Proton-Stabilized Photochemically Reversible E/ Z Isomerization of Spiropyrans

Spiropyran as Building Block in Peptide Synthesis and Modulation of Photochromic Properties

Light-controlled triggering of materials requires efficient embedding of molecular photoswitches into larger molecules. We herein present the synthesis of two new building blocks for the synthesis of photoswitchable peptides, embedding spiropyranes as a central unit into peptide-backbones via a novel, yet unreported approach. The synthesis presented here allows us to embed spiropyranes directly into solid-phase peptide synthesis (SPPS), further describing the resulting photophysical properties of the as-prepared photoswitchable peptides.

A Rhodamine-Spiropyran Conjugate Empowering Tunable Mechanochromism in Polymers under Multiple Stimuli

Mechanochromic functionality realized via the force-responsive mechanophores in polymers has great potential for damage sensing and information storage. Mechanophores with the ability to recognize multiple stimuli for tunable chromic characteristics are highly sought after for versatile sensing ability and color programmability. Nevertheless, the majority of mechanophores are based on single-component chromophores with limited sensitivity, or require additional fabrication technology for multi-modal chromism. Here, we report a novel multifunctional mechanophore capable of vividly detectable and tunable mechanochromism in polymers. This synergistic optical coupling relies on strategically fusing rhodamine and spiropyran (Rh-SP), and tethering polymer chains on both subunits. The mechanochromic behaviors of the Rh-SP-linked polymers under sonication and compression are thoroughly evaluated in response to changes in force and the light-controlled relaxation process. Non-sequential ring-opening of the two subunits under force is identified, endowing high-contrast mechanochromism. Light-induced differential ring-closing reactions of the two subunits, together with the acidichromism of the SP moiety, are employed to engineer elastomers with programmable and wide-spectrum colors. Our work presents an effective strategy for highly appreciable and regulable mechanochromic functionality, and also provides new insights into the rupture mechanisms of π-fused mechanophores, as well as how the stimuli history controls stress accumulation in polymers.

Polymer Free Volume-Controlled Molecular Clock and Emitter for Multicolored Transient Data Display in Advanced Photonic Cryptography

The booming demand on data security has aroused great interest for developing smart materials with temporal display feature and dynamic multicolor fluorescence. However, it remains challenging to implement both features on most responsive molecules. Herein, we construct a polymer free volume-controlled "molecular clock and emitter" via covalently embedding a multi-stimuli responsive molecular switch (i.e., spiropyran) into a polymer network (i.e., poly(pentafluorophenyl acrylate)) with programmable crosslink density and free volume. By the aminolysis of pentafluorophenyl ester with different amount of diamine crosslinkers, pPFPA-co-SP networks with controllable crosslink densities are generated, which have different confinement effects on the rate constant of SP/MC isomerization, thus leading to time-dependent photochromism. In addition, PTF1, a fluorescent probe that is sensitive to polymer rigidity, is introduced to further endow pPFPA-co-SP system with phototunable dynamic full-color emission. Therefore, relying on their synergistical responses to the rigidity of the polymer network, we have successfully developed a versatile molecular clock and emitter via an "one stone two birds" manner, which shows time-dependent data display along with dynamic multicolor fluorescence switching, providing great potential for advanced encryption and anticounterfeiting with a high security level.

Isomerization, Protonation, and Hydrolysis Properties of Naphthalimide-Containing Spiropyran in Aqueous Media

Synthesis of spiropyrans exhibiting ring-opening/closing isomerization driven by external stimuli is a challenging subject for the development of molecular sensors. A napthalimide-containing spiropyran (1) promotes rapid isomerization between the spirocyclic (SP) form and ring-opened merocyanine (MC) form by the change in solvent polarity even under the dark condition at room temperature. In this work, the effect of water on the isomerization behavior of 1 was studied. The addition of water caused an MC-water H-bonding interaction and shifted the resonance MC structure to the zwitterionic form with a lower ground-state energy. The stabilized MC form promoted spontaneous SP → MC isomerization and increased the equilibrium MC concentration. The effect of pH on the behavior of 1 was also studied. In acidic-neutral media, protonation/deprotonation of the naphthalimide moiety led to rapid and reversible changes in the absorption spectra. In contrast, strongly basic media (pH > 12) promoted irreversible base-catalyzed hydrolysis of the alkene moiety.

