TiO₂ Surface Functionalization to Control the Density of States

Surface functionalization of mesoporous nanocrystalline (anatase) TiO2 thin films with decyltriethoxysilane, octyltriethoxylsilane, hexyltriethoxysilane, decylphosphonic acid, undecanoic acid, and hemin was accomplished by room temperature reactions in toluene, acetonitrile, or DMSO. Surface functionalization was verified by attenuated total reflection infrared spectroscopy (ATR-FTIR) and the integrated density of unfilled TiO2 states (DOS) were probed by spectroelectrochemical, reactivity, and excited-state injection yield measurements. With the exception of hexyltriethoxysilane, all surface functionalizations were found to shift the DOS positive on an electrochemical scale (away from the vacuum level) in 0.1 M tetrabutylammonium ion containing electrolyte. The magnitude of the effect was found to be dependent on the surface coverage. The potential onset of the unfilled TiO2 states was not affected by functionalization in 0.1 M lithium ion containing electrolyte but the DOS at more negative potentials was significantly decreased. The 532 nm sensitized injection yield with Ru(dcb)(bpy)2(PF6), where dcb is 4,4′-(COOH)2-2,2′-bipyridine and bpy is bipyridine, was 0.89 ± 0.09 for all surface functionalizations. An enhancement of the open circuit photovoltage in regenerative solar cells with 0.5 M LiI/0.05 M I2 was measured after surface functionalization, and an analysis of this data with the diode equation indicated decreased rates for I3− reduction by factors of 7−330. The second-order rate constant for the reduction of carbon tetrachloride by electrochemically reduced TiO2 that had been surface functionalized with decytriethoxysilane, 0.21 ± 0.01 M−1 s−1, was decreased relative to an unfunctionalized TiO2 thin film, 1.02 ± 0.03 M−1 s−1, behavior attributed to the ability of the functionalized surface to prevent close encounters with electron acceptors

Characterization of Photoinduced Self-Exchange Reactions at Molecule–Semiconductor Interfaces by Transient Polarization Spectroscopy: Lateral Intermolecular Energy and Hole Transfer across Sensitized TiO₂ Thin Films

Transient anisotropy measurements are reported as a new spectroscopic tool for mechanistic characterization of photoinduced charge-transfer and energy-transfer self-exchange reactions at molecule–semiconductor interfaces. An anisotropic molecular subpopulation was generated by photoselection of randomly oriented Ru(II)−polypyridyl compounds, anchored to mesoscopic nanocrystalline TiO2 or ZrO2 thin films, with linearly polarized light. Subsequent characterization of the photoinduced dichromism change by visible absorption and photoluminescence spectroscopies on the nano- to millisecond time scale enabled the direct comparison of competitive processes: excited-state decay vs self-exchange energy transfer, or interfacial charge recombination vs self-exchange hole transfer. Self-exchange energy transfer was found to be many orders-of-magnitude faster than hole transfer at the sensitized TiO2 interfaces; for [Ru(dtb)2(dcb)](PF6)2, where dtb is 4,4′-(C(CH3)3)2-2,2′-bipyridine and dcb is 4,4′-(COOH)2-2,2′-bipyridine, anchored to TiO2, the energy-transfer correlation time was θent = 3.3 μs while the average hole-transfer correlation time was ⟨θh+⟩ = 110 ms, under identical experimental conditions. Monte Carlo simulations successfully modeled the anisotropy decays associated with lateral energy transfer. These mesoscopic, nanocrystalline TiO2 thin films developed for regenerative solar cells thus function as active “antennae”, harvesting sunlight and transferring energy across their surface. For the dicationic sensitizer, [Ru(dtb)2(dcb)](PF6)2, anisotropy changes indicative of self-exchange hole transfer were observed only when ions were present in the acetonitrile solution. In contrast, evidence for lateral hole transfer was observed in neat acetonitrile for a neutral sensitizer, cis-Ru(dnb)(dcb)(NCS)2, where dnb is 4,4′-(CH3(CH2)8)2-2,2′-bipyridine, anchored to TiO2. The results indicate that mechanistic models of interfacial charge recombination between electrons in TiO2 and oxidized sensitizers must take into account diffusion of the injected electron and the oxidized sensitizer. The phenomena presented herein represent two means of concentrating potential energy derived from visible light that could be used to funnel energy to molecular catalysts for multiple-charge-transfer reactions, such as the generation of solar fuels

A Nuclear Isotope Effect for Interfacial Electron Transfer: Excited-State Electron Injection from Ru Ammine Compounds to Nanocrystalline TiO₂

