Neutral Spiropyran Research Articles

Photochromism of neutral spiropyran in the crystalline state at room temperature

Solid-state photochromic materials are very attractive due to their promising future in advanced functional materials with reversible and tunable optical properties.

Phototropic Multiresponsive Active Nanogels.

Aqueous dispersions of nanogels that respond to switches in environmental pH and/or temperature by changes in their hydrodynamic radius (Rh ) and/or ζ-potential are prepared by reversible addition-fragmentation chain-transfer (RAFT) polymerization-induced thermal (70 °C) self-assembly (PITSA) of N-isopropylacrylamide (NIPAM) in the presence of a poly(methacrylic acid) (PMAA)-substituted macromolecular chain transfer agent and a cross-linker. Photochromic spiropyran (SP) moieties are coupled to the carboxylic acid groups of the nanogels. Upon UV irradiation, the neutral SP isomerizes to the zwitterionic merocyanine (ME) form. Upon UV light irradiation, microgels formed by assembly of SP nanogels undergo a collective motion toward the UV-light source.

Asymmetric heterostructured SiO2/Al2O3 nanofluidic diodes modulating ionic transport for highly efficient light-gating device

Bioinspired artificial asymmetric nanochannels for more precise controls of ion transport and ion pump at the nanometer scale have been attracting some attention for extensive application in biosensors, ions separation technologies, nanofluidic diodes and energy device. Herein, we construct the first asymmetric heterostructured SiO2/Al2O3 nanofluidic diodes based on the introduction of physical structures and chemical surface activity. Due to asymmetric chemically deposited coatings and surface charges distribution, the SiO2/Al2O3 nanofluidic diodes are capable of regulating ion transport selectivity and possessing ionic current rectification properties. These asymmetric heterostructured SiO2/Al2O3 nanofluidic diodes are further introduced spiropyran into nanochannels to create a lightgating device, which were triggered by ultraviolet light to be “ON” state from the neutral spiropyran blocked “OFF” state. This asymmetric heterostructured SiO2/Al2O3 nanofluidic diode epitomizes high sensitivity, fine selectivity, superior stability and highly reversibility. The combination with photoswitchable compounds spiropyrans shows great promise for easy application in the optical device such as highly sensitive light gating devices.

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.

Photoswitchable Hydrogel Surface Topographies by Polymerisation‐Induced Diffusion

Herein, we describe the preparation of patterned photoresponsive hydrogels by using a facile method. This polymer-network hydrogel coating consists of N-isopropylacrylamide (NIPAAM), cross-linking agent tripropylene glycol diacrylate (TPGDA), and a new photochromic spiropyran monoacrylate. In a pre-study, a linear NIPAAM copolymer (without TPGDA) that contained the spiropyran dye was synthesised, which showed relatively fast photoswitching behaviour. Subsequently, the photopolymerisation of a similar monomer mixture that included TPGDA afforded freestanding hydrogel polymer networks. The light-induced isomerisation of protonated merocyanine into neutral spiropyran under slightly acidic conditions resulted in macroscopic changes in the hydrophilicity of the entire polymer film, that is, shrinkage of the hydrogel. The degree of shrinkage could be controlled by changing the chemical composition of the acrylate mixture. After these pre-studies, a hydrogel film with spatially modulated cross-link density was fabricated through polymerisation-induced diffusion, by using a patterned photomask. The resulting smooth patterned hydrogel coating swelled in slightly acidic media and the swelling was higher in the regions with lower cross-linking densities, thus yielding a corrugated surface. Upon exposure to visible light, the surface topography flattened again, thus showing that a hydrogel coating could be created, the topography of which could be controlled by light irradiation.

