Comprehensive Theoretical Study of the Conversion Reactions of Spiropyrans: Substituent and Solvent Effects

Photo-responsive anti-fouling polyzwitterionic brushes: a mesoscopic simulation

The antifouling effects of a toothbrush-shaped photo-responsive polyzwitterionic membrane were studied via dissipative particle dynamics simulations in this work. The results reveal that the membrane modified by spiropyran methacrylate brushes displays photo-switchable and antifouling capability due to the photo-induced ring-opening reaction. Namely, surface morphology and hydrophilicity change in response to visible or UV light irradiation, which can be observed visually by protein adsorption and desorption. Further study indicates that: (1) brush-modification density can influence the structure and properties of the membrane. With low modification density, systems cannot establish an intact selective layer, which hinders the antifouling ability; as the modification density increases, the intact selective layer can be formed, which is conducive to the expression of photo-responsiveness and antifouling capability. (2) Factors of toothbrush-hair length and grafting ratio can influence the establishment of a light-responsive surface: as the grafting ratio and toothbrush-hair length increase, the light-responsive surface is gradually formed, meanwhile, the antifouling ability can be continuously reinforced under UV light irradiation. (3) As the brushes switch into a zwitterionic merocyanine state under UV exposure, the selective layer swelling becomes stronger than that with a hydrophobic spiropyran state under visible exposure. This is owing to the enhanced interaction between zwitterionic brushes and water, which is the root of the antifouling effect. The present work is expected to provide some guidelines for the design and development of novel antifouling membrane surfaces.

Is this how bromine spiropyran salt is converted to merocyanine under UV irradiation? A look through the prism of quantum chemical calculations

The stability of merocyanine forms formed under UV irradiation of a solution of a spiropyran salt, in which an organic part acts as a cation and a compact bromide ion as an anion, their photophysical properties, and the formation mechanism are studied in this work using time-dependent density functional theory. Theoretical calculations show that TTC and CTT are the most stable open forms (the difference in stability energies is 10.5 and 12.0 kcal mol-1 relative to the formation energy of spiropyran, respectively). The simulated absorption bands in the UV spectrum of the merocyanine forms are observed both in the UV region at 308-366 nm and in the visible region at 544-757 nm due to n → π* and π → π* type transitions. We found that the isomerisation mechanism of spiropyran into merocyanine forms includes two key stages: the ring opening to form cisoid merocyanine forms (except unstable TCC) through conical intersection and their subsequent isomerisation to form stable transoid isomers. The length of the Cspiro-O bond is 1.97 Å and the C1'-C2'-C3'-C4' angle is 70° in the structure close to conical intersection. The stage that determines the rate of this process is the isomerisation between transoid forms, as in the case of transformation of open merocyanine forms into spiropyran.

DELFI: a computer oracle for recommending density functionals for excited states calculations

Density functional theory (DFT) is the workhorse of computational quantum chemistry. One of its main limitations is that choosing the right functional is a non-trivial task left for human experts. The choice is particularly hard for excited state calculations when using its time-dependent formulation (TD-DFT). This is due to the approximations of the method, but also because the photophysical properties of a molecule are defined by a manifold of states that all need to be properly described. This includes not only the relative energy of the states, but also capturing the correct character, order, and intensity of the transitions. In this work, we developed a neural network to recommend functionals to be used on molecules for TD-DFT calculations, by simultaneously considering all these properties for a manifold of states. This was possible by developing a scoring system to define the accuracy of an excited state's calculation against a higher-accuracy reference. The scoring system is generalizable to any level of theory; we here applied it to evaluate the performance of common functionals of different rungs against a higher accuracy method on a large set of organic molecules. The results are collected in a database that we released and made open, providing four million data points to the community for future applications. The scoring system assigns a value between zero and one hundred to each functional for each molecule, transforming the complicated task of learning photophysical properties into a simpler regression task. We used the dataset to train a graph attention neural network to predict the scores for unseen molecules. We call this oracle DELFI (Data-driven EvaLuation of Functionals by Inference), which can be used to quickly screen and predict the ranking of functionals to calculate the optical properties of organic molecules. We validated DELFI in two in silico experiments: choosing a common functional for a series of spiropyran-merocyanine isomers and a unique functional to screen a large dataset of over 50 000 organic photovoltaic molecules, for which an extensive benchmark would be unfeasible. A corresponding web application allows DELFI to be easily run and the results to be analyzed, alleviating the hurdle of choosing the right functional for TD-DFT calculations.

