Template-directed synthesis, excited-state photodynamics, and electronic communication in a hexameric wheel of porphyrins

To investigate new architectures for molecular photonics applications, a shape-persistent cyclic hexameric architecture (cyclo-Zn(3)Fb(3)U-p/m) has been prepared that is comprised of three free base (Fb) porphyrins and three zinc porphyrins: linked at the mesa-positions via diphenylethyne units. The synthesis involves the Pd-mediated coupling of a p/p-substituted diethynyl Zn porphyrin and a m/m-substituted diiodo Fb porphyrin, forming p/m-substituted diphenylethyne linkages. The isolated yield of cyclo-Zn(3)Fb3U-p/m is 5.3% in the presence of a tripyridyl template. The array has C-3v, symmetry, 108 atoms in the shortest path, and a face-to-face distance of similar to 35 Angstrom across the cavity. The excited-state lifetime of the Zn porphyrin in cyclo-Zn(3)Fb(3)U-p/m is 17 ps, giving a rate of energy transfer to each adjacent Fb, porphyrin of k(trans) = (34 ps)(-1) and a quantum efficiency of Phi(trans) = 99.2%. This rate is comparable to that in a dimer (ZnFbU-p/m) having an identical linker, but slower than that of a p/p-linked ZnFb dimer, which has k(trans) = (24 ps)(-1). At ambient temperatures, the hole/electron hopping rate in [cyclo-Zn6U-p/m](+) is comparable to or faster than the EPR time scale (similar to 4 MHz). The hole/electron hopping rate in [cyclo-Zn6U-p/m](+) appears to be more than 2-fold larger than for [Zn2U-p/m](+); [Zn2U-p/m](+) has a rate at least 10-fold slower than for the p/p-linked dimer [Zn2U](+). Both excited state energy transfer and ground-stare hole/electron hopping proceed via through-bond mechanisms mediated by the diphenylethyne linker. The origin of the slightly slower energy-transfer rate, and substantially slower ground-state hole/electron hopping rate, in the p/m-linked arrays versus the pfa-linked analogues, is attributed primarily to the larger electron density of the frontier molecular orbitals at the p-versus m-position of the phenyl ring in the diphenylethyne linker. Collectively, these results indicate that the site of attachment of the porphyrin to the linker could be used to direct energy and/or hole/electron flow in a controlled manner among porphyrins in diverse 3-dimensional (linear, cyclic. tubular) architectures.