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‘Sequential potential field’ is a concept to produce a potential cascade by the organization of redox components, and to aim for an effective and vectorial electron and/or hole transfer. Construction of the sequential potential field is examined by double-layered thin film composed of two kinds of metallophthalocyanine having different oxidation potentials, i.e. copper phthalocyanine and zinc phthalocyanine. In the electrochromic device of double-layered metallophthalocyanine thin films, hole transfer can be controlled by the order of deposition, which suggests that the present system is suitable for the construction of a sequential potential field. Double-layered metallophthalocyanine thin films were also used as a hole transfer layer in all-solid organic electroluminescent devices. In the case of an indium-tin oxide/copper phthalocyanine/zinc phthalocyanine/tris(8-hydroxyquinoline)aluminum/Mg-Ag device for electroluminescence, a high brightness of more than 4000 cd m⁻² can be achieved. This double-layered hole transfer system is more effective than the corresponding single hole transfer system, which is due to the construction of a sequential potential field in the hole transfer layer.

Electrochromic redox reactions of vacuum-evaporated thin films of tetrapyridotetraazaporphyrinatozinc(II) (TPyTAPZn) have been studied in aqueous KCl electrolytes by in situ spectrocyclic voltammetry, which combines cyclic voltammetry and in situ monitoring of the absorption spectrum of the film. Although the voltammetric response of TPyTAPZn thin film electrodes can hardly be understood by the conventional electrochemical approach, the spectral information linked to the voltammetric information elucidated the presence of two-step reductions and reoxidations by each electron per TPyTAPZn molecule, which, however, take place with significant overlap during the voltammetric scan. Reduction of the film turned the color of the film from blue to purple and it changed reversibly to regenerate the original spectrum upon reoxidation. These reversible electrochromic reactions could be repeated many times without causing any noticeable change in the voltammetric and spectral responses. While use of electrolytes with various cations did not affect the redox behavior of the film, the effect of electrolyte pH was clearly seen, changing the ratio of the two reduction and two reoxidation peak magnitudes in the voltammogram. XPS analysis of the reduced film indicated that charge compensation upon reduction of the film occurred by protonation of reduced TPyTAPZn molecules at their aza and/or pyrido nitrogen atoms. A reaction scheme is presented to describe the observed redox reactions of the TPyTAPZn thin film from thermodynamic and kinetic viewpoints.

Enhancing the operation life time or the electrochemical durability is one of the key issues in electrochromic material studies. It is generally accepted that the inorganic–organic hybrid structure is one of the effective ways to enhance the chemical stability of the material. In this study, an electrochromic film made of silica-polyaniline core-shell composite nanoparticles was tested. The composite particles were prepared through a chemical dispersion polymerization of aniline in an aqueous colloidal solution of silica. The synthesized particles were then dispersed into ethanol and the solution was deposited onto an Indium Tin Oxide (ITO)-coated glass substrate. The electrochromic characterization on the prepared films was performed using the cyclovoltammetry and the optical response to a switching potential. The results showed that the inorganic–organic core-shell hybrid nanoparticle could be a promising choice for the enhancement of electrochromic durability.

In the multicolor regulation of visible-light band, the inorganic electrochromic multicolor modulation has always been the bottleneck of expanding the application of inorganic electrochromic reflection display. An electrochromic device (ECD) based on an ultracompact unsymmetric Fabry–Perot resonator was designed. Electrochromic oxides such as tungsten oxide, offer the possibility to tune their refractive index and extinction coefficient upon ion insertion, allowing active control over resonance conditions for Fabry–Perot cavity-type devices. The spectrum colors of red, yellow, green and blue can be obtained by adjusting the thickness of tungsten oxide in the W/(WO${}_{3-x}\cdot \mathit{\text{n}}$H₂O) electrode. The optical constants of tungsten oxide can be adjusted reversibly by using electrochemical ion insertion/stripping, and the reflection peak of W/(WO${}_{3-x}\cdot n$H₂O) electrode in the visible-light band can be adjusted. The dynamic color control of the ECDs can be achieved finely.

