Non-Nernstian Two-Electron Transfer Photocatalysis at Metalloporphyrin–TiO2 Interfaces

Excitation-Wavelength-Dependent Charge Injection and Hole Localization in Diblock Oligomers Anchored to TiO2

A series of diblock oligomers containing oligothiophene (Tn, n = 4, 5) and 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadizole (TBT) segments, functionalized with carboxylic acid anchoring groups, were prepared and anchored to mesoporous TiO2 films to study wavelength-dependent interfacial electron transfer mechanisms. Thin films of the surface-anchored diblock oligomers contained two absorption bands centered at 400 and 500 nm, corresponding to the Tn and TBT blocks, respectively. Pulsed-laser excitation of the oligomer-sensitized films yielded local excited-states that promoted electron injection into TiO2. The injection pathway was dependent on the excitation wavelength, as electron injection occurred from the oligomer block that was locally excited. Recombination between the injected electron and the oxidized oligomer was sensitive to the bridging unit that separates the oligomer conjugated segments (-C≡C- vs trans-Pt(PBu3)2-). When the bridge facilitated strong coupling between the two blocks (-C≡C- bridge), the excitation wavelength had no effect on the recombination pathway, as the hole was delocalized over the entire oligomer. However, in the weak coupling case (Pt(PBu3)2- bridge), selective excitation resulted in wavelength-dependent hole localization that persisted to the μs time scale, providing control over the recombination pathway by varying the excitation wavelength. Dye-sensitized solar cells (DSSCs) were fabricated by using the diblock oligomers as sensitizers. The photocurrent action spectra were measured, and the absorbed photon-to-current efficiency (APCE) provided further insight into the electron-transfer mechanisms that are operative under continuous illumination.

General Kinetic Model for pH Dependence of Proton-Coupled Electron Transfer: Application to an Electrochemical Water Oxidation System

The pH dependence of proton-coupled electron transfer (PCET) reactions, which are critical to many chemical and biological processes, is a powerful probe for elucidating their fundamental mechanisms. Herein, a general, multichannel kinetic model is introduced to describe the pH dependence of both homogeneous and electrochemical PCET reactions. According to this model, a weak pH dependence can arise from the competition among multiple sequential and concerted PCET channels involving different forms of the redox species, such as protonated and deprotonated forms, as well as different proton donors and acceptors. The contribution of each channel is influenced by the relative populations of the reactant species, which often depend strongly on pH, leading to complex pH dependence of PCET apparent rate constants. This model is used to explain the origins of the experimentally observed weak pH dependence of the electrochemical PCET apparent rate constant for a ruthenium-based water oxidation catalyst attached to a tin-doped In2O3 (ITO) surface. The weak pH dependence is found to arise from the intrinsic differences in the rate constants of participating channels and the dependence of their relative contributions on pH. This model predicts that the apparent maximum rate constant will become pH-independent at higher pH, which is confirmed by experimental measurements. Our analysis also suggests that the dominant channels are electron transfer at lower pH and sequential PCET via electron transfer followed by fast proton transfer at higher pH. This work highlights the importance of considering multiple competing channels simultaneously for PCET processes.

Electro- and Photoinduced Interfacial Charge Transfers in Nanocrystalline Mesoporous TiO2 and TiO2/Iron Porphyrin Sensitized Films under CO2 Reduction Catalysis

