Aromaticity Reversal in the Lowest Excited Triplet State of Archetypical Mobius Heteroannulenic Systems

The aromaticity reversal in the lowest triplet state (T-1) of a comparable set of Huckel/Mobius aromatic metalated expanded porphyrins was explored by optical spectroscopy and quantum calculations. In the absorption spectra, the T-1 states of the Mobius aromatic species showed broad, weak, and ill-defined spectral features with small extinction coefficients, which is in line with typical antiaromatic expanded porphyrins. In combination with quantum calculations, these results indicate that the Mobius aromatic nature of the S-0 state is reversed to Mobius antiaromaticity in the T-1 state. This is the first experimental observation of aromaticity reversal in the T-1 state of Mobius aromatic molecules.