Design and synthesis of light-harvesting rods for intrinsic rectification of the migration of excited-state energy and ground-state holes

We present the design of molecular materials for ultimate use in solid-state solar cells. The molecular materials are semi-rigid oligomeric rods of defined length with metalloporphyrins in the backbone and a carboxy group at one end for attachment to a surface. The rods are designed to absorb visible light, and then undergo excited-state energy transfer and ground-state hole transfer in opposite directions along the length of the rod. The rational synthesis of the multiporphyrin arrays relies on joining porphyrin building blocks in an efficient and controlled manner. Several porphyrin building blocks have been synthesized that bear bromophenyl, iodophenyl, trimethylsilylethynylphenyl and/or ethynylphenyl substituents for use in a copper-free Sonogashira reaction using Pd-2 (dba)(3) and P(o-tol)(3). Competition experiments performed on equimolar quantities of an iodoporphyrin and a bromo-porphyrin with an ethynyl-porphyrin show iodo 1 ethyne coupling with a low amount (35 degreesC) or undetectable amount (22 degreesC) of bromo + ethyne coupling. Efficient coupling of bromo-porphyrins with ethynyl-porphyrins was achieved using the same copper-free Sonogashira reaction conditions at higher temperature (50 degreesC or 80 degreesC). These findings allow successive coupling reactions to be achieved using substrates bearing iodo and bromo synthetic handles. Thus, a porphyrin-based tetrad (or pentad) was synthesized with a final convergent coupling of a bromo-substituted dyad (or triad) and an ethynyl-substituted dyad. A porphyrin triad was prepared by sequential iodo 1 ethyne coupling reactions. The triad, tetrad, and pentad each are comprised of a terminal magnesium porphyrin bearing one carboxy group (for surface attachment) and two pentafluorophenyl groups; the remaining porphyrins in each array are present as the zinc chelate. Electrochemical characterization of benchmark porphyrins indicates the presence of the desired electrochemical gradient for hole hopping in the arrays. Static absorption data indicate that the arrays are weakly coupled, while static fluorescence data indicate that the excited-state energy flows in high yield to the terminal magnesium porphyrin. Time-resolved spectroscopic analysis leads to rate constants in THF of (9 ps)(-1), (15 ps)(-1), and (30 ps)(-1) for ZnMg dyad 20, Zn2Mg triad 13, and Zn3Mg tetrad 15, respectively, and quantum efficiencies greater than or equal to 99% for energy flow to the magnesium porphyrin in each case. These design and synthesis strategies should be useful for the construction of materials for molecular-based solar cells.