Photochromic materials

Photochromic materials


Photochromic molecules can belong to various classes: triarylmethanes, stilbenes, azastilbenes, nitrones, fulgides, spiropyrans, naphthopyrans, spiro-oxazines, quinones.




Spiropyrans and Spriooxazines

Spiro-mero photochromism.
One of the oldest, and perhaps the most studied, families of photochromes are the spiropyrans.

Very closely related to these are the spirooxazines.

For example, the spiro form of an oxazine is a colorless leuco dye; the conjugated system of the oxazine and another aromatic part of the molecule is separated by a sp³-hybridized "spiro" carbon. After irradiation with UV light, the bond between the spiro-carbon and the oxazine breaks, the ring opens, the spiro carbon achieves sp² hybridization and becomes planar, the aromatic group rotates, aligns its π-orbitals with the rest of the molecule, and a conjugated system forms with ability to absorb photons of visible light, and therefore appear colorful. When the UV source is removed, the molecules gradually relax to their ground state, the carbon-oxygen bond reforms, the spiro-carbon becomes sp³ hybridized again, and the molecule returns to its colorless state.

This class of photochromes in particular are thermodynamically unstable in one form and revert to the stable form in the dark unless cooled to low temperatures. Their lifetime can also be affected by exposure to UV light. Like most organic dyes they are susceptible to degradation by oxygen and free radicals. Incorporation of the dyes into a polymer matrix, adding a stabilizer, or providing a barrier to oxygen and chemicals by other means prolongs their lifetime.

Diarylethenes

Dithienylethene photochemistry

The "diarylethenes" were first introduced by Irie and have since gained widespread interest, largely on account of their high thermodynamic stability. They operate by means of a 6-pi electrocyclic reaction, the thermal analog of which is impossible due to steric hindrance. Pure photochromic dyes usually have the appearance of a crystalline powder, and in order to achieve the color change, they usually have to be dissolved in a solvent or dispersed in a suitable matrix. However, some diarylethenes have so little shape change upon isomerization that they can be converted while remaining in crystalline form.

Photochromic quinones

Some quinones, and phenoxynaphthacene quinone in particular, have photochromicity resulting from the ability of the phenyl group to migrate from one oxygen atome to another. Quinones with good thermal stability have been prepared, and they also have the additional feature of redox activity, leading to the construction of many-state molecular switches that operate by a mixture of photonic and electronic stimuli.

Inorganic photochromics

Many inorganic substances also exhibit photochromic properties, often with much better resistance to fatigue than organic photochromics. In particular, silver chloride is extensively used in the manufacture of photochromic lenses. Other silver and zinc halides are also photochromic.

Fulgides

Triarylmethanes

Triphenylmethane, or triphenyl methane, is the hydrocarbon with the formula (C6H5)3CH. This colorless solid is soluble in nonpolar organic solvents, but not water. Triphenylmethane has the basic skeleton of many synthetic dyes called triarylmethane dyes, many of them are pH indicators, and some display fluorescence. A trityl group in organic chemistry is a triphenylmethyl group Ph3C, e.g. triphenylmethyl chloride — trityl chloride

Examples of triarylmethane dyes are bromocresol green or malachite green