Pentacene

Last updated
Pentacene
Pentacene.svg
Pentacene molecule spacefill.png
Names
Preferred IUPAC name
Pentacene
Other names
2,3:6,7-Dibenzanthracene
Benzo[b]naphthacene
Dibenz[b,i]anthracene
NSC 90784
lin-Dibenzanthracene
lin-Naphthoanthracene
Identifiers
3D model (JSmol)
1912418
ChEBI
ChemSpider
ECHA InfoCard 100.004.722 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 205-193-7
733903
PubChem CID
UNII
  • InChI=1S/C22H14/c1-2-6-16-10-20-14-22-12-18-8-4-3-7-17(18)11-21(22)13-19(20)9-15(16)5-1/h1-14H Yes check.svgY
    Key: SLIUAWYAILUBJU-UHFFFAOYSA-N Yes check.svgY
  • InChI=1/C22H14/c1-2-6-16-10-20-14-22-12-18-8-4-3-7-17(18)11-21(22)13-19(20)9-15(16)5-1/h1-14H
    Key: SLIUAWYAILUBJU-UHFFFAOYAR
  • c1ccc2cc3cc4cc5ccccc5cc4cc3cc2c1
Properties
C22H14
Molar mass 278.354 g·mol−1
AppearanceDark blue powder
Density 1.3 g cm−3
Melting point >300 °C (572 °F; 573 K) sublimes at 372 °C
Boiling point 40–43 °C (104–109 °F; 313–316 K) at 0.15 torr
-205.4 × 10−6 cm3 mol−1
Structure
Triclinic
P-1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)

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.

Contents

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. [1] [2] 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. [3] [4]

In 2012, pentacene-doped p-terphenyl was shown to be effective as the amplifier medium for a room-temperature maser. [5]

Synthesis

Scanning tunneling microscopy image of pentacene molecules on nickel. Pentacene on Ni(111) STM.jpg
Scanning tunneling microscopy image of pentacene molecules on nickel.
Pentacene powder Pentacene.jpg
Pentacene powder

Pentacene was first synthesized in 1912 by British chemists William Hobson Mills and Mildred May Gostling. [7] [8] A classic method for pentacene synthesis is by the Elbs reaction. [9] [10]

Elbs reaction to pentacene Elbs-Reaktion 4.svg
Elbs reaction to pentacene

Pentacenes can also be prepared by extrusion of a small volatile component (carbon monoxide) from a suitable precursor at 150 °C. [11]

Formation of pentacene by extrusion of carbon monoxide Formation of pentacene by extrusion of CO.jpg
Formation of pentacene by extrusion of carbon monoxide

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. [11]

Pentacene synthesis Pentacene synthesis.svg
Pentacene synthesis

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. [12] [13] [14] Another method is based on homologization of diynes by transition metals (through zirconacyclopentadienes) [15] [16] [17] [18] [19] Functionalization of pentacene has allowed for control of the solid-state packing of this chromophore. [20] [21] 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-methylene-6,13-dihydropentacene and 6-methylpentacene.

6-methylpentacene equilibrium 6-methylpentaceneEquilibrium.png
6-methylpentacene equilibrium

This equilibrium is entirely in favor of the methylene 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 [22] 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. [23] 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.

Hexathiapentacene Hexathiapentacene.png
Hexathiapentacene

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: [24]

Twisted acenes Twisted acene synth.png
Twisted acenes

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

Pentacene.Polymers.tif

Oligomers and polymers based on pentacene have been explored both synthetically as well as in device application settings. [25] [26] Polymer light emitting diodes (PLEDs) have been constructed using conjugated copolymers (1a–b) containing fluorene and pentacene. [27] A few other conjugated pentacene polymers (2a–b and 3) have been realized based on Sonogashira and Suzuki coupling reactions of a dibromopentacene monomer. [28] [29] Non-conjugated pentacene-based polymers have been synthesized via esterification of a pentacene diol monomer with bis-acid chlorides to form polymers 4a–b. [30] [31]

OLIGOMERS.tif
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, [32] 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. [33] 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. [34] Non-conjugated oligomers 9–10 based on pentacene have been synthesized, [30] [31] 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. [35]

Dendrimer11.tif Dendrimer12.Highres.PS.tif

Materials research

Pentacenes have been 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. [36] [37] Longer acenes align better in the nematic liquid crystal phase.

