A Novel Bis(zinc-porphyrin)-Oxoporphyrinogen Donor-Acceptor Triad:Synthesis,Electrochemical,Computational and Photochemical Studies

摘要

The first example of a porphyrin-quinonoid donor-acceptor triad featuring(tetraphenylporphinato)zinc(II)moieties co-valently attached to an oxoporphyrinogen through its macro-cyclic nitrogen atoms is reported.This arrangement of chro-mophores results in an interesting interplay between the electron-donating zinc-porphyrin(s)and the electron/energy accepting oxoporphyrinogen.The optical absorption of the triad reveals features corresponding to both the donor and acceptor entities.The geometry and electronic structure of the triad deduced from B3LYP/3-21G(*)calculations reveal an absence of inter-chromophoric interactions and localization of the HOMO on one zinc-porphyrin group and the LUMO on the oxoporphyrinogen scaffold.The electrochemical redox states of the triad were established from a comparative electrochemistry of the triad and the reference compounds.Both steady-state and time-resolved emission studies revealed guenching of the singlet excited state of zinc-porphyrin in the triad,and the free-energy calculations performed using Weller's approach indicate the possibility of electron transfer from the singlet excited zinc-porphyrin group to the oxoporphyrinogen in polar solvents.Time-resolved fluorescence studies reveal excited state energy transfer from zinc-porphyrin to oxoporphyrinogen in nonpolar solvents,while nanosecond transient absorption studies combined with time-resolved fluorescence studies in polar solvents are indicative of the occurrence of photoinduced charge separation from the singlet excited zinc-porphyrin to the oxoporphyrinogen.