Making photoactive molecular-scale wires

Abstract

A synthetic strategy is described that provides access to oligopyridine-based ditopic ligands bridged by an alkyne spacer comprising one to four ethynyl groups. The spacer serves as both a rigid girder to maintain structural inetgrity and a conduit for electron flow. These ditopic ligands, bearing 2,2′-bipyridine, 1,10-phenanthroline or 2,2′:6′,2″-terpyridine coordination sites, are used to construct a dichotomous series of polynuclear metal complexes. Judicious selection of ligand and metal cation permits the assembly of novel molecular architectures incorporating a logical gradient of redox units along the molecular axis. Different molecular shapes become possible by changing the position at which the alkyne bridge is connected to the terminal ligand and varying the nature of the complexing cation. The elecrochemical and photochemical properties of many such molecular arrays are enumerated with a view to the future design of molecular electronic devices. The alkyne bridge actively promotes long-range electronic coupling between remote cationic units, especially under illumination with visible light. Intramolecular triplet energy transfer, photon migration and light-induced electron transfer occur by way of extremely rapid superexchange involving LUMO and HOMO states localized on the bridge. It is also shown that facile electron delocalization over an extended π* orbital takes place in the triplet excited states of symmetrical binuclear complexes. This process extends the triplet lifetime and thereby facilitates secondary reactions that are otherwise unattainable. The level of electronic communication along the molecular wire can be controlled by insertion of suitable insulating groups into the bridge, these groups also providing further means by which to manipulate the stereochemistry. In certain cases, the alkyne bridge undergoes reductive electropolymerization to generate molecular films having metallo centres dispersed along a conjugated backbone.