DFT/MRCI calculations on the excited states of porphyrin, hydroporphyrins, tetrazaporphyrins and metalloporphyrins

A combination of density functional theory and multi-reference configuration interaction methods (DFT/MRCI) has been applied to the calculation of electronic absorption spectra in a series of porphyrin-type molecules. The calculated excitation energies and oscillator strengths for free-base porphyrin (PH2) are in excellent agreement with experiment for both lower and higher excited states which are characterized by a significant contribution of double excitations (>20%). The 41B2u, 41B3u, and 51B2u states are assigned to the L-band and the 71B3u state to the M-band. The results for the hydroporphyrins chlorin (CH2) and bacteriochlorin (BH2) are in agreement with the experimentally observed increase in intensity for the Q-bands relative to PH2. For BH2 we predict a red shift of the Qx-band (0.2 eV) and a blue shift of the B-band (0.5–0.7 eV) in comparison to both PH2 and CH2. For porphyrazine (PzH2) and the commercial pigment phthalocyanine (PcH2) the calculated oscillator strengths of the Q- and B-bands a...