Photochromism is the reversible change of color upon exposure to light. It is a transformation of a chemical species (photoswitch) between two forms through the absorption of electromagnetic radiation (photoisomerization), where each form has a different absorption spectrum. This reversible structural or geometric change in photochromic molecules affects their electronic configuration, molecular strain energy, and other properties.
History 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 synonym "tenebrescence" was coined by the RCA engineer Humboldt Leverenz in 1946, and continues to see occasional use in the field of minerology, where it was introduced by Philco scientist David Medved in 1954. 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). Transition metal complexes can also display photochromic properties due to linkage isomerizations. 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. 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. Photochromic systems rely on irradiation to induce the isomerization. Some rely on irradiation for the reverse reaction, others use thermal activation for the reverse reaction.
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. Photochromism in transition metal oxides is generally attributed to the redox reactions of the transition metal ion and the resulting electron transfer between its different valence states. When electrons are excited from the valence band to the conduction band, a hole is generated in the valence band. This photo-induced hole can decompose adsorbed water on the material's surface, producing protons. These protons can react with transition metal ions in different valence states, forming hydrogen-based compounds that exhibit color changes. Upon exposure to light of a different wavelength or an oxidizing atmosphere, the reduced transition metal ion can undergo re-oxidation. Various forms of tungsten trioxide (WO3), including bulk crystals, thin films, and quantum dots, have been studied for their photochromic properties. WO3 transitions between two optical states, shifting from transparent to blue when exposed to light, heat, or electricity. The reversible color change is associated with the tungsten center's ability to undergo oxidation-reduction reactions, alternating between different oxidation states (W6+ to W5+ or W5+ to W4+). 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.
… excerpt ends here. Continue reading the full article.
