Thermochromic properties

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... 

Preceding reviews on distibines, R2Sb-SbR25,7,66-68 feature mainly the structural aspects in relation to the thermochromic properties of some of these compounds, but also overviews on the earlier work of the coordination chemistry of distibine ligands have been reported.7,68 All the distibines that... 

In recent years, many spirochromenes such as (136) and (137) have been studied because of their photochromic or thermochromic properties, while derivatives of xanthene are useful as dyes and indicators, for example pyronine G (138), fluorescein (139) and rhodamine 6G (140). 

Figure 2.12 Schematic of a surface assembly of a pyrene-derivatized Anderson (left) that absorbs in the blue arranged next to a [Movllg054(S03)2]4 cluster species which has thermochromic properties. On the right there...

While tetraphenyldibismuthine (46) is not isostructural with tetraphenyl-distibine (1), it does not show any close intramolecular Bi—Bi contacts. Although no other structural data are available, it might be noted that there is a complete match of the thermochromic properties for the dibis-muthines with the corresponding distibines. Thus, in all cases if a distibine is thermochromic the analogous dibismuthine is as well. Conversely, nonthermochromic distibines correspond to nonthermochromic dibis-muthines. A structural correspondence also seems likely. 

It has therefore been concluded that any further modification of the photo-chromic compound 1 should be associated with structural variations in its 2,3-dihydroperimidine moiety. In the course of subsequent studies, the spirocyclohex-adienones belonging to structural types 1-9 have been found to display photo- and thermochromic properties. 

The isolobal relations R E RE and RE E " (E" = Sb,Bi E " = Se,Te) that are supported by the thermochromic properties of some ditellurides have stimulated the search for colour effects of compounds with bonds between Sb, Bi and Se or Te. As a result of these efforts, knowledge in synthetic chemistry in the field has expanded considerably and methods have been developed for the preparation of the following types of novel compounds - - ... 

V S. Marevtsev, L. L. Zaichenko, V D. Ermakova, S. I. Beshenko, V. A. Linskii, A. T. Gradyushko, and M. I. Cherkashin, Effect of electron-donating and electron-accepting substituents on photo- and thermochromic properties of indolino-spiropyrans, Bull. Acad. Sci. USSR, Chem. Sci. 29, 1591— 1596 (1980) Chem, Abstr. 94, 29901 (1980). 

S. M. Aldoshin, V. A. Lokshin, A. N. Rezonov, N. V. Volbushko, N. Shelepin, M. I. Knyazhanskii, L. O. Atovmyan, and V. I. Minkin, Structures and photo- and thermochromic properties of spiropyrans of the 2-oxaindan series with polycondensed chromene fragments, Chem, Heterocycl. Compd. 1987, 614-624 Chem. Abstr. 108, 131516 (1987). 

As distinct from photochromic spiropyrans and spirooxazines, [2//J-chro-menes do not contain a heterocyclic fragment such as an indoline nucleus. Consequently, in such compounds there are no specific n-a orbital interactions that elongate and weaken the C, iro—O bond, the ruptures of this bond in the excited and ground states being responsible for the photo- or thermochromic properties of spiropyrans and spirooxazines as well as [2// -chromcncs. 

The thermochromism of spiropyrans, discovered in 1926, has been extensively studied. Nearly every known compound of this class leads to deep color on melting (generally red, purple, or blue). However, heating solutions of spiropyrans also causes coloration. Day1 in his review gave a table reporting essentially thermochromic spiropyrans of the indoline and spirobipyran series. Bertelson6 summarized the main spectroscopic and physicochemical data obtained up to 1971. Thermochromic properties of spirooxazines have been more recently reported.7... 

During the past decade, new thermochromic spiropyrans have been described, particularly by Russian teams. For example, spiropyrans of the 2-oxaindane or azaindanone series with polycondensed chromene fragments (1-8), exhibiting photo- and thermochromic properties, have been synthesized by Minkin and co-workers.19,20... 

Krasieva and co-workers21 mentioned the thermochromic behavior of spiropyrans of the dithiolane series (9, 10). These data confirm that the annellation of the benzopyran moiety favors the thermochromic properties of this class of compounds. 

Although the thermochromic properties ofmonospiropyrans (with one pyran ring) have been described extensively, bis-spiropyrans with various structures have also been studied. For bis-spiropyrans in which the merocyanine fragments are... 

The molecular conformation and the possibility or impossibility of stack packing in the crystal have been shown to be responsible for photo- and thermochromism.60 Molecules for which the less favorable planar structure is stabilized by stack packing possess thermochromic properties. The strong inter-... 

Pistolis et al,73 have shown that the complexation of thermochromic N-5-chlorosalicylideneaniline and A-salicylidene-2-aminopyridine with cyclodextrins (CD) results in the disappearance of thermochromic properties and the appearance of photochromism. From -NMR, UV-visible, and fluorescence studies in dimethylformamide (DMF) solutions, it has been found that P- and a-cyclo-dextrins bind the thermodynamically more stable enol form of anils. Moreover, NMR data indicated that the binding site of the anil in the CD is the imino bridge, which dictates the position of the enol-keto equilibrium. 

Structural and optical studies74 of the thermochromism of /V-tetrachloro-salicylideneaniline (140) Table 3 and A-tetrachlorosalicylidene-l-pyrenylamine (171) Table 4 showed, in the latter case, an intermolecular charge-transfer interaction [the interplanar distances between the mean phases being short (3.38 A)]. The consequence is a decrease of the thermochromic properties due to a modification of the proton transfer by this charge transfer. 

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