Structural Gating Enhances Long-Distance Light-Driven Interfacial Electron Transfer.

Structural gating provides a molecular means to transfer electrons preferentially in one desired vectorial direction, a behavior needed for applications in artificial photosynthesis. At the interfaces utilized herein, visible-light absorption by a transition metal complex a "structural gate" by planarization of otherwise rotating phenyl rings in phenylene ethynylene (PE) bridge units. Planarization provides a conjugated pathway for electron flow toward a conductive oxide surface.

Interfacial electron transfer of perylenes: Influence of the anchor binding mode.

Interfacial electron transfer (IET) through saturated single-linker and dual-linker groups from a perylene chromophore into nanostructured TiO2 films was studied by ultrafast spectroscopy. Perylene chromophores with one and two propanoic acid linker groups in the peri and ortho positions were investigated. In comparison to previously studied perylenes bound via unsaturated acrylic acid linkers, the chromophores with saturated linkers showed bi-exponential IET dynamics.

Reorganization Energies for Interfacial Electron Transfer across Phenylene Ethynylene Rigid-Rod Bridges.

A family of three ruthenium bipyridyl rigid-rod compounds of the general form [Ru(bpy)(LL)](PF) were anchored to mesoporous thin films of tin-doped indium oxide (ITO) nanocrystals. Here, LL is a 4-substituted 2,2-bipyridine (bpy) ligand with varying numbers of conjugated phenylenethynylene bridge units between the bipyridine ring and anchoring group consisting of a bis-carboxylated isophthalic group. The visible absorption spectra and the formal potentials, (), of the surface anchored rigid-rods were insensitive to the presence of the phenylene ethynylene bridge units in 0.

Self-Assembled Monolayers for Improved Charge Injection of Silver Back Electrodes in Inverted Organic Electronic Devices.

Self-assembled monolayers (SAMs) formed from thiol compounds bound to Ag and Au electrodes have been used as an important strategy in improving the stability and efficiency of optoelectronic devices. Thiol compounds provide only one binding site with the metal electrode which limits their influence. Dithiolane/dithiol compounds can provide multiple binding sites and could be useful in enhancing the performance of the device.

Controlling and Optimizing Photoinduced Charge Transfer across Ultrathin Silica Separation Membrane with Embedded Molecular Wires for Artificial Photosynthesis.

Ultrathin amorphous silica membranes with embedded organic molecular wires (oligo(-phenylenevinylene), three aryl units) provide chemical separation of incompatible catalytic environments of CO reduction and HO oxidation while maintaining electronic and protonic coupling between them. For an efficient nanoscale artificial photosystem, important performance criteria are high rate and directionality of charge flow. Here, the visible-light-induced charge flow from an anchored Ru bipyridyl light absorber across the silica nanomembrane to CoO water oxidation catalyst is quantitatively evaluated by photocurrent measurements.

Synthesis and Properties of Perylene-Bridge-Anchor Chromophoric Compounds.

The quest to control chromophore/semiconductor properties to enable new technologies in energy and information science requires detailed understanding of charge carrier dynamics at the atomistic level, which can often be attained through the use of model systems. Perylene-bridge-anchor compounds are successful models for studying fundamental charge transfer processes on TiO, which remains among the most commonly investigated and technologically important interfaces, mostly because of perylene's advantageous electronic and optical properties. Nonetheless, the ability to fully exploit synthetically the substitution pattern of perylene with linker (= bridge-anchor) units remains little explored.

Helical Peptides Design for Molecular Dipoles Functionalization of Wide Band Gap Oxides.

The use of helical hexapeptides to establish a surface dipole layer on a TiO substrate, with the goal of influencing the energy levels of a coadsorbed chromophore, is explored. Two helical hexapeptides, synthesized from 2-amino isobutyric acid (Aib) residues, were protected at the N-terminus with a carboxybenzyl group (Z) and at the C-terminus carried either a carboxylic acid or an isophthalic acid (Ipa) anchor group to form Z-(Aib)-COOH or Z-(Aib)-Ipa, respectively. Using a combination of vibrational and photoemission spectroscopies, bonding of the two peptides to TiO surfaces (either nanostructured or single-crystal TiO(110)) was found to be highly dependent on the anchor group, with Ipa establishing a monolayer much more efficiently than COOH.