Superresolution imaging of antibiotic-induced structural disruption of bacteria enabled by photochromic glycomicelles

Bacterial evolution, particularly in hospital settings, is leading to an increase in multidrug resistance. Understanding the basis for this resistance is critical as it can drive discovery of new antibiotics while allowing the clinical use of known antibiotics to be optimized. Here, we report a photoactive chemical probe for superresolution microscopy that allows for the in situ probing of antibiotic-induced structural disruption of bacteria. Conjugation between a spiropyran (SP) and galactose via click chemistry produces an amphiphilic photochromic glycoprobe, which self-assembles into glycomicelles in water. The hydrophobic inner core of the glycomicelles allows encapsulation of antibiotics. Photoirradiation then serves to convert the SP to the corresponding merocyanine (MR) form. This results in micellar disassembly allowing for release of the antibiotic in an on-demand fashion. The glycomicelles of this study adhere selectively to the surface of a Gram-negative bacterium through multivalent sugar-lectin interaction. Antibiotic release from the glycomicelles then induces membrane collapse. This dynamic process can be imaged in situ by superresolution spectroscopy owing to the "fluorescence blinking" of the SP/MR photochromic pair. This research provides a high-precision imaging tool that may be used to visualize how antibiotics disrupt the structural integrity of bacteria in real time.

Reversibly Tuning the Viscosity of Peptide-Based Solutions Using Visible Light

Light can be used to design stimuli-responsive systems. We induce transient changes in the assembly of a low molecular weight gelator solution using a merocyanine photoacid. Through our approach, reversible viscosity changes can be achieved via irradiation, delivering systems where flow can be controlled non-invasively on demand.

Spiropyran-Based Photoisomerizable α-Amino Acid for Membrane-Active Peptide Modification

Photoisomerizable peptides are promising drug candidates in photopharmacology. While azobenzene- and diarylethene-containing photoisomerizable peptides have already demonstrated their potential in this regard, reports on the use of spiropyrans to photoregulate bioactive peptides are still scarce. This work focuses on the design and synthesis of a spiropyran-derived amino acid, (S)-2-amino-3-(6'-methoxy-1',3',3'-trimethylspiro-[2H-1-benzopyran-2,2'-indolin-6-yl])propanoic acid, which is suitable for the preparation of photoisomerizable peptides. The utility of this amino acid is demonstrated by incorporating it into the backbone of BP100, a known membrane-active peptide, and by examining the photoregulation of the membrane perturbation by the spiropyran-containing peptides. The toxicity of the peptides (against the plant cell line BY-2), their bacteriotoxicity (E. coli), and actin-auxin oscillator modulation ability were shown to be significantly dependent on the photoisomeric state of the spiropyran unit.

Photo- and halochromism of spiropyran-based main-chain polymers

Advanced functional polymeric materials based on spiropyrans (SPs) feature multi-stimuli responsive characteristics, such as a change in color with exposure to light (photochromism) or acids (halochromism). The inclusion of stimuli-responsive molecules in general - and SPs in particular - as main-chain repeating units is a scarcely explored macromolecular architecture compared to side chain responsive polymers. Herein, we establish the effects of substitution patterns on SPs within a homopolymer main-chain synthesized via head-to-tail Acyclic Diene METathesis (ADMET) polymerization. We unambiguously demonstrate that varying the location of the ester group (-OCOR) on the chromophore, which is essential to incorporate the SPs in the polymer backbone, determines the photo- and halochromism of the resulting polymers. While one polymer shows effective photochromism and resistance towards acids, the opposite - weak photochromism and effective response to acid - is observed for an isomeric polymer, simply by changing the position of the ester-linker relative to the benzopyran oxygen on the chromene unit. Our strategy represents a simple approach to manipulate the stimuli-response of main-chain SP bearing polymers and highlights the critical importance of isomeric molecular constitution on main-chain stimuli-sensitive polymers as emerging materials.

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.

Construction of Multi-Stimuli Responsive Highly Porous Switchable Frameworks by In Situ Solid-State Generation of Spiropyran Switches

Stimuli-responsive molecular systems support within permanently porous materials offer the opportunity to host dynamic functions in multifunctional smart materials. However, the construction of highly porous frameworks featuring external-stimuli responsiveness, for example by light excitation, is still in its infancy. Here a general strategy is presented to construct spiropyran-functionalized highly porous switchable aromatic frameworks by modular and high-precision anchoring of molecular hooks and an innovative in situ solid-state grafting approach. Three spiropyran-grafted frameworks bearing distinct functional groups exhibiting various stimuli-responsiveness are generated by two-step post-solid-state synthesis of a parent indole-based material. The quantitative transformation and preservation of high porosity are demonstrated by spectroscopic and gas adsorption techniques. For the first time, a highly efficient strategy is provided to construct multi-stimuli-responsive, yet structurally robust, spiropyran materials with high pore capacity which is proved essential for the reversible and quantitative isomerization in the bulk as demonstrated by solid-state NMR spectroscopy. The overall strategy allows to construct dynamic materials that undergoes reversible transformation of spiropyran to zwitterionic merocyanine, by chemical and physical stimulation, showing potential for pH active control, responsive gas uptake and release, contaminant removal, and water harvesting.