Solvent Polarity Effects on the Mechanochemistry of Spiropyran Ring Opening

The spiropyran mechanophore (SP) is employed as a reporter of molecular tension in a wide range of polymer matrices, but the influence of surrounding environment on the force-coupled kinetics of its ring opening has not been quantified. Here, we report single-molecule force spectroscopy studies of SP ring opening in five solvents that span normalized Reichardt solvent polarity factors (ETN) of 0.1-0.59. Individual multimechanophore polymers were activated under increasing tension at constant 300 nm s-1 displacement in an atomic force microscope. The extension results in a plateau in the force-extension curve, whose midpoint occurs at a transition force f* that corresponds to the force required to increase the rate constant of SP activation to approximately 30 s-1. More polar solvents lead to mechanochemical reactions that are easier to trigger; f* decreases across the series of solvents, from a high of 415 ± 13 pN in toluene to a low of 234 ± 9 pN in n-butanol. The trend in mechanochemical reactivity is consistent with the developing zwitterionic character on going from SP to the ring-opened merocyanine product. The force dependence of the rate constant (Δx‡) was calculated for all solvent cases and found to increase with ETN, which is interpreted to reflect a shift in the transition state to a later and more productlike position. The inferred shift in the transition state position is consistent with a double-well (two-step) reaction potential energy surface, in which the second step is rate determining, and the intermediate is more polar than the product.

Light-Switchable Biocatalytic Covalent-Organic Framework Nanomotors for Aqueous Contaminants Removal

Self-propelled nanomotors represent a promising class of adaptable and versatile technologies with broad applications in the realms of biomedicine and environmental remediation. Herein, we report a biocatalytic nanomotor based on a covalent-organic framework (COF) that demonstrates intelligent and switchable motion triggered by a blue-to-red light switch. Consequently, when exposed to blue light, the nanomotor significantly enhances the removal of contaminants in aqueous solutions due to its elevated mobility. Conversely, it effectively deactivates its motion and contaminant removal upon exposure to red light. This study explores the heterogeneous assembly strategy of the COF-based nanomotor and its light-controlled propulsion performance and provides a novel strategy for the regulation of movement, offering valuable insights for the design and practical applications of nanomotors.

New cationic spiropyrans with photoswitchable NIR fluorescence

Visible-light-mediated photochromic compounds with NIR absorption and fluorescence are of great interest for use in different biomedical applications. In this work, new representatives of spiropyrans with conjugated cationic 3H-indolium substituents in different positions of 2H-chromene moiety were synthesized. The electron-donating methoxy groups were introduced in the uncharged indoline and charged indolium cycles to form the effective conjugation chain between the hetarene moiety and the cationic fragment for reaching NIR absorption and fluorescence. The molecular structure and the effects of cationic fragment position on the mutual stability of the spirocyclic and merocyanine forms of compounds were carefully studied in the solutions and solid state by NMR, IR, HRMS, single-crystal XRD, and quantum chemical calculations. It was found that the obtained spiropyrans demonstrate positive or negative photochromism depending on the cationic fragment's position. One of spiropyrans has shown bidirectional photochromic properties induced exclusively by visible light of different wavelengths in both directions. The photoinduced merocyanine forms of compounds possessed far-red shifted absorption maxima and NIR fluorescence, which makes them prospective fluorescent probes for bioimaging.

