Visible-Light-Driven Photocatalytic H2 Production Using Composites of Co-Al Layered Double Hydroxides and Graphene Derivatives.

The direct conversion of solar energy into chemical energy represents an enormous challenge for current science. One of the commonly proposed photocatalytic systems is composed of a photosensitizer (PS) and a catalyst, together with a sacrificial electron donor (ED) when only the reduction of protons to H2 is addressed. Layered double hydroxides (LDH) have emerged as effective catalysts. Herein, two Co-Al LDH and their composites with graphene oxide (GO) or graphene quantum dots (GQD) have been prepared by coprecipitation and urea hydrolysis, which determined their structure and so their catalytic performance, giving H2 productions between 1409 and 8643 µmol g-1 using a ruthenium complex as PS and triethanolamine as ED at 450 nm. The influence of different factors, including the integration of both components, on their catalytic behavior, has been studied. The proper arrangement between the particles of both components seems to be the determining factor for achieving a synergistic interaction between LDH and GO or GQD. The novel Co-Al LDH composite with intercalated GQD achieved an outstanding catalytic efficiency (8643 µmol H2 g-1) and exhibited excellent reusability after 3 reaction cycles, thus representing an optimal integration between graphene materials and Co-Al LDH for visible light driven H2 photocatalytic production.

Benign-by-Design Solventless Mechanochemical Synthesis of Three-, Two-, and One-Dimensional Hybrid Perovskites.

Organic-inorganic hybrid perovskites have attracted significant attention owing to their extraordinary optoelectronic properties with applications in the fields of solar energy, lighting, photodetectors, and lasers. The rational design of these hybrid materials is a key factor in the optimization of their performance in perovskite-based devices. Herein, a mechanochemical approach is proposed as a highly efficient, simple, and reproducible method for the preparation of four types of hybrid perovskites, which were obtained in large amounts as polycrystalline powders with high purity and excellent optoelectronics properties. Two archetypal three-dimensional (3D) perovskites (MAPbI3 and FAPbI3 ) were synthesized, together with a bidimensional (2D) perovskite (Gua2 PbI4 ) and a "double-chain" one-dimensional (1D) perovskite (GuaPbI3 ), whose structure was elucidated by X-ray diffraction.

λ/20 axial control in 2.5D polymerized structures fabricated with DLW lithography.

An astonishing λ/20 height control is accomplished in 2.5D photopolymerized structures by taking advantage of the induced expansion of the resin. Our nanofabrication method is a one-pot approach with two processing steps: (i) regular 2.5D photopolymerization of the resin monomer by using multiphoton direct laser writing (DLW) lithography and (ii) spatially-selective irradiation of the photopolymerized features before development resulting in a nanometer-controlled height increase of the structure. The UV-visible-NIR sub-wavelength axial feature size (~40 nm) of this method allows fabricating devices with applications in multiple technological fields such as nanoelectronics and photonics.

Europium insertion into MgAl hydrotalcite-like compound to promote the photocatalytic oxidation of nitrogen oxides.

Easy synthesis of efficient, non-toxic photocatalysts is a target to expand their potential applications. In this research, the role of Eu3+ doping in the non-toxic, affordable, and easily prepared MgAl hydrotalcite-like compounds (HTlcs) was explored in order to prepare visible light semiconductors. Eu doped MgAl-HTlcs (MA-xEu) samples were prepared using a simple coprecipitation method (water, room temperature and atmospheric pressure) and europium was successfully incorporated into MgAl HTlc frameworks at various concentrations, with x (Eu3+/M3+ percentage) ranging from 2 to 15. Due to the higher ionic radius and lower polarizability of Eu3+ cation, its presence in the metal hydroxide layer induces slight structural distortions, which eventually affect the growth of the particles. The specific surface area also increases with the Eu content. Moreover, the presence of Eu3+ 4f energy levels in the electronic structure enables the absorption of visible light in the doped MA-xEu samples and contributes to efficient electron-hole separation. The microstructural and electronic changes induced by the insertion of Eu enable the preparation of visible light MgAl-based HTlcs photocatalysts for air purification purposes. Specifically, the optimal HTlc photocatalyst showed improved NOx removal efficiency, ∼ 51% (UV-Vis) and 39% (visible light irradiation, 420 nm), with excellent selectivity (> 96 %), stability (> 7 h), and enhanced release of •O2- radicals. Such results demonstrate a simple way to design photocatalytic HTlcs suitable for air purification technologies.