Electro- and photochemical CO2 reduction (CO2R) is the quintessence of modern-day sustainable research. We report our studies on the electro- and photoinduced interfacial charge transfer occurring in a nanocrystalline mesoporous TiO2 film and two TiO2/iron porphyrin hybrid films (meso-aryl- and β-pyrrole-substituted porphyrins, respectively) under CO2R conditions. We used transient absorption spectroscopy (TAS) to demonstrate that, under 355 nm laser excitation and an applied voltage bias (0 to -0.8 V vs Ag/AgCl), the TiO2 film exhibited a diminution in the transient absorption (at -0.5 V by 35%), as well as a reduction of the lifetime of the photogenerated electrons (at -0.5 V by 50%) when the experiments were conducted under a CO2 atmosphere changing from inert N2. The TiO2/iron porphyrin films showed faster charge recombination kinetics, featuring 100-fold faster transient signal decays than that of the TiO2 film. The electro-, photo-, and photoelectrochemical CO2R performance of the TiO2 and TiO2/iron porphyrin films are evaluated within the bias range of -0.5 to -1.8 V vs Ag/AgCl. The bare TiO2 film produced CO and CH4 as well as H2, depending on the applied voltage bias. In contrast, the TiO2/iron porphyrin films showed the exclusive formation of CO (100% selectivity) under identical conditions. During the CO2R, a gain in the overpotential values is obtained under light irradiation conditions. This finding was indicative of a direct transfer of the photogenerated electrons from the film to absorbed CO2 molecules and an observed decrease in the decay of the TAS signals. In the TiO2/iron porphyrin films, we identified the interfacial charge recombination processes between the oxidized iron porphyrin and the electrons of the TiO2 conduction band. These competitive processes are considered to be responsible for the diminution of direct charge transfer between the film and the adsorbed CO2 molecules, explaining the moderate performances of the hybrid films for the CO2R.

Unraveling Sequential Oxidation Kinetics and Determining Roles of Multi-Cobalt Active Sites on Co3O4 Catalyst for Water Oxidation

The multi-redox mechanism involving multi-sites has great implications to dictate the catalytic water oxidation. Understanding the sequential dynamics of multi-steps in oxygen evolution reaction (OER) cycles on working catalysts is a highly important but challenging issue. Here, using quasi-operando transient absorption (TA) spectroscopy and a typical photosensitization strategy, we succeeded in resolving the sequential oxidation kinetics involving multi-active sites for water oxidation in OER catalytic cycle, with Co₃O₄ nanoparticles as model catalysts. When OER initiates from fast oxidation of surface Co²⁺ ions, both surface Co²⁺ and Co³⁺ ions are active sites of the multi-cobalt centers for water oxidation. In the sequential kinetics (Co²⁺ → Co³⁺ → Co⁴⁺), the key characteristic is fast oxidation and slow consumption for all the cobalt species. Due to this characteristic, the Co⁴⁺ intermediate distribution plays a determining role in OER activity and results in the slow overall OER kinetics. These insights shed light on the kinetic understanding of water oxidation on heterogeneous catalysts with multi-sites.

Charge carrier dynamics and reaction intermediates in heterogeneous photocatalysis by time-resolved spectroscopies

Sunlight as the most abundant renewable energy holds the promise to make our society sustainable. However, due to its low power density and intermittence, efficient conversion and storage of solar energy as a clean fuel are crucial. Apart from solar fuel synthesis, sunlight can also be used to drive other reactions including organic conversion and air/water purification. Given such potential of photocatalysis, the past few decades have seen a surge in the discovery of photocatalysts. However, the current photocatalytic efficiency is still very moderate. To address this challenge, it is important to understand fundamental factors that dominate the efficiency of a photocatalytic process to enable the rational design and development of photocatalytic systems. Many recent studies highlighted transient absorption spectroscopy (TAS) and time-resolved infrared (TRIR) spectroscopy as powerful approaches to characterise charge carrier dynamics and reaction pathways to elucidate the reasons behind low photocatalytic efficiencies, and to rationalise photocatalytic activities exhibited by closely related materials. Accordingly, as a fast-moving area, the past decade has witnessed an explosion in reports on charge carrier dynamics and reaction mechanisms on a wide range of photocatalytic materials. This critical review will discuss the application of TAS and TRIR in a wide range of heterogeneous photocatalytic systems, demonstrating the variety of ways in which these techniques can be used to understand the correlation between materials design, charge carrier behaviour, and photocatalytic activity. Finally, it provides a comprehensive outlook for potential developments in the area of time-resolved spectroscopies with an aim to provide design strategies for photocatalysts.

