Silsesquioxane-Terpyridine Nano Building Blocks for the Design of Three-Dimensional Polymeric Networks.

Novel polyhedral oligomeric silsesquioxanes (POSS) decorated with eight terpyridine moieties were synthesized in a one-pot procedure via Heck coupling reaction with the aim of investigating the possible formation of three-dimensional extended supramolecular organizations. The monosubstituted analogue was also prepared and used as a model compound. Both POSS-based nanostructures were extensively characterized via 1H, 13C, and 29Si nuclear magnetic resonance (NMR), ultraviolet-visible spectroscopies and combustion chemical analysis. The assembly of these nanocaged bricks and two different metal ions (Zn2+ and Fe2+) was investigated via 1H NMR as well as absorption and emission spectroscopy. Both mono- and octa-terpyridine-functionalized POSS (O-POSS) displayed interesting photophysical properties. Moreover, under selected conditions, the O-POSS forms stable gels at room temperature and can easily be shaped in the form of a film with potential applications in nanotechnology.

Molecular recognition of nucleotides in water by scorpiand-type receptors based on nucleobase discrimination.

The detection of nucleotides is of crucial importance because they are the basic building blocks of nucleic acids. Scorpiand-based polyamine receptors functionalized with pyridine or anthracene units are able to form stable complexes with nucleotides in water, based on coulombic, π-π stacking, and hydrogen-bonding interactions. This behavior has been rationalized by means of an exploration with NMR spectroscopy and DFT calculations. Binding constants were determined by potentiometry. Fluorescence spectroscopy studies have revealed the potential of these receptors as sensors to effectively and selectively distinguish guanosine-5'-triphosphate (GTP) from adenosine-5'-triphosphate (ATP).

Activating multistep charge-transfer processes in fullerene-subphthalocyanine-ferrocene molecular hybrids as a function of π-π orbital overlap.

We have synthesized two different fullerene-subphthalocyanine-ferrocene conjugates. The molecules were designed so that the ferrocene unit is linked at the subphthalocyanine axial position through a phenoxy spacer while the C(60) is rigidly held close to the concave face of the macrocycle via a 3-fold C(3)-symmetrical anchoring. The Bingel trisaddition reaction leading to the final products proceeded with very high regioselectivities and full diastereoselectivity. The only difference between both systems is the length of the triple tether employed, which finely regulates the regioselectivity of the trisaddition reaction and the distance between the subphthalocyanine and the C(60) complementary π-π surfaces. Thus, when the tether is connected to the subphthalocyanine through a direct C-C bond, a short π-π distance of 3.25-3.30 Å was calculated. In contrast, when it is connected through a slightly longer C-O-C bond, the distance increases to 3.5-3.6 Å. This π-π distance has a strong influence on the ground-state electronic interactions between the subphthalocyanine and the C(60), as determined from electronic absorption and cyclic voltammetry measurements. In addition, fluorescence and time-resolved transient absorption experiments demonstrated that different mechanisms operate in the two systems after photoexcitation. Despite the similar HOMO-LUMO gaps, only when the two complementary π-π surfaces of the subphthalocyanine and the C(60) are held at a close distance, therefore showing a high degree of orbital overlap, is a multistep electron transfer process triggered, ultimately leading to the long-lived, spatially separated C(60) radical anion and ferrocenium radical cation pair. A full account of the synthesis, characterization, and studies of the ground- and excited-state electronic interactions occurring in these conjugates, as well as in their reference C(60)-subphthalocyanine and subphthalocyanine-ferrocene dyads, is presented in this article.

Tuneable Emission of Polyhedral Oligomeric Silsesquioxane Based Nanostructures that Self-Assemble in the Presence of Europium(III) Ions: Reversible trans-to-cis Isomerization.