Polarity-Driven Isomerization of a Hydroxynaphthalimide-Containing Spiropyran at Room Temperature

Design of spiropyrans showing spontaneous isomerization driven by the polarity of solvents is an important consideration for the synthesis of optical sensory materials. Although some spiropyrans undergo polarity-driven isomerization, they must be heated owing to the high activation energy required for isomerization. In this study, we describe that a spiropyran containing a hydroxynaphthalimide unit (1) exhibits a polarity-driven isomerization at room temperature. It exists as a colorless spirocyclic (SP) form in less polar solvents but is isomerized to a colored merocyanine (MC) form in polar solvents. The equilibrium amount of the MC form increases with an increase in the polarity of solvents. The MC form involves two resonance structures-the quinoidal and zwitterionic forms. In polar media, the zwitterionic form dominates mainly owing to solvation by polar molecules. Solvation stabilizes the negative charge of the zwitterionic form and decreases its ground state energy, thereby enhancing SP → MC isomerization. The SP ⇌ MC isomerization terminates within barely 30 s even at room temperature because the naphthol moiety with high π-electron density lowers the activation energy for the rate-determining rotational step.

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.

Reversible light-controlled fluorescence switch of block polymer-grafted carbon dots and cellular imaging

A novel type of aggregation-induced emission (AIE) nanoparticles, which are carbon dots (CDs) grafted with block polymer of tetraphenylethylene, spiropyran and N-isopropylacrylamide (CD-g-poly((TPE-co-SPA)-block-NIPAM)), was synthesized. The CD-g-poly((TPE-co-SPA)-block-NIPAM) nanoparticles can emit weak cyan fluorescence in tetrahydrofuran, while showing AIE-enhanced cyan fluorescence in water and solid film. The fluorescence of the CD-g-poly((TPE-co-SPA)-block-NIPAM) nanoparticles can reversibly transform cyan to red with UV/visible light irradiation, and functioned as a reversible fluorescence photoswitch. Importantly, the CD-g-poly((TPE-co-SPA)-block-NIPAM) nanoparticles have low cytotoxicity and, therefore, can be used for imaging in living cells.

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.

Photochromism of a Spiropyran in Low-Temperature Matrices: Unprecedented Bidirectional Switching between a Merocyanine and an Allene Intermediate

Photochromism of spiropyrans has attracted much attention due to its potential in many light-controlled system applications. However, several fundamental aspects regarding the structure, energetics, and mechanistic details of the transformations of spiropyrans are still not well understood. Here, we report the study of the photochromism of a 6-hydroxy-spiropyran (HBPS) under conditions of matrix isolation, where monomers of the compound are frozen in a solidified noble gas (krypton, at 15 K). The structure of the matrix-isolated HBPS was first elucidated by infrared (IR) spectroscopy supported by density functional theory computations. Then, the photochromism of HBPS, from the colorless spiropyran to the colored merocyanine, was induced by ultraviolet (UV) irradiation at 310 nm. The analysis of the IR spectrum of the photoproduced species revealed the exclusive formation of the most stable merocyanine MC-TTC stereoisomer. Subsequent visible-light (550 nm) irradiation of MC-TTC generated a new colorless allenic isomeric species ALN, where the UV irradiation (310 nm) of ALN was found to convert this species back to MC-TTC. This constitutes an unprecedented bidirectional transformation between a colored merocyanine and a colorless allene species. The newly observed photoswitching reaction (or photochromism) occurs along an intramolecular hydrogen bond existing in both merocyanine and allenic species, thus suggesting that it might be generally feasible in the chemistry of spiropyrans. On the other hand, the usual assumption that, as a general rule, merocyanines photochemically revert to spiropyrans is not supported in this work.

Mechanochromophore-linked Polymeric Materials with Visible Color Changes

Mechanical force as a type of stimuli for smart materials has obtained much attention in the past decade. Color-changing materials in response to mechanical stimuli have shown great potential in the applications such as sensors and displays. Mechanochromophore-linked polymeric materials, which are a growing sub-class of these materials, are discussed in detail in this review. Two main types of mechanochromophores which exhibit visible color change, summarized herein, involve either isomerization or radical generation mechanisms. This review focuses on their synthesis and incorporation into polymer matrices, the type of mechanical force used, factors affecting the mechanochromic properties, and their applications. This article is protected by copyright. All rights reserved.

