Pyrene is a polycyclic aromatic hydrocarbon (PAH) with the formula . Consisting of four fused benzene rings, it in a flat aromatic compound. This colorless compound is the smallest peri-fused PAH (one where the rings are fused through more than one face). Pyrene forms during incomplete combustion of organic compounds.
Occurrence and properties
Pyrene was first isolated from coal tar, where it occurs up to 2% by weight. As a peri-fused PAH, pyrene is much more resonance-stabilized than its five-member-ring containing isomer fluoranthene. Therefore, it is produced in a wide range of combustion conditions. For example, automobiles produce about 1 μg/km.
Reactions
Pyrene contains two kinds of ring subunits: two a-rings with three CH bonds and two b-rings with two CH bonds. The a-rings are more susceptible to reactions with electrophiles and oxidants. The b-rings can be partially hydrogenated to give tetrahydropyrene. Similarly the b-rings can be oxygenated to give the quinone-like derivative
Oxidation with chromate affords perinaphthenone and then naphthalene-1,4,5,8-tetracarboxylic acid. Pyrene undergoes a series of hydrogenation reactions and is susceptible to halogenation, Diels-Alder additions, and nitration, all with varying degrees of selectivity.
Reduction with sodium affords the radical anion. From this anion, a variety of pi-arene complexes can be prepared.
Pyrene and its derivatives are used commercially to make dyes and dye precursors, for example pyranine and naphthalene-1,4,5,8-tetracarboxylic acid.
Photophysics
Pyrene has been described as "one of the most studied organic molecules in terms of its photophysical properties ..., by far, the most frequently applied dye in fluorescence labeled polymers".
It is an electron donor in some donor-acceptor systems. Its potential as a photocatalyst has also been heavily investigated.
Pyrene was the first molecule for which excimer behavior was discovered. Such excimer appears around 450 nm. Theodor Förster reported this in 1954.
Pyrene's fluorescence emission spectrum is very sensitive to solvent polarity.thumb|190px|Diagram showing the numbering and ring fusion locations of pyrene according to [[IUPAC nomenclature of organic chemistry.]]
Safety and environmental factors
Although it is not as problematic as benzopyrene, animal studies have shown pyrene is toxic to the kidneys and liver. It is now known that pyrene affects several living functions in fish and algae.
Its biodegradation has been heavily examined. The process commences with dihydroxylation at each of two kinds of CH=CH linkages. Experiments in pigs show that urinary 1-hydroxypyrene is a metabolite of pyrene, when given orally.
See also
- List of interstellar and circumstellar molecules
- Perhydropyrene
- Fluoranthene, an isomeric PAH