Perturbing Pentalene: Aromaticity and Antiaromaticity in a Non-alternant Polycyclic Aromatic Hydrocarbon and BN-heteroanalogues.

Pentalene (C8H6) and NN- and BB-bridged heterocyclic analogues (BN)4H6, derived by replacement of CC pairs with BN, are taken as  paradigms for tuning of ring-current (anti)aromaticity by variation of π charge, electronegativity and substitution pattern.  Ab initio calculation of maps for the π current density induced in these model systems by a perpendicular external magnetic field exhibits the full range of tropicity, from diatropic aromatic to nonaromatic to paratropic antiaromatic, with a ready rationalisation in terms of an orbital model.  Further calculations on systems of varying charge in which these motifs are embedded in extended PAH systems with naphthalene and phenanthrene 'clamps' show promise for switching between current patterns and related  opto-electronic properties. Particular sensitivity to charge is found for the experimentally accessible NN-bridged heteropentalene hybrids.