UV-light and visible-light photochromism of inorganic–organic multilayer films based on polyoxometalate and poly(acrylamide)

Novel UV-light and visible-light photochromic inorganic–organic multilayers composed of polyoxometalates (phosphomolybdic acid (PMoA)) and poly(acrylamide) (PAM) were prepared by the layer-by-layer (LbL) self-assembly method. The grown process, composition, surface topography, and photochromic properties and mechanism of the multilayer films were investigated by ultraviolet–visible (UV–vis) spectra, Fourier transform infrared spectra (FT-IR), atomic force microscopy (AFM), electron resonance spectra (ESR), and X-ray photoelectron spectra (XPS). Irradiation with UV-light or visible-light, the transparent films changed from colorless to blue and showed reversible photochromism. PMoA/PAM LbL films had higher photochromic efficiency under UV-light irradiation than visible-light irradiation. The bleaching process occurred when the films were in contact with O₂ in the dark or heated in air. The photochromic process of PMoA/PAM LbL film was in accordance with radical mechanism.     

Sonochemical preparation of photochromic nanocomposite thin film based on polyoxometalates well dispersed in polyacrylamide

Novel photochromic nanocomposite thin film containing phosphotungstic acid entrapped in polyacrylamide was prepared using ultrasound technique. TEM image revealed that the polyoxometalates nanoparticles with narrow size distribution were finely dispersed in polymer matrix. IR spectra showed that the Keggin geometry of polyoxometalates was still preserved inside the composite film and strong Coulombic interaction was built between heteropolyoxometalates and polyacrylamide via hydrogen bond. It was found that the thermal stability of the hybrid film was lower than that of pure polymer but the film was stable enough for photochromic application from the TG-DTA curves. The transparent film changed from colorless to blue under UV irradiation and showed reversible photochromism. The bleaching process occurred when the film was in contact with air or O₂ in the dark. The photoreduced process was in accordance with radical mechanism.     

Photochromic inorganic-organic multilayer films based on polyoxometalates and poly(ethylenimine)

Novel photochromic inorganic-organic multilayers composed of polyoxometalates and poly(ethylenimine) have been prepared by the layer-by-layer (LbL) self-assembly method. The growth process, composition, surface topography, and photochromic properties of the multilayer films were investigated by UV-visible and Fourier transform infrared spectroscopy, atomic force microscopy, electrospin resonance (ESR), and X-ray photoelectron spectroscopy (XPS). Irradiated with ultraviolet light, the transparent films changed from colorless to blue. Moreover, the blue films showed good reversibility of photochromism and could recover the colorless state gradually in air, where oxygen plays an important role in the bleaching process. On account of the ESR and XPS results, parts of W6+ in multilayers were reduced to W5+, which exhibited a characteristic blue; a possible photochromic mechanism can be speculated. This work provides basic guideline for the assembly of multilayers with photochromic properties.     

Preparation and Visible-light Photochromism of Phosphomolybdic Acid/Polyvinylpyrrolidone Hybrid Film

The visible-light photochromic hybrid film was constructed by entrapping phosphomolybdic acid（PMoA） into polyvinylpyrrolidone（PVPd） networks. The microstructure, photochromic properties and mechanism were inves- tigated with transmission electron microscopy（TEM）, atomic force microscopy（AFM）, Fourier transform infrared （FTIR） spectroscopy, ultraviolet-visible（UV-Vis） spectra and X-ray photoelectron spectroscopy（XPS）. The results in- dicate that the Keggin geometry of PMoA and the basic structure of PVPd are not destroyed during the composite process. Irradiated with visible light, the transparent PMoA/PVPd film changes color from colorless to blue and ex- hibits reversible photochromism in the presence of oxygen. According to the XPS analysis, the charge-transfer bridge of N-H-O has been built between PMoA and PVPd matrix via non-covalent bonding, and the appearance of Mo5＋ species indicates that the photo-reduction process is in accordance with the proton transfer mechanism.     

