title: Directly meso-meso linked porphyrin rings: synthesis, characterization, and efficient excitation energy hopping.
abstract: Directly meso-meso linked porphyrin rings CZ4, CZ6, and CZ8 that respectively comprise four, six, and eight porphyrins have been synthesized in a stepwise manner from a 5,10-diaryl zinc(II) porphyrin building block. Symmetric cyclic structures have been indicated by their very simple (1)H NMR spectra that exhibit only a single set of porphyrin and their absorption spectra that display a characteristic broad nonsplit Soret band around 460 nm. Energy minimized structures calculated at the B3LYP/6-31G* level indicate that a dihedral angle between neighboring porphyrins decreases in order of CZ6 > CZ8 > CZ4, which is consistent with the (1)H NMR data. Photophysical properties of these molecules have been examined by the steady-state absorption, fluorescence, fluorescence lifetime, fluorescence anisotropy decay, and transient absorption measurements. Both the pump-power dependence on the femtosecond transient absorption and the transient absorption anisotropy decay profiles are directly related with the excitation energy migration processes within the porphyrin rings, where the exciton-exciton annihilation time and the polarization anisotropy rise time are well described in terms of the Forster-type incoherent energy hopping model. Consequently, the excitation energy hopping rates have been estimated for CZ4 (119 +/- 2 fs)(-)(1), CZ6 (342 +/- 59 fs)(-)(1), and CZ8 (236 +/- 31 fs)(-)(1), which reflect the magnitude of the electronic coupling between the neighboring porphyrins. Overall, these porphyrin rings serve as a well-defined wheel-shaped light harvesting antenna model in light of very efficient excitation energy hopping along the ring.
authors.affiliation: Department of Chemistry, Graduate School of Science, Kyoto University, and Core Research for Evolutional Science and Technology, Japan Science and Technology Agency, Sakyo-ku, Kyoto 606-8502, Japan.
authors.forename: Yasuyuki
authors.initials: Y
authors.lastname: Nakamura
authors.forename: In-Wook
authors.initials: IW
authors.lastname: Hwang
authors.forename: Naoki
authors.initials: N
authors.lastname: Aratani
authors.forename: Tae Kyu
authors.initials: TK
authors.lastname: Ahn
authors.forename: Dah Mee
authors.initials: DM
authors.lastname: Ko
authors.forename: Akihiko
authors.initials: A
authors.lastname: Takagi
authors.forename: Tomoji
authors.initials: T
authors.lastname: Kawai
authors.forename: Takuya
authors.initials: T
authors.lastname: Matsumoto
authors.forename: Dongho
authors.initials: D
authors.lastname: Kim
authors.forename: Atsuhiro
authors.initials: A
authors.lastname: Osuka