Porphyrins with paramagnetic metal ions

Other paramagnetic ions that can be incorporated in the porphyrin ring include Mn(iii), Fe(iii), Co(ii), Ni(ii), Cu(ii), and Ag(ii). Of these ions Co(ii) is the best shift probe, and Fe(iii) and Mn(iii) the best relaxation probes. This whole series of ion probes was used to study a variety of porphyrin-calTeine complexes. (655) Shift and line-broadening data suggest that the solution structures are of the plane-to-plane type with the metal situated above nitrogen-9 of the caffeine ring. 

Zr(IV), and Ce(IV) as the central metal ion. Copper(II) porphyrins are among the most studied of the paramagnetic metalloporphyrins. The Cu(II) complexes show a low-temperature luminescence that arises from the and states that exist in thermal equilibrium. These two states are derived from the lowest excited triplet state on the porphyrin ring, which is split because of the presence of a unpaired electron on the Cu(II) center. Transient absorption measurements show that the ambient temperature excited-state decay times are lowered when a ligand is associated with the axial coordination positions of the tetracoordinate Cu(Il) porphyrin complex. The excited state lifetimes of Cu(II) porphyrin complexes in solution can be either dependent or independent of the temperature and solvent. For the octaethylporphyrin complex Cu(OEP) the excited state lifetime increases as the temperature is lowered, and also as the solvent polarity is increased. By contrast, the excited state lifetime of the tetraphenylporphyrin Cu(TPP) is insensitive to both the temperature and the polarity of the solvent. This difference in their photophysical behavior is likely due to a difference in the energy gap between the charge transfer state and the T/ T states in the pair of complexes. 

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