Entropic changes control the charge separation process in triads mimicking photosynthetic charge separation

Laser-induced optoacoustic spectroscopy (LIOAS) measurements with carotene-porphyrin-acceptor "supermolecular" triads (C-P-A, with A = C60, a naphthoquinone NQ, and a naphthoquinone derivative, Q) were carried out with the purpose of analyzing the thermodynamic parameters for the formation and decay of the respective long-lived charge separated state C*+-P-A*-. The novel procedure of inclusion of the benzonitrile solutions of the triads in Triton X-100 micelle nanoreactors suspended in water permitted the separation of the enthalpic and structural volume change contributions to the LIOAS signals, by performing the measurements in the range 4-20 degrees C. Contractions of 4.2, 5.7, and 4.2 mL mol-1 are concomitant with the formation of C*+-P-A*- for A = C60, Q and NQ, respectively. These contractions are mostly attributed to solvent movements and possible conformational changes upon photoinduced electron transfer, due to the attraction of the oppositely charged ends, as a consequence of the giant dipole moment developed in these compounds upon charge separation ( approximately 110 D). The estimations combining the calculated free energies and the LIOAS-derived enthalpy changes indicate that entropy changes, attributed to solvent movements, control the process of electron transfer for the three triads, especially for C-P-C60 and C-P-Q. The heat released during the decay of 1 mol of charge separated state (CS) is much smaller than the respective enthalpy content obtained from the LIOAS measurements for the CS formation. This is attributed to the production of long-lived energy storing species upon CS decay.     

Simple STM tip functionalization for rapid DNA sequencing: an Ab initio Green's function study

An ab initio nonequilibrium Green's function study of the electron-transport properties of the adenine, thymine, cytosine, and guanine DNA bases located between gold electrodes has been performed. One-electron transmission spectra were calculated for both gold and sulfur-modified gold electrodes, which are model conditions of scanning tunneling microscopy (STM) experiments with the different tips. It is shown that the nature of chemical bonding between molecules and metal electrodes plays the most significant role in the overall conductance of the systems. The distance between electrodes and the size of molecules are less important, at least when both sides of the molecule form chemical contact with the electrodes. On the basis of the obtained results, a simple two-pass DNA sequencing scheme is suggested.     

Iptycene‐Based Fluorescent Sensors for Nitroaromatics and TNT

Small‐molecule fluorescent sensors ( 1 – 5 ) for the recognition of nitroaromatic compounds, such as 2,4‐dinitrotoluene and the explosive TNT, were obtained by using a three‐step dehydrohalogenation cycloaddition protocol. The interaction of the receptors and nitroaromatics was studied both in solution and in the solid state by using fluorescence spectroscopy and X‐ray crystallography, respectively. It is shown that the iptycene receptors 1 – 5 provide a cavity suitable for binding nitroaromatic compounds in an edge‐to‐face mode, rather than simple ring‐stacking interactions. The results obtained inspired us to develop an inexpensive, reliable and robust sensor for vapour detection of explosives. Polymer nanofibres are particularly suitable for the production of such TNT sensors as they accelerate the mass exchange between the polymer and the vapours of TNT. Quenching of the sensors took place within 1 min compared to 10 min for a glass‐slide assay. Hence, the sensor performance can be improved by optimising the matrix material and morphology without resynthesising the sensor moieties.     

Enhancement and control of H2 dissociative ionization by femtosecond VUV laser pulses

We report ab initio calculations of H2 ionization by VUV/fs 10(12) W/cm2 laser pulses including correlation and all electronic and vibrational degrees of freedom (DOF). Inclusion of the nuclear DOF leads to a substantial increase of resonance enhanced multiphoton ionization. By varying pulse duration, it is possible to control the ratio of dissociative to nondissociative ionization as well as the final H+(2) vibrational distribution. For pulses longer than 10 fs and proportional to omega>0.46 a.u., dissociative ionization entirely dominates, which is a very unusual situation in photoionization studies.     

