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

Abstract

<p>The present study aims to provide a solid ground for the identification, characterization and controlled design of pro-aromatic quinoidal organic compounds holding conjugated rings with Hückel and/or Baird (singlet and triplet) excited state aromatic character, and expects to become a reference work for future studies on Baird-aromaticity. Concretely, we explore a wide range of compounds with a central conjugated ring of different sizes and symmetric exocyclic substitutions. We employ a combination of different computational tools and we also introduce a new energy-based approach for the quantification of the Baird-aromatic character. </p> <p> </p> The key findings of this study indicate that Baird aromaticity is achieved in pro-aromatic quinoidal compounds having small anionic rings or in electron deficient rings with the proper exocyclic substitution. Low-lying states of these systems show very strong hole/electron overlaps, implying that their aromaticity cannot be related to intramolecular charge transfer.

Acenes and phenacenes in their lowest-lying triplet states. Does kinked remain more stable than straight?

“…1 Although this property does not have a clear definition, aromaticity is applied in molecular design for the modulation of photoreactivity or in the development of materials for singlet fission, triplet-triplet anhilation or thermally activated delayed fluorescence among others, through the (de)stabilization of ground and excited states. [2][3][4][5][6][7] In the closed-shell singlet ground state, aromaticity obeys a series of rules 8 that allow the characterization of a compound as aromatic, non-aromatic or antiaromatic.…”

Estimation of Singlet Oxygen Quantum Yield Using Novel Green‐Absorbing Baird‐type Aromatic Photosensitizers†

“…However, there have been synthetic efforts to create nonclassical polyaromatic chromophores with individual rings exhibiting either paratropic or diatropic ring current (14). Accordingly, Baird‐type chromophores are globally aromatic, but at least one ring (with a π‐conjugation) within these systems would be anti/non/proaromatic or exhibit paratropic ring current (15). When this condition is met, it is believed that the chromophore of interest would be able to access its triplet state following the Baird rule.…”