Selective visible-light photocatalysis of acetylene to ethylene using a cobalt molecular catalyst and water as a proton source

The production of polymers from ethylene requires the ethylene feed to be sufficiently purified of acetylene contaminant. Accomplishing this task by thermally hydrogenating acetylene requires a high temperature, an external feed of H₂ gas and noble-metal catalysts. It is not only expensive and energy-intensive, but also prone to overhydrogenating to ethane. Here we report a photocatalytic system that reduces acetylene to ethylene with ≥99% selectivity under both non-competitive (no ethylene co-feed) and competitive (ethylene co-feed) conditions, and near 100% conversion under the latter industrially relevant conditions. Our system uses a molecular catalyst based on earth-abundant cobalt operating under ambient conditions and sensitized by either [Ru(bpy)₃]²⁺ or an inexpensive organic semiconductor (metal-free mesoporous graphitic carbon nitride) under visible light. These features and the use of water as a proton source offer advantages over current hydrogenation technologies with respect to selectivity and sustainability.

Dye-sensitized solar cells strike back

Dye-sensitized solar cells (DSCs) are celebrating their 30th birthday and they are attracting a wealth of research efforts aimed at unleashing their full potential. In recent years, DSCs and dye-sensitized photoelectrochemical cells (DSPECs) have experienced a renaissance as the best technology for several niche applications that take advantage of DSCs' unique combination of properties: at low cost, they are composed of non-toxic materials, are colorful, transparent, and very efficient in low light conditions. This review summarizes the advancements in the field over the last decade, encompassing all aspects of the DSC technology: theoretical studies, characterization techniques, materials, applications as solar cells and as drivers for the synthesis of solar fuels, and commercialization efforts from various companies.

Reconciliation of Differences in Apparent Diffusion Coefficients Measured for Self-Exchange Electron Transfer between Molecules Anchored to Mesoporous Titanium Dioxide Thin Films

Redox-active sites present at large concentrations as part of a solid support or dissolved as molecules in fluid solutions undergo reversible self-exchange electron-transfer reactions. These processes can be monitored using a variety of techniques. Chronoamperometry and cyclic voltammetry are common techniques used to interrogate this behavior for molecules bound to mesoporous thin films of wide-bandgap semiconductors and insulators. In order to use these techniques to obtain accurate values for apparent diffusion coefficients, which are proxies for rate constants for self-exchange electron transfer, it is imperative to take into consideration nonidealities in redox titrations, parasitic currents, and ohmic resistances. Using spectroelectrochemical measurements taken concurrently with measurements of chronoamperometry data, we show that the spectroscopic data is not confounded from effects of parasitic currents or electroinactive dyes. However, we show that the thickness of the thin film over the region that is optically probed by the measurements must be known. When each of these considerations is included in data analyses, calculated apparent diffusion coefficients are, within error, independent of the method used to obtain the data. These considerations help reconcile variations in apparent diffusion coefficients measured using different techniques that have been reported over the past several decades and allow correct analyses to be performed in the future, independent of the method used to obtain the data.

Charge accumulation kinetics in multi-redox molecular catalysts immobilised on TiO2

Multi-redox catalysis requires the accumulation of more than one charge carrier and is crucial for solar energy conversion into fuels and valuable chemicals. In photo(electro)chemical systems, however, the necessary accumulation of multiple, long-lived charges is challenged by recombination with their counterparts. Herein, we investigate charge accumulation in two model multi-redox molecular catalysts for proton and CO2 reduction attached onto mesoporous TiO2 electrodes. Transient absorption spectroscopy and spectroelectrochemical techniques have been employed to study the kinetics of photoinduced electron transfer from the TiO2 to the molecular catalysts in acetonitrile, with triethanolamine as the hole scavenger. At high light intensities, we detect charge accumulation in the millisecond timescale in the form of multi-reduced species. The redox potentials of the catalysts and the capacity of TiO2 to accumulate electrons play an essential role in the charge accumulation process at the molecular catalyst. Recombination of reduced species with valence band holes in TiO2 is observed to be faster than microseconds, while electron transfer from multi-reduced species to the conduction band or the electrolyte occurs in the millisecond timescale. Finally, under light irradiation, we show how charge accumulation on the catalyst is regulated as a function of the applied bias and the excitation light intensity.

