Probing ground-state hole transfer between equivalent, electrochemically inaccessible states in multiporphyrin arrays...

A new strategy is described and implemented for determining the rates of hole-transfer between equivalent porphyrins in multiporphyrin architectures. The approach allows access to these rates between sites that are not the most easily oxidized components of the array. The specific architectures investigated with this new strategy are triads consisting of one zinc porphyrin (Zn) and two free base porphyrins (Fb). The triads employ a diphenylethyne linker (ZnFbFbU) and a phenylene linker (ZnFbFbPhi). The zinc porphyrin is selectively oxidized to produce Zn(+)FbFb, the free base porphyrins are excited to produce the excited-state mixture Zn(+)Fb*Fb and Zn(+)FbFb*, and the subsequent dynamics are monitored by ultrafast absorption spectroscopy. The system evolves by a combination of energy- and hole-transfer processes involving (adjacent and nonadjacent) zinc and free base porphyrin constituents that are complete within 100 ps of excitation; the rate constants of many of these processes are derived from prior studies of the oxidized forms of the benchmark dyads (ZnFbU and ZnFbPhi). One of the excited-state decay channels produces the metastable state ZnFbFb(+) that decays to a second metastable state ZnFb(+)Fb by the target hole-transfer process, followed by rapid hole transfer to produce the Zn(+)FbFb thermodynamic ground state of the system. The rate constant for hole transfer between the free base porphyrins in the oxidized ZnFbFb triads is found to be (0.5 ns)(-1) and (0.6 ns)(-1) across phenylene and diphenylethyne linkers, respectively. These rate constants are comparable to those recently measured, using a related but distinct strategy, for ground-state hole transfer between zinc porphyrins in oxidized ZnZnFb triads. The two complementary strategies provide unique approaches for probing hole transfer between equivalent sites in multiporphyrin arrays, with the choice of method being guided by the particular target process and the ease of synthesis of the necessary architectures.