Effect of Two Interacting Rings in Metalloporphyrin Dimers upon Stepwise Oxidations

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.