Structural tuning of ligand-based two-electron intervalence charge transfer

The lowest-energy optical transition of two-electron-oxidized porphyrinogens [(LM)-M-Delta] is a ligand-based charge transfer. The color of the intermediary, two-electron mixed-valent oxidation state shifts from vermilion (lambda(max) = 480 nm) to yellow (lambda(max) = 270 nm) upon increasing the ionic radius of the central metal dication from Mg2+ to Zn2+ and Ca2+. Structural, spectroscopic, and computational studies establish that the relative energies of the highest occupied and lowest unoccupied orbitals, between which the intervalence charge-transfer optical transition occurs, are modulated by the molecular dipole moment, which in turn depends on the only structural variable among the [(LM)-M-Delta] compounds, the position of M2+ relative to the dianionic dipyrrole unit.