Merocyanine Isomers Research Articles

Involvement of surface-adsorbed water in photochromism of spiropyran molecules deposited on NaY zeolite

The photochromic behavior of spiropyran molecules is very sensitive to the property of surrounding media. Enhanced isomerization of spiropyran to merocyanine was observed after storing functionalized zeolites under ambient temperature and moisture for an extended period of time. Experimental results based on infrared spectroscopy suggest that the enhanced isomerization is due to an increase in the amount of surface-adsorbed water. The presence of more surface-adsorbed water likely altered the isomerization by stabilizing the merocyanine isomer. This observation was utilized to improve reversibility of the isomerization between spiropyran and merocyanine on NaY zeolite surfaces for potential optical applications.

PHOTO- AND THERMOCHROMIC SPIRANS. 35. SYNTHESIS AND PHOTOCHROMIC PROPERTIES OF SPIRO[INDOLINE-2,3'-PYRANO[3,2- f ]QUINOLINES] AND THEIR CATIONIC DERIVATIVES

New photochromic spiropyranoquinolines and their cationic derivatives have been synthesized. Quaternization of the quinoline fragment leads to significant enhancement of the thermal stability of the merocyanine isomers of these cationic spiropyrans. Authors: N. A. Voloshin, S. O. Bezuglyi, A. V. Metelitsa, E. V. Solov'eva, K. E. Shepelenko, and V. I. Minkin. English Translation in Chemistry of Heterocyclic Compounds , 2012, 48 (3), pp 525-531 http://link.springer.com/article/10.1007/s10593-012-1025-z

Characterization of the thermal and photoinduced reactions of photochromic spiropyrans in aqueous solution.

Six water-soluble spiropyran derivatives have been characterized with respect to the thermal and photoinduced reactions over a broad pH-interval. A comprehensive kinetic model was formulated including the spiro- and the merocyanine isomers, the respective protonated forms, and the hydrolysis products. The experimental studies on the hydrolysis reaction mechanism were supplemented by calculations using quantum mechanical (QM) models employing density functional theory. The results show that (1) the substitution pattern dramatically influences the pKa-values of the protonated forms as well as the rates of the thermal isomerization reactions, (2) water is the nucleophile in the hydrolysis reaction around neutral pH, (3) the phenolate oxygen of the merocyanine form plays a key role in the hydrolysis reaction. Hence, the nonprotonated merocyanine isomer is susceptible to hydrolysis, whereas the corresponding protonated form is stable toward hydrolytic degradation.

Photo‐Controlled Synthesis of Responsive Polymer Capsules from Hybrid Core‐Shell Nanoparticles

SummaryHerein, we report for the first time the light‐driven supramolecular engineering of water‐dispersible polymer vesicles. The hollow capsules were controllably formed and ruptured by stimulating photosensitive, spherical polymer brushes on silica particles with the appropriate phototrigger. The polymer brushes were synthesized by surface‐initiated atom transfer radical polymerization (ATRP) and consisted of dual‐responsive 2‐(dimethylamino)ethyl methacrylate (DMAEMA) and light‐sensitive spiropyran (SP) moieties which undergo isomerization to merocyanine (MC) upon UV irradiation. Upon prolonged UV exposure of the sterically crowded polymer brushes, H‐type π − π stacks are formed between the MC isomers, which act as physical cross‐links between the polymer chains. After HF etching of the inorganic core, dual responsive polymeric vesicles are derived, whose walls' robustness is provided by the H‐type aggregates. The hydrophilic and pH‐sensitive properties of the DMAEMA comonomer, facilitate the formation of the hollow capsules in water and their size manipulation via alteration of the solution pH, respectively. The inherent ability of the nanocapsules to fluoresce in water opens new avenues for the development of nanoscale capsules for biomedical applications. Finally, disruption of the stacks and thus the polymer vesicles can be achieved remotely using harmless visible‐light irradiation.

