Novel sol–gel reversible thermochromic materials for environmental sensors

Anhydrous cobalt(II) chloride has been encapsulated in several inorganic sol–gel matrices with different solvent/water ratios. Sols were cast into cuvettes and hermetically closed. Such sol–gel materials were found to be sensitive to temperature in the 10–50 °C range showing a change of colour. General characterisation of the sensitive materials was made by immersion into a thermostatic water bath and recording of the corresponding visible spectra. The optical response consisted of a change in colour from light pink to deep blue as the temperature increases. These temperature detectors behave as sensors showing good optical sensitivity in the range mentioned above and reversibility for more than 30 cycles. The sensors response time is at about 15 min and their lifetime is 2 months at least. These sol–gel materials have been designed to be applied for preservation and conservation purposes. High temperatures and cyclical temperature changes can yield severe consequences for the correct preservation of cultural heritage materials (textiles, archaeological ceramics and other remains, metallic objects and statues, stained glass windows, etc.) both in museums and outdoors.     

Ferromagnetism in Metallocene-Doped Fullerenes

 Ferromagnetism in fullerene-based systems doped with metallocenes is reviewed. These compounds form a ferromagnetic state by spin-coupling between π electrons on fullerene units, while the metallocene molecules do not contribute to the spin ordering. One of these compounds has the highest critical temperature (19 K) for this class of compound. The magnetic properties of these materials are very strongly dependent on the crystallization conditions. Corresponding author. E-mail: Dragan.Mihailovic@ijs.si Received September 4, 2002; accepted September 6, 2002     

Synthesis and description of SrSn0.6Ln0.4O3 perovskite pigments

Inorganic pigments containing lanthanides based on pseudo cubic structure of SrSnO₃ have been prepared by solid state reaction between SrCO₃, SnO₂ and lanthanide oxides (Tb₄O₇, Pr₆O₁₁, CeO₂) in the temperature range 1300–1550°C. The resultant materials were characterised by XRD, TG-DTA and colourimetric techniques were used. The most interesting colour properties were provided by SrTb_0.4Sn_0.6O₃ prepared by firing at temperature 1500°C which has light yellow-green colour hue. The increase of temperature 1300–1500°C caused the creation of pigments with deeper and brighter colour hue. Colour properties of the samples prepared by calcination at 1550°C were of inferior quality due to structural changes from pseudo cubic to orthorhombic system.     

Phase behavior of low molecular weight polyethylenes from homogeneous and heterogeneous solutions

The pseudophase diagrams of solutions of low molecular weight polyethylene (PE) (number‐average molecular weight < 1500 g/mol) in octamethyl cyclotetrasiloxane (OCTS) and isododecane (IS) were determined by direct observation of cloud points and optical microscopy. In addition, melting temperatures were also determined by differential scanning calorimetry. In the range of single liquid–solid transitions, the data conformed to the classical melting temperature composition relation as a result of the formation of extended crystallites. The melting data were used to determine the interaction parameter of the PE in OCTS (1.4 ± 0.1) and IS (0.22 ± 0.05). The structural and thermal properties of the gels formed by a competing liquid–liquid and liquid–solid phase separation, under nonequilibrium conditions, contrast with the properties of the crystals formed from a single liquid–solid transition. Coarsening within the liquid phases was evidenced by optical microscopy, and insights about the mechanism of the kinetics of the coarsening process are given. The temporal changes of the melting temperature of crystallites formed from the heterogeneous phase (OCTS) reveal dynamics within a nonequilibrium state. In contrast, the crystallites formed from a homogeneous solution (IS) showed negligible melting‐temperature changes with time. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 878–889, 2002     

Chromogenic polymer gels for reversible transparency and color control with temperature at a constant volume

The preparation and characterization of novel chromogenic materials has developed extremely rapid in the last years. Among them, thermotropic and thermochromic polymer gel networks have met with growing interest, because of their advanced properties. These novel polymer gels exhibit pronounced changes in transparency and/or color intensity in a practically relevant temperature range. For many future technical applications it is an essential condition, that the volume of the used gel materials is independent of temperature. A current overview of this field is given. Copyright © 2002 John Wiley & Sons, Ltd.     

