Photochromic molecules and materials: presentation, characteristics and applications

When a material has the ability to change color following the absorption of light, it is called photochromic. Once irradiation has stopped, the material may return to its original color, either spontaneously, or through new light stimulation of a different nature. Light-induced color changes can also be accompanied by variations in other physico-chemical properties of the material (refractive index, fluorescence, mechanical response....), enabling these properties to be switched remotely and in a controlled manner. Thanks to this versatility, photochromism is constantly expanding the range of possibilities in terms of industrial applications.

First, the concept of photochromism and the different families of organic photochromes are introduced. Then, experimental determinations of photochromic parameters are detailed: quantum yield, thermal return rate constant, spectral signature of the photostationary state, fatigue resistance.

Then, through a few selected examples, this article discusses how to modulate the kinetic and photochemical characteristics of photochromes through a better understanding of reaction mechanisms. This understanding is based on the use of ultrafast transient spectroscopies in combination with modern computational chemistry focused on excited states. Once this understanding has been achieved, a constructive dialogue with organic synthesis enables the molecule to be judiciously modified to match its properties with the targeted applications. Since many preliminary studies are carried out in solution, the implementation of the material (in the form of a single crystal, a polymer film or a thin layer) that retains the target properties is a delicate stage. A few examples are given to illustrate the problem.

Finally, various applications are reviewed. In addition to the well-known example of variable-tint sunglasses, various examples of applications for photochromic molecules, which exploit much more than color change, are mentioned: the creation of molecular machines, intelligent textiles or new drug delivery methods.