Photochromism

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Hackmanite before and after exposure to sunlight (UV light)

Under photochromism to the light-induced understood reversible transformation of 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 thermal case, under the condition that A is more stable than B, the system will spontaneously return to state A after removing the exciting radiation source under standard conditions. In P-type photochromism, the reverse reaction is greatly accelerated under irradiation with a wavelength  ν 2 .

A ↔ B

This reversibility distinguishes photochromism from the well-known light-induced reactions. If the system shows an increase in color under the influence of electromagnetic radiation, one speaks of positive, in the opposite case of negative photochromism. In reality, however, secondary reactions and side reactions often occur that limit the shelf life of the photochromic materials and complicate the scheme.

Photochromism is also used naturally formed substances such as the positive photochromic minerals Kleinit and Tugtupite well as depending on the locality negative or positive photochromic sodalite - variety Hackmanit .

Gated photochromy

Fig. 1: Example of gated photochromism. The inactive diene form is only converted into the active photochromic species by a Diels-Alder reaction.

Under gated photochromism refers to the function of a photochromic reaction of an inhibitory factor. The photochromic reaction does not take place unless an external stimulus (e.g. electricity , photons of a different wavelength, heat, chemical activation) occurs. Photochromic saccharide sensors have been known for a long time. The example in Fig. 1 shows a new type of dienophile-dependent sensor molecule that could one day find applications in chemical dosimeters .

Dual mode photochromism

Fig. 2: Chiroptic switch system according to Huck et al.

Dual-mode photochromism is understood to mean systems that can be influenced by more than one external stimulus. Huck et al. presented a chiro-optical molecular switch in 1995, which allows the photochemically adjusted state to be secured chemically by making protons available, since the protonated species are no longer photoactive.

history

Hirshberg coined the term photochromism in 1950, but the observation of photochromic phenomena was already described by Fritzsche in 1867, who observed the fading of a solution of orange tetracene during the day and the return of color at night. Further early milestones in photochromism were set by ter Meer and Markwald. The former discovered the reversible coloration of fence posts coated with a zinc pigment in sunlight, the latter examined the change in color of 2,3,4,4-tetrachloronaphthalene-1 (4 H ) -one in crystalline form. It was not until 1940 that research came to the fore again, when Hirshberg and Fischer at the Daniel Sieff Research Institute in Rechowot , Israel, undertook systematic attempts to elucidate the synthetic pathways and mechanisms of photochromic compounds. The development of the field followed in the 1960s, especially the physical methods on which it was based. Unsolved problems of the then known photochromic compound classes, especially the eminent photodecomposition, caused research to stagnate before the first fatigue-free chromene and spirooxazine compounds could be shown in the 1980s. The popularity of this research area has been great since then, the number of publications has steadily increased, reviews have appeared, and even the Bundestag took notice in the form of a technology assessment of nanotechnology.

Excitation mechanisms

Fig. 3: Energy level representation of the photochromic absorption

In general, it can be assumed that the formation of species B takes place according to a one-photon mechanism, starting from the excitation of the molecule into a singlet or triplet state. Systems based on two-photon mechanisms are particularly interesting for applications such as three-dimensional optical storage media. Here, as shown in Fig. 3, the excitation can take place using two different wavelengths ν 1 and ν 2 . The absorption takes place either simultaneously via a virtual level or sequentially via a real level, from which the excitation into the higher level Sn takes place. In addition, there is the possibility of guiding the photochromic mechanism via an unstable intermediate that, after absorption of a second photon of the same wavelength, continues to react to form product B. Since 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.

Compound classes and mechanisms

Fig. 4: The isomerization reactions of the most important families of photochromic compounds

Today, a multitude of classes of compounds are known, each of which undergoes its own photochromic reactions. The most important ones are shown in Fig. 4 based on their isomerization reactions. Spiropyrans, spirooxazines, chromenes, hexa-1,3,5-trienes, diheteroarylethenes and cyclohex-1,3-dienes show electrocyclizations involving six π electrons and six atoms or, in the case of pyrazolines, five atoms. The light-induced E / Z isomerization of stilbene , the azo compounds , azines or thioindigo -type compounds is well known and is used in model systems to bring molecules to their target and release them in a directional manner. Other mechanisms are based on the intramolecular transfer of protons (aniles, benzylpyridines, acinitro compounds, salicylates, triazoles, oxazoles, metal dithizonates and perimidinspirohexadienones) or larger groups e.g. B. Acetyl groups in polycyclic quinones (Fig. 4). In addition, heterolytic bond cleavages of triarylmethanes and homolytic cleavages of triarylimidazole dimers, tetrachloronaphthalenes, perchlorotoluene, nitrosodimers and hydrazines are known. In Viologens we find electron transfer reactions. The Phytochromie and phototropy also based on photochromic reactions.