Host-guest complexation of [60]fullerenes and porphyrins enabled by "click chemistry".

Herein the synthesis, characterization, and organization of a first-generation dendritic fulleropyrrolidine bearing two pending porphyrins are reported. Both the dendron and the fullerene derivatives were synthesized by Cu(I) -catalyzed alkyne-azide cycloaddition (CuAAC). The electron-donor-acceptor conjugate possesses a shape that allows the formation of supramolecular complexes by encapsulation of C60 within the jaws of the two porphyrins of another molecule. The interactions between the two photoactive units (i.e., C60 and Zn-porphyrin) were confirmed by cyclic voltammetry as well as by steady-state and time-resolved spectroscopy. For example, a shift of about 85 mV was found for the first reduction of C60 in the electron-donor-acceptor conjugate compared with the parent molecules, which indicates that C60 is included in the jaws of the porphyrin. The fulleropyrrolidine compound exhibits a rich polymorphism, which was corroborated by AFM and SEM. In particular, it was found to form supramolecular fibrils when deposited on substrates. The morphology of the fibrils suggests that they are formed by several rows of fullerene-porphyrin complexes.

Triazole bridges as versatile linkers in electron donor-acceptor conjugates.

Aromatic triazoles have been frequently used as π-conjugated linkers in intramolecular electron transfer processes. To gain a deeper understanding of the electron-mediating function of triazoles, we have synthesized a family of new triazole-based electron donor-acceptor conjugates. We have connected zinc(II)porphyrins and fullerenes through a central triazole moiety--(ZnP-Tri-C(60))--each with a single change in their connection through the linker. An extensive photophysical and computational investigation reveals that the electron transfer dynamics--charge separation and charge recombination--in the different ZnP-Tri-C(60) conjugates reflect a significant influence of the connectivity at the triazole linker. Except for the m4m-ZnP-Tri-C(60)17, the conjugates exhibit through-bond photoinduced electron transfer with varying rate constants. Since the through-bond distance is nearly the same for all the synthesized ZnP-Tri-C(60) conjugates, the variation in charge separation and charge recombination dynamics is mainly associated with the electronic properties of the conjugates, including orbital energies, electron affinity, and the energies of the excited states. The changes of the electronic couplings are, in turn, a consequence of the different connectivity patterns at the triazole moieties.

Surface energy transfer in hybrid halide perovskite/plasmonic Au nanoparticle composites.

The incorporation of plasmonic metal nanoparticles (NPs) into the multilayered architecture of perovskite solar cells (PSCs) has been a recurrent strategy to enhance the performance of photovoltaic devices from the early development of this technology. However, the specific photophysical interactions between the metal NPs and the hybrid halide perovskites are still not completely understood. Herein, we investigate the influence of Au NPs on the photoluminescence (PL) signal of a thin layer of the CH3NH3PbI3 hybrid perovskite. Core-shell Au@SiO2 NPs with a tunable thickness of the SiO2 shell were used to adjust the interaction distance between the plasmonic NPs and the perovskite layer. Complete quenching of the PL signal in the presence of the Au NPs is measured together with the gradual recovery of the PL intensity at a thicker thickness of the SiO2 shell. A nanometal surface energy transfer (NSET) model is employed to reasonably fit the experimental quenching efficiency. Thus, the energy transfer deactivation is revealed as a detrimental process occurring in the PSCs since it funnels the photon energy into the non-active excited state of the Au NPs. This work indicates that tuning the distance between the plasmonic NPs and the perovskite materials by a silica shell may be a simple and straightforward strategy for further improving the efficiency of PSCs.

Amphiphilic polymers for aggregation-induced emission at air/liquid interfaces.

