Photochromism

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 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

:[[File:PentammineCoNO2.svg|thumb|220px|Photoisomerization of .Red-colored isomer (left) converts to the yellow isomer (right) upon UV irradiation.]] 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.

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

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.

References

References

1. (2016). "Ullmann's Encyclopedia of Industrial Chemistry".
2. Irie, M.. (2000). "Photochromism: Memories and Switches-Introduction". Chemical Reviews.
3. (2024). "Photochromic molecules and materials: design and development". Elsevier.
4. Ter Meer, Edm.. (1876). "Ueber Dinitroverbindungen der Fettreihe". Justus Liebigs Annalen der Chemie.
5. Phipson, T. L.. (1881). "Dangers of Pyrogallic Acid". Scientific American.
6. (2016). "Books on Photochromism".
7. (2024). "Inorganic photochromic materials: Recent advances, mechanism, and emerging applications". Responsive Materials.
8. Marckwald, W.. (1899). "Ueber Phototropie". Zeitschrift für Physikalische Chemie.
9. (2002). "Photoinduced Linkage Isomers of Transition-Metal Nitrosyl Compounds and Related Complexes". Chemical Reviews.
10. Bitterwolf, Thomas E.. (2006). "Photochemical nitrosyl linkage isomerism/metastable states". Coordination Chemistry Reviews.
11. Rack, J. (2009). "Electron transfer triggered sulfoxide isomerization in ruthenium and osmium complexes". Coordination Chemistry Reviews.
12. (2010). "Isomerization in Photochromic Ruthenium Sulfoxide Complexes". European Journal of Inorganic Chemistry.
13. (2009). "Photochemistry of Organic Compounds". Wiley.
14. (2014). "Photochromism".
15. (2011). "Azobenzene photoswitches for biomolecules". Chemical Society Reviews.
16. (2012). "Control over molecular motion using the cis – trans photoisomerization of the azo group". Beilstein Journal of Organic Chemistry.
17. (1980). "Azo-Crown Ethers. The Dyes with Azo Group Directly Involved in the Crown Ether Skeleton". Chemistry Letters.
18. (1988). "Thermally irreversible photochromic systems. Reversible photocyclization of diarylethene derivatives". The Journal of Organic Chemistry.
19. (2014). "Photochromism of Diarylethene Molecules and Crystals: Memories, Switches, and Actuators". Chemical Reviews.
20. (1993). "Comparative photodegradation study between spiro[indoline—oxazine] and spiro[indoline—pyran] derivatives in solution". Journal of Photochemistry and Photobiology A: Chemistry.
21. Ballet, Gilles. (1997). "Photodegradation of Organic Photochromes in Polymers - Naphthopyrans and Naphthoxazines Series -". Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals.
22. (1995). "Study of the Fatigue Process and the Yellowing of Polymeric Films Containing Spirooxazine Photochromic Compounds". Bulletin of the Chemical Society of Japan.
23. (2013). "Enhanced photoswitching and ultrafast dynamics in structurally modified photochromic fulgides". International Reviews in Physical Chemistry.
24. (2011). Molecular Switches
25. (2012). "Photomechanical Motion of Furylfulgide Crystals". Chemistry Letters.
26. (2017). "Photochromism of Fulgide Crystals: From Lattice-Controlled Product Accumulation to Phase Separation". Crystal Growth & Design.
27. (2014). "Hydrazone-based switches, metallo-assemblies and sensors". Chemical Society Reviews.
28. (2024). "1,2-BF 2 Shift and Photoisomerization Induced Multichromatic Response". Journal of the American Chemical Society.
29. Hartley, G. Spencer. (1938). "113. The cis-form of azobenzene and the velocity of the thermal cis→trans-conversion of azobenzene and some derivatives". J. Chem. Soc..
30. (2019). "Heteroaryl azo dyes as molecular photoswitches". Nature Reviews Chemistry.
31. (2018). "Twist and Return−Induced Ring Strain Triggers Quick Relaxation of a ( Z )-Stabilized Cyclobisazobenzene". The Journal of Physical Chemistry Letters.
32. (2022). "Azoheteroarenes". Wiley.
33. (2023). "Revisiting peri-aryloxyquinones: From a forgotten photochromic system to a promising tool for emerging applications".
34. (2011). "A new thin film photochromic material: Oxygen-containing yttrium hydride". Solar Energy Materials and Solar Cells.
35. (2018). "Advances on tungsten oxide based photochromic materials: strategies to improve their photochromic properties". Journal of Materials Chemistry C.
36. (2023). "Inserting Bi atoms on the [WO6] framework: Photochromic WO3/Bi2WO6 nanoparticles enable visual sunlight UV sensing". Applied Surface Science.
37. (1993). "On the fundamental role of oxygen for the photochromic effect of WO3". Journal of Applied Physics.
38. (1992). "Photochromism induced in an electrolytically pretreated Mo03 thin film by visible light". Nature.
39. (2005). "The generic enhancement of photochromic dye switching speeds in a rigid polymer matrix". Nature Materials.
40. (2006). "Rapid Photochromic Switching in a Rigid Polymer Matrix Using Living Radical Polymerization". Macromolecules.
41. Hirshberg, Yehuda. (1956). "Reversible Formation and Eradication of Colors by Irradiation at Low Temperatures. A Photochemical Memory Model". Journal of the American Chemical Society.
42. (2000). "Three-Dimensional Optical Data Storage Using Photochromic Materials". Chemical Reviews.
43. (2015). "Towards Solar Energy Storage in the Photochromic Dihydroazulene–Vinylheptafulvene System". Chemistry – A European Journal.