Applications

The possible uses of photochromic compounds are based for the most part on a few key figures, the main ones of which are explained here.

  • Colorability describes the ability of a system to develop color. When irradiated with light of wavelength λ, the initial absorption A 0 is proportional to the quantum yield of the photoreaction Φ, the molar absorption coefficient of the colored form Q, the concentration of the colorless form and the photon flux (contained in k ). A 0 (λ) = k · Φ · Q · c
  • Fatigue is understood to be the loss of efficiency of the photochromic reaction caused by side reactions (especially photochemically activated oxidation reactions).
  • The number of cycles or the Z50 value is understood as the number of switch-on and switch-off processes (conversion A → B → A) within a system after which 50% of the initial absorption is still present. Particularly stable compounds such as bacteriorhodopsin, which will be discussed in more detail later, survive 100,000 cycles without decomposition.
  • In photochromism, the half-life characterizes the time that the system needs for the absorption value to return to half of the maximum value within one cycle due to thermal bleaching.

The changes in directly observable physical properties such as luminescence , electrical resistance, refractive index , circular dichroism and viscosity associated with photochromism promise radical improvements in optoelectronics and materials science. Especially in the field of optical data storage, photochromic systems promise considerable advantages compared to previous thermo-optical or magneto-optical processes. However, not all substances which are photochromic in solution meet the necessary criteria, so that they do not enter into any side reactions even in the solid phase, but still be able to react as desired.

Sunglasses

The classic example of photochromic systems are certainly the silver halide -containing sunglass lenses invented by Armistead in 1962 and marketed by Corning Glass Works under the trade name Bestlite ™ . Despite disadvantages in terms of durability, plastic materials have almost completely replaced conventional glass lenses due to their weight advantages. Today's eyeglass materials are mostly based on the standard polymer CR 39 ( polyallyl diglycol carbonate ), which is produced by polymerizing allyl diglycol carbonate (ADC). To introduce the photochromic properties, either the functionality can be introduced with an additional hardening inorganic coating or built directly into the matrix. The latter offers advantages in terms of durability, but makes the manufacturing process more complex and expensive.

On the other hand, automatic welding protection screens and sports goggles that can be darkened within switching times of 0.01 to 0.1 seconds for journeys over routes with unlit road tunnels are based on electrically controlled liquid crystal technology .

Optical memories based on spiropyrans

Fig. 5: Scheme of a three-dimensional optical storage system based on spiropyran according to Parthenopoulos.

In 1989 the realization of a three-dimensional storage system based on spiropyran was shown for the first time in a well-regarded publication . The calculated optical limit of the data density for λ = 517 nm is approx. 812 GB / cm³ and is thus approximately 1000 times higher than that of a two-layer DVD. Fig. 5 shows how the split and half frequency doubled beam of an Nd: YAG laser is used to write location-specific data in a three-dimensional medium in which a photochromic spiropyran is enclosed. The disadvantage, however, is the low number of cycles that this system can withstand compared to chromenes , spiroxazines and bacteriorhodopsin . Nevertheless, this example was the first to document how data can be written in a two-photon process, read out using fluorescence and selectively erased again.

Bacteriorhodopsin

Fig. 6: Bacteriorhodopsin photocycle

So is z. B. the extensively researched 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. Using the photochemical excitation of the O intermediate of the BR photocycle into the state P, which is contained in the 9-cis-retinal state, and its thermal conversion into Q, which in turn can be converted photochemically into the B-ground state, any writing and erasing of Data within the medium possible. Process improvements in the biotechnological synthesis of the BR led to falling prices, which now make BR appear attractive for new applications. Nevertheless, the prices are currently still in the double-digit dollar range per gram. An application for encrypted optical storage of security-critical data in a WORM process was shown at the University of Marburg. Apart from the well-known photocycle, a thermally and photochemically stable state can be achieved through two-photon absorption (so-called F-state), the persistent polarization of which can be determined by modulating the polarization of the neodymium laser. The encryption takes place either simultaneously or sequentially by overwriting the user data with data from the key, here a one-time pad . All that is needed to read out the data is a vertically polarized light source and a rotatable polarization filter for the optics of the CCD camera. In the literature cited, the achievable data density remained limited to approx. 125 KB / cm due to the desire to use inexpensive plastic optics. In 1993 Popp et al. the two-dimensional and Birge et al. 1994 three-dimensional holographic data storage in materials containing BR.

