The redox chemistry of copper tetraphenylporphyrin revisited

Mixing Cu(II)Cl₂ with free base tetraphenylporphyrin ((H ₂(TPP)) in dry deaerated 1,2-dichloroethane surprisingly yields a mixture of Cu(II)(TPP) and Cu(III)(TPP). The later is long-lived in the absence of water and slowly decomposes within days to yield Cu(II)(TPP). Furthermore, in the presence of air, a slightly different Cu^III porphyrin species forms, presumably, Cu(III)(TPP)^•+, in which the Q-band is subject to a 10 nm red shift. The new Cu(III)(TPP) and Cu(III)(TPP) ^•+ differ in terms of absorption, fluorescence, electron paramagnetic resonance (EPR), and X-ray photoemission spectroscopy (XPS) when compared to the well-characterized Cu(II)(TPP), Cu(II)(TPP)^•+, and Cu(II)(TPP)²⁺ species.