Time-dependent Density Functional Calculations on the Optical Spectra of Metal-porphyrins and Metal-phthalocyanines

The spectroscopic properties of porphyrins and related macrocyles (tetraazaporphyrin, tetrabenzoporphyrin, phthalocyanine) can be calculated very accurately with time-dependent DFT [A. Rosa, G. Ricciardi, E. J. Baerends, S. J. A. van Gisbergen, JPhys Chem. A 105 (2001) 3311; A 105 (2001) 5242]. This leads to unambiguous assignment and resolves existing controversies. A special feature of Kohn-Sham DFT is the “physical” nature of the orbitals, occupied as well as virtual. This leads to an interpretation of the spectra with the same simplicity as the elementary (extended) Hückel models, but now corresponding to very accurate excitation energies. The original interpetation of the spectra in terms of the so-called four-orbital model (Gouterman, Simpson, Platt) is reviewed. The basis of the simple explanations for the intensity distribution in the spectrum is an idealized cyclic polyene model, but the actual ring systems consist of four pyrrole rings. This discrepancy is resolved [E. J. Baerends, G. Ricciardi, A. Rosa, S. J. A. van Gisbergen, Coord. Chem. Rev. 230 (2002) 5]