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 s ymmetry and spin-allowed E, excited states of the nickel tetrapyrrole serie s, NiP, NiPz, NiTBP, and NiPc. Optical spectra collected in noncoordinating solvents are presented for NiTBP, for the newly synthesized octaethyl nick el porphyrazine, NiOEPz, and for MgOEPz. The theoretical results prove to a gree very well with the experimental data, providing an accurate descriptio n of the W-vis spectra. The effects on the optical spectra of introducing i n the basic porphyrinic ring, aza bridges, benzo rings, both aza bridges an d benzo rings, and a transition metal are highlighted and interpreted on th e basis of the electronic structure changes occurring along the;series. The following results were found: (i) The near-degeneracy of the configuration s (a(lu)e(g)) and (a(2u)e(g)) of the four-orbital model leads to strong mix ing in NiP. The resulting low-energy state corresponds to the Q band with l ow intensity due to opposing transition dipoles of the contributing transit ions. In NiTBP, NiPz, and NiPc, the degeneracy is lifted, and the lowest tr ansition becomes increasingly purely (a(lu)e(g)) with concomitant larger in tensities 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 (a(lu )e(g)) and (a(2u)e(g)) configurations with parallel transition dipoles, hen ce a large intensity. In NiPz and NiPc, the B band can no longer be describ ed in terms of the four-orbital model, it has considerable MLCT character i n 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.