Spectroscopic and quantum chemical studies of excited states of one- and two-electron oxidation products of a lutetium triple-decker phthalocyanine complex

Electronic and MCD (magnetic circular dichroism) spectra of dilutetium(III) tris(phthalocyanate), Lu-2(Pc)(3) (Pc = phthalocyanine), its one-electron oxidation product [Lu-2(Pc)(3)](+), and its two-electron oxidation product [Lu-2(Pc)(3)](2+) are reported. Lu-2(Pc)(3) was prepared by a new synthetic route in which PcLu(CH3COO)(H2O)(2) is used as the starting material. This new route enabled a purification of the triple-decker compound to a level necessary for spectroscopic measurements. Each of the two chemical oxidation steps occurred keeping isosbestic points. The first oxidation product [Lu-2(Pc)(3)](+) shows a relatively intense band in the near-IR region. This is assigned to the LUMO-to-SOMO (singly occupied orbital) transition. The band has a transition moment perpendicular to the Pc planes. A single intense band was observed in the Q-band region in [Lu-2(Pc)(3)](+) in contrast to the two bands in Lu-2(Pc)(3). Between the Q band and the near-IR band, several weak bands are observed. Among them, two bands show clear MCD A-term dispersions. In the region between the Q band and the B band, [Lu-2(Pc)(3)](+) shows a characteristic band for Pc pi radicals. Its MCD pattern suggests the presence of a negative A term. The second oxidation product [Lu-2(PC)(3)](2+) displays an intense band with two distinct vibronic bands in the near-IR region. None of the bands in the region shows an A-term contribution in MCD. The Q band remains single. There is only one band which shows an MCD A term between the near-IR band and the Q band. Semiempirical quantum chemical calculations were carried out for the ground and excited states of [Lu-2(PC)(3)](+) and [Lu-2(PC)(3)](2+). Band assignments are discussed using the theoretical excitation energies, oscillator strengths, and MCD A terms.