Chiral,Thermally Irreversible and Quasi-Stealth Photochromic Dopant to Control Selective Reflection Wavelength of Cholesteric Liquid Crystal

A chiral and thermally irreversible photochromic fulgide derivative incorporating an (R)-binaphthol unit in its acid anhydride moiety was used for the photoswitching of the pitch length of cholesteric liquid crystals. Since the absorption maximum wavelengths of both thermally stable photoisomers are nearly in the UV region (quasi-stealth photochromism), it can be exposed to visible light without inducing photochromic reactions. Therefore, when the photoswitching molecule is added to a permanent cholesteric liquid crystal whose reflection light wavelength is in the visible region, the UV light-induced photochromic reaction of the photoswitching molecule changes the wavelength of the reflection light in the visible light region. We have succeeded in regulating the color of cholesteric liquid crystalline cells between red and blue upon UV light irradiation. Attempts to introduce this system in polymer dispersed cholesteric liquid crystals are also described.     

Synthesis, crystal structure and characterization of photochromic 1,2-bis{2-ethyl-5-[2-(1,3-dioxolane)]-3-thienyl}perfluorocyclopentene

A novel photochromic diarylethene, 1,2-bis{2-ethyl-5-[2-(1,3-dioxolane)]-3-thienyl}perfluorocyclopentene (BEDTP), was synthesized and its structure was determined by single-crystal X-ray diffraction analysis. Its photochromic and fluorescent properties were also investigated. The results showed that this compound exhibited reversible photochromism, changing from colorless to magenta after irradiation with UV light both in solution and in the crystalline phase. In hexane solution, the open-ring isomer of BEDTP exhibited relatively strong fluorescence at 400 nm when excited at 282 nm. The fluorescence intensity decreased along with the photochromism upon irradiation with 254 nm light and its closed-ring isomer showed almost no fluorescence.     

Photochromic polyoxotungstoeuropate K12[EuP5W30O110]/polyvinylpyrrolidone nanocomposite films

A novel photochromic nanocomposite film containing polyoxotungstoeuropate K₁₂[EuP₅W₃₀O₁₁₀] entrapped in polyvinylpyrrolidone has been prepared through a spin-on coating technique. Thus-obtained amorphous nanocomposite film was characterized by IR spectra, UV-vis absorption spectra, XRD, SEM, TG-DTA, and ESR. Results show that polyoxotungstoeuropate interacts with polyvinylpyrrolidone strongly and disperses homogeneously in the matrix. The composite film exhibits good photochromic properties. When irradiated with UV light, the transparent film changes from colorless to blue. Then, bleaching occurs when the film is in contact with ambient air or O₂ in the dark. The photochromism of the composite film is due to charge transfer by reduction of polyoxotungstoeuropate and oxidation of polyvinylpyrrolidone.     

Two-photon photochromism of two simple chromene derivatives

Two kinds of typical chromene derivatives, 5a,9-dimethyl-5aH-benzo[b]chromene (1) and 7a,11-dimethyl-7aH-dibenzo[b,f]chromene (2), were synthesized to understand the relationship between molecular structures and two-photon photochromism for chromene derivatives. The photochromic performances of 1 and 2 in hexane were investigated by measuring ultraviolet (UV)–visible absorption and ¹H nuclear magnetic resonance spectral changes upon UV irradiation. The results indicate that 1 showed the two-photon photochromic reaction, whereas the main photochemical reaction of 2 was the isomerization between the open-E and open-Z isomers. Density functional theory (DFT) calculations suggest that these different photochromic performances are attributed to the differences in relative stabilities of closed, open-Z and open-E isomers for 1 and 2. Combined with DFT calculations for 12aH-12a-methyl-naphtho[3,2]chromene (3), the most typical chromene derivative well exhibiting two-photon photochromism, we conclude that chromene derivatives show the best two-photon photochromic performances only if closed isomers are more stable than open-Z and open-E isomers.     

