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Aim. To summarize and analyze literature data on the polymethine dyes containing the 2,2-difluoro-1,3,2-dioxaborine ring.Results and discussion. Boron difluoride complex of β-diketone (2,2-difluoro-1,3,2-dioxaborine, F2DB) is a unique structural motif endowing organic compounds with prominent physicochemical properties, such as a strong fluorescence and high mo-lar attenuation coefficients. Incorporation of the F2DB core into a polymethine chromophore either as an end-group or as an integral part of the polymethine chain allows obtaining dyes with exceptional characteristics, highly appealing for design of up-to-date functional materials. This review focuses on the synthesis and spectral properties of the F2DB-containing polymethines along with the latest advancement in the synthesis of highly fluorescent polyanionic polymethines. A brief discussion of the effects of the structural modification of the π-conjugated system on the photophysical properties of dyes is included.Conclusions. The literature on the F2DB-containing polymethines demonstrates a high potential of the F2DB core for the development of strongly fluorescent and intensely absorbing dyes., Мета. Узагальнити та проаналізувати літературні дані про поліметинові барвники на основі 2,2-дифлуоро-1,3,2-діоксаборину.Результати та їх обговорення. Бородифлуоридний комплекс β-дикетону (2,2-дифлуоро-1,3,2-діоксаборин, F2DB) є унікаль-ним структурним елементом, який надає органічним сполукам особливі фізико-хімічні властивості, такі, як сильна флуоресценція та високі коефіцієнти молярної екстинкції. Уведення ядра F2DB до поліметинового хромофора як кін-цевої групи чи як складової частини поліметинового ланцюга дає змогу отримувати барвники з винятковими характе-ристиками, привабливі для розробки сучасних функціональних матеріалів. У цьому огляді висвітлено питання синтезу та спектральних властивостей поліметинів, що містять ядро F2DB, а також останні досягнення в синтезі високофлуо-ресцентних поліаніонних поліметинів. Додано коротке обговорення впливу структурних модифікацій π-спряженої системи на фотофізичні властивості барвників.Висновки. Аналіз літератури про поліметини, що містять ядро F2DB, свідчить про високий потенціал діоксаборино-вого комплексу для розробки барвників з яскравою флуоресценцією та інтенсивним світлопоглинанням.

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., A multilayer coating, with a different photochrome in each layer, enables predictable control over transmission and colour with a single wavelength of light, which is an important goal in functional optical materials and devices.

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.

This mini review presents a series of experiments in which light acts on two general phenomena: isomerization reactions and photoluminescence. The experiments described using the reversible photoactive molecular switch spiropyrane/merocyanine, is suited to teach fundamental concepts of photochemistry as well as to introduce innovative applications related to smart materials, molecular motors and logic gates.

Inflammation usually results in high-level reactive oxygen species (ROS) and reactive nitrogen species (RNS) not only in acidic tissue but also in alkaline tissue. However, noninvasively in vivo monitoring reactive species specifically within alkaline tissue remains a huge challenge. Here we introduce a dual activatable fluorescent probe PN910 located in the second near-infrared window (NIR-II, 900-1700 nm), which shows high selectivity toward H 2 O 2 and OONO - at pH beyond 7.4. Then we verified that PN910 could be used for the real-time, specific and accurate monitoring of cystitis and colitis for living animals. This report presents a unique approach to the development of dual activatable probe for in vivo biosensing.

The spiropyran derivatives are known for their good photochromic properties. These photochromic compounds are isomerized between two forms; merocyanine is the open-ring form and spiropyran is the closed-ring form. In this work, two spiropyran derivatives (compounds 1 and 2) with the N-alkynyl functional group are prepared. The click reactions of these spiropyran derivatives with 1-azido-4-nitrobenzene and (azidomethyl)benzene are studied. For this purpose, 3,3-dimethyl-2-methylene-1-prop-2-ynyl-2,3-dihydro-1H-indole is synthesized, which is then reacted with 3- and 5-nitrosalicylaldehyde. The FT-IR, 1H-NMR, and 13C-NMR spectra of the reaction products are used for their characterization . The solutions of the products in methanol are prepared, and their UV–visible spectra are investigated. The darkening of the crystals of compound 1 upon exposure to the UV irradiation indicates that the spiro C–O bond in this compound is weaker than that in compound 2, which is due to the presence of the nitro group in the para position relative to this bond, and consequently, the stabilization of the open-ring form.

This review is devoted to the features of isomerization reactions of indoline spiropyrans and the possibilities for their applications as building blocks in the design of molecular machines and ensembles capable of self-organization. Nanoscale systems functioning on the base of changes in geometric parameters and molecular dipole moment when switching between the spirocyclic and merocyanine forms have been considered. The factors that alter the system morphology may be electromagnetic irradiation or the acidity of the medium.

