Effects of multiple pathways on excited-state energy flow in self-assembled wheel-and-spoke light-harvesting architectures

Static and time-resolved optical measurements are reported for three cyclic hexameric porphyrin arrays and their self-assembled complexes with guest chromophores. The hexameric hosts contain zinc porphyrins and 0, 1, or 2 free base (Fb) porphyrins (denoted Zn-6, Zn(5)Fb, or Zn(4)Fb(2), respectively). The guest is a coremodified (O replacing one of the four N atoms) dipyridyl-substituted Fb porphyrin (DPFbO) that coordinates to zinc porphyrins of a host via pyridyl-zinc dative bonding. Each architecture is designed to have a gradient of excited-state energies for excitation funneling among the weakly coupled constituents of the host to the guest. Energy transfer to the lowest-energy chromophore(s) (coordinated zinc porphyrins or Fb porphyrins) within a hexameric host occurs primarily via a through-bond (TB) mechanism, is rapid (similar to 40 ps), and is essentially quantitative (>= 98%). Energy transfer from a pyridyl-coordinated zinc porphyrin of the host to the guest in the Zn-6 center dot DPFbO complex has a yield of similar to 75%, a rate constant of similar to(0.7 ns)(-1), and significant Forster through-space (TS) character. In the case of Zn5F center dot DPFbO, which has an additional TS route via the Fb porphyrin with a rate constant of similar to(20 ns)(-1), the yield of energy transfer to the guest is somewhat lower (similar to 50%) than that for Zn-6 center dot DPFbO. Complex Zn(4)Fb(2)center dot DPFbO has an identical TS pathway via the Fb porphyrin plus an additional TS pathway involving the second Fb porphyrin (closer to the guest) with a rate constant of similar to(0.5 ns)(-1). This complex exhibits an energy-transfer yield to the guest that is significantly enhanced over that for Zn(5)Fb center dot DPFbO and comparable to that for Zn-6 center dot DPFbO. Collectively, the results for the various arrays suggest designs for similar host-guest complexes that are expected to exhibit much more efficient light harvesting and excitation trapping at the central guest chromophore.