Photochromism

Photochromism is defined as the light-induced reversible transformation between two species in one another with a change of the absorption spectrum, and, consequently, their physical properties. The reverse reaction can take place thermally ( T-type photochromism ) or also photochemically ( P-type photochromism ). In the case the thermal system with the proviso that A is more stable than B return, after removal of the source of exciting radiation spontaneously at standard conditions to the state A. In P-type photochromism the reverse reaction is greatly accelerated under irradiation with a wavelength ν2.

This reversibility is different photochromism of the well-known light-induced reactions. Displays the system under the action of electromagnetic radiation to increase the color, it is called positive, in the opposite case of negative photochromism. In reality, however, occur often result and side reactions which limit the shelf-life of the photochromic materials and complicate the schema.

Gated photochromism

Under gated Photochromism is defined as the function of a photochromic reaction of an inhibitory factor. The photochromic reaction does not occur unless an external stimulus is performed (for example, electricity, photons of a different wavelength, heat, chemical activation). Has long been known photochromic Saccharidsensoren. The example in Figure 1 shows a novel dienophilabhängiges sensor molecule that could be found once applications in chemical dosimeters.

Dual -mode photochromism

Under dual-mode photochromism are systems that can be affected by more than one external stimulus. Huck et al. 1995 presented a chirooptischen molecular switch which allows the photochemically set state chemically to secure supply, makes by protons, since the protonated species are no longer photoactive.

History

The term photochromism coined Hirshberg 1950, the observation of photochromic phenomena but has been described by Fritzsche 1867, who observed the fading of a solution of orange colored tetracene during the day and the return of color in the night. Other early milestones photochromism set by ter Meer and Mark Forest. The former discovered the reversible staining of spread with a zinc pigment fence posts in the sunlight, the latter examined the color change from 2,3,4,4 - Tetrachlornaphtalin -1 ( 4H )-one in crystalline form. It was only after 1940 came the issue again in the focus of research, as Hirshberg and fishermen on the Daniel Sieff Research Institute in Rehovot undertook, Israel, systematic attempts to elucidate synthetic pathways and mechanisms of photochromic compounds. In the 1960s, followed by the development of the area, especially the physical methods on which it built. Unsolved problems of the known photochromic compound classes, especially the eminent photodecomposition could stagnate the research before the first anti-fatigue chromene and spirooxazine could be shown in the 1980s. The response to this field of research has since been large, the number of publications has increased steadily, reviews have been published, and even the Bundestag took the form of a technology assessment of nanotechnology note.

Excitation mechanisms

In general it can be assumed that the formation of species B, starting is based on a single-photon mechanism of the excitation of the molecule in a singlet or triplet state. However, are of particular interest for applications such as three-dimensional optical storage systems based on two-photon mechanisms. Here, as shown in Figure 3, carried out by two different excitation wavelengths, ν1 and ν2. The absorption occurs either simultaneously or sequentially via a virtual level on a real level, from the done of the excitation in the higher level Sn. Moreover, there is the possibility of implementing the mechanism of photochromic an unstable intermediate that after absorption of a second photon of the same wavelength is further responsive to the product B. Because the probability of excitation depends on the product of the photon densities EP ( 1), and EP (2 ), the use of ultrashort laser pulses in the femtosecond range is advantageous.

Classes of compounds and mechanisms

Today, a variety of classes of compounds are known, each received their own photochromic reactions. The most important are shown in Figure 4 based on their isomerization reactions. To view spiropyrans spirooxazines, chromenes, hexa -1 ,3,5 -trienes, Diheteroarylethene and cyclohex -1 ,3-dienes electrocyclizations with the participation of six π - electrons and six atoms or in the case of pyrazolines five atoms. The E / Z isomerization of stilbene light-induced, azo compounds, azines or Thioindigo -type compounds is well known and is used in model systems to bring molecules to their site of action and release directed. Other mechanisms are based on the intra-molecular transfer of protons ( anils, Benzylpyridine, Acinitroverbindungen, salicylates, triazoles, oxazoles, and Metalldithizonate Perimidinspirohexadienone ) or larger groups such as acetyl groups on the polycyclic quinones ( Fig. 4). In addition, heterolytic bond cleavages of triarylmethanes and homolytic known at Triarylimidazoldimeren, Tetrachlornaphthalinen, Perchlortoluol, Nitrosodimeren and hydrazines. In Viologens one finds electron transfer reactions. The Phytochromie and phototropism are also based on photochromic reactions.