The intercalation of ethidium bromide in the chromonic lyotropic phases of drugs and nucleic acids

Chromonic liquid crystalline phases are formed by a variety of drug and dye/water systems. In contrast to conventional lyotropic phases (where micelle formation underlies the mesogenic properties), in chromonic systems the molecules stack in columns. The different chromonic phases are different arrangements of these columns. We have examined the solution of ethidium bromide (EB) in the well-documented chromonic Intal/water system. EB is a widely used nucleic acid stain which changes colour when intercalated into DNA and which becomes fluorescent.

We have charted the changes in the temperature/composition phase diagram of the Intal/water system caused by adding EB. Although there are changes in the position of the phase boundaries, the overall pattern remains qualitatively the same—implying that the host phase is accepting EB as a similar chromonic molecule. The intercalation of EB molecules in the chromonic host phase results in optical effects—a metachromic colour change and fluorescence, similar to those occurring when the dye stains DNA.

These observations strengthen our belief that the central stack of bases in DNA can be regarded as being chromonic in nature.     

Single-Molecule White-Light-Emitting Starburst Donor-Acceptor Triphenylamine Derivatives and Their Application as Ratiometric Luminescent Molecular Thermometers

White-light emission (WLE) from a single molecule is a highly desirable alternative to a complex mixture of complementary colour emitters, which suffers from poor stability and reproducibility for potential use in organic electronic devices and lighting applications. We report single-molecule WLE both in solution and thin films by judiciously controlled π-electron delocalisation between the triarylamine subchromophoric units. Triphenylamine (TPA) forms the central core, and the phenyl rings are substituted with the electron-deficient acceptor 3-ethylrhodanine (Rh) and electron-rich donors triphenylamine or carbazole. The enforced biphenyl configuration of the TPA core and the other donors renders the π-conjugation across the entire chromophore poor, thus the individual subchromophoric units retain their individual emission characteristics, which cover all three primary colour emissions, that is, red, green and blue (RGB). TPA−Rh units exhibit broad fluorescence in the green-red region originating from the local excited (LE) state and intramolecular charge transfer state (ICT), strongly influenced by the solvent, water, and temperature. Different fluorescence parameters, including spectral maxima, ratiometric changes in ICT emission at the expense of blue emission from terminal donor units, and changes in lifetime, have a linear relationship with temperature between 180–330 K, thus the molecules can function as a multiparameter luminescent molecular thermometer. A temperature coefficient of 0.19 K⁻¹ in ratiometric fluorescence changes along with a spectral shift of 0.3 nm K⁻¹ and their workability over the wide temperature makes these molecules promising materials for potential applications. At lower temperatures, individual subchromophoric properties subside because of the reduced dihedral angle of biphenyl, and fluorescence from the whole molecule becomes dominant.     

Study of ceramic pigments based on Er2Ce2?x Mo x O7

This contribution is focused on the synthesis, characterization and optical properties of new inorganic pigments which are environment friendly and can substitute some toxic metals in interesting colour compounds. Pyrochlores belong to the group of high-temperature pigments, and are a variety of actual and potential applications for several materials. Examples include catalysts, thermal barrier coatings, solid electrolytes, nuclear waste forms and host materials for luminescence centers. The pigments were prepared by the solid state reaction and also by method of suspension mixing of materials in the series with increasing content of molybdenum. The pigments were applied into organic matrix and ceramic glaze. The colour properties of these applications were investigated depending on content of Mo, method of preparation and temperature of calcination (1350?C1550?°C after step 50?°C). The optimum conditions for their synthesis were determined. The pigments were evaluated from standpoint of their structure, colour and particle sizes. Characterization of Er₂Ce_2?x Mo _x O₇ pigments (x?=?0.1, 0.3, 0.5 and 0.7) suggests that they have a potential to be alternative yellow or orange colourants for paints, plastics, ceramics and building materials.     

Poly(amidoamine) dendrimers peripherally modified with 1,8-naphthalimides. Photodegradation and photostabilization on polyamide matrix

Some novel fluorescent poly(amidoamine) dendrimers supported by a polyamide-6 matrix have been prepared and studied for the first time. The colour characteristics of the novel materials have been determined and found to be dependent on the nature of 1,8-naphthalimides bonded to their periphery amino groups. The materials have been treated with protons and cuprum cations. The resulting into changes in the colour characteristics and photostability of the dendrimers has been investigated. It has been shown that these ions inhibit the photodegradation of dendrimers molecules studied.     