Multistate Switching of Some Ruthenium Alkynyl and Vinyl Spiropyran Complexes

To study the switching properties of photochromes, we undertook the synthesis and characterization of several ruthenium organometallic complexes of the type [Ru(Cp*)(dppe)(C≡C-SP)] or [Ru(CO)(dppe)(PPh3)Cl(CH═CH-SP)], where SP = spiropyran. The spectroscopic and electrochemical properties of the complexes were determined by careful cyclic voltammetric and spectroelectrochemical experiments. Whereas the mononuclear alkynyl ruthenium complexes undergo one-electron oxidations localized over the metal alkynyl moiety, the oxidation of the mononuclear vinyl ruthenium complexes is centered on the indoline moiety of the spiropyran. Through these studies, we demonstrate access to several stable redox states, in addition to switching states attained via acidochromism and/or photoisomerization.

Light-driven self-assembly of spiropyran-functionalized covalent organic framework

Controlling the number of molecular switches and their relative positioning within porous materials is critical to their functionality and properties. The proximity of many molecular switches to one another can hinder or completely suppress their response. Herein, a synthetic strategy involving mixed linkers is used to control the distribution of spiropyran-functionalized linkers in a covalent organic framework (COF). The COF contains a spiropyran in each pore which exhibits excellent reversible photoswitching behavior to its merocyanine form in the solid state in response to UV/Vis light. The spiro-COF possesses an urchin-shaped morphology and exhibits a morphological transition to 2D nanosheets and vesicles in solution upon UV light irradiation. The merocyanine-equipped COFs are extremely stable and possess a more ordered structure with enhanced photoluminescence. This approach to modulating structural isomerization in the solid state is used to develop inkless printing media, while the photomediated polarity change is used for water harvesting applications.

Spiropyran-based chromic hydrogels for CO2 absorption and detection

By enabling rapid, cost-effective, user-friendly and in situ detection of carbon dioxide, colorimetric CO₂ sensors are of relevance for a variety of fields. However, it still remains a challenge the development of optical chemosensors for CO₂ that combine high sensitivity, selectivity and reusability with facile integration into solid materials. Herein we pursued this goal by preparing hydrogels functionalized with spiropyrans, a well-known class of molecular switches that undergo different color changes upon application of light and acid stimuli. By varying the nature of the substituents of the spiropyran core, different acidochromic responses are obtained in aqueous media that allow discriminating CO₂ from other acid gases (e.g., HCl). Interestingly, this behavior can be transferred to functional solid materials by synthesizing polymerizable spiropyran derivatives, which are used to prepare hydrogels. These materials preserve the acidochromic properties of the incorporated spiropyrans, thus leading to selective, reversible and quantifiable color changes upon exposure to different CO₂ amounts. In addition, CO₂ desorption and, therefore, recovery of the initial state of the chemosensor is favored by irradiation with visible light. This makes spiropyran-based chromic hydrogels promising systems for the colorimetric monitorization of carbon dioxide in a diversity of applications.

Novel Indoline Spiropyrans Based on Human Hormones β-Estradiol and Estrone: Synthesis, Structure, Chromogenic and Cytotoxic Properties

The introduction of a switchable function into the structure of a bioactive compound can endow it with unique capabilities for regulating biological activity under the influence of various types of external stimuli, which makes such hybrid compounds promising objects for photopharmacology, targeted drug delivery and bio-imaging. This work is devoted to the synthesis and study of new spirocyclic derivatives of important human hormones-β-estradiol and estrone-possessing a wide range of biological activities. The obtained hybrid compounds represent an indoline spiropyrans family, a widely known class of organic photochromic compounds. The structure of the compounds was confirmed by ¹H and ¹³C NMR, IR, HRMS and single-crystal X-ray analysis. The intermolecular interactions in the crystals of spiropyran (3) were defined by Hirshfeld surfaces and 2D fingerprint plots, which were successfully acquired from CrystalExplorer (v21.5). All target hybrids demonstrated pronounced activity in the visible region of the spectrum. The mechanisms of thermal isomerization processes of spiropyrans and their protonated merocyanine forms were studied by DFT methods, which revealed the energetic advantage of the protonation process with the formation of a β-cisoid CCCH conformer at the first stage and its further isomerization to more stable β-transoid forms. The proposed mechanism of acidochromic transformation was confirmed by the additional NMR study data that allowed for the detecting of the intermediate CCCH isomer. The study of the short-term cytotoxicity of new spirocyclic derivatives of estrogens and their 2-formyl-precursors was performed on the HeLa cell model. The precursors and spiropyrans differed in toxicity, suggesting their variable applicability in novel anti-cancer technologies.

