The research of organic photochromic chemistry and materials has been expanding and deepening in recent years. Organic photochromic substances can be roughly divided into the following categories according to their reaction types.
- Heterolysis of the key
Both spiropyran and spirooxazine photochromic are of this type. Spiropyrane is a widely studied compound. When the colorless spiropyrane is excited by ultraviolet light, it can lead to the heterofersion of the spiropyrane carbon-oxygen bond, resulting in the formation of an open-ring metacyanine compound absorbed in the long-wave region. The study on 46 series spiropyran and its derivatives of 800 kinds shows that spiropyran has poor fatigue resistance and is easily degraded by oxidation.
After the 1980s, most of the research focused on spirazine compounds, and experiments showed that the fatigue resistance of these compounds was greatly improved compared with spiropyran. The results also indicate that the direct photochromic process of these compounds is only carried out by excitation of the singlet state, and oxygen has no obvious effect on the reaction, which is obviously a major reason for the increase of fatigue resistance.
Another type of photochromic compounds are triarylmethane derivatives, whose solutions produce dark colored triarylmethane positive ions under light excitation, and the resulting chromophores (positive ions) can quickly recombine with negative ions in the dark. The fatigue resistance of these compounds is poor. - Bond splitting
The hexaphenyl bisimidazole is cleaved under light and a very active triphenyl imidazole radical is formed. This photochromic product is easily combined with oxygen, and in the presence of oxygen, its color and achromatic cycles can only be repeated a few times. - Proton transfer tautomerism
Salicylaldehyde aniline schiff base is a class of photochromic compounds that are easy to prepare. Under ultraviolet light, the transfer of protons from oxygen to nitrogen occurs, often showing a color change from yellow to orange-red. Although this kind of compound has good fatigue resistance, at room temperature, in solution, its photochromic product stability is very poor, and even can only be observed by flash photolysis technology, and it is a class of fast photoresponsive materials.
In recent years, the study of retinal Schiff bases has been favored by people, mainly because of its important application prospects in high technology fields such as optical computing. - Cis-trans isomerism
The cis-trans isomerization reactions of stilbenes, benzylanilines and azobenzenes in biological systems were studied in detail. The process by which the human eye produces vision is actually caused by photoisomerization. Rhodopsin is a protein containing 11-cis-retinoid. Under the action of external light, the isomerization reaction causes the change of peptide bonds in the rhodopsin, which produces a nerve impulse that is transmitted to the brain and causes vision.
The cis-trans isomerization of indigo dyes has the advantages of good reversibility and high photoisomerization efficiency.
The cis-trans isomerization of supramolecular compounds has also been studied. The cis-thioindigo derivative can be designed as an exoreceptor, and metal ions such as potassium ions can be used as the substrate. The two can form supramolecule through complexation, thus obtaining a stable cis-isomer complex. When it is irradiated by 450nm light, the cis-isomer complex will develop Cis-trans isomerization, the trans isomer loses the ability to complex with metal ions, and the trans isomer is thermodynamically stable, and when it is exposed to 550nm light, it forms a cis-isomer, and then supramolecular resurrection. This cycle can be performed multiple times with no by-product formation observed. - REDOX reaction
Thermally stable dense ring aromatic compounds can also undergo photochromic reactions under the action of light and oxygen.
The cis-trans isomerization of azobenzene has long been well known, but its cis-structure is often unstable and cannot be composed of bistable state. Therefore, the development of photochromic materials based on the cis-trans isomerization property of azobenzene is limited. Shimizu et al. combined cis-trans isomerization with REDOX to make it possible for practical application.
In 1990, Tenjima et al. used the same reaction principle and L-B membrane technology to prepare multilayer single-molecule azobenzene compound membrane materials, and applied STM technology to the field of optical information storage, creating the first nanobit information. - Circumferential reaction system
Anhydride is one of the representatives of this class of compounds, and its reaction mechanism is cyclic reaction. In general
Anhydride reaction process does not produce active free radicals, ions or dipole intermediates, so its thermal stability and fatigue resistance and snail
Pyranes are much improved. Heterocyclic diarylethylene photochromic materials also belong to this type. Has received much attention in recent years. Among them, Japan’s Irie has done deep and detailed work.
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