photochromism

Not to be confused with Photochromism (a reversible color change induced by light energy).

Spiropyrans are one of the oldest families of photochromism.

Both processes are often applied in molecular switches and for photochromism (reversible state changes from exposure to light).

Tenebrescence, also known as reversible photochromism, is the ability of minerals to change colour when exposed to sunlight.

In 1952, Fisher and co-workers announced for the first time photochromism in spiropyrans.

As solids, the spiropyrans do not present photochromism.

Silver halides are also used to make corrective lenses darken when exposed to ultraviolet light (see photochromism).

Another requirement of photochromism is two states of the molecule should be thermally stable under ambient conditions for a reasonable time.

The photochromism is due to electrocyclic cleavage of the C-spiro-O bond with photoexcitation.

Scotophors show tenebrescence (reversible photochromism) and darken when subjected to an intense radiation such as sunlight.

The molecular component can be described as a molecular switch, and may perform this function by any of several mechanisms, including charge storage, photochromism, or changes in capacitance.

Dr. Staebler has performed research investigations on photochromism in materials, electrochromism, storage of holograms in electro-optic crystals, plus amorphous silicon solar cells produced by discharge.

Since photochromism is just a special case of a photochemical reaction, almost any photochemical reaction type may be used to produce photochromism with appropriate molecular design.

Some of the most common processes involved in photochromism are pericyclic reactions, cis-trans isomerizations, intramolecular hydrogen transfer, intramolecular group transfers, dissociation processes and electron transfers (oxidation-reduction).

He labeled this phenomenon "phototropy", and this name was used until the 1950s when Yehuda Hirshberg, of the Weizmann Institute of Science in Israel proposed the term "photochromism".

A number of sunglass manufacturers/retailers (Intercast, Oakley, Serengeti Eyewear, Persol to name a few) offer products that use photochromism to make lenses that go from a dark to a darker state.

Photochromism does not have a rigorous definition, but is usually used to describe compounds that undergo a reversible photochemical reaction where an absorption band in the visible part of the electromagnetic spectrum changes dramatically in strength or wavelength.

When the spiropyran is in a solution with polar solvents or when it receives heating (thermochromism) or radiation (photochromism) it becomes coloured because its structure has changed and it has been transformed into the merocyanine form.

In the 1970s, Valeri Barachevskii demonstrated that this photochromism could be produced by two-photon excitation, and finally at the end of the 1980s Peter M. Rentzepis showed that this could lead to three-dimensional data storage.