Photochromic Process Research Articles

Anion-Controlled Architecture and Photochromism of Naphthalene Diimide-Based Coordination Polymers.

Three new cadmium coordination polymers, namely [Cd(NO3)2(DPNDI)(CH3OH)]·CH3OH (1), [Cd(SCN)2(DPNDI)] (2), and [Cd(DPNDI)2(DMF)2]·2ClO4 (3) (DPNDI = N,N-di(4-pyridyl)-1,4,5,8-naphthalene diimide, DMF = N,N-dimethylformamide) have been synthesized by reactions of DPNDI with Cd(NO3)2, Cd(SCN)2, and Cd(ClO4)2, respectively. Compound 1 is a one-dimensional coordination polymer with strong lone pair-π interactions between the coordinated NO3− anions and the imide ring of DPNDI; while 2 is a two-dimensional network with a (4, 4) net topology. In the case of 3, due to the presence of uncoordinated perchlorate counter ions, it exhibits a non-interpenetrated square-grid coordination polymer containing one-dimensional rhomboid channels. The structural diversity in these compounds is attributed to different coordination abilities and geometries of counter anions. Due to the presence of electron-deficient NDI moiety, the photochromic behavior of these compounds was studied. Interestingly, only compounds 1 and 3 exhibit color changes under light irradiation. The influence of the anions on the photochromism process of the NDI-based materials has been discussed.

Reversible luminescence modulation of Ho‐doped K0.5Na0.5NbO3 piezoelectrics with high luminescence contrast

AbstractA significant luminescence modulation behavior based on photochromic reactions was observed in Ho3+‐doped (Na0.52K0.48)0.92Li0.08NbO3 ceramics, fabricated by the conventional solid‐state reaction method. Under visible light irradiation (407 nm) for 20 second, the samples changed pale gray from initial pale green, and returned to their original color by a thermal stimulus of 230°C for 10 minutes, showing typical photochromic phenomenon. Under 453 nm excitation, the samples exhibited strong green emission at 551 nm. Interestingly, their green emission intensity can be effectively tailored by controlling photochromic reaction processes (irradiation wavelength and time), and the luminescent modulation ratio (ΔRt) reaches up to 77%. And, the ΔRt value has no any obvious degradation after 10 cycles by alternating visible light irradiation and thermal stimulus, showing excellent reversibility. These results make it potential applications in many fields as a kind of multifunctional material.

Photoelectron-Regulated Redox Reaction of Polyaniline for Visual Detection of Trace Copper

Abstract A photochromic sensing platform composing of emeraldine salt of polyaniline (ES-PANI) and titanium dioxide nanoparticles (TiO 2 NPs) for visual detection of trace copper was developed. Under ultraviolet light irradiation, the greenish ES-PANI could be oxidized to dark blue pernigraniline salt by the photo-generated hole of excited TiO 2 NPs. In the presence of Cu 2+ , a light yellow leucoemeraldine salt was visually observed. The overall mechanism of color change was verified to be corresponding to the different redox states of PANI regulated by Cu species during the photochromic process. By integrating the advantages of both photoelectric property and visual detection, the redox reaction-based sensing mechanism led to a good sensitivity and high selectivity in detection of Cu 2+ with the limit of detection of 0.4 μM. Besides the naked eyes, two color recognition methods including reading mean green intensities in Photoshop and recording ultraviolet absorbance in microplate reader were also studied. This method was successfully applied to Cu 2+ detection in human hair, and satisfactory recoveries were achieved. More significantly, this sensing platform was really simple, low-cost and able to detect an array of analytes within several minutes without requiring sophisticated equipment. This photoelectron-regulated colorimetric strategy provides a novel concept for the design of visual sensing platform, and can develop the portable test kits for rapid detection in clinical diagnosis.

Flexible Viologen-Based Porous Framework Showing X-ray Induced Photochromism with Single-Crystal-to-Single-Crystal Transformation.

A viologen-based Borromean entangled porous framework was found to be sensitive to both CuKα and MoKα X-ray sources, showing rapid photochromic response and recovery within one minute. The X-ray-induced photochromic process is accompanied by a reversible single-crystal-to-single-crystal (SC-SC) structural transformation, an unprecedented phenomenon for X-ray sensitive materials. The complex can be further processed into portable thin films for detecting the dose of the X-ray exposure. Moreover, the photochromism can occur over a broad temperature range of 100-333 K, both in the form of single crystals and thin films, making it a potential candidate for practical indoor and outdoor applications.

