Distance Dependent Electron Transfer at TiO₂ Interfaces Sensitized with Phenylene Ethynylene Bridged Ru^II–Isothiocyanate Compounds

Abstract

Excess electrons present in semiconductor nanocrystallites generate a significant electric field, yet the role this field plays in molecular charge transfer processes remains poorly understood. Three ruthenium bipyridyl cis-Ru(bpy)(LL)(NCS)2 compounds, where LL is a 4-substituted bpy, with zero, one, or two phenylene ethynylene bridge units, were anchored to mesoporous nanocrystalline TiO2 thin films to specifically quantify interfacial charge transfer with chromophores designed to be set at variable distances from the surface. Injection of electrons into TiO2 resulted in a blue shift of the metal-to-ligand charge transfer absorption consistent with an underlying Stark effect. The electroabsorption data were used to quantify the electric field experienced by the compounds that decreased from 0.85 to 0.22 MV/cm as the number of OPE spacers increased from 0 to 2. Charge recombination on the 10(-8)-10(-5) s time scale correlated with the magnitude of the electric field with an apparent attenuation factor β = 0.12 Å(-1). Slow components to charge recombination observed on the 10(-4)-10(-1) s time scale that were unaffected by temperature, irradiance, or the bridge units present on the molecular sensitizer were attributed to electron tunneling between TiO2 acceptor states. The photocurrent efficiencies of solar cells based on these compounds decreased markedly when the bridge units were present on the sensitizer. Iodine was found to form adducts with all three compounds, K = 1.8 ± 0.2 × 10(4) M(-1), but only significantly lowered the excited state injection yield for those that possessed the bridge units.

Modulating Electron Transfer Dynamics at Dye–Semiconductor Interfaces via Self-Assembled Bilayers

“…While there is an exponential dependence of k bet on bridge length ( Figure S6, Supporting Information), the distance dependence is weak (β = 0.03 Å −1 ), suggesting that, despite the slowed interfacial electron transfer rate, k bet is dominated by TiO 2 (e − ) trapping/detrapping kinetics. 16 Electron injection efficiencies (Φ inj ) were obtained by using thin film actinometry with TiO 2 −RuC (Φ inj = 100%) as the reference. 42 ΔA values for all samples were evaluated at 20 ns following 532 nm excitation at 5.0 mJ/pulse.…”

“…Recently, Meyer and co-workers demonstrated thatwith at least some bridge-dyesincreasing bridge lengths can be effective in slowing electron transfer rates. 16 However, the covalent interaction between the dye and bridge influences the electrochemical and photophysical properties of the dye, making it difficult to differentiate the distance dependence from other variables. 8 Additionally, the multistep synthesis required for bridge-dye molecules limits the generalizability of these systems.…”

Virtual Eyes Designed for Quantitative Spectroscopy of Inorganic Complexes: Vibronic Signatures in the Phosphorescence Spectra of Terpyridine Derivatives

“…9 In particular, polypyridine Ruthenium complexes have been extensively studied from both theoretical and experimental points of views and remain preferential targets due to their strong luminescent properties and their ability to be entrapped in many different devices. [10][11][12][13][14][15][16] However, from a pure experimental point of view, it remains difficult to fully characterize those compounds with a single technique, and it is often necessary to deal with a multi-spectroscopic approach. In this context, quantum mechanical computations by methods rooted into the density functional theory (DFT) and its time dependent extension (TD-DFT) are increasingly used to obtain complementary electronic and vibrational information for medium to large-size systems thanks to terrific improvements of hardware and software.…”

Photoinitiated charge separation in a hybrid titanium dioxide metalloporphyrin peptide material

“…Much of the current work for varying the distance and orientation of the dye molecule relative to the surface of the semiconducting material relies extensively on the synthesis of new dye molecules [11][12][13][14] . We have developed an alternative, strategic approach to organizing the dye molecule and the semiconducting material by employing a self-assembled peptide amphiphile as the structural template.…”