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 eITect on the lowest excited singlet and, to a lesser extent, the lowest triplet ener!!:y. 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.

Laser-Desorption Supersonic Jet Spectroscopy of Phthalocyanines

“…to correlate with the hole size (half the average distance between diagonally opposite pyrrole nitrogens) of the macrocycle, and hence, with geometric distortion of the -electron system. It has been shown that the hole size of the porphyrin macrocycle is directly related to the effective ionic radius of the coordinated metal and that the geometry of the macrocycle is affected by the hole size (18). Also, semiempirical molecular orbital calculations (19) have indicated that hole size can influence electronic transition energies.…”

Equilibrium, Photophysical, Photochemical, and Quantum Chemical Examination of Anionic Mercury(II) Mono- and Bisporphyrins

“…For an extensive conjugation, planar structure is preferable. However, in porphyrins geometrical distortion can arise due to overcrowded peripheral substitution and special metalation, too, , because the size of the metal center is one of the most important factors influencing the geometry of porphyrin complexes . Too short of a metal−nitrogen bond (significantly shorter than 2 Å) generally induces ruffle or saddle distortion of the porphyrin ring. ,− Conversely, if the bond is considerably longer than one-half of the diagonal N−N distance in the free-base porphyrin, dome deformation can take place. , This happens if the radius of the metal center exceeds the critical value of about 80 pm, or it does not prefer the square planar coordination.…”

Molecular Structure of Phthalocyaninatotin(II) Studied by Gas-Phase Electron Diffraction and High-Level Quantum Chemical Calculations

“…The Sn atom is forced out of the macrocycle because the Sn(II) atom diameter is about 50 pm larger than the maximum size suitable for the Pc inner cavity. The four N-Sn bonds with an average length of 225.3 (10) pm and a lone pair form a square-pyramidal arrangement about the Sn atom. The Pc ligand described as "saucer-shaped" 14 is composed by four essentially planar isoindole subunits where the outer C atoms deviate slightly from the inner plane.…”