Salicylideneanilines

Sautet P, Joachim C (1989) The salicylideneaniline molecular switch revisited. Chem Phys 135 99... 

Higelin D, Sixl H (1983) Spectroscopic studies of the photochromism on N-salicylideneaniline mixed crystals and glasses. Chem Phys 77 391... 

Rate coefficients in the expected range have been observed in excited state intramolecular proton transfers involving 2-(2-hydroxyphenyl)benzothiazole (Barbara et al., 1980a) and salicylideneaniline (Barbara et al., 1980b). In the former, the observation of fluorescence was explained by Scheme 2 and the... 

It is interesting to note that the reaction of N-salicylideneaniline, which possesses a free phenolic hydroxyl group at the ortho position in the benzylidene portion, with the acylzirconocene chloride gives the a-amino ketone in 67 % yield in the absence of any additive (Scheme 5.16) [23]. Similarly, N-(p-hydroxybenzylidene)aniline reacts with the acylzirconocene chloride to give the a-amino ketone in 58 % yield. The reaction of N-(m-hydroxyben-zylidene)aniline, however, gives the a-amino ketone in just 12 % yield. Neither N-(o-MeO-benzylidenejaniline nor N-(p-MeO-benzylidene)aniline gives an appreciable amount of product in the absence of additive. 

A particular effect of an intramolecular hydrogen bond occurs in /V-salicylideneaniline derivatives119 and anils of hydroxynaphthaldehydes120, and their tautomerism (shown... 

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... 

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) ... 

An interesting futuristic application is in the field of molecular electronics where a one-dimensional molecular wire such as polyacetylene in combination with a suitable molecular switch, e.g. salicylideneanilines, would yield a molecular microchip whose information storage capacity would be about 10 times that of a conventional microchip. A new generation of high performance computer with memory elements of nanometre dimensions is visualized on the basis of such molecular microchips. 

The majority of electro syntheses have been carried out according to route A, for example, those reported in Refs. 594 to 597. In this respect, the electrochemical syntheses (3.252) with participation of 2-tosylamino-N-salicylideneaniline 640 (X — O, Y — NTS), 1-methyl-2-aminobenzimidazole (L), cobalt, copper, and zinc (M°) were carried out in acetonitrile at room temperature (10 mA, 20 V, 4 hr) in the presence of Et4NC104 [594,595]. The structures of copper [594] and cobalt [595] complexes 804 were proved by x-ray diffraction. 

Stumpe and his co-workers [ 122-124] synthesized several SCLC methaciy-late copolymers containing the photochromic N-salicylideneaniline (NSA) chro-mophore together with several other mesogenic and nonmesogenic groups. Copolymers, 58, had from 0 to 100% of the NSA chromophore. Most of these polymers had an unspecified 5 mesophase. The NSA chromophore undergoes an excited state proton transfer (Fig. 17) to the Z-keto (often called cw-keto form)... 

Figure 17 Enol (phenol)-keto tautomerism of the N-salicylideneaniline chromophore. Photochemical processes are indicated by solid arrows, thermal processes by dotted arrows.

Photo-enolization of 2-methylisophthalaldehyde 244 Photochemical vs. thermochemical behaviour of salicylideneanilines Photochromic behaviour of dinitrobenzylpyridines 249... 

This section describes the characteristics of the photo- and thermochromism derived from proton tautomerization in crystals, using the photo- and thermochromic behaviour of 2-alkylisophthalaldehydes, salicylideneanilines, and dinitrobenzylpyridines as typical examples. 

Salicylideneaniline [32] and its derivatives are known to exhibit photochromic behaviour. The colour of the crystal of salicylideneaniline changes from pale yellow to red, and the red colour changes back to the original one in a thermal process. On the other hand, other derivatives of salicylideneaniline show only thermochromic behaviour. It is usually the case that a photochromic crystal never exhibits thermochromism, and vice versa. 

In imine reductions rapid equilibration of dimeric dianions and the precursor radical anions leads to the thermodynamically more stable isomer [257]. In the reductive coupling of salicylideneanilines, however, rate-determining C-C bond formation is preceded by intramolecular H-bridging [258]. 

A kinetic study of the hydrolysis of JV-salicylideneaniline (113) in the presence and absence of cobalt(II), nickel(II), copper(II) and zinc(II), using 10% ethanol-water as solvent, has been carried out by Dash and Nanda. The (1 1) Schiff base-metal complexes (ML ) were found to undergo acid-catalyzed hydrolysis at rates decreasing with the thermodynamic stabilities of the complexes, the most thermodynamically stable complexes undergoing the slowest rate of hydrolysis. More recent measurements using high copper(II) to ligand ratios have indicated that the copper(II)-imine is quite stable to hydrolysis at pH 5. 

R. S. Becker, C. Lenoble, and A. Zein, Photophysics and photochemistry ofthe nitro derivatives of salicylideneaniline and 2-(2 -hydroxyphenyl)benzothiazole and solvent effects, J. Phys. Chem. 91, 3517-3524(1987). 

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. 

Figure 6. Relationship between the crystal structure and the thermo- and photochromic properties of A -salicylideneaniline.60 61...

On the other hand, in photochromic crystals, with no stack-packing, a nonadiabatic acoplanarization occurs following the phototransfer, leading to an absorption at wavelengths up to 540 nm. A comparison of the photo- and thermochromic conversions for salicylideneanilines is given in Figure 7. 

Figure 7. Representation of photo- and thermochromic conversions of JV-salicylideneaniline.62...

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. 

With the use of nuclear quadrupole resonance (NQR) and nuclear quadrupole double resonance techniques,83,92 94 the temperature dependence of 35C1,14N and 170 frequencies was measured at 77-289 K for a series of salicylideneanilines. All the experimental results agree with a model in which the exchange between the low-energy enol form and the high-energy tautomeric keto form is fast and reflects a dynamic equilibrium between the two forms. 

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