Merocyanine Isomers Research Articles

Re-usable colorimetric polymeric gel for visual and facile detection of multiple metal ions

For fast and visual detection of multiple metal ions, herein, a re-usable colorimetric polymeric gel with spiropyran moiety was fabricated. Under ultraviolet light, the spiropyran (SP) moiety could isomerize into the colored merocyanine (MC) isomer and chelate with divalent metal ion (M2+) and form MC-M2+ complexes. Chelated with different metal ions (Ca2+, Co2+, Zn2+, and Cu2+ions), the UV–Vis spectra of the gels could display the specific colorimetric response. When the metal-ions chelated gel was irradiated with Vis light, ions dissociated from MC and released from gel due to the MC moiety isomerized to the SP form. Furthermore, after 7 cycles of alternative UV–Vis irradiations, the gel showed excellent reversible chelation/dissociation of metal ion. The competitive tendency of metal ions to chelate with gel was as following: Cu2+ > Co2+ ≈ Zn2+ > Ca2+, which was in accordance with the density functional theory modeling. Using ionoprinting technology, the gel could achieve repeatability simultaneous detection of multiple metal ions. The light-triggered metal ion chelation/dissociation capability of the gel could be harnessed to construct reusable onsite platform for visual, fast, easy detection of multiple metal ions.

Synthesis, structure and photochromic properties of indoline spiropyrans with electron-withdrawing substituents

Indoline spiropyrans containing conjugated cationic vinyl-3H-indolium fragment and their analogues with formyl substituent in the 2H-chromene moiety were synthesized by a simple one-pot method. The structure of the obtained compounds was confirmed by 1H and 13C NMR (including 2D methods) and IR spectroscopy, as well as by element analysis and HRMS. Molecular structures of 1,3,3,6´-tetramethyl-8´-formyl-spiro[indoline-2,2´-2H-chromene] 8 and its cationic derivative 5 were refined by single crystal X-ray and compared. Notably, it was found that the crystals of formyl-substituted spiropyran contain both enantiomers (R- and S-) of the molecule in the unit cell. Photochromic properties of all the compounds were studied and compared using spectral-photokinetic methods. It was found out that electron-withdrawing substituents (vinyl-3H-indolium fragment and formyl group) located at the position 6´ of 2H-chromene moiety stabilize merocyanine isomer more effectively, than at the position 8´, leading even to the appearance of negative photochromism for the compound 3 with cationic substituent, which is quite unusual for this class of organic photochromes. Also, it was shown that chlorine atoms as σ-acceptors at the positions 5 and 5´´ of hetarene moiety and cationic fragment can suppress photochromic behavior.

Synthesis and study of new photochromic unsymmetrical bis-spiropyrans with nonequivalent heteroarene fragments conjugated through the common 2H,8H-pyrano[2,3-f]chromene moiety

Four novel bis-spiropyrans possessing two different photochromic units based on indoline and 1,3-benzoxazine heterocycles which are connected through the common 2H,8H-pyrano[2,3-f]chromene moiety and modified with electron donating substituents were obtained by multistep synthesis starting with 2,4-dihydroxy-iso-phthalic aldehyde. The structure of the compounds was confirmed by 1H and 13CNMR, FTIR and HRMS. All bis-spirocompounds exhibited photochromic activity at room temperature. Investigation of photochromic properties of bis-spiropyrans revealed that their once-opened merocyanine isomers were more stable than the double-opened ones under experimental conditions. It was also found that modification of the benzoxazine moiety of the molecule with electron donating substituents along with introduction of bulky benzyl group close to the oxazinespiro-center leads to an increase of the merocyanine lifetime up to 148 s.

Synthesis and structure of indoline spiropyrans based on benzo[f]coumarin

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.

Visible to near-IR molecular switches based on photochromic indoline spiropyrans with a conjugated cationic fragment.