Spontaneous Isomerization of a Hydroxynaphthalene-Containing Spiropyran in Polar Solvents Enhanced by Hydrogen Bonding Interactions

The synthesis of spiropyran dyes exhibiting solvent-driven isomerization even in the dark condition is an important subject for the design of optical materials. A conventional synthesis strategy involves the conjugation of indoline moieties with electron-deficient aromatic moieties. Herein, we report that a spiropyran conjugated with a hydroxynaphthalene moiety (1) is a new member exhibiting solvent-driven isomerization, even bearing an electron-donating -OH moiety. The dye exists as a colorless spirocyclic (SP) form in nonpolar media. It, however, shows a blue color in polar media, especially in aqueous media, due to the formation of ring-opened merocyanine (MC) forms, where the isomerization terminates in 10 s even at room temperature. The spontaneous SP → MC isomerization originates from the MC forms stabilized by the highly delocalized π-electrons on the hydroxynaphthalene moiety. The solvation in polar media and the hydrogen bonding interaction with water molecules decrease the ground-state energy of the MC forms, triggering spontaneous isomerization. The dye exhibits two MC absorption bands assigned to the trans-trans-cis (TTC) and cis-trans-cis (CTC) isomers. The absorbance of the CTC band increases more significantly with an increase in the water content, and the increase exhibits a linear relationship with a hydrogen-bond donor acidity of solvents. The phenolate oxygen of the CTC form has larger hydrogen-bond acceptor basicity, resulting in stronger stabilization by the water molecule.

Modulating the Optical Characteristics of Spiropyran@Metal-Organic Framework Composites as a Function of Spiropyran Substitution

Understanding the interactions between the single components of hybrid systems is essential to drive the development of advanced functional materials. A prerequisite for this is the systematic variation of the building blocks of such compounds. Focusing on spiropyran@metal-organic framework (MOF) composite materials with noncovalently attached spiropyran dyes, both the host scaffold and the dye molecules can be systematically tuned. In this work, a broad substitution pattern was applied to systematically elucidate the characteristics of the resulting hybrid materials as a function of the supplemental substitution on spiropyran. The newly developed 12 composites exhibit substitution and host-dependent optical characteristics, which are particularly affected by the substitution of the 6'-position on the chromene ring. Through the favorable combination of the MOF host's polarity and an adequate strength of the spiropyran's indoline_donor-chromene_acceptor pair, reversible conversion between photoisomers is efficiently accomplished, especially for nitro-substituted spiropyrans inside MIL-68(In).

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.

Substituent Effects Control Spiropyran-Merocyanine Equilibria and Mechanochromic Utility

Spiropyran (SP) derivatives can be converted into the colored merocyanine (MC) form using a variety of triggers. Optical switching by light for memories and dynamic systems is long known. Recently, mechanical force has been reported as an additional stimulus that converts SP into MC. SP-based mechanochromic systems are especially interesting for polymer scientists, as the covalent attachment of polymer chains is ideal to transduce force to the SP level. Whether such materials are investigated to better understand fundamental processes or long standing questions in polymer science, to design force sensors or to self-report damage, or simply pose fascinating materials which turn colored upon deformation, they have intrigued polymer scientists for more than a decade. With the chemistry of SPs being feasible and SP functionalization important to modulate SP/MC equilibria, a significant amount of work on SP structure- mechanochromic function relations has accumulated. SPs can be used as bifunctional initiators, cross-linkers, monomers, or be synthesized during polymerization. This feature article provides an overview of how the chemistry used sets the boundaries within which the mechanochromic response of SP containing polymers can be modulated.

