Effects of ring geometry on triplet and singlet energies of the metal mesoporphyrins.

Abstract

Correlations were found between the hole size of 10 metalloporphyrins, determined by x-ray crystallography, and the ionic radius of the central metal atom, derived from a new compilation of ionic radii as a function of coordination number, spin state, and valence. These correlations were used to define representative porphyrin geometries for each of three different hole sizes. We recommend the use of these geometries for future molecular-orbital calculations. Molecular orbital calculations based on these porphyrin geometries predict a decrease in the phosphorescence energy as the hole size increases, a prediction in agreement with data in the literature on metal mesoporphyrins. Experimental excitation energy of the singlet Soret (but not the visible) band also decreases with increasing hole size, a result not given by calculations using either four-orbital or extensive configuration-interaction models. Electronegativity has a statistically significant effect on the lowest excited singlet and, to a lesser extent, the lowest triplet energy. Changes in hole size produce little change in calculated bond order, indicating that structural changes induced by increased hole size are due to strain induced in sigma bonds. There is no correlation between either singlet or triplet energy and the charge density of the central metal atom as determined by previous molecular-orbital calculations. Thus, the primary effect of changing metals on the lowest triplet state of metal porphyrins is due to effects on the pi electrons of the geometric distortion of the porphyrin ring imposed by the steric requirements of the central metal atom. The effect of the central metal on the singlet energies remains to be fully explained.