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.

Molecular-Switch-Embedded Solution-Processed Semiconductors

Recent improvements in the performance of solution-processed semiconductor materials and optoelectronic devices have shifted research interest to the diversification/advancement of their functionality. Embedding a molecular switch capable of transition between two or more metastable isomers by light stimuli is one of the most straightforward and widely accepted methods to potentially realize the multifunctionality of optoelectronic devices. A molecular switch embedded in a semiconductor can effectively control various parameters such as trap-level, dielectric constant, electrical resistance, charge mobility, and charge polarity, which can be utilized in photoprogrammable devices including transistors, memory, and diodes. This review classifies the mechanism of each optoelectronic transition driven by molecular switches regardless of the type of semiconductor material or molecular switch or device. In addition, the basic characteristics of molecular switches and the persisting technical/scientific issues corresponding to each mechanism are discussed to help researchers. Finally, interesting yet infrequently reported applications of molecular switches and their mechanisms are also described.

Thermal and (Thermo-Reversible) Photochemical Cycloisomerization of 1H-2-Benzo[c]oxocins: From Synthetic Applications to the Development of a New T-Type Molecular Photoswitch

A novel T-type molecular photoswitch based on the reversible cyclization of 1H-2-benzo[c]oxocins to dihydro-4H-cyclobuta[c]isochromenes has been developed. The switching mechanism involves a light-triggered ring-contraction of 8-membered 1H-2-benzo[c]oxocins to 4,6-fused O-heterocyclic dihydro-4H-cyclobuta[c]isochromene ring systems, with reversion back to the 1H-2-benzo[c]oxocin state accessible through heating. Both processes are unidirectional and proceed with good efficiency, with switching properties─including reversibility and half-life time─easily adjusted via structural functionalization. Our new molecular-switching platform exhibits independence from solvent polarity, originating from its neutral-charge switching mechanism, a property highly sought-after for biological applications. The photoinduced ring-contraction involves a [2+2] conjugated-diene cyclization that obeys the Woodward-Hoffmann rules. In contrast, the reverse process initiates via a thermal ring-opening (T > 60 °C) to produce the original 8-membered 1H-2-benzo[c]oxocins, which is thermally forbidden according to the Woodward-Hoffmann rules. The thermal ring-opening is likely to proceed via an ortho-quinodimethane (o-QDM) intermediate, and the corresponding switching mechanisms are supported by experimental observations and density functional theory calculations. Other transformations of 1H-2-benzo[c]oxocins were found upon altering reaction conditions: prolonged heating of the 1H-2-benzo[c]oxocins at a significantly elevated temperature (72 h at 120 °C), with the resulting dihydronaphthalenes formed via the o-QDM intermediate. These reactions also proceed with good chemoselectivities, providing new synthetic protocols for motifs found in several bioactive molecules, but are otherwise difficult to access.

Spiropyran/Merocyanine Amphiphile in Various Solvents: A Joint Experimental–Theoretical Approach to Photophysical Properties and Self-Assembly

This joint experimental-theoretical work focuses on molecular and photophysical properties of the spiropyran-containing amphiphilic molecule in organic and aqueous solutions. Being dissolved in tested organic solvents, the system demonstrates positive photochromism, i.e., upon UV stimulus the colorless spiropyran form is transformed into colorful merocyanine isomer. However, the aqueous solution of the amphiphile possesses a negative photochromism: the orange-red merocyanine form becomes thermodynamically more stable in water, and both UV and vis stimuli lead to the partial or complete photobleaching of the solution. The explanation of this phenomenon is given on the basis of density functional theory calculations and classical modeling including thermodynamic integration. The simulations reveal that stabilization of merocyanine in water proceeds with the energy of ca. 70 kJ mol−1, and that the Helmholtz free energy of hydration of merocyanine form is 100 kJ mol−1 lower as compared to the behavior of SP isomer in water. The explanation of such a difference lies in the molecular properties of the merocyanine: after ring-opening reaction this molecule transforms into a zwitterionic form, as evidenced by the electrostatic potential plotted around the opened form. The presence of three charged groups on the periphery of a flat conjugated backbone stimulates the self-assembly of merocyanine molecules in water, ending up with the formation of elongated associates with stack-like building blocks, as shown in molecular dynamics simulations of the aqueous solution with the concentration above critical micelle concentration. Our quantitative evaluation of the hydrophilicity switching in spiropyran/merocyanine containing surfactants may prompt the search for new systems, including colloidal and polymeric ones, aiming at remote tuning of their morphology, which could give new promising shapes and patterns for the needs of modern nanotechnology.