Flexible Viologen‐Based Porous Framework Showing X‐ray Induced Photochromism with Single‐Crystal‐to‐Single‐Crystal Transformation

AbstractA viologen‐based Borromean entangled porous framework was found to be sensitive to both CuKα and MoKα X‐ray sources, showing rapid photochromic response and recovery within one minute. The X‐ray‐induced photochromic process is accompanied by a reversible single‐crystal‐to‐single‐crystal (SC‐SC) structural transformation, an unprecedented phenomenon for X‐ray sensitive materials. The complex can be further processed into portable thin films for detecting the dose of the X‐ray exposure. Moreover, the photochromism can occur over a broad temperature range of 100–333 K, both in the form of single crystals and thin films, making it a potential candidate for practical indoor and outdoor applications.

Photochemically driven luminescence switch of metal supramolecular assembly incorporating mixed lanthanides and photochromic guest molecule

We developed a model dual-light-responsive luminescent lanthanide supramolecular switch. A poly[2]pseudorotaxane was firstly constructed from a dilanthanide metal supramolecular assembly (EuTb·1EPO) and an asymmetric diarylperfluorocyclopentene (2), in which EuTb·1EPO exhibits a satisfactory and tunable lanthanide luminescence, while the 2 moiety in the poly[2]pseudorotaxane displays a reversible photochromic process upon light irradiation with an excellent resistance to fatigue. By switching between closed-form and open-form states of 2 using ultraviolet and visible light irradiation alternately, the lanthanide luminescence of poly[2]pseudorotaxane can be quenched and rapidly recovered, that is, the poly[2]pseudorotaxane can be used as switchable controlling unit.

Photochromic behavior of triangle trithiophene with π-extension at the bridge unit

ABSTRACTA photochromic triangle terthiophene with phenylethynyl attached was synthesized under Sonogashira coupling condition and fully characterized. The photochromic behavior was investigated in typical solvents. In hexane, the reversibly photochromic process was induced by UV and Visible light. The solution color could be toggled between colorless and pink color. The photo- and thermo-stability were also investigated at 273 and 323 K. It is stable in the dark at the room temperature, while the degradation accelerated with the temperature rising. Correspondingly, the optimized geometry structure and frontier molecular orbitals were calculated. Both the distance between reactive carbon atoms and the HOMO distribution of ring-open form support the photo-cyclization reaction.

Excellent photochromic properties of an oxygen-containing yttrium hydride coated with tungsten oxide (YHx:O/WO3)

Oxygen-containing yttrium hydride films (YHx:O) show interesting photochromic properties under ultraviolet irradiation. In this paper, oxygen-containing yttrium hydride films covered with tungsten oxide were prepared by a direct current magnetron sputtering method. This composite material showed reversible photochromic properties. Furthermore, its coloration speed was faster than that of a single layer of yttrium hydride, and its switching modulation was broader. The photochromic mechanism of the YHx:O/WO3 thin films was investigated by X-ray diffraction, X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy analysis. Oxygen and hydrogen played a key role in the photochromic process of the YHx:O/WO3 composite films.

Dual photo-electrochromic diimides derived from aliphatic aminothiols and π-electron deficient aromatic dianhydrides

The reaction of aliphatic 2-aminothiols with pyromellitic and benzophenone-3,3′,4,4'-tetracarboxylic dianhydrides endowed dual-response diimides with photo- and electrochromic properties in concentrated sulfuric acid solution. Upon stimulation, the yellow color photo-isomeric materials turned in a few seconds to dark blue or red color and subsequently reverted back within a few minutes. The color change was ascribed to the generation of radical-cation species which may be formed through a reversible photo-induced electron-transfer process from the thiol group to the proton-activated dianhydride moiety. Spectroelectrochemistry measurements in concentrated H2SO4 solution of the specified diimides were accompanied by a quite similar UV-vis spectral response as observed in photochromic process, indicating the equality of the created chromic centre in both phenomena. The two chromic activities showed a great sensitivity to the structure of the donor-acceptor couples.

Temperature dependence of the conversion efficiency of photochromic perylene bisimide dithienylcyclopentene triads embedded in a polymer.