Hybrid nanostructures with switchable and reversible "blue-red-green" emission were efficiently synthesized. These nanostructures comprise polyhedral oligomeric silsesquioxanes (POSS) that behave as a nanocage that can be functionalized with terpyridine-based organic ligands, which can be easily complexed with europium (III) ions. The complexes were characterized by UV-Vis and fluorescence spectroscopy and their stoichiometry was also confirmed by 1 H NMR spectroscopy. In the presence of the Eu(III) ions, the octafunctionalized nanocages self-assemble to form 3D architectures that display an intense red-emission, especially in the solid state. The presence of an alkenyl group bridging the inorganic core to the organic moiety was employed to tune the emission properties by trans-cis isomerization of the double bond. In the case of the octafunctionalized nanocages (O-POSS), this isomerization was monitored in the presence of Eu(III) cations and was accompanied by an evident colour change from blue (trans-O-POSS) to red (Eu@trans-O-POSS) and finally to green (cis-O-POSS) as consequence of the release of the metal cations. This behaviour, together with the easy dispersion of the dry powder and the possibility of coating as a film in presence of small amounts of solvent, makes the emissive solid promising for applications in materials science.

Synthesis, characterization, and photoinduced electron transfer processes of orthogonal ruthenium phthalocyanine-fullerene assemblies.

The convergent synthesis, electrochemical characterization, and photophysical studies of phthalocyanine-fullerene hybrids 3-5 bearing an orthogonal geometry (Chart ) are reported. These donor-acceptor arrays have been assembled through metal coordination of linear fullerene mono- and bispyridyl ligands to ruthenium(II) phthalocyanines. The hybrid [Ru(CO)(C(60)Py)Pc] (3) and the triad [Ru(2)(CO)(2)(C(60)Py(2))Pc(2)] (5) were prepared by treatment of the phthalocyanine 6 with the mono- and hexakis-substituted C(60)-pyridyl ligands 1 and 2, respectively. The triad [Ru(C(60)Py)(2)Pc] (4) was prepared in a similar manner from the monosubstituted C(60)-pyridyl ligand 1 and the phthalocyanine precursor 7. The simplicity of this versatile synthetic approach allows to determine the influence of the donor and acceptor ratio in the radical ion pair state lifetime. The chemical, electrochemical, and photophysical characterization of the phthalocyanine-fullerene hybrids 3-5 was conducted using (1)H and (13)C NMR, UV/vis, and IR spectroscopies, as well as mass spectrometry, cyclic voltammetry, femtosecond transient absorption studies, and nanosecond laser flash photolysis experiments. Arrays 3-5 exhibit electronic coupling between the two electroactive components in the ground state, which is modulated by the axial CO and 4-pyridylfulleropyrrolidine ligands. With respect to the excited state, we have demonstrated that RuPc/C(60) electron donor-acceptor hybrids are a versatile platform to fine-tune the outcome and dynamics of charge transfer processes. The use of ruthenium(II) phthalocyanines instead of the corresponding zinc(II) complexes allows the suppression of energy wasting and unwanted charge recombination, affording radical ion pair state lifetimes on the order of hundreds of nanoseconds for the C(60)-monoadduct-based complexes 3 and 4. For the hexakis-substituted C(60) unit 2, the reduction potential is shifted cathodically, thus raising the radical ion pair state energy. However, the location of the RuPc triplet excited state is not high enough, and still offers a rapid deactivation of the radical ion pair state.

Higher intrinsic photocatalytic efficiency of 2,4,6-triphenylpyrylium-based photocatalysts compared to TiO2 P-25 for the degradation of 2,4-dichlorophenol using solar simulated light.

The photocatalytic efficiency of two 2,4,6-triphenylpyrylium (TP(+)) based photocatalysts (supported on silica or incorporated inside zeolite Beta, 3wt%) for the degradation of 2,4-dichlorophenol (DCP) in aqueous media has been compared with TiO(2) (Degussa P-25). It was found that the efficiency of the degradation depends on the photocatalytic setup, recirculation through a tubular reactor being highly unfavorable for the TP(+)-based photocatalysts due to the deposition of the silica or zeolite particles. In contrast, high efficiency in DCP disappearance (up to 87%) and in the total organic content decrease (up to 62%) were obtained using a discontinuous batch reactor in which the TP(+) photocatalysts were uniformly suspended. Kinetic studies were also made and DCP degradation follows a first order kinetics. The obtained kinetic constants when corrected to account for the influence of the fraction of light absorbed and the amount of active sites shows that the intrinsic activity of TP(+) adsorbed on silica or incorporated inside zeolite Beta was over one order of magnitude higher than TiO(2) activity.