The concept of the ideal polymethine state lies at the heart of the theory of color of π-conjugated organic compounds. For merocyanines—neutral donor–acceptor polymethine dyes—an attainment of a structure close to the cyanine limit is usually traced in solutions by their solvatochromic behavior or 1H NMR spectra. Herein, we report an X-ray crystallographic study of a vinylogous series of merocyanines characterized by a close uniformity of the C − C-bond length in the polymethine chain in the solid state. It reveals that for merocyanines in the crystal the ideal polymethine state can be achieved via a judicious choice of the donor–acceptor properties of their end-groups. The polymethine chain length is found to have only a slight impact on the electronic symmetry of such dyes in the solid state, as distinct from both less and more dipolar merocyanines, the bond length alternation in the chromophore of which increases for higher vinylogues. The interaction energies between merocyanine molecules in the stacking dimers in the crystal have been analyzed using the Localized Molecular Orbital Energy Decomposition Analysis (LMOEDA) algorithm.

A visible light‐switchable buffer system based on a merocyanine photoacid is presented. Para‐substitution of the indolium side with a methoxy group affords a compound suitable for making hydrolytically stable aqueous buffers whose pH can be tuned between 7 and 4 using 500 nm light., We show that protonated merocyanines can act as reversible photoacids even when buffered with NaOH, attaining tunable and fully reversible jumps of up to 3 pH units between pH 7 and 4.

Spiropyran-based materials (SPBMs) can give responses to the stimulations induced by the light, heat, force, or pH, which have been used as triggers for many smart materials. Here, a cross-linkable SPBM containing mesogenic-units is synthesized, which is pale-colored, non-photoluminescent and non-mesogenic at a spiro form, but dark-colored, photoluminescent, and mesogenic at a merocyanine form. Moreover, the dynamic interconversion behavior of the form in the different chemical environments are distinct. Liquid crystalline polymers (LCPs) containing the SPBMs cross-linked via visible light, own a photoswitchable glass transition temperature (Tg ) and retain the switchable property; however, the SPBMs cross-linked via UV light will be locked at the MC state, because the molecular movement was frozen at the room temperature lower than the given Tg of the LCP. Thus, programmable chromism and photoluminescence based on the tunable Tg can be endowed to the functional materials prepared from the SPBMs.

Stimuli-responsiveness is an important characteristic that show promising potential in various applications. Herein, a novel ZIF-8-on-Tb-dpn (H3dpn = 5-(2',4'-dicarboxylphenyl)nicotic acid) heterostructure is constructed using a heteroepitaxial strategy combining the chemical-responsive (antibiotics) and light-responsive behaviors. The pyridine nitrogen of Tb-dpn acts as an anchor site for Zn2+, which helps to overcome the limit of lattice mismatch between two metal-organic frameworks (MOFs) and promotes the growth of ZIF-8 nanocrystals. Based on the synergy effect of two MOFs, ZIF-8-on-Tb-dpn exhibits an efficient turn-off response toward tetracycline and chloramphenicol via competitive absorption, Forster resonance energy transfer, and photoinduced electron transfer processes with limit of detection values of 5.6 and 37.6 nM, respectively, which are three- to -fivefold lower than those of Tb-dpn. Moreover, the nanocage of ZIF-8 is utilized to encapsulate photochromic spiropyran (SP) molecules and realize the reversible conversion between SP and merocyanine (MC) under visible light and ultraviolet light. The MC form is accompanied with strong adsorption at 555 nm, which can erase the emission of Tb3+. Therefore, a reversible invisible anticounterfeiting pattern is designed with SP ⊂ ZIF-8-on-Tb-dpn for information anticounterfeiting. The excellent stimuli-responsive ability makes the luminescent platform a potential candidate in luminescence applications.

Abstract Indoline spiropyran containing an σ-acceptor chlorine atom in 6′ position of the 2H-chromene part of the molecule was synthesized and studied. The use of 1,2,3,3-tetramethyl-3H-indolium perchlorate as a starting compound made it possible to achieve higher product yields as compared to previous studies. The molecular structure of the compound was established by single crystal X-ray diffraction analysis. The features of the crystal structure and intermolecular interactions were investigated using CrystalExplorer17 software package. The photochromic behavior in acetonitrile solution was studied for the first time. It was found that the merocyanine form of spiropyran is characterized by an absorption maximum at 592 nm, which is 37 nm closer to the range of the “biological window” in comparison with the nitro-substituted analog.

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 1H 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 1H 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., Controlled uptake and release: We investigate the photo‐ and acid‐induced isomerization of monomeric and homo‐dimeric tetra‐spiropyran tetra‐urea calix[4]arene and calix[4]pyrrole derivatives. The acid‐induced isomerization of the switches was coupled to the homo‐dimeric calix[4]arene capsule's disassembly and the release of the cationic guest to the bulk solution. The neutralization of the acidified solution with Et3N completely restored the original encapsulation complex.