Transient absorption spectroscopy on spiropyran monolayers using nanosecond pump—probe Brewster angle reflectometry

Self-assembled monolayers of 11-(3',3'-dimethyl-6,8-dinitrospiro[chromene-2,2'-indoline]-1'-yl) undecanoic acid (amphiphilic spiropyran) at the air-water interface are studied using Brewster angle reflectometry. Transient kinetics of the spiropyran to merocyanine conversion are recorded in a UV-pump, VIS-probe configuration. By varying the probe wavelength using an optical parametric oscillator, we are able to reconstruct absorption spectra of intermediate states with a time-resolution of 10 nanoseconds, limited by the temporal convolution of the two laser pulses. After UV irradiation, spiropyran converts to merocyanine in two stages. The first occurs within a timescale of several tens of nanoseconds and is heavily convoluted with the system response time, whereas the second stage occurs over a few hundred nanoseconds. During the rise time there is a small red shift in the transient absorption spectrum of ~20 nm. We assign the red shift and the slower kinetics to the isomerization of a merocyanine isomer cis about the central methine bond to those that are trans about the same bond.

Spiropyrans and spirooxazines 9. Photochromism of novel cationic spirooxazines

Novel photochromic spirooxazinoquinolines, their cationic derivatives, and cationic spirooxazine containing pyridinium cation in the aliphatic side chain of the oxazine moiety of the molecule were synthesized. Quaternization of the quinoline fragment leads to a substantial increase in the thermal stability of the merocyanine isomers of cationic spirooxazines and a decrease in the efficiency of photocoloration. Pyridinium cation in the aliphatic side chain of the spirooxazine does not exert a substantial effect on the kinetics of photocoloration and thermal bleaching.

Photochromism of a Merocyanine Dye Bound to Sulfonatocalixarenes: Effect of pH and the Size of Macrocycle on the Kinetics

The effect of 1:1 complex formation on the photochromic behavior of the merocyanine isomer of a nitro-substituted spirobenzopyran dye was studied in aqueous solution using 4-sulfonatocalixarene (SCXn) cavitands possessing four or eight phenol units. The binding constants were independent of the size of the macrocycle, and about 7-fold more stable associates were produced at pH 2.3 than in slightly alkaline solution. The complexation with SCXn diminished the acidity of protonated merocyanine (trans-MCH(+)) and precluded its photoinitiated transition to spirobenzopyran form, but did not affect the reactions in basic media. Upon exposure to light, the complexed trans-MCH(+) was converted to cis isomer. The association with 4-sulfonatocalix[4]arene slowed down the thermal back reaction in the dark to a larger extent than the confinement to 4-sulfonatocalix[8]arene. Both the activation energy and the Arrhenius A factor were significantly larger when the smaller, more rigid macrocycle served as a host.

Self-protonating spiropyran-co-NIPAM-co-acrylic acid hydrogel photoactuators

Up to now, photoresponsive hydrogels and ionogels based on poly(N-isopropylacrylamide) copolymerised with pendant spiropyran groups require exposure to external acidic solution (usually milimolar HCl) to generate the swollen gel prior to photo-triggered contraction. This serious functional limitation has been solved by copolymerising acrylic acid into the gel matrix, to provide an internal source of protons. Due to the relative pKa values of acrylic acid and the spiropyran and merocyanine isomers, the protonation and deprotonation occurs internally within the gel and there is no need for an external source of protons. Furthermore, the shrinking–expansion cycles of these gels in deionised water are repeatable, as protonation throughout the gel does not rely on movement of protons from an external acidic solution into the bulk gel. In contrast to previous formulations, these gels do not show degradation of their photo-induced shrinking ability after multiple washings in deionised water and repeated switching over a 2 month period.