Zinc Porphyrin-Functionalized Fullerenes for the Sensitization of Titania as a Visible-Light Active Photocatalyst: River Waters and Wastewaters Remediation

Zinc porphyrin-functionalized fullerene [C60] derivatives have been synthesized and used to prepare titania-based composites. The electrochemical properties and HOMO and LUMO levels of the photosensitizers were determined by electrochemical measurements. Raman and IR techniques were used to study chemical groups present on the titania surface. Absorption properties of the composites were measured in the solid state by diffuse reflectance UV-Vis spectra (DRS). The zeta potential and aggregate sizes were determined using dynamic light scattering (DLS) and electrophoretic light scattering (ELS) techniques. Surface areas were estimated based on Brunauer–Emmett–Teller (BET) isotherms. The photocatalytic activity of the photocatalysts was tested using two model pollutants, phenol and methylene blue. The composite with the highest photocatalytic potential (1/TiO₂) was used for river and wastewater remediation. The photodegradation intermediates were identified by LC-UV/Vis-MS/MS techniques.

Optimization of Photocatalyst Excited- and Ground-State Reduction Potentials for Dye-Sensitized HBr Splitting

Dye-sensitized bromide oxidation was investigated using a series of four ruthenium polypyridyl photocatalysts anchored to SnO₂/TiO₂ core/shell mesoporous thin films through 2,2'-bipyridine-4,4'-diphosphonic acid anchoring groups. The ground- and excited-state reduction potentials were tuned over 500 mV by the introduction of electron withdrawing groups in the 4 and 4' positions of the ancillary bipyridine ligands. Upon light excitation of the surface-bound photocatalysts, excited-state electron injection yielded an oxidized photocatalyst that was regenerated through bromide oxidation. High injection quantum yields (Φ_inj) and regeneration quantum yields (Φ_reg) were essential to obtain efficient bromide oxidation yet required a photocatalyst that is both a potent photoreductant and a strong oxidant after excited-state injection. The four photocatalysts utilized in this manuscript ranged from unity Φ_inj (1.0) and minimal Φ_reg (0.037) to minimal Φ_inj (0.09) and unity Φ_reg (1.0). The photocatalyst that displayed the highest overall dye-sensitized photoelectrosynthesis cell performances exhibited near unity Φ_reg (0.99), while a significant Φ_inj was still preserved (0.59). Thus, these results highlighted the delicate interplay between the ground- and excited-state reduction potentials of photocatalysts for dye-sensitized hydrobromic acid splitting.

Vibrational structure analysis of cobalt fluoro-porphyrin surface coatings on gallium phosphide

Grazing angle attenuated total reflectance Fourier transform infrared (GATR–FTIR) spectroscopy is used to characterize chemically modified gallium phosphide (GaP) surfaces containing grafted cobalt(II) porphyrins with 3-fluorophenyl substituents installed at the meso-positions. In these hybrid constructs, porphyrin surface attachment is achieved using either a two-step method involving coordination of cobalt fluoro-porphyrin metal centers to nitrogen sites on an initially applied thin-film polypyridyl surface coating, or via a direct modification strategy using a cobalt fluoro-porphyrin precursor bearing a covalently bonded 4-vinylphenyl surface attachment group at a [Formula: see text]-position. Both surface-attachment chemistries leverage the UV-induced immobilization of alkenes but result in distinct structural connectivities of the grafted porphyrin units and their associated vibrational spectra. In particular, the in-plane deformation vibrational frequency of metalloporphyrin components in samples prepared via coordination to the polymeric interface is characterized by an eight wavenumber shift to higher frequencies compared to that measured on metalloporphyrin-modified surfaces prepared using the one-step attachment method. The more rigid ring structure in the polymeric architecture is consistent with coordination of porphyrin cobalt centers to pyridyl-nitrogen sites on the surface graft. These results demonstrate the use of GATR–FTIR spectroscopy as a sensitive tool for characterizing porphyrin-modified surfaces with absorption signals that are close to the detection limits of many common spectroscopic techniques.

