Organic thermochromic materials

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

The commercial thermochromic products include multi-components, namely the colour former, an acidic catalyst and a non-polar co-solvent medium. The organic thermochromic materials are easy to obtain and cheaper than thermochromic liquid crystal, but they are not sensitive and their colour change range is narrow. These materials can be used on some thermochromic printing without high quality demands. 

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). 

Bernard Mark Heron was born in Workington, England in 1965. After graduation (GRSC) from Lancashire Polytechnic (Preston) in 1987 and a brief period in industry he obtained his PhD (CNAA) in Benzothiopyran Chemistry in 1992 under the supervision of Professor John Hepworth at the University of Central Lancashire. A postdoctoral fellowship in heterocyclic chemistry (1992-95) and an industrially funded lectureship at Central Lancashire (1995-98) were followed by appointment to a James Robinson Lectureship at the University of Hull (1998-2000). Dr. Heron was appointed as a senior lecturer (2000-present) in the Department of Colour and Polymer Chemistry at the University of Leeds. His research interests include the chemistry and applications of heterocyclic compounds, color chemistry, and organic photo- and thermochromic materials. 

Furthermore, exploitation of fundamental coordination chemistry for solid state synthesis affords a variety of metal ions with preferred coordination geometries and numerous ligand architectures for the construction of metal-organic composite materials. Such hybrid inorganic-organic materials exhibit useful physical properties with applications to optics," magnetism," transport," thermochromism," and conductance. ... 

Interest in polysilane polymers has been reawakened because they have appeared as new potential industrial raw materials for production of conducting and semi-conducting electronic devices, photomemories, photoresists, UV-absorbing and thermochromic materials, radical photoinitiators for polymerization, precursors for silicon carbide ceramics and fibers, organic glasses and m cal drugs [7-12]. 

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. 

Whilst this definition is academically accurate for many inorganic and organic materials, the label thermochromic has also been applied to important technical areas that involve other external influences as well as heat in the observed colour change, e.g. thermochromic pigments. 

Molecular self-organization in solution depends critically on molecular structural features and on concentration. Molecular self-organization or aggregation in solution occurs at the critical saturation concentration when the solvency of the medium is reduced. This can be achieved by solvent evaporation, reduced temperature, addition of a nonsolvent, or a combination of all these factors. Solvato-chromism and thermochromism of conjugated polymers such as regioregular polythiophenes are two illustrative examples, respectively, of solubility and temperature effects [43-45]. It should therefore be possible to use these solution phenomena to pre-establish desirable molecular organization in the semiconductor materials before deposition. Our studies of the molecular self-assembly behavior of PQT-12, which leads to the preparation of structurally ordered semiconductor nanopartides [46], will be described. These PQT-12 nanopartides have consistently provided excellent FETcharacteristics for solution-processed OTFTs, irrespective of deposition methods. 

This survey of organic photochromic and thermochromic compounds focuses on the main families that are involved in existing commercial applications, such as variable optical transmission materials (ophthalmic glasses and lenses), or in potential uses such as optical storage (optical disks or memories). 

The nanocomposite films were purified by prolonged dipping in acetone. In particular, polystyrene became swollen by acetone absorption, and all organic by-product completely diffused outside. In addition, thermochromic layers of very uniform thickness were obtained by dissolving the nanocomposite material in adequate quantities of chloroform and spin-coating the obtained viscous solutions on polymeric substrates (e.g., PET films). 

The ability of oxophenalenoxyl radicals to form functional organic materials was also examined by tetrathiafulvalene (TTF) molecule as an electron donor. It was fotmd that TTF-substituted oxophenalenoxyl radicals exhibited tunable intramolecular electron transfer (lET) by moderate change of external environments such as solvent and temperature in solution, leading to spin center transfer accompanied by solvato-/thermochromism (Fig. 10). There were two species involved in this process, a neutral radical 35 existing in dichloromethane solution with most of the spin localized on oxophenalenoxyl motif, and a zwitterionic radical 36 existing in trifluor-oethanol solution with spin localized on TTF moiety as radical cation species. 

Many experimental results have suggested that the conformational modification of the polythiophene backbone can be induced through order-disorder transitions of the side-chains [59]. It was then postulated that various external stimuli could pertuib the side-chain organization and consequently induce some chromic effects. These side-chain transitions can be induced by heating (thermochromism), varying the solvent quality (solvatochromism), ion complexation (ionochromism), photo-induced isomerization (photochromism) and affinity binding (affinity or biochromism) giving rise to a novel class of field responsive materials. 

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. ... 

---