Explanation of the sign inversion in the magnetic circular dichroism of Mِbius [28] hexaphyrin

Meso-phenyl hexaphyrin is an extended annulene system which can adopt both Hückel and Möbius topologies based on the oxidation state. In this contribution we analyze the remarkable sign inversion of the MCD spectrum which accompanies this topological switch. The treatment is based on the identification of the circular wave assignments of the relevant excited states; the corresponding magnetic moments are calculated using the CASCI (4,4) approach. In this approach wavefunctions are obtained from configuration interaction in the active space of the four Kohn–Sham frontier orbitals of the chromophore. The results reproduce the signs of the MCD spectra. The anomalous sign in the Möbius conformation is attributed to the quenching of the excited state orbital moment by the ring twist.