Photochromism is a phenomenon in which a single chemical species changes its molecular structure in the presence of light without changing its molecular weight, reversibly producing two isomers with different face absorption spectra. In many instances, changes in refractive index, dielectric characteristics, redox potential, and melting point follow changes in color and molecular structure.
Alfa Chemistry provides a variety of photochromic compounds for its customers, some of which are developed into functional inks, optical switches, optical recording media, and light modulating materials.
Classification of Photochromic Materials
From the perspective of thermal stability, photochromic compounds can be broadly classified into two types.
- Type T: This refers to processes in which the conversion process is caused by heat other than light. Typical examples are azo-benzene and propellant.
- Type P: The p-type photochromic compounds exist in two reversible forms under irradiation and have good thermal stability and better fatigue resistance. Typical examples are stilbene and fullerig
Fig 1. Photochromic reaction of spiropyrans (from closed ring structure to open ring structure).
Photochromic dyes are divided into inorganic and organic molecules. Metal oxides, alkaline earth metals, sulfides, copper compounds, and mercury compounds are among the crucial inorganic kinds.
The organic types, which include spiro-pyrans, spirooxazines, chromogenic alkenes, fulvic acids, fulvic gold, and diaryl ethylenes, are efficient and environmentally beneficial. Fulgides, fulgimides, and diarylethenes are thermally stable while spiro-pyrans, spirooxazines, and chromogenic alkenes are sensitive to thermal influences and transition to a colorless state when heated or exposed to light. Among these spiro-pyrans are of more scientific interest than any other class.
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Reference
- Gupta V. K, et al. (2020). "Photochromic Dyes for Smart Textiles." Dyes and Pigments.