Exceptional electronic modulation of porphyrins through meso-arylethynyl groups. Electronic spectroscopy, electronic structure, and electrochemistry of [5,15-bis](aryl)ethynyl]-10,20-diphenylporphinato]zinc(II) complexes

A new class of porphyrins in which intervening ethynyl moieties connect aryl groups to the porphyrin 5 and 15-positions is described. The synthesis and optical spectroscopy of five new compounds are reported: [5,15-bis[(4'-dimethylaminophenyl)ethynyl]-10,20-diphenylporphinato]zi nc(II), [5,15-bis[(4'-methoxyphenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) , [5,15-bis[(phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II), [5, 15-bis[(4'-fluorophenyl)ethynyl]-10,20-diphenylporphinato]zinc(II), and [5,15-bis[(4'-nitrophenyl)ethynyl]-10,20-diphenylporphinato]zinc(II). The X-ray structure of one of these complexes, [5,15-bis[(4'-fluorophenyl)ethynyl]-10,20-diphenylporphinato]zinc(II), is unusual in that it shows the arylethynyl-phenyl rings to be coplanar with the porphyrin macrocycle. This arrangement of aryl rings with respect to the porphyrin macrocycle underscores the fact that the steric barrier to rotation between the porphyrin and arylethynyl-phenyl aromatic systems has been removed and highlights the enhanced π-conjugative interactions responsible for the unusual electronic characteristics of these molecules. Electronic spectroscopy and electrochemistry of these systems evince the ability of this new porphyrin structural motif to modulate extensively chromophore electronic structure. ZINDO-determined (INDO-1 semiempirical parameters; CI level = 20) frontier orbital energies for the [5,15-bis[(aryl)ethynyl]-10,20-diphenylporphinato]zinc(II) complexes demonstrate that these properties derive from energetic differences between the porphyrin HOMO and HOMO-1 and a substantial splitting of the formerly degenerate porphyrin LUMO; these significant electronic perturbations (relative to simple (porphinato)zinc(II) complexes) are manifest in the fact that the low lying singlet excited states of these compounds are highly polarized.