Other Porphyrin-based Dye Assemblies for Energy Electron Transfer Studies

Other Porphyrin-based Dye Assemblies for Energy/Electron Transfer Studies 

Through coor nation motif, a tetra-pyrazinyl porphyrin-based assembly with four dimeric [meso-tetrakis(2-carboxy-4-nonylphenyl)porphyrinato]-zinc(II) as the antenna moiety was constructed by Kuroda et al. [113]. The efficiency of energy transfer from the zinc porphyrin-pyrazine complex to the free base porphyrin has been determined to be 82%. Also for this system, the observed efficiency of energy transfer is not so high compared with those reported for covalently finked multi-porphyrin systems [114]. Himter s group has designed a pentameric porphyrin array, wtiich was built up by the complementary coordination of a tetra-pyridyl porphyrin and two zinc-porphyrin dimers [115]. Steady-state and time-resolved fluorescence spectroscopic studies revealed that the energy transfer efficiency of this array is around 70%. 

Besides genuine metal-coordination, the cooperative effect of several weak forces has been utilized to create similar functional assemblies. Weiss and coworker have recently designed the noncovalent couple 42 by utilizing the unique recognition properties of phenanthrofine-strapped zinc porphyrin towards AT-unsubstituted imidazoles [116]. Metal coordination and hydrogenbonding interaction simultaneously drive the formation of the assembly and 

Imahori et al. have reported the electron transfer assemblies 43, in which a zinc porphyrin dimer acts as the electron donor and benzoquinone or py-romellitimide serves as the electron acceptor [117,118]. Because of the two-sites binding of the electron acceptors with high stabilities, the assembhes 43a and 43b may form exclusively even at low concentrations typically used for spectroscopic studies. The electron transfer rate constants were found to be 1.6 X 10 ° s for 43a [117] and 2.1 x 10 ° s for 43b [118]. These nearly identical values suggest similar barriers for electron transfer at a comparable distance between chromophore and quencher. As the shortest through-space and through-bond pathways between the chromophore and the quencher in 

43 are similar, it is not possible to distinguish whether the electron transfer in these systems is occurring through space or through bond. 

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