Pyrene

Pyrene is a polycyclic aromatic hydrocarbon (PAH) consisting of four fused benzene rings, resulting in a large, flat aromatic system. It is the smallest peri-fused polycyclic aromatic hydrocarbon - one where the rings are fused through more than one face. It forms during incomplete combustion of organic material and therefore can be isolated from coal tar along with a broad range of related compounds. As a peri-fused PAH, pyrene is much more resonance stabilized than its five-member-ring containing isomer fluoranthene. Thus, it is produced in a wider range of combustion conditions. Pyrene is a colorless solid. Animal studies have shown pyrene is toxic to the kidneys and the liver.

Pyrene is used commercially to make dyes, pesticides, pharmaceuticals, and plastics. It is also a valuable molecular probe for fluorescence spectroscopy, having a high quantum yield and lifetime (0.65 and 410 nanosecond, respectively, in ethanol at 293K). Its fluorescence emission spectrum is very sensitive to the solvent's polarity, so pyrene has been used as a probe to determine solvent environments. This is due to its excited state having a different, non-planar structure than the ground state. Certain emission bands are unaffected, but others vary in intensity due to the strength of interaction with a solvent.

Pyrene adopts a dimeric structure, with molecules arranged in sandwiched pairs in order to form lattice units. This results in a high degree of symmetry, belonging to the C2h symmetry group. In solution, dimers exhibit high wavelength fluorescence and also excimer formation.