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We employ time-resolved terahertz (THz) spectroscopy (TRTS) to directly monitor the picosecond dynamics of electron transfer in dye-sensitized oxides in the presence of an electrolyte phase. Understanding the time scale on which electrons are injected from the dye into the oxide phase in the presence of electrolyte is important for optimization of the solar cell efficiency. We quantify injection dynamics from two different dyes into both mesoporous TiO2 and SnO2 films. Measurements are performed in inert media (air, acetonitrile), in the presence of two different electrolytes (the conventional iodine/iodide couple and the recently reported disulfide/thiolate redox couple), and in the presence of two different electrolyte additives (Li+ ions and tert-butyl pyridine). Electron injection dynamics in TiO2 is found to occur on two time scales: sub-150 fs and ∼10 ps, attributed to injection from the singlet and lower-lying triplet state, respectively. For SnO2, injection is slower, despite the lower energy of the band edge. The slow injection observed for SnO2 is attributed to the reduced density of electronic states in the material. We observe that for both oxides electron injection can be strongly retarded by changing the composition of the medium in which the sensitized oxide film is immersed. In particular, our results indicate that injection dynamics can be significantly slowed down in the presence of the disulfide/thiolate redox couple and/or tert-butyl pyridine.

The interactions between the [Ru(bipy)(CN)(4)](2-) anion and N-methyl-halopyridinium cations have been examined in both the solid state and in solution. In the solid state, crystal structures of [Ru(bipy)(CN)(4)](2-) salts containing iodinated cations (N-methyl-3-iodopyridinium and N-methyl-3,5-diiodopyridinium) show clear C-I...NC(Ru) halogen bonds between the externally directed cyanide lone pairs of the anion and the iodine atoms of the cation which dominates the structures. In contrast the analogous brominated cations (N-methyl-3-bromopyridinium and N-methyl-3,5-dibromopyridinium) do not exhibit C-Br...NC(Ru) interactions in the solid state, with the cyanide groups instead involved in hydrogen bonding, principally to lattice water molecules. The charge-assisted C-I...NC(Ru) interactions are therefore clearly of value as synthons in crystal engineering applications. In CH(2)Cl(2) solution, spectroscopic titrations between [Ru(4,4'-(t)Bu(2)-bipy)(CN)(4)](2-) and both N-methyl-3-iodopyridinium and N-methyl-3-bromopyridinium cations show clear evidence for formation of distinct 1:1, 3:2, and then 2:1 cation/anion adducts with high association constants (>10(7) M(-1) for the first 1:1 association constant). However the presence of identical results using the non-halogenated cation N-methyl-pyridinium indicates that this strong cation/anion association in CH(2)Cl(2) is dominated by electrostatic effects: either C-H...NC(Ru) hydrogen bonds or C-X...NC(Ru) halogen bonds could be involved in the ion pairs but it is the charge-assistance that makes the association strong. This is confirmed by a titration between [Ru(4,4'-(t)Bu(2)-bipy)(CN)(4)](2-) and the neutral halogen-bond acceptor C(6)F(5)I for which the first association constant is very low (ca. 6 M(-1)). The formation of adducts between [Ru(4,4'-(t)Bu(2)-bipy)(CN)(4)](2-) and the various N-methyl-pyridinium cations in solution results in a clear blue-shift of the (1)MLCT absorption maxima associated with the Ru(II) unit, a characteristic consequence of interaction of the cyanide lone pairs with a Lewis-acidic site on the cation. The (3)MLCT luminescence from the [Ru(4,4'-(t)Bu(2)-bipy)(CN)(4)](2-) center, however, does not show the usual associated increase in intensity associated with this blue shift in the (1)MLCT absorptions, most likely because of electron-transfer quenching by the N-methyl-pyridinium cations in the assemblies.

The combination of cis-protected metal fragments with linear linkers is expected to yield molecular squares. We found instead that treatment of the 90 degrees angular precursor trans-[RuCl2(dmso-S)4] (1) with an equivalent amount of the linear and rigid pyrazine (pyz) linker unexpectedly yields, in a number of different experimental conditions, the molecular triangle [{trans,cis-RuCl2(dmso-S)2(mu-pyz)}3] (3), together with polymeric material. Very similar results were also obtained from the reaction between 1 and the preformed corner fragment trans,cis,cis-[RuCl2(dmso-S)2(pyz)2] (6). In both cases, the expected molecular square [{trans,cis-RuCl2(dmso-S)2(mu-pyz)}4] (4) was observed only as a transient species. These results suggest that 3, which is the first example of a neutral molecular triangle with octahedral metal corners and pyrazine edges, is both the thermodynamic and the kinetic product of the reactions described above. The X-ray structure of 3 shows that the main distortions from ideal coordination geometry concern the N-Ru-N angles, which are narrower than 90 degrees , and the coordination bonds of pyz. The pyrazine molecules, which are basically planar, are significantly tilted from linearity. Calculations performed on 6 indicated that the N-Ru-N angle is ca. six times more rigid than the tilt angle of pyrazine. The structural and theoretical findings on 3 and 6, together with the previous examples of molecular triangles and squares with cis-protected metal corners and linear pyz edges, suggest that the entropically favored molecular triangles might be preferred over the expected molecular squares with metal corner fragments that spontaneously favor Npyz-M-Npyz angles narrower than 90 degrees because of the presence of ancillary ligands with significant steric demand on the coordination plane. The rather-flexible coordination geometry of pyrazine can accommodate the moderate distortions from linearity required to close the small metallacycle with modest additional strain.

The complex cations [RuL2(H2biim)]2+ (L=bipy, 4,4'-tBu2-bipy) interact with cyanometallate anions via a chelating hydrogen-bonding interaction between the two N-H donors of the complex cation and the N lone pair of one cyanide ligand in the complex anion; the anion hexacyanoferrate(III) quenches the Ru(II)-based luminescence in CH2Cl2 solution by photoinduced electron-transfer within the H-bonded assembly, whereas hexacyanocobaltate(III) enhances the Ru(II)-based luminescence.