Theoretical and Experimental Study of New Photochromic Bis-Spiropyrans with Hydroxyethyl and Carboxyethyl Substituents

Two new asymmetrical bis-spiropyrans with nonequivalent spiropyran units have been investigated. Hydroxyethyl and carboxyethyl substituents have been used to improve thermal stability of the photoinduced merocyanine forms of the bis-spiropyrans. 2-Hydroxyethyl substituted compound is characterized by 4 times more stable merocyanine isomer. 2-Carboxyethyl substituent in the hetarene part enables chelation by metal ions and controllably stabilizes the merocyanine from thermal decay. As a result of theoretical modeling and photochemical experiments, it was shown that obtained compounds are perspective prototypes for multistate light-driven switches with improved stability of photoinduced forms.

Bis-spironaphthooxazine based photochromic polymer materials

The photochromism and guest binding of spiro-naphtho-oxazine dimer (SNOD) based materials were investigated. The specifically angled, conjugated dimer, SNOD, was covalently connected to α-methoxy-ω-amino-poly(ethylene glycol) (PEG) and studied under UV irradiation. Results showed the specific effects of the polymer chain on their photochromic properties and relaxation kinetics. These new materials exhibited repeatable photo-switchability and little kinetics dependency on solvent polarity. The nanoscale micelles were prepared by self-assembly of amphiphilic SNOD–PEG conjugates in aqueous medium. A palladium catalyst as a guest agent was encapsulated in the micelle core, which reduced the thermal closing rate of the merocyanine isomer of the spirooxazine dimer. SNOD was also conjugated onto solid polymer beads and investigated for the binding affinity with dyes and catalysts. The stable binding affinity towards the palladium catalyst indicated potential applications for selective recognition, extraction, and controlled release.

Selective Acceleration of the Protonated Merocyanine‐Spiropyran Photochromic Transformation by Inclusion in Cucurbit[7]uril

The effect of cucurbit[7]uril (CB7) on the spiropyran-merocyanine photochromic interconversion was studied in acidic and alkaline aqueous solutions. The merocyanine (MC) isomer was found to be the thermodynamically most stable form both in water and in the presence of CB7. A preferential binding of the protonated merocyanine (MCH(+)) to CB7 was observed with an equilibrium constant of 7.4×10(4) m(-1), and the complex formation led to significant diminution of acidity of the guest. The photoinduced transformation of MCH(+) to the spiropyran isomer was accelerated 2.3-fold upon addition of CB7, whereas the rates of the other photochromic processes were not affected. The partial inclusion of MCH(+) in CB7 led to dual fluorescence due to the incomplete deprotonation in the singlet-excited state.

Photo- and thermochromic spirans. 35.* Synthesis and photochromic properties of spiro[indoline-2,3′-pyrano[3,2-f]quinolines] and their cationic derivatives

New photochromic spiropyranoquinolines and their cationic derivatives have been synthesized. Quaternization of the quinoline fragment leads to significant enhancement of the thermal stability of the merocyanine isomers of these cationic spiropyrans.

Reversible low-light induced photoswitching of crowned spiropyran-DO3A complexed with gadolinium(III) ions.

Photoswitchable spiropyran has been conjugated to the crowned ring system DO3A, which improves its solubility in dipolar and polar media and stabilizes the merocyanine isomer. Adding the lanthanide ion gadolinium(III) to the macrocyclic ring system leads to a photoresponsive magnetic resonance imaging contrast agent that displays an increased spin-lattice relaxation time (T1) upon visible light stimulation. In this work, the photoresponse of this photochromic molecule to weak light illumination using blue and green light emitting diodes was investigated, simulating the emission spectra from bioluminescent enzymes. Photon emission rate of the light emitting diodes was changed, from 1.75 × 1016 photons·s−1 to 2.37 × 1012 photons·s−1. We observed a consistent visible light-induced isomerization of the merocyanine to the spiropyran form with photon fluxes as low as 2.37 × 1012 photons·s−1 resulting in a relaxivity change of the compound. This demonstrates the potential for use of the described imaging probes in low light level applications such as sensing bioluminescence enzyme activity. The isomerization behavior of gadolinium(III)-ion complexed and non-complexed spiropyran-DO3A was analyzed in water and ethanol solution in response to low light illumination and compared to the emitted photon emission rate from over-expressed Gaussia princeps luciferase.

