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Guidelines for Tuning the Excited State Hückel–Baird Hybrid Aromatic Character of Pro‐Aromatic Quinoidal Compounds **
Author(s) -
Escayola Sílvia,
Tonnelé Claire,
Matito Eduard,
Poater Albert,
Ottosson Henrik,
Solà Miquel,
Casanova David
Publication year - 2021
Publication title -
angewandte chemie
Language(s) - English
Resource type - Journals
eISSN - 1521-3757
pISSN - 0044-8249
DOI - 10.1002/ange.202100261
Subject(s) - aromaticity , conjugated system , chemistry , excited state , molecule , character (mathematics) , singlet state , resonance (particle physics) , computational chemistry , stereochemistry , organic chemistry , physics , atomic physics , geometry , mathematics , polymer
Pro‐aromatic molecules have higher‐energy diradicaloid states that are significantly influenced by resonance structures in which conjugated rings take on Hückel‐aromatic character. Recently, it has been argued that there are also pro‐aromatic molecules that adopt central units with 4 n π‐electron Baird‐aromatic character in the T 1 state, although detailed analysis suggests that these compounds are better labelled as T 1 Hückel–Baird hybrid molecules where Hückel‐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 Hückel/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.