photochromism

Photochromism is the reversible change of color upon exposure to light. It is a transformation of a chemical species (photoswitch) between two forms through the absorption of electromagnetic radiation (photoisomerization), where each form has a different absorption spectrum. This reversible structural or geometric change in photochromic molecules affects their electronic configuration, molecular strain energy, and other properties.

==History== In 1867, Carl Julius Fritzsche reported the concept of photochromism, indicating that orange tetracene solution lost its color in daylight but regained it in darkness. Later, similar behavior was observed by both Edmund ter Meer and Phipson. Ter Meer documented the color change of the potassium salt of dinitroethane, which appeared red in daylight and yellow in the dark. Phipson also recorded that a painted gatepost appeared black during the day and white at night due to a zinc pigment, likely lithopone. In 1899, Willy Markwald, who studied the reversible color change of 2,3,4,4-tetrachloronaphthalen-1(4H)-one in the solid state, named this phenomenon "phototropy". However, this term was later considered misleading due to its association with the biological process "phototropism". In 1950, Yehuda Hirshberg (from the Weizmann Institute of Science in Israel) proposed the term "photochromism", derived from the Greek words phos (light) and chroma (color), which remains widely used today. The phenomenon extends beyond colored compounds, encompassing systems that absorb light across a broad spectrum, from ultraviolet to infrared, and includes both rapid and slow reactions. Photochromism can take place in both organic and inorganic compounds, and also has its place in biological systems (for example retinal in the vision process). The use of photochromic materials has evolved beyond protective eyewear to applications including 3D optical data storage, photocatalysis, and radiation dosimetry.