Light-Regulated Supramolecular Engineering of Polymeric Nanocapsules

This article describes the light-driven supramolecular engineering of water-dispersible nanocapsules (NCPs). The novelty of the method lies in the utilization of an appropriate phototrigger to stimulate spherical polymer brushes, consisting of dual-responsive 2-(dimethylamino)ethyl methacrylate (DMAEMA) and light-sensitive spiropyran (SP) moieties, for the development or disruption of the NCPs in a controlled manner. The fabrication of the nanocarriers is based on the formation of H-type π-π interactions between merocyanine (MC) isomers within the sterically crowded environment of the polymer brushes upon UV irradiation, which enables the SP-to-MC isomerization of the photosensitive species. After HF etching of the inorganic core, dual-responsive polymeric vesicles whose walls' robustness is provided by the MC-MC cross-link points are formed. Disruption of the vesicles can be achieved remotely by applying a harmless trigger such as visible-light irradiation. The hydrophilic nature of the DMAEMA comonomer facilitates the engineering of the vesicles in environmentally benign aqueous media and enables the controlled alteration of the NCPs size upon variation of the solution pH. The inherent ability of the NCPs to fluoresce in water opens new possibilities for the development of addressable nanoscale capsules for biomedical applications.

Physicochemical study of spiropyran–terthiophene derivatives: photochemistry and thermodynamics

The photochemistry and thermodynamics of two terthiophene (TTh) derivatives bearing benzospiropyran (BSP) moieties, 1-(3,3″-dimethylindoline-6'-nitrobenzospiropyranyl)-2-ethyl 4,4″-didecyloxy-2,2':5',2″-terthiophene-3'-acetate (BSP-2) and 1-(3,3″-dimethylindoline-6'-nitrobenzospiropyranyl)-2-ethyl 4,4″-didecyloxy-2,2':5',2″-terthiophene-3'-carboxylate (BSP-3), differing only by a single methylene spacer unit, have been studied. The kinetics of photogeneration of the equivalent merocyanine (MC) isomers (MC-2 and MC-3, respectively), the isomerisation properties of MC-2 and MC-3, and the thermodynamic parameters have been studied in acetonitrile, and compared to the parent, non-TTh-functionalised, benzospiropyran derivative, BSP-1. Despite the close structural similarity of BSP-2 and BSP-3, their physicochemical properties were found to differ significantly; examples include activation energies (E(a(MC-2)) = 75.05 kJ mol(-1), E(a(MC-3)) = 100.39 kJ mol(-1)) and entropies of activation (ΔS = 43.38 J K(-1) mol(-1), ΔS = 37.78 J K(-1) mol(-1)) for the thermal relaxation from MC to BSP, with the MC-3 value much closer to the unmodified MC-1 value (46.48 J K(-1) mol(-1)) for this latter quantity. The thermal relaxation kinetics and solvatochromic behaviour of the derivatives in a range of solvents of differing polarity (ethanol, dichloromethane, acetone, toluene and diethyl ether) are also presented. Differences in the estimated values of these thermodynamic and kinetic parameters are discussed with reference to the molecular structure of the derivatives.

Microwave-assisted crystallization inclusion of spiropyran molecules in indium trimesate films with antidromic reversible photochromism

An indium trimesate metal–organic framework (JUC-120) which possesses a cubic zeolitic MTN topology, as a new analogue to MIL-100(Al, Fe or Cr) compounds, has been successfully synthesized. Its structure exhibits a mesoporous cage (26 A) and high thermal stability. The assembly of nitrobenzospiropyran derivatives (BSP) into JUC-120 nanocrystals (BSP/JUC-120) is achieved by a microwave-assisted crystallization inclusion approach. The BSP/JUC-120 films have been prepared on quartz wafers by a spin-coating method. The successful encapsulation of BSP molecules into the mesopores of the JUC-120 structure has been verified by N2 adsorption and TGA measurements. The photochromic properties of BSP/JUC-120 films are studied by the UV-Vis and fluorescence spectroscopies. More interestingly, metastable open merocyanine (OMC) species are directly generated from the closed spiropyran form (CSP) without photoirradiation and stabilized for a long period in the BSP/JUC-120 film. The open merocyanine isomer bleaches to the closed spiropyran form by ultraviolet or visible light, and the coloration is regained upon standing in the dark, exhibiting antidromic photochromism. Moreover, the BSP/JUC-120 film shows high reversibility and thermal stability of photochromism. This highly efficient MOF film is expected to be promising in the applications of optical devices.