Prussian Blue (PB) films were directly grown on FTO glass by a hydrothermal method only using potassium ferricyanide and hydrobromic acid as raw reagents. Hydrobromic acid plays the role of both providing acidic conditions and as a reducing agent which improves the atomic utilization of the raw materials. The as-prepared PB devices exhibited multicolor electrochromic properties: Blue, green and transparent states, reversibly. The maximum optical modulations of PB device could reach the range of 47.7%. The PB films also have a fast coloration/bleaching time of 1.9/1.3s, respectively. This study provided a novel method for preparing PB films by a facile hydrothermal method.

The red V₂O₅ sol was prepared by facile alternating stirring and ultrasonic dispersion route. A V₂O₅ film fabricated on ITO-PET by electrophoresis method was used to demonstrate a flexible electrochromic device. The effects of applied voltage and electrophoresis time for V₂O₅films were investigated in detail. When the electrophoresis time was kept 30 s, the applied voltage was above 3 V to deposit uniform V₂O₅-PET film. The electrophoresis time could more significantly regulate the deposition of V₂O₅ on ITO-PET film. A flexible electrochromic device with “sandwich” structure was fabricated using V₂O₅ film, and its electrochromic performance of the device was evaluated. The flexible V₂O₅ electrochromic devices show the multicolor electrochromic performance of yellow, yellow-green, green and orange-red. The coloration efficiency values of V₂O₅ devices were 21.6 cm²C${}^{-1}$ (from initial yellow to green) and 26.9 cm²C${}^{-1}$ (from green to orange-red), respectively, and the highest transmittance modulation range was 51% at 741 nm. The work provided a facile and green method for fabrication of flexible V₂O₅ electrochromic device.

The optical properties of palladium phthalocyanine in dimethylformamide, toluene, acetonitrile, and dichloromethane solutions were studied in the visible range of energy. Palladium phthalocyanine in organic medium presented itself as a mixture of monomeric and dimeric species in equilibrium, and it was only in dichloromethane that the monomeric, non-aggregated form predominated. Palladium phthalocyanine films were prepared by casting, and studied by cyclic voltammetry and spectroelectrochemistry. The films showed an electrochromic response from blue to purple, and stability that depended on the applied potential. Electrochemical quartz crystal microbalance experiments indicated that the film exfoliates into solution after applying high values of final potentials. Raman spectroscopy was used to structurally characterize the palladium phthalocyanine film, which showed a central metal with +2 oxidation state that is not affected even when the film is oxidized.

The group of the University of Valladolid is a multidisciplinary team formed by chemists, physicists and engineers. The activities of the group are focused to the study of the physicochemical properties of nanostructured Langmuir-Blodgett thin films based on phthalocyanines and their applications. Films of a variety of phthalocyanine molecules including several metallophthalocyanines, lanthanide double decker phthalocyanines and heteroleptic derivatives have been prepared. Their spectroelectrochemical properties have been described in detail and compared with those observed in disordered casted films or microcrystalline evaporated films. The group has dedicated special attention to films based on rare earth double decker compounds due to their unique semiconducting, optical and electrochemical properties. A rich electrochromism has been demonstrated in thin films of this family of compounds. The reversibility is improved in nanostructured Langmuir-Blodgett films. This has permitted development of an electrochromic display that can change its color from blue to green and finally to red. At the present moment, our main objective is the design of sensors able to detect gases and liquids. It has been demonstrated that thin film assemblies based on rare earth bisphthalocyanines modify their conductivity and their optical properties in the presence of electron donor or electron acceptors gases. Changes are also observed when the devices are exposed to Volatile Organic Compounds such as esters, alcohols or aldehydes which are responsible of odors in foods and beverages. Liquid sensors have also been developed. Their working principle is based in the fact that the rich electrochemical properties of phthalocyanine thin films are extremely sensitive to the nature of the electrolytic solution. Arrays of phthalocyanines have been used to construct an electronic nose able to discriminate odors from a variety of foods and beverages. Similarly, phthalocyanines have also been used to construct an electronic tongue based on voltammetric sensors. This is one of the main contributions of the group to the field of sensors.

