Photochromism of Salicylideneanilines

Figure 3. Photochromism of salicylideneaniline (SAN). Adapted from references 19,20. A number of photochemical species can form after the initial proton transfer reaction, the distribution of which depends upon solvent conditions and temperature (a) excited enol (b) excited quinoid (c) excited state after solvent relaxation. The ns decay process shown is not observed at room temperature. Instead, species (c) is reactivated to (b) followed by emission to (d), the ground state quinoid (e) other photochemical products. The intermediate (d) can be exploited in rotational studies because it has a lifetime in the ls range, one which is also very dependent upon environmental conditions (e.g. polarity, temperature).

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)... 

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

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. 

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. 

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

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. 

X-ray analysis of structural changes in photochromic salicylideneaniline crystals. Solid-state reaction induced by two-photon excitation. J. Am. Chem. Soc., 121, 5809-5810. 

The first example is the structure of the metastable red species of a salicylideneaniline derivative, A-3,5-di-rert-butylsalicylidene-3-nitroaniline, which, when the pale yellow crystal is irradiated with a mercury lamp, shows photochromism with the largest lifetime among various salicylideneaniline derivatives [9]. Much research on this subject has been undertaken but the structure of the photo-coloured red species remains controversial [10]. 

N-salicylideneaniline is photochromic the enol configuration can be converted to the trans-keto conformation by using light of frequency Vj, and converted back by either heat, or by light of frequency V2 here the tautomerism causes an intramolecular rotation [67]. 

Keywords Pyridine derivatives Salicylideneaniline derivatives Structure of excited molecule Structure of lophyl radical Unstable species of photochromism... 

More interesting results were obtained when the complex crystals between the salicylideneanilines with carboxyl group and the dibenzyl- or dicyclohexylamine were examined [40, 41], The compound of A -saUcylidene-3-carboxyaniline, 3, makes acid-base complexes with dibenzylamine, 3-a, and dicyclohexylamine, 3-b, whereas Af-salicylidene-4-carboxyaniline, 4, forms an acid-base complex with dibenzylamine, 4-a, which are shown in Scheme 7.5. The crystals of 3 and 4 have orange color and show no photochromism, whereas the colorless, orange, and colorless crystals of 3-a, 3-b, and 4-a, respectively, turned red when they were irradiated with UV light. The UV-vis spectra before and after photo-irradiation for 3,3-a, and 3-b are shown in Fig. 7.13, which are very similar to those of 2y, 2a, and 2p, respectively. The spectra of 4 and 4-a are similar to those of 3 and 3-b. 

The salicylideneaniline (SAN) molecule and related molecules are highly usefiil model compounds for the investigation of the role of structural flexibihty in Kght-induced structural transformations. This photochromic transformation is connected with adiabatic ESIPT OH—>NH in the Sj state of the structure (E, Scheme 6), which is the origin of the fluorescence anomalous Stokes shifL The anomalous Stokes-shifted fluorescence and absorption spectral maxima of SAN observed under several conditions are listed in Table 68.9. The anomalous Stokes-shifted emission was found to be observed only in rigid matrices, especially in a glassy solvent at 77 K, as well as in crystals. This implies that the necessary step... 

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