Chemistry and applications of flavylium compounds: a handful of colours

Flavylium compounds are versatile molecules that comprise anthocyanins, the ubiquitous colorants used by Nature to confer colour to most flowers and fruits. They have found a wide range of applications in human technology, from the millenary colour paints described by the Roman architect Vitruvius, to their use as food additives, combining colour and antioxidant effects, and even as light absorbers in solar cells aiming at a greener solar energy conversion. Their rich complexity derives in part from their ability to switch between a variety of species (flavylium cations, neutral quinoidal bases, hemiketals and chalcones, and negatively charged phenolates) by means of external stimuli, such as pH, temperature and light. This critical review describes (i) the historical advancements in the understanding of the equilibria of their chemical reaction networks; (ii) their thermodynamics and kinetics; (iii) the mechanisms underlying their colour development, such as co-pigmentation and host-guest interactions; (iv) the photophysics and photochemistry that lead to photochromism; and (v) applications in solar cells, models for optical memories, photochromic soft materials such as ionic liquids and gels, and their properties in solid state materials (274 references).     

Preparation and selected properties of pigments on base of Ln-doped CaSnO<Subscript>3</Subscript>

Inorganic pigments containing lanthanides based on orthorhombic perovskite structure of CaSnO₃ have been prepared by solid state reaction of CaCO₃, SnO₂ and lanthanide oxides (Tb₄O₇, Pr₆O₁₁, CeO₂). The TG-DTA analysis indicates the formation of Ca-stannates around of temperature 1200°C, but from the pigmentary — application point of view, it is better to synthesize the product at higher temperature (1400 or 1500°C). The resultant materials were characterised by XRD, particle size distribution and measurement of colour properties. The doping of Ca-stannates by ions of rare earth elements (Tb, Pr, Ce) brings the production of two-and three-phase systems. The most interesting colour properties provided the stannate doped by ions of terbium and cerium and synthesized by heating at temperature 1400°C. The pigment has reddish brown colour hue.     

Towards the Design of New Materials: Regio- and Stereoselective (Cyclo-) Polymerization of 1-Alkynes and 1,6-Heptadiynes

Summary. The metathesis polymerization, respectively cyclopolymerization, of 1-alkynes and 1,6-heptadiynes using well-defined group VIA transition metal initiators is summarized. For purposes of comparison, selected quaternary catalytic systems used as alternatives as well as the properties of the resulting materials will be presented. Special consideration is given to the mechanistic advancements that have been made during the last decade, which allow the tailor-made synthesis of conjugated materials with interesting optical and electronic properties. E-mail: michael.r.buchmeiser@uibk.ac.at Received November 20, 2002; accepted November 21, 2002     

Stability of chromogenic colour prints in polluted indoor environments

Chromogenic colour prints are known to be sensitive to storage environments. However, limited research is available on the effect of atmospheric pollutants on these materials, especially pollutants generated indoors. The stability of photographic dyes is of particular interest and the rate of their change can be best described using the standard RGB colour model. Therefore, the colourimetric method was compared to dye extraction and liquid chromatographic analysis to justify its use as a rapid, non-destructive method for quantitative assessment of the rate of change in dye content of colour photographs during degradation. The effects of typical indoor (acetic acid, formaldehyde) and outdoor (nitrogen dioxide) generated pollutants on chromogenic colour prints were then investigated at 80 °C, 60% RH. It was identified that acetic acid leads to the most pronounced changes in photographic dye concentrations, which is significant considering that acetic acid is often the most prominent pollutant in archival environments. On the other hand, formaldehyde exhibited a slight protective effect in comparison to the blank experiment.     

Determination of nitrogen in steel

The accuracy of the measurement of small amounts of nitrogen in steel by chemical methods is dependent on satisfactory procedures for converting the nitrides into ammonia, and for measuring the ammonia thus formed.In the method proposed for the latter, Nessler's reagent is used and the intensity of colour is measured on an absorptiometer. For maximum accuracy, variables such as temperature and time of standing before measurement, must be carefully controlled.     