Long-lived charge separation in dye–semiconductor assemblies: a pathway to multi-electron transfer reactions

Key approaches to achieve long-lived charge separation and promote conduction band mediated electron transfer in dye-sensitized semiconductor assemblies.

Amino acid appended perylene bisimides: self-assembly, immobilization on nanocrystalline TiO2, and electrochromic properties

Titanium dioxide (TiO₂) nanoparticle films have been used as a conducting support for the immobilisation of alanine-appended perylene bisimides (PBI-A) via dip-coating and carboxylate chemisorption.

Probing single-molecule electron-hole transfer dynamics at a molecule-NiO semiconductor nanocrystalline interface

Interfacial charge transfer dynamics in dye-sensitized NiO nanoparticles are being investigated for photocathodes in p-type dye-sensitized solar cells. In the photoreaction, after fast electron transfer from NiO to a molecule, the recombination of the hole in the nanoparticles with the electron in a reduced molecule plays an important role in the charge separation process and solar energy harvesting. Nevertheless, knowledge of the interfacial charge recombination (CR) rate and its mechanism is still limited due to the complex photoinduced electron and hole dynamics and lack of characterization of the inhomogeneity of the dynamics. Here, we report our work on probing interfacial charge recombination dynamics in Zn(ii)-5,10,15,20-tetra(3-carboxyphenyl)porphyrin (m-ZnTCPP) dye-sensitized NiO nanoparticles by correlating single-molecule fluorescence blinking dynamics with charge transfer dynamics using single-molecule photon-stamping spectroscopy. The correlated analyses of single-molecule fluorescence intensity, lifetime, and blinking reveal the intrinsic distribution and temporal fluctuation of interfacial charge transfer reactivity, which are closely related to site-specific molecular interactions and dynamics.

Probing the inhomogeneity and intermediates in the photosensitized degradation of rhodamine B by Ag3PO4 nanoparticles from an ensemble to a single molecule approach

This work reports the single-molecule observations of the photosensitized process and intermediates generation in the photodegradation of RhB by Ag₃PO₄ nanoparticles.

Carbon Nitride Supramolecular Hybrid Material Enabled High-Efficiency Photocatalytic Water Treatments

Surface defects in relation to surface compositions, morphology, and active sites play crucial roles in photocatalytic activity of graphitic carbon nitride (g-C₃N₄) material for highly reactive oxygen radicals production. Here, we report a high-efficiency carbon nitride supramolecular hybrid material prepared by patching the surface defects with inorganic clusters. Fe (III) {PO₄[WO(O₂)₂]₄} clusters have been noncovalently integrated on surface of g-C₃N₄, where the surface defects provide accommodation sites for these clusters and driving forces for self-assembly. During photocatalytic process, the activity of supramolecular hybrid is 1.53 times than pure g-C₃N₄ for the degradation of Rhodamine B (RhB) and 2.26 times for Methyl Orange (MO) under the simulated solar light. Under the mediation of H₂O₂ (50 mmol L^-1), the activity increases to 6.52 times for RhB and 28.3 times for MO. The solid cluster active sites with high specific surface area (SSA) defect surface promoting the kinetics of hydroxide radicals production give rise to the extremely high photocatalytic activity. It exhibits recyclable capability and works in large-scale demonstration under the natural sunlight as well and interestingly the environmental temperature has little effects on the photocatalytic activity.

Metalloporphyrin-modified semiconductors for solar fuel production

A novel synthetic method is used to prepare metalloporphyrin-modified gallium phosphide photocathodes for solar-driven hydrogen evolution from water.

