Antiaromaticity-Aromaticity Interplay in Fused Benzenoid Systems Using Molecular Electrostatic Potential Topology

The phenomenon of antiaromaticity-aromaticity interplay in aromatic-antiaromatic (A-aA)-fused systems is studied using molecular electrostatic potential (MESP) analysis, which clearly brings out the electron-rich pi-regions of molecular systems. Benzene, naphthalene, phenanthrene, and pyrene are the aromatic units and cyclobutadiene and pentalene are the antiaromatic units considered to construct the A-aA-fused systems. The fused system is seen to reduce the antiaromaticity by adopting a configuration containing the least number of localized bonds over antiaromatic moieties. This is clearly observed in 25 isomers of a fused system composed of three naphthalene and two cyclobutadiene units. Denoting the number of pi-bonds in the cyclobutadiene rings by the notation (n, n'), the systems belonging to the class (0, 0) and (2, 2) turn out to be the most and least stable configurations, respectively. The stability of the fused system depends on the empty pi-character of the antiaromatic ring, hence naphthalene and benzene prefer to fuse with cyclobutadiene in a linear and angular fashion, respectively. Generally, a configuration with the maximum number of 'empty' rings (0, 0, 0, ...) is considered to be the most stable for the given A-aA system. The stability and aromatic/antiaromatic character of A-aA-fused systems with pentalene is also interpreted in a similar way. MESP topology, clearly bringing out the distribution of double bonds in the fused systems, leads to a simple interpretation of the aromatic/antiaromatic character of them. Also, it leads to powerful predictions on stable macrocyclic A-aA systems.