Insights into the Origin of Solid‐State Photochromism and Thermochromism of N‐Salicylideneanils: The Intriguing Case of Aminopyridines

Abstract

The relationships between the crystal structure and optical properties of switchable N-salicylideneanils have been revised and discussed on the basis of new experimental results and a computational approach. N-salicylidene-3-aminopyridine (L(3)) is a versatile thermo- and photochromic molecule. It also exhibits an infinitely slow thermal back relaxation (k = 9.9x10(-8) s(-1)) after photoswitching that is suitable for optical memories. Contrary to reports in the literature, N-salicylidene-4-aminopyridine (L(4)) is exclusively thermochromic. To explain these unexpected optical properties in the solid state, crystallography combined with UV-visible spectroscopic data was exploited. L(3) was also used as a ligand in new thermochromic coordination complexes [M(CH(3)OH)(2)(L(3))(2)(NCX)(2)], in which M(II) = Fe, Co, Ni, Cu or Mn and X = S or Se (1-6), which allowed the fine-tuning of the electron density in the photochromic moiety. The influence of the coordination through the nitrogen of the pyridine ring is also fully discussed.

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

“…In the past decade, many non-planar o-hydroxy Schiff bases derived from benzylamine derivatives were synthesized and reported as thermochromic. There is still a general consensus that the deviation from the planarity, the dihedral angle between the aromatic subunits, can be up to 25 • [25,26] for aniline derivatives, while o-hydroxy imines with a spacer between the imine N-atom and the aromatic rings of the corresponding amine are excluded from that empirical rule [27,28]. The accepted mechanism for thermochromism is a temperature-dependent keto-enol tautomeric change between an uncolored enol form and a yellow cis-keto form, while the existence of the trans-keto form changes the color to red [27,28].…”

“…There is still a general consensus that the deviation from the planarity, the dihedral angle between the aromatic subunits, can be up to 25 • [25,26] for aniline derivatives, while o-hydroxy imines with a spacer between the imine N-atom and the aromatic rings of the corresponding amine are excluded from that empirical rule [27,28]. The accepted mechanism for thermochromism is a temperature-dependent keto-enol tautomeric change between an uncolored enol form and a yellow cis-keto form, while the existence of the trans-keto form changes the color to red [27,28]. In many cases, keto-amine forms were not found in the crystal structure o-hydroxy Schiff bases; in some cases, the molecules are in keto tautomeric form, and the samples are not of the expected color; and in some cases, keto-enol tautomeric equilibrium was found, and its position is affected by a temperature change without a color change; the postulates of Schiff base thermochromism are still taken as definite facts.…”

“…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 (dihedral angle between aromatic subunits φ < 25°) should be thermochromic and ones with non-planar molecules (φ > 25°) should be photochromic [18][19][20][21]. The above-mentioned reasons and facts are the motives why scientists are still trying to either reaffirm them or to find o-hydroxy imines in which these reasons cannot be strictly determined, but the imines do show such a chromic change [22][23][24][25][26][27][28][29][30][31][32]. It should be pointed out that the empirical rule on the molecular (non)planarity should not be applied to o-hydroxy Schiff bases obtained from benzylamine derivatives and aliphatic amines, as stated before in the literature [25,26,28,30].…”

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.…”

“…[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…”