Comparison of ZnO surface modification with gas-phase propiolic acid at high and medium vacuum conditions.

Recent advances in preservation of the morphology of ZnO nanostructures during dye sensitization required the use of a two-step preparation procedure. The first step was the key for preserving ZnO materials morphology. It required exposing clean ZnO nanostructures to a gas-phase prop-2-ynoic acid (propiolic acid) in vacuum.

Vibrational Spectroscopy on Photoexcited Dye-Sensitized Films via Pump-Degenerate Four-Wave Mixing.

Molecular sensitization of semiconductor films is an important technology for energy and environmental applications including solar energy conversion, photocatalytic hydrogen production, and water purification. Dye-sensitized films are also scientifically complex and interesting systems with a long history of research. In most applications, photoinduced heterogeneous electron transfer (HET) at the molecule/semiconductor interface is of critical importance, and while great progress has been made in understanding HET, many open questions remain.

Morphology-Preserving Sensitization of ZnO Nanorod Surfaces via Click-Chemistry.

Films of ZnO nanorods grown by chemical vapor deposition were functionalized with a chromophore in a stepwise process that preserves the surface morphology. In the first step, the ZnO nanorods were functionalized by exposure to prop-2-ynoic acid (propiolic acid) in vacuum, which did bind through the COOH group leading to a ZnO surface functionalized with ethyne moieties (ethyne/ZnO). In the second step, 9-(4-azidophenyl)-2,5-di-tert-butylperylene (DTBPe-Ph-N) was reacted with the ethyne/ZnO surface via copper-catalyzed azide-alkyne click reaction (CuAAC) in solution to form the DTBPe-functionalized surface (DTBPe/ZnO).

Functionalization of MgZnO nanorod films and characterization by FTIR microscopic imaging.

Metal organic chemical vapor deposition grown films consisting of MgZnO (4% < x < 5%) nanorod arrays (MgZnO) were functionalized with 11-azidoundecanoic acid (1). The MgZnO was used instead of pure ZnO to take advantage of the etching resistance of the MgZnO during the binding and subsequent sensing device fabrication processes of sensor devices, while the low Mg composition level ensures that selected ZnO properties useful for sensors development, such as piezoelectricity, are retained. Compound 1 was bound to the MgZnO surface through the carboxylic acid group, leaving the azido group available for click chemistry and as a convenient infrared spectroscopy (IR) probe.

Heterogeneous Electron-Transfer Dynamics through Dipole-Bridge Groups.

Heterogeneous electron transfer (HET) between photoexcited molecules and colloidal TiO has been investigated for a set of Zn-porphyrin chromophores attached to the semiconductor via linkers that allow to change level alignment by 200 meV by reorientation of the dipole moment. These unique dye molecules have been studied by femtosecond transient absorption spectroscopy in solution and adsorbed on the TiO colloidal film in vacuum. In solution energy transfer from the excited chromophore to the dipole group has been identified as a slow relaxation pathway competing with S-S internal conversion.

Photoelectrochemical properties of porphyrin dyes with a molecular dipole in the linker.

The electronic properties of three porphyrin-bridge-anchor photosensitizers are reported with (1a, 1e, 3a and 3e) or without (2a and 2e) an intramolecular dipole in the bridge. The presence and orientation of the bridge dipole is hypothesized to influence the photovoltaic properties due to variations in the intrinsic dipole at the semiconductor-molecule interface. Electrochemical studies of the porphyrin-bridge-anchor dyes self-assembled on mesoporous nanoparticle ZrO2 films, show that the presence or direction of the bridge dipole does not have an observable effect on the electronic properties of the porphyrin ring.

Synthesis of Zinc Tetraphenylporphyrin Rigid Rods with a Built-In Dipole.

Three Zn(II) tetraphenylporphyrins (ZnTPP) were synthesized to study the influence of a molecular dipole on the energy level alignment of a chromophore bound to a metal oxide semiconductor: ZnTPP-PE(DA)-IpaOMe (1), ZnTPP-PE-IpaOMe (2), and ZnTPP-PE(AD)-IpaOMe (3). Each contained a rigid-rod linker made of a p-phenylene ethynylene (PE) moiety terminated with the methyl ester of an isophthalic acid unit (Ipa). Porphyrins 1 and 3 contained an intramolecular dipole in the central phenyl ring, which was built by introducing electron donor (D, NMe2) and acceptor (A, NO2) substituents in para position to each other.