Applications

The applications of photochromic compounds based largely on some indicators, of which the principal are to be explained here.

• Dyeability describes the ability of a system to develop color. The initial absorbance A0 at the irradiation with light of wavelength λ is proportional to the quantum efficiency of the photoreaction Φ, the molar absorption coefficient of the colored form of Q, the concentration of the colorless form and the photon flux (included in k). A0 ( λ ) = k · Φ · Q · c
• Under fatigue is defined as the result of side reactions (by a photochemically activated oxidation reactions) resulting loss of efficiency of the photochromic reaction.
• Among the number of cycles or the Z50 - value refers to the number of one - off operations (conversion A → B → A) within a system according to which only 50 % of the initial absorption are present. Particularly stable compounds, such as discussed in more detail later bacteriorhodopsin survive 100,000 cycles without degradation.
• The half-life features in the photochromism the time required by the system, so that the absorption value is again returned to one-half of the maximum value within one cycle by thermal bleaching.

The already mentioned with the photochromism associated changes directly observable physical properties such as luminescence, electrical resistance, refractive index, viscosity and circular dichroism promise dramatic improvements in optoelectronics and materials science. Especially in the field of optical data storage promise photochromic systems in comparison to the previous thermo- or magneto-optical method significant benefits. However, not all meet in solution photochromic substances the criteria necessary to take in solid phase, no side reactions, but still be able to respond as desired.

The classic example of photochromic systems are certainly the Armistead in 1962 invented and marketed by Corning Glass Works under the trade name BestliteTM silver halide sunglass lenses. Despite drawbacks in durability plastic materials have almost completely replaced conventional glass lenses due to weight advantages. Today's glasses materials are usually based on standard polymer CR 39, which is prepared by polymerization of diethylene glycol bisallyl carbonate. To introduce the photochromic properties can be the functionality introduced with an addition -curing inorganic coating or incorporated directly into the matrix either. The latter has advantages in durability, makes the manufacturing process complicated and expensive, however.

Optical memory based on spiropyrans

In 1989, in a much-publicized publication for the first time the realization of a three-dimensional memory system on Spiropyranbasis shown. The calculated limit of the optical density data for λ = 517 nm is about 812 Gb / cm ³ and is therefore approximately a factor of 1000 over that of a two-layer DVD. In figure 5 it is shown how the divided and frequency-doubled beam half of an Nd: YAG laser is used, a site-specific to write data in a three-dimensional medium in which a photochromic spiropyran is included. However, a disadvantage is the low compared to chromenes, spiroxazines bacteriorhodopsin and number of cycles, which is the system. However, it was documented in this example, the first time how to write data in a two - photon process, read out by fluorescent and can selectively delete again.

Bacteriorhodopsin

Thus, for example, is comprehensively researched the bacteriorhodopsin (BR ), which occurs in Halobacterium salinarium and there serves as a light-driven proton pump, for storing large amounts of data in small volumes. Taking advantage of the photochemical excitation of the O - intermediate of the BR photocycle in containing the state 9 -cis -retinal condition P and its thermal conversion to Q, which in turn can be converted photochemically to the B ground state, is any writing and erasing data within the medium possible. Process improvements in biotechnological synthesis of BR led to falling prices, which can be BR now appear attractive for new applications. Nevertheless, the prices are currently in the double- digit dollars per gram reports. An application for encrypted optical storage safety-critical data in a WORM method was shown at the University of Marburg. Away from the known photocycle may by two-photon absorption, a thermally and photochemically stable state can be achieved (so-called F- state), whose persistent polarization can be determined by the modulation of the polarization of the neodymium laser. The encryption is performed either simultaneously or sequentially by overwriting the data payload with the key, in this case a one-time- pads. To read data only a vertically polarized light source and a rotatable polarizing filter for the optics of the CCD camera is needed. The achievable data density remained in the literature cited by the desire to use cheaper plastic optics limited to about 125 KB / cm. 1993 showed Popp et al. the two-dimensional and Birge et al. In 1994, the three-dimensional holographic data storage in BR- containing materials.

Foldamers

Foldamers are credited with great potential in the field of drug delivery and the photo- adaptive smart materials on the current state of research. It has been known that foldamers can enjoy stable helical structures in solution. Khan et al. succeeded for the first time by incorporating a diazo group in the middle of the Polyphenylenethinylenrückrats foldamer to present a photoswitchable foldamers. Located at the diazo bond in the E configuration, the structure is severely disturbed, the helix unfolds and possibly trapped molecules are released.