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Pyrolysis Products Intrigue

Blasting polycyclic aromatic hydrocarbons gives unusual biphenylene dimer

by Elizabeth K. Wilson
March 23, 2009 | A version of this story appeared in Volume 87, Issue 12

Here's one new way to produce an unexpected isomer: Subject a highly energetic polycyclic aromatic hydrocarbon (PAH) to a flash vacuum photolysis treatment. This result lends insight into the tangle of carbon-rearrangement mechanisms these fused carbon compounds undergo. PAHs are generated by burning fossil fuels and are ubiquitous pollutants on Earth. In space they permeate the interstellar medium. Peter C. Vollhardt of the University of the California Berkeley, William L. Karney of the University of San Francisco, and their colleagues blasted one particular PAH, angular [4]phenylene—a bent molecule composed of alternating four- and six-membered rings—and examined the aftermath (Chem. Commun., DOI: 10.1039/b902648g). The combustion unexpectedly produced an intermediate "biphenylene dimer," an unusual ring of alternating four-, six-, and eight-carbon rings, the smallest unit of a phenylene sheet believed to have potential semiconducting properties. The authors also found coronene, the "superbenzene," composed of a ring of six benzenes. They're hoping to find ways to generate larger phenylene substructures by this method. "This work nicely illustrates how highly strained molecules invariably find their way to more stable structures when given the opportunity to climb out of their local energy wells," says Lawrence T. Scott of Boston College.


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