Ab initio electronic and optical spectra of free-base porphyrins: The role of electronic correlation.

We present a theoretical investigation of electronic and optical properties of free-base porphyrins based on density functional theory and many-body perturbation theory. The electronic levels of free-base porphine (H2P) and its phenyl derivative, free-base tetraphenylporphyrin (H2TPP) are calculated using the ab initio GW approximation for the self-energy. The approach is found to yield results that compare favorably with the available photoemission spectra. The excitonic nature of the optical peaks is revealed by solving the Bethe–Salpeter equation, which provides an accurate description of the experimental absorption spectra. The lowest triplet transition energies are in good agreement with the measured values. [ABSTRACT FROM AUTHOR]