Structural control of photoinduced energy transfer between adjacent and distant sites in multiporphyrin arrays

A family of diphenylethyne-linked porphyrin dimers and trimers has been pre pared via a building block approach for studies of energy-transfer processe s. The dimers contain Mg and Zn porphyrins (MgZnU); the trimers contain an additional free base porphyrin (MgZnFbU). In both the dimers and trimers, s ites of attachment to the Mg porphyrin (at the meso- or beta-position) and diphenylethyne linker (at the para- or meta-positions) were varied, produci ng four Mg porphyrin-Zn porphyrin arrangements with the following linker co nfigurations: meso-p/p-meso, meso-m/p-meso, beta-p/p-meso, and beta-m/p-mes o. All four trimers employ a meso-p/p-meso Zn porphyrin-Fb porphyrin connec tion. The ground- and excited-state properties of the porphyrin dimers and trimers have been examined using static and time-resolved optical technique s. The rate of energy transfer from the photoexcited Zn porphyrin to the Mg porphyrin decreases according to the following trend: meso-p/p-meso (9 ps) (-1) > beta-p/p-meso (14 ps)(-1) > meso-m/p-meso (19 ps)(-1) > beta-m/p-mes o (27 ps)(-1) In each compound, energy transfer between adjacent porphyrins occurs through a linker-mediated through-bond process. The rate of energy transfer between Zn and Fb porphyrins is constant in each trimer ((24 ps)(- 1)). Energy transfer from the photoexcited Zn porphyrin branches to the adj acent Fb and Mg porphyrins, with nearly one-half to three-fourths proceedin g to the Mg porphyrin (depending on the linker). Energy transfer from the e xcited Mg porphyrin to the nonadjacent Fb porphyrin occurs more slowly, wit h a rate that follows the same trend in linker architecture and porphyrin c onnection site: meso-p/p-meso (173 ps)(-1) > beta-p/p-meso (225 ps)(-1) > m eso-m/p-meso (320 ps)(-1) > beta-m/p-meso (385 ps)(-1). The rate of transfe r between nonadjacent Mg and Fb porphyrins does not change significantly wi th temperature, indicating a superexchange mechanism utilizing orbitals/sta tes on the intervening Zn porphyrin. Energy transfer between nonadjacent si tes may prove useful in directing energy flow in multiporphyrin arrays and related molecular photonic devices.