A theoretical treatment of the absorption and emission properties of Cu(II) porphyrin

We examine the electronic spectrum of Cu(II) porphyrin using the intermediate neglect of differential overlap model. The isolated molecule is predicted to have trip-doublet absorption, corresponding to porphyrin triplets, at 9000 and 13900 cm−1. The Q band is calculated at 15900 cm−1, in good agreement with experiment. We find that coordinating with ligands such as pyridine in the fifth position leads to a state in energy below the Q band that is mostly d(z2) → d(x2−y2) but with appreciable a2u(π) → d(x2−y2) character. We postulate that the presence of this state quenches the emission when Cu(II) porphyrins are complexed with strong base ligands.