Fluorescent acenequinones Pentacenequinone.png
Fluorescent acenequinones

Combined with buckminsterfullerene, pentacene is used in the development of organic photovoltaic prototypes. [38] [39] Organic photovoltaic cells are cheaper and more flexible than traditional inorganic cells, which could potentially open doors to solar cells in new markets. [40]

Pentacene is a popular choice for research on organic thin-film transistors and OFETs, being 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 cm2/(V·s), which exceeds that of amorphous silicon. [41] [42] [43]

Pentacene, as well as other organic conductors, is subject to rapid oxidation in air, which precludes commercialization. If the pentacene is preoxidized, the pentacene-quinone is a potential gate insulator, then the mobility can approach that of rubrene – the highest-mobility organic semiconductor – namely, 40 cm2/(V·s). This pentacene oxidation technique is akin to the silicon oxidation used in the silicon electronics. [42]

See also

Related Research Articles

<span class="mw-page-title-main">Organic electronics</span> Field of materials science

Organic electronics is a field of materials science concerning the design, synthesis, characterization, and application of organic molecules or polymers that show desirable electronic properties such as conductivity. Unlike conventional inorganic conductors and semiconductors, organic electronic materials are constructed from organic (carbon-based) molecules or polymers using synthetic strategies developed in the context of organic chemistry and polymer chemistry.

<span class="mw-page-title-main">Pyridine</span> Heterocyclic aromatic organic compound

Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one methine group (=CH−) replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid with a distinctive, unpleasant fish-like smell. Pyridine is colorless, but older or impure samples can appear yellow. The pyridine ring occurs in many important compounds, including agrochemicals, pharmaceuticals, and vitamins. Historically, pyridine was produced from coal tar. As of 2016, it is synthesized on the scale of about 20,000 tons per year worldwide.

<span class="mw-page-title-main">Conductive polymer</span>

Conductive polymers or, more precisely, intrinsically conducting polymers (ICPs) are organic polymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors. The biggest advantage of conductive polymers is their processability, mainly by dispersion. Conductive polymers are generally not thermoplastics, i.e., they are not thermoformable. But, like insulating polymers, they are organic materials. They can offer high electrical conductivity but do not show similar mechanical properties to other commercially available polymers. The electrical properties can be fine-tuned using the methods of organic synthesis and by advanced dispersion techniques.

<span class="mw-page-title-main">Polythiophene</span>

Polythiophenes (PTs) are polymerized thiophenes, a sulfur heterocycle. The parent PT is an insoluble colored solid with the formula (C4H2S)n. The rings are linked through the 2- and 5-positions. Poly(alkylthiophene)s have alkyl substituents at the 3- or 4-position(s). They are also colored solids, but tend to be soluble in organic solvents.

Antiaromaticity is a characteristic of a cyclic molecule with a π electron system that has higher energy due to the presence of 4n delocalised electrons in it. Unlike aromatic compounds, which follow Hückel's rule and are highly stable, antiaromatic compounds are highly unstable and highly reactive. To avoid the instability of antiaromaticity, molecules may change shape, becoming non-planar and therefore breaking some of the π interactions. In contrast to the diamagnetic ring current present in aromatic compounds, antiaromatic compounds have a paramagnetic ring current, which can be observed by NMR spectroscopy.

Organic semiconductors are solids whose building blocks are pi-bonded molecules or polymers made up by carbon and hydrogen atoms and – at times – heteroatoms such as nitrogen, sulfur and oxygen. They exist in form of molecular crystals or amorphous thin films. In general, they are electrical insulators, but become semiconducting when charges are either injected from appropriate electrodes, upon doping or by photoexcitation.

In organometallic chemistry, acetylide refers to chemical compounds with the chemical formulas MC≡CH and MC≡CM, where M is a metal. The term is used loosely and can refer to substituted acetylides having the general structure RC−CM. Acetylides are reagents in organic synthesis. The calcium acetylide commonly called calcium carbide is a major compound of commerce.

<span class="mw-page-title-main">Organic field-effect transistor</span> Type of field-effect transistor

An organic field-effect transistor (OFET) is a field-effect transistor using an organic semiconductor in its channel. OFETs can be prepared either by vacuum evaporation of small molecules, by solution-casting of polymers or small molecules, or by mechanical transfer of a peeled single-crystalline organic layer onto a substrate. These devices have been developed to realize low-cost, large-area electronic products and biodegradable electronics. OFETs have been fabricated with various device geometries. The most commonly used device geometry is bottom gate with top drain and source electrodes, because this geometry is similar to the thin-film silicon transistor (TFT) using thermally grown SiO2 as gate dielectric. Organic polymers, such as poly(methyl-methacrylate) (PMMA), can also be used as dielectric. One of the benefits of OFETs, especially compared with inorganic TFTs, is their unprecedented physical flexibility, which leads to biocompatible applications, for instance in the future health care industry of personalized biomedicines and bioelectronics.

