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 Mobius aromatic and doubly twisted Huckel 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 Mobius [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 Huckel [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.