Magnetic circular dichroism of phthalocyanine (M = Mg, Zn) and tetraazaporphyrin (M = Mg, Zn, Ni) metal complexes. A computational study based on time-dependent density functional theory

We present here simulated magnetic circular dichroism (MCD) spectra of MTAP (M = Mg, Ni, Zn) and MPc (M = Mg, Zn) where TAP = tetraazaporphyrin and Pc = phthalocyanine. The study is based on magnetically perturbed time-dependent density functional theory (MP-TDDFT) and a newly implemented method for the calculation of A and B terms from the theory of MCD. It follows from our investigation that the MCD spectrum for the MTAP and MPc systems in the Q-band region consists of a single positive A term augmented by a positive B term, in agreement with experiment where available. The Q band can be fully characterized in terms of the 2a(1u) -> 2e(g) one-electron excitation. For the aza systems MgTAP and ZnTAP, the simulated MCD spectra in the Soret region are dominated by the two one-electron excitations 2a(2u) -> 2e(g) and 1a(2u) -> 2e(g) and has the appearance of a positive A term (with values between 1.33-1.55, depending on the MTAP system) made asymmetric by a negative B term, in good agreement with experiment. We find, in agreement with all available experimental findings on MPc (M = Mg, Zn) type systems, that the MCD spectra in the Soret region are dominated by two transitions with positive A/D-term values and two negative B/D-term values. The major contribution to the two transitions comes from the 2a(2u) -> 2e(g) and 1a(2u) -> 2e(g) one-electron excitations. It appears that, the ratio of A/B for the term parameters is underestimated by theory.