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., Transient absorption studies of protonated pyridine‐ and nitro‐merocyanines reveal a pH‐gated photoacidic behaviour. The O‐protic site shows an ultrafast excited state super‐photoacid reactivity at low pH values that is even capable of proton transfer to alcoholic solvents. Remarkably, the pyridine compound exhibits an additional N‐protic site which functions as photoacid generator at pH 5.5 due to its lowered ground state pKa value after photoisomerization to the ring‐closed spiropyran structure.

Cyanine dyes have been used in industrial applications since their discovery over 200 years ago. Today, they are still viewed as highly valuable small molecules with applications in biotechnology and nanotechnology. One class of cyanine dyes is called merocyanines (MCs). MCs exhibit impressive sensitivity to their environment via negative solvatochromism as well as impressive fluorescent characteristics. Alone they can be employed to sense nucleophiles in solution by losing their bright colours upon addition of the nucleophile. Beyond sensing small molecules independently, they are capable of sensing different DNA topologies including duplex, quadruplexes (GQs), and three-way junctions (3WJs), and sensing these topologies can provide insight into essential biological processes. Their affinity to different DNA topologies can give rise to other applications involving them being utilized in aptamer platforms. DNA aptamers are a functional form of nucleic acids that can adopt shapes that can enable them to engage in target-specific binding. They are derived using an iterative process known as in vitro selection, where many DNA sequences are exposed to the target ligand, and those that do not bind the target are removed while the binders are isolated. This process has produced many different aptamers for a variety of targets (metals, small molecules, and proteins) capable of binding at low concentrations (nM to μM range). Aptamers have recently seen a rapid uptake in a broad array of biotechnological applications, due to their simple, economical, and efficient design. In this body of work, a novel family of MC dyes is synthesized, and their applications and properties are explored. In Chapter 2, temperature-sensitive thiolate sensing MCs will be investigated, and thermodynamic contributions characterized. These studies result in the development of Chapter 3 where a substituted phenolic merocyanine dye with a benzothiazolium acceptor is shown to be capable of selectively binding DNA 3WJs. The MC also exhibits impressive turn-on fluorescent signalling ability and has been utilized for sensitive small molecule detection in complex matrices. In Chapter 4, the MC family is expanded where the development and structure-activity relationship analysis results in improved sensing and binding characteristic for a variety of small molecule targets demonstrating merocyanine dye versatility. 2023-07-20

In this letter, we report a new method of controllable supramolecular polymerization, taking advantage of host-enhanced photodimerization. The low-molecular-weight supramolecular oligomers were formed by noncovalent complexation between cucurbit[8]urils (CB[8]) and the bifunctional monomers (DBN) with Brooker’s merocyanine moiety (MOED) on either end. Interestingly, when irradiated with UV light, the supramolecular oligomers could transform into supramolecular polymers with high molecular weight. The molecular weight of supramolecular polymers could be controlled by varying the irradiation time. It is highly anticipated that this work can enrich the methods on the modulation of supramolecular polymerization.

We report the discovery of a 2H-naphtho[1,2-b]pyran mechanophore that produces a permanent merocyanine dye upon mechanochemical activation, in contrast to the reversible product generated photochemically. Experiments suggest that the irreversibility of the mechanically generated merocyanine is due to a unique reaction in which the scission of an ester C–O bond reveals a β-hydroxy ketone that locks the merocyanine through an intramolecular H-bonding interaction. In addition to demonstrating the reactivity using solution-phase ultrasonication, permanent merocyanine generation is also achieved in solid polymeric materials. The permanent coloration achieved with the naphthopyran mechanophore affords unique opportunities for sensing and force-recording applications as well as fundamental studies limited by the reversibility of typical colorimetric force probes.

The spiropyran derivatives can exist in two forms, the closed-ring spiropyran form and the open-ring merocyanine (MC) form. The SP form could be converted into the MC form upon UV irradiation. In this work, some 1′,3′,3′-trimethylspiro[chromene-2,2′-indoline] derivatives containing the nitro and formyl groups are synthesized via the reaction of 1,3,3-trimethyl-2-methylene-5-nitroindoline with the corresponding salicylaldehyde derivatives. These compounds have different photochromic behaviors. The synthesized photochromic molecules are characterized by the FT-IR, 1H-NMR, and 13C-NMR spectroscopic techniques. In order to investigate the photochromic properties of these compounds, the UV–visible spectroscopic analyses of their methanolic solutions before and after exposure to a UV lamp (in the spectral range of 360–400 nm) are studied.

