Coronene (also known as superbenzene and cyclobenzene) is a polycyclic aromatic hydrocarbon (PAH) comprising seven peri-fused benzene rings. Its chemical formula is C 24H 12. It is a yellow material that dissolves in common solvents including benzene, toluene, and dichloromethane. Its solutions emit blue light fluorescence under UV light. It has been used as a solvent probe, similar to pyrene.
The compound is of theoretical interest to organic chemists because of its aromaticity. It can be described by 20 resonance structures or by a set of three mobile Clar sextets. In the Clar sextet case, the most stable structure for coronene has only three isolated outer sextets as fully aromatic although superaromaticity would still be possible when these sextets are able to migrate into next ring.
Occurrence and synthesis
Carpathite
Coronene occurs naturally as the very rare mineralcarpathite, 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 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, where it can dimerize to a fifteen ring PAH, trivially named "dicoronylene". Centimeter-long crystals can be grown from a supersaturated solution of molecules in toluene (~2.5 mg/ml), which is slowly cooled (~0.04K/min) from 328K (55°C) to 298K (25°C) over a period of 12 hours.
Structure
Crystals of β- and γ-coronene under daylight (left) and UV light (right)
Coronene is a planar circulene. 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 (~1tesla) or by phase transition from γ decreasing the temperature below 158K (−115°C). The structure containing two C–H groups adjacent to each other on one benzene ring, so-called DUO, can be analyzed by infrared spectroscopy due to the out-of-plane bending vibration of C-H.
Coronene has been used in the synthesis of graphene. 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.
Coronene can also be carbonized without metal catalysts such as copper to form carbon materials with SOLO, which includes one C-H group in one benzene ring. Since coronene is symmetrical, carbon materials with controlled structures can be prepared.
See also
Cyclooctadecanonaene, the compound consisting of just the outer ring without the benzene core
↑Wan, Xi; etal. (2013). "Enhanced Performance and Fermi-Level Estimation of Coronene-Derived Graphene Transistors on Self-Assembled Monolayer Modified Substrates in Large Areas". The Journal of Physical Chemistry C. 117 (9). ACS Publications: 4800–4807. doi:10.1021/jp309549z.