Structural and Electronic Characterization of a Photoresponsive Lanthanum(III) Complex Incorporated into Electrospun Fibers for Phosphate Ester Catalysis

Herein, we present the light-induced synthesis and characterization of a La³⁺/spiropyran derivative complex (LaMC) and its application as a catalyst when incorporated into electrospun polycaprolactone (PCL) fibers. In addition to experimental methods, computational calculations were also essential to better understand the structure and electronic characteristics of LaMC. The LaMC complex was identified as a 10-coordinated structure with the La³⁺ ion coordinated by four oxygens from the phenolate and the carbonyl of the carboxyl acid group from both MC ligands and by six oxygens from three nitrate ligands. In addition, LaMC was capable of getting reversibly isomerized by UV or visible light cycling. All PCL fibers were successively obtained, and their morphologies, surface properties, and catalytic behavior were studied. Results showed that PCL/LaMC fibers were capable of catalyzing bis(2,4-dinitrophenyl)phosphate degradation efficiently. Complete hydrolysis was accomplished in only 1.5 days relative to the half-life time of 35 days for the uncatalyzed hydrolysis at pH 8.1 and 25 °C.

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.

Design of Collective Motions from Synthetic Molecular Switches, Rotors, and Motors

Precise control over molecular movement is of fundamental and practical importance in physics, biology, and chemistry. At nanoscale, the peculiar functioning principles and the synthesis of individual molecular actuators and machines has been the subject of intense investigations and debates over the past 60 years. In this review, we focus on the design of collective motions that are achieved by integrating, in space and time, several or many of these individual mechanical units together. In particular, we provide an in-depth look at the intermolecular couplings used to physically connect a number of artificial mechanically active molecular units such as photochromic molecular switches, nanomachines based on mechanical bonds, molecular rotors, and light-powered rotary motors. We highlight the various functioning principles that can lead to their collective motion at various length scales. We also emphasize how their synchronized, or desynchronized, mechanical behavior can lead to emerging functional properties and to their implementation into new active devices and materials.

Chiral Symmetry Breaking of Spiropyrans and Spirooxazines by Dynamic Enantioselective Crystallization

Dynamic enantioselective crystallization enabled the chiral symmetry breaking of two spiropyrans and one spirooxazine. The three spiro compounds afforded racemic conglomerate crystals, and easily racemized in alcoholic solution without irradiation. Optically pure enantiomorphic crystals were obtained by vapor-diffusion crystallization or attrition-enhanced deracemization (Viedma ripening). Their absolute configurations were determined by single-crystal X-ray analysis and each enantiomorphic crystal was correlated with its solid-state circular dichroism (CD) spectrum.

DNA-binding mechanism of spiropyran photoswitches: the role of electrostatics

The binding mechanism of the protonated open form of three spiropyran derivatives into a 12-mer (poly-dAT)₂ has been unveiled by means of computational methods.

The binding mechanism of the protonated open form of three spiropyran derivatives into a 12-mer (poly-dAT)₂ has been unveiled by means of computational methods. It is found that the electrostatic term in the probe:DNA binding energy, modulates the binding mode, providing new guidelines for the design of spiropyran photoswitches with specific binding modes to DNA.

Roles of Water Molecules and Counterion on HS- Sensing Reaction Utilizing a Pyrylium Derivative: A Computational Study

In this paper, we present a comprehensive computational study on the hydrogen sulfide ion (HS^-) sensing mechanism in aqueous solution using pyrylium-thiopyrylium transformation. Explicit water molecules up to three water molecules are considered using supramolecular models. The effect of water bulk solvent is also taken into account according to the polarizable continuum model. Our results demonstrate that water molecules are directly involved in the sensing reactions by altering reaction mechanisms and dramatically lower the activation energies. The most favorable HS^- sensing mechanism involves a 10-membered ring transition structure formed by three water molecules and one hydronium. The catalytic effects of water molecule(s) due to the alleviation of ring strain and the stabilization from deprotonated hydronium significantly lower the activation energy. The activation energies in aqueous solution decrease from 40.2 kcal/mol for the hydronium-only-catalyzed reaction to 15.7, 14.8, and 7.4 kcal/mol for one-water-, two-water-, and three-water-catalyzed mechanisms, respectively. In addition, the effect of the counterion tetrafluoroborate (BF₄^-) on the reaction mechanisms was also investigated. Our results demonstrate that the counterion BF₄^- most likely behaves as a spectator and has minor influence on the reaction mechanism.

