Catalytic Thermal Decomposition of NO2 by Iron(III) Nitrate Nonahydrate-Doped Poly(Vinylidene Difluoride).

The products of thermal decomposition of iron nitrate nonahydrate doped into poly(vinylidene difluoride) are examined using Mössbauer spectroscopy. Very little of the expected nitrogen dioxide product is observed, which is attributed to Fe3+ catalysis of the decomposition of NO2. The active site of the catalysis is shown to be Fe(OH)3 in the polymer matrix, which is, unexpectedly, reduced to Fe(OH)2. Thermodynamic calculations show that the reduction of Fe3+ is exergonic at sufficiently high temperatures. A reaction sequence, including a catalytic cycle for decomposition of NO2, is proposed that accounts for the observed reaction products. The role of the polymer matrix is proposed to inhibit transport of gas-phase products, which allows them to interact with Fe(OH)3 doped in the polymer.

Catalysis of the Thermal Decomposition of Transition Metal Nitrate Hydrates by Poly(vinylidene difluoride).

Poly(vinylidene difluoride) (PVDF) doped with transition metal nitrate hydrates are cast into thin films giving a high ß-phase content. Analysis of the thermal behavior of the doped PVDF shows that the decomposition of the metal (II) nitrate hydrates to metal (II) oxides is catalyzed by the PVDF, as evidenced by reduction in the decomposition temperature by as much as 170 °C compared to the pure metal salts. In contrast, there is little to no apparent catalysis for the decomposition of the metal (III) nitrate hydrates. The FTIR spectra of the gas phase decomposition products show H2O and NO2 are the major components for both PVDF-doped material and the pure metal nitrate hydrates. A mechanism for the role of PVDF is proposed that uses the internal electric field of the ferroelectric phase to orient the nitrate ions and polarize the N-O bonds.

Evolution of Surface Morphology of Spin-Coated Poly(methyl methacrylate) Thin Films.

The morphology of sub-micron poly(methyl methacrylate) films coated to glass supports by spin coating from toluene is examined using surface profilometry. Wrinkled surfaces with local quasi-sinusoidal periodicity were seen on the surfaces of films with thicknesses of larger than 75 nm. The surface wrinkles had large aspect ratios with wavelengths in the tens of microns and amplitudes in the tens of nanometers. Wrinkles that formed during spin-coating are attributed to surface perturbations caused by Rayleigh-Bénard-Marangoni convective instabilities. The effects of film thickness, coating solution concentration, and drying rate on the thin film surface morphology are investigated. The results can be used to prepare surfaces with controlled morphology, either smooth or with periodic wrinkles.

A Spectroscopic Study of Xanthene Dyes on a Polystyrene Surface: an Investigation of Ion-π Interactions at Polymer Interfaces.

Thin films of three differently charged xanthene dyes: rhodamine 6G (Rh6G, cationic), fluorescein 27 (F27, neutral), and disodium fluorescein (DSF, anionic) were cast onto a polystyrene (PS) coated glass substrate to investigate ion-π interactions. Absorbance spectroscopy was used to determine the aggregation state of the dyes on the PS surface. Deconvolution of the spectra of films revealed multiple peaks for all dyes assigned to isolated monomers, aggregates, and complexes between the dye and the polymer substrate. The shift of the low energy peak relative to the monomer peak was used as an indication of interaction strength of that species with the PS π system, which followed the trend of Rh6G > DSF > F27. Increase in the interaction energy is attributed to stabilization from ion-π interactions. Steady-state emission spectra and excited state lifetime measurements were performed on all films. The formation of a weakly emissive exciplex was found for Rh6G and DSF, consistent with ion-π interactions, but no evidence of an exciplex is found for the F27 films.

Rhodamine 6G Structural Changes in Water/Ethanol Mixed Solvent.

In water rhodamine 6G (Rh6G) tends to form aggregates at higher concentrations while in ethanol the aggregation is minimal. The extent of aggregation can be controlled by changing the water to ethanol ratio. In ethanol the absorption spectra have a low energy peak and a higher energy shoulder, which are assigned to the S1 π-π* transition and vibronic side band, respectively, of Rh6G monomers. In water the same two peaks absorption peaks are observed at low concentrations but at higher concentrations a new peak grows in, which is assigned to an H-dimer. Emission spectra are in agreement with these assignments, but also develop a third peak at higher concentrations that is assigned to emission from excimer aggregates. For the first time, the monomer and dimer average diameters were measured by light scattering to be 1.4 ± 0.2 nm and 3.3 ± 0.6 nm, which form in the ground state, leading to the observed excited states. In a mixed solvent the extent of aggregation can be controlled by selecting the ethanol to water ratio, even at the highest concentrations. Graphical Abstract ᅟ.

A comparison of SERS and MEF of rhodamine 6G on a gold substrate.