Polymersomes and related self-assembled nanostructures displaying Aggregation-Induced Emission (AIE) are highly relevant for plenty of applications in imaging, biology and functional devices. Experimentally simple, scalable and universal strategies for on-demand self-assembly of polymers rendering well-defined nanostructures are highly desirable. A purposefully designed combination of amphiphilic block copolymers including tunable lengths of hydrophilic polyethylene glycol (PEGm) and hydrophobic AIE polymer poly(tetraphenylethylene-trimethylenecarbonate) (P(TPE-TMC)n) has been studied at the air/liquid interface. The unique 2D assembly properties have been analyzed by thermodynamic measurements, UV-vis reflection spectroscopy and photoluminescence in combination with molecular dynamics simulations. The (PEG)m-b-P(TPE-TMC)n monolayers formed tunable 2D nanostructures self-assembled on demand by adjusting the available surface area. Tuning of the PEG length allows to modification of the area per polymer molecule at the air/liquid interface. Molecular detail on the arrangement of the polymer molecules and relevant molecular interactions has been convincingly described. AIE fluorescence at the air/liquid interface has been successfully achieved by the (PEG)m-b-P(TPE-TMC)n nanostructures. An experimentally simple 2D to 3D transition allowed to obtain 3D polymersomes in solution. This work suggests that engineered amphiphilic polymers for AIE may be suitable for selective 2D and 3D self-assembly for imaging and technological applications.

[2]Catenanes decorated with porphyrin and [60]fullerene groups: design, convergent synthesis, and photoinduced processes.

A new class of [2]catenanes containing zinc(II)-porphyrin (ZnP) and/or [60]fullerene (C(60)) as appended groups has been prepared. A complete description of the convergent synthetic approach based on Cu(I) template methodology and "click" 1,3-dipolar cycloaddition chemistry is described. This new electron donor-acceptor catenane family has been subjected to extensive spectroscopic, computational, electrochemical and photophysical studies. (1)H NMR spectroscopy and computational analysis have revealed that the ZnP-C(60)-[2]catenane adopts an extended conformation with the chromophores as far as possible from each other. A detailed photophysical investigation has revealed that upon irradiation the ZnP singlet excited state initially transfers energy to the (phenanthroline)(2)-Cu(I) complex core, producing a metal-to-ligand charge transfer (MLCT) excited state, which in turn transfers an electron to the C(60) group, generating the ZnP-[Cu(phen)(2)](2+)-C(60)(*-) charge-separated state. A further charge shift from the [Cu(phen)(2)](2+) complex to the ZnP subunit, competitive with decay to the ground state, leads to the isoenergetic long distance ZnP(*+)-[Cu(phen)(2)](+)-C(60)(*-) charge-separated radical pair state, which slowly decays back to the ground state on the microsecond time scale. The slow rate of back-electron transfer indicates that in this interlocked system, as in previously studied covalently linked ZnP-C(60) hybrid materials, this process occurs in the Marcus-inverted region.

Synthesis and photophysical studies of an indigo derivative: N-octyl-7,7'-diazaindigo.

In this paper, we explore the synthesis, characterization, and photophysical properties of a novel indigo derivative, N-octyl-7,7'-diazaindigo, being the first time that diazaindigos have been studied as photophysically-active chemical entities. Reduction of the neutral "keto-form" to the so-called "leuco-form" changes the global spectroscopic and photophysical behaviors. Both species have been investigated by different photophysical studies, such as analysis of absorption and emission spectra, fluorescence quantum yields (Φ F) and lifetimes. Finally, to appraise in depth the deactivation of the excited state of the keto form, femtosecond transient absorption (TA) experiments and Density Functional Theory (DFT) and Time Dependent (TD)-DFT calculations were performed. In an organic aprotic solvent (N,N-dimethylformamide), TA experiments showed a fast deactivation channel (τ 1 = 2.9 ps), which was ascribed to solvent reorganization, and a longer decay component (τ 2 = 86 ps) associated with an internal conversion (IC) process to the ground-state, in opposition to the excited state proton transfer (ESPT) mechanism that takes place in the indigo molecules but in protic solvents. A comparative study was also carried out on the parent molecule, 7,7'-diazaindigo, corroborating the previous conclusions obtained for the alkyl derivative. In agreement with experimental observations, DFT and TD-DFT calculations revealed that the deactivation of the S1 state of the keto form takes place through an internal conversion process.

Cr3+ substituted Zn-Al layered double hydroxides as UV-Vis light photocatalysts for NO gas removal from the urban environment.

