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 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 pi,pi* 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.

The photophysical properties of a julolidene-based molecular rotor.

The photophysical properties of 9-dicyanovinyljulolidine are sensitive to solvent viscosity but are little affected by changes in polarity. In fluid solution, the lifetime of the first-excited singlet state is very short and triplet state formation cannot be detected by laser flash photolysis. Decay of the excited singlet state is strongly activated and weak phosphorescence can be observed in a glassy matrix at 77 K. Temperature dependent 1H NMR studies indicate that the molecule undergoes slow internal rotation in solution, for which the activation energy has a value of ca. 35 kJ mol(-1). This process is unlikely to account for the poor fluorescence quantum yield found in fluid solution. Instead, it is considered that the target compound undergoes rapid rotation around the dicyanovinyl double bond from the excited singlet state. The rate of rotation depends weakly on the viscosity of the solvent in a range of linear alcohols at room temperature. This might represent the fact that the rotor is relatively small and can pack into cavities in the solvent structure. In glycerol, the rate of rotation is more sensitive to viscosity effects but a quite complex temperature dependence is observed in ethanol. Here, the rate is almost activationless in a glassy matrix and in fluid solution at high temperature but strongly activated at intermediate temperatures.

Reversible luminescence switching in a ruthenium(II) bis(2,2':6',2''-terpyridine)-benzoquinone dyad.

An electroactive luminescent switch has been synthesized that comprises a hydroquinone-functionalized 2,2':6',2''-terpyridine ligand coordinated to a ruthenium(II) (4'-phenylethynyl-2,2':6',2''-terpyridine) fragment. The assembly is sufficiently rigid that the hydroquinone-chromophore distance is fixed. Excitation of the complex via the characteristic metal-to-ligand charge-transfer (MLCT) absorption band produces an excited triplet state in which the promoted electron is localized on the terpyridine ligand bearing the acetylenic group. The triplet lifetime in butyronitrile solution at room temperature is 46 +/- 3 ns but increases markedly at lower temperature. Oxidation of the hydroquinone to the corresponding benzoquinone switches on an electron-transfer process whereby the MLCT triplet donates an electron to the quinone. This reaction reduces the triplet lifetime to 190 +/- 12 ps and essentially extinguishes emission. The rate of electron transfer depends on temperature in line with classical Marcus theory, allowing calculation of the electronic coupling matrix element and the reorganization energy as being 22 cm(-1) and 0.84 eV, respectively. The switching behavior can be monitored using luminescence spectroelectrochemistry. The on/off level is set by temperature and increases as the temperature is lowered.