Rhodamine 6G is spin-cast onto gold surfaces and the reflectance, emission, excitation, and SERS spectra are reported. Electron microscopy shows that the particle sizes of the gold are uniform for all preparations. Reflection spectra demonstrate the spectroscopic signature for Rh6G aggregates for thicker films and that the gold plasmon band shifts due to the refractive index change on the surface. The intensity of the SERS spectra increases with increasing surface coverage but the change is nonlinear between submonolayer and multilayer surface densities. The SERS resonance frequencies are unchanged as a function of Rh6G thickness, indicating that there is no coupling between Rh6G molecules in the ground state. The emission spectra behave unexpectedly as a function of Rh6G coverage. At submonolayer coverage the emission is relatively strong, decreases as the surface density increases to a monolayer, and then increases as the Rh6G thickness increases. Excitation spectra demonstrate that the emitting species at low surface density is monomeric but for thicker layers the moiety responsible for emission is Rh6G excited state aggregates. For the thicker films, the Rh6G acts as its own dielectric layer for metal enhanced fluorescence of the aggregates, which is the first example of a system where the fluorophore acts as its own dielectric for metal enhanced fluorescence. The intensity of the aggregate emission on gold intensity is three times of that found when Rh6G is deposited on glass. The gold induces emission in the Rh6G excited state aggregates that are quenched in the absence of the plasmon field.

The Influence of Interfacial Effects on the Photophysics of Rhodamine 6G Thin Films on a Poly(vinylidene fluoride) Surface.

The spectral response of ultrathin films of rhodamine 6G (Rh6G) cast onto polyvinylidenefluoride (PVDF) coated glass slides is studied to investigate a perceived fluorescence emission enhancement. Varying the thickness of the Rh6G layer (submonolayer to multiple layers) on the PVDF layer revealed the existence of multiple Rh6G species on the surface, similar to previous reports on glass. Excitation spectra show that J-type excitons are not responsible for an emission enhancement. Trends in the fluorescence emission intensity show that the surface roughness of the PVDF layer prevents the Rh6G molecules from organizing the way they would on a smooth glass surface. The PVDF surface roughness preserves the emissive monomers and excited-state excimers of Rh6G while reducing the self-quenching of aggregates. Coupled with this is an internal reflection effect that causes light to be trapped between the Rh6G/PVDF and PVDF/glass interfaces. This effect leads to multiple absorption events, and thus more efficient use of the incident light.

Speciation of the products of and establishing the role of water in the reaction of TNT with hydroxide and amines: structure, kinetics, and computational results.

The reaction of trinitrotoluene (TNT) with bases has been investigated by NMR and visible spectroscopy methods. Hydroxide ion was found to react in one of two ways, either by deprotonation of the methyl group or by nucleophilic attack on the aromatic ring to form a σ adduct. The rate of each mode of reaction depends upon the polarity of the solvent. In tetrahydrofuran (THF), σ adduct formation is rapid and the long-term equilibrium product is deprotonation of the methyl group. When the solvent is methanol (MeOH), the two reactions have similar rates and the σ adduct becomes the majority product. Amines were found to be ineffective in directly deprotonating TNT or in forming σ adducts. Rather, the amines react with ambient water to generate hydroxide ion, which then reacts with TNT. The solvent choice and water content are crucial to understanding the reactivity of bases with TNT. To assist in the interpretation of the experimental results, computational analysis was performed at the B3LYP/6-311+G**//HF/6-311+G** level to determine the thermodynamics of the reactions of TNT. The SM8 implicit solvation model was applied to converged geometries and suggested a strong solvation effect upon product formation. Thermodynamic analysis suggested a significant preference of alkoxide or hydroxide attack versus amine attack in any modeled dielectric, consistent with the experimental observations.

Light trapping to amplify metal enhanced fluorescence with application for sensing TNT.

Metal Enhanced Fluorescence (MEF) typically produces enhancement factors of 10 to 50. By using a polymer layer as the dielectric spacer enhancements as high as 1,600 can be observed. The effect occurs with a variety of different polymers and substrates, all of which act to trap light in the dielectric layer. This allows the fabrication of sensors with improved sensitivity as demonstrated for detection of trinitrotoluene (TNT).

Fluorescent species formed by reaction of trinitroaromatics with N,N-dimethylformamide and hydroxide.

Trinitrobenzene (TNB) and trinitrotoluene (TNT) react in N,N-dimethylformamide (DMF) to form multiple species in solution. Despite structural similarities, electronic spectra show that the reactivity is different for TNB and TNT. In addition to reaction with the DMF solvent, residual water in nominally dry DMF generates sufficient hydroxide for reaction with TNB and TNT. Multiple sigma adducts are formed and observed to be fluorescent, which has not been previously reported. Both TNB and TNT show the capacity to form sigma adducts with hydroxide and DMF, while methyl hydrogens of TNT can be deprotonated by hydroxide.

Hybrid solar cells based on single-walled carbon nanotubes/Si heterojunctions.

Photovoltaic devices based on single-walled carbon nanotubes (SWNTs) and n-silicon heterojunctions have been fabricated by a spray deposition process. We provide direct evidence that nanotubes serve as an active photosensing material involved directly in the photon absorption process as well as contributing to charge separation, transport and collection. The characteristic band of the SWNT band in the photoconductivity spectrum matches the S(11) absorption band of semiconducting SWNTs of 7,6 chirality. Centrifugation of the SWNTs provides two fractions. The sediment fraction exhibits a conversion efficiency ( approximately 1.7%) higher by a factor of eight compared to the supernatant fraction. SEM images and conductivity measurements show that the SWNT network morphology of the sediment fraction has longer and thicker nanotube bundles forming highly porous films, accounting for the enhanced conductivity and higher transparency.

