Optimization and chemical control of porphyrin-based molecular wires and switches

The design rationale is described as are some experimental and theoretical developments concerning rigidly fused porphyrin molecular wires. These materials consist of porphyrin units fused to acene-type bridges and have been synthesized in a range of topologies including linear porphyrin octamers of length ca. 120 Angstrom. Next we demonstrate, for some linear oligoporphyrins, how the electronic coupling between the end porphyrin units can be modulated by simple (possibly in situ) chemical modulation of the bridging units. Specifically, the chemical systems considered involve either pi-I-controlled protonation of bridge azines or conversion of bridge quinone or quinone dioxime rings to or from benzenoid or hydroquinone rings. In the most general terms, the electronic coupling through oligoporphyrin molecular wires is discussed in terms of a simple model in which complete end-to-end pi electronic delocalization is required in order to provide strong long-range interactions. Computationally, we monitor interorbital coupling using an appropriate mixture of density functional (B3LYP) and ab initio SCF computational schemes. Finally, we examine bridge modulation of the intermetallic coupling in three homovalent bis-metallic oligoporphyrin systems, two in which Ru(CO)(2) is complexed within terminal porphyrin rings, the other in which [Cu(I)Cl-2](-) is tethered using phenanthroline end groups. Results are obtained both using an effective two-level model, appropriate for spectroscopic properties, and using a more general scheme, appropriate for molecular conduction.