389. Topochemistry. Part VII. The photoactivity of anils of salicylaldehydes in rigid solutions

Abstract

By M. D. COHEN, (the late) Y. HIRSHEERG, and G. M. J. SCHMIDT. All anils of salicylaldehydes investigated undergo colour changes whenThe In some solvents the change their solutions in rigid glasses are irradiated with ultraviolet light. effect is reversed when the solutions soften. can be reversed photochemically even while rigidity is maintained.Anils of aldehydes lacking an o-hydroxyl group show no photoactivity.PART VI * has discussed experimental work on the photoactivity of solid salicylideneanilines. It was concluded that the photo-behaviour of these compounds is topochemically controlled since photochromy and thermochromy, shown to be mutually exclusive properties in the solid state, may be found in different polymorphic forms of the same anil.It was further shown that the *' coloured " species obtained by the photochemical or thermal mechanisms resemble each other in their spectral properties : however, little information on their molecular structures could be derived from the spectra in solid films. The present and succeeding (Part VIII) Papers deal with the properties of salicylideneand hydroxynaphthylidene-anilines in solution and in rigid organic glasses. (Stobbe's experiments1 on anils incorporated in films of resins had suggested to him that photochromic properties are not maintained in the amorphous state; however, we felt that those experiments had been too crude in their choice of *' solvent " to be significant.) Our experiments were designed to test the photo-properties of the isolated anil molecules in an environment of lesser rigidity than that imposed by the crystalline lattice, and to obtain better spectral data than was possible with crystalline materials. The initial rigid-state experiments were conducted with solutions of salicylidene-mtoluidine in methyl cyclohexane-ligroin at liquid-air temperature. They showed that irradiation causes, in such solutions, a colour-change similar to that occurring in the crystal (yellow to red). Later experiments proved this to be a general effect occurring with most anils in a variety of rigid solvents. The photo-colour is stable as long as rigidity * Part VI, preceding paper.

Supramolecular Influence on Keto-Enol Tautomerism and Thermochromic Properties of o-Hydroxy Schiff Bases

“…[25] In all three cases of o-hydroxy imines reported here, the dihedral angles between aromatic subunits are well under 25° (Table 1), the value which has been declared in literature and several times disproved as an important criterion for thermochromic behaviour. [12][13][14][15][16][17][18][19][20][21][22][23] A molecular overlay of molecules of all three Schiff bases is given in Figure 3b and shows the small deviation of the molecules from planarity. The influence of temperature on the planarity of these three imines is neglectable.…”

“…[11] The microscopic reasons of such macroscopic properties are still under investigation, [7,8,[12][13][14][15][16][17][18][19] and this work is one of them. There are three described mutually dependant reasons for such behaviour in the solid state: proton transfer via intramolecular O⋯N hydrogen bonds and thus the change of the tautomeric form, [12][13][14][15][16][17] the contribution of fluorescence and not only of light absorption as a consequence of the tautomeric change and the change in molecular geometry. [18] The scientists are still trying to either reaffirm the mentioned reasons or to find imines in which the reasons cannot be strictly determined but they do show such chromic change.…”

A spectroscopic study of proton transfer and photochromism in N-(2-hydroxybenzylidene)aniline

“…Cohen et a/. (5,6) report the spectra of irradiated and nonirradiated low temperature solutions of several N-salicylideneanilines and in all cases it appears that the increase in absorbance at the visible wavelength is greater than the decrease in absorbance at the near ultraviolet wavelength. In no case, however, is the ratio so large as in the case of the ultrathin polycrystalline film reported here.…”

“…One equilibrium involves proton transfer and tautomerization and the other involves a trans + cis isomerization about the central carbon-nitrogen double bond. Other authors (5,6,22) have also suggested that more than three molecular species may be involved in these processes. Hadjoudis et a / .…”

Four Thermochromic o-Hydroxy Schiff Bases of α-Aminodiphenylmethane: Solution and Solid State Study

“…N-substituted imines can be easily synthesized by condensation of aldehydes (or ketones) and primary amines [12] and used not only for the investigation of their physico-chemical properties, but also because of their well-recognized biological and pharmacological properties [9,[13][14][15][16], as well as their metal complexes [17]. There are three mutually-dependent reasons for such behavior in the solid state described in the literature: (a) proton transfer via intramolecular O···N hydrogen bonds and thus (b) the change of the tautomeric form [18][19][20][21][22][23] and (c) the contribution of fluorescence and not only of light absorption as a consequence of the tautomeric change and the change in molecular geometry [24]. Another intriguing "well-known fact" about thermo-and/or photochromism of o-hydroxy Schiff bases is that these two phenomena are mutually exclusive and that imines with planar molecules light absorption as a consequence of the tautomeric change and the change in molecular geometry [24].…”