Pentacene

Chembox new
ImageFile = Pentacene.png ImageSize =
IUPACName = Pentacene
OtherNames =
Section1 = Chembox Identifiers
CASNo = 135-48-8
PubChem = 24886995
SMILES = c45cc3cc2cc1ccc
cc1cc2cc3cc4cccc5
Section2 = Chembox Properties
Formula = C₂₂H₁₄
MolarMass = 278.36 g/mol
Appearance = Dark powder
Density =
MeltingPt = > 300 °C; sublimes at 372-374 °C
BoilingPt =
Solubility =
Section3 = Chembox Hazards
MainHazards =
FlashPt =
Autoignition = ; making it sensitive to oxidation. For this reason, its originally purple color in the solid state (a powder) slowly turns green upon prolonged exposure to air and light.

Pentacene is a promising candidate for the use in organic thin film transistors and OFETs. It is one of themost thoroughly investigated conjugated organic molecules with a high application potential due to a hole mobility in OFETs of up to 5.5 cm²V^-1s^-1 (almost comparable to amorphous silicon). ["Organic Electronic Devices and Their Functional Interfaces, N. Koch, ChemPhysChem, 2007, 8(10) pp 1438-1455," [http://www3.interscience.wiley.com/cgi-bin/abstract/114275266/ABSTRACT Link] ]

Combined with buckminsterfullerene Pentacene is used in the development of organic photovoltaic devices. ["Nanoimprinted large area heterojunction pentacene-C60 photovoltaic device, D. M. Nanditha, M. Dissanayake, A. A. D. T. Adikaari, Richard J. Curry, Ross A. Hatton, and S. R. P. Silva, Applied Physics Letters, 2007 90 pp 253502. [http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=APPLAB000090000025253502000001&idtype=cvips&gifs=yes] ] " ["Efficiently Organic: Researchers Use Pentacene To Develop Next-generation Solar Power" sciencedaily.com [http://www.sciencedaily.com/releases/2004/12/041220005834.htm Link] ]

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).

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."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] The precursor is reported to have some solubility in chloroform and is therefore amiable to spin coating. Pentacene is reported to be soluble in hot chlorinated benzenes, such as 1,2,4-trichlorobenzene, from which it can be recrystallized to form platelets

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Pentacene derivatives

Although Pentacene looks just like any aromatic compound such as anthracene 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 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 ["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] 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. ["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.] 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: ["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]

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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.

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. [Synthesis and Characterization of Fluorescent Acenequinones as Dyes for Guest-Host Liquid Crystal DisplaysZhihua 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] The longer the acene the better it is able to align itself in a nematic liquid crystal phase.

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External links

* [http://www.chm.bris.ac.uk/motm/pentacene/pentacene.htm facts about pentacene] , retrieved Apr. 17, 2006
* [http://www.nature.com/news/2007/070423/full/070423-11.html] , retrieved May 2, 2007
* [http://technology.newscientist.com/channel/tech/mg19626325.400-organic-transistor-improves-with-age.html?feedId=online-news_rss20 Organic transistor improves with age] , New Scientist, "2 December 2007"

References