Photochromic molecules which can absorb and emit light within the "biological window" (650-1450 nm) are of great interest for using in various important biomedical applications such as bio-imaging, photopharmacology, targeted drug delivery, etc. Here we present three new indoline spiropyrans containing conjugated cationic fragments and halogen substituents in the 2H-chromene moiety which were synthesized by a simple one-pot method. The molecular structure of the obtained compounds was confirmed by FT-IR, 1H and 13C NMR spectroscopy (including 2D methods), HRMS, elemental and single crystal X-ray analysis. Photochemical studies revealed the photochromic activity of spiropyrans at room temperature which caused photoswitchable fluorescence in the near-IR region after UV-irradiation. While the spirocyclic forms of compounds demonstrated absorption bands in the UV-Vis spectra with maxima in the visible region at about 445 nm and were not fluorescent, the photogenerated merocyanine isomers absorbed in the near-IR range at 708-738 nm and emitted at 768-791 nm. It was found that compound 1a with fluorine substituent possesses the most red-shifted absorption and emission bands of merocyanine form among all the known photochromic spiropyrans with maxima at 738 and 791 nm correspondingly. TD DFT calculations have shown that the longest wavelength absorption maxima of the merocyanine forms correspond to S0-S1 transitions of the isomers with at least one trans-trans-trans-configured vinylindolium fragment which brings them closer to cyanine-like structure and causes an appearance of the absorption and emission bands in the near-IR region.

Merocyanines in a Halogen-Bonded Network Involving Inorganic Building Blocks

In this work, we present spiropyran derivatives (SP) involving a halogen bonded network with the addition of inorganic building blocks, i.e. inorganic acids and bivalent metal salts. In solution, a ring-opening isomerization of the spiropyran results in the colored merocyanine isomer (MC), which is only rarely observed in the solid state. By detaining the phenolate oxygen atom of the merocyanine in a hydrogen (HB) or halogen bond (XB) we can successfully obtain this form and access a variation in chromic properties. The O atom of the open MC form unfortunately only represents a weak XB acceptor which led us to introduce supplementary building blocks to strengthen this type of interaction. Fourteen new crystal forms were determined with 3 spiropyran derivatives (1,3,3-trimethylindolinospiropyran (SPH/MCH), 1,3,3-trimethylindolino-6′-nitrobenzopyrylospiran (SPNO2/MCNO2), 1,3,3-trimethylindolino-β-naphthopyrylospiran (SPBenz/MCBenz)) and several di- and tritopic XB donors (1,3,5-triiodotrifluorobenzene (135tfib), 1,4-diiodotetrafluorobenzene (14tfib), 1,3-diiodotetrafluorobenzene (13tfib), 1,2-diiodotetrafluorobenzene (12tfib), abd 1,2-diiodotetrafluoroethan (12tfe)) as well as a polymorphic form of the parent compound with hydrochloric acid. We successfully expanded the XB interaction possibilities with hydrochloric acid or zinc/cobalt chloride and found proof of the tunability of the chromic properties in the solid state using these building blocks.

Investigating the Photochemistry of Spiropyran Metal Complexes with Online LED-NMR.

As one of the best studied photoswitches, spiropyrans (SPs) have attracted significant interest in the scientific community. Among the many stimuli to alter the isomerization into the merocyanine (MC) isomer, such as temperature, pH, solvent polarity, redox potential, or mechanical force, the ability of the MC form to act as a ligand site for metal complexation has recently raised new attention. We herein synthesize hitherto undescribed coordination compounds of 8-methoxy-1',3',3'-trimethyl-6-nitrospiro-[chromene-2,2'-indoline] with s-block ([Ca(MC)4](ClO4)2), d-block ([Zn(MC)2(MeCN)2](ClO4)2, [Ni(MC)2(MeCN)2](ClO4)2), as well as f-block ([La(NO3)3(MC)2]) metals. All complexes are structurally described by X-ray crystallography and systematically investigated in solution via Job's method of continuous variations (Job plots), as well as online NMR spectroscopic kinetic experiments with in situ irradiation of the analyte solution inside the NMR spectrometer (LED-NMR). We can unambiguously identify the photoresponsive nature of the complexes in solution, which is a crucial step toward the application of these promising molecules in material science.

