Aromatic - not so antiaromatic redox pair

01 July 2008 - Aromaticity

antiaromaticityMiao2008
Acenes and in particular heteroacenes are in the picture as organic electron-carrier materials. The pyrazine compound on the left recently described by Shaobin Miao et al. (DOI) is a promising candidate. In the solid state the intrastack distance is only 330 picometer compared to 380 pm for the same acene without the alkyne side groups and better pi stacking means better electron mobility.

As a sort of side show the researchers compared the diazine with its dihydroazine cousin which can be obtained by hydrogenation using Wilkinson's catalyst. This compound is unstable and easily converts back to the diazine by oxydation with air.

This redox pair is theoretically also an aromatic (4n+2) - antiaromatic 4n pair (both planar) but it turns out both systems are aromatic with aromaticity reduced in the dihydrodiazine. In fact the HOMO-LUMO band gap for the dihydrodiazene (in silico and based on fluorescence) is actually the higher of the two reflecting higher stability.

This system and the main conclusion (there are limits to antiaromaticity) is comparable to one described in 2007 by the same group (DOI).