Molecular switch

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A molecular switch is a molecule that can be reversibly shifted between two or more electronic states. The molecules are shifted between the different states, depending on environmental influences, such as changes in pH , light, temperature, electrical current, the microenvironment or the presence of a ligand . In some cases a combination of several influences is required. The oldest form of synthetic molecular switches are pH indicators , which show different colors depending on the pH value. At the moment, synthetic molecular switches in the field of nanotechnology are of interest for application in molecular computers. Molecular switches are also important in biology , as many biological functions are based on them, for example optical sensory perception with the eye. They are the simplest examples of molecular machines .

Photochromic molecular switches

An extensively studied class of substances are photochromic compounds that are able to switch back and forth between electron configurations as soon as they are irradiated with light of a certain wavelength. Each state has a specific absorption maximum that can be characterized using UV / VIS spectroscopy . Examples of this substance class are: azobenzene , diaryl ethane , Ditionylethan , fulvenes , stilbene , spiropyran and Phenoxynaphthochinone .

Switching process in dithienylethane

Chiroptic molecular switches are a special subgroup in which switching between enantiomers takes place. This is more likely to be demonstrated by circular dichroism (with a polarization spectrometer) than by simple spectroscopy. Sterically hindered alkenes, such as those shown below, change their helicity in response to exposure to circularly polarized light .

Sterically hindered alkenes

Chiroptic molecular switches exhibit movement that is considered to be a synthetic molecular motor :

TBu Helicenemolecularmotor.png

Molecular host-guest switches

In host-guest chemistry , the metastable states differ in their affinity for guests. Some early examples of such systems are based on crown ether chemistry. The first switchable host was described by Desvergne and Bouas-Laurent in 1978. They synthesized a crown ether through a photochemical anthracene dimerization. Although the crown ether is not a switch in the strict sense, this substance is able to absorb cations after a photochemical reaction, which are released again after the addition of acetonitrile.

Anthracene crown, Desvergne 1978

As early as 1980, Yamashita et al. represented a crown ether that already contained anthracene units (an anthracenophane) and examined the ion uptake photochemically.

Anthracene crown, Misumi 1980

Also in 1980 Shinkai synthesized an anthracene unit as a photo antenna molecule with an azobenzene group. and proved for the first time the existence of molecules with on-off switches. In this molecule, light triggers a cis-trans isomerization of the azo group, which results in ring enlargement. This means that the crown ether in the trans form preferentially binds ammonium , lithium and sodium ions, while in the cis form it absorbs potassium and rubidium . The reverse reaction of isomerization takes place in the dark reaction.

Molecular Switch, Shinkay 1980

Shinkai uses this reference to ion transport to copy the biochemical reaction of monensin and nigericin : In a two-phase system, the ions are absorbed in one phase, initiated by light, and released again in the dark in the other phase.

Mechanically interlocking molecular switches

Some of the most advanced molecular switches are based on mechanically interlocking molecular architectures , with the bistable state varying in the position of the macromolecule . In 1991, Stoddart pointed out a pendulum molecule that, based on rotaxane, swings between two docking stations like a pearl on a string. Stoddart predicted that the pendulum will become a molecular machine if the stations are similar to the externally excited stations. In 1993 Stoddart was overtaken by Fritz Vögtle , the pioneer of supramolecular chemistry, who developed a molecular switch based not on rotaxane but on catenane .

Photo-switchable catenane, Vögtle 1993
Molecular switch, Kaifer and Stoddart 1994

This connection is based on a two-ring system: one ring holds the photoswitchable azobenzene ring and two docking stations and the other ring is a polyether with two aromatic rings with a binding affinity to the paraquat units. In this system, NMR spectroscopy shows that the azo-trans form of the polyether can rotate freely around the partner ring, but the rotation is stopped as soon as light initiates isomerization to the cis form.