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.

Light-responsive biomaterials for ocular drug delivery

Light-responsive biomaterials can be used for the delivery of therapeutic drugs and nucleic acids, where the tunable/precise delivery of payload highlights the potential of such biomaterials for treating a variety of conditions. The translucency of eyes and advances of laser technology in ophthalmology make light-responsive delivery of drugs feasible. Importantly, light can be applied in a non-invasive fashion; therefore, light-triggered drug delivery systems have great potential for clinical impact. This review will examine various types of light-responsive polymers and the chemistry that underpins their application as ophthalmic drug delivery systems.

Open-Form Configurational Isomers of a Tricyanofuran-Type Metastable-State Photoacid

We determine the presence of four open-form configurational isomers for an unsubstituted metastable-state photoacid (mPAH) of the tricyanofuran (TCF) type in solution, at room temperature, via 2D NMR experiments. Electronic structure calculations are carried out to predict the relative stability of the isomers found experimentally and their isomerization barriers. According to the calculated rate constants for isomerization, the molecule can freely interconvert between the open-form isomers, thereby providing a thermal pathway between the isomers that might be better suited to access the cyclized closed-form configuration and those that are not. In establishing the open form isomeric makeup of the TCF mPAH under study, this work establishes the need to consider the four isomers in further studies on the thermal and excited-state isomerization processes and substituent effect thereon.

A Dual pH‐ and Light‐Responsive Spiropyran‐Based Surfactant: Investigations on Its Switching Behavior and Remote Control over Emulsion Stability

A cationic surfactant containing a spiropyran unit is prepared exhibiting a dual‐responsive adjustability of its surface‐active characteristics. The switching mechanism of the system relies on the reversible conversion of the non‐ionic spiropyran (SP) to a zwitterionic merocyanine (MC) and can be controlled by adjusting the pH value and via light, resulting in a pH‐dependent photoactivity: While the compound possesses a pronounced difference in surface activity between both forms under acidic conditions, this behavior is suppressed at a neutral pH level. The underlying switching processes are investigated in detail, and a thermodynamic explanation based on a combination of theoretical and experimental results is provided. This complex stimuli‐responsive behavior enables remote‐control of colloidal systems. To demonstrate its applicability, the surfactant is utilized for the pH‐dependent manipulation of oil‐in‐water emulsions.

Stimuli-Modulated Metal Oxidation States in Photochromic MOFs

Tuning metal oxidation states in metal-organic framework (MOF) nodes by switching between two discrete linker photoisomers via an external stimulus was probed for the first time. On the examples of three novel photochromic copper-based frameworks, we demonstrated the capability of switching between +2 and +1 oxidation states, on demand. In addition to crystallographic methods used for material characterization, the role of the photochromic moieties for tuning the oxidation state was probed via conductivity measurements, cyclic voltammetry, and electron paramagnetic resonance, X-ray photoelectron, and diffuse reflectance spectroscopies. We confirmed the reversible photoswitching activity including photoisomerization rate determination of spiropyran- and diarylethene-containing linkers in extended frameworks, resulting in changes in metal oxidation states as a function of alternating excitation wavelengths. To elucidate the switching process between two states, the photoisomerization quantum yield of photochromic MOFs was determined for the first time. Overall, the introduced noninvasive concept of metal oxidation state modulation on the examples of stimuli-responsive MOFs foreshadows a new pathway for alternation of material properties toward targeted applications.

Single wavelength colour tuning of spiropyran and dithienylethene based photochromic coatings

Controlling the transmission of thin films with external stimuli is an important goal in functional optical materials and devices. Tuning is especially challenging where both broad band (neutral density filtering) and spectrally varied (colour) transmission are required. The external control provided by photochemically driven switching, between transmission levels and colours, is functionally simple from a device perspective. The limits due to the spectral ranges of individual photochromic compounds can be overcome by combining several photochromes within one material or device. Here we show that a combination of photochromic molecular switches immobilised in a PMMA polymer matrix enables tuning of colour and transparency. We show that only a single excitation wavelength is required through the use of the primary inner filter effect and the layered construction of the films in which the photochromes nitrospiropyran (NSP), and nitrothiospiropyran (TSP) or 1,2-bis-terthienyl-hexafluorocyclopentene (DTE) are separated spatially. The approach taken circumvents the need to match photochemical quantum yields and thermal reactivity of the component photochromes. The photochemical switching of the films was characterised by UV/vis absorption spectroscopy and shows that switching rates and photostationary states are limited by inner filter effects rather than the intrinsic properties of photochromes, such as photochemical quantum yields and thermal stability. The photochemical behaviour and stability of the photochromes in solution and in the PMMA films were compared and the concentration range over which self-inhibition of photochemical switching occurs was established. The rate of photochemical switching and the difference in transmission between the spiropyran and merocyanine forms in solution enable prediction of the performance in the films and enable rational design of colour tuning ranges and responsivity in thin film filters.

