The Extension of Baird's Rule to Twisted Heteroannulenes: Aromaticity Reversal of Singly and Doubly Twisted Molecular Systems in the Lowest Triplet State.

We have investigated the aromaticity of singly twisted Möbius aromatic and doubly twisted Hückel antiaromatic bis(palladium(II)) [36]octaphyrins in the lowest triplet state (T ₁ ) by spectroscopic measurements and quantum calculations. The T ₁ state of the singly twisted Möbius [36]octaphyrin shows broad and weak absorption spectral features that are analogous to those of antiaromatic expanded porphyrins while the T ₁ state of the doubly twisted Hückel [36]octaphyrin exhibits intense and distinct spectral features, indicating the aromatic nature. These results along with theoretical calculations support the hypothesis that the aromaticity is reversed in the T ₁ state. Furthermore, we show that the degree of structural smoothness affects the aromaticity reversal in the T ₁ state.
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