Self-Assembly of Giant Polymer Vesicles by Light-Assisted Solid Hydration.

An amphiphilic copolymer containing a terminal spiropyran (P-SP) forms giant polymer vesicles when exposed to ultraviolet (UV) light. The process involves photoisomerization of the spiropyran to the corresponding hydrophilic merocyanine isomer, which significantly improves the efficiency of film hydration and facilitates the self-assembly of the polymer in water. Giant vesicles formed by light-assisted hydration have diameters ranging from 5 to 25 micrometers, and can be observed and quantified by confocal fluorescence microscopy. Rapid and efficient formation of giant vesicles only occur during exposure of P-SP to UV light and within the area delimited by the light beam. Light-assisted hydration offers high spatial and temporal control over vesicle formation, conditions not easily fulfilled by other techniques.

Effect of Chemical Microenvironment in Spirothiopyran Monolayer Direct-Write Photoresists.

We study the effect of the microenvironment on writing chemical patterns into spirothiopyran monolayers over large areas in a single step with light. Surfaces functionalized with photoresponsive spirothiopyran are fabricated by chemically modifying amine-terminated monolayers. The merocyanine isomer selectively participates in a thiol-Michael addition reaction with maleimide-functionalized molecules, rendering these surfaces ideal for fast, mask-less direct writing. The local microenvironment of spirothiopyran is found to strongly influence the kinetics of photoswitching. The quantum yield of ring opening is found to be 17 times faster for spirothiopyran surrounded by a locally charged environment rich in guanidinium diluent molecules as compared to a closed-packed monolayer without diluents. Hydrophilic environments are also found to improve the kinetics of ring closing. Optimization of the diluent concentration leads to dramatic improvements in both contrast and yield of direct writing. This enables the monolayer to be used for maskless two-color photopatterning in which spatial control over patterning is obtained by varying the relative intensity of incident UV and green light. These experiments demonstrate the capacity of spirothiopyran monolayers to serve as a versatile toolbox for rapid, large-area surface functionalization.

New Photochromic Spiropyrans with ortho-Hydroxyaldimine Substituent

A series of new spiropyrans of indoline series with ortho-hydroxyaldimine substituent has been synthesized, their structure and photochromic properties have been studied. The structure of the obtained compounds was confirmed by the data of 1H NMR and IR spectroscopy and elemental analysis. Photochromic properties of the prepared compounds have been studied. The introduction of ortho-hydroxy group into aldimine fragment of molecule (in the 6 position of the 2H-chromene fragment) has increased the lifetime of merocyanine isomer by 40 times as compared with amino-substituted spiropyran.

Spiropyrans and spirooxazines 13. Synthesis and photochromic properties of benzoxazolyl-substituted spirobenzopyrans

New benzoxazolyl-substituted spiroindoline-benzopyrans with electron-withdrawing groups in the indoline fragment were obtained. These compounds exhibit positive P- and T-type photochromism. Structural correlations of the spectrokinetic properties depending on the variation of substituents in the indoline part of the spiropyran molecules were established. Electronwithdrawing groups were found to cause a bathochromic shift of the long-wavelength absorption maxima of the merocyanine isomers as compared to the unsubstituted spiropyran and decrease their lifetime. Conversely, an introduction of electron-donating groups results in a hypso chromic shift of the absorption maxima and an increase in the lifetime of the colored isomers of spiropyrans. Electron-withdrawing substituents at the position 5 of the indoline fragment cause the increase of the colorability of the spiropyrans.

Self-Assembled Pd(II) Barrels as Containers for Transient Merocyanine Form and Reverse Thermochromism of Spiropyran.

