Robert C. Snoeberger

Hydroxamate anchors for water-stable attachment to TiO2 nanoparticles

Surface functionalization of nanoparticles is of broad interest, such as for dye attachment in dye-sensitized solar cells (DSSCs) and photocatalysis. Visible-light photoexcitation of the dye gives interfacial electron transfer (IET) into the conduction band of a semiconductor host. In a Gr€atzel cell, TiO2 is functionalized with Ru polypyridyl complexes that attach via carboxylate substituents that permit ultrafast IET but are unstable in aqueous conditions. We now report on hydroxamate anchors for robust TiO2 functionalization even in aqueous conditions. Hydroxamate ligands bind tightly to transition metals, even in water. For example, bacterial siderophores that contain hydroxamates can dissolve Fe(III) from the oxide. Recent studies have reported binding of hydroxamic acids to TiO2. 8 Here, we investigate their potential as robust anchors for functionalization of TiO2 thinfilms commonly used in solar energy conversion and photocatalysis. We synthesize and deposit a hydroxamate-functionalized terpyridine and demonstrate visible-light sensitization of TiO2 and activation of Mn adsorbates by ultrafast IET by using spectroscopy and molecular modeling. The synthesis (Scheme 1) builds on prior methods and proceeds in two steps in good yield. The methyl ester (1) reacts with O-Bn hydroxylamine (BnONH2) in the presence of LiHMDS to give the corresponding ester. The ester is then deprotected with H2 and Pd/C to give the product 2. Degussa P25 TiO2 nanoparticles (NPs) were sensitized with a solution of 2 in dry EtOH using known techniques. The resulting sensitized nanoparticles were characterized using UV-visible and FTIR spectroscopy (see Fig. S1 and S2†). The spectroscopic data are consistent with 2 anchoring to TiO2 via the hydroxamate. Upon binding, the disappearance of a C]O stretch at 1635 cm 1 present in the IR of unbound 2 is consistent with a O–CR]N–O unit

Deposition of an oxomanganese water oxidation catalyst on TiO2 nanoparticles: computational modeling, assembly and characterization

Inexpensive water oxidation catalysts are needed to develop photocatalytic solar cells that mimic photosynthesis and produce fuel from sunlight and water. This paper reports the successful attachment of a dinuclear di-µ-oxo manganese water oxidation catalyst [H2O(terpy)MnIII(µ-O)2 MnIV(terpy)H2O](NO3)3 (1, terpy = 2,2′:6′2″-terpyridine) onto TiO2 nanoparticles (NPs) via direct adsorption, or in situ synthesis. The resulting surface complexes are characterized by EPR and UV-visible spectroscopy, electrochemical measurements and computational modeling. We conclude that the mixed-valence (III,IV) state of 1 attaches to near-amorphous TiO2 NPs by substituting one of its water ligands by the TiO2 NP, as suggested by low-temperature (7 K) EPR data. In contrast, the analogous attachment onto well-crystallized TiO2 NPs leads to dimerization of 1 forming Mn(IV) tetramers on the TiO2 surface as suggested by EPR spectroscopy and electrochemical studies.

Water-stable, hydroxamate anchors for functionalization of TiO2 surfaces with ultrafast interfacial electron transfer

A novel class of derivatized hydroxamic acid linkages for robust sensitization of TiO2 nanoparticles (NPs) under various aqueous conditions is described. The stability of linkages bound to metal oxides under various conditions is important in developing photocatalytic cells which incorporate transition metal complexes for solar energy conversion. In order to compare the standard carboxylate anchor to hydroxamates, two organic dyes differing only in anchoring groups were synthesized and attached to TiO2 NPs. At acidic, basic, and close to neutral pH, hydroxamic acid linkages resist detachment compared to the labile carboxylic acids. THz spectroscopy was used to compare ultrafast interfacial electron transfer (IET) into the conduction band of TiO2 for both linkages and found similar IET characteristics. Observable electron injection and stronger binding suggest that hydroxamates are a suitable class of anchors for designing water stable molecules for functionalizing TiO2.

Characterization of an amorphous iridium water-oxidation catalyst electrodeposited from organometallic precursors

Upon electrochemical oxidation of the precursor complexes [Cp*Ir(H(2)O)(3)]SO(4) (1) or [(Cp*Ir)(2)(OH)(3)]OH (2) (Cp* = pentamethylcyclopentadienyl), a blue layer of amorphous iridium oxide containing a carbon admixture (BL) is deposited onto the anode. The solid-state, amorphous iridium oxide material that is formed from the molecular precursors is significantly more active for water-oxidation catalysis than crystalline IrO(2) and functions as a remarkably robust catalyst, capable of catalyzing water oxidation without deactivation or significant corrosion for at least 70 h. Elemental analysis reveals that BL contains carbon that is derived from the Cp* ligand (∼ 3% by mass after prolonged electrolysis). Because the electrodeposition of precursors 1 or 2 gives a highly active catalyst material, and electrochemical oxidation of other iridium complexes seems not to result in immediate conversion to iridium oxide materials, we investigate here the nature of the deposited material. The steps leading to the formation of BL and its structure have been investigated by a combination of spectroscopic and theoretical methods. IR spectroscopy shows that the carbon content of BL, while containing some C-H bonds intact at short times, is composed primarily of components with C═O fragments at longer times. X-ray absorption and X-ray absorption fine structure show that, on average, the six ligands to iridium in BL are likely oxygen atoms, consistent with formation of iridium oxide under the oxidizing conditions. High-energy X-ray scattering (HEXS) and pair distribution function (PDF) analysis (obtained ex situ on powder samples) show that BL is largely free of the molecular precursors and is composed of small, <7 Å, iridium oxide domains. Density functional theory (DFT) modeling of the X-ray data suggests a limited set of final components in BL; ketomalonate has been chosen as a model fragment because it gives a good fit to the HEXS-PDF data and is a potential decomposition product of Cp*.

