What is a photochromic material?
A photochromic coating (from the Greek photôs light and chromos color) is, by definition, made of a pigment/colorant whose optical properties change according to the light.
These photochromic materials belong to the vast family of X-chromic materials, i.e. materials that change color under the effect of an external excitation (temperature, pressure, humidity level, etc.).
More specifically, a photochromic material is a chemical substance, organic or inorganic, whose coloration is modified under the effect of a light excitation following the appearance of a metastable state. This phenomenon can be reversible or not, and the kinetics is strongly dependent on the material and the nature of the excitation.
Photochromism is a reversible transformation, photo-induced in at least one direction, between two states A and B of a chemical species with different absorption spectra. The electromagnetic radiation inducing the transformation must belong to the UV, visible or near IR range.
The A → B transition is accomplished by irradiation at a wavelength λA corresponding to an absorption region of the photochromism in state A, and that of B → A by irradiation at a wavelength λB in the absorption region of the photochromism in state B, or by thermal reaction (Δ).
There are two types of photochromism if we consider the return reaction from B to A: – type P, if the return takes place only by photo-chemical way; – type T, if the return can also take place by thermal way.
What are the main families of photochromic materials ?
Historically, the first description of a photochromic effect dates back to 1867. Fritzsche (C. R. l’Academie Sci. 1867, 69, 1035) reported the loss of color of a solution of tetracene in sunlight, then its return to its initial color (orange) in the dark.
Today, photochromism is a property observed in purely organic molecules, in biological derivatives (notably associated with the mechanism of retinal vision) but also in inorganic compounds. Reactions that do not require large structural changes during the light-induced effect can be observed in solution as well as in crystalline phase.
Photochromic organic materials
These are certainly the most studied photochromic molecules. The chemical processes involved during irradiation, which lead to the change of color of the compound, are numerous. It may be a structural change: cis-trans isomerization around double bonds, proton transfer, opening and closing of rings, but also a redox process.
Photochromism by redox reaction was discovered in the family of quinones and naphthalenes (Z. F Phys. Chem., 1899, 30, 140). In these systems, the color change phenomenon is caused by the redox reaction which strongly changes the conjugation of the arenes.
Cis-trans isomerization is encountered in the derivatives of stilbenes, azobenzenes. They present an isomerization of the double bond C=C or N=N under UV irradiation. An example of cis-trans photoisomerization is the “Disperse Red one” (DR1).
Intramolecular proton transfer isomerization is described for salicylidene-aniline compounds. Intramolecular proton transfer allows the transition from the enol form (yellow in color) to the ketone form (red in color due to an n-p* transition of the oxygen free electron pair). Depending on the compound, the return to the enol form may take a few seconds or several months.
The isomerization by closing or opening of ring is a mechanism met in two forms: – Molecules of type T (reversibility by thermal way) like spiropyranes and spirooxazines and – Molecules of type P (reversibility by photochemical way) like fulgides and diarylethenes.
Spiropyrans and spirooxazines have been widely studied. Under ultraviolet irradiation, the carbon-oxygen bond breaks, followed by a cis-trans isomerization leading to the colored form called merocyanine.
The first examples of reversible diarylethenes were described by Irie (J. Org. Chem. 1988, 53, 803-808). This family is now widely studied. The diarylethene derivatives combine both thermal irreversibility and high strength. Light-induced color cycles can be performed on some diarylethenes more than 104 times without observing photo-degradation (J. Org. Chem. 1988, 53, 6136-6138).
Hybrid photochromic materials
Organic ligands based on photochromic antenna. Many hybrid complexes are composed of one (or more) organic photochromic antenna(s) linked to a metal ion. The structural change of these antennas by isomerization and/or photo-induced cyclization can cause the color change of the complex.
Among the most described photochromic antennas are azobenzene, spiropyran, diarylethene or quinone derivatives. The ligands used to connect the metal complex are frequently bipyridine, terpyridine as well as cyclopentadienyl substituted by an azobenzene group.
If we consider as an illustration the diarylethenes associated with metal complexes, the obtained compounds present the photochromic property thanks to the mechanism of photo-cyclisation of the ligand. The described complexes involve rhenium, platinum and zinc.
The photochromic complexes with geometry change. The most striking example is the copper complex, [Cu(dieten)2](BF4)2 having two N,N-diethylethylenediamine (dieten) ligands. Under irradiation, the complex changes from a square planar conformation to a slightly distorted conformation that places the metal in a slightly tetrahedral conformation. The color change from red to violet is stable for several hours at a temperature of 35 K.
The photochromic complexes with spin transition. Some complexes have the particularity to change their spin state according to an irradiation. This change from high spin to low electron spin configuration can be accompanied by a color change. There are several types of transitions caused by irradiation. The LIESST (Light-Induced Excited Spin State Trapping) effect is the most studied photo-induced effect. This effect was first observed in 1984 (Inorg. Chem. 1985, 24, 2174) in the iron(II) complex with six propyltetrazole (ptz) ligands, [Fe(ptz)6](BF4)2.
Mixed-valence photochromic complexes. Some Prussian Blue analogues are known for their photochromic and magnetic properties. These materials, composed of two divalent and trivalent metal centers connected by cyano ligands, show a magnetization induced by irradiation or for temperature variations. This is notably the case for cobalt-iron analogues (Polyhedron 2001, 20, 1339) which change from the FeIIILS-CN-CoIIILS form to the FeIILS-CN-CoIIHS form by irradiation at low temperature. This electron transfer from one metal center to the other is accompanied by a structural change, a change in the absorption spectrum of the material (from green to red) and a modulation of the magnetic properties.
The photochromic complexes with intra-conversion of a ligand. The most studied compounds of this family are the complexes with a nitrosyl ligand, such as sodium nitroprusside: Na2[Fe(CN)5(NO)].
This photochromic complex presents two metastable states: – an MS1 state characterized by the inversion of the nitrosyl into an isonitrosyl ligand, where the metal is directly coordinated to the oxygen; and – an MS2 state, where the linear nitrosyl ligand in the ground state becomes “angled”. In this so-called bent conformation, the metal-ligand bond is considered a η2 -NO conformation.
Photochromic inorganic materials
The property of photochromism has also been described in inorganic materials, notably, based on silver halides, zinc and molybdenum oxides.
Under the effect of light radiation, the redox reaction, Ag+ + X–→ Ag0 + X0 (X = Cl, Br, I) leads to the formation of silver colloids. The initially light-colored material becomes dark gray. When the excitation ceases, the reverse reaction takes place. The material returns to its stable configuration and recovers its initial coloring.
Another class of inorganic materials with photochromic effect is the molybdenum-based oxides. Under UV excitation, MoO3 changes from pale yellow to an intense blue. The change in color is due to the photo-reduction of metal cations Mo6+ (4d0 configuration) to Mo5+ (4d1 configuration) under illumination.
Through this brief analysis of the existing, we have just reported the main materials and mechanisms responsible for the photochromic properties. Each of these families has advantages and disadvantages.
The property of a photochromic system is dependent on the chemical process involved, and four main factors must be taken into account: – the rate of conversion from the ground state to the photo-induced state, – the lifetime of the photo-induced states, – the temperature range, – the fatigability of the process.
These main factors vary according to the nature of the chemical process and the nature of the molecules used (organic, inorganic, biological). Our chemists and photochemists experts are at your disposal to select the best candidate according to the specificity of your specifications.
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