Theoretical investigation of the molecular and electronic structures and excitation spectra of iron phthalocyanine and its derivatives, FePc and FePcLn (L = Py, CN−; n = 1, 2)

Abstract

The effects of axial ligands on the ground-state geometries, electronic structures and the characteristic optical properties of iron phthalocyanine and its derivatives, FePc and FePcL_n (L = pyridine (Py) and cyanide (CN⁻); n = 1, 2), were investigated using the density functional theory (DFT) method. The geometries of FePc with a triplet spin state and of FePc(Py), FePc(Py)₂, FePc(CN⁻) and FePc(CN⁻)₂ with singlet spin states were optimized under D_4h, C_2v, D_2h, C_4v, and D_4h molecular symmetries, respectively. The highest occupied molecular orbitals (HOMOs) of FePc, FePc(Py), FePc(Py)₂, and FePc(CN⁻) are π-type orbitals, which have no contribution from the p_z atomic orbitals of all nitrogen atoms, whereas the HOMO of FePc(CN⁻)₂ is the 7e_g orbital, which has contributions from the d_xz and the d_yz orbitals of the Fe atom mixing with the π-orbitals of the axial CN⁻ ligands. The time-dependent (TD) DFT method gives many optically allowed excitations for FePc, FePc(Py), FePc(Py)₂, FePc(CN⁻), and FePc(CN⁻)₂ in the UV-VIS region. Our calculated bands corresponded well with the experimental results. In FePc(Py)₂, the metal–ligand charge transfer (MLCT) transitions from the metal d to the axial-ligand π*-type orbitals contributed to the B band region. In FePc(CN⁻)₂, the MLCT transitions from the metal d to the Pc-ring π*-type orbitals contributed mainly to the first B band region, but those from the metal d to the axial-ligand π*-type orbitals did not appear in the energy regions of the Q and B bands. Thus, the axial ligands caused a spectral change in FePc through orbital mixing.