Pentacene

 Pentacene is a polycyclic aromatic hydrocarbon consisting of 5 linearly-fused benzene rings. This extended conjugation, together with a favorable crystal structure is responsible for its properties as an organic semiconductor. The purple-colored compound generates excitons upon absorption of ultra-violet (UV) or visible light; 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 the most 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).^[1]

Combined with buckminsterfullerene Pentacene is used in the development of organic photovoltaic devices.^{[2] [3]}

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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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.^[4] 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

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^[5] 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.^[6] 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:^[7]

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.^[8][9] The longer the acene the better it is able to align itself in a nematic liquid crystal phase.

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