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In this work, we examine the use of environment-sensitive fluorescent dyes in fluorescence lifetime imaging microscopy (FLIM) biosensors. We screened merocyanine dyes to find an optimal combination of environment-induced lifetime changes, photostability, and brightness at wavelengths suitable for live-cell imaging. FLIM was used to monitor a biosensor reporting conformational changes of endogenous Cdc42 in living cells. The ability to quantify activity using phasor analysis of a single fluorophore (e.g., rather than ratio imaging) eliminated potential artifacts. We leveraged these properties to determine specific concentrations of activated Cdc42 across the cell.

Recently the need for smart materials has been extremely growing, and the development of materials with appropriate characteristics has become one of the main challenges. Heptamethine cyanine dyes are widely used in theranostics and photovoltaics due to their remarkable optical properties. However, they are characterized by "rigid" non–changable properties. The main aim of the current study is to unveil correlation between structure and optical properties in the range of vinyl‐3H‐indolium substituted spiropyrans, which can be considered as the closest analogs of heptamethine cyanines with the photocontrolable optical and fluorescent properties. The effects of substitution in indoline and 2H‐chromene cycles, as well as anion change are under the study. To achieve this goal, a series of spiropyran iodides and perchlorates with methyl, methoxy substituents or fluorine atom in position 6′, as well as a conjugated vinyl‐3H‐indolium fragment in position 8′, were synthesized. The structure of the compounds was carefully studied using SC XRD, NMR, IR and mass spectroscopy. Then, studies on absorption and photoluminescence characteristics, as well as photochromic behavior were carried out. [ABSTRACT FROM AUTHOR]

Merocyanines (MC) usually adopt ring opened zwitterionic structures that are interconvertible with their ring‐closed spiropyran photoisomers. By methylating the phenolate oxygen, and thereby blocking the ring‐closure reaction, a cis/trans‐only MC photoswitch was obtained, yielding a perfect candidate for a detailed examination of the cis/trans isomerization mechanism for this class of compounds. This photoswitch displays outstanding properties including excellent photoreaction quantum yields and photoswitching turnovers. Due to the central polymethine bridge of MC, in principle eight cis (C)/trans/(T) isomers are possible. Density Functional Theory (DFT) calculations revealed the CCT and TTT‐isomers of the studied compound as most stable cis and trans ground state isomers, respectively. UV/vis transient absorption studies combined with conical intersection computations with the complete active space self‐consistent field (CASSCF) method show that both trans/cis‐ and cis/trans‐photoisomerizations are initiated by a rotation of the central doubled bond fragment. A hot ground state species is then formed, which undergoes a second isomerization. Thus, the cis/trans reaction proceeds via a CCT‐CTT‐TTT sequence and the reverse reaction via TTT‐TCT‐CCT. [ABSTRACT FROM AUTHOR]

Eine neue Generation löslicher Phenothiazinyl‐Merocyanin‐substituierter Polyacetylene wurde mittels Rhodium‐katalyisierter Polymerisation der entsprechenden 3‐Ethinyl‐Phenothiazine dargestellt. Die Monomere sind über Sonogashira‐Kupplung und Knoevenagel‐Kondensation zugänglich. UV/Vis‐ und fluoreszenzspektroskopische Untersuchungen der 7‐Acceptor‐substituierten Phenothiazinyl‐Polyacetylene zeigen, dass diese Polymere mit konjugiert verknüpften Merocyaninen in Lösung mit positiver Emissionssolvatochromie lumineszieren und in bestimmten Fällen eine ausgeprägte Festkörperlumineszenz aufweisen. [ABSTRACT FROM AUTHOR]

The most important area of modern pharmacology is the targeted delivery of drugs, and one of the most promising classes of chemical compounds for creating drugs of this kind are the photochromic spiropyrans, capable of light-controlled biological activity. This work is devoted to the synthesis and study of the photochromic properties of new triphenylphosphonium salts of spiropyrans. It was found that all the synthesized cationic spiropyrans have high photosensitivity, increased resistance to photodegradation and the ability for photoluminescence. [ABSTRACT FROM AUTHOR]

