ELECTRONIC STRUCTURES AND ABSORPTION SPECTRA Gouterman Four-Orbital Model

Martin Gouterman first proposed the four-orbital model in the 1960s to explain the absorption spectra of porphyrins [17, . According to this theory, the absorption bands in porphyrin systems arise from transitions between two HOMOs and two LUMOs, and it is the identities of the metal center and the substituents on the ring that affect the relative energies of these transitions. The HOMOs were calculated to be an a1u and an a2u orbital, while the LUMOs were calculated to be a degenerate set of eg orbitals (see figure 3.1). Transitions between these orbitals gave rise to two excited states, both of Eu character. Orbital mixing splits these two states in energy, creating a higher energy Eu state with greater oscillator strength, giving rise to the Soret band, and a lower energy Eu state with less oscillator strength, giving rise to the Q-bands.