Photochromic materials refer to a class of materials that can change color after being excited by a light source. The first successful commercial application of photochromic materials began in the 1960s. Subsequently, a lot of research was done on its mechanism and application. Later on, color-changing glasses were developed.

Photochromic compounds can generally be divided into two types: organic and inorganic. The first developed photochromic compounds are mainly organic compounds, and their photochromic activity mainly comes from the color change caused by bond cleavage, reorganization or conformational change. Among them, the basis of molecular motors, photo-induced cis-trans isomerization, is a typical system in the field of photochromism. In addition, spiropyran can hetero-cleavage the carbon-oxygen bond under UV irradiation to produce cyanine compounds that can be photoexcited to open the ring, which is a typical bond-cleavage photochromic system. In addition, salicylaldehyde aniline Schiff bases can transfer protons from oxygen to nitrogen to change color under illumination. Inorganic materials rely on metal elements as coordination centers and photochromic organic molecules as ligands to generate photochromic functions by providing triplet excited states and electron/energy transfer. For example, by absorbing stilbene, azo groups, or alike as ligands into transition metals to form a composite system, triplet photosensitive photochromic compounds can be produced. Due to the existence of triplet excited states, the photochromic properties of these compounds can be shifted from the high-energy ultraviolet region to the visible light and even to the near-infrared region, which has greatly opened up the potential application scenarios of photofunctional molecular materials.

Classification of photochromic materials

Organic photochromic materials

Spiropyrans: Spiropyran is one of the earliest and most widely used systems in organic photochromic materials. Under ultraviolet light irradiation, the C-O bond in the colorless spiropyran structure is broken and the ring is opened, and the molecule rotates locally and forms a coplanar merocyanine structure with indole to develop color, and the absorption spectrum is red-shifted accordingly. Under the action of visible light or heat, the ring-opening body can return to the spiro-ring structure. The breaking time of the C-O bond is at the picosecond level, and the discoloration speed is extremely fast. However, merocyanine will automatically transform into a colorless spiro structure when stored at room temperature for a few minutes to a few hours. In addition, photochemical side reactions will occur during the reversible process, thereby affecting the number of cycles of reversible transformation. These shortcomings limit the application of spiropyrans in photomolecular switches.

Pyrrole substituting fulgide photochromic material: It is one of the earliest organic photochromic compounds to be synthesized. In 1999, Kiji et al. reported the synthesis of bis-heterocyclic fulgine compounds by carbonylation of 1,4-bis-heterocyclic substituted butyne-1,4-diol. The reaction is carried out under high temperature and high pressure using Pd as a catalyst. This method opens up a new route for the synthesis of diheterocyclic fulgides, but the synthesis conditions are harsh and difficult to scale up.

Diarylethenes: Diarylethenes have very good thermal stability, chemical stability, excellent sensitivity and fatigue resistance, and they are receiving more and more attention from material researchers at home and abroad.

Azobenzenes: Azobenzene compounds have good photochromic properties, and have the characteristics of ultra-high storage density and non-destructive information readout. The action of light or heat can cause the conversion between cis and trans azobenzene, and the trans structure is generally more stable than the cis structure. The cis-trans isomerization reaction under the action of heat is usually from cis to trans, but both isomerization directions can be carried out under the action of light.

Inorganic photochromic materials

Transition metal oxides: These substances mainly include WO3, MoO3, TiO2, etc. As an important inorganic photochromic material, WO3 tungsten oxide has the advantages of good stability and low cost, but its photochromic efficiency is low.

Metal halides: Metal halides have certain photochromic properties, such as calcium iodide and mercury iodide mixed crystals, copper chloride, chlorination pot, silver chloride, etc. When the calcium fluoride doped with La, Ce, Gd or Tb is irradiated, the spectral characteristic absorption of rare earth impurities will occur, and the discoloration mechanism is the valence of metal ions. For example, Ce-doped calcium fluoride crystals will produce lattice defects, which turn colorless Ce3+ into pink defects.

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