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An answer, somewhat, on graphenes versus PAHs February 8, 2009

Posted by fetzthechemist in PAHs.

I had an interesting exchange of emails with Ivan Gutman. He is one of the leading theoreticians on PAHs and their electron structures.

He had a paper with Jerry Ray Dias, Polycyclic Aromatic Compounds, 18, 221-229 (2000), entitled “Cyclic Conjugation in Total Resonance Sextet Benzenoid Hydrocarbons”. Their calculations show that the core pi electrons in aromatic rings of very large PAHs become more graphite-like with more rings. That is they become less localized as Clar sextets and there are no longer filled and empty rings.

The periphery, however, remains PAH in behavior. This explains why Klaus Muellen’s isomers of 60, 78, or even 120 carbons look different from each other. The peripheral conjugation follows the different shapes of the isomers, giving different UV and fluorescence spectra and colors.

This does add an interesting perspective of the so called “graphene ribbons”. If these are very long in one direct, but narrow in the other then it is just one long, thin PAH structure because the periphery is close enough to all parts of the structure. A graphene ribbon is, thus, just a one dimensional electron conduit that behaves oddly because of its dimensionality, not that it is a graphitic structure.



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