Dye-sensitized electron transfer from TiO2 to oxidized triphenylamines that follows first-order kinetics

Abstract

Two sensitizers, [Ru(bpy)₂(dcb)]²⁺ (RuC) and [Ru(bpy)₂(dpb)]²⁺ (RuP), where bpy is 2,2'-bipyridine, dcb is 4,4'-dicarboxylic acid-2,2'-bipyridine and dpb is 4,4'-diphosphonic acid-2,2'-bipyridine, were anchored to mesoporous TiO₂ thin films and utilized to sensitize the reaction of TiO₂ electrons with oxidized triphenylamines, TiO₂(e^-) + TPA⁺ → TiO₂ + TPA, to visible light in CH₃CN electrolytes. A family of four symmetrically substituted triphenylamines (TPAs) with formal E^o(TPA^+/0) reduction potentials that spanned a 0.5 eV range was investigated. Surprisingly, the reaction followed first-order kinetics for two TPAs that provided the largest thermodynamic driving force. Such first-order reactivity indicates a strong Coulombic interaction between TPA⁺ and TiO₂ that enables the injected electron to tunnel back in one concerted step. The kinetics for the other TPA derivatives were non-exponential and were modelled with the Kohlrausch-William-Watts (KWW) function. A Perrin-like reaction sphere model is proposed to rationalize the kinetic data. The activation energies were the same for all of the TPAs, within experimental error. The average rate constants were found to increase with the thermodynamic driving force, consistent with electron transfer in the Marcus normal region.

Scheme 1. Mechanism for the photoinitiation of the desired reaction. Visible light absorption by the ruthenium sensitizer induced rapid excited-state electron injection to the acceptor state of TiO₂, k_inj > 10⁸ s^–1. The oxidized sensitizer is then regenerated by triphenylamine (TPA) with a rate constant k_reg. This sequence provides the reactants for the desired charge recombination reaction of the injected electron with the oxidized triphenylamine redox mediator (k_rec) that was quantified over a 0.5 eV change in driving force.

Fig. 1. Ruthenium sensitizers (RuP and RuC) and triphenylamines (TPAs) used in this study.

Fig. 2. Cyclic voltammograms (a) and TPA⁺ absorption spectra (b) of the indicated TPA measured in 0.1 M NaClO₄ acetonitrile electrolyte.

Fig. 3. Transient absorption spectra measured at indicated delay times after pulsed 532 nm excitation of TiO₂|RuC submerged in 0.1 M NaClO₄ acetonitrile electrolyte with 8 mM MeO-TPA (a) or 8 mM Cl-TPA (b). Overlaid as solid lines are the simulated spectra.

Fig. 4. Single wavelength absorption changes measured after pulsed 532 nm laser excitation of TiO₂|RuP in 0.1 M LiClO₄ acetonitrile electrolyte (a) and TiO₂|RuC in 0.1 M NaClO₄ acetonitrile electrolyte (b) with 8 mM of the indicated TPA mediator. Kinetics were monitored at the TPA⁺ absorption peak. Fits to the KWW model are overlaid on the data as solid yellow lines.

Fig. 5. Single wavelength absorption changes measured after pulsed 532 nm laser excitation of TiO₂|RuP in 0.1 M LiClO₄ with 8 mM of Br-TPA at indicated temperatures (a). Kinetics were monitored at the TPA⁺ absorption peak. Arrhenius plots for the recombination from TiO₂|RuP to the indicated TPAs in 0.1 M LiClO₄ (b) and from TiO₂|RuP to Br-TPA in the indicated 0.1 M CH₃CN electrolytes (c).

Scheme 2. A reaction sphere model for interfacial charge recombination. Schematic representation of a proposed Perrin-like model. When the driving force for recombination is large, as seen for Br- and Cl-TPA, electron transfer occurs over relatively large distances (blue sphere). With smaller driving force, the electron must hop closer to the TPA⁺ acceptor before electron transfer can occur (red sphere), leading to a decreased β value in fits to the KWW function.

Fig. 6. Calculated Lévy distribution of the charge recombination rate constants abstracted from transient data for electron transfer from TiO₂(e^–)|RuC (top) and TiO₂(e^–)|RuP (bottom) to MeO-TPA⁺ (red) or to Me-TPA⁺ (black). An average rate constant based on eqn (2) is shown as a vertical line. Also shown as vertical lines are the first-order rate constants for recombination to Br-TPA⁺ (blue) and Cl-TPA⁺ (green).