Effect of Two Interacting Rings in Metalloporphyrin Dimers upon Stepwise Oxidations

Abstract

The interaction between two porphyrin macrocycles, connected covalently through either a rigid ethylene or a flexible ethane bridge, in the metalloporphyrin dimers (M: 2H, Zn2+) have been investigated upon stepwise oxidations. Upon 1e-oxidation, two porphyrin macrocycles come closer and cofacial to each other while 2e-oxidation forces them to be separated as far as possible. This has resulted in the conversion of the cis isomer to trans for the ethylene bridged porphyrin dimer with the stabilization of an unusual “U” form, which has unique spectral and geometrical features. Detailed ultraviolet–visible–near-infrared (UV-vis-NIR), infrared (IR), electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) spectroscopic investigations, along with X-ray structure determination of the 2e-oxidized complexes, have demonstrated strong electronic communications between two porphyrin π-cation radicals through the bridging ethylene group. Such extensive π-conjugation also results in strong antiferromagnetic coupling between the radical spins of both of the macrocycles, which generates a diamagnetic compound. The experimental observations are also strongly supported by density functional theory (DFT) calculations.