In Search of the Inverted Region:  Chromophore-Based Driving Force Dependence of Interfacial Electron Transfer Reactivity at the Nanocrystalline Titanium Dioxide Semiconductor/Solution Interface

Abstract

Intentional variations of the driving force for back electron transfer from titanium dioxide to a surface-bound redox couple, via synthetic alteration of the couple's formal potential, show that the reaction takes place in the Marcus normal region; i.e., rates become faster as the driving force increases. Variable-temperature rate measurements show that back ET is thermally activated, with the activation barrier decreasing with increasing driving force, as expected for a normal region process. The observation of normal region behavior, despite the existence of overall reaction driving forces that significantly exceed the likely reorganization energy, is attributed to the occurrence of a sequential electron-and proton-transfer process. The sequential process yields a driving force for the rate-determining electron-transfer step that is considerably smaller than the overall reaction driving force. The sequential electron-and proton-transfer mechanism also provides an explanation for the pH independence of the back-ET kinetics. Variable-temperature rate measurements additionally point toward a high degree of nonadiabaticitys3-5 orders of magnitude of rate attenuation due solely to inefficient electronic coupling. The physical basis for the effect presumably is in the need to traverse eight bonds, some of them saturated, in the back-ET process. Together with the barrier activation effects, the reaction nonadiabaticity accounts for the slow rates for back ET (ca. 10 7 s -1) and, therefore, much of the ability of metal-to-ligand charge-transfer type chromophores to function effectively as sensitizers in TiO 2 -based photoelectrochemical cells. The findings also suggest that more efficient cells could be constructed by extending the chemical linkage between the dye and semiconductor and by further decreasing the driving force for the back-ET process.

A surface science perspective on TiO2 photocatalysis

“…A more detailed understanding of the dynamics of electron transfer from the TiO 2 , potentially addressing the trap vs. CB issue, can be obtained from a wealth of information involving back-electron transfer events in molecular sensitization studies [163,164,433,493,494,496,497,507,532,545,601,608,609,618,621,624,650,662,667,670,675,[688][689][690][691][692][693][694][695][696]. Typically, backelectron transfer in the DSSC setting is viewed as detrimental to solar light harvesting, but the dynamics of such events provide unique insights into the photoreduction aspects of TiO 2 .…”

“…Weng et al [493] have shown that back-electron transfer rates following electron injection from adsorbed Fe(CN) 3− 6 were independent of TiO 2 particle size and preparation method, suggesting electron traps in the vicinity of the complex and the surface. Timescales for back-electron transfer processes span the picosecond to nanosecond ranges, which are considerably slower than the electron injection and thermalization times [163,164,493,494,496,497,532,609,618,667,675,691,693,695]. However, sub-picosecond back-electron transfer timescales have been observed [650,670,675], as have lifetimes in the microsecond and longer timescales [163,494,601,621,624,667].…”

“…However, back-electron transfer into the less stable triplet state was found to fall into the normal region, which accounted for it being observable. Yan and coworkers [695] observed normal Marcus back-electron transfer kinetics (faster electron transfer as the driving force increased) for a series of ligand-modified, phosphate-anchored Ru-based dyes. They also observed that these back-electron transfer processes were thermally activated, putting them in the normal regime.…”

Amphiphilic Ruthenium Sensitizer with 4,4‘-Diphosphonic Acid-2,2‘-bipyridine as Anchoring Ligand for Nanocrystalline Dye Sensitized Solar Cells

“…Grafting sensitizers on the surface of TiO 2 with the phosphonic acid group has been reported earlier by us [4][5][6] and other groups. [7][8][9][10][11][12][13] Recently, we have demonstrated the significance of using an amphiphilic polypyridyl ruthenium sensitizer to achieve unprecedented stability in DSCs at elevated temperatures. 14 Also, self-assembling of ω-alkylphosphonic acids on a wide range of metal oxide surfaces has been of great scientific and practical interest due to the ability to form robust and compact monolayers.…”

Supramolecular Solar Cells

“…Magnesium ion's abilty to axially coordinate helps to strengthen and improve the stability of the dimer in photosynthetic reaction center. Axial coordination of Maganesium ion containing porphyrin for DSSC applications have not been investigated, although, solution based studies have been reported 173 The free energy for charge recombination calculated as shown below 175 Under similar condition, the Gibbs' free energy change for recombination depends mainly on the oxidation potential of MgTPP and ZnTPP. Hence the driving force for charge recombination observed in case of ZnTPP will be higher compared to MgTPP.…”