Optical humidity sensing and ultrasound effect for mesoporous silicon one-dimensional photonic crystals.

Mesoporous silicon (PSi) microcavities (MC) based on one-dimensional photonic crystals have been studied as optical sensors for relative humidity (RH). Oxidized PSi modified the structure of the MC such that the spectral position of the MC resonance peak depended on the humidity. A spectral shift of the MC resonance peak of up to 6 nm to longer wavelengths was observed as the RH increased from 20% to 85%. Ultrasound affects the MC peak spectral position in the reverse direction as a result of water removal from mesoporous structure. This effect can be used for the stabilization of the peak spectral position for an optical interconnect and fast recovery of the optical gas sensors.

Photoactuation from a carbon nanotube-nafion bilayer composite.

A bilayer composite of single walled carbon nanotubes (SWNTs) deposited onto Nafion exhibits substantial mechanical motion upon exposure to visible or near-infrared light. The magnitude of the actuation parallels the absorption spectrum of the SWNTs in the near-infrared, but the actuation diminishes in the visible and disappears in the UV portions of the spectrum. In the near-infrared region, the photoactuation is linear with respect to the light intensity. The photoactuation also appears to be associated with a photocurrent across the nanotube/Nafion interface. The proposed mechanism for the actuation is that band bending of the semiconducting SWNTs induces polarization of mobile hydrogen ions at the Nafion interface, which then causes swelling of the polymer.

Effect of residual monomer on the spectroscopic properties of polythiophenes.

The addition of some small molecules can red shift UV-Visible absorption and quench the fluorescence of poly(3-octadecylthiophene).

Bis(phosphine imide)s: easily tunable organic electron donors.

[graph: see text] The electrochemical, structural, and spectroscopic properties of bis(phosphine imide)s have been investigated. p-Phenylenebis(phosphine imide)s Ar3PNC6H4NPAr3 (1a-d) have two reversible single-electron oxidations. The first oxidation potentials can be varied from -0.05 to 0.15 V (versus SCE) by modification of the substituents on phosphorus (Ar). Electron-donating substituents lower the oxidation potential, while electron-withdrawing substituents increase the oxidation potential. The difference between the first and second oxidation potential (deltaE, 0.41-0.50) and the electronic coupling (Hab, 1.1 eV) are similar for 1a-d. Computational (DFT) and UV-visible-NIR spectroscopic investigations of 1a-d suggest that the first oxidation leads to a delocalized radical cation 1a*+ while the second oxidation leads to a quinonoidal dicationic state 1a2+. The aromatic linker between phosphine imides has also been modified. Upon oxidation, N,N'-4,4'-biphenylene(bis(triphenyl)phosphine imide) (3) forms radical cationic and a dicationic species similar to 1a-d. While deltaE (0.18 V) and Hab (0.63 eV) are smaller, suggesting weaker electronic communication between the two P=N units in the radical cationic state, the presence of NIR absorptions with vibrational fine structure (768, 861, and 983 nm) supports the formation of delocalized radical cation for 3*+.

Electromechanical actuation of composite material from carbon nanotubes and ionomeric polymer.

An actuating system composed of nafion ionomeric polymer coated with single-walled carbon nanotubes electrodes was studied as an electromechanical actuator. The actuator gives a sizable mechanical response to low voltages (turn-on voltage of approximately 2.5 V) under open-air conditions, i.e., in the absence of a surrounding supporting electrolyte. The actuator is active under both dc and ac bias and has a strong resonance at low frequencies which is dependent upon the size of the actuator. The actuator was studied using Fourier transform infrared and vis-NIR spectroscopies, cyclic voltammetry, and by the current-time response under an applied step voltage. An analytical model is proposed to understand the electrical behavior, which is consistent with the spectroscopic results.

Unusual chromic and doping behavior of ether substituted polythiophenes.

Poly[3-(oligoethylene oxide)-4-methylthiophene] is doped by HCl in aqueous solution in the absence of oxygen and undergoes dramatic solvatochromism in water-ethanol mixtures.

Synthesis and solution characterization of [Ru(bpy)2]2+ modified polyazines.

A series of [Ru(bpy)(2)](2+) complexes linked by a controlled number of azine units (one to seven) were synthesized and studied in the solution phase. Polymers and dimer model compounds were examined by cyclic voltammetry and IR, NMR, and visible-NIR spectroscopies. The NMR spectra and the cyclic voltammograms indicated that the Ru(2+) sites influenced the main chain properties at least 15 A from the metal site. The first oxidation in each material was assigned to a ligand-centered process, but DFT calculations suggested that the Ru(2+) has an important influence. The first oxidized state of the polymers has a spectroscopic band that is consistent with an intervalence transfer (IT) transition, but this absorption is not seen in the dimer model compounds. Thus, the IT feature is assigned to a ligand-ligand transition that spans several repeat units in the polymer.