The ZnAl-CO3, ZnAlCr-CO3 and ZnCr-CO3 LDH samples were studied as De-NOx photocatalysts in this work. Samples without Cr and increasing the presence of Cr3+ in the LDH framework in the 0.06, 0.15 and 0.3 Cr/Zn ratio were prepared by co-precipitation method, all of them constituted by pure LDH phase. The increase of chromium content in the LDH framework leads to lower crystallinity and higher specific surface area in the samples. Moreover, the CrO6 octahedron centres expand the photo-activity from UV to Visible light and assist to decrease the recombination rate of the electrons and holes. The favourable textural, optical and electronic properties of Cr-containing LDH samples explain the good NO removal efficiency (55%) and outstanding selectivity (90%) found for the analysed De-NOx process.

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.

Mechanochemical synthesis of three double perovskites: Cs2AgBiBr6, (CH3NH3)2TlBiBr6 and Cs2AgSbBr6.

A wide majority of the organic-inorganic hybrid perovskites employed in photovoltaics contain Pb, which is a negative issue due to its high toxicity and the low stability of the Pb-based three-dimensional (3D) perovskites. The double perovskites or "elpasolites" with the formula A2BB'X6 arise as an alternative to avoid the use of Pb, however, not many of the theoretically predicted structures have been synthesized so far due to several synthetic issues, such as, the formation of stable side products. Herein, we report the synthesis of three double perovskites, Cs2AgBiBr6, MA2TlBiBr6 and Cs2AgSbBr6, through a highly efficient and reproducible mechanochemical approach: the high energy ball milling. This synthetic approach does not require the use of organic solvents, so it is a greener method compared to those reported for other double perovskites. The Cs2AgBiBr6 and MA2TlBiBr6 double perovskites were synthesized with high purity as proved by X-ray diffraction (XRD) and X-ray fluorescence (XRF) measurements. However, the Cs2AgSbBr6 double perovskite was obtained in mixture with Cs3Sb2Br9, a side product of the reaction. Several attempts to prepare the Cs2AgSbBr6 double perovskite by using other synthetic methods have been unsuccessful due to the low formation enthalpy of the Cs3Sb2Br9 side product and only the hydrothermal method afforded Cs2AgSbBr6 in mixture with other compounds. We believe that the low temperature required in the ball milling synthesis is the key factor that allows the formation of the antimony double perovskite. Cs2AgSbBr6 is a brown powder with a bandgap energy of 1.93 eV as shown with diffuse reflectance measurements. The three powders exhibit a very high stability with no changes at all in the crystal structure after several months of storage at room temperature and ambient humidity.

5,10-Dihydrobenzo[ a]indolo[2,3- c]carbazoles as Novel OLED Emitters.

The excited-state dynamics of four 5,10-dihydrobenzo[ a]indolo[2,3- c]carbazoles in solution and in films were studied with stationary and time-resolved spectroscopies. The solvent dependency of the photophysics reveals no appreciable dipole moment in the ground state. In the excited state, electron-withdrawing substituents contribute to an outspoken charge-transfer character. In films, although the molecules are mostly present as monomers, the excited-state dynamics are characterized by a cascade of energy-transfer processes to excited dimers and aggregates which dominate the photoluminescence (PL) spectra. The properties of the aggregates depend on the used substituents. The electroluminescence spectra obtained from single-layer and multilayer devices mostly resemble the PL spectra, but show contributions from other species such as electromers or electroplexes. It is inferred that the different substituents lead to a different packing of the carbazole moieties, each of which has different mobilities and recombination probabilities.

Molecular organization of a water-insoluble iridium(III) complex in mixed monolayers.