Large, Tunable, and Reversible pH Changes by Merocyanine Photoacids

Molecular photoswitches capable of generating precise pH changes will allow pH-dependent processes to be controlled remotely and noninvasively with light. We introduce a series of new merocyanine photoswitches, which deliver reversible bulk pH changes up to 3.2 pH units (pH 6.5 to pH 3.3) upon irradiation with 450 nm light, displaying tunable and predictable timescales for thermal recovery. We present models to show that the key parameters for optimizing the bulk pH changes are measurable: the solubility of the photoswitch, the acidity of the merocyanine form, the thermal equilibrium position between the spiropyran and the merocyanine isomers, and the increased acidity under visible light irradiation. Using ultrafast transient absorption spectroscopy, we determined the quantum yields for the ring-closing reaction and found that the lifetimes of the transient cis-merocyanine isomers ranged from 30 to 550 ns. Quantum yields did not appear to be a limitation for bulk pH switching. The models we present use experimentally determined parameters and are, in principle, able to predict the change in pH obtained for any related merocyanine photoacid.

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.

Acid-catalysed liquid-to-solid transitioning of arylazoisoxazole photoswitches

Molecular photoswitches play a vital role in the development of responsive materials. These molecular building blocks are particularly attractive when multiple stimuli can be combined to bring about physical changes, sometimes leading to unexpected properties and functions. The arylazoisoxazole molecular switch was recently shown to be capable of efficient photoreversible solid-to-liquid phase transitions with application in photoswitchable surface adhesion. Here, we show that the arylazoisoxazole forms thermally stable and photoisomerisable protonated Z- and E-isomers in an apolar aprotic solvent when the pK a of the applied acid is sufficiently low. The tuning of isomerisation kinetics from days to seconds by the pK a of the acid not only opens up new reactivity in solution, but also the solid-state photoswitching of azoisoxazoles can be efficiently reversed with selected acid vapours, enabling acid-gated photoswitchable surface adhesion.

Hydrogen-Bonded Dimeric Capsules with Appended Spiropyran Units: Towards Controlled Cargo Release

We report the synthesis of unprecedented tetra-urea derivatives of calix[4]arene and calix[4]pyrrole containing four spiropyran (SP) units at their upper rim. We investigate the photo- and acid-induced isomerization of the monomeric and homo-dimeric tetra-ureas derivatives using UV-Vis and ¹ H NMR spectroscopies. At micromolar concentration, irradiation of the samples with 365 nm light induces changes in their absorption spectra that are consistent with SP→merocyanine (MC) isomerization. However, analogous experiments at millimolar concentration do not produce noticeable changes in the ¹ H NMR spectra. The addition of triflic acid to micromolar and millimolar solutions of the tetra-ureas produces the quantitative isomerization of the SP units to the protonated merocyanine form (E-MCH⁺ ) and the simultaneous disassembly of the capsular dimers to form ill-defined aggregates. The neutralization of the acid solutions resets the SP form. Under these acid/base treatment conditions, the controlled release of the included guest and the reassembly of the all-SP tetra-urea dimers occurs at different extents depending on its calix[4]arene or calix[4]pyrrole scaffold.

Prevent or Cure-The Unprecedented Need for Self-Reporting Materials

Self-reporting smart materials are highly relevant in modern soft matter materials science, as they allow for the autonomous detection of changes in synthetic polymers, materials, and composites. Despite critical advantages of such materials, for example, prolonged lifetime or prevention of disastrous material failures, they have gained much less attention than self-healing materials. However, as diagnosis is critical for any therapy, it is of the utmost importance to report the existence of system changes and their exact location to prevent them from spreading. Thus, we herein critically review the chemistry of self-reporting soft matter materials systems and highlight how current challenges and limitations may be overcome by successfully transferring self-reporting research concepts from the laboratory to the real world. Especially in the space of diagnostic self-reporting systems, the recent SARS-CoV-2 (COVID-19) pandemic indicates an urgent need for such concepts that may be able to detect the presence of viruses or bacteria on and within materials in a self-reporting fashion.