Synthesis and electronic properties of 1,2-hemisquarimines and their encapsulation in a cucurbit[7]uril host.

The synthesis of a new class of robust squaraine dyes, colloquially named 1,2-hemisquarimines (1,2-HSQiMs), through the microwave-assisted condensation of aniline derivatives with the 1,2-squaraine core is reported. In CH3CN, 1,2-HSQiMs show a broad absorption band with a high extinction coefficient and a maximum at around λ=530 nm, as well as an emission band centered at about λ=574 nm, that are pH dependent. Protonation of the imine nitrogen causes a redshift of both absorption and emission maxima, with a concomitant increase in the lifetime of the emitting excited state.

Homoleptic "star" Ru(II) polypyridyl complexes: shielded chromophores to study charge-transfer at the sensitizer-TiO2 interface.

Three homoleptic star-shaped ruthenium polypyridyl complexes, termed Star YZ1, Star YZ2, and Star YZ3, where the Ru(II) center is coordinated to three bipyridine ligands each carrying two oligo(phenylene ethynylene) (OPE) rigid linker units terminating with isophthalic ester (Ipa) groups for binding to metal-oxide surfaces were synthesized. In Star YZ3, each OPE linker was substituted with two n-butoxy (n-BuO) solubilizing groups. Star complex YZ4, which is homoleptic but lacks the octahedral symmetry, was synthesized as a reference compound.

Distance dependent electron transfer at TiO2 interfaces sensitized with phenylene ethynylene bridged Ru(II)-isothiocyanate compounds.

Excess electrons present in semiconductor nanocrystallites generate a significant electric field, yet the role this field plays in molecular charge transfer processes remains poorly understood. Three ruthenium bipyridyl cis-Ru(bpy)(LL)(NCS)2 compounds, where LL is a 4-substituted bpy, with zero, one, or two phenylene ethynylene bridge units, were anchored to mesoporous nanocrystalline TiO2 thin films to specifically quantify interfacial charge transfer with chromophores designed to be set at variable distances from the surface. Injection of electrons into TiO2 resulted in a blue shift of the metal-to-ligand charge transfer absorption consistent with an underlying Stark effect.

Functionalization of nanostructured ZnO films by copper-free click reaction.

The copper-free click reaction was explored as a surface functionalization methodology for ZnO nanorod films grown by metal organic chemical vapor deposition (MOCVD). 11-Azidodecanoic acid was bound to ZnO nanorod films through the carboxylic acid moiety, leaving the azide group available for Cu-free click reaction with alkynes. The azide-functionalized layer was reacted with 1-ethynylpyrene, a fluorescent probe, and with alkynated biotin, a small biomolecule.

Morphology effects on the biofunctionalization of nanostructured ZnO.

A stepwise surface functionalization methodology was applied to nanostructured ZnO films grown by metal organic chemical vapor deposition (MOCVD) having three different surface morphologies (i.e., nanorod layers (ZnO films-N), rough surface films (ZnO films-R), and planar surface films (ZnO films-P).

Fluorescence enhancement of di-p-tolyl viologen by complexation in cucurbit[7]uril.

A viologen derivative, 1,1'-di-p-tolyl-(4,4'-bipyridine)-1,1'-diium dichloride (DTV(2+)), was studied in solution and encapsulated in cucurbit[7]uril (CB7), a macrocyclic host. Upon encapsulation, DTV(2+) exhibited dramatically enhanced fluorescence. Aqueous solutions of DTV(2+) were weakly fluorescent (Φ = 0.

Photoinduced electron transfer across a molecular wall: coumarin dyes as donors and methyl viologen and TiO2 as acceptors.

Coumarins C-153, C-480, and C-1 formed 1:2 (guest:host) complexes with a water-soluble cavitand having eight carboxylic acid groups (OA) in aqueous borate buffer solution. The complexes were photoexcited in the presence of electron acceptors (methyl viologen, MV(2+), or TiO(2)) to probe the possibility of electron transfer between a donor and an acceptor physically separated by a molecular wall. In solution at basic pH, the dication MV(2+) was associated to the exterior of the complex C-153@OA(2), as suggested by diffusion constants (~1.