| We have been interested in the design and synthesis of large porphyrin assemblies, namely the preparation of photosynthetic reaction center (PRC) mimics and porphyrin networks. The majority of our work has focused on synthesizing a molecular model of the PRC that is structurally similar to the bacterial reaction center. We have designed and synthesized the first bis(porphyrin)-quinone triad, Zr(TTP)(TTP-NH-PAQ), that mimics structural and electronic features of the photosynthetic reaction center: (1) the bis(porphyrin) interplanar separation of ∼3Å is similar to the separation of the special pair, (2) the oxidized bis(porphyrin), Zr(TTP)2+, has a near IR band at 1074 nm, which is similar in energy to that of the oxidized special pair, (3) the porphyrin spacer molecule mimics the bacteriopheophytins of the reaction center and provides for the possibility of a multi-step electron transfer pathway, (4) the quinone molecule mimics the quinone electron acceptor in natural reaction centers.; The AQ triad, Zr(TTP)(TTP-NH-PAQ) was synthesized by coupling Zr(TTP)(TTP-NH2) with PAQ carboxylic acid using oxalyl chloride as the coupling agent. Other common coupling agents such as DCC and thionyl chloride were not effective. Column chromatography was used to separate AQ triad from the starting materials. Zr(TTP)(TTP-NH-PAQ) was analyzed by FD-MS, 1H-NMR, UV-vis spectroscopy, and HPLC.; Photophysical measurements were carried out in collaboration with Professor Dewey Holten and Robin Lammi at Washington University. The lifetimes for the initial photoexcited states of the triad, protected triad, and Zr(TTP)(TTP-NH 2) in a given solvent are the same within experimental error. This result means that an electron is not transferring to the monomeric spacer porphyrin in high yield. Presumably, the ΔG for this process is too unfavorable. Instead, the excited zirconium bis(porphyrin) sandwich complex simply returns to the ground state. One way to improve upon this system would be to make the bridging porphyrin a better electron acceptor by replacing the mesityl substituents with pentafluorophenyl groups. |
