Spectroscopic and Quantum Chemical Studies of Excited States of One- and Two-Electron Oxidation Products of a Lutetium Triple-Decker Phthalocyanine Complex

Abstract

Electronic and MCD (magnetic circular dichroism) spectra of dilutetium(III) tris(phthalocyanate), Lu2(Pc)3 (Pc = phthalocyanine), its one-electron oxidation product [Lu2(Pc)3]+, and its two-electron oxidation product [Lu2(Pc)3]2+ are reported. Lu2(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 [Lu2(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 [Lu2(Pc)3]+ in contrast to the two bands in Lu2(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, [Lu2(Pc)3]+ shows a characteristic band for Pc π radicals. Its MCD pattern suggests the presence of a negative A term. The second oxidation product [Lu2(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 [Lu2(Pc)3]+ and [Lu2(Pc)3]2+. Band assignments are discussed using the theoretical excitation energies, oscillator strengths, and MCD A terms.