Coronene (also known as superbenzene and cyclobenzene) is a

polycyclic aromatic hydrocarbon A polycyclic aromatic hydrocarbon (PAH) is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. ...

(PAH) comprising seven peri-fused

benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...

rings. Its chemical formula is . It is a yellow material that dissolves in common solvents including

toluene Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon. It is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) ...

, and

dichloromethane Dichloromethane (DCM or methylene chloride, methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odour is widely used as a solvent. Although it is not miscible wit ...

. Its solutions emit blue light

fluorescence Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...

under

UV light Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...

. It has been used as a solvent probe, similar to

pyrene Pyrene is a polycyclic aromatic hydrocarbon (PAH) consisting of four fused benzene rings, resulting in a flat aromatic system. The chemical formula is . This yellow solid is the smallest peri-fused PAH (one where the rings are fused through mo ...

. The compound is of theoretical interest to organic chemists because of its

aromaticity In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...

. It can be described by 20

resonance structures In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or ''forms'', also variously known as ''resonance structures'' or '' ...

or by a set of three mobile Clar sextets. In the Clar sextet case, the most stable structure for coronene has only the three isolated outer sextets as fully aromatic although superaromaticity would still be possible when these sextets are able to migrate into the next ring.

Occurrence and synthesis

Coronene occurs naturally as the very rare

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

carpathite, which is characterized by flakes of pure coronene embedded in sedimentary rock. This mineral may be created from ancient hydrothermal vent activity. In earlier times this mineral was also called karpatite or pendletonite. The presence of coronene putatively formed from contact of magma with fossil fuel deposits has been used to argue that the Permian-Triassic “ Great Dying” event was caused by a greenhouse gas warming episode triggered by large-scale Siberian vulcanism. Coronene is produced in the petroleum-refining process of

hydrocracking In petrochemistry, petroleum geology and organic chemistry, cracking is the process whereby complex organic molecules such as kerogens or long-chain hydrocarbons are broken down into simpler molecules such as light hydrocarbons, by the breaking o ...

, where it can dimerize to a fifteen ring PAH, trivially named " dicoronylene" (formally named benzo 0,11henanthro ',3',4',5',6':4,5,6,7hryseno ,2,3-bcoronene or benzo ,2,3-bc:4,5,6-b'c'icoronene). Centimeter-long crystals can be grown from a supersaturated solution of the molecules in toluene (ca. 2.5 mg/ml), which is slowly cooled (ca. 0.04 K/min) from 328 K to 298 K over a period of 12 hours.

Structure

Coronene is a planar

circulene A circulene is a macrocyclic arene in which a central polygon is surrounded and fused by benzenoids. Nomenclature within this class of molecules is based on the number of benzene rings surrounding the core, which is equivalent to the size of the ...

. It forms needle-like crystals with a monoclinic, herringbone-like structure. The most common polymorph is γ, but β form can also be produced in an applied magnetic field (ca. 1 Tesla) or by phase transition from γ decreasing the temperature below 158 K. The structure containing two C-H groups on one benzene ring, so-called DUO, was analyzed by infrared spectroscopy. File:Coronene crystals luminescence.jpg, Crystals of β and γ coronene under daylight (left) and UV light (right).

Other uses

Coronene has been used in the synthesis of

graphene Graphene () is an allotrope of carbon consisting of a Single-layer materials, single layer of atoms arranged in a hexagonal lattice nanostructure.

. For example, coronene molecules evaporated onto a copper surface at 1000 degrees Celsius will form a graphene lattice which can then be transferred onto another substrate.

References

{{PAHs Polycyclic aromatic hydrocarbons

Occurrence and synthesis

Structure

Other uses

See also

References