A heterocyclic compound formally derived from an arene by replacement of one or more methine (-C=) and/or vinylene (-CH=CH-) groups by trivalent or divalent heteroatoms, respectively, in such a way as to maintain the continuous pi-electron system characteristic of aromatic systems and a number of out-of-plane pi-electrons corresponding to the Hueckel rule (4n+2). [ ANON:ANON ]