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

To investigate new architectures for molecular photonics applications, a sh ape-persistent cyclic hexameric architecture (cyclo-Zn(3)Fb(3)U-p/m) has be en 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-substi tuted 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, symmetr y, 108 atoms in the shortest path, and a face-to-face distance of similar t o 35 Angstrom across the cavity. The excited-state lifetime of the Zn porph yrin 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 effici ency of Phi(trans) = 99.2%. This rate is comparable to that in a dimer (ZnF bU-p/m) having an identical linker, but slower than that of a p/p-linked Zn Fb dimer, which has k(trans) = (24 ps)(-1). At ambient temperatures, the ho le/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-star e hole/electron hopping proceed via through-bond mechanisms mediated by the diphenylethyne linker. The origin of the slightly slower energy-transfer r ate, and substantially slower ground-state hole/electron hopping rate, in t he p/m-linked arrays versus the pfa-linked analogues, is attributed primari ly to the larger electron density of the frontier molecular orbitals at the p-versus m-position of the phenyl ring in the diphenylethyne linker. Colle ctively, these results indicate that the site of attachment of the porphyri n 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, cyc lic. tubular) architectures.