In this paper, the relaxation isomerization properties of a spiropyran-doped poly(methyl methacrylate) (PMMA) UV light-responsive dynamic material are presented. The polar liquid (acetonitrile) and solid (PMMA matrix) medium may contain two merocyanine isoforms of the selected spiropyran. A complex equilibrium of the colored and colorless spiropyran isoforms resulted in specific transformations of the absorption spectra during the relaxation process. Bands at 522 nm and 580 nm characterized the absorption of the fabricated dynamic material in the ground (non-activated) state in the visible range. The presence of colored and colorless isoforms of spiropyran incorporated into the PMMA matrix using Raman spectroscopy was revealed. The photosensitive polymer samples were irradiated with a UV LED (365 nm and 390 nm) under various temperatures (0–30 °C). Spectral changes within the relaxation process demonstrated their complex dynamics. Surprisingly, the photodegradation of the dynamic material was estimated at about 25% (using 390 nm) and 7% (using 365 nm) after eight photoswitch cycles for one isoform, and no fatigue after an additional one (using 390 nm) was discovered. The presented light-responsive material is of interest for UV sensors and new hybrid material design. [ABSTRACT FROM AUTHOR]

Low-molecular-weight peptide gelators are a versatile class of compounds able to form gels under a variety of conditions, even via simple ultrasound sonication. In this paper, the ability of Boc-L-Phe-D-Oxd-L-Phe-OBn to gelate three organic solvents (toluene, tert-butyl methyl ether, and ethanol) was evaluated. The rheological behaviour of the materials was assessed via strain sweep analysis, while the fibrous network was analysed via optical microscopy on the wet gels. The gel obtained from toluene is a highly transparent material, and the one from ethanol appears translucent, while the one from tert-butyl methyl ether is opaque. These gels were used to study the reversible light-induced transformation from spyropiran (SP) to merocyanine (MC) and back, as a model system to check the effect of the gel medium onto the rection kinetic. We observed that the solvent used to form the organogels has a crucial effect on the reaction, as gels from aprotic solvents stabilize the SP form, while the ones from protic solvents stabilize the MC form. We thus obtained a solid support to stabilize the two photochromic species just by changing the solvent polarity. Moreover, we could demonstrate that the self-assembled gels do not interfere with the light-driven conversion process, either starting from SP or MC, thus representing a valid and economical photochromic material. [ABSTRACT FROM AUTHOR]

A promising spiro[pyridobenzopyran–indoline] molecular platform for the development of ionochromic systems with a high potential for metal cation chemosensor applications is developed. Evidence for this conclusion is provided the established characteristics and properties of 6-bromo-substituted spiro[pyridobenzopyran–indoline], including ionochromism accompanied by a chromogenic effect in the form of a red shift that reaches 200 nm. The obtained Zn(II), Ni(II), Cu(II), Co(II), and Cd(II) complexes are characterized by a high stability with constants of 105.6–108.6 and 105.0–106.9 for the first and second steps, respectively. The possibilities to obtain selective sensors are associated with the fluorescence detected in the Zn(II) and Cd(II) complexes only. The photostability of the studied complexes, on the one hand, and the high spectral contrast of complex formation of the dynamic 6-bromo-substituted spiro[pyridobenzopyran–indoline] systems on the other, open up the possibility of using chemosensors based on these molecular platforms in the naked-eye colorimetric mode. [ABSTRACT FROM AUTHOR]

[Display omitted] The computational modeling results are presented in the defining points of the isomerization process of the spiropyrans with cationic vinyl-3 H -indolium substituent in the positions 6 ' and 8 ' of the 2 H -chromene moiety taking into account DMSO solvent. The role of effective conjugation is revealed, that determines the significant difference in the behavior of the studied systems, based on the data of NBO energetic analysis and the atomic charge transfer analysis. The model shows that the interaction between an anion and a cation in these compounds has predominantly electrostatic nature without chemical bonds in the NBO terms [ABSTRACT FROM AUTHOR]

Petrosamine (1)—a colored pyridoacridine alkaloid from the Belizean sponge, Petrosia sp., that is also a potent inhibitor of acetylcholine esterase (AChE)—was investigated by spectroscopic and computational methods. Analysis of the petrosamine-free energy landscapes, pKa and tautomerism, revealed an accurate electronic depiction of the molecular structure of 1 as the di-keto form, with a net charge of q = +1, rather than a dication (q = +2) under ambient conditions of isolation-purification. The pronounced solvatochromism (UV-vis) reported for 1, and related analogs were investigated in detail and is best explained by charge delocalization and stabilization of the ground state (HOMO) of 1 rather than an equilibrium of competing tautomers. Refinement of the molecular structure 1 by QM methods complements published computational docking studies to define the contact points in the enzyme active site that may improve the design of new AChE inhibitors based on the pyridoacridine alkaloid molecular skeleton. [ABSTRACT FROM AUTHOR]

