Pentacene
From Wikipedia, the free encyclopedia
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General |
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| Name | Pentacene |
| Chemical formula | C22H14 |
| CAS-number | [] |
| ACX number | X1001757-9 |
| SMILES | c45cc3cc2cc1ccc cc1cc2cc3cc4cccc5 |
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Physical |
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| Molar mass | 278.36 g/mol |
| Melting point | < 300 °C |
| Chemical decomposition | 350 °C [1] |
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Information from Chemfinder. SI units were used where possible. Unless otherwise stated, standard conditions were used. |
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Pentacene is an polycyclic aromatic hydrocarbon molecule consisting of 5 linearly-fused benzene rings. It has the appearance of a dark-blue powder. It is an organic semiconductor. It is sensitive to light and oxygen. It is a purple powder, but its oxidation product is greenish. Most commercial samples of pentacene are greenish due to oxidized outer layer. Vacuum sublimation or adsorption chromatography are used to purify pentacene.
Researchers from Georgia Institute of Technology are developing new approach for using pentacene combined with buckminsterfullerene in solar panels. [2] Pentacene is also a subject of research for use in organic thin film transistors and OFETs.
Pentacene is one of a series of linear acenes, the previous one being tetracene ( four fused benzene rings) and the next one being hexacene (six fused benzene rings).
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[edit] Synthesis
Pentacenes can be prepared in the laboratory by extrusion of a small volatile component. In one such experimental procedure carbon monoxide is liberated from a precursor at 150°C [1]. The precursor is reported to have some solubility in chloroform and is therefore amiable to spin coating.
[edit] Pentacene derivatives
Although Pentacene looks just like any aromatic compound such as antracene its aromatic properties are ill defined and for this reason the compound and its derivatives are topic of research.
A tautomeric chemical equilibrium exists between 6-methylene-6,13-dihydropentacene and and 6-methylpentacene.
This equilibrium is entirely in favor of the methylene compound. Only by heating to a solution of the compound to 200°C does a small amount of the pentacane develop as evidenced by the emergence of a red-violet color. According to one study [3] the reaction mechanism for this equilibrium is not based on an intramolecular 1,5-hydride shift but on a bimolecular free radical hydrogen migration. In contrast, isotoluenes with the same central chemical motif easily aromatize.
Pentacene reacts with elemental sulfur in 1,2,4-trichlorobenzene to the compound hexathiapentacene [4]. X-ray crystallography shows that all the carbon-to-sulfur bond lengths are roughly equal (170 picometer) and from there it follows that resonance structures B and C with complete charge separation are more significant than structure A.
In the crystal phase the molecules display aromatic stacking interactions whereby the distance between some sulfur atoms on neighboring molecules can become less (337 pm) than the sum of two Van der Waals radii (180 pm)
Like the related tetrathiafulvalene, this compound is studied in the field of organic semiconductors.
The acenes may appear as planar and rigid molecules, but in fact they can be very distorted. The pentacene depicted below [5]:
has an end-to end twist of 144° and is sterically stabilized by the six phenyl groups. The compound can be resolved into its two enantiomers with an unusually high reported optical rotation of 7400° although racemization takes place with a chemical half-life of 9 hours.
[edit] Other uses
Pentacenes are examined as potential dichroic dyes. The pentacenoquinone displayed below is fluorescent and when mixed with liquid crystal E7 mixture a dichroic ratio of 8 is reached [6] [7]. The longer the acene the better it is able to align itself in a nematic liquid crystal phase.
[edit] External links
[edit] References
- ^ a b A new type of soluble pentacene precursor for organic thin-film transistors Kew-Yu Chen, Hsing-Hung Hsieh, Chung-Chih Wu, Jiunn-Jye Hwang and Tahsin J. Chow Chem. Commun., 2007, 1065 - 1067, DOI:10.1039/b616511g 10.1039/b616511g
- ^ Efficiently Organic: Researchers Use Pentacene To Develop Next-generation Solar Power sciencedaily.com Link
- ^ Why 6-Methylpentacene Deconjugates but Avoids the Thermally Allowed Unimolecular Mechanism Joseph E. Norton, Brian H. Northrop, Colin Nuckolls, and K. N. Houk Org. Lett.; 2006; 8(21) pp 4915 - 4918; (Letter) DOI:10.1021/ol062012g
- ^ Hexathiapentacene: Structure, Molecular Packing, and Thin-Film Transistors Alejandro L. Briseno, Qian Miao, Mang-Mang Ling, Colin Reese, Hong Meng, Zhenan Bao, and Fred Wudl J. Am. Chem. Soc.; 2006; 128(49) pp 15576 - 15577; (Communication) DOI:10.1021/ja066088j.
- ^ Synthesis, Structure, and Resolution of Exceptionally Twisted Pentacenes Jun Lu, Douglas M. Ho, Nancy J. Vogelaar, Christina M. Kraml, Stefan Bernhard, Neal Byrne, Laura R. Kim, and Robert A. Pascal, Jr. J. Am. Chem. Soc.; 2006; 128(51) pp 17043 - 17050; (Article) DOI:10.1021/ja065935f
- ^ Synthesis and Characterization of Fluorescent Acenequinones as Dyes for Guest-Host Liquid Crystal Displays Zhihua Chen and Timothy M. Swager Org. Lett.; 2007; 9(6) pp 997 - 1000; (Letter) DOI:10.1021/ol062999m
- ^ in the synthesis of this compound, the starting material is reacted with 1,4-naphthoquinone and DPT. DTP converts the oxo-norbornadiene to an intermediary furan. The second step is oxidation by PPTS