Photochromic Polyamide 6 Based on Spiropyran Synthesized via Hydrolyzed Ring-Opening Polymerization

We report photochromic polyamide 6 (PA6) which was synthesized by hydrolyzed ring-opening polymerization of ε-caprolactam with spiropyran (SP) embedded in the polymer chains. It indicated that crystallinity degree of the resulting copolymers was decreased since only PA6 segments can crystallize with increasing content of SP modifier. Meanwhile, toughness of photochromic PA6 was decreased. The photochromic property analysis indicated that the sample with more flexibility and more content of SP was more sensitive to UV light at the beginning of irradiation than other samples and its color after being irradiated for 1 min tended to reddish. Investigation revealed that the UV-vis absorbance of SP-PA6-3 had negligible decay after 10 cycles, which indicated SP-modified PA6 possessed excellent photoresponse reversibility and fatigue resistance.

Proton-Transfer Dynamics of Photoacidic Merocyanines in Aqueous Solution

Photoacids attract increasing scientific attention, as they are valuable tools to spatiotemporally control proton-release reactions and pH values of solutions. We present the first time-resolved spectroscopic study of the excited state and proton-release dynamics of prominent merocyanine representatives. Femtosecond transient absorption measurements of a pyridine merocyanine with two distinct protonation sites revealed dissimilar proton-release mechanisms: one site acts as a photoacid generator as its pKa value is modulated in the ground state after photoisomerization, while the other functions as an excited state photoacid which releases its proton within 1.1 ps. With a pKa drop of 8.7 units to -5.5 upon excitation, the latter phenolic site is regarded a super-photoacid. The 6-nitro derivative exhibits only a phenolic site with similar, yet slightly less photoacidic characteristics and both compounds transfer their proton to methanol and ethanol. In contrast, for the related 6,8-dinitro compound an intramolecular proton transfer to the ortho-nitro group is suggested that is involved in a rapid relaxation into the ground state.

Solid Multiresponsive Materials Based on Nitrospiropyran-Doped Ionogels

The application of molecular switches for the fabrication of multistimuli-responsive chromic materials and devices still remains a challenge because of the restrictions imposed by the supporting solid matrices where these compounds must be incorporated: they often critically affect the chromic response as well as limit the type and nature of external stimuli that can be applied. In this work, we propose the use of ionogels to overcome these constraints, as they provide a soft, fluidic, transparent, thermally stable, and ionic-conductive environment where molecular switches preserve their solution-like properties and can be exposed to a number of different stimuli. By exploiting this strategy, we herein pioneer the preparation of nitrospiropyran-based materials using a single solid platform that exhibit optimal photo-, halo-, thermo-, and electrochromic switching behaviors.

Electrochemical Ring-Opening and -Closing of a Spiropyran

The bistability of molecular switches is an essential characteristic in their use as functional components in molecular-based devices and machines. For photoswitches, light-driven switching between two stable states proceeds via short-lived changes of the bond order in electronically excited states. Here, bistable switching of a ditertbutyl-substituted spiropyran photoswitch is instead demonstrated by oxidation and subsequent reduction in an overall four-state cycle. The spiropyran structure chosen has reduced sensitivity to the effect of secondary electrochemical processes such as H⁺ production and provides transient access to a decreased thermal Z–E isomerization barrier in the one electron oxidized state, akin to that achieved in the corresponding photochemical path. Thus, we show that the energy needed for switching spiropyrans to the merocyanine form on demand, typically delivered by a photon, can instead be provided electrochemically. This opens up further opportunities for the utilization of spiropyrans in electrically controlled applications and devices.

Spiropyran Meets Guanine Quadruplexes: Isomerization Mechanism and DNA Binding Modes of Quinolizidine-Substituted Spiropyran Probes

The recent delivery of a fluorescent quinolizidine-substituted spiropyran, which is able to switch in vivo and bind to guanine quadruplexes (G4) at physiological pH values, urged us to elucidate its molecular switching and binding mechanism. Combining multiscale dynamical studies and accurate quantum chemical calculations, we show that, both in water and in the G4 environment, the switching of the spiropyran ring is not promoted by an initial protonation event-as expected by the effect of low pH solutions-but that the deprotonated merocyanine form is an intermediate of the reaction leading to the protonated open species. Additionally, we investigate the binding of both deprotonated and protonated open forms of merocyanine to c-MYC G4s. Both species bind to G4s albeit with different hydrogen-bond patterns and provide distinct rotamers around the exocyclic double bond of the merocyanine forms. Altogether, our study sheds light on the pharmacophoric points for the binding of these probes to DNA, and thereby, contributes to future developments of new G4 binders of the remarkable family of quinolizidine-substituted spiropyrans.

