Cohen et al. systematically studied the absorption spectra of salicylaldehyde aniline compounds, and found that some crystals of these compounds were photochromic and some were thermochromic. According to the spectral properties and crystal structure, they divided the crystals of these compounds into two categories.
There are many differences between α-type and β-type crystals: the molecules in thermochromic crystals are planar and stacked face to face, and the distance between molecules is small, about 3.3A; In photochromic crystals, the salicylaldimine part of the molecule is flat, and the angle between aniline ring and the salicylaldimine plane is 40 ~ 50, which makes the molecules pile up loosely in the crystal, and there is no face-to-face molecular dense packing structure.
In order to explain this photochromic and thermotropic phenomenon, Talon Yunebeli’s dream shows that there are intramolecular hydrogen bonds in the molecule of Schiff base. When the photochromic reaction occurs, protons are transferred from oxygen atoms to helium atoms.
Intramolecular proton transfer can occur in both ground state (thermochromism) and excited state (photochromism). Photochromism and thermochromism involve two isomers, one is Enol form with proton covalently bonded to oxygen atom, and the other is ketone form with proton covalently bonded to nitrogen atom (including cis-Keto cis-keto and trans-Keto trans-keto). The absorption of keto isomers is located in a longer wavelength region, so photochromic and thermochromism cause the red shift of crystal absorption spectrum. As far as photochromic crystals are concerned, in the electronic ground state, due to the distortion of molecular conformation, proton transfer requires high energy, so proton transfer in the ground state cannot occur; However, in the excited state, its crystal cavity is large, which can be generated after proton transfer.
Subsequent geometric isomerization (cis-trans isomerization) produces trans-Keto structure, which is stable due to the breaking of intramolecular hydrogen bonds. However, for thermochromism, in the ground state, the molecules are planar, and proton transfer is easy, and proton transfer in the ground state can occur to form cis-Keto structure. They concluded that the photochromic products were trans-Keto and the thermochromic products were cis-Keto. Hadjouds et al. extended the study on the relationship between photochromic and thermochromism of Schiff bases and crystal structure to three kinds of heterocyclic Schiff bases.
In the Schiff base of salicylaldehyde condensation-aminopyridine, there is no mutual repulsion between the nitrogen atom on the pyridine ring and the hydrogen atom on the N=C double bond outside the ring. On the contrary, the hydrogen atoms on the N=C double bond outside the benzene rings of 103 and 104 and the ortho-hydrogen atoms on the pyridine ring repel each other. So the molecules in the crystal are flat, and these compounds are thermochromic. In the crystal of 3- aminopyridine Schiff Base (103), there is a certain deviation (~15′) between the plane of pyridine ring and the plane of salicylaldehyde imine, so it is weakly thermochromic. The nitrogen atom on the pyridine ring in the crystal molecule of salicylaldehyde-4- aminopyridine Schiff base (104) has little effect on the crystal structure, so it may be photochromic or thermochromic.
Recently, Kawato et al. synthesized and studied a series of tert-butyl substituted Schiff bases (105), and studied the relationship between their crystal structures and photochromism. It is found that tert-butyl substitution increases the space of molecular movement in the crystal, which is beneficial to photochromic reaction and increases the stability of photochromic products.
Cohen and others once thought that photochromic and thermochromism are two mutually exclusive properties of Schiff base crystals. but
Hadjouds found that 4- methoxyhydroxaldehyde benzylamine (106) is thermochromic and photochromic. From this point of view, the planarity of molecules in the crystal is not the only factor that determines the thermochromism and photochromic of Schiff base. In solid solution and solvent, the crystallization factor is eliminated and the freedom of molecules is increased. Therefore, all Schiff bases can be photochromic.
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