New spiropyrans with chromene and indoline cores bearing alkoxycarbonyl group and cationic π-acceptor in the chromene moiety were synthesized in one step from 1,2,3,3-tetramethylindolinium and isophthalic dialdehyde derivatives. The appending of a 8′-positioned conjugated cationic fragment causes a significant red-shift of the photoinduced isomers absorption maxima and provides them with extremely long lifetimes up to 75 min. The obtained spiropyrans exhibit both positive and negative photochromism in solutions, demonstrating the properties of ‘photochromic balance’.

UV-visible transient absorption spectroscopy and quantum mechanical simulations are combined to elucidate the photochemical mechanism of two metastable merocyanine/spiropyran photoacids, 2-[(E)-2-(2-hydroxyphenyl)ethenyl]-3,3-dimethyl-1-(3-sulfopropyl)-3H-indol-1-ium (phenylhydroxy-MCH) and 2-[(E)-2-(1H-indazol-7-yl)ethenyl]-3-(3-sulfopropyl)-1,3-benzothiazol-3-ium (indazole-MCH). Transient absorption spectra demonstrate that trans-acid isomerization to the cis form results in deprotonation on a picosecond time scale. Ring closure to form spiropyran follows promptly from the appropriate conformation or follows at longer time delays (≫3.5 ns) following a barrier crossing for single-bond isomerization to the appropriate conformation. Consistent with the results of Berton et al. [ Chem. Sci. 2020, 11, 8457-8468] , we find that cis-phenylhydroxy-MCH is a stronger acid than trans-phenylhydroxy-MCH. The decrease in pKa upon isomerization is further investigated to benchmark quantum chemical methods for their accuracy. Calculations were performed with nine levels of theory including continuum solvent models and explicit water. The calculations are not sufficient to describe the ΔpKa following isomerization of these photoacids, and more work is necessary to properly evaluate the physical basis for the acidity of the cis photoacids.

Nonsymmetric bis-spiropyran derivatives based on 1,3-dihydroxy-6-oxo-6H-benzo[c]chromene-2,4-dicarbaldehyde were synthesized. The obtained compounds with substituents R = H, Cl at positions 5,5′ of the indoline fragment in DMSO solution exist in a fully open merocyanine form. Their change for the electron-acceptor group NO2 results to the existence of the corresponding bis-spiropyran in the form of a tautomeric mixture of spirocyclic and merocyanine forms. The resulting compounds in DMSO solution undergo photoinduced cyclization. The nitro derivative demonstrates both positive and negative photochromism.

Herein we report an X-ray crystallographic characterization of tetramethine merocyanine based on benzimidazole and thiobarbituric acid, which shows negative solvatochromism in the entire polarity range of organic solvents. In the crystal, there are three types of non-equivalent dye molecules (A, B, C) and one water molecule in the independent part of the unit cell. The former constitute the AACBBC π-stacked columns, while the latter serves as a hydrogen-bonded bridge between the B and C molecules of the adjacent stacks. The negative bond-length alternation (BLA) values for the polymethine chain of all merocyanine species indicate a greater contribution of the zwitterionic limiting structure in their electronic structure in the crystal. The dipolarity is somewhat lesser for the molecule A, which does not form H-bonds with water. The DFT-B3LYP calculations of the studied merocyanine, its monohydrate, and the π-stacked dimer in vacuo and in a model environment (IEFPCM) have shown that a single hydrogen bond should have lesser effect on its electronic structure in the crystal than π-stacking and other multiple close-range interactions.

The fluorescence intensity of the ring-opened merocyanine (MC) form of spiropyran can be enhanced by metal nanostructures. However, direct contact between spiropyran and the metal results in fluore...

Summary In this study, the optical properties and excited-state dynamics of the unique self-assembled donor-acceptor (DA) merocyanine dye stacks from dimer up to octamer, prepared via dipole-dipole interactions, are reported in terms of coherent exciton dynamics and formation of an excimer-like state. Our findings are based on the steady-state absorption/emission, time-resolved fluorescence, and transient absorption (anisotropy) measurements, including wavepacket analysis and quantum mechanical calculations. Coherent exciton of torsional motions-restricted dye stacks rapidly localizes into the weakly emissive excimer-like state, by shortening the inter-moiety distance and changing the bond-length alternation pattern. The inner merocyanine moiety, having two neighboring units, has a reversed resonance character (non-polar (N) Z) in the ground state. This difference has led to two conclusions: (1) tetramers and octamers exhibit different features of excimer-like state than the dimer, and (2) octamers exhibit slower localization dynamics due to the enhanced homogeneity (six inner-moieties) compared with tetramers (two inner moieties).

A spectral-kinetic investigation was carried out to study the fluorescent properties of complexes of molecules of photochromic nitro-substituted spiropyran with metal cations differing in electron affinity in polymer matrices of various nature. It is shown that the photoinduced dynamic color change is determined by the change not only in the absorption, but also in the luminescence of complexes exhibiting negative photochromism. The role of proton complexes in the photochromic transformations of the merocyanine form of spiropyran was revealed.