Aromaticity and antiaromaticity

Aromatic molecules are cyclic and pi-conjugated, and exhibit some interesting effects. Most well-known nowadays is the behaviour of aromatic molecules in a magnetic field: in an NMR spectrometer, we see that the chemical shifts of protons inside and outside the ring appear (de)shielded. This effect arises from the establishment of a persistent ring current of circulating pi-electrons, which itself generates a molecular magnetic field which opposes the applied field.

What can we learn about the fundamental nature of aromaticity and antiaromaticity? Despite nearly a hundred years of study, aromaticity remains poorly defined. In addition to answers to this fundamental question, we are looking for materials applications of (anti)aromatic materials.