Elena Galoppini

Zinc(II) Tetraarylporphyrins Anchored to TiO2, ZnO, and ZrO2 Nanoparticle Films through Rigid-Rod Linkers

A series of six Zn(II) tetraphenylporphyrins (ZnTPP), with a phenyl (P) or oligophenyleneethynylene (OPE = (PE) n ) rigid-rod bridge varying in length (9-30 A) and terminated with an isophthalic acid (Ipa) anchoring unit, were prepared as model dyes for the study of sensitization processes on metal oxide semiconductor nanoparticle surfaces (MO(n) = TiO(2), ZnO, and insulating ZrO(2)). The dyes were designed such that the electronic properties of the central porphyrin chromophore remained consistent throughout the series, with the rigid-rod anchoring unit allowing each porphyrin unit to be located at a fixed distance from the metal oxide nanoparticle surface. Electronic communication between the porphyrin and the rigid-rod unit was not desired. Rigid-rod porphyrins ZnTPP-Ipa, ZnTPP-P-Ipa, ZnTPP-PE-Ipa, ZnTPP-(PE)(2)-Ipa, ZnTPP-(PE)(3)-Ipa, and ZnTMP-Ipa (with mesityl substituents on the porphyrin ring) were synthesized using combinations of mixed aldehyde condensations and Pd-catalyzed cross-coupling reactions. Their properties, in solution and bound, were compared with that of Zn(II) 5,10,15,20-tetra(4-carboxyphenyl)porphyrin ( p-ZnTCPP) as the reference compound. Solution UV-vis and steady-state fluorescence spectra for all six rigid-rod-Ipa porphyrins were almost identical to each other and to that of p-ZnTCPP. Cyclic voltammetry and differential pulse voltammetry scans of the methyl ester derivatives of the six rigid-rod-Ipa porphyrins, recorded in dichloromethane/electrolyte, exhibited redox behavior typical of ZnTPP porphyrins, with the first oxidation in the range +0.99 to 1.09 V vs NHE. All six rigid-rod-Ipa porphyrins and p-ZnTCPP were bound to metal oxide (MO(n) = TiO(2), ZnO, and insulating ZrO(2)) nanoparticle films. The Fourier transform infrared attenuated total reflectance spectra of all compounds bound to MO n films showed a broad band at 1553-1560 cm(-1) assigned to the v(CO(2)(-)) asymmetric stretching mode. Splitting of the Soret band into two bands at 411 and 423 nm in the UV-vis spectra of the bound compounds, and broadening and convergence of both fluorescence emission bands in the fluorescence spectra of the porphyrins bound to insulating ZrO(2) were also observed. Such changes were less evident for ZnTMP-Ipa, which has mesityl substituents on the porphyrin ring to prevent aggregation. Steady-state fluorescence emission of rigid-rod-Ipa porphyrins bound to TiO(2) and ZnO through the longest bridges (>14 A) showed residual fluorescence emission, while fluorescence quenching was observed for the shortest compounds.

Distance Dependent Electron Transfer at TiO2 Interfaces Sensitized with Phenylene Ethynylene Bridged RuII–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. The electroabsorption data were used to quantify the electric field experienced by the compounds that decreased from 0.85 to 0.22 MV/cm as the number of OPE spacers increased from 0 to 2. Charge recombination on the 10(-8)-10(-5) s time scale correlated with the magnitude of the electric field with an apparent attenuation factor β = 0.12 Å(-1). Slow components to charge recombination observed on the 10(-4)-10(-1) s time scale that were unaffected by temperature, irradiance, or the bridge units present on the molecular sensitizer were attributed to electron tunneling between TiO2 acceptor states. The photocurrent efficiencies of solar cells based on these compounds decreased markedly when the bridge units were present on the sensitizer. Iodine was found to form adducts with all three compounds, K = 1.8 ± 0.2 × 10(4) M(-1), but only significantly lowered the excited state injection yield for those that possessed the bridge units.

Molecular host–guest complexes: Shielding of guests on semiconductor surfaces

A recent and promising development in the field of renewable energy is the use of molecular containers (cyclodextrins, cucurbiturils, Cram-type hosts, and calixarenes) to bind encapsulated chromophores or redox-active compounds to the surface of TiO2 nanoparticles and other nanostructured wide bandgap metal oxide semiconductors. The host provides an alternative method of adsorption of molecules on semiconductors and shields the guest from the heterogenous interface. The novel hybrid systems exhibited photophysical and electrochemical properties that differ from the properties of layers obtained by directly attaching the chromophore to the semiconductor through binding groups, including slower and homogeneous interfacial electron recombination dynamics. In addition to DSSCs, the guest@host/semiconductor systems find applications as sensors, electrochromic windows, and LEDs.

Synthesis of a molecular tripod to anchor molecular coordination compounds to semiconductor nanoparticles

A 1,3,5,7-tetraphenyladamantane derivative having three arms terminating with carboxylic acids and a fourth substituted with a 9,10-phenanthrolinyl ligand was prepared as a new kind of molecular linkage to study dynamics of electron injection at the interface of TiO2 nanoparticles.

A ZnO Nanostructure-Based Quartz Crystal Microbalance Device for Biochemical Sensing

We report a ZnO-nanostructure-based quartz crystal microbalance (nano-QCM) device for biosensing applications. ZnO nanotips are directly grown on the sensing area of a conventional QCM by metalorganic chemical vapor deposition (MOCVD). Scanning electron microscopy (SEM) shows that the ZnO nanotips are dense and uniformly aligned along the normal to the substrate surface. By using superhydrophilic nano-ZnO surface, more than tenfold increase in mass loading sensitivity of the nano-QCM device is achieved over the conventional QCM. The ZnO nanotip arrays on the nano-QCM are functionalized. The selective immobilization and hybridization of DNA oligonucleotide molecules are confirmed by fluorescence microscopy of the nano-QCM sensing areas.

