Photochromism

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. == Principles ==

: Photochromism often is associated with pericyclic reactions, cis-trans isomerizations, intramolecular hydrogen transfer, intramolecular group transfers, dissociation processes and electron transfers (oxidation-reduction). Important properties of photochromic compounds include quantum yield, fatigue resistance, and the lifetime of the photostationary state (PSS). The quantum yield of the photochemical reaction determines the efficiency of the photochromic change relative to the amount of light absorbed. In photochromic materials, the loss of photochromic component is referred to as fatigue, and it is observed by processes such as photodegradation, photobleaching, photooxidation, and other side reactions. All photochromic compounds suffer from fatigue to some extent, and its rate is strongly dependent on the activating light and the sample conditions. ==Classes of photochromic materials==

Molecular photoswitches Photochromic quinones Some quinones, and phenoxynaphthacene quinone in particular, have photochromicity resulting from the ability of the phenyl group to migrate from one oxygen atom 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 photochromic materials 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. Yttrium oxyhydride is another inorganic material with photochromic properties. Some inorganic photochromic materials include oxides such as BaMgSiO4, Na8[AlSiO4]6Cl2, and KSr2Nb5O15. Additionally, rare-earth (RE)-doped compounds like CaF2:Ce, CaF2:Gd, as well as transition metal oxides such as WO3, TiO2, V2O5, and Nb2O5 have been explored. Molybdenum trioxide (MoO3) is widely used in UV sensing applications due to its selective absorption of UV light. Upon UV exposure, MoO3 undergoes a photochromic transformation, which can be reversed in the presence of an oxidizing agent. MoO3 nanosheets exhibit a stronger photochromic effect than the bulk materials due to enhanced carrier mobility and structural flexibility. Photochromic coordination compounds Photochromic coordination complexes are relatively rare compared to the organic compounds listed above. There are two major classes of photochromic coordination compounds: those based on sodium nitroprusside and the ruthenium sulfoxide compounds. The ruthenium sulfoxide complexes were created and developed by Rack and coworkers. The mode of action is an excited-state isomerization of a sulfoxide ligand on a ruthenium polypyridine fragment from S to O or O to S. The difference in bonding between Ru and S or O leads to the dramatic color change and change in Ru(III/II) reduction potential. The ground state is always S-bonded, and the metastable state is always O-bonded. Typically, absorption maxima changes of nearly 100 nm are observed. The metastable states (O-bonded isomers) of this class often revert thermally to their respective ground states (S-bonded isomers), although a number of examples exhibit two-color reversible photochromism. Ultrafast spectroscopy of these compounds has revealed exceptionally fast isomerization lifetimes ranging from 1.5 nanoseconds to 48 picoseconds. ==Applications: sunglasses and related materials ==

, after exposure to sunlight while part of the lens remained covered by paper.''' Reversible photochromism is the basis of color changing lenses for sunglasses. The largest limitation in using photochromic technology is that the materials cannot be made stable enough to withstand thousands of hours of outdoor exposure so long-term outdoor applications are not appropriate at this time. The switching speed of photochromic dyes is highly sensitive to the rigidity of the environment around the dye. As a result, they switch most rapidly in solution and slowest in the rigid environment like a polymer lens. In 2005 it was reported that attaching flexible polymers with low glass transition temperature (for example siloxanes or polybutyl acrylate) to the dyes allows them to switch much more rapidly in a rigid lens. Some spirooxazines with siloxane polymers attached switch at near solution-like speeds even though they are in a rigid lens matrix. ==Aspirational applications==

Data storage Photochromic compounds for data storage has long been a topic of speculation. The area of 3D optical data storage promises discs that can hold a terabyte of data. Solar energy storage Photochromism is a potential mechanism to store solar energy. The photochromic dihydroazulene–vinylheptafulvene system is a proof-of-concept. ==See also==