Exploitation of Baird Aromaticity and Clar’s Rule for Tuning the Triplet Energies of Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAH) are a prominent substance class with variety of applications in molecular materials science. Their electronic properties crucially depend on the bond topology ways that often highly non-intuitive. Here, we study, using density functional theory, triplet states four biphenylene-derived PAHs finding dramatically different excitation energies for closely related isomeric structures. These differences rationalised qualitative description Clar sextets and Baird quartets, quantified terms nucleus independent chemical shifts, represented graphically through recently developed method visualising shielding tensors (VIST). The results further interpreted 2D rigid rotor model aromaticity an analysis natural transition orbitals involved excited showing good consistency between viewpoints. We believe this work constitutes important step consolidating these varying viewpoints electronically states.