Control of Photoinduced Charge Transfer Lifetimes in Porphyrin Arrays by Ligand Addition

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We demonstrate that the binding of N-donor ligands (4-picoline, imidazole) or anions (chloride) to the terminal zinc(II) porphyrin of selectively metalated triad and tetrad porphyrin arrays allows unprecedented control of photoinduced energy and charge transfer processes. Detailed electrochemical and photophysical studies of the arrays, and also of dyadic and monomeric reference compounds, show that the main effect of ligation is to alter the redox potentials of the zinc(II) porphyrin, making the final step of trans-array charge transfer (between the zinc(II) porphyrin and an adjacent free-base porphyrin) more exothermic. The result of this increased exothermicity is a reduced reversibility for this step and an increase in the trans-array charge transfer lifetime, correlating with the basicity of the ligand employed. For the tetrad, ligation with chloride extends the trans-array charge transfer lifetime to millisecond time scales in polar solvents. For the triad, chloride ligation “switches on” trans-array charge transfer in the nonpolar solvent toluene. The porphyrin arrays studied have geometries that match closely the arrangement of chromophores in natural photosynthetic reaction centers. Using metal ligation to control and optimize interporphyrin energy and electron transfer processes mimics another important aspect of natural light harvesting systems.