Polybenzenoid Hydrocarbons in the S1 State: Simple Structural Motifs Predict Electronic Properties and (Anti)aromaticity

Polybenzenoid hydrocarbons (PBHs) are widely studied for their semiconductive properties and potential applications in organic electronics and photochemistry. Understanding their behavior in excited states is crucial for optimizing their performance in these applications. Here, we computationally investigate a dataset of 43 unbranched cata-condensed PBHs in their first singlet excited state (S₁), revealing clear correlations between molecular structure and electronic properties. By analyzing these molecules through their annulation patterns—specifically the arrangement of linear (L) and angular (A) tricyclic subunits and tetracyclic zigzag (Z) and curve (C) motifs—we establish a predictive hierarchy (L > Z > C > A) for the location of unpaired electrons and Baird-antiaromaticity. This structural approach enables semiquantitative prediction of key properties, including excitation energies, magnetic response, and singlet fission capability. Notably, we find that singlet fission propensity is dependent on both the length of the Longest L sequence and the position of the L motifs within the sequence. These insights, derived from the analysis of small tri- and tetracyclic components and validated on larger systems, provide a practical framework for understanding and designing PBH-based materials.