Mathematical models for the concentration‐, temperature‐ and solvent‐dependent analysis of self‐assembly equilibria are derived for the most simple case of dimer formation, to highlight the assumptions these models and the thus determined thermodynamic parameters are based on. The three models were applied to UV/Vis absorption data for the dimerization of a highly dipolar merocyanine dye in 1,4‐dioxane. Isothermal titration calorimetry (ITC) dilution experiments were performed as an independent reference technique. While the concentration‐dependent analysis is according to our studies the most reliable method, also the less time‐consuming temperature‐dependent evaluation can give accurate results in the present example, despite small thermochromic effects. In contrast, the strong negative solvatochromism of the merocyanine tampers with the results from the solvent‐dependent evaluation. Even though the studies presented in this work are limited to the monomer‐dimer equilibrium of a dipolar dye, the basic principles can be transferred to other chromophores and different self‐assembly models, including those for supramolecular polymerization. [ABSTRACT FROM AUTHOR]

The most important area of modern pharmacology is the targeted delivery of drugs, and one of the most promising classes of chemical compounds for creating drugs of this kind are the photochromic spiropyrans, capable of light-controlled biological activity. This work is devoted to the synthesis and study of the photochromic properties of new triphenylphosphonium salts of spiropyrans. It was found that all the synthesized cationic spiropyrans have high photosensitivity, increased resistance to photodegradation and the ability for photoluminescence.

Flexible conductive fibers have attracted significant attention in the wearable electronics field because of light weight, small size, and easy shape morphing. To date, there are few reports on multifunctional flexible conductive fibers. Finding a facile and effective solution for large-scale production of flexible conductive fibers is still a challenge. In this work, a flexible core-sheath fiber with excellent electrical conductivity (24.71 Ω·m) and visible light sensing is fabricated, which contains a merocyanine diol (MC(OH)2)/CrCl3·6H2O/ethanol aqueous solution core layer and a silicon rubber outer layer. The synthesized MC(OH)2 is an intelligent molecular switch with a visible light response that can reversibly convert from open-form MC(OH)2 to closed-form spiropyran diol (SP(OH)2) after short visible light irradiation. In the reverse process, the transition from SP(OH)2 to MC(OH)2 can occur gradually in the dark. This can be used to sense visible light in addition to electrical conductivity. The reversible change in the electrical resistance of the MC(OH)2/CrCl3·6H2O/ethanol aqueous solution reaches 15.59%. This fiber shows excellent electrical conductivity, good flexibility, a sensitive visible light response, good photochromism, a simple structure, low cost, easy preparation, and suitability for industrial large-scale production, offering promise for applications in the fields of flexible and wearable electronics.

The trans‐cis isomerization in the excited state of linear merocyanine L‐Mero4 and phenyl substituted linear merocyanine P‐L‐Mero4 in salt solution and in ionic liquid was investigated using frequency upconversion measurements. Strontium chloride and cesium iodide were added to solvent dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) to vary the ionic strength. The time‐resolved fluorescence curves of merocyanines displayed multiple exponential decay behavior. The second temporal component with time constant τ2 ≈ 2.8 (11) ps of L‐Mero4 (P‐L‐Mero4) in DMSO was assigned to the duration to reach the isomerization equilibrium between the trans and the twisted conformers. The τ2 increased at higher salt concentrations and was explained by the attachment of salt ions on the polar excited merocyanines decelerating the isomerization rate. The rotational correlation time constants obtained from the anisotropy decay of fluorescence were 360 and 240 ps in neat DMSO for L‐Mero4 and P‐L‐Mero4, respectively, and they increased to 790 and 450 ps in the most concentrated SrCl2. Using Perrin relation, we estimated the increase in the rotating volume at [SrCl2] = 536 mM, revealing ≈15 SrCl2 molecules surrounding L‐Mero4 and 7 SrCl2 on P‐L‐Mero4. The experimental data indicated that the ion–molecule interaction was stronger with SrCl2 and on L‐Mero4 than on P‐L‐Mero4. [ABSTRACT FROM AUTHOR]

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 molar 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.

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For the first time a pyrrolidinofullerene salt containing a spiropyran group and an ammonium group, capable of reversibly reacting to UV radiation, has been synthesized. Photoinduced reactions of the synthesized compounds were studied using absorption and luminescence spectroscopies, spectral and kinetic characteristics were measured. The hybrid molecule was found to exhibit intrinsic fluorescence even in the spirocyclic form. The C60 derivative showed a higher stability and better spectral and luminescent properties than the precursor. [ABSTRACT FROM AUTHOR]

Three one‐pot three‐step four‐component reaction sequences for spiropyran synthesis have been developed, based around three different methods for in situ generation of 1,2,3‐trisubstituted indoles (indole N‐alkylation, Fischer indolisation and indole C‐alkylation). The reaction sequences give access to a broad swathe of spiropyran structures. Each sequence is operationally straightforward, rapid and high yielding. We have synthesized 58 structurally diverse spiropyrans bearing a wide range of useful functional groups, and their utility were demonstrated in the targeted synthesis of potential ratiometric fluorescence probes for metal ions and spiropyran‐cholesterol conjugates. Finally, we showed that our one‐pot N‐alkylation‐C‐alkylation‐condensation sequence can underpin combinatorial spiropyran synthesis through generation of a 16‐member spiropyran library. [ABSTRACT FROM AUTHOR]

