Reversible thermochromic materials

Carmona N., Herrero-Hernandez E., Llopis J. and ViUegas M.A. (2008), Novel sol-gel reversible thermochromic materials for environmental sensors. Journal of Sol-gel Science and Technology, 47 pp. 31-37. 

The effects of temperature on the color development of the porous film in chlorobenzene were shown in Table 6 [23]. The coloration was reversible thermochromism. The refractive index of the materials generally decreases as the temperature increases, and the temperature dependence of the liquid is greater than that of the solid. For example, the temperature dependence (A/id/°C) of PVA and chlorobenzene was found to be 3.0 x 10 and 4.5 x 10" at 589.3 nm. Consequently, it is interpreted that the wavelength of the crosspoint between the dispersion curves of PVA and chlorobenzene shifts from the long side to the short side with increasing tem-... 

Typically the chromophores used are pH sensitive, i.e. acidochromic or ionochromic (see section 1.4), and the medium is one whose pH varies with temperature. A variety of pH sensitive chromophores are known that are readily synthesised and can be modified to provide a wide range of shades. Application of these pH sensitive dyes in pressure and thermally sensitive papers for digital printing, e.g. fax papers, where the whole object is for the process to be irreversible, will be covered under ionochromism. The main area of application for reversible indirect thermochromic materials is as thermochromic organic pigments, as will be described below (section 1.3.4.1). 

Maclaren, D.C., White, M.A., 2003h. Competition between dye-developer and solvent-developer interaction in a reversible thermochromic system. Journal of Materials Chemistry 13, 1701-1704. 

Ultrafast and reversible thermochromism of a conjugated polymer material based on the assembly of peptide amphiphiles. Chem. Sci. 5,4189-4195. 

Zhang, X. Manufacture of reversible organic thermochromic materials from neutral red. Faming Zhuanli Shenqing Gongkai Shuomingshu CN 1546603, 2004 Chem. Abstr. 2005,143, 356252. 

Molecular rearrangement based thermochromic materials include spiro-lactones, fluorans, spiropyrans, and fulgides. These thermochromic materials normally consist of three components a dye precursor, a colour developer and a non-polar solvent. The colourless dye precursor and colour developer are both microencapsulated. Figure 14.6 shows the rearrangement of the molecular structure of spirolactone, which leads to the reversible thermochromic effect. A proton is donated to the spirolactone by the colour developer to form the dye. Before applying to textiles, thermochromic materials are normally encapsulated. Under some temperatures, bisphenol A emits proton, and crystal violet lactone opens rings and combines with the proton to make n system and shows colour. The colour varies with the substituent when it is H, the colour is violet when R is CH3 and X is OCH3, the colour is blue. 

A new free-standing silica-based composite film that exhibits reversible thermochromic reflection, induced by a L.C. guest in the pores of iridescent mesoporous films, has been described. The authors have demonstrated that selective reflection from the novel mesoporous organosilica material with chiral nematic organization can be reversibly switched by thermal cycling of the 8CB guest between its isotropic and L.C. states, which was proven by SS NMR experiments. ... 

The main object of such analysis is to use the vibrational IR and Raman spectra of the pristine polymers to understand the molecular structure of the systems and their evolution when the material is dissoved in chloroform or when the solid material is heated. It has been shown that the class of polyalkylthiophenes show reversible thermochromism [155] whose origin at the molecular level needed an explanation. 

The subject of thermochromism in organic and polymeric compounds has been reviewed in some depth previously (8,16,18), and these expansive overviews should be used by readers with deeper and more particular interest in the subject. Many more examples can be found in the reviews that further illustrate the pattern of association between thermochromism and molecular restmcturing of one kind or another. The specific assignment of stmctures is still Open to debate in many cases, and there are still not many actual commercial uses for these or any of the other thermally reversible materials discussed herein. Temperature indicators have been mentioned, though perhaps as much or more for irreversible materials. 

Colour formers such as compounds 243 and 245 are not inherently thermochromic. For example, they melt without any change in colour. However, they may be used to generate colour thermally, either irreversibly or reversibly, as composite materials. In thermally sensitive paper, the colour former and an acidic developer, usually a phenol, are dispersed as insoluble particles in a layer of film-forming material. When brought into contact with a thermal head at around 80-120 °C, the composite... 

Those crystalline salicylideneanilines that are not thermochromic are photo-chromic that is, they undergo a reversible color change on exposure to light. In these materials the molecules are markedly nonplanar (45b see discussion on biphenyls in Section II-D) and the crystal structures are very open. It has been suggested that the color change here is due to a two-step process (102) ... 

WO3 is also used as an addition to VO2 in smart windows. Thermochromism [4.36] is the ability of a material to be transparent for infi d rays below a certain transition temperature and to reflect above, due to a crystallographic phase transition. The process is strictly reversible and there is almost no influence on visible light. VO2 has a transition temperature at 68 °C, which can be lowered by doping with WO3. Glass coated with a thin film of the above mixed oxide is called a smart window and is a promising candidate for use in automobiles and buildings for heat protection. 

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