Photochromic Synthesis

The term photochromic originates from the Greek words “phos” (light) and “chroma” (color) and is used to describe a substance that changes color on exposure to light. More formally, photochromism is a reversible transformation of a chemical species, induced by the absorption of electromagnetic radiation, involving two forms, A (inactive) and B (active), each having different absorption spectra. Generally, form A is colorless while form B is a colored solution. Most dyes, inevitably return to the more stable form, the colorless state. Each dye is unique and varies greatly depending on the matrix in which it is dissolved in.

Figure 1 - Visual inspection of the two forms of a photochromic dye solution (on the left: form A, inactive

From something as familiar and mundane as color changing prescription glasses, the deactivated photochromic dye in the glasses absorbs UV wavelengths from the sun, transforming to its activated state and consequently turning regular glasses into sunglasses. To the innovative field of supramolecular chemistry, where photochromic units could be attached to enzymes, thereby effectively creating an on and off switch. Further applications involve novel 3D optical data storage, where photochromic compounds could be used to potentially store petabytes of information on a CD allowing us to keep up with the ever-increasing amount of data produced in the

There is a diverse range of chemical groups and reactions that give rise to the exquisite photochromic effect. For instance, cis-trans isomerizations or geometric isomerism, where the functional group of a molecule is rotated into a different position or orientation, retaining the same molecular formula. Then there are pericyclic reactions, following UV exposure, the molecule switches to form B by the rearrangement of bonds. The Spiropyrans, the most prevalent class of photochromes, are a good example. When exposed to high energy UV, the sp³-hybridized carbon-oxygen bond in a Spiropyran breaks and opens the ring such that the carbon becomes sp² hybridized. This process of bond rearrangement forms a colored compound called merocyanine (Figure