Closed shell metalloporphyrins

The rates of radiationless transitions between electronic states of porphyrins and their derivatives play a dominant role in their photochemistry because they are the major decay channels of the electronically excited states. Radiative channels, such as fluorescence, rarely exceed 10% of the overall decay rate constant at room temperature. The lifetimes of the lowest electronic states of free-base porph3nins and closed-shell metalloporphyrins vary by more than 10 orders of magnitude with the nature of the substituents. The understanding of such variations is essential to design and control the photochemistry of porphyrins and justifies an incursion on the fundamentals of radiationless transitions. 

Closed shell metalloporphyrins such as those having divalent ions (e.g., magnesium or zinc) readily undergo oxidation to radical cations/ These closed shell ionic metalloporphyrins can be considered to be porphyrin dianions, and therefore they are oxidized more readily than are the free porphyrins themselves. The redox potentials of metalloporphyrins increase with increasing electronegativity of the metal ion. Metalloporphyrins with trivalent or tetravalent metal ions are particularly difficult to oxidize because of the high electropositive character of the metal center. 

Emission. The fluorescence properties of metalloporphyrin and porphyrin-like compounds depend on the character of central metal ion. Fluorescence of porphyrins is generally observed from the free-base or closed-shell metal complexes. The... 

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