The Unusual Photochromic and Hydrochromic Switching Behavior of Cellulose-Embedded 1,8-Naphthalimide-Viologen Derivatives in the Solid-State

Stimuli-responsive chromic materials such as photochromics, hydrochromics, thermochromics, and electrochromics have a long history of capturing the attention of scientists due to their potential industrial applications and novelty in popular culture. However, hybrid chromic materials that combine two or more stimuli-triggered color changing properties are not so well known. Herein, we report a design strategy that has led to a series of emissive 1,8-naphthalimide-viologen dyads which exhibit unusual dual photochromic and hydrochromic switching behavior in the solid-state when embedded in a cellulose matrix. This behavior manifests as reversible solid state fluorescence hydrochromism upon changes in atmospheric relative humidity (RH), and reversible solid state photochromism upon generation of a cellulose-stabilized viologen radical cation. In this design strategy, the bipyridinium unit serves as both a water-sensitive receptor for the hydrochromic fluorophore-receptor system, and a photochromic group, capable of eliciting its own visible colorimetric response, generating a fluorescence quenching radical cation with prolonged exposure to ultraviolet (UV) light. These dyes can be inkjet-printed onto cellulose paper or drop-cast as cellulose powder-based films and can be unidirectionally cycled between three different states which can be characteristically visualized under UV light or visible light. The material's photochromism, hydrochromism, and underlying mechanism of action was investigated using computational analysis, dynamic vapor sorption/desorption isotherms, electron paramagnetic resonance spectroscopy, and variable humidity UV-Vis adsorption and fluorescence spectroscopies.     

Photochemically-induced refractive index and fluoresence patterning on polymer films

The molecular design for large photo-induced refractive index changes in transparent visible light region was proposed and realized with norbornadiene polymers and poly(vinyl cinnamate). The patterning of pure refractive-index contract on their transparent films was made with near-field scanning optical microscopy (NSOM). Reversible fluorescence patterning on polymer films is also presented by using controlled energy transfer from a fluorescent pyromethene to a photochromic diarylethene.     

Electron transport in silver-semiconductor nanocomposite films exhibiting multicolor photochromism

In the multicolor photochromism of TiO2 nanoporous films loaded with photocatalytically deposited Ag nanoparticles, visible light-induced electron transfer from Ag to oxygen molecules plays an essential role. Here we examined the effect of TiO2 on the electron transfer. We found that not only photocatalytically deposited Ag, but also electrodeposited Ag and commercially available Ag nanoparticles in a nanoporous TiO2 film exhibit the multicolor photochromism. The electrodeposited Ag exhibits the multicolor photochromism also in a nanoporous ZnO film, but not in nanoporous indium-tin oxide (ITO) and SiO2 matrices. Photoelectrochemical measurements for the Ag-TiO2 nanocomposite elucidated that some of the photo-excited electrons on Ag are transferred to oxygen molecules via TiO2 and non-excited Ag. Thus, an n-type semiconductor plays an important role in the charge separation between the excited electrons and Ag+. Non-excited Ag on TiO2 also plays an important role in the charge separation and/or catalysis of oxygen reduction. Replacement of the non-excited Ag with Pt accelerated the electron transport from the photo-excited Ag to oxygen molecules and the photochromic behavior.     

Facile fabrication of snowman-like Janus particles with asymmetric fluorescent properties via seeded emulsion polymerization

This paper presents a facile method for the preparation of snowman-like Janus particles (SJP) with asymmetric fluorescent property via seeded emulsion polymerization, in which in situ formed raspberry-like cadmium sulfide/poly(styrene–divinylbenzene–acrylic acid) nanocomposite particles (RNP) were used as the seeds. The as-prepared RNP and SJP have been thoroughly characterized by transmission electron microscopy, field-emission scanning electron microscopy, thermogravimetric analysis, X-ray powder diffraction, Fourier transform infrared, ultraviolet visible, and photoluminescent spectrometry. It is found that the size ratio of the polymer bulge/inorganic seed part could be continuously tuned as well as the composition of polymer bulges by changing the composition of monomer mixtures and monomer/seed weight ratio. The obtained Janus particles possess amphiphilic properties which can be further used as solid surfactants to stabilize W/O emulsions and successively to construct hierarchical structured materials. Meanwhile, their asymmetric fluorescent properties may be exploited to detect their assembled situation and orientation at the oil–water interface of emulsions as well as at the surface of hierarchical structured materials.