We report a direct one-step method to chemically graft metalloporphyrins to a visible-light-absorbing gallium phosphide semiconductor with the aim of constructing an integrated photocathode for light activating chemical transformations that include capturing, converting, and storing solar energy as fuels. Structural characterization of the hybrid assemblies is achieved using surface-sensitive spectroscopic methods, and functional performance for photoinduced hydrogen production is demonstrated via three-electrode electrochemical testing combined with photoproduct analysis using gas chromatography. Measurements of the total per geometric area porphyrin surface loadings using a cobalt-porphyrin based assembly indicate a turnover frequency ≥3.9 H₂ molecules per site per second, representing the highest reported to date for a molecular-catalyst-modified semiconductor photoelectrode operating at the H⁺/H₂ equilibrium potential under 1-sun illumination.

Simultaneous Spectroscopic and Topographic Imaging of Single-Molecule Interfacial Electron-Transfer Reactivity and Local Nanoscale Environment

The fundamental information related to the energy flow between molecules and substrate surfaces as a function of surface site geometry and molecular structure is critical for understanding interfacial electron-transfer (ET) dynamics. The inhomogeneous nanoscale molecule-surface and molecule-molecule interactions are presumably the origins of the complexity in interfacial ET dynamics; thus, identifying the environment of molecules at nanoscale is crucial. We have developed atomic force microscopy (AFM) correlated single-molecule fluorescence intensity/lifetime imaging microscopy (AFM-SMFLIM) capable of identifying and characterizing individual molecules distributed across the heterogeneous surface at the nanometer length scale. Using the novel AFM-SMFLIM imaging, we are able to obtain nanoscale morphology and interfacial ET dynamics at a single-molecule level. Moreover, the observed blinking behavior and lifetime of each molecule in combination with the topography of the environment at nanoscale provide the location of each molecule on the surface (TiO2 vs cover glass) at nanoscale and the coupling strength of each molecule with TiO2 nanoparticles.

Aqueous Hydricity of Late Metal Catalysts as a Continuum Tuned by Ligands and the Medium

Aqueous hydride transfer is a fundamental step in emerging alternative energy transformations such as H2 evolution and CO2 reduction. "Hydricity," the hydride donor ability of a species, is a key metric for understanding transition metal hydride reactivity, but comprehensive studies of aqueous hydricity are scarce. An extensive and self-consistent aqueous hydricity scale is constructed for a family of Ru and Ir hydrides that are key intermediates in aqueous catalysis. A reference hydricity is determined using redox potentiometry and spectrophotometric titration for a particularly water-soluble species. Then, relative hydricity values for a range of species are measured using hydride transfer equilibria, taking advantage of expedient new synthetic procedures for Ru and Ir hydrides. This large collection of hydricity values provides the most comprehensive picture so far of how ligands impact hydricity in water. Strikingly, we also find that hydricity can be viewed as a continuum in water: the free energy of hydride transfer changes with pH, buffer composition, and salts present in solution.

A single crystalline porphyrinic titanium metal-organic framework

We have successfully synthesized a single crystalline porphyrinic titanium MOF, namely PCN-22. PCN-22 represents an important step towards mimicking dye sensitized TiO₂ in MOFs.

We successfully assembled the photocatalytic titanium-oxo cluster and photosensitizing porphyrinic linker into a metal–organic framework (MOF), namely PCN-22. A preformed titanium-oxo carboxylate cluster is adopted as the starting material to judiciously control the MOF growth process to afford single crystals. This synthetic method is useful to obtain highly crystalline titanium MOFs, which has been a daunting challenge in this field. Moreover, PCN-22 demonstrated permanent porosity and photocatalytic activities toward alcohol oxidation.