<span class="mw-page-title-main">Macrocycle</span> Molecule with a large ring structure

Macrocycles are often described as molecules and ions containing a ring of twelve or more atoms. Classical examples include the crown ethers, calixarenes, porphyrins, and cyclodextrins. Macrocycles describe a large, mature area of chemistry.

<span class="mw-page-title-main">Hexacene</span> Chemical compound

Hexacene is an aromatic compound consisting of six linearly-fused benzene rings. It is a blue-green, air-stable solid with low solubility.

<span class="mw-page-title-main">Heptacene</span> Chemical compound

Heptacene is an organic compound and a polycyclic aromatic hydrocarbon and the seventh member of the acene or polyacene family of linear fused benzene rings. This compound has long been pursued by chemists because of its potential interest in electronic applications and was first synthesized but not cleanly isolated in 2006. Heptacene was finally fully characterized in bulk by researchers in Germany and the United States in 2017.

<span class="mw-page-title-main">Acene</span> Class of chemical compounds

In organic chemistry, the acenes or polyacenes are a class of organic compounds and polycyclic aromatic hydrocarbons made up of benzene rings which have been linearly fused. They follow the general molecular formula C4n+2H2n+4.

<span class="mw-page-title-main">Fullerene chemistry</span>

Fullerene chemistry is a field of organic chemistry devoted to the chemical properties of fullerenes. Research in this field is driven by the need to functionalize fullerenes and tune their properties. For example, fullerene is notoriously insoluble and adding a suitable group can enhance solubility. By adding a polymerizable group, a fullerene polymer can be obtained. Functionalized fullerenes are divided into two classes: exohedral fullerenes with substituents outside the cage and endohedral fullerenes with trapped molecules inside the cage.

[n]Radialenes are alicyclic organic compounds containing n cross-conjugated exocyclic double bonds. The double bonds are commonly alkene groups but those with a carbonyl (C=O) group are also called radialenes. For some members the unsubstituted parent radialenes are elusive but many substituted derivatives are known.

<span class="mw-page-title-main">Polyfluorene</span> Chemical compound

Polyfluorene is a polymer with formula (C13H8)n, consisting of fluorene units linked in a linear chain — specifically, at carbon atoms 2 and 7 in the standard fluorene numbering. It can also be described as a chain of benzene rings linked in para positions with an extra methylene bridge connecting every pair of rings.

The Buchner ring expansion is a two-step organic C-C bond forming reaction used to access 7-membered rings. The first step involves formation of a carbene from ethyl diazoacetate, which cyclopropanates an aromatic ring. The ring expansion occurs in the second step, with an electrocyclic reaction opening the cyclopropane ring to form the 7-membered ring.

Singlet fission is a spin-allowed process, unique to molecular photophysics, whereby one singlet excited state is converted into two triplet states. The phenomenon has been observed in molecular crystals, aggregates, disordered thin films, and covalently-linked dimers, where the chromophores are oriented such that the electronic coupling between singlet and the double triplet states is large. Being spin allowed, the process can occur very rapidly and out-compete radiative decay thereby producing two triplets with very high efficiency. The process is distinct from intersystem crossing, in that singlet fission does not involve a spin flip, but is mediated by two triplets coupled into an overall singlet. It has been proposed that singlet fission in organic photovoltaic devices could improve the photoconversion efficiencies.

<span class="mw-page-title-main">Phenacene</span>

Phenacenes are a class of organic compounds consisting of fused aromatic rings. They are polycyclic aromatic hydrocarbons, related to acenes and helicenes from which they differ by the arrangement of the fused rings.

Diketopyrrolopyrroles (DPPs) are organic dyes and pigments based on the heterocyclic dilactam 2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione, widely used in optoelectronics. DPPs were initially used as pigments in the painting industry due to their high resistance to photodegradation. More recently, DPP derivatives have been also investigated as promising fluorescent dyes for bioimaging applications, as well as components of materials for use in organic electronics.

<span class="mw-page-title-main">Contorted aromatics</span>

Contorted aromatics or more precisely contorted polycyclic aromatic hydrocarbons are polycyclic aromatic hydrocarbons (PAHs) in which the fused aromatic molecules deviate from the usual planarity.