Foldamers

According to the current state of research, foldamers are ascribed great potential in the field of drug delivery and photoadaptive intelligent materials. It has long been known that foldamers can adopt stable helical structures in solution. Khan et al. it was now possible for the first time to introduce a photoswitchable foldamer by incorporating a diazo group in the middle of the polyphenyleneethynylene backbone of the foldamer. If the diazo bond is in the E configuration, the structure is severely disturbed, the helix unfolds and any trapped molecules are released.

See also

literature

Individual evidence

  1. ↑ Michinori Takeshita, Kingo Uchida, Masahiro Irie: Novel saccharide tweezers with a diarylethene photoswitch . In: Chemical Communications . tape 1996 , no. 15 , 1996, pp. 1807-1808 .
  2. ↑ a b Vincent Lemieux, Neil R. Branda: Reactivity-gated photochromism of 1,2-dithienylethenes for potential use in dosimetry applications . In: Org. Lett . tape 7 , no. 14 , 2005, pp. 2969-2972 .
  3. ↑ a b Nina PM Huck, Ben L. Feringa: Dual-mode photoswitching of luminescence . In: Journal of the Chemical Society, Chemical Communications . tape 1995 , no. 11 , 1995, pp. 1095-1096 .
  4. ↑ Yehuda Hirshberg: Photochromy dans la serie de la bianthrone . In: Comptes Rendus de l'Academie des Sciences . tape 231 , no. 903 , 1950.
  5. ^ J. Fritzsche: Note sur les carbures d'hydrogène solides, tirés du gaudron de houille . In: Comptes Rendus de l'Académie des Sciences . tape 69 , 1867, pp. 1035-1037 .
  6. ↑ Edm. ter Meer: About Dinitro Compounds of the Fat Series . In: Justus Liebig's Annals of Chemistry . tape 181 , no. 1 , 1876, p. 1-22 , doi : 10.1002 / jlac.18761810102 .
  7. ^ Heinz Duerr: Organic Photochromy . In: Angewandte Chemie . tape 116 , no. 25 , 2004, pp. 3404-3418 .
  8. ↑ H. Paschen, C. Coenen, T. Fleischer, R. Grünwald, D. Oertel: TA project nanotechnology . In: Office for Technology Assessment of the German Bundestag (Ed.): Final report (TAB-Arbeitsberichte 92) . Bonn 2003.
  9. ↑ a b Anzar Khan, Christian Kaiser, Stefan Hecht: Prototype of a photoswitchable foldamer . Angewandte Chemie, 118 (12): 1912-1915, 2006.
  10. ^ William H. Armistead, Stanley D. Stookey: Phototrophic Material And Article Made Therefrom . U.S. Patent 3,208,860, July 1962
  11. ↑ Hybrid polymer layer systems . ( Memento of the original from September 21, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Fraunhofer Institute for Silicate Research @1@ 2Template: Webachiv / IABot / www.isc.fraunhofer.de
  12. ↑ Archived copy ( memento of the original from July 24, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Ctrl Eyewear> Specs, © 2015, accessed July 24, 2016. @1@ 2Template: Webachiv / IABot / www.ctrl-eyewear-shop.com
  13. ^ Dimitri A. Parthenopoulos, Peter M. Rentzepis: Three-Dimensional Optical Storage Memory . Science, 245: 843-845, 1989
  14. ↑ Thorsten Fischer, Martin Neebe, Thorsten Juchem, Norbert A. Hampp: Biomolecular Optical Data Storage and Data Encryption . IEEE Transactions on Nanobioscience, 2: 1-5, 2003
  15. ^ A. Popp, M. Wolperdinger, N. Hampp, C. Bruchle, D. Oesterhelt: Photochemical conversion of the O-intermediate to 9-cis-retinal-containing products in bacteriorhodopsin films . In: Biophysical Journal . tape 65 , no. 4 , 1993, p. 1449-1459 .
  16. ^ Robert R. Birge: Branched photocycle optical memory device . U.S. Patent 5,559,732, 1994
  17. ↑ David J. Hill, Matthew J. Mio, Ryan B. Prince, Thomas S. Hughes, Jeffrey S. Moore: A field guide to foldamers . In: Chem Rev. . tape 101 , no. 12 , 2001, p. 3893-4012 .