Probing Metal Ion Complexation of Ligands with Multiple Metal Binding Sites: The Case of Spiropyrans

Photoswitchable Photochromic Chelating Spironaphthoxazines: Synthesis, Photophysical Properties, Quantum-Chemical Calculations, and Complexation Ability

The stable and efficient photochromic and photoswitchable molecular systems designed from spirooxazines are of increasing scientific and practical interest because of their present and future applications in advanced technologies. Among these compounds, chelating spironaphthoxazines have received widespread attention due to their efficient optical response after complexation with some metal ions being of biomedical interest and environmental importance, as well as their good cycle performance and high reliability, especially by metal ion sensing. In this mini-review, we summarize our results in the design of novel photoswitchable chelating spironaphthoxazines with specific substituents in their naphthoxazine or indoline ring systems in view of recent progress in the development of such molecular systems and their applications as metal ion sensors. The design, synthesis methods, and photoresponse of such spirooxazine derivatives relevant to their applications, as well as quantum-chemical calculations for these compounds, are presented. Examples of various design concepts are discussed, such as sulfobutyl, hydroxyl, benzothiazolyl, or ester and carboxylic acid as substituents in the chelating spironaphthoxazine molecules. Further developments and improvements of this interesting and promising kind of molecular photoswitches are outlined.

A Modern Look at Spiropyrans: From Single Molecules to Smart Materials

Photochromic compounds of the spiropyran family have two main isomers capable of inter-switching with UV or visible light. In the current review, we discuss recent advances in the synthesis, investigation of properties, and applications of spiropyran derivatives. Spiropyrans of the indoline series are in focus as the most promising representatives of multi-sensitive spirocyclic compounds, which can be switched by a number of external stimuli, including light, temperature, pH, presence of metal ions, and mechanical stress. Particular attention is paid to the structural features of molecules, their influence on photochromic properties, and the reactions taking place during isomerization, as the understanding of the structure-property relationships will rationalize the synthesis of compounds with predetermined characteristics. The main prospects for applications of spiropyrans in such fields as smart material production, molecular electronics and nanomachinery, sensing of environmental and biological molecules, and photopharmacology are also discussed.

A dinoflagellate-inspired mechanochromic film for fast and reversible information encryption and display

Inspired by dinoflagellates, we developed a flexible film consisting of spiropyran-based soft polyacrylate and Zn(OTf)₂. The open-ring form of spiropyran coordinated with Zn(OTf)₂ under stretching to produce a visible fluorescent color change from colorless to yellow. The potential of this film was demonstrated for fast and reversible information encryption and decryption.

Molecular Switches—Tools for Imparting Control in Drug Delivery Systems

Cancer is a globally prevalent cause of premature mortality. Of growing interest is the development of novel anticancer therapies and the optimisation of associated risks. Major issues presently facing conventional anticancer therapies include systemic toxicity, poor solubility, membrane permeability, and multidrug resistance Nanocarriers have been employed to address these issues. Nanocarriers encapsulate anticancer drugs, enabling them to bypass biological barriers and minimise their adverse side effects. These drug delivery systems offer extensive benefits as they can be modified to gravitate towards specific environmental conditions. To further enhance the safety and efficacy of these drug carriers, modern developments have included incorporating a molecular switching mechanism into their structure. These molecular switches are responsive to endogenous and exogenous stimuli and may undergo reversible and repeatable conformational changes when activated. The incorporation of molecular switches can, therefore, impart stimuli-responsive drug-release control on a DDS. These stimuli can then be manipulated to offer precise dosage control over the drug release at a specific target site. This review discusses recent developments in the design of DDSs incorporating light and pH-responsive molecular switches as drug release controllers.

Complexation of 1,3-dihetaryl-5-phenyl-2-pyrazoline Derivatives with Polyvalent Metal Ions: Quantum Chemical Modeling and Experimental Investigation

1,3,5-Triaryl-2-pyrazoline derivatives with a pyridine ring in position 1 and 2-benzimidazolyl or 2-benzothiazolyl bicycles in position 3 were synthesized. Spectral properties in solvents of similar polarity, i.e. aprotic acetonitrile and in protic methanol, were studied, complexation with cadmium and mercury ions in acetonitrile was elucidated as well. Quantum-chemical modeling with application of the elements of Bader's atoms-in-molecules (AIM) theory of the title molecules conformational structure and 1:1 stoichiometry complexes formed with polyvalent metals of various nature (Mg, Zn, Cd, Pb, Hg, Ba) was conducted. The principal possibility of “nitrogen-sulfur” switching of the metal ions binding sites for the benzothiazole derivative was revealed, and makes possible to classify this compound as “smart ligand”.

Spiropyran-Based Drug Delivery Systems

Nanocarriers are rapidly growing in popularity in the field of drug delivery. The ability of nanocarriers to encapsulate and distribute poorly soluble drugs while minimising their undesired effects is significantly advantageous over traditional drug delivery. Nanocarriers can also be decorated with imaging moieties and targeting agents, further incrementing their functionality. Of recent interest as potential nanocarriers are spiropyrans; a family of photochromic molecular switches. Due to their multi-responsiveness to endo- and exogenous stimuli, and their intrinsic biocompatibility, they have been utilised in various drug delivery systems (DDSs) to date. In this review, we provide an overview of the developments in spiropyran-based DDSs. The benefits and drawbacks of utilising spiropyrans in drug delivery are assessed and an outline of spiropyran-based drug delivery systems is presented.

