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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. (2025). 9783527306732 ISBN 9783527306732 This reversible structural or geometric change in photochromic molecules affects their electronic configuration, molecular strain energy, and other properties.
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.
| Azobenzene | The photochromic trans-cis ( E/Z) isomerization of azobenzenes has been used extensively in molecular switches. Upon isomerization, azobenzenes experience changes in physical properties, such as molecular geometry, absorption spectra, or dipole moment. Azobenzene groups incorporated into crown ethers give switchable receptors and azobenzenes. | |
| Diarylethene | Diarylethenes undergo a fully reversible transformation between "ring-open" and "ring-closed" isomeric forms when exposed to light of suitable wavelength. Diarylethene-based photoswitches exhibit high photofatigue resistance, enabling them to undergo many photoswitching cycles with minimal degradation. These compounds have been evaluate for long-lasting photochemical memory devices due to the thermal stability of both photoforms of diarylethenes. | |
| Spiropyran and spirooxazines | Spiropyrans, among the oldest photochromic compounds, are closely related to spirooxazines. Irradiation with UV light induce ring-opening, forming a colorful isomer. When the UV source is removed, the chromophore gradually relax to their colorless ground state, the carbon-oxygen bond reforms, and the molecule. This class of photochromes, in particular, is thermodynamically unstable in one form and revert to the stable form in the dark unless cooled to low temperatures. | |
| Fulgide and Fulgimides | Similar to diarylethenes, the photochromic behavior of fulgides and fulgimides is based on 6π-electrocyclic ring-opening and ring-closing reactions. They are highly photochromic photoswitches and reversibly interconvert between two isomeric forms when exposed to light of different wavelengths. (2025). 9783527313655 ISBN 9783527313655 These compounds exhibit low photochemical fatigue, high thermal stability, as well as high conversion yields. | |
| Hydrazone | Hydrazone photoswitches can be activated by light and undergo efficient and reversible E/Z isomerization around the C=N double bond. | |
| Certain naphthopyrans, such as 3,3-diphenyl-3H-naphthopyran, convert from their colorless form to a colored isomer via a ring-opening process. Such materials are used in self-darkening glasses. | ||
| Azoheteroarenes | Azoheteroarenes, structural analogues of azobenzene, are photoswitches capable of reversible E–Z photoisomerization. In these compounds, one or both Phenyl group of azobenzene are replaced by a heterocycle, while maintaining similar structural and mechanistic properties. Like azobenzenes, their thermal isomerization follows three main pathways: inversion, rotation, or Tautomer. Typically, the Z-isomer of azoheteroarenes exists as the Metastability state. The incorporation of heteroatoms into the ring system enhances functionality as well as improves Bioisostere, polarity, lipophilicity, and solubility, making azoheteroarenes promising alternatives to azobenzenes. |
Some inorganic photochromic materials include Oxide such as BaMgSiO4, Na8AlSiO46Cl2, 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.
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 Siloxane 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.
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