This work focuses on the formyl derivative of 2-oxaindane series spiropyran under its condensation reaction. The data indicated that all products are thermodynamically stable enamine-ketone tautomeric form in both DMSO solution and solid state. This finding can be explained by the migration of proton in 7-hydroxy group to the azomethine nitrogen, which is conjugated with electrocyclic opening of the spirounit in the non-stable Schiff-bases. We observe the enamine-ketones dynamic equilibrium of Z-, E-isomers in the DMSO solution. X-Ray diffraction study was performed for single crystalline sample of aminoethanol derivative. The synthesized compounds were evaluated in vitro cytotoxic activity against four typical human cell lines, showing moderate activity against lung carcinoma (LU-1), hepatocellular carcinoma (Hep-G2), breast carcinoma (MCF-7), and undifferentiated thyroid carcinomas (8505c). Among them, the compound with R = i-Pr showed the best activity with IC50 values 10.60 (MCF-7), and 10.55 µM (8505c). The in vitro antioxidant activity test using 1,1-diphenyl-2-picrylhydrazyl (DPPH) showed that all the synthesized compounds have weak or no activity. [ABSTRACT FROM AUTHOR]

Low-molecular-weight peptide gelators are a versatile class of compounds able to form gels under a variety of conditions, even via simple ultrasound sonication. In this paper, the ability of Boc-L-Phe-D-Oxd-L-Phe-OBn to gelate three organic solvents (toluene, tert-butyl methyl ether, and ethanol) was evaluated. The rheological behaviour of the materials was assessed via strain sweep analysis, while the fibrous network was analysed via optical microscopy on the wet gels. The gel obtained from toluene is a highly transparent material, and the one from ethanol appears translucent, while the one from tert-butyl methyl ether is opaque. These gels were used to study the reversible light-induced transformation from spyropiran (SP) to merocyanine (MC) and back, as a model system to check the effect of the gel medium onto the rection kinetic. We observed that the solvent used to form the organogels has a crucial effect on the reaction, as gels from aprotic solvents stabilize the SP form, while the ones from protic solvents stabilize the MC form. We thus obtained a solid support to stabilize the two photochromic species just by changing the solvent polarity. Moreover, we could demonstrate that the self-assembled gels do not interfere with the light-driven conversion process, either starting from SP or MC, thus representing a valid and economical photochromic material.

A straightforward approach for the construction of the new class of conjugated pyrans based on enamination of 2-methyl-4-pyrones with DMF-DMA was developed. 2-(2-(Dimethylamino)vinyl)-4-pyrones are highly reactive substrates that undergo 1,6-conjugate addition/elimination or 1,3-dipolar cycloaddition/elimination followed by substitution of the dimethylamino group without ring opening. This strategy includes selective transformations leading to conjugated and isoxazolyl-substituted 4-pyrone structures. The photophysical properties of the prepared 4-pyrones were determined in view of further design of novel merocyanine fluorophores. A solvatochromism was found for enamino-substituted 4-pyrones accompanied by a strong increase in fluorescence intensity in alcohols. The prepared conjugated structures demonstrated valuable photophysical properties, such as a large Stokes shift (up to 204 nm) and a good quantum yield (up to 28%). [ABSTRACT FROM AUTHOR]

A new spiropyran containing a cationic 3Н-indolium substituent and methoxy groups at positions 5 and 5" of the indoline cycles was synthesized. Hydrolysis of this compound was observed during crystallization from EtOH, leading to the formation of protonated merocyanine form of spiropyran containing a free formyl group. The hydrolysis product was characterized by X-ray structural analysis, the intermolecular interactions in crystal were studied with the CrystalExplorer 21.5 software suite. Quantum-chemical modeling based on the Fukui function distribution was used to establish the preferred site for nucleophilic attack, and a mechanism for hydrolysis was proposed. [ABSTRACT FROM AUTHOR]

New salts of photochromic indoline spiropyrans capable of reversibly responding to UV radiation were synthesized to develop light-controlled materials. Photoinduced reactions of the synthesized compounds were studied using absorption and luminescence spectroscopies, and the quantum yields of photoisomerization and other spectral and kinetic characteristics were measured. It was shown that the light sensitivity and photostability of the synthesized compounds are considerably influenced by the length of the spacer between the indole and ammonium nitrogen atoms. [ABSTRACT FROM AUTHOR]

