Highly Charged Porphyrin Molecular Wires

Successive oxidations, or reductions, of an extended molecular π-system can give rise to significant changes in the electronic structure compared to the neutral or singly charged counterparts. In this work, we investigated highly reduced states of butadiyne-linked porphyrin oligomers with up to nine porphyrin units and one negative charge per porphyrin. CW-EPR and 1H Mims ENDOR spectroscopy were used to probe the proton hyperfine interactions in these polyanions. Both techniques show a continuous narrowing of the spectra with simultaneously increasing charge and oligomer length corresponding to a decrease in the magnitudes of the largest hyperfine interactions. This is consistent with density functional theory calculations that support delocalization of spin density over the entire lengths of the molecules with high degrees of spin polarization and nonuniform spin density distributions. These spin distributions can also be described by a Hubbard model with antiferromagnetic spin coupling between electrons on adjacent sites. Finding systems with coherent wave functions that span the entire length of π-conjugated molecules, up to a length of more than 10 nm, will help identifying suitable candidates capable of coherent electron tunneling through molecular junctions.