Interpretation of the electronic absorption spectrum of free-base porphin using time-dependent density-functional theory

The electronic absorption spectrum of free-base porphin has been studied at density-functional theory (DFT) level using the time-dependent perturbation theory approach. The optimization of the molecular structure was carried out using the Becke–Perdew functional and split-valence quality basis sets augmented by polarization functions. In the calculation of the electronic excitation energies, the same functional was employed while the basis set was further augmented by diffuse s, p, and d functions. The calculated absorption spectrum agrees with recent ab initio and DFT calculations and with experiment. The B-band must be assigned to the strong 3 1B2u and the 3 1B3u transitions. Two additional weak transitions (2 1B2u and 2 1B3u) are found below the B band. However, these states have not been obtained at CASSCF (CASPT2) and DFT/MRCI levels and might be an artifact of the functional used. The lowest symmetry-allowed triplet–triplet transition is calculated to be 1.58 eV as compared to the experimental value of 1.56–1.58 eV.