Electrochromic graphene molecules

Polyclic aromatic hydrocarbons also called Graphene Molecules (GMs), with chemical composition C132H36(COOH)2 were synthesized in situ on the surface of transparent nanocrystalline indium tin oxide (nc-ITO) electrodes and their electronic structure was studied electrochemically and spectro-electrochemically. Variations in the potential applied onto the nc-ITO/GM electrodes induce only small changes in the observed current, but they produce dramatic changes in the absorption of the GMs, which are associated with their oxidation and reduction. Analysis of the absorption changes using a modified Nernst equation is used to determine standard potentials associated with the individual charge transfer processes. For the GMs prepared here, these were found to be E1,ox(0) = 0.77 ± 0.01 V and E2,ox(0) = 1.24 ± 0.02 V vs NHE for the first and second oxidation and E1,red(0) = -1.50 ± 0.04 V for the first reduction. The charge transfer processes are found to be nonideal. The nonideality factors associated with the oxidation and reduction processes are attributed to strong interactions between the GM redox centers. Under the conditions of potential cycling, GMs show rapid (seconds) color change with high contrast and stability. An electrochromic application is demonstrated wherein the GMs are used as the optically active component.

Ultrafast electron and energy transfer in dye-sensitized iron oxide and oxyhydroxide nanoparticles

An emerging area in chemical science is the study of solid-phase redox reactions using ultrafast time-resolved spectroscopy. We have used molecules of the photoactive dye 2',7'-dichlorofluorescein (DCF) anchored to the surface of iron(III) oxide nanoparticles to create iron(II) surface atoms via photo-initiated interfacial electron transfer. This approach enables time-resolved study of the fate and mobility of electrons within the solid phase. However, complete analysis of the ultrafast processes following dye photoexcitation of the sensitized iron(III) oxide nanoparticles has not been reported. We addressed this topic by performing femtosecond transient absorption (TA) measurements of aqueous suspensions of uncoated and DCF-sensitized iron oxide and oxyhydroxide nanoparticles, and an aqueous iron(III)-dye complex. Following light absorption, excited state relaxation times of the dye of 115-310 fs were found for all samples. Comparison between TA dynamics on uncoated and dye-sensitized hematite nanoparticles revealed the dye de-excitation pathway to consist of a competition between electron and energy transfer to the nanoparticles. We analyzed the TA data for hematite nanoparticles using a four-state model of the dye-sensitized system, finding electron and energy transfer to occur on the same ultrafast timescale. The interfacial electron transfer rates for iron oxides are very close to those previously reported for DCF-sensitized titanium dioxide (for which dye-oxide energy transfer is energetically forbidden) even though the acceptor states are different. Comparison of the alignment of the excited states of the dye and the unoccupied states of these oxides showed that the dye injects into acceptor states of different symmetry (Ti t2gvs. Fe eg).

Photocatalytic CO2 reduction using a molecular cobalt complex deposited on TiO2 nanoparticles

A macrocyclic cobalt catalyst is effectively coupled with TiO₂ nanoparticles for photocatalytic CO₂ reduction.

Excited state electron transfer after visible light absorption by the Co(i) state of vitamin B12

The Co(i) form of vitamin B₁₂ was found to undergo excitation wavelength-dependent excited state electron transfer to TiO₂.

The photocatalytic activity and degradation mechanism of methylene blue over copper(ii) tetra(4-carboxyphenyl) porphyrin sensitized TiO2under visible light irradiation

Copper(ii)tetra(4-carboxyphenyl) porphyrin were chemically sensitized on TiO₂, its electronic relaxation dynamics and the visible-light induced catalytic mechanism are discussed.

Ligand coordination and spin crossover in a nickel porphyrin anchored to mesoporous TiO2 thin films

The coordination and spin equilibrium of a Ni(II) meso-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 S = 0 ((1)A1g) Ni(II) compound and a high-spin S = 1 ((3)B1g) 6-coordinate compound. Temperature-dependent equilibrium constants were consistently smaller for the surface-anchored NiP/TiO2, as were the absolute values of ΔH and ΔS. 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, Ea = 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(II)P*/TiO2 indicates that internal conversion to ligand field states was at least 10 times greater than that of excited-state injection into TiO2.