PAHs are formed through the fusion of benzene rings, leading to an extended delocalised pi-electron system.
To be aromatic a molecule must have (4n + 2) delocalised pi electrons, where n = 0, 1, 2, 3...
Astrochemists tend to be a bit naughty and give the label PAH to anything that resembles the true PAHs! For example, fluorene (C13H10) is not aromatic, but is often called a PAH. Generally, when the term PAH is used in astrochemistry, it refers to PAH-like material and is not used in its strictest chemical sense.
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