Schiff bases, thermochromism

Scheelite, 25 349-350 Scheelite sorting, 16 626 Scheibel column, 10 777-778 Scheibler filter, 11 364 Schiff base(s), 21 203, 204, 25 100-101 chelating agents, 5 712t reaction with amino acids, 2 567 reaction with aniline, 2 786 thermochromic materials, 6 622-623 Schiff base chemistry, 24 42 Schiff base (reductive amination) method, for covalent ligand immobilization, 6 396t... 

The thermochromic crystalline salicylideneanilines, 45a, show behavior reminiscent of that described above. (In 45a and 45b the Schiff base on the left is as in 15a.) These materials are light yellow at low temperatures, but become orange-red at elevated temperatures. This change is associated with proton transfer between O and N in the near-planar, close-packed molecules (101). The... 

The potentially tridentate Schiff bases derived from ring-substituted SAL and aminopyridine so far structurally characterized are reported in ref. 95. The molecules are not strictly planar. Since the relatively small deviations from planarity correspond to a weak thermochromism, it has been suggested95 that thermochromism should be a monotonic function of the angles which define the relative orientation of the six-membered rings with respect to each other. 

The photochromism and thermochromism of methyl 12-aminodehydroabiatate Schiff bases have been studied in crystalline states, rigid glasses, and in solution. The E-Z photoisomerization of benzophenone-9-anthraldehyde... 

The Schiff bases of salicylaldehydes with arylamines, aminopyridines, and aryl- or thienylalkylamines show thermochromism and photochromism in the solid state due to hydrogen transfer.54 At the beginning of the century, Senier and Shepheard55 observed that many of the colored crystalline salicylideneanilines were thermochromic. In most cases, the color changed, upon heating, from yellow to orange or red. 

The major thermochromic Schiff bases prepared by condensation of salicyl-aldehydes with anilines are reported in Table 3. Replacement of the phenyl in the aniline part by a 2-pyridyl group64 leads only to thermochromic molecules (Table 4). From X-ray crystallographic studies of compounds in this series, it has been suggested that planarity is achieved because of the position of the N atom of the pyridine ring, which is always cis with respect to the (N=C)-H hydrogen atom. 

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-... 

E. Hadjoudis, Spectroscopic studies on the thermochromic and photochromic Schiff bases, Presented at the 9th National Conference on Molecular Spectroscopy. Sofia, Bulgaria 1980. 

T. Inabe, I. Gautier-Luneau, N. Hoshino, K. Okaniwa, H. Okamoto, T. Mitani, U. Nagashima, and Y. Maruyama, Structure and optical properties of thermochromic Schiff bases. Charge transfer interaction and proton transfer in the V-tetrachlorosalicylideneaniline and JV-tetrachlorosalicyli-dene-l-pyrenylamine crystals, Bull. Chem. Soclpn. 64, 801-810 (1991). 

N. Hoshino, T. Inabe, T. Mitani and Y. Mamyama, Structure and optical properties of a thermochromic Schiff base Thermally induced-intramolecular proton transfer in the N,N bis(salicylidene)-p-phenylenediamine crystals, Bull. Chem. Soc. Jpn. 61, 4207-4214 (1988). 

The solid state of the Schiff bases is of great interest because of their photochromic and thermochromic properties. A few studies of Schiff bases in the solid state exist. Salman and coworkers found for aniline Schiff bases of 2-hydroxy-1-naphthaldehyde that at... 

Hadjoudis, E., Vittorakis, M. and Moustakali-Mavridis, I. (1987) Photochromism and thermochromism of schiff-bases in the solid-state and in rigid glasses. Tetrahedron, 43, 1345-1360. 

Hajoudis, E. and Mavridis, I.M. (2004) Photochromism and thermochromism of schifF bases in the solid state structural aspects. Chem. Soc. Rev., 33, 579-588. 

Certain known color reactions cannot be included unequivocally into these groups, because they represent a combination of several of the processes mentioned. For example, in the reaction of primary aromatic amines with aromatic aldehydes (p. 215), Schiff s bases are formed which give strongly colored salts (halochromism) in the presence of acids, the color of which deepens on heating (thermochromism). We should also keep in mind that the color reaction with the organic substance can take place either directly, or it may be neccessary to transform the tested substance... 

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