Theoretical Study of the Q and B Bands of Free-Base, Magnesium, and Zinc Porphyrins, and Their Derivatives

The multireference Møller-Plesset perturbation (MRMP) theory with complete active space self-consistent field (CASSCF) reference functions is applied to the study of the Q and B bands of free-base, magnesium, and zinc porphyrins and their derivatives. The Q band of free-base porphyrin is so weak because the transition is pseudoparity-forbidden because of the alternancy symmetry. The Q band in the visible region can be intensified if the pairing property is broken. This is achieved by various chemical modifications to the basic structure of free-base porphyrin such as external substituents, changes in conjugation pathway, and change in central substituent. The intensity and color of the Q band as well as the macrocyclic porphyrin skeleton are perturbed to a greater or lesser extent by various chemical modifications. Insertion of aza groups in meso position leads to the shrinking of the size of the central hole, whereas hydrogenation to the exo double bond of pyrrole rings expands the central hole. Substitution of nitrogen atoms for meso carbon atoms significantly intensifies the oscillator strength of the higher energy side transition of the Q band with the peak position unchanged. Ring reduction widens the splitting of two peaks of the Q band, giving rise to the characteristically strong far-red band and the weaker visible band. Changes from metal-free to metalloporphyrins have a less dramatic effect on the spectra.