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 (T₁) by spectroscopic measurements and quantum calculations. The T₁ 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 T₁ 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 T₁ state. Furthermore, we show that the degree of structural smoothness affects the aromaticity reversal in the T₁ state.