Photochromic molecules that are covalently linked to a strong fluorophore combine the requirements of external control and strong fluorescence, which will become increasingly important for super-resolution microscopy techniques based on single molecules. However, given the bulky structure of such constructs, steric hindrance might affect their photoconversion efficiencies upon immobilising them for imaging purposes. In this study the efficiencies of the photochromic conversion processes of molecular triads that are embedded in a polymer have been studied as a function of temperature. The triads consist of two perylene bisimide dye molecules that are connected via a dithienylcyclopentene photochromic bridge that undergoes a cyclization/cycloreversion reaction upon appropriate illumination. It is found that photochromic switching remains active, even at 5 K, yet with reduced but finite efficiency for the cycloreversion reaction. This might even be an advantage for the achievement of high labelling densities in super-resolution microscopy.

Spectral and Color Changes of Ag/TiO2 Photochromic Films Deposited on Diffusing Paper and Transparent Flexible Plastic Substrates

Giving paper and polymer photochromic properties under laser irradiation is challenging due to the low resistance of these materials to heat, their flexibility, and their possibly irregular structure. However, we could successfully deposit TiO2/Ag/TiO2 layers stacking on flexible white glossy paper and transparent polyethylene terephalate (PET) substrates using a reactive magnetron sputtering technique, and tailor coloration changes after laser irradiation, alternating visible and ultraviolet (UV) wavelengths. The sample colors are characterized by a panel of chromas depending on the irradiation conditions. We demonstrate that these chroma changes are due to morphological changes of Ag nanoparticles (NPs) after visible laser irradiation of the colored as-deposited sample. The process exhibits a good reversibility after subsequent UV irradiation due to the growth of new metallic Ag NPs. The colors displayed in diffuse reflection by the paper samples are more saturated than the ones displayed in regular transmission by PET samples. We demonstrate the efficiency of the photochromic process on such support by printing high resolution patterns exhibiting different colors depending on the observation conditions.

Molybdenum oxide nanosheets prepared by an anodizing-exfoliation process and observation of photochromic properties

Anodizing-exfoliation of molybdenum foil was performed in 0.02M HCl electrolyte at 30V. In this process, the electrolyte rapidly turned into a blue colloidal solution of molybdenum oxide nanosheets with fragmented edges observed on transmission electron microscope (TEM). X-ray Diffraction (XRD) pattern of particles was free of peak while annealing at a temperature range of 100–500°C led to formation of monoclinic (for T<300 °C) and orthorhombic (for T>300°C) phases of MoO3. Moreover, addition of PdCl2 (0.2g/l) salt solution caused a spontaneous bleaching of the initial blue colloid. Annealing of powders extracted from these bleached solutions with different PdCl2 concentrations at 500°C led to a preferential growth of (0k0) orientation. X-ray photoelectron spectroscopy (XPS) revealed that the blue nanosheets solution contains mainly Mo5+ with slightly Mo6+ oxidation states and each of annealing or salt bleaching procedures can entirely convert Mo5+ to Mo6+. When the bleached solutions was exposed to KrF laser beam (λ=248nm) a strong photochromic coloration with a deep blue color was occurred. Regardless of Pd:Mo ratio, the primary and laser irradiated solutions showed analogues optical absorption bands in the 1–3nm photon energy range while the photochromic process led to a broader absorption band.

A new dithienylethene dimer with terminal tertiary amine redox centers: Electrochemical, UV–vis–NIR spectral and electronic transfer charges induced by a stepwise photochromic process

Photochromic diarylethene dimers (R-DTE-CCCC-DTE-R, R = benzene, 1oo; R = triphenylamine, 2oo; DTE = dithienylethene) with remarkable stepwise photochromic process were synthesized successfully. Dimer 2oo, whose two terminal redox-active triphenylamine centers were separated by two photochromic DTE-acetylide moieties, could accomplish photoswitchable charge delocalization between the N⋯N electro-active centers. Upon UV-light irradiation, due to the progressive increase of π-conjugated systems as the order of 2oo → 2co → 2cc, the electronic interaction of two N-centers in 2oo was increasingly facilitated following the stepwise photocyclization. Furthermore, for monocationic species [2oo]+/[2co]+/[2cc]+, their absorption strength was gradually increased during the conversion of [2oo]+→[2co]+→[2cc]+, which demonstrated that the extent of charge delocalization along the molecular backbones was progressively enhanced during the stepwise photocyclization. The lower electronic density of reactive carbon atoms during the oxidation of N0 to N+ could result more difficult photocyclization reaction to produce oxidized species [2oo]2+/[2co]2+. Additionally, compound 2 exhibited eight switchable states through stepwise photochromic and reversible redox processes.

