Structural Tuning of Ligand-Based Two-Electron Intervalence Charge Transfer

The lowest-energy optical transition of two-electron-oxidized porphyrinogens [LΔM] is a ligand-based charge transfer. The color of
the intermediary, two-electron mixed-valent oxidation state shifts
from vermilion (λmax = 480 nm) to yellow (λmax = 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 [LΔM] compounds, the position of
M2+ relative to the dianionic dipyrrole unit.