I have investigated the lowest triplet state (T1) aromaticity of singly-twisted Möbius aromatic and doubly-twisted Hückel antiaromatic bis(palladium(ll)) octaphyrins using spectroscopic measurements and quantum calculations. In the T1 state absorption spectrum, singly-twisted Möbius octaphyrin showed broad and weak absorption spectral features, which is analogous to those of antiaromatic expanded porphyrins (Fig. 1a). On the other hand, the T1 state absorption spectrum of doubly-twisted Hückel octaphyrin exhibited intense and distinct features, representing aromatic nature (Fig. 1b). The quantum calculations based on magnetic and topological indices support the reversed aromaticity in the T1 state. Furthermore, I have revealed that the degree of structural smoothness affects the aromaticity reversal in the T1 state.
Figure 1. The S0 and T1 absorption spectra of a) MOc and b) HOc in toluene. The asterisks indicate experimental errors that are induced by different spectral resolutions between spectrometers used for ground-state absorption and TA measurement.