Noncommutative Switching of Double Spiropyrans

The spiropyran family of photochromes are key components in molecular-based responsive materials and devices, e.g., as multiphotochromes, covalently coupled dyads, triads, etc. This attention is in no small part due to the change in properties that accompany the switch between spiropyran and merocyanine forms. Although the spiropyran is a single structural isomer, the merocyanine form represents a family of isomers (TTT, TTC, CCT, etc.) and protonation states. Combining two spiropyrans into one compound increases the number of possible structures dramatically and the interaction between the units determines, which are impeded due to intramolecular quenching of excited states. Here, we show that the coupling of two spiropyran photochromes through their phenol units yields favorable interactions (crosstalk) between the components that provides access to species inaccessible with the component monospiropyran alone. Specifically, the ring opening of one spiropyran unit, which is thermally stable at −30 °C, prevents ring opening of the second spiropyran unit. Furthermore, whereas protonated E- and Z-monomerocyanines were previously shown to undergo thermal- and photo-equilibration, the corresponding protonated E- and Z- bimerocyanines are thermally stable and show one-way photoisomerization from the Z,Z- to an emissive E,E-bimerocyanine form. Subsequent deprotonation at room temperature resets the system to the bispiro ring-closed form, but deprotonation at −30 °C yields the otherwise inaccessible bimerocyanine form. This form is photochemically inert but undergoes a two-step thermal relaxation via the merocyanine-spiropyran form, showing that the connection at the phenol units provides sufficient intramolecular interaction to fine-tune the complex isomerization pathways of spiropyrans and demonstrating noncommutability in photo- and pH-regulated multistep isomerization pathways.

Thermodynamics and kinetics of protonated merocyanine photoacids in water

Metastable-state photoacids (mPAHs) are chemical species whose photo-activated state is long-lived enough to allow for proton diffusion. Liao's photoacid (1) represents the archetype of mPAHs, and is being widely used on account of its unique capability to change the acidity of aqueous solutions reversibly. The behavior of 1 in water, however, still remains poorly understood. Herein, we provide in-depth insights on the thermodynamics and kinetics of 1 in water through a series of comparative 1H NMR and UV-Vis studies and relative modelling. Under dark conditions, we quantified a three-component equilibrium system where the dissociation (K a) of the open protonated form (MCH) is followed by isomerization (K c) of the open deprotonated form (MC) to the closed spiropyran form (SP) - i.e., in the absence of light, the ground state acidity can be expressed as K GS a = K a(1 + K c). On the other hand, under powerful and continuous light irradiation we were able to assess, for the first time experimentally, the dissociation constant (K MS a) of the protonated metastable state (cis-MCH). In addition, we found that thermal ring-opening of SP is always rate-determining regardless of pH, whereas hydrolysis is reminiscent of what is found for Schiff bases. The proposed methodology is general, and it was applied to two other compounds bearing a shorter (ethyl, 2) and a longer (butyl, 3) alkyl-1-sulfonate bridge. We found that the pK a remains constant, whereas both pK c and pK MS a linearly increase with the length of the alkyl bridge. Importantly, all results are consistent with a four-component model cycle, which describes perfectly the full dynamics of proton release/uptake of 1-3 in water. The superior hydrolytic stability and water solubility of compound 3, together with its relatively high pK GS a (low K c), allowed us to achieve fully reversible jumps of 2.5 pH units over 18 consecutive cycles (6 hours).

Nonadiabatic Dynamics Simulations on Early-Time Photochemistry of Spirobenzopyran

Photoinduced ring-opening, decay, and isomerization of spirobenzopyran have been explored by the OM2/MRCI nonadiabatic dynamics simulations based on Tully's fewest-switches surface hopping scheme. The efficient S₁ to S₀ internal conversion as observed in experiments is attributed to the existence of two efficient excited-state decay pathways. The first one is related to the C-N dissociation, and the second one is done to the C-O dissociation. The C-O dissociation pathway is dominant, and more than 90% trajectories decay to the S₀ state via the C-O bond-fission related S₁/S₀ conical intersections. Near these regions in the S₀ state, trajectories can either return to spirobenzopyran or proceed to various intermediates including merocyanine via a series of bond rotations. Our nonadiabatic dynamics simulations also demonstrate that the hydrogen-out-of-plane (HOOP) motion is important for efficient and ultrafast excited-state deactivation. On the other hand, we have also found that the replacement of methyl groups by hydrogen atoms in spirobenzopyran can artificially introduce different intramolecular hydrogen transfers leading to hydrogen-transferred intermediates. This finding is important for the community and demonstrates that such a kind of structural truncation, sometimes, could be problematic, leading to incorrect photodynamics. Our present work provides valuable insights into the photodynamics of spirobenzopyran, which could be helpful for the design of spiropyran-based photochromic materials.