recent endeavors of molecular design, replacing one or both phenyl rings with heteroaromatics turned out to be a good strategy to access compounds with improved photoswitching properties, as well as to expand molecular diversity. One of the challenges related to the design of new azobenzene photoswitches is that it often includes the synthesis of large libraries of compounds due to limited methods for prediction of their properties. Herein, we present a computationally driven workflow for the design and synthesis of a novel class of azobenzene photoswitches, heteroaryl azobenzenes with N-bridgehead heterocycles─pyrazolo[1,5-a]pyrimidine and 1,2,4-triazolo[1,5-a]pyrimidine. A small library of heteroaryl photoswitches was synthesized, and their photochemical properties were evaluated. Subsequently, these results were used to validate a computational approach, which included the in silico evaluation of a large library of designed photoswitch candidates leading to the synthesis of a new photoswitch with improved spectral properties, red-shifted λmax values.

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.

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.

Dicyanorhodanine-Pyrrole Conjugates for Visible Light-Driven Quantitative Photoswitching in Solution and the Solid State

Small molecule photoswitches capable of toggling between two distinct molecular states in response to light are versatile tools to monitor biological processes, control photochemistry, and design smart materials. In this work, six novel dicyanorhodanine-based pyrrole-containing photoswitches are reported. The molecular design avails both the Z and E isomers from synthesis, where each can be isolated using chromatographic techniques. Inter- and intramolecular hydrogen bonding (H-bonding) interactions available to the E and Z isomers, respectively, uniquely impart thermal stability to each isomer over long time periods. Photoisomerization could be assessed by solution NMR and UV-vis spectroscopic techniques along with complementary ground- and excited-state computational studies, which show good agreement. Quantitative E → Z isomerization occurs upon 523 nm irradiation of the parent compound (where R = H) in solution, whereas Z → E isomerization using 404 nm irradiation offers a photostationary state (PSS) ratio of 84/16 (E/Z). Extending the π-conjugation of the pyrrole unit (where R = p-C6H4-OMe) pushes the maximum absorption to the yellow-orange region of the visible spectrum and allows bidirectional quantitative isomerization with 404 and 595 nm excitation. Comparator molecules have been prepared to report how the presence or absence of H-bonding affects the photoswitching behavior. Finally, studies of the photoswitches in neat films and photoinactive polymer matrices reveal distinctive structural and optical properties of the Z and E isomers and ultimately afford reversible photoswitching to spectrally unique PSSs using visible light sources including the Sun.

Easily Accessible Substituted Heterocyclic Hemithioindigos as Bistable Molecular Photoswitches

Thioaurone chromophores, part of the indigoid family and commonly named hemithioindigos, have recently gained attention due to their interesting photoswitching properties. The study focuses on heterocyclic hemithioindigos (Het-HTIs) and investigates their synthesis using electron-rich and electron-poor heterocycles and modifications to the thioindigo moiety. Furthermore, it aims to evaluate the photoswitching performances of these synthesised compounds, with a particular emphasis on the influence of the heterocycles on the photoisomerization capabilities, which was found to be more prominent than the modifications made to the thioindigo moiety. Among the 44 Het-HTIs tested, several exhibited highly efficient photoswitchable properties, demonstrating Z-to-E photoisomerization in the blue region, and E-to-Z photoisomerization in the green or the red regions. Additionally, the metastable E-isomer displayed an impressive half-life of up to 54 days in a polar solvent (DMSO). These results suggest that heterocyclic hemithioindigos hold great promise as photoswitches for researchers interested in light-controlled molecular mechanisms.

Proton-Coupled Photochromic Hemithioindigo: Toward Photoactivated Chemical Sensing and Imaging

We report photoswitchable fluorescent hemithioindigos (HTIs) where the metastable E isomers were stabilized by the proton-bridged intramolecular hydrogen bond. Titration experiments and computational analysis indicated that the E isomers were much more basic than the Z isomers, which enabled photoactivated colorimetric and fluorescent pH response in solvents and polypropylene films. The HTIs exhibited reversibly switchable fluorescence with the Z isomers being the most fluorescent. Moreover, the HTIs were lysosomotropic and the kinetic fluorescence evolution during photoswitching was able to differentiate subcellular compartments with different pH. The combination of photoenhanced basicity, switchable fluorescence, and proton-coupled photochromism lay the groundwork for a broad range of chemical and biological applications.