New Sol-Gel Photochromic Thin Films Made by Super-Fine Particles of Organic Photochromic Compounds

New photochromic thin gel films have been prepared by the dipping method from a sol-gel coating solution containing Malachite Green Leucocyanide soluble in sol and super-fine dispersed solution of spiropyran or spironaphthooxazine, which was effective to increase the concentration of photochromic compounds in gel film. Photochromism of these gel films is compared with that in sol under the conditions of acid catalysis. Though in sols using spiropyran and Malachite Green Leucocyanide normal photochromism was not observed, photochromism in thin gel films prepared from the same sol was confirmed. Normal photochromism of spironaphthooxazine in thin gel films was similar to that in solution. The reaction rate from the colored form to the spiro form of spiropyran and spironaphthooxazine in thin gel film is dependent on the gel matrix, which is influenced by the reaction time of the sol.     

Remediation of Wastewater with Ultraviolet Irradiation Using a Novel Titanium (IV) Oxide Photocatalyst

Photocatalysis has become common and nanomaterials having photocatalytic functions have been widely characterized. At present, among the many candidates for photocatalysis, TiO₂ is almost the only material suitable for industrial use. In this paper, we present a TiO₂ synthesis starting from Ti sheets put in contact with a mixture of 0.1 N NaOH and acetone for 72?hours under ambient conditions. The obtained sheets were washed with distilled water and ethanol, and the surface was analyzed for its structural and morphological properties. Thus, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) investigations indicated the formation of TiO₂ on the edges of nanometer circles on the surface of the Ti sheets. For characterizing the photocatalytic capacity for wastewater treatment, Ti sheets with TiO₂ on the surface contacted with methylene blue solutions at room temperature under ultraviolet light. The degradation of the methylene blue concentration was measured by ultraviolet–visible spectroscopy, demonstrating 99.94% efficiency for wastewater treatment using the obtained material.     

Cotton fabrics treated with acylhydrazone-based polyviologen to create innovative multi-stimulus responsive textiles

Novel multi-stimuli responsive cotton fibers were developed via spray-coating with an acylhydrazone-based polyviologen (AHPV). Polyviologen was prepared by supramolecular condensation polymerization of bipyridinium dialdehyde with a hydroxyl-substituted aryldihydrazide in an acidified aqueous medium. Transparent AHPV/resin nanocomposite film was deposited onto the surface of cotton fabric by well-dispersion of AHPV as a chromogenic substance in a resin binding agent. Increasing the temperature of the AHPV-coated cotton fabric from room temperature to 85 °C reversibly triggered a change in color from pale yellow (437 nm) to green (607 nm), respectively. The transparent layer immobilized onto the white cotton surface transformed into green under ultraviolet source as demonstrated by CIE Lab parameters. The photochromic impacts were explored at various AHPV. In addition, the AHPV-coated cotton immediately displayed a vapochromic activity upon exposure to NH_3(g), and then recovered to pale yellow after removing the ammonia source away. The current AHPV-coated cotton fabric displayed a limit of detection (LOD) to NH_3(aq) in the range of 50–150 ppm. The spray-coated cotton fabrics demonstrated a reversible photochromism, thermochromism and vapochromism with high stability. The produced AHPV nanoparticles were also studied by transmission electron microscopy (TEM), demonstrating particle diameter of 74–92 nm. The mechanical and morphological properties of the spray-coated cotton fabrics were also explored. The surface morphology of AHPV-finished samples was examined by Fourier-transform infrared (FTIR) and scanning electron microscopy (SEM). No considerable defects were observed in permeability to air and bending length of AHPV-finished samples. Additionally, high colorfastness was monitored for the AHPV-finished cotton substrates. The cytotoxic activity of the AHPV-finished cotton was also examined. Mechanistic study accounting for the multichromic activity of acylhydrazone-based polyviologen is explored.     

Efficient One‐step Synthesis and Properties of Photochromic Diarylethenes Having an Indene Bridging Unit

Two novel isomeric photochromic diarylethenes with an indene bridging unit have been prepared by a simple and efficient one‐step synthesis method. Their properties, including photochromic behavior, fluorescent properties and fatigue resistance, have been investigated. These two isomeric compounds showed photochromic back‐and‐forth reactions with ultraviolet and visible light both in solution and in PMMA film. Their ring‐open forms exhibited appreciable fluorescence, which was quenched by the ring‐closed forms. All results indicated that diarylethenes derivatives with indene‐aryl bridges exhibited rather high fatigue resistance and good thermally irreversible photochromic properties.     

