Triplet State Baird-Aromaticity in Macrocycles: Scope, Limitations and Complications

The aromaticity of cyclic 4np-electron molecules in their first pp* triplet state (T₁), labelled Baird-aromaticity, has gained growing attention in the last decade. Here we explore computationally the limitations of T₁ state Baird-aromaticity in macrocyclic compounds, [n]CM’s, which are cyclic oligomers of four different monocycles (M = para-phenylene (PP), 2,5-linked furan (FU), 1,4-linked cyclohexa-1,3-diene (CHD), and 1,4-linked cyclopentadiene (CPD)). We strive for conclusions that are general for various DFT functionals, although for macrocycles with up to 20 p-electrons in their main conjugation paths we find that for their T₁ states single-point energies at both canonical UCCSD(T) and approximative DLPNO-UCCSD(T) levels are lowest when based on UB3LYP over UM06-2X and UCAM-B3LYP geometries. This finding is in contrast to what has earlier been observed for the electronic ground state of expanded porphyrins. Yet, irrespective of functional, macrocycles with 2,5-linked furans ([n]CFU’s) retain Baird-aromaticity until larger n than those composed of the other three monocycles. Also, when based on geometric, electronic and energetic aspects of aromaticity, a ³[n]CFU with a specific n is more strongly Baird-aromatic than the analogous ³[n]CPP while the magnetic indices tell the opposite. To construct large T₁ state Baird-aromatic [n]CM’s the design should be such that the T₁ state Baird-aromaticity of the macrocyclic perimeter dominates over a situation with local closed-shell Hückel-aromaticity of one or a few monocycles and semi-localized triplet diradical character. Monomers with lower Hückel-aromaticity in S₀ than benzene (e.g., furan) that do not impose steric congestion are preferred. Structural confinement imposed by, e.g., methylene bridges is also an approach to larger Baird-aromatic macrocycles. Finally, by using the Zilberg-Haas description of T₁ state aromaticity we reveal the analogy to the Hückel-aromaticity of the corresponding closed-shell dications, yet, observe stronger Hückel-aromaticity in the macrocyclic dications than Baird-aromaticity in the T₁ states of the neutral macrocycles.