title: Effects of multiple pathways on excited-state energy flow in self-assembled wheel-and-spoke light-harvesting architectures.
abstract: 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 core-modified (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 ( approximately 40 ps), and is essentially quantitative (>or=98%). Energy transfer from a pyridyl-coordinated zinc porphyrin of the host to the guest in the Zn(6)*DPFbO complex has a yield of approximately 75%, a rate constant of approximately (0.7 ns)(-1), and significant Förster through-space (TS) character. In the case of Zn(5)Fb*DPFbO, which has an additional TS route via the Fb porphyrin with a rate constant of approximately (20 ns)(-1), the yield of energy transfer to the guest is somewhat lower ( approximately 50%) than that for Zn(6)*DPFbO. Complex Zn(4)Fb(2)*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 approximately (0.5 ns)(-1). This complex exhibits an energy-transfer yield to the guest that is significantly enhanced over that for Zn(5)Fb*DPFbO and comparable to that for Zn(6)*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.
authors.affiliation: Department of Chemistry, Washington University, St. Louis, Missouri 63130-4889, USA.
authors.forename: Hee-eun
authors.initials: HE
authors.lastname: Song
authors.forename: Christine
authors.initials: C
authors.lastname: Kirmaier
authors.forename: Jennifer K
authors.initials: JK
authors.lastname: Schwartz
authors.forename: Eve
authors.initials: E
authors.lastname: Hindin
authors.forename: Lianhe
authors.initials: L
authors.lastname: Yu
authors.forename: David F
authors.initials: DF
authors.lastname: Bocian
authors.forename: Jonathan S
authors.initials: JS
authors.lastname: Lindsey
authors.forename: Dewey
authors.initials: D
authors.lastname: Holten