Excited-state energy flow in phenylene-linked multiporphyrin arrays

The dynamics and pathways for excited-state energy transfer in three dyads and five triads composed of combinations of zinc, magnesium, and free base porphyrins (denoted Zn, Mg, Fb) connected by p-phenylene linkers have been investigated. The processes in the triads include energy transfer between adjacent nonequivalent porphyrins, between adjacent equivalent porphyrins, and between nonadjacent nonequivalent porphyrins using the intervening porphyrin as a superexchange mediator. In the case of the triad ZnZnFbPhi, excitation of the zinc porphyrin (to yield Zn) ultimately leads to production of the excited free base porphyrin (Fb) via the three processes with the derived rate constants as follows: (2.8 ps)(-1) for ZnZn Fb --> ZnZnFb, (4 ps)(-1) for Zn ZnFb left arrow over right arrow ZnZn Fb, and (14 ps)(-1) for Zn ZnFb --> ZnZnFb. These results and those obtained for the other four triads show that energy transfer between nonadjacent sites is significant and is only 5-7-fold slower than between adjacent sites. This same scaling was found previously for arrays joined via diphenylethyne linkers. Simulations of the energy-transfer properties of fictive dodecameric arrays based on the data reported herein show that nonadjacent transfer steps make a significant contribution to the observed performance of such larger molecular architectures. Collectively, these results indicate that energy transfer between nonadjacent sites has important implications for the design of multichromophore arrays for molecular-photonic and solar-energy applications.