Javier Martí-Rujas

Hyperbranched quasi-1D nanostructures for solid-state dye-sensitized solar cells.

In this work we demonstrate hyperbranched nanostructures, grown by pulsed laser deposition, composed of one-dimensional anatase single crystals assembled in arrays of high aspect ratio hierarchical mesostructures. The proposed growth mechanism relies on a two-step process: self-assembly from the gas phase of amorphous TiO2 clusters in a forest of tree-shaped hierarchical mesostructures with high aspect ratio; oriented crystallization of the branches upon thermal treatment. Structural and morphological characteristics can be optimized to achieve both high specific surface area for optimal dye uptake and broadband light scattering thanks to the microscopic feature size. Solid-state dye sensitized solar cells fabricated with arrays of hyperbranched TiO2 nanostructures on FTO-glass sensitized with D102 dye showed a significant 66% increase in efficiency with respect to a reference mesoporous photoanode and reached a maximum efficiency of 3.96% (among the highest reported for this system). This result was achieved mainly thanks to an increase in photogenerated current directly resulting from improved light harvesting efficiency of the hierarchical photoanode. The proposed photoanode overcomes typical limitations of 1D TiO2 nanostructures applied to ss-DSC and emerges as a promising foundation for next-generation high-efficiency solid-state devices comprosed of dyes, polymers, or quantum dots as sensitizers.

Formation of a Thermally Stable, Porous Coordination Network via a Crystalline-to-Amorphous-to-Crystalline Phase Transition

A porous coordination network was cleanly obtained after a crystalline-to-amorphous-to-crystalline phase transition, and the crystal structure was unambiguously solved by ab initio powder X-ray diffraction. After the solid-state phase transition, the porous structure was remarkably stable up to 673 K and capable of reabsorbing various guest molecules, such as aromatic solvents and I(2).

Hydrogen and halogen bonding drive the orthogonal self-assembly of an organic framework possessing 2D channels

Orthogonal self-assembly of an open organic framework showing 2D channels has been obtained by combining hydrogen and halogen bonding. The framework is able to host various guest molecules with a diverse set of steric demands and substitution patterns, and survives single-crystal-to-single-crystal guest exchanges from liquid and gas phases.

Dramatic Structural Rearrangements in Porous Coordination Networks

With the use of ab initio X-ray powder diffraction, a family of isostructural crystalline porous coordination networks, [(ZnX(2))(3)(TPT)(2)](n)· (solvent) (X = I, Br, Cl), has been studied at elevated temperatures of 573-723 K. Upon heating, all three networks exhibited crystalline-to-amorphous-to-crystalline (CAC) phase transformations to three new networks, [(ZnI(2))(3)(TPT)(2)](n), [(ZnBr(2))(3)(TPT)(2)](n)·(H(2)O) and [(ZnBr(2))(μ-Br)(ZnBr)(TPT)](n), and [(ZnCl(2))(μ-Cl)(ZnCl)(TPT)](n), respectively. A set of control experiments was used to obtain detailed mechanistic aspects of the CAC transformations. We demonstrate how bonds are broken and formed in these significant molecular rearrangements and how the initial arrangement plays a crucial role in the formation of the new networks after the CAC transformations. The structural information in the amorphous phase is retained and passed from a metastable to a more stable crystal, thus, reinforcing the notion that coordination networks are flexible and chemically active.

An Adaptable and Dynamically Porous Organic Salt Traps Unique Tetrahalide Dianions

Bis(I2) adducts of hexamethonium dihalides are pre-organized to respond dynamically to heating and reach a functional structure that favors the formation of the poorly stable and virtually unknown [I2Br2]2− and [I2Cl2]2− tetrahalides which could not be obtained in solution (see picture). The cavity-directed reactivity affords new opportunities for synthesis and interconversion of polyhalogen anions.

Dimensional caging of polyiodides: cation-templated synthesis using bipyridinium salts

The potential of bipyridinium derivatives in the cation templated synthesis of polyiodides has been explored by applying the strategy of size-matching between cations and anions. Bipyridinium cations 1–4, bearing benzyl and functionalized benzyl pendants at nitrogen atoms, are able to template the selective formation of I42− and I3− species. Thanks to the supramolecular space compartmentation induced by the benzyl pendants, the formation of I42−and I3− is independent of the stoichiometry adopted in the crystallization procedure. Bipyridinium cation 5, bearing methyl pendants, is unable to induce space compartmentation and different polyiodides are obtained depending on the stoichiometry used in the crystallization process as the cation–anion size-matching alone does not control the polyiodide formation.

