Magnetic ground state and its control in porphyrin-based magnets

Alternating metalloporphyrin-acceptor systems provide a 'laboratory' for study of quasi-one-dimensional magnetic phenomena. For example, the [octaethylporphyrinatomanganate(III)][hexacyanobutadiene], [MnOEP][HCBD], compound is a model alternating classical (spin 2) and quantum (spin 1/2) spin system. While the high temperature susceptibility is well fit to Seiden's model for alternating S = 2 and S = 1/2 chains with antiferromagnetic exchange J/k(B) = 172 K, and g for [MnOEP] = 1.92, the low temperature data reveal a transition to a two-dimensional antiferromagnet, followed by a transition to a three-dimensional canted antiferromagnet The similarly structured [tetraphenyl-porphyrinatomaganate(III)][tetracyanoethenide], [MnTPP][TCNE], also has ferrimagnetic behavior within the chains, though it has ferromagnetic interactions between the chains. We associate the change in sign of interchain exchange (from antiferromagnetic to ferromagnetic) with an increase in interchain separation and a change in the interchain exchange pathways. Subtle disorder, the magnitude of which depends upon the solvent incorporated within the structure, causes cluster glass-like behavior at low temperatures. The roles of the various constituents of these compounds in determining the ferromagnetic or antiferromagnetic nature of the ground state are discussed.