Membranes of poly(vinylidene difluoride) (PVDF) and relatedpolymersare used as piezoelectric materials to decompose rhodamine B (RhB)under low power sonication. The change in the visible spectrum ofRhB allows determination of the rate constants for different polymercompositions and membrane thicknesses. The results indicate that thepiezoelectric field generated by mechanical deformation of the polymerscatalyzes the scission of the C-N bond in RhB. When PVDF isdoped with transition-metal nitrates to increase the fraction of thepiezoelectric beta-phase, the rate constants increase, but theanalysis is complicated by concurrent adsorption of RhB onto the membrane.A mechanism consistent with the observed data is proposed where thepiezocatalytic effect of the membrane is to heterolytically cleavethe C-N bond followed by formation of a C-Cl bond onthe new xanthene ring.

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.

The absorption and emission spectra of six rhodamine dyes are examined in dilute aqueous solution and as thin films on a glass substrate. The absorption and emission spectra in water can be described using the displaced harmonic oscillator (DHO) model. The changes of the absorption maxima in aqueous solution are controlled by the substitution of alkyl groups on the xanthene nitrogen atoms, which is supported by density functional theory calculations. In addition, the nature of the alkyl substitution influences the excited state lifetimes. In contrast, the photophysical behavior of thin films of the dyes cannot be described using the DHO model. Rather, the spectral changes depend on interactions with the glass substrate for thin films and by aggregation for thicker films. The intensity of the emission spectra can be maximized by controlling the film thickness to be less than a monolayer.

The absorbance, excitation, and emission spectra of ultra-thin films of slides are examined as a function of both PMMA and Rh6G thickness. The thickness of PMMA has a little effect on the absorption or emission properties of Rh6G. At low surface coverage, the spectral properties of Rh6G are dominated by isolated molecules. As the Rh6G thickness increases, there is no evidence for exciton formation. Instead, the Rh6G molecules aggregate, which is responsible for the observed absorption and emission spectra. The aggregates significantly quench the emission so that the maximum emission intensity is found just below a monolayer surface coverage. The lack of exciton formation, which is unexpected, is attributed to the surface morphology of PMMA, which has a periodic surface structure.

Poly(vinylidene difluoride) (PVDF) doped with transition metal nitrate hydrates are cast into thin films giving a high β-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.

Thin films of poly(vinylidene difluoride) (PVDF) doped with zinc nitrate spin-cast onto silicon substrates were studied by optical profilometry, infrared and x-ray photoelectron spectroscopy (XPS). The spin-cast films were found to be significantly nonuniform having variations on the order of hundreds of nanometers across tens of microns for films with low zinc ion content and with pores that extend through the entire film thickness. As the Zn2+ content increases both the porosity and the amount of β-phase of the PVDF in the bulk of the films increase. The depth of some of the pores allowed investigation of both the polymer/air and polymer/substrate interface. The increased porosity of the doped films is tentatively attributed to Marangoni effects.

A predictive film thickness model based on an accepted equation of state is applied to the spin-coating of sub-micron poly(methylmethacrylate) viscous thin films from toluene. Concentration effects on density and dynamic viscosity of the spin-coating solution are closely examined. The film thickness model is calibrated with a system-specific film drying rate and was observed to scale with the square root of spin speed. Process mapping is used to generate a three-dimensional design space for the control of film thickness.

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.

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.

The surface morphology of polystyrene thin films formed from various molecular weight polystyrene and solvent conditions is studied. When spin-cast from tetrahydrofuran (THF) wrinkles are formed at the extremities that have periodicity with wavelengths in the μm range and amplitudes in the nm range but varies with molecular weight. A mixed solvent system consisting of THF and dimethylformamide (DMF) leads to periodic structures only with THF-rich compositions. THF and DMF have similar properties relevant to spin-casting: density, surface tension, molecular weight, and viscosity but different boiling points and room temperature vapor pressures, demonstrating that formation periodicity requires a volatile solvent. The formation of the surface structures is attributed to the Marangoni effect and the film thicknesses and wave parameters are shown to be consistent with literature models.

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.

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 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.