Interfacial Electron Transfer into Functionalized Crystalline Polyoxotitanate Nanoclusters

Interfacial electron transfer (IET) between a chromophore and a semiconductor nanoparticle is one of the key processes in a dye-sensitized solar cell. Theoretical simulations of the electron transfer in polyoxotitanate nanoclusters Ti(17)O(24)(OPr(i))(20) (Ti(17)) functionalized with four p-nitrophenyl acetylacetone (NPA-H) adsorbates, of which the atomic structure has been fully established by X-ray diffraction measurements, are presented. Complementary experimental information showing IET has been obtained by EPR spectroscopy. Evolution of the time-dependent photoexcited electron during the initial 5 fs after instantaneous excitation to the NPA LUMO + 1 has been evaluated. Evidence for delocalization of the excitation over multiple chromophores after excitation to the NPA LUMO + 2 state on a 15 fs time scale is also obtained. While chromophores are generally considered electronically isolated with respect to neighboring sensitizers, our calculations show that this is not necessarily the case. The present work is the most comprehensive study to date of a sensitized semiconductor nanoparticle in which the structure of the surface and the mode of molecular adsorption are precisely defined.

Single-molecule interfacial electron transfer in donor-bridge-nanoparticle acceptor complexes.

Photoinduced interfacial electron transfer (IET) in sulforhodamine B (SRhB)-aminosilane-Tin oxide (SnO(2)) nanoparticle donor-bridge-acceptor complexes has been studied on a single molecule and ensemble average level. On both SnO(2) and ZrO(2), the sum of single molecule fluorescence decays agree with the ensemble average results, suggesting complete sampling of molecules under single molecule conditions. Shorter fluorescence lifetime on SnO(2) than on ZrO(2) is observed and attributed to IET from SRhB to SnO(2). Single molecule lifetimes fluctuate with time and vary among different molecules, suggesting both static and dynamic IET heterogeneity in this system. Computational modeling of the complexes shows a distribution of molecular conformation, leading to a distribution of electronic coupling strengths and ET rates. It is likely that the conversion between these conformations led to the fluctuation of ET rate and fluorescence lifetime on the single molecule level.

Thioflavin T and Its Photoirradiative Derivatives: Exploring Their Spectroscopic Properties in the Absence and Presence of Amyloid Fibrils

In this work, we found that, during storage or after UV irradiation, ThT is demethylated or oxidized, forming three derivatives. These three derivatives were purified by high performance liquid chromatography and characterized by mass and nuclear magnetic resonance spectroscopy and the spectroscopic properties of pure ThT and the derivatives carefully compared. Our results show that the emission peak at 450 nm results from oxidized ThT and not from the monomeric form of ThT, as previously proposed. The partial conversion of ThT into oxidized and demethylated derivatives has an effect on amyloid detection using ThT assay. Irradiated ThT has the same lag time as pure ThT in the amyloidogenesis of insulin, but the intensity of the emitted fluorescence is significantly decreased.

Pharmaceutical formulation affects titanocene transferrin interactions

Since the discovery of the anticancer activity of titanocene dichloride (TDC), many derivatives have been developed and evaluated. MKT4, a soluble, water-stable formulation of TDC, was used for both Phase I and Phase II human clinical trials. This formulation is investigated here by using (1)H and (13)C NMR, FT-ICR mass spectrometry, and UV/vis-detected pH-dependent speciation. DFT calculations are also utilized to assess the likelihood of proposed species. Human serum transferrin has been identified as a potential vehicle for the Ti anticancer drugs; these studies examine whether and how formulation of TDC as MKT4 may influence its interactions, both thermodynamic and kinetic, with human serum transferrin by using UV/vis absorption and fluorescence quenching. MKT4 binds differently than TDC to transferrin, showing different kinetics of binding as well as a different molar absorptivity of binding (7500 M(-1) cm(-1) per site). Malate, used in the buffer for MKT4 administration, acts as a synergistic anion for Ti binding, shifting the tyrosine to Ti charge transfer energy and decreasing the molar absorptivity to 5000 M(-1) cm(-1) per site. These differences may have had consequences after the change from TDC to MKT4 in human clinical trials.

The influence of surface hydration on the interfacial electron transfer dynamics from Rhodamine B into SnO2

The influence of surface hydration on the time-scales and mechanisms of interfacial electron transfer from rhodamine B into SnO2 is investigated. We combine molecular dynamics simulations and quantum dynamics propagation of transient electronic excitations to analyze the regulatory role of water molecules affecting the adsorbate-semiconductor interactions and the underlying electronic couplings that determine the electron injection times. The reported results are essential to advance our understanding of interfacial electron transfer dynamics in dye sensitized semiconductor surfaces at the molecule level, including fundamental interactions that affect the efficiency of interfacial electronic processes in dye-sensitized solar cells as well as in a wide range of other technological applications.