Tuning the photoreactivity of Z-hexatriene photoswitches by substituents - a non-adiabatic molecular dynamics study

To understand how substituents can be used to increase the quantum yield of photochemical electrocyclic ring-closing of the Z-hexa-1,3,5-triene (HT) photoswitch forming cyclohexadiene (CHD), we investigate the S1 photo dynamics of HT and its derivatives 2,5-dimethyl-HT (DMHT), 2-isopropyl-5-methyl-HT (2,5-IMHT), 1-isopropyl-4-methyl-HT (1,4-IMHT), and 2,5-diisopropyl-HT (DIHT) using time-dependent density functional theory surface hopping dynamics. We report detailed photoproduct distributions, formation mechanisms, branching ratios, and wavelength-dependent product quantum yields. Most products have been confirmed experimentally and include all-trans HT derivatives, cyclopropanes, cyclobutenes, cyclopentene, cyclohexadienes, and bicyclic compounds. Regarding CHD formation, we find that for the 2,5-substituted derivatives DMHT, 2,5-IMHT, and DIHT, the branching ratios increase with increasing size of the substituents. In contrast the branching ratios of the E/Z-isomerization decrease with increasing size of the substituents. Due to steric interactions, increasing the size of the substituents increases the amount of gZg rotamers in the ground state, which are prone to CHD formation and have lower E/Z-isomerization probability. Furthermore, we find [1,4], [1,5], and [1,6]-sigmatropic hydrogen shift reactions occurring at large percentages (5% to 15%); for sterical reasons these reactions stem from tZg conformers. DIHT shows the lowest percentage of side product formation among the 2,5-substituted molecules and highest CHD branching ratio; its CHD quantum yield can be increased up to more than 64%, by excitation of DIHT on the red tail of its absorption spectrum, whereas the Z/E-isomerization is reduced below 5% and side reactions practically vanish. This makes DIHT the best candidate for applications in molecular switches.

Substituent Effects and Mechanism in a Mechanochemical Reaction

We report the effect of substituents on the force-induced reactivity of a spiropyran mechanophore. Using single molecule force spectroscopy, force-rate behavior was determined for a series of spiropyran derivatives substituted with H, Br, or NO₂ para to the breaking spirocyclic C-O bond. The force required to achieve the rate constants of ∼10 s^-1 necessary to observe transitions in the force spectroscopy experiments depends on the substituent, with the more electron withdrawing substituent requiring less force. Rate constants at 375 pN were determined for all three derivatives, and the force-coupled rate dependence on substituent identity is well explained by a Hammett linear free energy relationship with a value of ρ = 2.9, consistent with a highly polar transition state with heterolytic, dissociative character. The methodology paves the way for further application of linear free energy relationships and physical organic methodologies to mechanochemical reactions, and the characterization of new force probes should enable additional, quantitative studies of force-coupled molecular behavior in polymeric materials.

Ultrafast Spintronics: Dynamics of the Photoisomerization-Induced Spin-Charge Excited-State (PISCES) Mechanism in Spirooxazine-Based Photomagnetic Materials

The optical control of spin state is of interest in the development of spintronic materials for data processing and storage technologies. Photomagnetic effects at the single-molecule level have recently been observed in the thin film state at 300 K in photochromic cobalt dioxolenes. Visible light excitation leads to ring-closure of a photochromic spirooxazine bound to a cobalt dioxolene, which leads to generation of a high magnetization state. Formation of the photomagnetic state occurs through a photoisomerization-induced spin-charge excited-state process and is dictated by the spirooxazine ligand dynamics. Here, we report a mechanistic investigation by ultrafast spectroscopy in the UV-vis region of the photochemical ring-closing process in the parent spirooxazine, azahomoadamantylphenanthroline spirooxazine, and the photomagnetic spirooxazine cobalt-dioxolene complex. The cobalt appears to stabilize a photomerocycanine transient intermediate, presumably the TCC isomer, formed along the ground-state potential energy surface (PES). Structural changes associated with the TCC isomer induces formation of the high-spin Co(II) form, suggesting that magnetization dymanics can occur along the excited-state PES, leading to ultrafast switching on the ps time scale. We demonstrate the full ring closure of the spiro-oxazine ligand is not required to switch magnetization states which can be induced with a higher yielding isomerization reaction. The ability of this system to undergo optically induced spin state switching on the ps time scale in the solid state makes it a promising canididate for resistive nonvolatile memory technologies.