The reaction of fluorescein-substituted 2-hydroxybenzaldehyde with 3H-indolium perchlorate has led to the formation of 1,3-dihydrospiro[indole-2,2′-chromene] bearing a fluorescein fragment at position 8 of chromene. In a solution, the obtained compound has existed in the cyclic form and has exhibited photochromic properties. Adding transition metal perchlorates to the spiropyran solution has led to intense coloration due to the complex formation. Zinc and cadmium complexes of the spiropyran have exhibited red fluorescence and negative photochromism. [ABSTRACT FROM AUTHOR]

Petrosamine (1)—a colored pyridoacridine alkaloid from the Belizean sponge, Petrosia sp., that is also a potent inhibitor of acetylcholine esterase (AChE)—was investigated by spectroscopic and computational methods. Analysis of the petrosamine-free energy landscapes, pKa and tautomerism, revealed an accurate electronic depiction of the molecular structure of 1 as the di-keto form, with a net charge of q = +1, rather than a dication (q = +2) under ambient conditions of isolation-purification. The pronounced solvatochromism (UV-vis) reported for 1, and related analogs were investigated in detail and is best explained by charge delocalization and stabilization of the ground state (HOMO) of 1 rather than an equilibrium of competing tautomers. Refinement of the molecular structure 1 by QM methods complements published computational docking studies to define the contact points in the enzyme active site that may improve the design of new AChE inhibitors based on the pyridoacridine alkaloid molecular skeleton.

Among photo‐switchable compounds, spiropyrans have attracted significant attention owing to their multi‐responsiveness towards a change in the pH, solvent polarity, and temperature, along with light. Upon exposure to the external stimulants, the merocyanine form is obtained due to bond cleavage between spiro carbon and oxygen, which transforms the optically active spiropyrans into an achiral form. The back conversion of the resulting achiral merocyanine form to the spiropyrans brings about a racemic mixture. In this study, D and L enantiomers of alanine and phenylalanine derivatives were incorporated onto a spiropyran bearing hydroxyl group to obtain optically active spiropyrans. Photochemical and photophysical properties of these compounds were studied in methanol, isopropanol, dichloromethane, and hexane. The effect of the benzyl and methyl units on the ring closure rate of the merocyanines was also investigated. For this purpose, ring closure kinetics of different merocyanines containing hydroxyl group, D‐alanine, and D‐phenylalanine derivatives were compared. The results reveal that the ring closure rate of the merocyanine with benzyl group attached is faster than the rate of the structures with methyl or hydroxyl groups. [ABSTRACT FROM AUTHOR]

[Display omitted] Isomeric forms of indoline spiropyrans show unusual behavior compared with similar compounds, according to experimental data. DFT modeling for gas phase was made to consider the simplest case without environmental effects, which revealed the intramolecular reasons for occurrence of ring opening reaction depending on the particular structure of the compound. The questions of charge redistributions, the changes of geometry and chemical bonds in the structures are also discussed. [ABSTRACT FROM AUTHOR]

The pseudopeptide backbone provided by N‐(2‐aminoethyl)‐glycine oligomers with attached nucleobases has been widely utilized in peptide nucleic acids (PNAs) as DNA mimics. Here we demonstrate the suitability of this backbone for the formation of structurally defined dye stacks. Toward this goal a series of peptide merocyanine (PMC) dye oligomers connected to a N‐(2‐aminoethyl)‐glycine backbone were prepared through peptide synthesis. Our concentration‐, temperature‐ and solvent‐dependent UV/Vis absorption studies show that under the control of dipole–dipole interactions, smaller‐sized oligomers consisting of one, two or three dyes self‐assemble into defined duplex structures containing two up to six chromophores. In contrast, upon further extension of the oligomer, the chosen peptide backbone cannot direct the formation of a defined duplex architecture anymore due to intramolecular aggregation between the dyes. For all aggregate species a moderate aggregation‐induced emission enhancement is observed. [ABSTRACT FROM AUTHOR]

