Guidelines for Tuning the Excited State Hückel–Baird Hybrid Aromatic Character of Pro‐Aromatic Quinoidal Compounds**

Abstract

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. 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. File list (2) download file view on ChemRxiv manuscript.pdf (5.62 MiB) download file view on ChemRxiv supp_info.pdf (38.90 MiB)

Excited state character of Cibalackrot-type compounds interpreted in terms of Hückel-aromaticity: a rationale for singlet fission chromophore design

“…As Hückel-aromaticity implies a larger number of paired electrons in the aromatic cycle it should, when possible, be preferred over Baird-aromaticity. 41 Hence, care needs to be exercised and one needs to explore what type of aromaticity is at hand in a particular system when in its lowest triplet or singlet excited state. There is otherwise a clear risk of the overuse of the Baird-aromaticity concept to molecules to which it does not apply.…”

“…Now, the value of DFLU ab /FLU claries whether a cycle is Baird-aromatic or Hückel-aromatic because DFLU ab is zero or negligible for a Hückel-aromatic cycle while it has a nonzero value for a Baird-aromatic cycle. 40,41 As references, indole and pyrrole in their closed-shell Hückel-aromatic S 0 states have DFLU ab /FLU values which are exactly zero while the Baird-aromatic indole and pyrrole dications in their triplet states have calculated values of 2.34 and 4.02, respectively (Table S1 †). We now nd that the DFLU ab /FLU ratios of the AB/A 0 B 0 moieties and the B/B 0 rings of T 1 state CIBA are low (À0.180 and À0.186, respectively), supporting an interpretation of the T 1 state in terms of Hückelaromaticity.…”

“…For example, a pp* triplet state can be of Hückel-Baird hybrid type as earlier found for a quinoid compound (TMTQ) in its T 1 state. 40,41 The T 1 state of such a compound has a central unit simultaneously inuenced by both Hückel-aromaticity and Baird-aromaticity. Also, electronically excited states can be described by pure Hückel-type aromaticity, as observed for the charge-transfer state of a 6aminosubstituted fulvene-based molecular motor where a 6pelectron cyclopentadienyl anionic ring makes an important contribution.…”

Retaining Hückel Aromaticity in the Triplet Excited State of Azobenzene

“…[8] Later the influence of Hückel aromaticity in the excited state was thoroughly investigated in the exo-substituted 1,6methano [10] annulene derivatives, establishing guidelines to tune Hückel/Baird aromaticity in the excited state. [9] Hückel aromaticity can be observed in both S 0 and in lowest excited states for certain polycyclic systems by discrete distribution of aromatic character between the rings. [10] For example, in the case of dipyrrolonaphthyridinedione derivatives adaptive aromatic character was claimed with Baird type excited state [5] in an earlier study and was later computationally shown to be of Hückel type.…”

“…Later the influence of Hückel aromaticity in the excited state was thoroughly investigated in the exo‐substituted 1,6‐methano [10] annulene derivatives, establishing guidelines to tune Hückel/Baird aromaticity in the excited state [9] . Hückel aromaticity can be observed in both S 0 and in lowest excited states for certain polycyclic systems by discrete distribution of aromatic character between the rings [10] .…”