The coordination compounds Ru(deeb)(NH3)4(PF6)2 and Ru(deeb)(NH2(CH2)2NH2)4(PF6)2, where deeb is 4,4‘-(CO2CH2CH3)2-2,2‘-bipyridine, were synthesized and attached to optically transparent nanocrystalline (anatase) TiO2 films. The compounds were found to be nonemissive in fluid acetonitrile and when attached to TiO2 with excited-state lifetimes 2 thin films. A small 10−15 mV shift in the RuIII/II reduction potentials was measured upon deuteration. Metal-to-ligand charge-transfer (MLCT) excitation resulted in interfacial electron transfer into the TiO2 semiconductor with quantum yields that were dependent on the excitation wavelength and deuteration of the ammine ligands. The quantum yields were optimized with blue light excitation (417 nm) and deuterium substitution. In contrast, the kinetic rate constants for charge recombination were insensitive to deuteration and the excitation wavelength. Control experiments with Ru(deeb)(bpy)2(PF6)2 indicated that deuteration of the TiO2 surface alone does not affect the injection or recombination processes. A model is proposed wherein electron injection occurs in competition with vibrational relaxation and/or intersystem crossing of the excited states. Exchange of hydrogen by deuterium slows vibrational relaxation and/or intersystem crossing, resulting in higher injection yields

Ligand Coordination and Spin Crossover in a Nickel Porphyrin Anchored to Mesoporous TiO₂ Thin Films

The coordination and spin equilibrium of a Ni^II <i>meso</i>-tetra­(4-carboxy­phenyl)­porphyrin compound, NiP, was quantified both in fluid solution and when anchored to mesoporous, nanocrystalline TiO₂ thin films. This comparison provides insights into the relative rate constants for excited-state injection and ligand field population. In the presence of pyridine, the spectroscopic data were consistent with the presence of equilibrium concentrations of a 4-coordinate low-spin <i>S</i> = 0 (¹A_1g) Ni^II compound and a high-spin <i>S</i> = 1 (³B_1g) 6-coordinate compound. Temperature-dependent equilibrium constants were consistently smaller for the surface-anchored NiP/TiO₂, as were the absolute values of Δ<i>H</i> and Δ<i>S</i>. In the presence of diethylamine (DEA), the ground-state 6-coordinate compound was absent, but evidence for it was present after pulsed light excitation of NiP. Arrhenius analysis of data, measured from −40 to −10 °C, revealed activation energies for ligand dissociation that were the same for the compound in fluid solution and anchored to TiO₂, <i>E</i>_a = 6.6 kcal/mol, within experimental error. At higher temperatures, a significantly smaller activation energy of 3.5 kcal/mol was found for NiP­(DEA)₂/TiO₂. A model is proposed wherein the TiO₂ surface sterically hinders ligand coordination to NiP. The lack of excited-state electron transfer from Ni^IIP*/TiO₂ indicates that internal conversion to ligand field states was at least 10 times greater than that of excited-state injection into TiO₂

Dynamic Quenching of Porous Silicon Excited States

Porous silicon samples have been prepared from p-type single-crystal silicon 〈100〉 by a galvanostatic and an open-circuit etch in 50% HF. The materials display bright red-orange room-temperature photoluminescence (PL) in air and toluene solution. Infrared measurements show that the porous silicon surface is partially oxidized. Exposure to anthracene (An) or 10-methylphenothiazine (MPTZ) results in dynamic quenching of the material's excited state(s). Nanosecond time-resolved PL decays are complex and wavelength dependent, with average lifetimes in neat toluene of 0.3−16 μs. Quenching by An and MPTZ is more efficient and rapid at short observation wavelengths. The steady-state and time-resolved quenching data are well fit to the Stern−Volmer model. The PL decays are well described by a skewed distribution of recombination rates

Stark Spectroscopic Evidence that a Spin Change Accompanies Light Absorption in Transition Metal Polypyridyl Complexes

The “Franck–Condon” (FC) excited state is the first state created when a molecule absorbs a visible photon. Here we report Stark and visible absorption spectroscopies that interrogate the FC state of rigorously diamagnetic [M­(bpy)3]2+ complexes, where bpy is 2,2′-bipyridine and M = Fe, Ru, and Os. Direct singlet-to-triplet metal-to-ligand charge transfer (MLCT) transitions are evident in the 550–750 nm region of the absorbance spectrum of [Os­(bpy)3]2+, yet are poorly resolved or absent for [Ru­(bpy)3]2+ and [Fe­(bpy)3]2+. In the presence of a strong 0.4–0.8 MV/cm electric field, well-resolved transitions are observed for all the complexes in this same spectral region. In particular, an electroabsorption feature at 633 nm (15 800 cm–1) provides compelling evidence for the direct population of a high spin [Fe­(bpy)3]2+* MLCT excited state. Group theoretical considerations and Liptay analysis of the Stark spectra revealed dramatic light-induced dipole moment changes in the range Δμ⇀ = 3–9 D with the triplet transitions consistently showing shorter charge transfer distances. The finding that the spin of the initially populated FC excited state differs from that of the ground state, even with a relatively light first row transition metal, is relevant to emerging applications in energy up-conversion, dye sensitization, spintronics, photoredox catalysis, and organic light emitting diodes (OLEDs)

Ostwald Isolation to Determine the Reaction Order for TiO₂(e^–)|S⁺→ TiO₂|S Charge Recombination at Sensitized TiO₂ Interfaces