In this study, detailed structural and vibrational analysis of merocyanine has been investigated using Raman, surface enhanced Raman scattering (SERS) and surface-enhanced resonance Raman scattering (SERRS). The Raman, SERS and SERRS studies aided by density functional theoretical (DFT) calculations clearly established the prevalence of the trans- and cis-conformers of the protonated form of merocyanine (MCH+) in solid and acetonitrile solution. The binding characteristics of merocyanine adsorbed on nanostructured silver-coated films (SCFs) were investigated using excitation-dependent SERS, concentration-dependent SERRS and DFT studies. The conformers of merocyanine involved in the surface adsorption processes were recognized. The prominent marker bands observed at 1538 (ethylenic C=C stretch) and 1133 cm−1 (pyridinium C-N stretch) in the Raman spectrum of merocyanine in acetonitrile shifted to 1540 and 1126 cm−1, respectively on the nanostructured SCFs. The shift in the marker bands is associated with either the preferential binding of selective conformer or change in resonance equilibrium between the benzenoid and quinoid forms. The excitation wavelength dependent SERS spectrum infers that in addition to the major contribution from the electromagnetic enhancement, chemical (resonance) effect leads to the amplification of the 1540 cm−1 band. The concentration-dependent SERRS study showed maximum enhancement for the nanostructured SCFs functionalized with 1 μM concentration of merocyanine, indicative of monolayer coverage. For lower concentrations of merocyanine, the SERRS signal intensity reduced without any alteration in the peak positions. The SERRS study thus, revealed sub-nanomolar (0.1 nM) sensing of merocyanine using nanostructured SCFs with the analytical enhancement factor (AEF) of ∼ 1010 for the 1126 cm−1 and 1540 cm−1 Raman bands for MC concentration of 0.1 nM. In this study, combination of SERRS and DFT have clearly established the predominance of trans-MCH+ on the nanostructured silver surface with minor contribution from cis-MCH+, which remain exclusively bound to the surface via the phenoxyl ring O atom. This conformational surface selectivity of geometrical isomers of merocyanine using nanostructured surfaces can be further explored for energy efficient and economical separation of geometrical isomers.

Polyurethanes represent a versatile class of polymers and are one of the most employed in the thermoset market. However, due to their thermodynamically stable carbamate bond, they suffer from a lack of reprocessability, recyclability, and degradability, and are therefore usually discarded after use. Recently, transcarbamoylation in polyurethane thermosets has been demonstrated via associative exchange in the presence of a strong organic acid. Intending to introduce spatiotemporal control in the processability of polyurethane materials, in this work we investigate dynamic transcarbamoylation by the addition of a latent acidic catalyst into an aromatic and an aliphatic polyurethane network. We introduce three photoswitches based on merocyanine/spiropyran compounds which, upon exposure to visible light, undergo a reversible cyclization reaction, resulting in the formation of strong acids. The use of such photoswitches prevents the presence of a permanent acidic species in the network, which would eventually lead to its degradation over time. Importantly, due to the reversible nature of the cyclization reaction, the release of the acidic catalyst is not permanent, as evidenced by stress relaxation measurements. Overall, the evidence of the light-induced, acid-catalyzed transcarbamoylation is supported by stress relaxation, optical microscopy, and self-healing measurements. [Display omitted] • Reversible Spiropyran photoswitches can catalyze the dynamic transcarbamoylation in polyurethanes. • The use of such photoswitches prevents the presence of a permanent acidic species in the network, preventing degradation over time. • Due to the reversible nature of the photoswitch, the release of acid is not permanent, as evidenced by stress-relaxation measurements. [ABSTRACT FROM AUTHOR]

For the first time a pyrrolidinofullerene salt containing a spiropyran group and an ammonium group, capable of reversibly reacting to UV radiation, has been synthesized. Photoinduced reactions of the synthesized compounds were studied using absorption and luminescence spectroscopies, spectral and kinetic characteristics were measured. The hybrid molecule was found to exhibit intrinsic fluorescence even in the spirocyclic form. The C60 derivative showed a higher stability and better spectral and luminescent properties than the precursor.

Exciton coupling between two or more chromophores in a specific environment is a key mechanism associated with color tuning and modulation of absorption energies. This concept is well exemplified by natural photosynthetic proteins, and can also be achieved in synthetic nucleic acid nanostructures. Here we report the coupling of barbituric acid merocyanine (BAM) nucleoside analogues and show that exciton coupling can be tuned by the double helix conformation. BAM is a nucleobase mimic that was incorporated in the phosphodiester backbone of RNA, DNA and GNA oligonucleotides. Duplexes with different backbone constitutions and geometries afforded different mutual dye arrangements, leading to distinct optical signatures due to competing modes of chromophore organization via electrostatic, dipolar, π–π‐stacking and hydrogen‐bonding interactions. The realized supramolecular motifs include hydrogen‐bonded BAM–adenine base pairs and antiparallel as well as rotationally stacked BAM dimer aggregates with distinct absorption, CD and fluorescence properties. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Herein, we report that an efficient nanohybrid material consists of Cd-doped ZnS nanocrystals (NCs), merocyanine and fullerene (C60-pyrrolidine tris acid). Cd-ZnS NCs serve as a substrate for supramolecular complexation between merocyanine 540 and fullerenes. The impact of grafting and co-grafting of dyes on the NCs surface was checked by Fourier transform infrared (FTIR), photoluminescence and ultraviolet–visible (UV–Vis) spectroscopic studies. Cd-doped ZnS NCs were synthesized by wet chemical approach and described by powdered X-ray diffraction, UV–Vis spectroscopy, field emission scanning electron microscopy and transmission electron microscopy. The grafted and co-grafted NCs were then used as an active blend in hybrid solar cells. The hybrid solar cell of grafted material blend (Cd-ZnS-MC540) shows the maximum short circuit current density 4.60 mA cm–2 and power conversion efficiency of 0.83%. The open circuit voltage, fill factors and cell conversion efficiency of all photovoltaic devices based on co-grafted Cd-doped ZnS NCs and P3HT decreases with the increase in concentration of donor and acceptor species. We note that by co-grafting, dye/dye interaction is replaced by dye/fullerene interaction but unfortunately co-grafting may have led to the formation of big clusters. Hence, the lack of morphological homogeneity may be held responsible for the weak performance of the solar cells. [ABSTRACT FROM AUTHOR]

