Gregorio García

Two-electron versus one-electron reduction of chalcogens by uranium(III): synthesis of a terminal U(V) persulfide complex

The reaction of the tripodal tris-amido U(III) complex [U{(SiMe2NPh)3–tacn}] (tacn = 1,4,7-triazacyclononane), 1, with 0.0625 and 0.25 equiv. of elemental sulfur affords the sulfide-bridged U(IV) complex [{U((SiMe2NPh)3–tacn)}2(μ-S)], 2, and the terminal persulfide U(V) complex [U{(SiMe2NPh)3–tacn}(η2-S2)], 4, respectively, in good yield. Two different electronic structures, U(V) persulfide and U(IV) supersulfide, were computed for complex 4 at the DFT level. The results show that complex 4 is best described as a U(V) persulfide species with a significant sulfur contribution. X-ray, magnetism and electrochemistry data support this description. Complex 4 is the first example of a terminal U(V) persulfide and of a two-electron reduction of S8 by a U(III) complex. Complex 4 behaves as a S-atom transfer agent when reacted with PPh3, affording the persulfide-bridged diuranium(IV) complex [{U((SiMe2NPh)3–tacn)}2(μ-η2:η2-S2)], 5, and SPPh3.

Evaluating push-pull dye efficiency using TD-DFT and charge transfer indices†

The performances of different functionals in the prediction of Charge Transfer excitations (CT) have been assessed, both in terms of quantitative agreement with experimental absorption data and on the basis of a recently developed density based diagnostic index, for a family of 18 recently synthesized push-pull compounds, containing 4-5-dicyannoimidazole (DCI) as an acceptor moiety, six different bridges and three different donor groups. The index used also allows obtaining an estimate of the charge transferred upon excitation (qCT) and of the spatial extent associated with a given electronic transition (DCT). From the computed values of these indices an estimate of the transition energy considering a purely electrostatic model (wCT) can be computed and compared to that expected for an ideal CT between the donor and the acceptor, thus enabling us to estimate the efficiency of the CT transition for the different push-pull systems.

Density functional theory study of the optical and electronic properties of oligomers based on phenyl-ethynyl units linked to triazole, thiadiazole, and oxadiazole rings to be used in molecular electronics

In the present work, we have studied from a theoretical perspective the geometry and electronic properties of the series of related compounds 2,5-bis(phenylethynyl)-1,3,4-thiadiazole, 2,5-bis(phenylethynyl)-1,3,4-oxadiazole, and 2,5-bis(phenylethynyl)-1,2,4-triazole as candidates for electron-conducting polymers and compounds with desirable (opto)electronic properties. The effect of the ethynyl group (-C[Triple Bond]C-) on the structure and electronic properties was also studied. The influence of planarity on electrical conductivity has been studied by a natural-bond-orbital analysis. The (opto)electronic properties and conducting capability were investigated through the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap, excitation energy, bond length alternation, LUMO energy, electron affinities, and intramolecular reorganization energy. Finally, the evolution of some properties such as optical bandgap and electron affinity with the increase of the number of repeat units in the oligomer chain has been checked.

Theoretical study of the effect of ethynyl group on the structure and electrical properties of phenyl-thiadiazole systems as precursors of electron-conducting materials

2,5-Bis(phenylethynyl)-1,3,4-thiadiazole (PhEtTh) and 2,5-diphenyl-1,3,4-thiadiazole (PhTh) are expected to be building blocks for polymer materials that could be employed to conduct electricity due to their narrow highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gaps. In this work, a theoretical, comparative study about the effect of the ethynyl group on the planarity and electrical conductivity of this kind of systems has been carried out. Thus, several ab initio (Hartree-Fock, Moller-Plesset) and DFT (B3LYP, B3PW91, M05, M05-2X) methods and basis sets (6-31G(*), 6-31G+G(**), 6-311G(**), cc-pVDZ, cc-pVTZ) have been tested. As a result, PhEtTh showed better properties for its use as electric conducting material relative to PhTh due to its smaller HOMO-LUMO gap, as well as its enhanced trend to retain the planarity provided the reduction in steric hindrances that the ethynyl group (-C[triple bond]C-) permits. Solvent effects were also modeled for ethanol and chloroform under the conductor-like polarizable continuum model approximation. Finally, electronic transitions in gas and solution phases were predicted by using TDDFT approximation in order to compare the theoretical lambda(max) with the experimental values reported in literature for both compounds.

Confinement Effects on UV-Visible Absorption Spectra: β-Carotene Inside Carbon Nanotube as a Test Case.

The effect of the confinement in a single-wall carbon nanotube on the optical properties of β-carotene is studied at the time-dependent density functional theory level. A complex computational protocol has been developed, based on a multilayered ONIOM approach making use of a recent range-separated hybrid functional as well as dispersion corrections. The role of both mechanical and electronic embedding has been clearly pointed out, showing how the inclusion of the latter is mandatory for a correct description of the experimental data. The correct calculation of the bathochromic shift experimentally observed upon encapsulation (0.23 eV) shows the ability of this computational protocol to reproduce all the physics behind such a complex host-guest interaction. From a more chemical point of view, this study allows one to show how such a shift is related to both geometrical and polarization effects.