Low-order aberration sensitivity of an optical vortex coronagraph

We describe a high-contrast imaging technique capable of directly measuring light from a terrestrial planet by using a vortex mask of topological charge m = 5. We demonstrate that this technique is relatively insensitive to low-order aberrations and compare its performance to that of a band-limited Lyot coronagraph.     

Synthesis, structure, anion binding, and sensing by calix[4]pyrrole isomers

The synthesis, structure, and anion binding properties of chromogenic octamethylcalix[4]pyrroles (OMCPs) and their N-confused octamethylcalix[4]pyrrole isomers (NC-OMCPs) containing an inverted pyrrole ring connected via alpha'- and beta-positions are described. X-ray diffraction analyses proved the structures of two synthesized isomeric pairs of OMCPs and NC-OMCPs. The addition of anions to solutions of chromogenic OMCPs and NC-OMCPs resulted in different colors suggesting different anion-binding behaviors. The chromogenic NC-OMCPs showed significantly stronger anion-induced color changes compared to the corresponding chromogenic OMCP, and the absorption spectroscopy titrations indicated that chromogenic OMCPs and NC-OMCPs also possess different anion binding selectivity. Detailed NMR studies revealed that this rather unusual feature stems from a different anion-binding mode in OMCPs and NC-OMCPs, one where the beta-pyrrole C-H of the inverted pyrrole moiety participates in the hydrogen-bonded anion-NC-OMCP complex. Preliminary colorimetric microassays using synthesized chromogenic calixpyrroles embedded in partially hydrophilic polyurethane matrices allow for observation of analyte-specific changes in color when the anions are administered in the form of their aqueous solutions and in the presence of weakly competing anions.     

Energetics of formation of TiGa3As4 and TiGa3P4 intermediate band materials

Using density functional theory quantum methods, total energy values and vibrational properties have been computed, and thermodynamic properties evaluated, for Ti-substituted GaAs and GaP, proposed as candidates for intermediate band photovoltaic cells. The calculations predict that the formation of these materials from the binary compounds implies an increase in total energy (that is ascribed largely to the change in coordination undergone by Ti, from six-fold to four-fold), and thus phase separation rather than mixed compound formation would be favored. However, the mentioned increase is not larger (for the arsenide case it is actually smaller) than that predicted for Mn-substituted GaAs, a material which has been experimentally made, and therefore the obtention of these Ti-substituted materials is expected to be feasible as well. Vibrational and disorder entropy contributions to the formation free energy of the ternary compounds have been also computed; they compensate partially for the total energy increase, and indicate that the thermodynamic feasibility of the materials synthesis improves for low Ti concentrations and high temperature conditions.     

Spin-filtered edge states in graphene

Spin–orbit coupling changes graphene, in principle, into a two-dimensional topological insulator, also known as quantum spin Hall insulator. One of the expected consequences is the existence of spin-filtered edge states that carry dissipationless spin currents and undergo no backscattering in the presence of non-magnetic disorder, leading to quantization of conductance. Whereas, due to the small size of spin–orbit coupling in graphene, the experimental observation of these remarkable predictions is unlikely, the theoretical understanding of these spin-filtered states is shedding light on the electronic properties of edge states in other two-dimensional quantum spin Hall insulators. Here we review the effect of a variety of perturbations, like curvature, disorder, edge reconstruction, edge crystallographic orientation, and Coulomb interactions on the electronic properties of these spin filtered states.     

Chiral Single-Molecule Potentiometers Based on Stapled ortho- Oligo(phenylene)ethynylenes

We report on the chemical design of chiral molecular junctions with stress-dependent conductance, whose helicity is maintained during the stretching of a single molecule junction due to the stapling of both ends of the inner helix. In the reported compounds, different conductive pathways are observed, with clearly different conductance values and plateau-length distributions, attributed to different conformations of the helical structures. The large chiro-optical responses and the potential use of these molecules as unimolecular spin filters have been theoretically proved using state-of-the-art Density Functional Theory (DFT) calculations, including a fully ab-initio estimation of the CISS-originating spin polarization which is done, for the first time, for a realistic molecular system.