Combined theoretical and time-resolved photoluminescence investigations of [Mo6Brⁱ8Br(a)6]²â» metal cluster units: evidence of dual emission.

The combined time-resolved photoluminescence (PL) and theoretical study performed on luminescent [Mo6Br(i)8Br(a)6](2-)-based systems unambiguously shows that their NIR-luminescence is due to at least two emissive states. By quantum chemical studies, we show for the first time that important geometrical relaxations occur at the triplet states either by the outstretching of an apex away from the square plane of the Mo6 octahedron or by the elongation of one Mo-Mo bond. Experimental PL measurements demonstrate that the external environment (counter-ions, crystal packing) of the cluster has a noticeable impact on its relaxation processes. Temperature and excitation wavelength dependence of the two components of the luminescence spectra is representative of multiple competitive de-excitation processes in contradiction with Kasha's rule. Our results also demonstrate that the relaxation processes before and after emission can be tracked via fast time-resolved spectroscopy. They also show that the surroundings of the luminescent cluster unit and the excitation wavelength could be modulated for target applications.

Quantum mechanisms for selective detection in complex gas mixtures using conductive sensors.

In this paper, we consider new quantum mechanisms for selective detection in complex gaseous media which provide the highest possible efficiency of quantum sensors and for the first time analyze their nature. On the basis of these quantum mechanisms, the concepts of quantum detection and innovative methods of analysis are developed, which are virtually impossible to implement in the conventional conductive sensors and nanosensors. Examples of original solutions to problems in the field of detection and analysis of human breath using point-contact sensors are considered. A new method of analysis based on detection of metastable quantum states of the "point-contact sensor-breath" system in dynamic mode is proposed. The conductance histogram of dendritic Yanson point contacts recorded for this system is a unique energy signature of breath which allows differentiation between the states of human body. We demonstrate that nanosized Yanson point contacts, which, thanks to their quantum properties, can replace a massive spectrometer, open up wide opportunities for solving complex problems in the field of breath analysis using a new generation of portable high-tech quantum sensor devices.

Coaxial nickel/poly(p-phenylene vinylene) nanowires as luminescent building blocks manipulated magnetically.

A template-based strategy was developed which combines a wet-chemical technique and electrodeposition within nanoporous membranes. Morphological, structural and chemical characterization by means of electron microscopy and related techniques demonstrate unambiguously that coaxial nickel/poly(p-phenylene vinylene) (PPV) nanowires have been successfully synthesized. Moreover, modification of their optical and magnetic properties due to the nanoscale and the core-shell structure has been studied. The nickel-PPV nanowires exhibit a slightly blueshifted photoluminescence (PL), which is directly related to the tubular morphology of the PPV shell. The effect of the nickel core on the PL intensity of the PPV shell is discussed. The ferromagnetic behavior has been shown with the magnetization easy axis along the wire axis. These arrays of coaxial semiconducting polymer-metal nanowires embedded in a polymer membrane are interesting for flexible electronics and photovoltaic devices. Furthermore, their magnetic manipulation has been demonstrated, which opens the way to use them as multifunctional building blocks for bio-applications.

Elaboration of conjugated polymer nanowires and nanotubes for tunable photoluminescence properties.

Prototypal photoluminescent nanofibres of poly-(p-phenylene-vinylene) (PPV) were prepared by the wetting template method in polycarbonate nanoporous membranes with an easy all-in solution polymer precursor route. Both nanowires and nanotubes were obtained by varying the dilution of the polymer precursor in methanol prior to thermal conversion. PPV nanotubes exhibit unique features, such as blue-shifted emission at 2.80 eV, higher quantum yield, and longer fluorescence lifetime with respect to PPV films. These effects are attributed to the cancellation of interchain interactions that are consistent with nanoscale tubular structures formed from weakly interacting and short polymer chain segments. The synthesis of these objects opens up perspectives for tunable photoluminescence properties in the blue spectral range and for biochemical applications.

Steady state and transient photoluminescence in poly-p-phenylene vinylene films and nanofibers.

We report in this paper experimental data on steady state and transient photoluminescence of poly-p-phenylene vinylene in the form of nanofibers prepared with a template method and converted at 110 degrees C. Results are compared to those obtained from films of different thicknesses converted at the same temperature. Data are analyzed by a model of bimodal distribution of conjugation lengths and the photoluminescence band shapes, evaluated in the framework of this model, are also presented.

Dynamics of charge migration in poly(para-phenylene vinylene) films and nanocomposites with single walled carbon nanotubes.

We present in this paper a comprehensive study of the migration dynamics of the charges underlying transient photoluminescence (PL) processes in poly(para-phenylene vinylene) (PPV) samples from room temperature to 13 K. In order to interpret experimental data, we have modelled the long-time PL decays (from 100 to 1000 ps) using a time function proportional to [Formula: see text] in which the parameter α is evaluated in a Monte Carlo simulation on polymeric chains. The one dimensional chains (2000 sites long) are formed by random sequences of long and short conjugated segments whose bimodal distributions have been elaborated in previous works in order to reproduce the PL band shapes and peak positions. Intra-chain and inter-chain dynamics are taken into account in the migration of the photogenerated charges from short to long conjugated segments. The statistical analysis is performed by averaging over a total of 10(6) trials for each initial conditions. The values of α have been determined for pristine PPV films and PPV composite films with single-walled carbon nanotubes. This theoretical analysis is in good agreement with experimental data and provides a coherent description for the migration of the photogenerated charges in such inhomogeneous polymeric systems.