Imidazolium-Functionalized Carbon Nanohorns for the Conversion of Carbon Dioxide: Unprecedented Increase of Catalytic Activity after Recycling.

Six new hybrid materials composed of carbon nanohorns (CNHs) and highly cross-linked imidazolium salts were easily synthesized using a one-step procedure based on the radical oligomerization of bis-vinylimidazolium salts (bVImiX) in the presence of pristine CNHs. The hybrid materials were characterized and employed as the sole catalysts for the conversion of carbon dioxide into cyclic carbonate by reaction with epoxides. The solids displayed excellent turnover number and productivity. Moreover, four catalysts were investigated in recycling experiments. Two catalysts containing an octyl linker between the imidazolium units and a bromide or an iodide anion showed no loss in activity after three cycles. The other two catalysts containing a p-xylyl linker and a bromide anion and different CNHs/bVImiX ratios showed an unprecedented increase of activity after recycling.

Photocatalytic activity of structured mesoporous TiO2 materials.

Starting from colloidal TiO2 nanoparticles in combination with tetraethyl orthosilicate using neutral Pluronic or cationic cetyltrimethylammonium as templates, a series of structured mesoporous silicas has been obtained. The structure of the mesoporous titania was confirmed by isothermal gas adsorption, transmission electron microscopy, and X-ray diffraction. The pore diameter ranged between 3.8 and 10.9 nm, and the BET surface area varied from 99 to 584 m2 g(-1). The photocatalytic activity of these samples for the degradation of phenol in aqueous solution has been compared with that of standard P-25 TiO2. Even though the activity of these new mesostructured materials is lower that those found for P-25 TiO2, the turnover frequency of the photocatalytic activity (moles of phenol degraded per Ti atom present at initial reaction time) is much higher for the mesoporous titania, particularly with low titanium contents for those materials (mpTiO2-5 and TiO2SBA15-5).

Ionic liquids as a novel medium for photochemical reactions. Ru(bpy)(3)2+/ viologen in imidazolium ionic liquid as a photocatalytic system mimicking the oxido-reductase enzyme.

Ionic liquids are suitable media which stabilize charged intermediates favoring those mechanisms that occur through charge separation. We have used ionic liquids to develop a photocatalytic system to perform the reduction of a carbonyl group to alcohol, thus mimicking the behavior of the reductase enzymes. The photochemical cycle is based on the well-known electron transfer from the Ru(bpy)(3)2+ complex in its excited state, acting as electron donor to MV2+, which acts as electron acceptor. The initial electron transfer process can be promoted upon selective Ru(bpy)(3)2+ excitation by visible light. By means of laser flash photolysis we have provided evidence of the nature and lifetimes of the intermediates involved in the photocatalytic system. Thus, the initial electron transfer between Ru(bpy)(3)2+ triplets and viologen MV2+ forms the MV*+ radical cation, which upon accepting an H* atom from a suitable hydrogen atom donor, forms the corresponding dihydropyridine MVH+ reducing agent.

Synthesis and photophysical properties of a hydrogen-bonded phthalocyanine-perylenediimide assembly.

A supramolecular phthalocyanine-perylenediimide donor-acceptor array has been assembled by using a melamine/perylenediimide motif. Photoexcitation of the perylenediimide component affords transduction of singlet excited state energy to the energetically lower lying phthalocyanine.

Microsecond charge separation upon photoexcitation of gold nanoparticles in imidazolium ionic liquids.

Gold nanoparticles (npAu, 5 nm diameter) stabilized by hexanethiol and dodecanethiol do not exhibit in THF upon laser flash excitation (355 nm) any detectable transient in the microsecond timescale. However, using octadecanethiol as npAu ligand, a weak signal attributed to THF solvated electrons was observed after excitation 355 nm. In contrast, irrespective of the alkyl length of the thiol ligand, irradiation of npAu in 1-butyl-3-methylimidazolium ionic liquid allows detecting charge separation in the microsecond time scale. By quenching with methyl viologen (MV) and determining the concentration of MV(+) , it was estimated that the quantum yield of charge separation state in ionic liquid was 0.12.