Thermochromism of polypropylene gels

It is found that polypropylene gels in some solvents of benzene-derivatives show a striking change of colour. The colour changes from blue to yellow as the temperature rises from the melting point of the solvent to around 70–80 °C. The apparent characteristic of the phenomenon resembles the thermochromism of cholesteric liquid crystals although polypropylene itself is colourless and has no asymmetric carbons in the molecules. Polypropylene flakes swelled in these solvents also show similar thermochromism.     

Structure:function relationships in molecular spin-crossover complexes

Spin-crossover compounds are becoming increasingly popular for device and sensor applications, and in soft materials, that make use of their switchable colour, paramagnetism and conductivity. The de novo design of new solid spin-crossover compounds with pre-defined switching properties is desirable for application purposes. This challenging problem of crystal engineering requires an understanding of how the temperature and cooperativity of a spin-transition are influenced by the structure of the bulk material. Towards that end, this critical review presents a survey of molecular spin-crossover compounds with good availability of crystallographic data. A picture is emerging that changes in molecular shape between the high- and low-spin states, and the ability of a lattice to accommodate such changes, can play an important role in determining the existence and the cooperativity of a thermal spin-transition in the solid state (198 references).     

On the Conformation of Bilirubin Ditaurate

Summary.  The first optically active taurine conjugate of a bilirubin was prepared by reaction of taurine sodium salt with the mixed anhydride formed from reaction of (βS,β′S)-dimethylmesobilirubin-XIIIα with isobutyl chloroformate. Analysis of the circular dichroism spectra of the conjugate in water and chloroform indicate a conformational preference for the (M)-helical ridge-tile conformation, thus providing the first spectroscopic evidence on the conformation of ditaurobilirubins. Corresponding author. E-mail: lightner@scs.unr.edu Received July 5, 2002; accepted July 15, 2002     

Processing of Thermal Sensitive Materials – a Case Study for Wood Plastic Composites

Summary. The influence of processing parameters, e.g. throughput and temperature, of the compounding step of wood plastic composites was investigated in this work. The effects on the mechanical properties as well as the influence on the colour of the produced composites were correlated with the different processing parameters. The composites showed a wide range of good processability, only at high temperature and low throughput strong darkening occurred, as well as a loss of tensile strength was found for these samples.     

Effect of added zinc on the properties of cobalt-containing ceramic pigments prepared from layered double hydroxides

Layered double hydroxides (LDHs) with the hydrotalcite-type structure containing Co and Al, or Zn, Co and Al in the brucite-like layers and carbonate in the interlayer have been prepared by coprecipitation. The Zn/Co molar ratio was kept to 1 in all samples, while the divalent/trivalent molar ratio was varied from 2/1 to 1/2. The samples have been characterised by element chemical analysis, powder X-ray diffraction, differential thermal and thermogravimetric analysis, temperature-programmed reduction and FT-IR spectroscopy. A single hydrotalcite-like phase is formed for samples with molar ratio 2/1, which crystallinity decreases as the Al content is increased, developing small amounts of diaspore and dawsonite and probably an additional amorphous phase. Calcination at 1200 °C in air led to formation of spinels; a small amount of NaAlO₂ was observed in the Al-rich samples, which was removed by washing. The nature of the spinels formed (containing Co^II, Co^III, Al^III and Zn^II) strongly depends on the cations molar ratio in the starting materials and the calcination treatment, leading to a partial oxidation of Co^II species to Co^III ones. Colour properties (L*a*b*) of the original and calcined solids have been measured. While the original samples show a pink colour (lighter for the series containing Zn), the calcined Co,Al samples show a dark blue colour and the Zn,Co,Al ones a green colour. Changes due to the different molar ratios within a given calcined series are less evident than between samples with the same composition in different series. These calcined materials could be usable as ceramic pigments.     

Microstructure of Nickel Aluminides Formed in Situ in Aluminium Matrix Composites

Nickel aluminides formed in situ in aluminium matrix composites due to the interaction between nickel powder and the matrix were examined. The composite materials were obtained from mixtures of nickel powder and aluminium powder by hot pressing in vacuum in the temperature range of 500–640°C. Depending on hot pressing parameters, the morphology of nickel aluminide particles was layered or homogenous, and their size for the same precursor powder was growing as the pressing temperature increased. XRD, SEM, TEM and EDS characterization methods were used for the microstructure investigation.