References

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  2. ↑ Gross, L.; Mohn, F; Moll, N; Liljeroth, P; Meyer, G (2009). "The Chemical Structure of a Molecule Resolved by Atomic Force Microscopy". Science . 325 (5944): 1110–1114. Bibcode:2009Sci...325.1110G. doi:10.1126/science.1176210. PMID   19713523. S2CID   9346745.
  3. ↑ Krieger, K. (23 August 2011). "Molecules Imaged Most Intimately". Science Now . Retrieved 29 June 2017.
  4. ↑ Gross, L.; Moll, N.; Mohn, F.; Curioni, A.; Meyer, G.; Hanke, F.; Persson, M. (2011). "High-Resolution Molecular Orbital Imaging Using a p-Wave STM Tip". Phys. Rev. Lett. 107 (8): 86101–86104. Bibcode:2011PhRvL.107h6101G. doi:10.1103/PhysRevLett.107.086101. PMID   21929180.
  5. ↑ Brumfiel, G. (2012). "Microwave laser fulfills 60 years of promise". Nature . doi:10.1038/nature.2012.11199. S2CID   124247048.
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  30. 1 2 Lehnherr, D.; Tykwinski, R. R. (2007). "Pentacene Oligomers and Polymers: Functionalization of Pentacene to Afford Mono-, Di-, Tri-, and Polymeric Materials". Organic Letters. 9 (22): 4583–4586. doi:10.1021/ol702094d. PMID   17918951.
  31. 1 2 Lehnherr, Dan; McDonald, Robert; Ferguson, Michael J.; Tykwinski, Rik R. (2008). "Synthesis of soluble oligo- and polymeric pentacene-based materials". Tetrahedron. 64 (50): 11449–11461. doi:10.1016/j.tet.2008.09.041. ISSN   0040-4020.
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  33. ↑ Gao, J.; Hegmann, F. A (2008). "Bulk photoconductive gain in pentacene thin films". Applied Physics Letters. 93 (22): 223306. Bibcode:2008ApPhL..93v3306G. doi:10.1063/1.3043431.
  34. ↑ Lehnherr, D.; Murray, A. H.; McDonald, R.; Tykwinski, R.R. (2010). "A Modular Synthetic Approach to Conjugated Pentacene Di-, Tri-, and Tetramers". Angewandte Chemie International Edition. 49 (35): 6190–6194. doi:10.1002/anie.201000555. PMID   20645363.
  35. ↑ Lehnherr, D.; Gao, J.; Hegmann, F. A.; Tykwinski, R. R. (2009). "Pentacene-based dendrimers: synthesis and thin film photoconductivity measurements of branched pentacene oligomers". Journal of Organic Chemistry. 74 (14): 5017–5024. doi:10.1021/jo9007089. PMID   19489566.
  36. ↑ Chen, Zhihua; Swager, TM (2007). "Synthesis and Characterization of Fluorescent Acenequinones as Dyes for Guest−Host Liquid Crystal Displays". Organic Letters. 9 (6): 997–1000. doi:10.1021/ol062999m. PMID   17298074.
  37. ↑ in the synthesis of this compound, the starting material is treated with 1,4-naphthoquinone and DPT. DTP converts the oxo-norbornadiene to an intermediary furan. The second step is oxidation by PPTS
  38. ↑ Dissanayake, D. M. Nanditha M. (2007). "Nanoimprinted large area heterojunction pentacene-C[sub 60] photovoltaic device" (PDF). Applied Physics Letters. 90 (25): 253502. Bibcode:2007ApPhL..90y3502D. doi:10.1063/1.2749863.
  39. ↑ Efficiently Organic: Researchers Use Pentacene To Develop Next-generation Solar Power sciencedaily.com Link
  40. ↑ "Efficiently Organic: Researchers Use Pentacene To Develop Next-generation Solar Power". ScienceDaily. Retrieved 2017-11-14.
  41. ↑ Norbert Koch (2007). "Organic Electronic Devices and Their Functional Interfaces". ChemPhysChem. 8 (10): 1438–55. doi:10.1002/cphc.200700177. PMID   17539032.
  42. 1 2 Tatsuo Hasegawa & Jun Takeya (2009). "Organic field-effect transistors using single crystals". Sci. Technol. Adv. Mater. 10 (2): 024314. Bibcode:2009STAdM..10b4314H. doi:10.1088/1468-6996/10/2/024314. PMC   5090444 . PMID   27877287.
  43. ↑ Yoshiro Yamashita (2009). "Organic semiconductors for organic field-effect transistors". Sci. Technol. Adv. Mater. 10 (2): 024313. Bibcode:2009STAdM..10b4313Y. doi:10.1088/1468-6996/10/2/024313. PMC   5090443 . PMID   27877286.
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