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Structural Control of Photoinduced Energy Transfer between Adjacent and Distant Sites in Multiporphyrin Arrays

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journal contribution
posted on 21.07.2000, 00:00 by Robin K. Lammi, Arounaguiry Ambroise, Thiagarajan Balasubramanian, Richard W. Wagner, David F. Bocian, Dewey Holten, Jonathan S. Lindsey
A family of diphenylethyne-linked porphyrin dimers and trimers has been prepared via a building block approach for studies of energy-transfer processes. The dimers contain Mg and Zn porphyrins (MgZnU); the trimers contain an additional free base porphyrin (MgZnFbU). In both the dimers and trimers, sites of attachment to the Mg porphyrin (at the meso- or β-position) and diphenylethyne linker (at the para- or meta-positions) were varied, producing four Mg porphyrin−Zn porphyrin arrangements with the following linker configurations:  meso-p/p-meso, meso-m/p-meso, β-p/p-meso, and β-m/p-meso. All four trimers employ a meso-p/p-meso Zn porphyrin−Fb porphyrin connection. The ground- and excited-state properties of the porphyrin dimers and trimers have been examined using static and time-resolved optical techniques. 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 > β-p/p-meso (14 ps)-1 > meso-m/p-meso (19 ps)-1 > β-m/p-meso (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 adjacent Fb and Mg porphyrins, with nearly one-half to three-fourths proceeding to the Mg porphyrin (depending on the linker). Energy transfer from the excited Mg porphyrin to the nonadjacent Fb porphyrin occurs more slowly, with a rate that follows the same trend in linker architecture and porphyrin connection site:  meso-p/p-meso (173 ps)-1 > β-p/p-meso (225 ps)-1 > meso-m/p-meso (320 ps)-1 > β-m/p-meso (385 ps)-1. The rate of transfer between nonadjacent Mg and Fb porphyrins does not change significantly with temperature, indicating a superexchange mechanism utilizing orbitals/states on the intervening Zn porphyrin. Energy transfer between nonadjacent sites may prove useful in directing energy flow in multiporphyrin arrays and related molecular photonic devices.