In 1994 Kaifer and Stoddart modified their molecular pendulum in such a way that an electron-poor four-fold charged cyclophane cation bead can choose between two docking stations: a biphenol and a benzidine . At room temperature in solution, NMR spectroscopy shows that the bead oscillates at a rate that is on the NMR time scale. If the temperature is reduced to 229 K, the signals show that the benzidine station is preferred with 84%. However, when trifluoroacetic acid is added, the nitrogen atoms of the benzidine ring are protonated so that the bead is permanently attached to the biphenol station. The same effect is obtained with an electrochemical oxidation in which the benzidine radical ion is formed, and of course both processes are reversible.

In 2007 pendulums were used in an experimental DRAM circuit. The circuit consists of 400 layers of silicon nanometer electrodes (16nm wide by 33nm space) crossed by another 400 titanium nanometer electrodes of similar dimensions that comprise a simple layer of the rotaxane as shown below:

Molecular switch in an electronic data storage device

Each bit of the device consists of a silicone and a titanium crossbar with 100 rotaxane molecules that fill the space between them arranged at right angles. The hydrophilic stopper on the left (gray) is specially made to anchor the silicone cable (made hydrophilic through phosphorus doping), while the hydrophobic tetraphenylmethane stopper attaches the hydrophobic titanium electrode accordingly. In the ground state, the paraquat ring is around the tetrathiafulvene unit (red) and moves to the naphthalene unit (green) as soon as the fulvalene unit is oxidized by electricity. When the fulvalene is reduced back to metastable conductivity, state 1 is formed again, which reverts to the ground state with a half-life of approximately one hour. The problem of a defect is circumvented by using a defect-tolerant architecture, which can be found in the Teramac project. In this way you get a circuit of 160,000 bits the size of a leukocyte , that is 10 11 bits per square centimeter.

Organic light emitting transistors

Scientists have realized light-emitting organic transistors that can be remotely controlled by light pulses. For this purpose, luminescent polymers are combined with photoswitchable molecules. Organic light-emitting transistors, a kind of symbiosis of organic transistor (OTFT) and organic light-emitting diode (OLED) , are key components for various optoelectronic applications in the display sector.

literature

  • Ben Feringa, WR Brown: Molecular switches , Wiley-VCH 2011, ISBN 978-3-527-63442-2
  • W. Velema, W. Szymanski, B. Feringa: Pharmakology: Beyond Proof of Principle , J. Am. Chem. Soc 2014, 136 (6), 2178-2191, doi: 10.1021 / ja413063e
  • Lili Hou, Xiaoyan Zhang, Giovanni F. Cotella, Giuseppe Carnicella, Martin Herder, Bernd M. Schmidt, Michael Pätzel, Stefan Hecht, Franco Cacialli & Paolo Samorì: Optically switchable organic light-emitting transistors , Nature Nanotechnology (2019), 18. February 2019, DOI: 10.1038 / s41565-019-0370-9 .

