Supramolecular triad and pentad composed of zinc-porphyrin(s), oxoporphyrinogen, and fullerene(s): Design and electron-transfer studies

By adopting a "covalent-co-ordinate" bonding approach, novel supramolecular pentad and triad molecules composed of zinc-porphyrin(s), fullerene(s), and oxoporphyrinogen redox-/photoactive entities have been constructed, and also characterized by means of spectral and electrochemical techniques. The geometry and electronic structures of the pentad and the triad were deduced by means of DFT calculations. Free-energy calculations suggested that the photoinduced electron/energy transfer from the zinc-porphyrin (ZnP) singlet-excited state to the imidazole modified fullerene (ImC₆₀) acceptor and oxoporphyrinogen (OxP) entities is feasible for both the triad and the pentad. The charge-separation rates (k_CS) determined from picosecond time-resolved emission studies were higher for pentad (C ₆₀Im:ZnP)₂-OxP than for the corresponding triad. C ₆₍)Im:ZnP-OxP. A comparison of the k_CS values previously reported for the covalently linked bis-(zinc-porphyrin)-oxoporphyrinogen triad suggests that employing a fullerene acceptor improves the electron-transfer rates. Nanosecond transient absorption studies provide evidence for the occurrence of electron-transfer processes. Lifetimes of the radical ion pairs (τ_RIP) are in the range of hundreds of nanoseconds, which indicates that there is charge stabilization in the supramolecular systems.