A concise synthesis of thioaurones via NBS-induced cyclization of MOM-protected 2'-mercaptochalcones

A mild and efficient approach for the synthesis of thioaurones via NBS-induced cyclization of methoxymethyl-protected mercapto-chalcones has been developed. This simple method is highly functional group tolerant and provides straightforward access to thioaurones in good to high yields.

Redshifted and thermally bistable one-way quantitative hemithioindigo-derived photoswitches enabled by isomer-specific excited state intramolecular proton transfer

We report bistable indole-containing hemithioindigos (HTIs) with one-way quantitative photoswitching properties. Supported by state-averaged CASPT2/CASSCF calculations, we propose a mechanism for the observed one-way photoswitching that involves an isomer-specific excited state intramolecular proton transfer (ESIPT). Additionally, we developed a thermally bistable oligomer-inspired bipyrrole-containing HTI, which displays large band separation and bidirectional near-quantitative photoisomerization in the near-infrared, bio-optical window.

Heterocyclic Hemithioindigos: Highly Advantageous Properties as Molecular Photoswitches

A survey of heterocyclic hemithioindigo photoswitches is presented identifying a number of structural motives with outstanding property profiles. The highly sought-after combination of pronounced color change, quantitative switching in both directions, exceptional high quantum yields, and tunable high thermal stability of metastable states can be realized with 4-imidazole, 2-pyrrole, and 3-indole-based derivatives. In the former, an unusual preorganization using isomer selective chalcogen- and hydrogen bonding allows to precisely control geometry changes and tautomerism upon switching. Heterocyclic hemithioindigos thus represent highly promising photoswitches with advanced capabilities that will be of great value to anyone interested in establishing defined and reversible control at the molecular level.

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.

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.

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.

Pyrrole Hemithioindigo Antimitotics with Near-Quantitative Bidirectional Photoswitching that Photocontrol Cellular Microtubule Dynamics with Single-Cell Precision*

We report the first cellular application of the emerging near-quantitative photoswitch pyrrole hemithioindigo, by rationally designing photopharmaceutical PHTub inhibitors of the cytoskeletal protein tubulin. PHTubs allow simultaneous visible-light imaging and photoswitching in live cells, delivering cell-precise photomodulation of microtubule dynamics, and photocontrol over cell cycle progression and cell death. This is the first acute use of a hemithioindigo photopharmaceutical for high-spatiotemporal-resolution biological control in live cells. It additionally demonstrates the utility of near-quantitative photoswitches, by enabling a dark-active design to overcome residual background activity during cellular photopatterning. This work opens up new horizons for high-precision microtubule research using PHTubs and shows the cellular applicability of pyrrole hemithioindigo as a valuable scaffold for photocontrol of a range of other biological targets.

Photoresponsive molecular tools for emerging applications of light in medicine

Light-based therapeutic and imaging modalities, which emerge in clinical applications, rely on molecular tools, such as photocleavable protecting groups and photoswitches that respond to photonic stimulus and translate it into a biological effect. However, optimisation of their key parameters (activation wavelength, band separation, fatigue resistance and half-life) is necessary to enable application in the medical field. In this perspective, we describe the applications scenarios that can be envisioned in clinical practice and then we use those scenarios to explain the necessary properties that the photoresponsive tools used to control biological function should possess, highlighted by examples from medical imaging, drug delivery and photopharmacology. We then present how the (photo)chemical parameters are currently being optimized and an outlook is given on pharmacological aspects (toxicity, solubility, and stability) of light-responsive molecules. With these interdisciplinary insights, we aim to inspire the future directions for the development of photocontrolled tools that will empower clinical applications of light.

Oxidized Hemithioindigo Photoswitches-Influence of Oxidation State on (Photo)physical and Photochemical Properties

The photophysical and photochemical properties of sulfoxide and sulfone derivatives of hemithioindigo photoswitches are scrutinized and compared to the unoxidized parent chromophores. Oxidation results in significantly blue-shifted absorptions and mostly reduction of photochromism while thermal stabilities of individual isomers remain largely unaltered. Effective photoswitching takes place at shorter wavelengths compared to parent hemithioindigos and high isomeric yields can be obtained reversibly in the respective photostationary states. Reversible solid-state photoswitching is observed for a twisted sulfone derivative accompanied by visible color changes. These results establish oxidized hemithioindigo photoswitches as promising and versatile tools for robust light-control of molecular behavior for a wide range of applications.