Pentacene

Key takeaways

Pentacene belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Pentacene to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Pentacene from memory before moving on to harder problems.

Reference excerpt

Pentacene (C22H14) is a polycyclic aromatic hydrocarbon consisting of five linearly-fused benzene (C6H6) rings. This highly conjugated compound is an organic semiconductor. The compound generates excitons upon absorption of ultra-violet (UV) or visible light; this makes it very sensitive to oxidation. For this reason, this compound, which is a purple powder, slowly degrades upon exposure to air and light. Structurally, pentacene is one of the linear acenes, the previous one being tetracene (four fused benzene rings) and the next one being hexacene (six fused benzene rings). In August 2009, a group of researchers from IBM published experimental results of imaging a single molecule of pentacene using an atomic force microscope. In July 2011, they used a modification of scanning tunneling microscopy to experimentally determine the shapes of the highest occupied and lowest unoccupied molecular orbitals. In 2012, pentacene-doped p-terphenyl was shown to be effective as the amplifier medium for a room-temperature maser.

Synthesis

The compound, originally called dinaphthanthracene after naphthalene and anthracene (modern nomenclature for polyacenes, including pentacene, was only introduced in 1939 by Erich Clar), was first synthesized in 1912 by British chemists William Hobson Mills and Mildred May Gostling. A classic method for pentacene synthesis is by the Elbs reaction.

Pentacenes can also be prepared by extrusion of a small volatile component (carbon monoxide) from a suitable precursor at 150 °C.

The precursor itself is prepared in three steps from two molecules of α,α,α',α'-tetrabromo-o-xylene with a 7-tert-butoxybicyclo[2.2.1]hepta-2,5-diene by first heating with sodium iodide in dimethylformamide to undergo a series of elimination and Diels–Alder reactions to form the ring system, then hydrolysing the tert-butoxy group to an alcohol and followed by its oxidation to the ketone.

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

Pentacene derivatives

Monomeric pentacene derivatives 6,13-Substituted pentacenes are accessible through pentacenequinone by reaction with an aryl or alkynyl nucleophile (for example Grignard or organolithium reagents) followed by reductive aromatization. Another method is based on homologization of diynes by transition metals (through zirconacyclopentadienes) Functionalization of pentacene has allowed for control of the solid-state packing of this chromophore. The choice of the substituents (both size and location of substitution on the pentacene) influences the solid-state packing and can be used to control whether the compound adopts 1-dimensional or 2-dimensional cofacial pi-stacking in the solid-state, as opposed to the herringbone packing observed for pentacene. Although pentacene's structure resembles that of other aromatic compounds like anthracene, its aromatic properties are poorly defined; as such, pentacene and its derivatives are the subject of much research. A tautomeric chemical equilibrium exists between 6-methylidene-6,13-dihydropentacene and 6-methylpentacene.

This equilibrium is entirely in favor of the methylidene compound. Only by heating a solution of the compound to 200 °C does a small amount of the pentacene develop, as evidenced by the emergence of a red-violet color. According to one study the reaction mechanism for this equilibrium is not based on an intramolecular 1,5-hydride shift, but on a bimolecular free radical hydrogen migration. Pentacene reacts with elemental sulfur in 1,2,4-trichlorobenzene to the compound hexathiapentacene. X-ray crystallography shows that all the carbon-to-sulfur bond lengths are roughly equal (170 pm); from this, 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:

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.

Oligomers and polymers of pentacene

Oligomers and polymers based on pentacene have been explored both synthetically as well as in device application settings. Polymer light emitting diodes (PLEDs) have been constructed using conjugated copolymers (1a–b) containing fluorene and pentacene. A few other conjugated pentacene polymers (2a–b and 3) have been realized based on Sonogashira and Suzuki coupling reactions of a dibromopentacene monomer. Non-conjugated pentacene-based polymers have been synthesized via esterification of a pentacene diol monomer with bis-acid chlorides to form polymers 4a–b.

Various synthetic strategies have been employed to form conjugated oligomers of pentacene 5a–c including a one-pot-four-bond forming procedure which provided a solution-processable conjugated pentacene dimer (5c) which exhibited photoconductive gain >10, placing its performance within the same order of magnitude as thermally evaporated films of non-functionalized pentacene which exhibited photoconductive gain >16 using analogous measurement techniques. A modular synthetic method to conjugated pentacene di-, tri- and tetramers (6–8) has been reported which is based on homo- and cross-coupling reactions of robust dehydropentacene intermediates. Non-conjugated oligomers 9–10 based on pentacene have been synthesized, including dendrimers 9–10 with up to 9 pentacene moieties per molecule with molar absorptivity for the most intense absorption > 2,000,000 M−1•cm−1. Dendrimers 11–12 were shown to have improved performance in devices compared to analogous pentacene-based polymers 4a–b in the context of photodetectors.

… excerpt ends here. Continue reading the full article.

Illustrations

Pentacene: Scanning tunneling microscopy image of pentacene molecules on nickel.[6] — Scanning tunneling microscopy image of pentacene molecules on nickel.[6]

Worked examples

Example 1 — a first encounter with Pentacene

Start with the simplest possible case. Write down what Pentacene claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Pentacene before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Pentacene ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Pentacene

In research: Pentacene appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Pentacene in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Pentacene is common in secondary-school and first-year university syllabi. It links to neighbouring topics Acenes, Organic semiconductors, Pentacyclic compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Pentacene outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Pentacene in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Pentacene means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Pentacene out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Pentacene in simple terms?

Pentacene (C22H14) is a polycyclic aromatic hydrocarbon consisting of five linearly-fused benzene (C6H6) rings. This highly conjugated compound is an organic semiconductor.

Why does Pentacene matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Pentacene?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Pentacene.