Excited-state energy transfer and ground-state hole/electron hopping in p-phenylene-linked porphyrin dimers

The ground- and excited-state properties of a series of p-phenylene-linked porphyrin dimers have been examined using a variety of static and time-resolved spectroscopic techniques. The dimers consist of a zinc porphyrin and a free base (Fb) porphyrin (ZnFbΦ), two zinc porphyrins (Zn₂Φ), or two Fb porphyrins (Fb₂Φ). In each array, the porphyrins are joined by the p-phenylene linker at one meso position, with the nonlinking meso positions bearing mesityl groups. Three analogous dimers in which the mesityl groups are replaced with pentafluorophenyl groups (F₃₀ZnFbΦ, F₃₀Zn₂Φ and F₃₀Fb₂Φ) were also synthesized and characterized. The excited-state energy-transfer rate from the photoexcited Zn porphyrin to the Fb porphyrin is (3.5 ps)^-1 for ZnFbΦ and (10 ps)^-1 for F₃₀ZnFbΦ. The quantum yields of excited-state energy transfer are ≥99% for both complexes. The energy-transfer rates in the p-phenylene-linked dimers are considerably faster than those observed for the analogous dimers containing a diphenylethyne linker ((24 ps)^-1, ZnFbU; (240 ps)^-1, F₃₀ZnFbU). At these distances, both through bond and through space contributions to the electronic coupling are important. The faster energy-transfer rates in the p-phenylene- versus diarylethyne-linked dimers are attributed to enhanced electronic coupling between the porphyrins in the former dimers arising primarily from the shorter inter-porphyrin separation. The electronic coupling in the p-phenylene-linked dimers is sufficient to support ultrafast energy transfer in both ZnFbΦ and F₃₀ZnFbΦ, but is not so large as to significantly perturb the redox or inherent lowest excited-state photophysical properties of the porphyrin constituents. Electronic perturbations resulting from fluorination have little effect on the energy-transfer rates in the p-phenylene-linked dimers, but the rates of room-temperature ground-state hole/electron hopping processes in the corresponding monocation radicals of the bis-Zn-analogues of the p-phenylene-linked dimers (≥(0.05 μ)^-1, [Zn₂Φ]⁺; ≤(2.5 μs)^-11, [F₃₀Zn₂Φ]⁺) are significantly influenced by the fluorination-induced changes in the electronic structure. Collectively, these characteristics make these constructs attractive candidates for incorporation into extended multi-porphyrin arrays for a variety of molecular photonics applications.