Tuning (Anti)Aromaticity: Variations on the [8]-Circulene Framework

Optoelectronic properties of organic molecules are underpinned by delocalisation and delocalisability of pi-electrons. These properties are sensitive to small changes in electron count, whether achieved by heteroatom substitution or redox chemistry. One measure of the delocalisability of pi-electrons is the current induced by an external magnetic field, which is diagnostic of (anti)aromaticity. The ab initio ipsocentric method is used here to model diverse ring-current patterns in the family of [8]-circulenes based on tetracyclopenta[def,jkl,pqr,vwx]tetraphenylene (TCPTP), in different charge states, with disjoint hetero-atom substitution, and with CC units systematically replaced by BN pairs. Maps calculated at the CHF/CTOCD-DZ2/6-31G** level reveal that these modifications of the TCPTP framework access the full range of possibilities for current from concentric global circulations (typically counter rotating) to full (non-aromatic) localisation. In the ipsocentric approach, induced current density is partitioned into robust orbital contributions that obey selection rules based on orbital symmetry, energy and nodal character. The selection rules are applied here to interpret current-density and exploit insights gained from simpler models to suggest design strategies for fine-tuning of pi-delocalisability (aromaticity and antiaromaticity) in macrocyclic frameworks. Depending on substitution pattern and charge, this single versatile pi framework (tetracyclopenta[def,jkl,pqr,vwx] tetraphenylene (TCPTP)) can host any combination of aromaticity and antiaromaticity, as judged from tropicity of currents, ranging from concentric global circulations of both senses to localised eddies, all with an intuitive orbital explanation. image