The photophysical properties of rhodamine 6G (Rh6G) deposited on the surface of Zn2+-doped polyvinylidene difluoride (PVDF) are reported. The zinc ion induces an increased amount of the ferroelectric β-phase in the PVDF. The doping suppresses aggregation of the Rh6G, even for the thickest films, as indicated by the lack the low energy signature in the absorption spectra. This effect has not been previously observed. The absorption spectra also show an unusual increase in intensity as the amount of Zn2+ is increased in the substrate while the Rh6G thickness is kept constant. The excitation spectra match the absorption spectra. As the Zn2+ ion concentration increases in the PVDF the emission spectra maxima shift to lower energy and the intensity of the emission increases. The excited states are assigned to monomers, excimers, and excitons. The coupling between the dimeric species is modulated by the electric field created by the ionic dopant and the ferroelectric phase.

Self-organized rhodamine 6G (Rh6G) thin films on a glass substrate were prepared by spin-coating, dip-coating, and drop-coating. The thickness of the Rh6G layer strongly influences both the absorption and emission spectra, which are accounted for by monomers, exciton and excimer formation, and molecular aggregation. Submonolayer films of Rh6G show one maximum and one apparent shoulder in the absorption spectrum, but three peaks are required for deconvolution of the spectrum. As the thin film becomes thicker, the observed maximum shifts to lower energy, and a fourth peak is required for deconvolution. The emission spectra show similar features. In addition, the relative intensity of the emission is strongly dependent on the film thickness with thinner films being substantially more emissive than thick films.

A sensing system for explosives is demonstrated. The sensor is based on a layered structure of approximately a monolayer of a fluorophore deposited onto a few hundred nm of a transparent polymer, supported by a glass slide. The fluorophores are inexpensive xanthene laser dyes, which have high quantum yields, and the polymers are commodity materials polymethylmethacrylate and polyvinylidene difluoride. The different fluorophore/polymer combinations give different emission responses to analytes, including both signal quenching and enhancement. The pattern of responses can be used to identify the analyte. The common explosives TNT, PETN, RDX, HMX, and TATP as gas phase species can all be uniquely identified at room temperature using only the natural vapor pressure of the explosive to deliver sample to the sensor.

In Chapter 1, Fluorescent enhancements have been achieved using a simple layered structure: fluorophore, polymer/metal ion, glass substrate. The polymer/metal ion layer apparently has a strong influence on the emission response of the fluorophore by removing the dye aggregation. This data supports that the addition of higher concentration of hydrated transition metal salt increases the production of β-phase in the Polyvinylidene Difluoride (PVDF). The absorption spectra intensity increased as the amount of Zn2+ is increased in the substrate while the Rhodamine 6G (Rh6G) thickness is kept constant. Investigation into the means of β-phase production and the influence of the interfacial region effect on the fluorescence enhancement was completed and reported in this work. The goal of this study is to understand the interfacial properties that control the nature of the fluorescent emission and determine the structure of the fluorophore on different substrates.

The solution Structutes of organic carbonate solvents (ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), and diethyl carbonate (DEC)) as electrolyte solutions of LiPF6 were investigated with FTIR and NMR spectroscopy and DFT computational methods. Both coordinated and uncoordinated solvents are observed by IR spectroscopy, allowing the determination of solvent coordination numbers, which a range from 2 to 5. The predominant species in solution changes as a function of LiPF6 Concentration. At low salt concentrations (<1.2 M), the predominant species is a solvent-separated ion pair, Whereas at high salt concentrations (>2.0 M) the predominant species in solution is the contact ion pair. In mixed solvent systems (PC-DMC, PC-DEC, EC-DMC, or EC-DEC) the mixed solvated cations are observed in the presence of high concentrations of uncoordinated cyclic carbonate despite the lunch larger dielectric constant of the cyclic carbonates than dielectric constant of linear carbonate.

The reactions of dinitrotoluenes (DNT) with hydroxide ion or wet alkylamines can be used to detect and identify each isomer. The reaction products give visible spectra that can be used directly to determine the isomer speciation with sensitivities in the low micromolar range. NMR spectroscopy in dimethylformamide provides unambiguous identification of the products of each DNT when the base is hydroxide. Kinetic studies show that the initial product of this reaction is the deprotonated anion followed by subsequent formation of sigma-adducts and, in some cases, a dimer, with the exception of 2,4-DNT, which does not react beyond the acid-base reaction. Alkylamines do not react with any DNT, even when the amine is acting as the solvent, unless there is water present. Water is the limiting reagent in these cases, implying that in all cases the reacting species is the hydroxide ion, not the free amine. Computational studies are consistent with the inability of the alkylamine to deprotonate the methyl group in any of the DNT isomers. A general mechanism that is applicable to hydroxide and amine bases is proposed. (C) 2015 Elsevier B.V. All rights reserved.