Characterization of the redox properties of TiO2 interfaces sensitized to visible light by a series of cyclometalated ruthenium polypyridyl compounds containing both a terpyridyl ligand with three carboxylic acid/carboxylate or methyl ester groups for surface binding and a tridentate cyclometalated ligand with a conjugated triarylamine (NAr3) donor group is described. Spectroelectrochemical studies revealed non-Nernstian behavior with nonideality factors of 1.37 ± 0.08 for the Ru(III/II) couple and 1.15 ± 0.09 for the NAr3(•+/0) couple. Pulsed light excitation of the sensitized thin films resulted in rapid excited-state injection (k(inj) > 10(8) s(-1)) and in some cases hole transfer to NAr3 [TiO2(e(-))/Ru(III)-NAr3 → TiO2(e(-))/Ru(II)-NAr3(•+)]. The rate constants for charge recombination [TiO2(e(-))/Ru(III)-NAr3 → TiO2/Ru(II)-NAr3 or TiO2(e(-))/Ru(II)-NAr3(•+) → TiO2/Ru(II)-NAr3] were insensitive to the identity of the cyclometalated compound, while the open-circuit photovoltage was significantly larger for the compound with the highest quantum yield for hole transfer, behavior attributed to a larger dipole moment change (Δμ = 7.7 D). Visible-light excitation under conditions where the Ru(III) centers were oxidized resulted in injection into TiO2 [TiO2/Ru(III)-NAr3 + hν → TiO2(e(-))/Ru(III)-NAr3(•+)] followed by rapid back interfacial electron transfer to another oxidized compound that had not undergone excited-state injection [TiO2(e(-))/Ru(III)-NAr3 → TiO2/Ru(II)-NAr3]. The net effect was the photogeneration of equal numbers of fully reduced and fully oxidized compounds. Lateral intermolecular hole hopping (TiO2/Ru(II)-NAr3 + TiO2/Ru(III)-NAr3(•+) → 2TiO2/Ru(III)-NAr3) was observed spectroscopically and was modeled by Monte Carlo simulations that revealed an effective hole hopping rate of (130 ns)(-1).

Ion-pair interactions between a cationic ruthenium complex, [Ru(dtb)(dea)][PF], C1 where dea is 4,4'-diethanolamide-2,2'-bipyridine and dtb is 4,4'-di-tert-butyl-2,2'-bipyridine, and chloride, bromide, and iodide are reported. A remarkable result is that a 1:1 iodide:excited-state ion-pair, [C1, I], underwent diffusional electron-transfer oxidation of iodide that did not occur when ion-pairing was absent. The ion-pair equilibrium constants ranged 10-10 M in CHCN and decreased in the order Cl > Br > I. The ion-pairs had longer-lived excited states, were brighter emitters, and stored more free energy than did the non-ion-paired states. The H NMR spectra revealed that the halides formed tight ion-pairs with the amide and alcohol groups of the dea ligand. Electron-transfer reactivity of the ion-paired excited state was not simply due to it being a stronger photooxidant than the non-ion-paired excited state. Instead, work term, ΔG was the predominant contributor to the driving force for the reaction. Natural bond order calculations provided natural atomic charges that enabled quantification of ΔG for all the atoms in C1 and [C1, I] presented herein as contour diagrams that show the most favorable electrostatic positions for halide interactions. The results were most consistent with a model wherein the non-ion-paired C1 excited state traps the halide and prevents its oxidation, but allows for dynamic oxidation of a second iodide ion.

Supramolecular assembly is shown to provide control over excited-state chloride release. Two dicationic chromophores were designed with a ligand that recognizes halide ions in CHCl and a luminescent excited state whose dipole was directed toward, 1, or away, 2, from an associated chloride ion. The dipole orientation had little influence on the ground-state equilibrium constant, K ∼ 4 × 10 M, but induced a profound change in the excited-state equilibrium. Light excitation of [1,Cl] resulted in time-dependent shifts in the photoluminescence spectra with the appearance of biexponential kinetics consistent with the photorelease of Cl. Remarkably, the excited-state equilibrium constant was lowered by a factor of 20 and resulted in nearly 45% dissociation of chloride. In contrast, light excitation of [2,Cl] revealed a 45-fold increase in the excited-state equilibrium constant. The data show that rational design and supramolecular assembly enables the detection and photorelease of chloride ions with the potential for future applications in biology and chemistry.

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The visible absorption and photoluminescence (PL) properties of the four neutral ruthenium diimine compounds [Ru(bpy)2(dcb)] (B2B), [Ru(dtb)2(dcb)] (D2B), [Ru(bpy)2(dcbq)] (B2Q), and [Ru(dtb)2(dcbq)] (D2Q), where bpy is 2,2'-bipyridine, dcb is 4,4'-(CO2(-))2-bpy, dtb is 4,4'-(tert-butyl)2-bpy, and dcbq is 4,4'-(CO2(-))2-2,2'-biquinoline, are reported in the presence of Lewis acidic cations present in fluid solutions at room temperature. In methanol solutions, the measured spectra were insensitive to the presence of these cations, while in acetonitrile a significant red shift in the PL spectra (≤1400 cm(-1)) was observed consistent with stabilization of the metal-to-ligand charge transfer (MLCT) excited state through Lewis acid-base adduct formation. No significant spectral changes were observed in control experiments with the tetrabutylammonium cation. Titration data with Li(+), Na(+), Mg(2+), Ca(2+), Zn(2+), Al(3+), Y(3+), and La(3+) showed that the extent of stabilization saturated at high cation concentration with magnitudes that scaled roughly with the cation charge-to-size ratio. The visible absorption spectra of D2Q was particularly informative due to the presence of two well-resolved MLCT absorption bands: (1) Ru → bpy, λmax ≈ 450 nm; and (2) Ru → dcbq, λmax ≈ 540 nm. The higher-energy band blue-shifted and the lower-energy band red-shifted upon cation addition. The PL intensity and lifetime of the excited state of B2B first increased with cation addition without significant shifts in the measured spectra, behavior attributed to a cation-induced change in the localization of the emissive excited state from bpy to dcb. The importance of excited-state localization and stabilization for solar energy conversion is discussed.