The Synthesis of Hexaazatrinaphthylene-Based 2D Conjugated Copper Metal-Organic Framework for Highly Selective and Stable Electroreduction of CO2 to Methane

2D conjugated MOFs have attracted significant interests in recent years owing to their special structural features and promising physical and chemical properties. These intriguing attributes, to a large extent, stem from the nature of incorporated ligands. The available ligands for the construction of 2D conjugated MOFs are still limited, especially those that have heteroatoms included and exposed to the pores. In this work, we designed and synthesized a highly symmetric hexaazatrinaphthylene (HATNA)-based ligand with two different coordination sites. Through selective coordination, a highly crystalline and porous 2D conjugated copper metal-organic framework was constructed. Due to the synergic effects of HATNA and copper catecholate node, this HATNA-based 2D conjugated MOF can mediate the electrocatalytic reduction of CO₂ to methane with high selectivity of 78 % at high current density of 8.2 milliamperes per square centimetre (mA cm^-2 ) for long durability over 12 hours.

Spiropyrans for light-controlled drug delivery

The supramolecular assembly of acetylsalicylic acid to a merocyanine-Zn complex translates into a photo-controllable drug delivery system. Herein, we present the synthesis and spectroscopic analysis of a three-component assembly combining a spiropyran derivative, a zinc(ii) cation, and acetylsalicylic acid. The ON/OFF system can be modulated by safe and easily affordable stimuli, for the specific delivery of more than one active compound. Our results confirm the formation of a ternary system in solution, where the behaviour is dominated by photo-induced responses, and pave the way for the use of such smart platforms for medicinal chemistry purposes.

A coloring tool for spiropyrans: solid state metal–organic complexation versus salification

Controlling the type of merocyanine metal interaction for broad solid-state coloration.

Photo-Responsive Graphene and Carbon Nanotubes to Control and Tackle Biological Systems

Photo-responsive multifunctional nanomaterials are receiving considerable attention for biological applications because of their unique properties. The functionalization of the surface of carbon nanotubes (CNTs) and graphene, among other carbon based nanomaterials, with molecular switches that exhibit reversible transformations between two or more isomers in response to different kind of external stimuli, such as electromagnetic radiation, temperature and pH, has allowed the control of the optical and electrical properties of the nanomaterial. Light-controlled molecular switches, such as azobenzene and spiropyran, have attracted a lot of attention for nanomaterial's functionalization because of the remote modulation of their physicochemical properties using light stimulus. The enhanced properties of the hybrid materials obtained from the coupling of carbon based nanomaterials with light-responsive switches has enabled the fabrication of smart devices for various biological applications, including drug delivery, bioimaging and nanobiosensors. In this review, we highlight the properties of photo-responsive carbon nanomaterials obtained by the conjugation of CNTs and graphene with azobenzenes and spiropyrans molecules to investigate biological systems, devising possible future directions in the field.

Light-responsive polymers for microfluidic applications

While the microfluidic device itself may be small, often the equipment required to control fluidics in the chip unit is large e.g. pumps, valves and mixing units, which can severely limit practical use and functional scalability. In addition, components associated with fluidic control of the device, more specifically the valves and pumps, contribute significantly to the overall unit cost. Here we sketch the problem of a gap between high end accurate, but expensive sensor platforms, versus less accurate, but widely employable hand-held low-cost devices. Recent research has shown that the integration of light-responsive materials within microfluidic devices can provide the function of expensive fluidic components, and potentially enable sophisticated measurements to be made using much less expensive equipment. An overview of the most recent developments will be presented for valves, mixers, transport and sample handling inside microfluidic devices.

trans to cis photo-isomerization in merocyanine dysprosium and yttrium complexes

A unique light-switching behavior is revealed in Yttrium(iii) and Dysprosium(iii) merocyanine complexes through NMR and AC magnetometry experiments. Its impact on slow relaxation of magnetization is described.

A Rationally Designed Reversible 'Turn-Off' Sensor for Glutathione

γ-Glutamyl-cysteinyl-glycine (GSH) plays a critical role in maintaining redox homeostasis in biological systems and a decrease in its cellular levels is associated with diseases. Existing fluorescence-based chemosensors for GSH acts as irreversible reaction-based probes that exhibit a maximum fluorescence (‘turn-on’) once the reaction is complete, regardless of the actual concentration of GSH. A reversible, reaction-based ‘turn-off’ probe (1) is reported here to sense the decreasing levels of GSH, a situation known to occur at the onset of various diseases. The more fluorescent merocyanine (MC) isomer of 1 exists in aqueous solution and this reacts with GSH to induce formation of the ring-closed spiropyran (SP) isomer, with a measurable decrease in absorbance and fluorescence (‘turn-off’). Sensor 1 has good aqueous solubility and shows an excellent selectivity for GSH over other biologically relevant metal ions and aminothiol analytes. The sensor permeates HEK 293 cells and an increase in fluorescence is observed on adding buthionine sulfoximine, an inhibitor of GSH synthesis.