Photochromic and Nonphotochromic Luminescent Supramolecular Isomers Based on Carboxylate-Functionalized Bipyridinium Ligand: (4,4)-Net versus Interpenetrated (6,3)-Net.

Photochromic and nonphotochromic luminescent bipyridinium-based supramolecular isomers [Cd(CPBPY)(BDC)(H2O)]n (1) and {[Cd(CPBPY)(BDC)]·H2O}n (2) (CPBPY = N-(3-carboxyphenyl)-4,4'-bipyridinium, BDC= terephthalate) have been successfully obtained by solvothermal reactions at 100 °C via tuning stoichiometric ratios of starting reagents. Isomer 1 features (4,4)-topological layer constructed by edge-shared Cd2 SBUs and BDC linkers attached by N-pendent CPBPY groups. Isomer 2 has (6,3)-topological layers with Cd atoms as nodes and BDC and double CPBPY as linkers, which are 4-fold interpenetrated into 3D network. Although both 1 and 2 contain bipyridinium ligands, only isomer 1 possesses reversible photochromic behavior with quick-switchable luminescence in the solid state. Compound 2 does not show photochromic behavior even after exposure to UV light for more than 2 h. Photochromism process of 1 originates from photostimulated reduction of CPBPY ligands to generate CPBPY(•-) radicals after irradiation, confirmed by EPR spectra. Careful check on structure reveals that the offset π-π stacking interaction between the pyridine ring of CPBPY and benzene ring of BDC with inter-ring shortest C···C distance of 3.214 Å in 1 is responsible for electron transfer to form the CPBPY(•-) radicals. The speculation is further supported by DFT calculation of frontier orbital matching of electron donor and acceptor. HOMO and HOMO-2 orbitals of BDC involve the carbon atoms of benzene ring while LUMO and LUMO+1 orbitals of CPBPY involve the carbon atoms of pyridinium ring. Importantly, the photoinduced formed CPBPY(•-) radicals in 1 have a long-lived lifetime (at least six months in air and room temperature condition), which is mainly attributed to the close packing mode.

Kinetics of Coloration in Photochromic Tungsten(VI) Oxide/Silicon Oxycarbide/Silica Hybrid Xerogel: Insight into Cation Self-diffusion Mechanisms

Silicon oxycarbide/silica composites with well-dispersed tungsten(VI) oxide (WO3) nanoparticles were obtained as transparent hybrid xerogels via an acid-catalyzed sol-gel process (hydrolysis/condensation polymerization) of 3-(triethoxysilyl)propyl methacrylate (TESPMA) and tetraethoxysilane (TEOS). The self-diffusion mechanism of alkali-metal cations and the kinetics of the photochromic coloration process in the WO3/TESPMA/TEOS hybrid xerogel systems have been systematically investigated. Under continuous UV illumination, a gradual color change (colorless → blue) corresponding to the reduction of W(6+) into W(5+) states in WO3 nanoparticles can be confirmed from the WO3/TESPMA/TEOS hybrid xerogels containing alkali-metal sulfates, although no coloration of the hybrid xerogel without alkali-metal sulfate was observed. The coloration behavior depended exclusively on a variety of alkali-metal cations present in the hybrid xerogel system. Furthermore, a detailed analysis of the self-diffusion mechanism confirmed that the alkali-metal cations electrostatically interact with a layer of unreacted silanol groups on the TESPMA/TEOS matrix surface, and subsequently pass through the interconnected pore network of the hybrid xerogel. More interestingly, in the context of an Arrhenius analysis, we found a good coincidence between the activation energies for alkali-metal cation self-diffusion and UV-induced coloration in the WO3/TESPMA/TEOS hybrid xerogel system containing the corresponding alkali-metal sulfate. It is experimentally obvious that the photochromic properties are dominated by the diffusion process of alkali-metal cations in the WO3/TESPMA/TEOS hybrid xerogel system. Such hybrid materials with cation-controlled photochromic properties will show promising prospects in applications demanding energy-efficient "smart windows" and "smart glasses".

