We found that axial ligation of pyridine occurs more efficiently in the excited singlet state than in the ground state for five-coordinated magnesium tetraphenylporphine (MgTPP) at room temperature in methylcyclohexane solution. The rates of association and dissociation of an axial ligand were determined from the decay of the fluorescence. The former process is faster than the latter by a factor of 8.5 in the ligand of 1 M pyridine. The results are interpreted in terms of the changes in electronegativity of the central Mg cation through the metal-porphyrin interaction.
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