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 4(1)B(2u), 4(1)B(3u), and 5(1)B(2u), states are assigned to the L-band and the 7(1)B(3u) 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 Q(x)-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 (PzH(2)) and the commercial pigment phthalocyanine (PcH2) the calculated oscillator strengths of the Q-and B-bands are of comparable size explaining the intense color of PcH2. For the metalloporphyrins with magnesium (PMg) and zinc (PZn), the x- and y-polarized components of the Q- and B-bands collapse, due to the higher D-4h symmetry of the molecules. The calculations reproduce the slight, experimentally observed increase in the oscillator strength of the Q-band and the decrease for the B-band. These effects are ascribed to the electropositive nature of the metals relative to hydrogen. Except for the Q-bands, which are adequately described by the 'four-orbital model,' it is essential to account for excitations outside the four frontier orbitals as well as double and triple excitations for accurate reproduction of experimental data. We compare our results both with experiment and, where available, recent first-principle SAC-CI, MRMP, and TDDFT calculations. Copyright (C) 2001 John Wiley & Sons, Ltd.