Evidence for sensory effects of a 1D organic conductor under gas exposure.

This work describes the gas-sensitive properties of a one-dimensional organic conductor before and after exposure to carbon monoxide and human breath. A sensitive material, an anion-radical salt of tetracyanoquinodimethane, has been investigated by infrared spectroscopy and electrical resistivity measurements. Drastic spectral and electrical changes are found after gas exposure showing that the compound interacts strongly with human breath, carbon monoxide, and ammonia. Under breath action the resistance changes by more than three orders of magnitude while the adsorption of CO, one of the components of breath, results in a decrease in both IR absorption and electrical conductivity. The intensity of the IR absorption spectrum of the material in the CO medium decreases down to 30% in the 2180-2500 cm(-1) range. This absorption varies by about 10% between 750 and 2500 cm(-1) after breath action. Direct electrical measurements show that actions of donor or acceptor gas result in opposite changes of electric resistance. The electrical resistance of the sample can drop down to 0.4 MΩ due to the pulse action of ammonia at 4 ppm concentration, while it increases upon exposure to carbon monoxide media at concentrations of 6-25 ppm. The response signal of the investigated samples changes proportionally to the concentration of the acting gas. The results substantiate prominent gas sensitivity of the investigated material, which might find applications for breath analysis, in particular, for the development of noninvasive diagnosis of gastric diseases.

Evidence of temperature dependent charge migration on conjugated segments in poly-p-phenylene vinylene and single-walled carbon nanotubes composite films.

We present new results of temperature dependence of photoluminescence spectra carried out on poly-p-phenylene vinylene (PPV) and on PPV composite films with single-walled carbon nanotubes. By performing studies at different temperatures (87 and 300 K), we show that a distribution of conjugated PPV segments is needed to interpret experimental data. At the microscopic scale, such a distribution corresponds to the morphological picture of poorly packed short chain segments and well-packed ordered long chain segments. Within this scheme, a new interpretation emerges for explaining the specific behavior of the photoluminescence bands. In particular, the two most intense components of the photoluminescence spectra of PPV thermally converted at 300 degrees C (2.23 and 2.43 eV at 300 K) change drastically their relative intensity when the observation temperature decreases. This effect is interpreted as due to the inhibition of charge migration to longer segments and to radiative recombination occurring mainly on n = 5 conjugated segments.

Two successive single crystal phase transitions involving the coordination sphere of antimony in PhSb(dmit), the first organo-antimony(III) dithiolene complex.

PhSb(dmit) (dmit(2)(-), 4,5-dithiolato-1,3-dithiole-2-thione), the first neutral organo-antimony dithiolene complex, has been synthesized by addition of PhSbCl(2) on a suspension of Na(2)(dmit). The complex was characterized by spectroscopic ((1)H and (13)C NMR and IR) methods and elemental analysis. Its crystal structure was determined by X-ray diffraction at room temperature in the monoclinic P2(1)/c space group, with a = 12.580(3), b = 8.9756(18), c = 15.905(3) A, beta = 109.06(3) degrees, V = 1697.5(6) A(3), Z = 4. A coordinating THF molecule was found in the structure and the coordination geometry around the antimony atom is of distorted pseudopentagonal bipyramid type, if taking into account the Sb.O and secondary Sb.S interactions, as well as the stereochemically active 5s(2) lone pair. The intermolecular Sb.S and S.S contacts, shorter than the sum of van der Waals radii of corresponding atoms, lead to the formation of a three-dimensional polymeric network in the solid state. A second X-ray diffraction experiment, performed at 85 K, revealed a very similar monoclinic unit cell with the noncentrosymmetrical space group P2(1) with a = 12.613(3), b = 8.9876(18), c = 15.109(3) A, beta = 107.01(3) degrees, V = 1637.8(6), Z = 4. The structural differences with the first one are basically due to the rotation of the THF ligand in the coordination sphere of the antimony center, leading to the loss of every inversion center found at room temperature. A temperature variable X-ray diffraction study on a PhSb(dmit) single-crystal allowed the detection, with a remarkable accuracy, of two successive first-order phase transitions, the first occurring at T = 162.5 K, while the second was observed at T = 182.5 K. Subsequently, a third set of X-ray data was collected at 180 K and the resulting structure (monoclinic, P2(1)/c, a = 16.736(3), b = 8.9653(18), c = 33.132(7) A, beta = 91.98(3) degrees, V = 4968.2(17), Z = 12) derives from the two others by a common b axis, a 3-fold cell volume increase, and the presence of only one-third of the inversion centers present at room temperature. A DSC analysis, showing two endothermic peaks at the expected temperatures, confirms the occurrence of the two structural phase transitions, also in agreement with preliminary Raman data.