Self-assembly of a cis-blocked Pd(II) 90° ditopic acceptor [ cis-(tmeda)Pd(NO3)2] (M) with a tetradentate donor L1 [benzene-1,4-di(4-terpyridine)] in 2:1 molar ratio yielded two isometric molecular barrels MB1 and MB3 in DMSO [tmeda = N, N, N' N'-tetramethylethane-1,2-diamine]. Exclusive formation of the symmetrical tetrafacial barrel (MB1) was achieved when the self-assembly was performed in aqueous medium. The presence of a large confined cavity makes MB1 a potential molecular container. Spiropyran (SP) compounds exist in stable closed spiro form in visible light and convert to transient open merocyanine (MC) form upon irradiation with UV-light or upon strong heating. The transient MC form readily converts to the stable closed SP form in visible light. MB1 has been employed as a safe container to store the planar and unstable merocyanine isomers (MC1/2) of different spiropyran molecules (SP1/2) [SP1/2 = 6-bromo-spiropyran and 6-nitrospiropyran] for several days. The transient MC forms (MC1 and MC2) were found to be stable inside the molecular container MB1 under visible light and even in the presence of different stimuli such as heat and UV light for a long time. Such stabilization of MC forms inside the confined cavity of MB1 is noteworthy. This phenomenon was generalized by utilizing a carbazole-based molecular barrel (MB2) as a host, which also showed a similar stabilization of transient MC form in visible light at room temperature. Moreover, reverse thermochromism was observed as a result of heating of the MC1 ⊂ MB2 complex, which de-encapsulates the guest in the form of SP1 to give a colorless solution. Moreover, both the host molecules (MB1, MB2) were capable of stabilizing transient MC2 even in the solid state. Such stabilization of transient MC forms in the solid state and transformation of SP forms to MC forms in the solid state in the presence of molecular barrel are remarkable, and these properties have been employed in developing a magic ink.

Enhanced Photogeneration of Reactive Oxygen Species and Targeted Photothermal Therapy of C6 Glioma Brain Cancer Cells by Folate-Conjugated Gold-Photoactive Polymer Nanoparticles.

Tumor-selective photodynamic therapy is a successful method for ablation of malignant and cancerous cells. Herein, we introduce the design and preparation of functionalized acrylic copolymer nanoparticles with spiropyran (SP) and imidazole groups through a facile semicontinuous emulsion polymerization. Then, Au3+ ions were immobilized and reduced on their surface to obtain photoresponsive gold-decorated polymer nanoparticles (PGPNPs). The prepared PGPNPs were surface-modified with folic acid as a site-specific tumor cell targeting agent and improve intracellular uptake via endocytosis. Fourier transform infrared spectroscopy and energy dispersive X-ray spectroscopy analyses, UV-vis spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy images were employed to characterize their spectral and morphological properties. Fluorescence microscopy images and inductively coupled plasma analysis demonstrated the cell line labeling capability and improved targeting efficiency of folate-conjugated PGPNPs (FA-PGPNPs) toward rat brain cancer cells (C6 glioma) with 71.8% cell uptake in comparison with 28.8% for the nonconjugated ones. Nonpolar SP groups are converted to zwitterionic merocyanine isomers under UV irradiation at 365 nm and their conjugation with Au nanoparticles exhibited enhanced photogeneration of reactive oxygen species (ROS). These were confirmed by intracellular ROS analysis and cytotoxicity evaluation on malignant C6 glioma cells. Owing to the strong surface plasmon resonance absorption of gold nanoparticles, FA-PGPNPs provided elevated local photothermal efficiency under near-IR irradiation at 808 nm. The prepared multifunctional FA-PGPNPs with a comprehensive integration of prospective materials introduced promising nanoprobes with targeting ability, enhanced tumor photodynamic therapy, cell tracking, and photothermal therapy.

Structure Investigation of New Condensation Products of 1,2,3,3-Tetramethylindolenium with Metoxysubstituted Diformylphenols

New spiropyran salts in the row of 1,3,3-trimethylspiro-indoline-2,2'-[2Н]-chromene are prepared and their structures are investigated. Molecular structures of obtained compounds are confirmed by X-ray diffraction and NMR spectroscopy. An open merocyanine isomer is found to be the most stable structure for one of these compounds.

