The mechanism of long-range electron exchange in molecular-scale photonic wires
Several disparate systems have been examined as putative molecular-scale wires able to conduct charge over relatively long distances under illumination. The key feature of such systems concerns the level of electronic coupling between the terminals, which are themselves formed from photoactive metal poly(pyridine) complexes. In particular, a series of linear polynuclear ruthenium(II) tris(2,2′-bipyridine) complexes has been synthesized whereby individual chromophores are separated by 1,4-diethynylenebenzene subunits bearing dialkoxy groups for improved solubility. These arrays contain two, three, four or five metal centres. The compounds are reasonably soluble in polar organic solvents and possess optical absorption spectral properties that are dominated by transitions associated with the polytopic ligand. Weak luminescence is observed for each complex in deoxygenated acetonitrile at room temperature that appears to be characteristic of emission from a metal-to-ligand, charge-transfer triplet state. The emission lifetime is essentially independent of temperature, at least over a modest range. There is no indication for interaction between close-lying triplet states and no obvious sign of a low-energy π,π* triplet associated with the polytopic ligand. The photophysical properties suggest, however, that the longer arrays are segmented due to intramolecular charge-transfer interactions. The ligands bind zinc(II) cations in solution and thereby affect the absorption and emission spectra.