Colorants thermochromic

The sohd-state, transition-metal example in Table 1 of [(CH2)2NH2]3CuCl illustrates another form of thermochromism the color shifts gradually and continuously because of changes in bandwidth with either heating or cooling (6). It is not unique, as this behavior has been mentioned for the class of... 

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-... 

The various sohd sulfur allotropes are all more or less yellow at 20 °C but the scale ranges from the deep orange-yellow of to the pale yellow of S. The color also depends on the particle size As finer the particles as lighter the color impression. Chemically produced Ss has in fact occasionally been obtained as a colorless material at room temperature which only slowly turns yellow as the initially tiny crystals grow [61]. The sample temperature is also of decisive influence (thermochromic effect) [10] (see above). 

For thermographic recording materials, thermochromic properties of the spiroindolino- and spirobenzothiazolino-benzopyrans have been utilized. As an example, thermal paper patented by National Cash Register38 can be cited. In this paper, the colored merocyanine form is fixed by reacting with phenols or metallic salts.2... 

Thermochromism of spiroindolinonaphthooxazine is observed only in high-concentration solution.77 A high concentration ( 10 3 M) of 33 in ethanol produces a bluish solution, and its color intensity increases as the temperature of such concentrated solution increases. This thermal equilibrium is also affected by substituent groups. Donor substituent groups promote the formation of the colored form. 

CuFeS2 (chalcopyrite) is one of the most important copper minerals. Red /3-Cu2HgI4 and yellow )3-Ag2HgI4 (CdGa2S4 type) are thermochromic they transform at 70 °C and 51 °C, respectively, to modifications having different colors (black and orange) in these the atoms and the vacancies have a disordered distribution. 

The trinuclear complexes [Au3(/7-MeC6H4N = OEt)3] and [Au3(Bzim)3] (Bzim= 1-benzylimida-zolate), which are colorless, can produce brightly colored materials by sandwiching naked Ag+ or Tl+ ions form linear-chain complexes (Figure 27) with luminescence properties as luminescence thermochromism.3173,3174... 

At room temperature, zinc oxide is white, but when heated it becomes yellow. A compound that changes color on heating is said to be thermochromic. 

Many distibines and dibismuthines have lighter colors in solutions or melts than in the solid state. Crystals of these thermochromic distibines or dibismuthines consist of linear chains of the dimetal compounds with short intermolecular metal-metal contacts. Delocalization of electrons along the chains is possibly responsible for the bathochromic shift between fluid and solid phases. Usually, the /raor-conformation is adopted by the tetraorganodimetal compounds in the solid state. (CF3)4As2 shows the /ra r-conformation also in the gas phase. Photoelectrospectroscopic measurements on Me4Sb2 revealed the presence of gauche- (12%) and trans- (88%) conformed in the gas phase.52... 

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) ... 

Cu(II) Complexes with Diethylethylenediamine. The complexes [Cu(dieten)2]X2, where X is an anion and dieten is the bivalent ligand N, V-diethylethylendiamine, well illustrate the effect of the medium on the extent of cooperativity of the transformation. These systems present interesting properties, and they have been investigated extensively [498-503]. When X is BF4, CIO4, or NO3 the complexes have thermochromic behavior and the color changes from red, at low temperature, to blue in the high-temperature form [499, 504]. 

A number of poly(arylene vinylene) (PAV) derivatives have been prepared. Attachment of electron-donating substituents, such as dimethoxy groups (structure 19.3), acts to stabilize the doped cationic form and thus lower the ionization potential. These polymers exhibit both solvatochromism (color changes as solvent is changed) and thermochromism (color is temperature-dependent). 

Solvatochromism and thermochromism are also characteristic of spirooxazines (Scheme 3) <1994RCB780>. The two forms 116 and 117 are in equilibrium in solution and more polar solvents shift the equilibrium more to the colored, acyclic form 117. Higher temperatures have the same effect for both solid spirooxazines and their solutions. A comprehensive review of spirooxazines <2002RCR893> has a collection of the absorption maxima for a large number of spirooxazines and their colored forms, which have their absorption maxima in the visible range at 480-670 nm. 

In solution [M(CO)5 Se—C(Aryl)H ] complexes show a thermochromic behavior. The color of the compounds is caused by a MLCT of d electrons into the LUMO localized mainly in the Se = C(Aryl)H ligand. In rf complexes this transition is at considerably lower energy than in if complexes. The observed color of solutions of [M(CO)5 Se = C(Aryl)H ] therefore depends on the rflif equilibrium, which is temprature dependent. Thus, solutions of, e.g., [W(CO)5 Se = C(Ph)H ] are blue at room temperature and green at -78°C.45... 

In certain cases the system may be thermochromic as well as photochromic thus giving rise to a residual background absorbance of the colored form which is temperature dependent. 

---