A method has been developed to obtain good quality Langmuir-Blodgett (LB) films of mixtures of lutetium bisphthalocyanine and carbon nanotubes. The π-A isotherms exhibit high monolayer stability and the compressed monolayers are easily transferred to solid substrates by Y-type deposition. The electronic absorption spectra of the composite LB films show the characteristic bands of both compounds; the bands appear broader and shifted to higher wavelengths than those observed in solutions or in disordered films prepared by the casting method. The films are sensitive to oxidant or reducing vapors, whose presence cause modifications in the electronic absorption spectra of the films. These fast and reversible changes can be the basis of optical sensors based on thin films of lutetium bisphthalocyanine and carbon nanotubes. LB film electrodes immersed in KCl and KClO₄ solutions show electroactivity associated to the oxidation and reduction of the phthalocyanine ring. Important changes in the UV-vis-NIR spectra registered in situ simultaneously to the application of the electrical potential have been observed. The electrochromism observed opens the possibility of using such thin films in electrochromic devices. Composite films show characteristic electrochemical responses when exposed to solutions of antioxidants, where a certain degree of electrocatalytic effect is observed. The enhanced degree of selectivity can be used in the development of voltammetric sensors.

Electrochromism synchronous to the charge/discharge of a novel Li ion battery having Li₃Fe₂(PO₄)₃ and Li₄Ti₅O₁₂ thin-film electrodes fabricated by a chemical process, the molecular precursor method, was discovered. A cathode of transparent Li₃Fe₂(PO₄)₃ thin film with a thickness of 80 nm was fabricated by heat treating a precursor ethanol solution including a Li(I) complex of nitrilotriacetic acid, an Fe(III) complex of ethylenediaminetetraacetic acid, and (dibutylammonium)₂H₂P₂O₇ ⋅ 0.5H₂O at 550°C for 10 min in air. An anode of transparent Li₄Ti₅O₁₂ thin film with a thickness of 90 nm was fabricated by heat treating a precursor ethanol solution including a Li(I) complex of nitrilotriacetic acid, a Ti(IV) complex of the identical organic ligand, and hydrogen peroxide at 550°C for 30 min in air. The precursor films for both electrodes were fabricated with a spin-coating method. The thermal reactions of the novel precursors were examined in detail by means of thermogravimetry and differential thermal analysis in order to examine the components and heat-treatment temperature. The crystal structure and surface morphology of the thin-film electrodes fabricated on glass substrates pre-coated with a fluorine-doped tin oxide film were examined with X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). The rechargeable function of the assembled sandwich-type battery using an electrolytic solution containing LiPF₆ was measured by the repeated charge and discharge test at a constant current of 10 μA; a maximum voltage of 3.6 V was recorded. The color changes of the transparent thin-film battery between colorless before charging and a blue-gray color after charging occurred synchronously and repeatedly with the charge/discharge cycles. The intercalation of Li⁺ ions into the Li₄Ti₅O₁₂ thin-film anode may be related to the drastic color change and the unprecedented visualization of the electrochemical reaction of a novel Li ion battery.

The electrochromic properties of certain transition metal oxides have been studied for several years resulting in commercial films are deposited as thin layers (0.1 to 0.4 microns) onto a transparent conductive automotive mirror and sun-glass products. The largest potential application of electrochromics is in window to regulate heat and light flow. Fabrication cost is one of the greatest barriers for large area development of the smart windows. Tungsten trioxide (WO₃) can be colored deeply in with an optical irradiation of appropriate energy (photochromism) or with an applied electric field (electrochromism). These processes have received considerable attention because of their potential application in electrochromic windows, display devices, sensors, and so on. For these purposes, tungsten trioxide films prepared by various physical methods such as molecular beam epitaxy, CVD, etc have been reported. These methods are generally expensive and it is difficult to form large area films. However electrodeposition method is probably most economical method for making the films in addition to its relative ease in forming in large area films. In this paper, tungsten trioxide (WO₃) films are prepared through the electrodeposition route and these films are used to study the electrochromic behavior in the various electrolytes by changing the concentrations. When coloration, the film attains deep blue color and in reduced state it becomes colorless. After the ion intercalation, the optical properties are also studied in the UV-Vis-NIR region.

The electrochromic properties of an electrochemical polymerized composite consisted of polyaniline doped with acid red dye (PANI-ARD) are reported. The structures of PANIARD were characterized via cycle voltammograms, spectroelectrochemistry and colorimetric analysis. Film of the PANI-ARD composites of different concentrations appears violet, aubergine in the neutral state and darkblue in the oxidized state, which are different from the pure PANI of yellow (-0.8V) and blue (1.0V). The oxidation and reduction response speed of PANI-ARD was a bit lower than those obtained in pure PANI. It is shown that acid dye doping is an effective method to broaden the color change range of the electrochromicmateials.