Photocatalytic activity of MCM-organized TiO(2) materials in the oxygenation of cyclohexane with molecular oxygen.

The photooxygenation of cyclohexane by molecular oxygen has been investigated on two mesoporous TiO(2) materials, which have been prepared using colloidal nanoparticles as building blocks. One of the structured systems (mpTiO(2)-50) is a mixture of 50% TiO(2) and 50% SiO(2); the second one (mpTiO(2)-100) is constituted by 100% of TiO(2). Both mpTiO(2)-100 and mpTiO(2)-50 can induce cyclohexane photooxidation in repeated cycles, but with the former the yield in cyclohexanone is higher and only traces of cyclohexanol are observed. The results of experiments with different incident light intensities are reported: contrary to mpTiO(2)-50, the selectivity of mpTiO(2)-100 towards cyclohexanone is not significantly affected by the photonic flux. Based on the substrate conversion rates, incident photonic flux effects, photoluminescence and EPR spectra of the mesoporous materials, we infer that the photoreactivity of mpTiO(2)-100 and mpTiO(2)-50 is mainly controlled by textural effects. In particular, we propose that the inter-particle electron mobility that characterizes the mpTiO(2)-100 material, which is constituted exclusively of TiO(2) nanoparticles, entails a better utilization of electron traps for converting the photogenerated cyclohexyl-peroxide radicals to cyclohexanone.

Direct time-resolved detection of singlet oxygen in zeolite-based photocatalysts.

Singlet oxygen has been characterized spectroscopically as a product of the exposure of suspensions of zeolites containing oxidation catalysts. Spectroscopic and lifetime studies show that a part of the singlet oxygen formed reacts within the zeolite porous structure, while a significant fraction escapes and becomes available for reaction in the bulk media. The liquid phase plays a key role in determining intra- and extracavity dynamics.

Enhancement of TiO2 photocatalytic activity by structuring the photocatalyst film as photonic sponge.

A TiO2 film having "photonic sponge" architecture (ps-TiO2) has been prepared using titania Degussa P25 nanoparticles that have been infiltrated into a template obtained through the arrangement of a mixture of different size latex spheres. The resulting photonic sponge exhibits 3.3 fold higher initial photocatalytic degradation rate for succinonitrile disappearance than analogous films of the same thickness made of unstructured P25 TiO2 nanoparticles. When corrected for the three fold lower mass of the ps-TiO2 films with respect to more dense P25, the enhancement of the photocatalytic activity by the photonic sponge morphology for the same mass of photocatalyst is about one order of magnitude. It was also observed that films of photonic sponge of 3 microm depth are more efficient than thicker films (5 and 7 microm).

Screening electronic communication through ortho-, meta- and para-substituted linkers separating subphthalocyanines and C60.

We have prepared two complementary series of SubPc-C(60) (SubPc=subphthalocyanine) electron/energy donor-acceptor systems, in which the two constituents are linked through ortho-, meta-, or para-substituted phenoxy spacers. In one of the series (1 a) the SubPc units bear iodine atoms, while in the other series (1 b) diphenylamino groups are linked to the SubPc macrocycles. The iodine atoms and diphenylamino groups both influence the resulting oxidation potentials of the electron-donating SubPc. They also modulate the outcome of excited state interactions, namely, energy and/or charge transfer. In addition, we have studied the impact that the substitution pattern in the phenoxy spacer exerts onto intramolecular processes in the ground and excited states. Although some of these processes are governed by the spatial separation between both components, the different electronic coupling through ortho-, meta-, or para- connections also plays decisive roles in some cases.

Sensitizers on inorganic carriers for decomposition of the chemical warfare agent yperite.

Sulfur-containing compounds, such as mercaptans, alkali sulfides, alkali sulfites, and alkali thiosulfates, are byproducts of industrial processes and pollutants of waste and natural waters. Other sulfur-containing compounds such as yperite are dangerous chemical weapons. Efficient photocatalytic decomposition of these molecules is a process that can find applications in emergency situations or for the controlled destruction of chemical warfare stockpiles. A series of heterogeneous photocatalysts consisting of a metal phthalocyanine or 2,4,6-triphenylpyrylium as photoactive components encapsulated inside the cavities of zeolite Y or the mesoporous channels of MCM-41 or supported on silica or titania-silica was tested for the photocatalytic decomposition of yperite. Two types of photoreactors, either an open reactor naturally aerated or a closed quartz tube with a constant airflow using UV or visible ambient light were used. These tests demonstrated that iron and manganese phthalocyanine and 2,4,6-triphenylpyrylium embedded in NaY or titania-silica can be suitable solid photocatalysts for the degradation of yperite using UV and visible irradiation.