Reversibly Light-Modulated Dirac Point of Graphene Functionalized with Spiropyran

Graphene has been functionalized with spiropyran (SP), a well-known photochromic molecule. It has been realized with pyrene-modified SP, which has been adsorbed on graphene by π-π interaction between pyrene and graphene. The field-effect transistor (FET) with SP-functionalized graphene exhibited n-doping effect and interesting optoelectronic behaviors. The Dirac point of graphene in the FET could be controlled by light modulation because spiropyran can be reversibly switched between two different conformations, a neutral form (colorless SP) and a charge-separated form (purple colored merocyanine, MC), on UV and visible light irradiation. The MC form is produced during UV light irradiation, inducing the shift of the Dirac point of graphene toward negative gate voltage. The reverse process back to the neutral SP form occurred under visible light irradiation or in darkness, inducing a shift of the Dirac point toward positive gate voltage. The change of the Dirac point by UV and visible light was reproducibly repeated. SP molecules also improved the conductance change in the FET device. Furthermore, dynamics on conversion from MC to SP on graphene was different from that in solution and solid samples with SP-grafted polymer or that on gold nanoparticles.

Stabilization of merocyanine by protonation, charge, and external electric fields and effects on the isomerization of spiropyran: a computational study

Protonation, charging, and field effects on the thermal isomerization of a nitrospiropyran (SP) modified by a thiolated etheroxide chain into merocyanine (MC) are computationally studied at the DFT level. The ring opening leads to cis-MC conformers that then isomerize to the more stable trans forms. While the closed neutral spiropyran is more stable than the conjugated open forms, the merocyanine conformers are significantly stabilized by protonation, electron attachment, and ionization. For protonation on the pyran oxygen atom and electron attachment, the MC conformers are more stable than SP, and unlike for the neutral species, the ring opening is spontaneous at room temperature. Moreover, for the pyran oxygen-protonated form, the ring opening to the cis-merocyanine becomes barrierless. On the other hand, barriers comparable to the neutral remain along the thermal pathway to the cis-merocyanine conformer for ionization or electron attachment, and the barrier for isomerization is significantly higher for the N-protonated SP form. External field effects on the neutral reaction path show that ring opening to the cis-merocyanine is favored when the field reduces the electron density on the pyran part, as also induced by the local field due to O protonation.

1-Benzyl-3,3,5′,6′-tetramethylspiro[indoline-2,2′-[2H]pyrano[3,2-b]-pyridinium] iodide, its hydrate, and a neutral precursor of the salts: synthesis, crystal structure, photochromic transformations in solutions and in crystals

A new 1-benzyl-3,3,5′,6′-tetramethylspiro[indoline-2,2′-[2H]pyrano[3,2-b]pyridinium] iodide, photochromic in the crystal state, was obtained. X-ray diffraction analysis was used for determination of crystal structures of the salt, its hydrate, and a neutral precursor. It was found that a replacement of the substituent in the indoline fragment leads to a considerable change in the crystal structure of both the neutral spiropyran and the salts as compared to the analogs studied earlier. Effects of crystal structure on photochromic properties were studied.

Optically Triggered Dissociation of Kinetically Stabilized Block Copolymer Vesicles in Aqueous Solution

We demonstrate a strategy for using an optical stimulus to trigger the dissociation of block copolymer (BCP) vesicles in aqueous solution. The BCP, comprising hydrophilic poly(ethylene oxide) (PEO) and a block of poly(methacrylic acid) bearing a number of spiropyran methacrylate comonomer units (P(MAA-co-SPMA)), was allowed to firstly self-assemble into large vesicles in aqueous solution at pH=3 with protonated carboxylic acid groups, and then become kinetically stable at pH=8 due to the glassy vesicle membrane of P(MAA-co-SPMA). Fast dissociation of the vesicles was achieved through a cascade of events triggered by UV-induced isomerization from neutral spiropyran to charged merocyanine in the membrane.

AFM Observation of Stimuli-Responsive Stretching Behavior for Spirobenzopyran/Crown-Ether Copolymer

Abstract We have observed stretching behavior of a stimuli-responsive polymer chain bearing two functional groups, spirobenzopyran (SP) and crown ether (CR), at the side chain (SP/CR copolymer) by atomic force microscopy (AFM). The spirobenzopyran side chain can undergo photoisomerization from its electrically neutral spiropyran form to zwitterionic merocyanine by UV irradiation. The crown ether side chain can complex with metal ions. These reactions in the side chains would cause changes in their geometric structure and/or physical properties, resulting in the enhanced bifunctionality of SP/CR copolymer based on their stimuli-responsive ability. In this study, SP/CR copolymer, which was adsorbed on mica substrate, was stretched by AFM tip, when external stimuli (i.e., photoirradiation or metal ion addition) were applied. The force–extension curves of SP/CR copolymer were then analyzed by curve fitting based on a theoretical model of polymer chain elasticity to elucidate the external-stimuli effect on the polymer stretching behavior at the single molecule level.

