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

Abstract

The aromaticity of cyclic 4 nπ-electron molecules in their first ππ* triplet state (T ₁), labeled Baird aromaticity, has gained growing attention in the past decade. Here we explore computationally the limitations of T ₁ state Baird aromaticity in macrocyclic compounds, [ n ]CM’s, which are cyclic oligomers of four different monocycles (M = p-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 π-electrons in their main conjugation paths we find that for their T ₁ 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 T ₁ state Baird-aromatic [ n ]CM’s, the design should be such that the T ₁ state Baird aromaticity of the macrocyclic perimeter dominates over a situation with local closed-shell Hückel aromaticity of one or a few monocycles and semilocalized triplet diradical character. Monomers with lower Hückel aromaticity in S ₀ 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 T ₁ 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 T ₁ states of the neutral macrocycles.