Anion-Mediated Architecture and Photochromism of Rigid Bipyridinium-Based Coordination Polymers

Four metal–organic coordination compounds Zn2(bcbp)Cl4 (1), Zn(bcbp)(PO3F) (2), [CdCl(Hbcbp)(bcbp)]·(ClO4)2 (3), and [Cd2(bcbp)4]·(ClO4)·F3 (4) (H2bcbp·2PF6 = 1,1′-bis(4-carboxyphenyl)-(4,4′-bipyridinium) hexafluorophosphate) have been synthesized via a solvothermal method and structurally characterized. Compound 1 is a 1D coordination polymer, while compound 2 shows a 2D + 2D → 3D inclined polycatenation structure. In the case of compound 3, due to the capped chlorine atoms and uncoordinated carboxyl of bcbp ligand, it exhibits a 1D T-shaped coordination configuration. Compound 4 is an unprecedented 13-fold interpenetrating structure with huge diamondoid frameworks. Because of the presence of electron-deficient bipyridinium moieties, the photochromic behaviors of these compounds have also been studied. Interestingly, only compounds 1–3 exhibit color changes under light irradiation. The impact of these anions on the photochromic process is discussed.

Reversible Disassembly-Assembly of Octa Acid-Guest Capsule in Water Triggered by a Photochromic Process.

Octa acid (OA), a calixarene-based cavitand, forms a 1:2 capsular assembly with neutral 1,3,3-trimethyl-6'-nitrospiro[2H-1]benzopyran-2,2'-indoline and 1:1 cavitandplex with its open zwitterionic merocyanine form. Photochromic interconversion between the spiropyran and merocyanine leads to unprecedented reversible capsular disassembly and assembly. OA provides stability to the merocyanine in both the ground and excited states. The photochemically controlled disassembly and assembly process established here points toward the opportunity of using the OA capsule in delivering small molecules at the desired locations.

A photochromic UVI indication card and the colorimetric analysis system built on smartphones

The present study offers a simple and real-time method to measure the ultraviolet index (UVI) via a photochromic UVI indication card and a colorimetric analysis system built on the smartphone. A photochromic Ormosil (Organically Modified Silicates) coating doped by the spirooxazine dye (SO-G) was prepared. The coating, with the broad absorption at 450–650nm, exhibited as a grey tone on its appearance under ultraviolet irradiation. The spectrokinetics of the SO-G doped Ormosil coating was investigated by monitoring the changes of absorption values at 495nm with time. During its photochromic process, the equilibrium time for both photostationary states were less than 20s. Tests of photochromic reversibility showed the reproducibility of at least 90 cycles. The UVI indication card was fabricated using the photochromic Ormosil coating and a printed standard depth scale was used for the UVI sensing strip to evaluate the instantaneous ultraviolet index. The relationship between the relative grayscales (Gy*) and the ultraviolet index was studied, and the linearity was obtained. The colorimetric analysis system including acquisition and processing of the image for colorimetric analysis was then built on the smartphone. By applying the UVI indication card as well as the Android application developed which brought the user experiences of visualization, convenience and participation, users can monitor fluctuating of the ultraviolet indexes all day long. This method is advantageous for application in smartphones used by nonprofessional consumers and is valuable to be investigated in-depth for the commercial importance.

Fluorescence and photochromic properties of a series of new Zn(ii)/Cd(ii) coordination compounds with a flexible semi-rigid tetrazole–viologen derivative

Diverse luminescence has been realized in four Zn(ii)/Cd(ii) coordination compounds. The isolated binuclear compound displays fluorescence and second-order nonlinear optical property variation in the photochromic process.

Combined aggregation induced emission (AIE), photochromism and photoresponsive wettability in simple dichloro-substituted triphenylethylene derivatives.

A dichloro-substituted triphenylethylene derivative (TrPECl2) with aggregation-induced emission (AIE), photochromism and photoresponsive wettability has been synthesized. The new compound shows fast-response photochromic behaviour with good ON/OFF repeatability by utilizing a proposed stilbene-type intramolecular photocyclization in the solid state. Compared with the more usual diphenylethylene derivatives, the photochromic properties of the triphenylethylene derivative are much more striking and easier to achieve. The triphenylethylene derivative also displays AIE properties leading to strong fluorescence in the solid state. Therefore, both the ultraviolet-visible absorption and fluorescence emission are drastically changed during the photochromic processes. Furthermore, the morphology of the TrPECl2 microcrystalline surface could be controlled by irradiation. The wettability of the surface could be drastically decreased with contact angles of a water droplet changing from 73° to 118°. The triphenylethylene derivative with a simple molecular structure is, therefore, attractive for multifunctional materials.