Crystal Structure Change for the Thermochromy of N-Salicylideneanilines. The First Observation by X-ray Diffraction

Bis[4-(salicylideneamino)phenyl]methane

“…The title compound, (I), has been shown to form helicate supramolecular complexes of the form [M 2 L 2 ] with transition metals (Yoshida & Ichikawa, 1997;Yoshida et al, 2000;Kruger et al, 2001). Free N-salicylideneanilines are often thermochromic (Ogawa et al, 1998;Filarowski et al, 2002;Popovic  et al, 2002;Ogawa & Harada, 2003) due to a temperature-dependent equilibrium between the keto±amine and enol±imino forms.…”

“…The factors determining whether a given molecule will occur in the keto±amine or enol±imino form are manifold. It has been shown to depend on the substitution on the benzene rings (Filarowski et al, 2002;Popovic  et al, 2002) and intermolecular hydrogen bonding (Ogawa et al, 1998;Ogawa & Harada, 2003), and aggregation (packing) of the molecules plays an important role for the equilibrium in solution (Ogawa et al, 2001;Ogawa & Harada, 2003 The molecular structure of (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.…”

“…The title compound, (I), has been shown to form helicate supramolecular complexes of the form [M 2 L 2 ] with transition metals (Yoshida & Ichikawa, 1997;Yoshida et al, 2000;Kruger et al, 2001). Free N-salicylideneanilines are often thermochromic (Ogawa et al, 1998;Filarowski et al, 2002;Popovic  et al, 2002;Ogawa & Harada, 2003) due to a temperature-dependent equilibrium between the keto±amine and enol±imino forms.The location of the H atoms showed unequivocally that (I) occurs in the enol±imino form in the crystalline state, in agreement with previous IR results ( Kruger et al, 2001;Pui et al, 2001). This is also the form found in CHCl 3 solution (Yoshida et al, 2000;Kruger et al, 2001).…”

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

“…Their optical properties in the solid state can be switched by various stimuli, for example temperature change in thermochromic or by changing the wavelength of irradiation in photochromic Schiff bases. [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. [7,8,[18][19][20][21][22][23] In this work, we report both the mechanochemical synthesis and the investigation of structurethermochromism correlation of three Schiff bases (Scheme 1) derived from three aromatic aldehydeso-vanillin (ovan), o-hydroxysalicylaldehyde (oOH) and p-hydroxysalicylaldehyde (pOH) and three aromatic amines -2-amino-4-methylphenol (2a4mp), 3-aminoacetophenone (3aa) and 4-aminoacetophenone (4aa). The aldehydes and amines were selected according to the…”