In-situ crystal structure analysis and control of photochromism with dual-mode photoreactive soft crystals

Photochromic compounds are of great interest currently owing to their potential applications. Their three-dimensional structure must be clarified to understand the photochromism mechanism. In-situ X-ray crystal structure analysis is a powerful tool for determining the structure directly after the photochromic reaction. In this review, I discuss about solid-state photochromic compounds, their direct X-ray observation in the crystal form, and in-situ control of photochromism in “dual photoreactive soft crystals”, which is a novel approach.     

A novel Ni/Na – Containing inorganic-organic hybrid supramolecule based on polyoxometalate and EDTA with ultraviolet-visible light photochromism

A novel Ni/Na – containing inorganic-organic hybrid supramolecule {(PW₁₂O₄₀)·[Na₂(NiH₂EDTA·H₂O)(H₄EDTA)·2H₂O]·2H₂O·H₃O}_n (short for NiEDTA-PW₁₂) has been successfully synthesized by solution method, and investigated by thermogravimetric-differential thermal analysis (TG-DTA), ultraviolet visible (UV-Vis) spectroscopy, cyclic voltammetry (CV), photoluminescence (PL), ultraviolet visible diffuse reflectance spectrum (UV-vis DRS) and single-crystal X-ray diffraction (XRD). NiEDTA-PW₁₂ exhibits intriguing infinite supramolecular structure with Na⁺ ions as linker. Furthermore, NiEDTA-PW₁₂ displays a fast-responsive reversible photochromism under ultraviolet or visible light. The photochromic property of NiEDTA-PW₁₂ has been investigated by techniques of UV-vis DRS and PL, and the impact of the O₂ on fading of the colored NiEDTA-PW₁₂ has been investigated.     

Multistate/multifunctional switches based on photochromic Schiff base

The salicylidene Schiff base derivative, namely, N-salicylidene-(S)-alpha-naphthylethylamine (SNEA) has been synthesized to study the characterization of multistate/multifunctional switches. Upon the stimulations of optical inputs (UV light and visible light) and chemical inputs (pH and Zn(2+)), SNEA undergoes reversible photochromism, deprotonation and complexation reactions. In this case, four chemical species are involved. These interconversions of SNEA between four states have been systematically investigated by the absorption and the emission spectra. Spectroscopic studies indicate that the photochromic switch, pH switch and fluorescent switch can be realized using the single molecular entity of SNEA.     

Synthesis of Pt/ionic liquid/graphene nanocomposite and its simultaneous determination of ascorbic acid and dopamine

A water-soluble and electroactive composite - Pt nanoparticles/polyelectrolyte-functionalized ionic liquid (PFIL)/graphene sheets (GS) nanocomposite was synthesized in one pot. The structure and composition of the Pt/PFIL/GS nanocomposite were studied by means of ultraviolet-visible (UV-vis) and X-ray photoelectron spectra (XPS). Scanning electron microscopy (SEM) and transmission electron microscope (TEM) images reveal Pt nanoparticles are densely dispersed on the transparent thin PFIL-functionalized graphene sheets. The obtained Pt/PFIL/GS nanocomposite-modified electrode was fabricated to simultaneously determine ascorbic acid (AA) and dopamine (DA) by cyclic voltammetry. It is worthwhile noting that the difference between the two peak potentials of AA and DA oxidation is over 200 mV, which leads to distinguishing AA from DA. The detection of increasing concentrations of AA in the presence of DA and the oxidation of continuous addition of DA in the presence of AA were also studied using differential pulse voltammetry. The proposed sensor in real sample analysis was also examined in human urine samples. Three independent oxidation peaks appear in urine sample containing AA and DA. Therefore, the Pt/PFIL/GS nanocomposite might offer a good possibility for applying it to routine analysis of AA and DA in clinical use.