Structure–Photoluminescence Correlation for Two Crystalline Polymorphs of a Thiophene–Phenylene Co-Oligomer with Bulky Terminal Substituents

Two crystal polymorphs of a thiophene-phenylene hexamer with bulky terminal substituents are characterized by different molecular conformations and parallel versus herringbone packing. Irrespective of their similar emissive spectra and common H-aggregate features, evidenced by crystal structure analysis and confirmed by solid-phase and excited-state first-principles calculations, their luminescence is relatively high and, for one form, nearly double than that for the other. Interaromatic packing energy contributions are established by quantum chemical calculations and can be compared quantitatively as the same species in different crystal environments is examined. The different luminescence efficiency of the two phases highlights the crucial role of the interaromatic packing for the luminescence properties of polyaromatic oligomers.

Mechanochemical dehydrochlorination and chelation reaction in the solid state: from a molecular salt to a coordination complex

We report the solid state structural transformation of a hydrogen bonded complex salt into a metal complex via dehydrochlorination using mechanochemistry. A crystalline salt containing a large and flexible bidentate dication hydrogen bonded to a tetrachlorometalate (II) anion has been ground in the presence of KOH. Substitution of charge-assisted hydrogen bonding interactions by coordination bonds via chelation has been demonstrated by single-crystal and powder X-ray diffraction analysis. By-product water molecules are included in the structure, playing an important role establishing electrostatic interactions. The irreversibility property of the transformation of the coordination complex into a hydrogen bonded complex salt was determined experimentally. Density functional calculations were used to attempt a rationalisation of the structural results into the mechanochemical reactions.

Solid state transformations in stoichiometric hydrogen bonded molecular salts: ionic interconversion and dehydration processes

We report structural aspects of three new stoichiometric 5-sulfosalicylate benzimidazolium (A1B1), 5-sulfonatosalicylate bis(benzimidazolium) monohydrate (A1B2·H2O) and 5-sulfonatosalicylate bis(benzimidazolium) (A1B2) molecular salts. Using single crystal and powder X-ray diffraction, we describe the reversible solid state ionic interconversion between A1B1 and A1B2·H2O upon neat grinding by adding one equivalent of benzimidazole or 5-sulfosalicilic acid dihydrate (B or A·2H2O), respectively. In the process water molecules are included/excluded upon grinding suggesting that water could mediate the proton transfer. We have also studied the hydrogen bond network rearrangement after the dehydration of A1B2·H2O by growing single crystals of the anhydrous salt using the seeding method. X-ray powder diffraction analysis confirmed that the product obtained directly upon heating corresponds to the structure obtained by the seeding method. In order to rationalize the experimental results and to gain insights into the role of water molecules in the formation of crystalline salts, the relative stabilities of the crystalline phases were compared by quantum mechanics calculations which are specific for solid state systems.

Ab initio powder diffraction structure analysis of a host-guest network: short contacts between tetrathiafulvalene molecules in a pore.

Many crystal structures of porous coordination networks have been solved by single-crystal X-ray crystallography, providing detailed molecular structural information on framework, guest arrangement, and even reactive intermediates generated in situ. Such detailed structural analysis facilitated the explosive development of porous coordination networks in the last 15 years. On the other hand, although there are many examples of ab initio powder X-ray diffraction (PXRD) analysis of inorganic materials, organic solids, nonporous coordination networks, and discrete small molecules, there are only few reports of ab initio PXRD analysis of porous coordination networks describing guest behavior (guest exchange, gas adsorption, etc.). This is because ab initio PXRD analysis is considerably more challenging than singlecrystal structure determination. Porous coordination networks usually have large unit cells and often low symmetry that contribute to severe peak overlap which hampers accurate structure determination. By using high-resolution synchrotron PXRD and the simulating annealing method, we have now succeeded in solving the crystal structure of a biporous coordination network having an unprecedented arrangement of tetrathiafulvalene (TTF) molecules and a unit cell larger than 15000 . The PXRD analysis revealed very short intermolecular S···S contacts between TTF molecules (3.370 ) that have a nonplanar shape indicating a neutral form. Those findings agree with solid-state spectroscopic features. A key issue in inducing intriguing physical properties in TTF is how to arrange the molecules. We tried to achieve a unique arrangement of TTF molecules by using the pores of a porous coordination network. We used the previously reported network [(ZnI2)3(1)2(2)]n·xC6H5NO2·yCH3OH (3, where 1 is 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPT) and 2 is triphenylene; x 4 and y 2) and prepared new isostructural ZnBr2 network 4 (Scheme 1) by instant synthesis. [3b]