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 (~1.2 × 10(-6) cm(2)/s) determined by DOSY NMR. The fluorescence of C-153@OA(2) was quenched in the presence of increasing amounts of MV(2+) and Stern-Volmer plots of I(o)/I and τ(o)/τ vs [MV(2+)] indicated that the quenching was static. As per FT-IR-ATR spectra, the capsule C-153@OA(2) was bound to TiO(2) nanoparticle films. Selective excitation (λ(exc) = 420) of the above bound complex resulted in fluorescence quenching. When adsorbed on insulating ZrO(2) nanoparticle films, excitation of the complex resulted in a broad fluorescence spectrum centered at 500 nm and consistent with C-153 being within the lipophilic capsule interior. Consistent with the above results, colloidal TiO(2) quenched the emission while colloidal ZrO(2) did not.

Large Footprint Pyrene Chromophores Anchored to Planar and Colloidal Metal Oxide Thin Films

Sensitization and binding of a large footprint pyrene chromophore to planar (sapphire) and colloidal metal oxide films (TiO2 and ZrO2) is investigated. The model compound combines a 1-pyrenyl-ethynylenephenylene unit with a new adamantane-tripodal linker that binds to the surface. The linker design, combining a large footprint (approximately 2 nm2) of the tripodal linker with the meta position of the COOH anchoring groups, was suggested from atomistic models, and it aims to provide improved spacing control. The pyrene chromophore unit provides a probe of sensitizer-sensitizer interactions through its propensity to form excimers, unless neighboring pyrene units are sufficiently spaced (>or=3.5 A). Absorption and fluorescence studies, and a comparison with a pyrene-rigid rod model compound, suggest that the new tripodal anchor group allows spacing control on planar surfaces. On colloidal films, the linker provides spacing control at low surface coverage but sensitizer-sensitizer interactions are still observed on colloidal films at high surface coverage. Implications for the functionalization of metal oxide films in hybrid molecule-metal oxide semiconductor material systems are discussed.

Calculations of interfacial interactions in pyrene-Ipa rod sensitized nanostructured TiO2.

Pyrene chromophores carrying different rigid rod spacer groups (ethynylene, ethynylene-phenylene-ethynylene, and ethynylene-bicyclo[2.2.2]octylene-ethynylene) and bound to TiO2 nanostructured materials via an isophthalic acid (Ipa) anchor group have been investigated using quantum chemical calculations in order to elucidate structural and electronic properties of dye-sensitized semiconductor structures capable of long-range photoinduced interfacial electron transfer. The calculations are used to study firstly the effect of the anchor and spacer groups on the electronic properties of the pyrene-dyes, secondly the binding of isophthalic acid to nanostructured TiO2, and thirdly the interfacial electronic interactions for dye-sensitized nanostructured TiO2 relevant to dye-sensitized solar cell applications. Together, these calculations provide theoretical insights into the effect of incorporating rigid rod anchor-cum-spacer group motifs in sensitizers for e.g. solar cell applications. In particular, the calculations help to rationalize the strong influence of the rods on the photophysical properties of the sensitizers in terms of electronic interactions between the individual chromophore, spacer, and anchor segments, as well as to provide information about interfacial electronic interactions of interest for the capabilities of the rods to act as efficient mediators of photoinduced interfacial charge separation.

Organic polyaromatic hydrocarbons as sensitizing model dyes for semiconductor nanoparticles.

The study of interfacial charge-transfer processes (sensitization) of a dye bound to large-bandgap nanostructured metal oxide semiconductors, including TiO(2), ZnO, and SnO(2), is continuing to attract interest in various areas of renewable energy, especially for the development of dye-sensitized solar cells (DSSCs). The scope of this Review is to describe how selected model sensitizers prepared from organic polyaromatic hydrocarbons have been used over the past 15 years to elucidate, through a variety of techniques, fundamental aspects of heterogeneous charge transfer at the surface of a semiconductor. This Review does not focus on the most recent or efficient dyes, but rather on how model dyes prepared from aromatic hydrocarbons have been used, over time, in key fundamental studies of heterogeneous charge transfer. In particular, we describe model chromophores prepared from anthracene, pyrene, perylene, and azulene. As the level of complexity of the model dye-bridge-anchor group compounds has increased, the understanding of some aspects of very complex charge transfer events has improved. The knowledge acquired from the study of the described model dyes is of importance not only for DSSC development but also to other fields of science for which electronic processes at the molecule/semiconductor interface are relevant.

Synthesis of rigid-rod linkers to anchor chromophores to semiconductor nanoparticles

Abstract Four rigid-rod sensitizers, made of a phenylethynyl spacer substituted with a chromophore and two COOR binding groups, were prepared to study dynamics of electron injection at the interface of metal oxide semiconductor nanoparticles. Dimethyl Ru(bpy)2(5-(5-1,10-phenanthrolinyl)ethynyl)isophthalate)2+ ( 4a ), dimethyl Ru(bpy)2(5-(4-(2,2′-bipyridinyl)ethynyl)isophthalate)2+ ( 4b ), dimethyl 5-(1-pyrenylethynyl)isophthalate ( 4c ), and dimethyl 5-(9-anthracenylethynyl)isophthalate ( 4d ), were synthesized and characterized. Their absorption spectra, emission spectra, and electrochemical properties have been studied in acetonitrile and hexane solutions at room temperature.