Photo-control of the mitotic kinesin Eg5 using a novel photochromic inhibitor composed of a spiropyran derivative.

In this study, we synthesized a novel photochromic inhibitor of the mitotic kinesin Eg5, which is composed of the photochromic compound spiropyran to photo-control the function of Eg5. The compound (S)-2, 3-dispiropyran propionic acid (DSPPA) exhibits reversible spiropyran-merocyanine photo-isomerization upon irradiation with visible or ultra-violet light. DSPPA induced reversible changes in the inhibitory effect on Eg5 ATPase and motor activities, which correlates with the spiropyran-merocyanine photo-isomerization. Microtubule-dependent ATPase activity was significantly more inhibited by the spiropyran isomer of DSPPA than by the merocyanine isomer. Additionally, an in vitro motility assay revealed that the microtubule gliding velocity was reduced more by the spiropyran isomer than by the merocyanine isomer. This indicates that the spiropyran derivative may be useful in regulating the function of the mitotic kinesin.

Chromogenic systems based on 8-(1,3-benzoxazol-2-yl) substituted spirobenzopyrans undergoing ion modulated photochromic rearrangements

8-(1,3-Benzoxazol-2-yl) substituted spirobenzopyrans 1A appear as a chromogenic system susceptible of switching between positive and negative photochromism under complexation with metal ions. The thermal and photochemical reactions accompanying metal ion-induced transformation of spiropyrans 1A to the complexes formed by their photoinduced merocyanine isomers 1B were investigated. In the dark, the colorless spiropyrans interact with Mn2+, Cd2+ and Zn2+ transition metal ions to yield deeply colored 1:1 complexes possessing fluorescence, while Co2+, Ni2+, Cu2+ ions prefer to form non-fluorescent complexes with two merocyanines. Under irradiation with visible light solutions of the merocyanine-metal complexes undergo photobleaching. This effect was quantified using photokinetic method.

Light-fuelled reversible expansion of spiropyran-based vesicles in water.

We present the design and synthesis of spiropyran-based dynamic vesicles, for which the building block is the amphiphilic merocyanine isomer. Under irradiation with visible light, the photo-conversion of the protonated and charged merocyanine to the neutral spiropyran form leads to the transient and reversible expansion of these vesicles.

Reversible Switching of Spiropyran Molecules in Direct Contact With a Bi(111) Single Crystal Surface

AbstractPhotochromic molecular switches immobilized by direct contact with surfaces typically show only weak response to optical excitation, which often is not reversible. In contrast, here, it is shown that a complete and reversible ring‐opening and ring‐closing reaction of submonolayers of spironaphthopyran on the Bi(111) surface is possible. The ring opening to the merocyanine isomer is initiated by ultraviolet light. Switching occurs in a two‐step process, in which after optical excitation, an energy barrier needs to be overcome to convert to the merocyanine form. This leads to a strong temperature dependence of the conversion efficiency. Switching of the merocyanine isomer back to the closed form is achieved by a temperature increase. Thus, the process can be repeated in a fully reversible manner, in contrast to previously studied nitrospiropyran molecules on surfaces. This is attributed to the destabilization of the merocyanine isomer by the electron‐donating nature of the naphtho group and the reduced van der Waals interaction of the Bi(111) surface. The result shows that molecules designed for switching in solutions need to be modified to function in direct contact with a surface.

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

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

Spectrally Resolved Super-Resolution Microscopy Unveils Multipath Reaction Pathways of Single Spiropyran Molecules.

By recording both the images and emission spectra of thousands of single fluorescent molecules stochastically generated from the ring-opening reaction of a spiropyran, we provide mechanistic insights into its multipath reaction pathways. Through statistics of the measured single-molecule spectra, we identify two spectrally distinct isomers, presumably TTC and TTT cis-trans isomers, for the open-ring merocyanine product, and discover a strong solvent polarity-dependence for the relative population of the two isomers. From single-molecule spectral time traces, we further examine isomerization between the two product merocyanine isomers, as well as their ring-closure reaction back to the spiropyran form.