Individual evidence

  1. ^ Jean-Pierre Sauvage, Valeria Amendola (ed.): Molecular machines and motors . Springer, Berlin / Heidelberg / New York 2001, ISBN 3-540-41382-0 .
  2. ↑ Angela Mammana, Gregory T. Carroll, Ben L. Feringa: Circular Dichroism of Dynamic Systems: Switching Molecular and Supramolecular Chirality . In: Nina Berova, Prasad L. Polavarapu, Koji Nakanishi, Robert W. Woody (Eds.): Comprehensive Chiroptical Spectroscopy . John Wiley & Sons, Inc., 2012, ISBN 978-1-118-12039-2 , pp. 289-316 , doi : 10.1002 / 9781118120392.ch8 .
  3. ↑ Ben L. Feringa, Richard A. van Delden, Nagatoshi Koumura, Edzard M. Geertsema: Chiroptical Molecular Switches . In: Chemical Reviews . tape 100 , no. 5 , May 1, 2000, ISSN  0009-2665 , pp. 1789-1816 , doi : 10.1021 / cr9900228 .
  4. ↑ Jean-Pierre Desvergne, Henri Bouas-Laurent: Cation complexing photochromic materials Involving bisanthracenes linked by a polyether chain. Preparation of a crown ether by photocycloisomerization . In: Journal of the Chemical Society, Chemical Communications . No. 9 , January 1, 1978, doi : 10.1039 / C39780000403 .
  5. ↑ Henri Bouas-Laurent, Alain Castellan, Jean-Pierre Desvergne: From anthracene photodimerization to jaw photochromic materials and photo crowns . In: Pure and Applied Chemistry . tape 52 , no. 12 , 1980, pp. 2633–2648 ( iupac.org [PDF; accessed September 18, 2015]).
  6. ↑ Isamu Yamashita, Mieko Fujii, Takahiro Kaneda, Soichi Misumi, Tetsuo Otsubo: Synthetic macrocyclic ligands. II. Synthesis of a photochromic crown ether . In: Tetrahedron Letters . tape 21 , no. 6 , 1980, pp. 541-544 , doi : 10.1016 / S0040-4039 (01) 85550-7 .
  7. ↑ Seiji Shinkai, Takahiro Nakaji, Yoshihiro Nishida, Toshiyuki Ogawa, Osamu Manabe: Photoresponsive crown ethers. 1. Cis-trans isomerism of azobenzene as a tool to enforce conformational changes of crown ethers and polymers . In: Journal of the American Chemical Society . tape 102 , no. 18 , August 1, 1980, p. 5860-5865 , doi : 10.1021 / ja00538a026 .
  8. ↑ Seiji Shinkai, Takahiro Nakaji, Toshiyuki Ogawa, Kazuyoshi Shigematsu, Osamu Manabe: Photoresponsive crown ethers. 2. Photocontrol of ion extraction and ion transport by a bis (crown ether) with a butterfly-like motion . In: Journal of the American Chemical Society . tape 103 , no. 1 , January 1, 1981, pp. 111-115 , doi : 10.1021 / ja00391a021 .
  9. ↑ Seiji Shinkai: Switch-functionalized systems in biomimetic chemistry . In: Pure and Applied Chemistry . tape 59 , no. 3 , 1987, pp. 425-430 ( iupac.org [PDF; accessed September 18, 2015]).
  10. ^ Pier Lucio Anelli, Neil Spencer, J. Fraser Stoddart: A molecular shuttle . In: Journal of the American Chemical Society . tape 113 , no. 13 , June 1, 1991, pp. 5131-5133 , doi : 10.1021 / ja00013a096 .
  11. ^ Fritz Vögtle, Walter Manfred Müller, Ute Müller, Martin Bauer, Kari Rissanen: Photoswitchable Catenanes . In: Angewandte Chemie International Edition in English . tape 32 , no. 9 , September 1, 1993, pp. 1295-1297 , doi : 10.1002 / anie.199312951 .
  12. ↑ Andrew C. Benniston, Anthony Harriman: A Light-Induced Molecular Shuttle Based on a [2] Rotaxane-Derived Triad . In: Angewandte Chemie International Edition in English . tape 32 , no. 10 , October 1, 1993, p. 1459–1461 , doi : 10.1002 / anie.199314591 .
  13. ^ Richard A. Bissell, Emilio Córdova, Angel E. Kaifer, J. Fraser Stoddart: A chemically and electrochemically switchable molecular shuttle . In: Nature . tape 369 , no. 6476 , May 12, 1994, pp. 133-137 , doi : 10.1038 / 369133a0 .
  14. ↑ Jonathan E. Green et al. a .: A 160-kilobit molecular electronic memory patterned at 1011 bits per square centimeter . In: Nature . tape 445 , no. 7126 , January 25, 2007, p. 414-417 , doi : 10.1038 / nature05462 .
  15. ^ Press report from the Humboldt University of Berlin