Anils thermochromism

The above properties (a and b) are interpreted by Cohen and Schmidt (21) on the basis of a detailed crystallographic study of photochromic and thermochromic anils. They conclude that photochromic crystals involve structures in which the central portion of adjacent molecules are essentially isolated from one another, so that, to a first approximation the energetics are that of an isolated molecule. On the other hand, when the alignment of the molecular dipoles is such as to give strong intermolecular interactions then the transition to the quinoid form requires much less energy and can occur thermally. For crystals in which thermochromism occurs, the photochemical isomerization is still possible but the reverse reaction is so rapid that no buildup of color is observed. In fact, fluorescence measurements on the thermochromic 5 -chlorosalicylidene-aniline (Fig. 5) indicate that photochemical isomerization precedes the luminescence process via the photochromic route ... 

The previously mentioned photochromic spiropyrans also exhibit thermochromic activity upon melting [56-58], The melts are usually purple, red, or blue [19] and the color change is reversible however, the stability of the spiropyran/merocyanine composition is low at the thermochromic temperature transition range. As a consequence, the useful thermochromic lifetime of the spiropyrans is relatively short. Other molecular types that exhibit both photochromic and thermochromic activity are the indenone oxides, anils, spirooxazines, and chromemes. 

In thermochromic crystals, on the other hand, the planar form of N-5-chlorosalicylideneaniline [34 X = 5-Cl, Y = H] (Bregman et al., 1964b) is packed tightly with an interplanar distance of 3.4 A (Fig. 17). Consequently, in the planar anils, the proton transfer from the phenolic OH to the imino nitrogen can take place thermally. The two forms, the OH form and the NH one, are thermally equilibrated, and as the temperature increases, the contribution of the cd-keto form becomes larger (Scheme 4). 

Kawato et al. (1985, 1986, 1994a) prepared t-butyl derivatives of thermochromic anils [36] and they found that these compounds exhibit photochromic behaviour in crystals. Introduction of bulky substituents is considered to provide enough space for the cis—trans isomerization to occur. 

When thermochromic anils are incorporated into host crystals of deoxycholic acid or its derivatives, the incorporated anils have been found to exhibit photochromic behaviour as well (Koyama et al., 1994 Kawato et al., 1994b). The incorporated anils are evidently allowed to isomerize photochemically as in the case of derivatives of anils carrying bulky substituents. 

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. 

Intensive investigations have been conducted to elucidate the nature of the mechanism of thermochromism of salicylidene Schiff bases. Different techniques or methods suitable for the study of the tautomeric equilibrium between the end form and the (Z)-keto form have been used, including X-ray diffraction, NMR, infrared (IR) and Raman spectroscopy, and theoretical calculations. In the anil-... 

During the past 12 years (the period covered by this review), very few new thcrmochromic molecular systems have been reported. The main families of compounds have continued to be the major focus of interest, with only a shift in the balance between them for example, spiropyrans have been less studied (in terms of development of new compounds) while interest in anils has increased. There has also been a shift away from work essentially devoted to the description of new compounds, as, in recent years, the attention of authors has mainly been directed toward thermodynamic, kinetic, structural, and mechanistic aspects. For this purpose, a large variety of techniques, including not only spectroscopic techniques (UV-visible, NMR) but also X-ray diffraction and theoretical calculations, have been employed, leading to a better knowledge of the fundamental aspect of the thermochromism of organic compounds. 

An interesting property of SALAI and its substituted derivatives is their ability to colour markedly under UV irradiation. This photochromism has been attributed to the possible formation of a geometric isomer of the quinoid tautomer. On the other hand, these compounds also exhibit thermochromiSm, which consists of a thermally induced change in colour which increases with an increase of the temperature. This phenomenon has been attributed to intramolecular hydrogen transfer, such as that depicted in Scheme 1. Both processes are reversible and mutually exclusive for the same compound in a given crystalline form. However, since the same anil may exhibit polymorphism, it may be thermochromic in one crystalline form and photochromic in the other. Therefore, these processes should be correlated with differences in the crystal structure rather than with inherent properties of the molecule. All the crystallographic results, which will be reported in Section 20.1.2.3, confirm the earlier hypothesis that molecules which exhibit thermo-chromism are planar, while the others are non-planar, as shown in Figure 1. 

Rosenfeld, T., Ottolenghi, M., and Meyer, A.Y., Photochromic anils. Structure of photoisomers and thermal relaxation process. Mol. Photochem., 5, 39, 1973 Lambi, E., Gegiou, D., and Hadjoudis, E., Thermochromism and photochromism of N-sahcyhdenebenzylamines and N-salicylidene-2-aminomethylpyridine, /. Photochem. Photohiol, A Chem., 86, 241, 1995 Goto, T. and Tashiro, Y, Photochromism of N-salicylideneaniline single crystal, /. Luminescence., 72—74, 921, 1997. 

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