Kinetic isolation conditions were identified that enabled determination of the reaction order for interfacial charge recombination at a sensitized mesoporous TiO₂ thin film. An external bias was used to maintain a fixed and known number of oxidized sensitizers, S⁺, or TiO₂ electrons, TiO₂(e^–)­s. Pulsed laser excitation resulted in excited state injection and the subsequent TiO₂(e^–)|S⁺ → TiO₂|S reaction was quantified spectroscopically. The data provide compelling evidence that the rate law for charge recombination under reverse bias is <i>r</i> = <i>k</i>[S⁺]¹[TiO₂(e^–)]¹ with <i>k</i> = 5.0 × 10^–16 cm³ s^–1 (∼3 × 10⁵ M^–1 s^–1). Under forward bias, the data were more complex. A recombination mechanism that incorporates a pre-equilibrium diffusional encounter between injected electrons and oxidized sensitizers is proposed. This and previously reported data indicate that diffusion limits recombination when the number of TiO₂(e^–)­s is small and electron transfer becomes more dominant when the number is large

Intramolecular Electronic Coupling Enhances Lateral Electron Transfer across Semiconductor Interfaces

The control of lateral electron-transfer reactions is important for many solar energy conversion strategies. Herein, four compounds with two redox-active groups, a bis­(tridentate) cyclometalated Ru^II metal center and a substituted triphenylamine (TPA) donor separated by an organic bridge, were anchored to TiO₂ surfaces to facilitate study of lateral <i>inter</i>molecular electron transfer. An important finding was that the TPA^+/0 diffusion coefficients were about 1.6 times larger when the bridge promoted <i>intra</i>molecular electronic coupling between the Ru metal center and TPA. Under conditions where TPA⁺ was able to oxidize the Ru^II center or <i>intra</i>molecular electronic coupling was large, the Ru^III/II electron transfer was facilitated by TPA^+/0 transport. These findings indicate that synergistic interactions between redox-active groups can be tailored to control electron transfer at the molecular level across metal oxide surfaces

Reductive Electron Transfer Quenching of MLCT Excited States Bound To Nanostructured Metal Oxide Thin Films

The ruthenium compounds Ru(deeb)(bpz)2(PF6)2, Ru(deeb)2(bpz)(PF6)2, and Ru(deeb)2(dpp)(PF6)2, where deeb is 4,4‘-(CO2CH2CH3)2-2,2‘-bipyridine, bpz is 2,2‘-bipyrazine, and dpp is 2,3-bis(2-pyridyl)pyrazine, have been prepared, characterized, and anchored to mesoporous nanoparticle thin films comprised of the wide band gap semiconductor TiO2 or the insulator ZrO2. The metal-to-ligand charge-transfer (MLCT) excited states of these compounds are potent photooxidants (E°(RuII*/+) > +1.0 V vs SCE) with long lifetimes (τ > 1 μs) that efficiently oxidize iodide and phenothiazine with rate constants that approach the diffusion limit in acetonitrile. Photogalvanic cells based on the sensitized TiO2 materials yield photocurrent action spectra that agree well with the Ru(II) absorptance spectra. The photocurrent efficiency was very low, φ -4. Transient absorption data show that neither the excited nor the reduced state of the ruthenium compounds efficiently inject electrons into the TiO2 particles. The cage escape yields following excited-state electron transfer are approximately 2/3 lower in the mesoporous thin films than in fluid solution. Intermolecular energy transfer across the nanoparticle surfaces is manifest in a second-order component to the excited-state relaxation kinetics

Factors that Control the Direction of Excited-State Electron Transfer at Dye-Sensitized Oxide Interfaces

Molecular excited states at conductive and semiconductive interfaces were found to transfer an electron to the oxide (injection) or accept an electron from the oxide (hole transfer). The direction of this electron transfer was determined by the energetic overlap of the metal oxide and sensitizer redox-active states and their electronic coupling. Potentiostatically controlled mesoporous thin films based on a nanocrystalline conductive metal oxide [tin-doped indium oxide (ITO)] and semiconducting metal oxides (TiO2 and SnO2) were utilized with the sensitizers (S) [Ru­(bpy)2(P)]­Br2 and [Ru­(bpz)2(P)]­Br2, where bpy is 2,2′-bipyridine, bpz is 2,2′-bipyrazine, and P is 2,2′-bipyridyl-4,4′-diphosphonic acid. For dye-sensitized TiO2, excited-state injection [TiO2|S* → TiO2(e–)|S+] was exclusively observed, and the injection yield decreased at negative applied potentials. In contrast, evidence for both injection [ITO|S* → ITO­(e–)|S+] and hole transfer ([ITO|S* → ITO­(h+)|S–] is reported for ITO and SnO2. Hole transfer became more efficient with negative applied potentials. The direction of electron flow between the metal oxide and excited state sensitizer was correlated with the energetic overlap and the electronic coupling as predicted by Marcus−Gerischer theory. The data reveal that control of the Fermi level enables conductive oxides to function as a photocathode or as a photoanode for solar energy conversion applications