On the O2 binding of Fe-porphyrin, Fe-porphycene, and Fe-corrphycene complexes.

Based on our previous study for the O2 binding of the Fe-Por complex, this study investigates the O2 binding mechanism in the Fe-porphyrin isomers, Fe-porphycene (FePc), and Fe-corrphycene (FeCor) complexes. By calculating the potential energy surface of the O2 binding, the present study explains the reason for the dramatic increase of O2 affinities observed in the FePc complex. In the case of FeCor-O2, the O2 binding process includes the intersystem crossing from a triplet to singlet state, as in the FePor-O2 complex. However, FePc-O2 uses only a singlet surface. This is because the ground state of the FePc complex in the deoxy state is a triplet state, while those of FePor and FeCor are a quintet state. Such difference originates from character of the SOMO. We estimated an equilibrium constant for the O2 binding that reasonably reproduced the trend observed in the experiments.