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 H2O2 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A straightforward approach for the construction of the new class of conjugated pyrans based on enamination of 2-methyl-4-pyrones with DMF-DMA was developed. 2-(2-(Dimethylamino)vinyl)-4-pyrones are highly reactive substrates that undergo 1,6-conjugate addition/elimination or 1,3-dipolar cycloaddition/elimination followed by substitution of the dimethylamino group without ring opening. This strategy includes selective transformations leading to conjugated and isoxazolyl-substituted 4-pyrone structures. The photophysical properties of the prepared 4-pyrones were determined in view of further design of novel merocyanine fluorophores. A solvatochromism was found for enamino-substituted 4-pyrones accompanied by a strong increase in fluorescence intensity in alcohols. The prepared conjugated structures demonstrated valuable photophysical properties, such as a large Stokes shift (up to 204 nm) and a good quantum yield (up to 28%).

New salts of photochromic indoline spiropyrans capable of reversibly responding to UV radiation were synthesized to develop light-controlled materials. Photoinduced reactions of the synthesized compounds were studied using absorption and luminescence spectroscopies, and the quantum yields of photoisomerization and other spectral and kinetic characteristics were measured. It was shown that the light sensitivity and photostability of the synthesized compounds are considerably influenced by the length of the spacer between the indole and ammonium nitrogen atoms.

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

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. [ABSTRACT FROM AUTHOR]

[Display omitted] 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'. [ABSTRACT FROM AUTHOR]

Four indolone-3-aminopropenylidene merocyanine dimers and a reference α-indolonyl-γ-amino merocyanine are readily synthesized in a consecutive multicomponent insertion-alkynylation-addition sequence in a onepot fashion. The rotational barriers of the terminal amino moieties were estimated by variable temperature NMR measurements and the observed coalescence at room temperature to lie in the same margin as for β-pyrrolidinyl enoates. While the mono merocyanine, chosen as a reference, displays the expected 1300 fold increase in emission intensity upon induced aggregation, the merocyanine dimers, although intense in their absorption behavior and redshifted in their emission maxima, only show a 50-60 fold fluorescence increase. The considerable attenuation of emission of the merocyanine dimers in the amorphous solid state supports the finding that unimolecular symmetrical merocyanine dimers of this type are not intensive AIE systems. [ABSTRACT FROM AUTHOR]

Solvent polarity is important data being used in solvent selections for preliminary engineering design of chemical processes. In this work, a predictive model is proposed for estimating the solvatochromic polarity of electronic transition energy (ET) of Reichardt indicator for aqueous mixtures. To validate the model, the ET values of eighteen aqueous mixtures collected from the literature were used. The predictive model provided a good estimation of ET values with an overall deviation of 2.1%, compared with an ideal model (5.1%) from the mole fraction average. The linear relationship of the contribution factor of hydrogen bond donor interactions (CFHBD) in the predictive model with Kamlet–Taft acidity was newly proposed in order to extend the model for other aqueous mixtures. The predictive model is applicable to many aqueous mixtures and simply requires three properties of pure components as: (i) ET values, (ii) gas-phase dipole moment and (iii) Kamlet–Taft acidity. [ABSTRACT FROM AUTHOR]

In this work, we present a novel spiropyran–containing photochromic pyrrolidinofullerene as a promising agent for the photodynamic therapy of cancer.

