Flash vacuum thermolysis (FVT, 1000 °C ≥ T ≥ 1200 °C) of acenaphtho[1,2‐a]acenaphthylene (3, C22H12) gave the C22H12 cyclopenta‐fused polycyclic aromatic hydrocarbon (CP‐PAH) acenaphtho[1,2‐e]acenaphthylene (4), cyclopenta[cd]perylene (5) and cyclopenta[def]benzo[hi]chrysene (6). Whereas the formation of 4 is explained by a ring contraction/ring expansion rearrangement of 3, the identification of 5 and 6 suggests that 3 also undergoes homolytic scission of a five‐membered ring's Carbon‐Carbon single bond furnishing the transient diradical intermediate 7. Ring closure of 7to form 8 after rotation around the Carbon‐Carbon single bond of the intact five‐membered ring followed by hydrogen shifts will give 6. The latter can rearrange subsequently into 5by ring contraction/ring expansion. The structural assignment of 4 and 5 was supported by independent FVT of 6,12‐bis(1‐chloroethenyl)chrysene (14) and 3‐(1‐chloroethenyl)perylene (23), respectively. FVT of 14 (900–1200 °C) gave in a consecutive process 6,12‐bis(ethynyl)chrysene (15), 9‐ethynylbenz[j]acephenanthrylene (16) and bis(cyclopenta[hi,qr])chrysene (17). Although at T ≥ 900 °C 17 selectively rearranges into 4 by ring contraction/ring expansion, at 1200 °C the latter is converted into 5 presumably via a diradical intermediate obtained by homolytic scission of a single Carbon‐Carbon bond of a five‐membered ring. FVT of 23 gave in situ 3‐ethynylperylene (25), which at 1000 °C is nearly quantitatively converted into 5. The propensity of internal cyclopenta moieties to undergo homolytic scission of a five‐membered ring′s Carbon‐Carbon single bond was corroborated by independent FVT of benzo[k]‐ (11) and benzo[j]fluoranthene (12). Previously unknown thermal pathways to important (CP)‐PAH combustion effluents are disclosed at T ≥ 1000 °C.

No abstract