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

The first example of a porphyrin‐quinonoid donor–acceptor triad featuring (tetraphenylporphinato)zinc(II) moieties covalently attached to an oxoporphyrinogen through its macrocyclic nitrogen atoms is reported. This arrangement of chromophores 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 quenching 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)