Indoline spiropyrans containing benzo[f]coumarin moiety annulated to the 2H-pyran cycle were synthesized. The obtained compounds exist in solutions as a tautomeric mixture of their spirocyclic and merocyanine forms. The spiro form exhibits an enhanced resistance to UV irradiation and does not rearranges into the merocyanine isomer in the temperature range of 278–303 K. [ABSTRACT FROM AUTHOR]

Spiropyrans are a class of photochromic compounds that feature two main isoforms, a closed spiropyran (SP) and an open merocyanine (MC), which are inter-switchable with UV and visible light. The double bond containing MC isomer has several resonance forms with different optical spectra. Consequently, the overall spectral characteristic of the spiropyran solution depends on the relative contribution of each resonance form. In addition, the reversible changes of the absorbance or fluorescence spectra of MC are dictated by the environment. By utilizing these properties, spiropyran and its derivatives have been employed to monitor a variety of molecules and ions that can alter the resonance forms of MC. In this review, we provide a comprehensive discussion on a range of species that can be detected by spiropyran including metal ions, anions, acids, solvents, and even biomolecules, the major analytical detection mechanisms involved, and the novelty of such techniques. This study establishes spiropyran as a versatile platform for the detection of both organic and inorganic target molecules. Image 1 • Two isoforms of spiropyran exhibit starkly different physical properties. • Reversible isomerization can be induced by multiple stimuli. • Properties of spiropyran can be tuned by introducing various functional groups. • Spiropyran is able to detect a broad range of organic and inorganic molecules. • Emerging applications include core-material for chemosensors and biosensors. [ABSTRACT FROM AUTHOR]

Three new photochromic spiropyrans containing a carboxylic group in the indoline fragment and an aldehyde substituent in the [2H]-chromene moiety were synthesized. The structure of the compounds obtained was studied using 1H and 13C NMR, IR and elemental analysis. Single crystal X-ray analysis was used to refine the molecular structure of 8′-formyl-1,3,3,6′-tetramethyl-spiro[indoline-2,2′-2H-chromene]-5-carboxylic acid. It was found out that in the solid state the molecules of that compound are arranged in dimeric associates due to the formation of intermolecular hydrogen bonds between carboxylic groups of two adjacent molecules. All the spiropyrans were shown to be photochromic with the lifetimes of the open forms in the range of 1.5–46.0 s. Image 1 • New photochromic carboxy- and formyl-substituted spiropyrans were synthesized. • Photochromic properties of spiropyrans can be tuned by the location of carbonyl groups. • Carboxy-substituted spiropyrans form dimeric associates in their single crystals. [ABSTRACT FROM AUTHOR]

Creation of a stable and reliable molecular device is an important task of nanotechnology. Photochromic molecules are the most promising candidates for such a purpose. One of the most prospective classes of photochromic compounds is spiropyran and its open form merocyanine. To use the spiropyranmericyanine system it is essential to know properties of these molecules. The transformation between spiropyran and merocyanine molecules can occur in different ways. Therefore, it is important to have a complete picture of the merocyanine structures and transition routes between them. Thus, the investigation of geometric structure of the merocyanine molecule was carried out. It shows that the merocyanine molecule can be in one of the eight possible isomers taking into account the rotation around the three central bonds C(25)-C(26), C(26)-C(27), C(27)-C(28). TTT, TTC, CTT and CTC isomers have a flat geometric structure and similar absorption spectra (from 490 nm to 510 nm), thus they cannot be separated in experimental studies. All these isomers exhibit dark stability, since the energy barrier for the transition between them is more than 1 eV. The other four isomers TCT, TCC, CCT and CCC have a spiral structure, at which each of these isomers can have right-handed or left-handed structure. Transformations between neighboring isomers have been studied. The results have shown that spiral structure isomers are thermally unstable, and they turn into one of the flat isomers rapidly. That is why their contribution to the absorption spectrum of the merocyanine molecule is absent. At the same time, all flat structures are characterized by identical conductivity properties along the π-electron system and the same length of the long axis of the molecule. The obtained results are important in terms of using the spiropyran-merocyanine system as a memory element or switch in future computer systems. [ABSTRACT FROM AUTHOR]

[Display omitted] One of the most significant species that are present in large quantities in biological cells is reactive oxygen species (ROS). They are essential for cell motility, cell differentiation, and cellular immunity. In this paper, we introduce that (E)-1-methyl-4-(4-(propionyloxy)styryl)pyridin-1-ium (MPSPI) is an "on" type fluorescent probe regulated by the ICT mechanism. After the addition of hydrogen peroxide, the fluorophore (E)-4-(4-hydroxystyryl)-1-methylpyridin-1-ium (HMI-OH) is released. The simple probe MPSPI not only exhibited good selectivity and low cytotoxicity, but also demonstrated a detection limit as low as 35 nM and reaction times as quick as 20 s. Moreover, it has been successfully applied to the imaging of hydrogen peroxide in RAW 264.7 cells. [ABSTRACT FROM AUTHOR]