The Optical Spectra of NiP, NiPz, NiTBP, and NiPc: Electronic Effects of Meso-tetraaza Substitution and Tetrabenzo Annulation

Time-dependent density functional calculations have been performed on the symmetry and spin-allowed Eu excited states of the nickel tetrapyrrole series, NiP, NiPz, NiTBP, and NiPc. Optical spectra collected in noncoordinating solvents are presented for NiTBP, for the newly synthesized octaethyl nickel porphyrazine, NiOEPz, and for MgOEPz. The theoretical results prove to agree very well with the experimental data, providing an accurate description of the UV−vis spectra. The effects on the optical spectra of introducing in the basic porphyrinic ring, aza bridges, benzo rings, both aza bridges and benzo rings, and a transition metal are highlighted and interpreted on the basis of the electronic structure changes occurring along the series. The following results were found:  (i) The near-degeneracy of the configurations (a1ueg) and (a2ueg) of the four-orbital model leads to strong mixing in NiP. The resulting low-energy state corresponds to the Q band with low intensity due to opposing transition dipoles of the contributing transitions. In NiTBP, NiPz, and NiPc, the degeneracy is lifted, and the lowest transition becomes increasingly purely (a1ueg) with concomitant larger intensities in the Q band, which is most intense in NiPc. (ii) The B band is calculated to correspond in NiP to a strong mixture of the degenerate (a1ueg) and (a2ueg) configurations with parallel transition dipoles, hence a large intensity. In NiPz and NiPc, the B band can no longer be described in terms of the four-orbital model, it has considerable MLCT character in NiPz and corresponds to a more complicated configuration mixing in NiPc. (iii) Transitions involving metal 3d states, either MLCT or LMCT, influence notably the spectrum to the blue of the B band.