Semiconductor behavior of a metal-organic framework (MOF).

Upon light excitation MOF-5 behaves as a semiconductor and undergoes charge separation (electrons and holes) decaying in the microsecond time scale. The actual conduction band energy value was estimated to be 0.2 V versus NHE with a band gap of 3.4 eV. Photoinduced electron transfer processes to viologen generates the corresponding viologen radical cation, while holes of MOF-5 oxidizes N,N,N',N'-tetramethyl-p-phenylenediamine. One application investigated for MOF-5 as a semiconductor has been the shape-selective photocatalyzed degradation of phenol in aqueous solutions.

A novel concept for photovoltaic cells: clusters of titanium dioxide encapsulated within zeolites as photoactive semiconductors.

Discrete clusters of TiO(2) (of only a few titanium atoms) are prepared within the internal micropore space of zeolite Y (4.8 wt % Ti loading) and characterized by Raman spectroscopy (rutile- and anatase-like structures), electron microscopy combined with elemental analyses (coincident Si and Ti spatial distribution), and X-ray diffraction (minor zeolite crystallinity decrease). The parent TiO(2)@Y sample is modified either by adsorption of acid-organic compounds (benzoic and 4-aminobenzoic acids or catechol) or by nitrogen doping. After modification, the optical UV/Vis spectrum of the parent TiO(2)@Y (onset of the absorption band at wavelengths <300 nm and bandgap of 4.2 eV) changes, and the appearance of new bands expanding to the visible region is observed. In contrast to the inactive zeolite Y matrix, all the zeolite-encapsulated TiO(2) species exhibit a photovoltaic response. The influence of the I(2)/I(3) (-) concentration in the electrolyte solution on the temporal profile of the photovoltage clearly shows that I(2)/I(3) (-) is also a suitable carrier for the positive charge in zeolite-based photovoltaic devices. The photocurrent response and the efficiency of the photovoltaic cell based on zeolite-encapsulated TiO(2) materials depend on the nature of the organic modifier and on the N-doping. The most efficient photovoltaic cell is that based on N-doped TiO(2)@Y, which exhibits a V(OC) (voltage at open circuit) of 270 mV, an I(SC) of 5.8 muA (current at short circuit), and a fill factor (FF) of 0.4. Although these values are low compared to current dye-sensitized TiO(2) solar cells, our findings could open up a promise for a stimulating research on the photovoltaic activity of zeolite-based host-guest solids.

Enhanced photocatalytic activity of zeolite-encapsulated TiO2 clusters by complexation with organic additives and N-doping.

In the literature it was found that titanium oxide clusters of a few metal atoms encapsulated inside the micropores of zeolite Y exhibit large blue shifts in the Ti-O ligand-to-metal charge-transfer band as compared to non-encapsulated bulk titanium dioxide particles. This blue shift of the Ti-O absorption band is believed to have a negative effect on the photocatalytic activity of zeolite-encapsulated TiO2. We report here on circumventing this problem and increasing visible-light absorption by means of a red shift of the absorption band caused by addition of some organic molecular modifiers containing acidic OH groups that can strongly bind with titanol groups TiOH. In the studied series of zeolite-encapsulated TiO2 samples, the red shift of the optical spectrum follows the order: catechol > 4-aminobenzoic acid > benzoic acid. Also N-doping of zeolite-encapsulated TiO2 clusters by thermal treatment with urea leads to a red shift of the TiO2 absorption band that depends on the annealing and hydration conditions. By comparison to the degradation of phenol in aqueous solution, we have demonstrated that these changes in the absorption spectrum on addition of the organic modifier are also reflected in the photocatalytic activity of the samples; a greater increase in photocatalytic activity (about 30%) was observed for the additive catechol.