Guidelines for Tuning the Excited State Huckel-Baird Hybrid Aromatic Character of Pro-Aromatic Quinoidal Compounds**

Pro-aromatic molecules have higher-energy diradicaloid states that are significantly influenced by resonance structures in which conjugated rings take on Huckel-aromatic character. Recently, it has been argued that there are also pro-aromatic molecules that adopt central units with 4n pi-electron Baird-aromatic character in the T-1 state, although detailed analysis suggests that these compounds are better labelled as T-1 Huckel-Baird hybrid molecules where Huckel-aromaticity dominates. Herein, we consider a series of symmetrically substituted conjugated rings with potential Baird aromaticity in the lowest excited triplet and singlet states. Our computational results allow us to establish general guidelines for the rational design of molecules with excited state Huckel/Baird aromaticity in pro-aromatic quinoidal compounds. We found two main strategies to promote high Baird aromatic character: 1) anionic and small conjugated rings with electron donating groups as substituents and small exocyclic groups with electron withdrawing substituents, or 2) electron deficient conjugated rings with exocyclic electron-donor substitution.

Automated summary

Pro-aromatic molecules have distinct diradicaloid and T-1 states with potential Baird aromaticity, which can be designed using strategies involving electron-donating substituents on small conjugated rings, electron withdrawing substituents on small exocyclic groups, or electron-deficient conjugated rings with exocyclic electron-donor substitution.