Electronic Tera-Order Stabilization of Photoinduced Metastable Species: Structure of the Photochromic Product of Spiropyran Determined with in situ Single Crystal X-ray Photodiffraction

The extraordinary stability of the photoinduced red form of a cationic spiropyran ( k approximately 10 (-6) s (-1) in water and approximately 10 (-6) to less than 10 (-8) s (-1) in the solid state) was employed to obtain in situ X-ray diffraction evidence of its molecular structure. By UV excitation under selected experimental conditions, on average, approximately one third of the cations in a single crystal of spiropyran iodide salt was converted and retained as the red form during the experiment. According to the structure of the mixed crystal, the ring opening, which is due to increased distance between the spiro oxygen and carbon atoms, is associated with slight molecular flattening caused by concurrent out-of-plane shift (11.2(5) degrees ) of the pyranopyridinium half and in-plane shift (4.8(7) degrees ) of the indoline half. The overall geometry change of the cation fits the steric requirements imposed by the ion packing in the crystal and can be viewed as molecular flattening caused by breaking of the spiroconjugation. The structure of the cation confirms that (at least in the case of cationic spiropyrans) the product is confined in the crystal mainly as a zwitterionic resonance structure in cis configuration similar to the (early) transition state. Although the positive charge of the closed form facilitates the ring-opening reaction by moving the reactant closer to the transition state, neither the weakening of the spiropyran C-O bond nor the space provided by the iodide alone can account for the stability of the product. Instead, the density functional theory calculations indicate that the stabilization of the red form of the cationic relative to the neutral spiropyran is thermodynamically controlled, probably through compensation of the charge within the zwitterion by the methylpyridinium group.

Photo-and thermochromic spiranes. 29. New photochromic indolinospiropyrans containing a quinoline fragment

A new photochromic spiropyran of the indoline series, containing a quinoline fragment, has been synthesized. The photochromic properties of the spiropyran and its iodomethylate have been studied. Features of the molecular structure of the compounds were determined by X-ray structural investigations. It was established that the pyridopyran fragments of the cations in the crystal of the iodomethylate salt, unlike those in the crystal of the neutral spiropyran, emerge from 21-stacks, disposed jointly with I− ions, which causes their higher mobility and aids the course of reversible photoconversions on UV irradiation in solution.

Application of Photoresponsive Polymers Carrying Crown Ether and Spirobenzopyran Side Chains to Photochemical Valve

A vinyl copolymer carrying crown ether and spirobenzopyran side chains, which undergoes significant photoinduced rheology changes, i.e., contraction and extension of its polymer chain, was applied to a material for photocontrol system of solvent permeation rate, so-called, photochemical valve. Macroporous polyethylene membranes coated by the crown ether-spirobenzopyran copolymer can work as a functional membrane controlling solvent permeation rate photochemically. UV-light irradiation on the photoresponsive membrane decreased the permeation rate of hexane, due to the increased polarity of the membrane pore, which was in turn derived from the photoisomerization of its spirobenzopyran moiety to the corresponding ionic merocyanine form. The following visible-light irradiation on the membrane restored the permeation rate by isomerization back to the electrically neutral spiropyran form. To the contrary, the permeation of ethanol through the membrane was enhanced by UV-light irradiation due to the increase in the apparent membrane pore size induced by the polymer chain contraction and vice versa by visible light. Similar photoresponses in the permeation rate of nonpolar and polar solvents were also observed with a sintered glass filter modified chemically by both silane-coupling reagents containing crown ether and spirobenzopyran moieties.