Theoretical study on the solvation of C60 fullerene by ionic liquids.

The solvation of C60 fullerene by 24 different ionic liquids belonging to the imidazolium, piperazinium, and cholinium families was analyzed from a nanoscopic viewpoint using classic molecular dynamics simulations and Density Functional Theory (DFT) methods. Charge transfer between the ions and fullerene were computed by DFT. Force field parametrization used in molecular dynamics simulations was corrected to reproduce DFT ion-C60 interaction mechanism. Structural, dynamic, and energetic factors were analyzed to infer the role of the studied ions on the behavior of fullerenes in ionic liquids. The intermolecular ion-C60 interaction energy controls the behavior of these fluids, leading to prevailing roles by interaction mechanism through the π system of C60 nanoparticle, both for anions and cations.

A Tuned LRC-DFT Design of Ambipolar Diketopyrrolopyrrole- Containing Quinoidal Molecules Interesting for Molecular Electronics

This work presents a Density Functional Theory (DFT) study on the charge transport related properties of two quinoidal diketopyrrolopyrrole (DPP) based systems. System A, recently synthesized, shows high efficiency as n-type organic semiconductor material while system B, not synthesized yet, has a linking benzothiadiazole (BT) unit between DPP moieties and would display an ambipolar character. The use of tuned, long-range corrected (LRC) functionals allows one to predict HOMO, LUMO, and charge transport properties for compound A in concordance with those experimentally observed. The use of BT building blocks allows for a conclusion that compound B is expected to display balanced and efficient charge injection along with high mobilities both for holes and electrons, which points to its potential to obtain high performances as an ambipolar semiconductor.

Influence of the alkyl and alkoxy side chains on the electronic structure and charge-transport properties of polythiophene derivatives.

Density Functional Theory has been used to study the structural, electronic and charge-transport properties of two regio-regular head-to-tail polythiophene derivatives, i.e. poly(3-hexyl-thiophene), P3HT, and poly(3-oxyhexyl-thiophene), P3OHT. The effect of substituents on the electronic structure was analyzed by means of bandwidth, bandgap, effective mass, total and partial densities of states and crystal orbital overlap populations. Electronic couplings were estimated from band diagrams as the splitting of the valence band. The neutral and cationic states of isolated oligomers were optimized using the supercell approximation. The hole-transfer rates and mobilities were evaluated according to Marcuss theory. Results provide a compelling illustration of the effect of side chains on the crystal packing, electronic structure and charge-transport properties. Thus, the hole mobility calculated for the alkyl derivative was 0.15 cm(2) V(-1) s(-1) (experimental mobility is 0.10 cm(2) V(-1) s(-1)), while the alkoxy derivative has a theoretical mobility of 0.49 cm(2) V(-1) s(-1). The obtained results hopefully could motivate experimentalists to try out P3OHT for an improved charge carrier mobility.

Solution-based synthesis and processing of Sn- and Bi-doped Cu3SbSe4 nanocrystals, nanomaterials and ring-shaped thermoelectric generators

Copper-based chalcogenides that comprise abundant, low-cost, and environmental friendly elements are excellent materials for a number of energy conversion applications, including photovoltaics, photocatalysis, and thermoelectrics (TE). In such applications, the use of solution-processed nanocrystals (NCs) to produce thin films or bulk nanomaterials has associated several potential advantages, such as high material yield and throughput, and composition control with unmatched spatial resolution and cost. Here we report on the production of Cu3SbSe4 (CASe) NCs with tuned amounts of Sn and Bi dopants. After proper ligand removal, as monitored by nuclear magnetic resonance and infrared spectroscopy, these NCs were used to produce dense CASe bulk nanomaterials for solid state TE energy conversion. By adjusting the amount of extrinsic dopants, dimensionless TE figures of merit (ZT) up to 1.26 at 673 K were reached. Such high ZT values are related to an optimized carrier concentration by Sn doping, a minimized lattice thermal conductivity due to efficient phonon scattering at point defects and grain boundaries, and to an increase of the Seebeck coefficient obtained by a modification of the electronic band structure with Bi doping. Nanomaterials were further employed to fabricate ring-shaped TE generators to be coupled to hot pipes, which provided 20 mV and 1 mW per TE element when exposed to a 160 °C temperature gradient. The simple design and good thermal contact associated with the ring geometry and the potential low cost of the material solution processing may allow the fabrication of TE generators with short payback times.

DFT study of the effect of fluorine atoms on the crystal structure and semiconducting properties of poly(arylene-ethynylene) derivatives.

The effect of fluorine substitution on the molecular structure, crystal packing, and n-type semiconducting properties of a set of poly(arylene-ethynylene) polymers based on alternating thiadiazole and phenyl units linked through ethynylene groups has been studied by means of Density Functional Theory. As a result, an enlargement in the interplanar distance between cofacial polymer chains, as well as a decrease of the electronic coupling and electron mobility is predicted. On the other hand, fluorination could facilitate electron injection into the material. A polymer containing both alkoxy pendant chains and fluorine atoms is proposed as a compromise solution between efficiency of electron injection and charge transport within the material.