In this work, organized mixed monolayers containing a cationic water-insoluble iridium(III) complex, Ir-dye, [Ir(ppy)(2)(tmphen)]PF(6), (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline, and ppy = 2-phenylpyridine), and an anionic lipid matrix, DMPA, dimyristoyl-phosphatidic acid, with different molar proportions, were formed by the co-spreading method at the air-water interface. The presence of the dye at the interface, as well as the molecular organization of the mixed films, is deduced from surface techniques such as pi-A isotherms, Brewster angle microscopy (BAM) and reflection spectroscopy. The results obtained remark the formation of an equimolar mixed film, Ir-dye/DMPA = 1:1. BAM images reveal a whole homogeneous monolayer, with gradually increasing reflectivity along the compression process up to reaching the collapse of this equimolecular monolayer at pi approximately equal to 37 mNm(-1). Increasing the molar ratio of DMPA in the mixture, the excess of lipid molecules organizes themselves forming dark flower-like domains of pure DMPA at high surface pressures, coexisting with the mixed Ir-dye/DMPA = 1:1 monolayer. On the other hand, unstable mixed monolayers are obtained by using an initial dye surface concentration higher than the equimolecular one. These mixed Langmuir monolayers have been successfully transferred onto solid substrates by the LB (Langmuir-Blodgett) technique.

Towards the photophysical studies of humin by-products.

Biomass conversion into chemicals, materials and fuels emerged in the past decade as the most promising alternative to the current petroleum-based industry. However, the chemocatalytic conversion of biomass and bio-derived sugars often leads to numerous side-products, such as humins. The limited characterization of humin materials restricts their study for possible future applications. Thus, herein photophysical studies on humins and separated humin fractions were carried out using steady-state and time-resolved fluorescence techniques. This paper aims to add to the literature important information for scientists involved in the photophysical studies.

Conformational changes of a calix[8]arene derivative at the air-water interface.

The particular behavior of a p-tert-butyl calix[8]arene derivative (C8A) has been studied at the air-water interface using surface pressure-area isotherms, surface potential-area isotherms, film relaxation measurements, Brewster angle microscopy (BAM), and infrared spectroscopy for Langmuir-Blodgett films. Thus, it is observed that the properties of the film, for example, isotherms, domain formation, and FTIR spectra, recorded during the first compression cycle differ appreciably from those during the second compression and following cycles. The results obtained are interpreted on the basis of the conformational changes of the C8A molecules by surface pressure, allowing us to inquire into the inter- and intramolecular interactions (hydrogen bonds) of those molecules. Thus, the compression induces changes in the kind of hydrogen bonds from intra- and intermolecular with other C8A molecules to hydrogen bonds with water molecules.

Selective fluorescence functionalization of dye-doped polymerized structures fabricated by direct laser writing (DLW) lithography.

The continuous development of the vast arsenal of fabrication techniques is a pivotal factor in the breakthrough of nanotechnology. Although the broad interest is generally focused on the reduction of the dimensions of the fabricated structures, localized functionalization of the nanomaterials emerges as a key factor closely linked to their potential applications. In particular, fabrication of spatially selective fluorescence nanostructures is highly demanded in nanophotonics, as for example in three-dimensional (3D) optical data storage (ODS), where massive storage capacity and fast writing-reading processes are promised. We have developed an innovative method to control the location and intensity of the fluorescence signal in dye-doped photopolymerized structures fabricated with Direct Laser Writing (DLW) lithography. Well-defined fluorescent pixels (area = 0.24 µm(2)) were written inside a polymer matrix with the help of a femtosecond pulsed laser (multiphoton absorption) via a thermally-induced di-aggregation of a fluorescent dye. Moreover, we have accomplished a fine control of the fluorescence intensity which can increase the storage capacity of ODS systems fabricated with this approach.

UV-Vis Reflection-Absorption Spectroscopy at air-liquid interfaces.

UV-Visible Reflection-Absorption Spectroscopy (UVRAS) technique is reviewed with a general perspective on fundamental and applications. UVRAS is formally identical to IR Reflection-Absorption Spectroscopy (IRRAS), and therefore, the methodology developed for this IR technique can be applied in the UV-visible region. UVRAS can be applied to air-solid, air-liquid or liquid-liquid interfaces. This review focuses on the use of UVRAS for studying Langmuir monolayers. We introduce the theoretical framework for a successful understanding of the UVRAS data, and we illustrate the usage of this data treatment to a previous study from our group comprising an amphiphilic porphyrin. For ultrathin films with a thickness of few nm, UVRAS produces positive or negative bands when p-polarized radiation is used, depending on the incidence angle and the orientation of dipole absorption. UVRAS technique provides highly valuable information on tilt of chromophores at the air-liquid interface, and moreover allows the determination of optical parameters. We propose UVRAS as a powerful technique to investigate the in situ optical properties of Langmuir monolayers.