Switching ability of nitro-spiropyran on Au(111): electronic structure changes as a sensitiveprobe during a ring-opening reaction

Spiropyran is a prototype molecular switch which undergoes a reversible ring-opening reactionby photoinduced cleavage of a C–O bond in the spiropyran (SP) to the merocyanine (MC)isomer. While the electronic states and switching behavior are well characterized insolution, adsorption on metal surfaces crucially affects these properties. Using two-photonphotoemission and scanning tunneling spectroscopy, we resolve the molecular energy levelson a Au(111) surface of both isomeric forms. Illumination at various wavelengths does notyield any observable switching rate, thus evidencing a very small upper limit of thequantum efficiency. Electron-induced switching from the SP to the MC isomer viageneration of a negative ion resonance can be detected with a quantum yield of(2.2 ± 0.2) × 10−10 events/electron in tunneling spectroscopy. In contrast, the back reaction could not be observed. This studyreveals that the switching properties of surface-bound molecular switches can be verydifferent compared with free molecules, reflecting the strong influence of the interactionwith the metal substrate.

Photoinduced variable stiffness of spiropyran-based composites

A quantitative demonstration of reversible stiffness upon appropriate light stimulus in a spiropyran-polymeric composite is presented. The polymeric films containing 3% wt. of the photochromic spiropyran were irradiated with alternating ultraviolet and visible light and the storage modulus was measured. A reversible change in modulus of about 7% was observed. The modulus change was attributed to an interaction of the polar merocyanine with the polymeric chains and/or to a variation of effective free volume induced by merocyanine aggregates formed in the polymer upon ultraviolet irradiation. The effect is fully reversed when the merocyanine isomers turn back to the spiropyran state after visible irradiation.

Photochromism in Cucurbit[8]uril Cavity: Inhibition of Hydrolysis and Modification of the Rate of Merocyanine–Spiropyran Transformations

The effect of inclusion complex formation on the photochromic behavior of a spirobenzopyran dye and its merocyanine isomer was studied in aqueous solution using cucurbit[8]uril (CB8) as a host. The merocyanine (MC) and protonated merocyanine (MCH(+)) were the most stable forms both in water and inside the cavity of CB8. The equilibrium constant of 1:1 complexation with CB8 was found to be 1.7 × 10(5) and 2.0 × 10(6) M(-1) for the former and latter species, respectively. The encapsulation led to significant change in the rate of the photoinduced and thermal photochromic transformations and hindered the hydrolysis of MC. The effect of CB8 on the reaction kinetics strongly altered with pH. The transition from the spiropyran form to trans-MC in a thermal reaction had 33 kJ mol(-1) lower activation energy and more than 5 orders of magnitude smaller Arrhenius pre-exponential factor in CB8 than in water.

Spiropyrans and spirooxazines 8. 5′-(1,3-Benzothiazol-2-yl)-substituted spiro[indoline-2,3′-naphthopyrans]: synthesis and spectral and photochromic properties

Novel photochromic 5′-(1,3-benzothiazol-2-yl)-substituted spiro[indoline-2,3′-naphthopyrans] possessing luminescence properties in both the cyclic and merocyanine forms were synthesized. Unlike the 8′-(1,3-benzothiazol-2-yl)-substituted benzopyran analogs, the synthesized compounds are characterized by the lower relative stability of the merocyanine isomers.