Many studies have proved that the photochromic mechanism of Schiff base proposed by Cohen, namely the excited state proton transfer reaction mechanism, is reasonable. However, the structure of photochromic and thermochromic products has been a topic of debate. Cohen et al. believed that thermochromic products have a (cis-Keto) structure, while photochromic products are (trans-Keto). However, later studies have suggested that photochromic products may have different structures in different media.
Russian researchers were the first to point out that the assumption of photochromic and thermochromic products as Keto structures has certain limitations. They propose that the photochromic and thermochromic products of Schiff bases can also be Zwitterion structures.
Nakagaki et al. ‘s study of salicylaldehyde aniline by time-resolved Fourier transform infrared spectroscopy showed that the photochromic reaction product was a (Keto-amine) structure, that is, trans-Keto pointed out by Cohen. Later, Lewis et al. proposed a different interpretation of the infrared spectral structure of Nakagaki, reassigned the infrared vibration frequency of each covalent bond, and believed that the photochromic product was a zwitterion. The properties of the medium for the thermal history of Schiff base products tend to be ketoid structure; In polar solvents, especially in mass polar solvents, the ground state of zwitterions is unstable, and the products tend to be ketone structure. In polar solvents, the products tend to be zwitterionic.
In addition, from the point of view of information storage application,Andes et al. studied the photochromic properties of crystal salicylaldehyde aniline. They believe that in general, the photochromic process of Schiff bases rarely causes side reactions unless impure substances are present. The crystalline derivatives of salicylaldehyde show very good fatigue resistance and can be cycled between yellow and red more than 50,000 times with ultraviolet and visible light.
Recently, Ming Yangfu et al. studied the transient absorption spectra and fluorescence spectra of double Schiff bases. The results show that only one Schiff base is involved in the photochromic process. The photoproducts include anti-cis-dimorphic ion Ⅱ, anti-trans-dimorphic ion Ⅲ and anti-waking configuration Ⅳ. The decay process accords with the first-order reaction kinetics formula respectively, and its half-life in cyclohexane
3.00,6.45 and 33.2us, respectively. The effect of solvent polarity on fluorescence spectra was studied. In both polar and non-polar solvents, the fluorescence is derived from the excited state of the dimorphic ion of the proton transfer product. Therefore, the photophysical and photochemical behavior of a double Schiff base induced by a general light source is similar to that of a single Schiff base; one Schiff base of a double Schiff base can only be regarded as a substitute for another Schiff base, and there is no integrated common system in the double Schiff base molecule.
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