Excited states of the lutetium phthalocyanine trimer: Semiempirical molecular orbital and localized orbital study

The electronic structure of the lutetium triple-decker phthalocyanine (Pc) trimer is studied by semiempirical quantum chemical calculation. The study employs a localized orbital (LO) basis set which is obtained by a transformation of the trimer's canonical MOs, such that the orbital populations on a certain ligand are maximum, Characteristic energy differences between LOs on the central Pc ring and those on outer Pc rings are discussed with regard to environmental changes around each Pc ring. Assignment of the electronic absorption bands of the trimer is discussed using the configuration interaction (CI) method with the LO basis, which provides characterization of the excited state in terms of inter- and intraligand excitation. The most intense band in the Q-band region is assigned to an excited state mainly of exciton coupling character, The second intense band can be assigned to a charge resonance state, which obtains spectral intensity from the exciton component. The near-IR bands and the band at higher energy of the most intense Q band are also characterized by combinations of excitations from highest occupied LOs to lowest unoccupied LOs. A comparison between MO basis CI calculation and that of the LO basis is carried out. The effects of the molecular structure distortions (twisting of Pc rings around the C-4 asis and elongation of interplanar distance) on the excited states are discussed.