Structure and photochromic properties of single crystals of spiropyran salts

Molecular and crystalline structure has been studied for the single crystals of neutral spiropyran (SP) 1,3,3,6′-tetramethyl-spiro[indoline-2,2′-[2H]pyrano[3,2-b]pyridine] (Ia) and its salts, 1,3,3,5′,6′-pentamethyl-spiro[indoline-2,2′-[2H]pyrano[3,2-b]pyridinium] iodide (I) and 5′-hydroxymethyl-1,3,3,7′,8′-pentamethyl-spiro[indoline-2,2′-[2H]pyrano[3,4-b]pyridinium] iodide (II). The crystalline structure of salts I and II differs from that of the neutral SP being formed by the cationic layers separated by I− ions, thus providing a sufficient volume for spiropyran phototransformations. Photochromism of I and II single crystals has been first studied. The quantum yield of the open form formation is about 0.1, and of bleaching 0.04. Ten-fold reversible photocycling reduces absorption of the colored form by 10–15%. Photochromic properties have been correlated with the single crystals structure.

Cations of Photochromic Spiropyrans as Promising Blocks for Polyfunctional Materials

ABSTRACT Molecular and crystalline strucutre has been studied for the single crystals of neutral spiropyran (SP) 1,3,3,6′-tetramethylspiro[indoline-2,2′-[2H]pyran[3,2-b] pyridine ( Ia ) and its salts, 1,3,3,5′,6′-pentamethylspiro[indoline-2,2′-[2H]pyran[3,2-b]pyridinium] iodide ( I ) and 5′-hydroxymethyl-1,3,3,7′,8′-pentamethylspiro[indoline-2,2′-[2H]pyran[3,4-b]-pyridinium iodide ( II ). Compounds are monoclinic, i.e., for Ia : a = 11,942(2), b = 11,733(2), c = 11,493(2) A˚, α = 90°. β = 96,96(3)°, γ = 90°, V = 1568,5(5) A˚, ρcalc. = 1.215 g/cm3, Z = 4; I : a = 24.107(5), b = 10.959(2), c = 15.557(2) A˚, α = 90°, β = 108.74°(3), γ = 90°, V = 3892.1(13), A˚3, ρ calc. = 1.482 g/cm3, Z = 8, space group P2(1)/c, while for compound II a = 12.789(3), b = 10.807(2), c = 15.612(3) A˚, α = 90°, β = 108.09°(3), γ = 90°, V = 2050.8(7) A˚3, ρcalc = 1.578 g/cm3, space group P2(1)/n, Z = 4. The crystalline structure of salts I and II differs from that of the neutral SP being formed by the cationic layers separated by I− ions, thus providing a sufficient volume for spiropyran phototransformations. Photochromism of I and II single crystals has been first studied. The quantum yield of the open form formation is about 0.1, and of bleaching 0.04. Tenfold reversible photocycling reduces absorption of the colored form by 10% Photochromic properties have been correlated with the single crystals structure.

Photoswitching of Single Association Force between a Pair of Photoionizable Spirobenzopyrans

We have measured the single intermolecular force of a typical photoionizable molecule, spirobenzopyran, by means of atomic force microscopy, which has proven to be useful in measuring directly single molecular forces. The spirobenzopyran moiety was immobilized covalently on both Au-coated probe tips and substrates by use of a self-assembled monolayer of a hexanethiol derivative incorporating a terminal spirobenzopyran moiety, 1'-(6'-mercaptohexyl)-3',3'-dimethylindolino-6-nitrospiro-(2H-1-benzopyran-2,2'-indoline). Force curve measurements were carried out using the spirobenzopyran-modified probe tip and substrate under dark conditions and in situ UV light irradiation. The adhesion force observed in a polar solvent (i.e., ethanol) was increased substantially under in situ UV light irradiation, which caused photoisomerization of the spirobenzopyran moiety bound to both tip and substrate from its electrically neutral spiropyran form to the corresponding zwitterionic merocyanine one. Statistical analyses of the observed force by autocorrelation technique have revealed that the photoionization enhanced by UV light caused a remarkable increase in the single intermolecular force of the photochromic compound.