Quenching of semiconductor quantum dot photoluminescence by a pi-conjugated polymer

In this communication we discuss the possibility of hole transfer between a photoexcited semiconductor quantum dot and a pi-conjugated polymer. This charge-transfer event will be investigated (exploited) on the basis of its implication toward a solar energy conversion scheme. Experimentally, we show that the steady-state photoluminescence (PL) of a solution of InP quantum dots is quenched by the introduction of solvated poly(3-hexylthiophene). Time-resolved PL experiments on these solutions are also presented. It was observed that the PL transients did not significantly change upon the addition of the conductive polymer. These new results indicate that said PL quenching is static in nature. This suggests that in solution, the quantum dot and the polymer exhibit a strong intermolecular interaction. As the two species encounter each other through diffusion, the polymer quenches the quantum dot photoluminescence without altering the population's PL lifetime. This new evidence suggests that the polymer and the quantum dot form a relatively stable complex.     

Excited-state symmetry breaking in cofacial and linear dimers of a green perylenediimide chlorophyll analogue leading to ultrafast charge separation

Photoexcitation of chromophoric dimers constrained to a symmetric pi-stacked geometry by their molecular structure usually produces excimers independent of solvent polarity, while dimers with edge-to-edge perpendicular pi systems undergo excited-state symmetry breaking in highly polar solvents leading to intradimer charge separation. We present direct evidence for symmetry breaking in the lowest excited singlet state of a symmetric cofacial dimer of 1,7-bis(pyrrolidin-1'-yl)-perylene-3,4:9,10-bis(dicarboximide) (5PDI) in the low polarity solvent toluene to produce a radical ion pair quantitatively. This dimer, cof-5PDI2, was synthesized by attaching two 5PDI chromophores via imide groups to a xanthene spacer. For comparison, a linear symmetric dimer, lin-5PDI2, was prepared in which the 5PDI chromophores are linked end-to-end via a N-N single bond between their imides. The edge-to-edge pi systems of the 5PDI chromophores within lin-5PDI2 are perpendicular to one another. Ground-state absorption spectra of both 5PDI dimers show exciton coupling, which is consistent with the orientation of the 5PDI chromophores relative to one another. Ultrafast transient absorption spectroscopy following excitation of the dimers with 700 nm, 100 fs laser pulses shows that quantitative intradimer electron transfer occurs in cof-5PDI2 in toluene with tau = 0.17 ps followed by charge recombination to the ground state with tau = 222 ps. Similar measurements on lin-5PDI2 reveal that photoinduced electron transfer does not occur in toluene, but occurs in more polar solvents such as 2-methyltetrahydrofuran, wherein tau = 55 ps for charge separation and tau = 99 ps for charge recombination. Excited-state symmetry breaking in 5PDI dimers provides new routes to biomimetic charge separation and storage assemblies that can be more easily prepared and modified than those based on multiple tetrapyrrole macrocycles.     

The Effect of Background Gas on the Excitation Temperature and Electron Number Density of Basalt Plasma Induced by 10.6 Micron Laser Radiation

Plasma Chemistry and Plasma Processing - Time-integrated optical emission spectroscopy was applied for the analysis of emission spectra, and determination of electron densities and excitation...     

Superposition of dynamic mechanical properties in the glassy state

Superposition of the loss tangent curves could be achieved for the β-transition of a series of homologous epoxy resins. It was found that both a vertical and horizontal shift were necessary to achieve superposition when the curves were plotted as the logarithm of the loss tangent versus reciprocal absolute temperature. Resins from the diglycidyl ether of bisphenol A (DGEBA) were prepared with five different curing agents and their loss tangent curves measured on a free-oscillation torsion pendulum (ca. 1 cps). The β transition is caused by DGEBA, which was found via molecular models to contain a mobile group. The intensity of the loss for three of the resins was found to be proportional to the concentration of DGEBA, molecular models revealing that no additional mobile groups were introduced by these curatives. The remaining two curing agents introduced mobile groups into their systems and for these two, no separate transitions were identified but the intensity of the DGEBA β transition was increased. This may be caused by a coupling of the DGEBA mobile groups through the flexibility of the curative-introduced mobile groups.     

Correlation analysis of kinetic parameters of the free radical polymerization of alkylphenyl methacrylates

Data from previous articles on the initial polymerization rates of a series of monoalkyl substituted phenyl methacrylates were found to depend strongly on the size and position of the monoalkyl substituent in the phenyl ring. In order to investigate the possibility of correlation of all the rate data obtained, all those for phenyl methacrylate and ten other alkylphenyl substituted methacrylate monomers were fitted into empirical linear free energy relationships (LFER). Assuming steady-state kinetics and separating out the initiator decomposition contribution to the initial overall rate R_p, it was demonstrated that k′ = k_p/k_t^½ can be successfully correlated with LFER equations containing predominantly steric parameters. The steric influence is more pronounced in the o- than in the p-substitution, due to the proximity of the o-substituent to the reaction propagation site. The results are corroborated by the existence of an isokinetic relationship of ΔH? vs. ΔS?, providing the proportionality of electronic effects, however small, and the predominating steric effects. The treatment is based on the assumption that the initiator efficiency ? equals 1 and that the termination rate constants k_t are equal for monomers of similar structure, what might represent an approximation of the true situation in the system investigated. © John Wiley & Sons, Inc.     

The lead tetraacetate reaction of alcohols containing a small ring. Part II. Cyclobutane-methanols and cyclopropane-ethanols

The leadtetraacetate and lead tetraacetate/metal chloride oxidations of cyclobutane-methanol, cyclopropane-ethanol and the corresponding α, α-diniethyl alcohols have been investigated and compared with the oxidative reactions of cyclobutane-carboxylic acid, cyclopropane-acetic acid and 4-pentenoic acid, performed with the same reagents and under similar conditions. It was found that alcohol β-fragmentation and acid decarboxylation follow a remarkably similar mechanistic course, affording comparable results when the substrates are of the same structural type ( 1, 2 and 5; 3, 4 and 6 ) or are converted to the same intermediate alkyl radical fragments ( 3, 4, 6 and 7 ). In addition, cyclization products formed from cyclopropane-ethanol (dihydropyran derivative 31 ) and 4-penteuoic acid (γ-lactones 38 ) have been isolated and identified.     

Seasonal indoor radon concentration in FYR of Macedonia

This paper presents the results of the seasonal indoor radon concentration measurements in dwellings in all regions of the Former Yugoslav Republic (FYR) of Macedonia. The measurements were made in 437 dwellings using CR-39 track detectors over four successive three-month periods (winter, spring, summer and autumn) throughout 2009. The results of analysis of variance showed statistically significant differences between indoor radon concentrations in different seasons. The geometric mean values and geometric standard deviations of indoor radon concentrations in winter, spring, summer and autumn were obtained to be: 115 Bq m^?3 (2.02), 72 Bq m^?3 (1.97), 46 Bq m^?3 (1.95), 92 Bq m^?3 (2.02), respectively. The geometric mean values of spring, summer and autumn to winter ratios were found to be: 0.63 (1.50), 0.40 (1.81), and 0.80 (1.58), respectively. The results of the analysis of the variance showed statistically significant differences among the indoor radon measurements for the regions in different seasons. The influence of the factors linked to building characteristics in relation to radon measurements in different seasons was examined. The factors which enable a differentiation into subgroups (significance level p < 0.05) are the floor level, basement and building materials.