Nanosecond laser flash photolysis of a 6-nitroindolinospiropyran in solution and in nanocrystalline suspension under single excitation conditions

Nanosecond transient absorption spectroscopy was used to study the photochemical ring-opening reaction for a 6-nitroindolinospiropyran (SP1) in solution and in nanocrystalline (NC) suspension at 298 K. We measured the kinetics in argon purged and air saturated acetonitrile and found that the presence of oxygen affected two out of the three components of the kinetic decay at 440 nm. These are assigned to the triplet excited states of the Z- and E-merocyanines (3Z-MC* and 3E-MC*). In contrast, a long-lived growth component at 550 nm and the decay of a band centered at 590 nm showed no dependence on oxygen and are assigned, respectively, to the ground state Z- and E-merocyanines (Z-MC0 and E-MC0). Laser flash photolysis studies performed in NC suspensions initially showed a very broad, featureless absorption spectrum that decayed uniformly for ca. 70 ns before revealing a more defined spectrum that persisted for greater than 4 ms and is consistent with a mixture of the more stable Z- and E-MC0 structures. We performed quantum mechanical calculations on the interconversion of E- and Z-MCs on the S0 and S1 potential energy surfaces. The computed UV-vis spectra for a scan along the Z → E interconversion reaction coordinate show substantial absorptivity from 300-600 nm, which suggests that the broad, featureless transient absorption spectrum results from the contribution of the transition structure and other high-energy species during the Z to E isomerization.

Modulation of the carboxamidine redox potential through photoinduced spiropyran or fulgimide isomerisation

Carboxamidines functionalized with either a spiropyran or fulgimide photoswitch were prepared on multigram scales. The thermal, electrochemical, and photochemical ring isomerizations of these compounds were studied and the results compared with related systems. The photochemical isomerisations were found to be reversible and could be followed by 1H NMR and UV-vis spectroscopy. The spiropyran/merocyanine couple was thermally active and an activation enthalpy of 116 kJ mol-1 was measured for ring-opening. These measurements yielded an enthalpy difference of 25 kJ mol-1 between the open and closed states which is consistent with DFT calculations. DFT calculations predicted a charge transfer to the carboxamidine group upon ring closure in the fulgimide and a charge transfer from the carboxamidine group upon switching the spiropyran to the merocyanine form. This was confirmed experimentally by monitoring the change in the oxidation potential assigned to the carboxamidine group. The potential of these molecules to therefore act as a new class of photoresponsive ligands that can modulate the ligand field of a complex is discussed.

Role of Solvent Polarity and Hydrogen-Bonding on Excited-State Fluorescence of 3-[(E)-{4-[Dimethylamino]benzylidene}amino]-2-naphthoic Acid (DMAMN): Isomerization vs Rotomerization

The present experimental and theoretical study on a new chromophore DMAMN of the type push-π-pull (push = dimethylaniline, π = imine, pull = 2-naphthoic acid), allows understanding of the mechanism by which the molecular conformational undergoes isomerization/rotomerization following electronic excitation. The steady-state fluorescence spectra of this compound, carried out in solvents of different polarities and proticities, showed significant changes in both the shape and peak positions. The wavelength and intensity change depend on the polarity and hydrogen-bonding environment. In highly polar solvents, the emission is weak and red-shifted compared to that for cyclohexane, but it is more red-shifted in moderate aprotic polar solvents. In hydroxyl solvents, a new weak low-energy emission band appears at ∼525 nm, attributed to the intermolecularly H-bonded open conformer. On the basis of the generated potential energy landscapes of the ground state and low-lying excited state in the gas phase and solution, we found that selective photon absorption, brings this molecule to a "bright" state, from which N═C isomerization Z → E, takes place. This isomerization in gas-phase and low-polarity solvents leads to two minima with a barrier, whereas in highly polar-protic media, it gives one minimum on the S₁ surface with low ΔE_S1/T1 (0.17 eV), facilitating deactivation via ISC.

