Linker Dependence of Energy and Hole Transfer in Neutral and Oxidized Multiporphyrin Arrays

The excited-state photodynamics of the neutral and one-electron-oxidized forms of five porphyrin dyads were studied in benzoilitrile containing tetrabutylammonium hexafluorophosphate as the supporting electrolyte. Each dyad contains a zinc porphyrin (Zn) and a free base porphyrin (Fb) joined by a linear biphenylene (Phi(2)), terphenylene (Phi(3)), quaterphenylene (Phi(4)), diphenylbutadiyne (L), or phenylethyne (E) linker (ZnFb Phi(2), ZnF Phi(3), ZnF Phi(4), ZnFbL, ZnFbE). The findings along with recent results on the neutral and oxidized forms of ZnFb dyads containing a diphenylethyne or phenylene linker (ZnFbU, ZnFb Phi) and steric hindrance to porphyrin-linker internal rotation at one or both ends of a diarylethyne linker (ZnFbD, ZnFbP, ZnFbB) give insights into the effects of linker characteristics (length, orbital energies, orbital overlap with the porphyrins) on the rate constants for excited-state energy transfer, excited-state hole transfer, and ground-state hole transfer. Analysis of the results is aided by density functional theory molecular orbital calculations and Forster energy-transfer calculations. Although the rate constants for linker-mediated through-bond excited-state energy transfer can be modulated significantly using a number of molecular design criteria (e.g., linker characteristics, interplay between porphyrin orbital characteristics, and linker attachment site), ground-state hole transfer, which also occurs via a linker-mediated through-bond electron-exchange mechanism, is primarily affected by the free-energy driving force for the process as dictated by the redox characteristics of the interacting porphyrins. The insights gained from this study should aid in the design of next-generation multichromophore arrays for solar energy applications.