Energy- and hole-transfer dynamics in oxidized porphyrin dyads

The mechanisms and dynamics of quenching of a photoexcited free base porphynn (Fb^*) covalently linked to a nearby oxidized zinc porphyrin (Zn⁺) have been investigated in a set of five dyads using time-resolved absorption spectroscopy. The dyads include porphyrins joined at the meso-positions by a diphenylethyne linker or a diarylethyne linker with 2,6-dimethyl substitution on either one or both of the aryl rings. Another dyad is linked at the β-pyrrole positions of the porphyrins via a diphenylethyne linker. The type of linker and attachment site modulate the interporphyrin through-bond electronic coupling via stenc hindrance porphyrin-linker orbital overlap) and attachment motif (porphyrin electron density at the connection site). For each ZnFb dyad the zinc porphyrin is selectively electrochemically oxidized (to produce Zn⁺Fb), the free base porphyrin is selectively excited with a 130 fs flash (to produce Zn⁺Fb^*), and the subsequent dynamics monitored The Zn⁺Fb^* excited state has a lifetime of ∼3 to ∼30 ps (depending on the linker stenc hindrance and attachment site) and decays by parallel excited-state energy- and hole-transfer pathways. The relative yields of the two channels depend on a number of factors including the linker-mediated through-bond electronic coupling and a modest (≤20%) Förster through-space contribution for the energy-transfer route. One product of Zn⁺Fb ^* decay is the metastable ground-state ZnFb⁺, which decays to the Zn⁺Fb preflash state by ground-state hole transfer with a linker-dependent rate constant of (20 ps)^-1 to (150 ps) ^-1. Collectively, these results provide a detailed understanding of the mechanism and dynamics of quenching of excited porphyrins by nearby oxidized sites, as well as the dynamics of ground-state hole transfer between nonequivalent porphyrins (Zn and Fb) The findings also lay the foundation for the study of ground-state hole transfer between identical porphyrins (e.g., Zn/Zn, Fb/Fb) in larger multiporphyrin arrays wherein a hole is selectively placed via electrochemical oxidation.