Spironaphthoxazine switchable dyes for biological imaging

Recent developments in super-resolution microscopy have significantly expanded the requirements for switchable dyes, leading to demand for specially designed molecular switches. We report the synthesis and characterization of a spironaphthoxazine photochromic switch (a derivative of palatinate purple) displaying high photoconversion (85-95%) under readily accessible 405 nm light, broad absorption in the visible, and excellent fatigue resistance. The indole substituent on this spironaphthoxazine is twisted out of conjugation with the naphthalene unit, yet it is crucial for activation with visible light. The open colored merocyanine form of the spironaphthoxazine reverts to the closed form with a lifetime of 4.7 s in dichloromethane at 20 °C; this thermal reversion is even faster in more polar solvents. The photochemical quantum yields for ring-opening and ring-closing are approximately 8% and 1%, respectively, in dichloromethane. The ring-opening and ring-closing reactions have been characterized by time-resolved infrared and transient absorption spectroscopies. Ring opening occurs rapidly (τ = 2.1 ns) and efficiently (∼90%) from the singlet excited state to form an intermediate (assigned as a cisoid merocyanine), which returns to the closed ground state (τ = 4.5 ns) in competition with relaxation to the transoid open form (τ = 40 ns). Photochemical ring closing is a faster and simpler process: the excited state proceeds to the closed spirooxazine with a time constant of 0.28 ns. This photochromic switch can be used in conjunction with commercial fluorescent dyes to create a small-molecule switchable fluorescent dyad that shows high contrast and good fatigue resistance in living cells. These properties make the dyads suitable for application in RESOLFT microscopy.

Reversible chromism of spiropyran in the cavity of a flexible coordination cage

Confining molecules to volumes only slightly larger than the molecules themselves can profoundly alter their properties. Molecular switches-entities that can be toggled between two or more forms upon exposure to an external stimulus-often require conformational freedom to isomerize. Therefore, placing these switches in confined spaces can render them non-operational. To preserve the switchability of these species under confinement, we work with a water-soluble coordination cage that is flexible enough to adapt its shape to the conformation of the encapsulated guest. We show that owing to its flexibility, the cage is not only capable of accommodating-and solubilizing in water-several light-responsive spiropyran-based molecular switches, but, more importantly, it also provides an environment suitable for the efficient, reversible photoisomerization of the bound guests. Our findings pave the way towards studying various molecular switching processes in confined environments.

Inheritance of Photochromic Properties of Nitro-Substituted and Halogenated Spiropyrans Containing the Pyrrolidino[60]fullerene

The photophysical and isomerization properties of hybrid molecular compounds that consist of photochromic nitro-substituted and halogenated spiropyran derivatives bonded to the surface of the [60]fullerene cage through the pyrrolidine bridge were investigated using various functionals and basis sets of TD-DFT and semiempirical quantum-chemical approaches. The role of nπ* states formed by the lone pairs of substituents in changing of the electronic structure and photochromic properties of spiropyran derivatives was evaluated. The S_ππ(spiropyran) → intermediate nπ* states → S_ππ(merocyanine) channel for phototransformation of the hybrid compound containing a nitro-substituted spiropyran moiety was established and compared with similar systems of halogenated spiropyrans attached to the [60]fullerene bulk where photoinduced isomerization does not process due to high probability of internal conversion from the excited electronic state localized on the spiropyran fragment to the states of the pyrrolidino[60]fullerene.

Lanthanide complexes of spiropyran photoswitch and sensor: spectroscopic investigations and computational modelling

A number of novel lanthanide (Gd³⁺, Sm³⁺, and Tb³⁺) complexes of the 1′,3′-dihydro-8-methoxy-1′,3′,3′-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-(2H)-indole] (spiropyran; SP), a widely studied molecular photoswitch, were investigated.