text: 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. In many cases, an absorbance band is present in only one form. The degree of change required for a photochemical reaction to be dubbed "photochromic" is that which appears dramatic by eye, but in essence there is no dividing line between photochromic reactions and other photochemistry. Therefore, while the trans-cis isomerization of azobenzene is considered a photochromic reaction, the analogous reaction of stilbene is not. 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).
  Another requirement of photochromism is two states of the molecule should be thermally stable under ambient conditions for a reasonable time. All the same, nitrospiropyran (which back-isomerizes in the dark over ~10 minutes at room temperature) is considered photochromic. All photochromic molecules back-isomerize to their more stable form at some rate, and this back-isomerization is accelerated by heating. There is therefore a close relationship between photochromic and thermochromic compounds. The timescale of thermal back-isomerization is important for applications, and may be molecularly engineered. Photochromic compounds considered to be "thermally stable" include some diarylethenes, which do not back isomerize even after heating at 80 C for 3 months.
  Since photochromic chromophores are dyes, and operate according to well-known reactions, their molecular engineering to fine-tune their properties can be achieved relatively easily using known design models, quantum mechanics calculations, and experimentation. In particular, the tuning of absorbance bands to particular parts of the spectrum and the engineering of thermal stability have received much attention.
  Sometimes, and particularly in the dye industry, the term irreversible photochromic is used to describe materials that undergo a permanent color change upon exposure to ultraviolet or visible light radiation. Because by definition photochromics are reversible, there is technically no such thing as an "irreversible photochromic"—this is loose usage, and these compounds are better referred to as "photochangable" or "photoreactive" dyes.
  Apart from the qualities already mentioned, several other properties of photochromics are important for their use. These include quantum yield, fatigue resistance, photostationary state, and polarity and solubility. The quantum yield of the photochemical reaction determines the efficiency of the photochromic change with respect to the amount of light absorbed. The quantum yield of isomerization can be strongly dependent on conditions. In photochromic materials, fatigue refers to the loss of reversibility by processes such as photodegradation, photobleaching, photooxidation, and other side reactions. All photochromics suffer fatigue to some extent, and its rate is strongly dependent on the activating light and the conditions of the sample. Photochromic materials have two states, and their interconversion can be controlled using different wavelengths of light. Excitation with any given wavelength of light will result in a mixture of the two states at a particular ratio, called the photostationary state. In a perfect system, there would exist wavelengths that can be used to provide 1:0 and 0:1 ratios of the isomers, but in real systems this is not possible, since the active absorbance bands always overlap to some extent. In order to incorporate photochromics in working systems, they suffer the same issues as other dyes. They are often charged in one or more state, leading to very high polarity and possible large changes in polarity. They also often contain large conjugated systems that limit their solubility.