Photochromic Materials

Photochromism refers to a chemical and physical phenomenon in which a substance undergoes a specific photochemical reaction to generate another substance after being excited by light of a certain wavelength, and a photochemical reaction occurs to restore the original structure under the irradiation of light of another wavelength. Photochromic materials refer to a class of materials that can change color after being excited by a light source.

Applications:

Due to the advantages of special physical and chemical properties and abundant types, photochromic materials have a wide range of applications in many fields such as construction, optical information storage, anti-counterfeiting and biomedicine.

• Architecture: Glass has long been widely used in doors, windows, skylights, curtain walls and other parts of the building due to its high visible light transmittance, high hardness, and wear resistance. It has become an indispensable main material for buildings, and it is also an important raw material for transportation industries such as automobiles, airplanes, ships, and many other economic sectors. Compared with ordinary glass, photochromic material coated glass can not only prevent ultraviolet rays and glare, but also protect eyesight for car window glass or building curtain wall, and has functions such as intelligent adjustment of indoor temperature, energy saving and environmental protection, and beautification. Therefore, the photochromic material coated glass has a very wide range of applications in the construction field. In addition, the application of photochromic materials in construction also includes wall coatings and building signs, etc.

Figure 1. Photochromic glass 

• Optical information storage: Under the effect of light of different intensities and certain wavelengths, the color changing process of photochromic materials is reversible and can be repeated cyclically. Owing to their reversible color change properties and good fatigue resistance properties, the use of photochromic materials in computer storage devices can achieve the reading, writing and erasing of information. Therefore, photochromic materials have very good application prospects in the field of information storage, and are often used to make storage elements in computers.

Figure 2. Optical information storage element made of photochromic material

• Anti-counterfeiting technology: As the technology becomes more and more developed, the patterns of counterfeit and shoddy products are also changing, and the popularization of anti-counterfeiting trademarks and the application of anti-counterfeiting technology are increasingly necessary. In the process of trademark printing, proper addition of photochromic materials with excellent performance can produce special anti-counterfeiting trademarks. Such trademarks are usually not easily noticeable. In the detection, only by irradiating the trademark with light of a specific wavelength and observing whether there is discoloration, the authenticity of the product can be identified.
• Biology: The spatial structure of biological macromolecules is specific and closely related to the biological activity of biological macromolecules. Therefore, the biological activity of biological macromolecules changes with the structure. By grafting the reactive group of the photochromic compound onto the polypeptide, a biomacromolecule with photochromic properties can be formed. Using the photochromic properties of this biomacromolecule to regulate the structure of biomolecules can be used to detect biological disease or treatment. In addition, due to its unique molecular structure, photochromic materials can coordinate and complex with metal ions or anions, so it also has great application prospects in biological probes.

Figure 3. Polymer photochromic materials for biology

Classification:

There are mainly two different classification methods for photochromic materials. One is that it can be divided into inorganic photochromic materials and organic photochromic materials according to the different basic processes of photochromism. The other is that it can be divided into positive photochromic materials and reverse photochromic materials according to different photochromic properties.

• Positive photochromic material: The material that can be changed from colorless to colored under the irradiation of ultraviolet light with higher energy, and then restored from colored to original colorless under the action of visible light with lower energy is called positive light color-changing material.
• Backlight photochromic material: The material that can be changed from colored to colorless under the irradiation of ultraviolet light with higher energy, and then restored from colored to the original colored material under the action of visible light with lower energy is called backlight color-changing material.

References:

1. Raphael C. L. (2019). "Nanostructured and Photochromic Material for Environmental Detection of Metal Ions." Molecules 24, 4243-3254.
2. Ziyong Li. (2019), "Aldehyde-functionalized dithienylethenes with extended p-systems as versatile building blocks for NIR photochromic materials." Tetrahedron Letters 60 151166-151171.
3. Céu M. (2018), "Photochromic hybrid materials doped with vinylidene-naphthofurans." Progress in Organic Coatings 125, 146-152.