Diffuse Reflectance Absorption Spectra Research Articles

Novel Charge-Transfer Materials via Cocrystallization of Planar Aromatic Donors and Spherical Polyoxometalate Acceptors

Spherical polyoxometalates (POMs) such as M6O192- and SiM12O404- (with M = Mo or W) and planar arene donors (anthracenes and pyrenes) can be cocrystallized (despite their structural incompatibility) by attaching a cationic “anchor” onto the arene which then clings to the POM anion by Coulombic forces. As a result, novel charge-transfer (CT) salts are prepared from arene donors and Lindqvist-type [M6O19]2- and Keggin-type [SiM12O40]4- acceptors with overall 2:1 and 4:1 stoichiometry, respectively. The CT character of the dark-colored (yellow to red) crystalline materials is confirmed by the linear Mulliken correlation between the CT transition energies and the reduction potentials of the POM acceptors, as well as by the transient (diffuse reflectance) absorption spectra (upon picosecond laser excitation) of anthracene or pyrene cation radicals (in monomeric and π-dimeric forms). X-ray crystallographic studies reveal a unique “dimeric” arrangement of the cofacially oriented arene couples which show contact ...

UV−Vis Absorption, Luminescence, and X-ray Photoelectron Spectroscopic Studies of Rhodamine Dyes Adsorbed onto Different Pore Size Silicas

Sulforhodamine 101 and rhodamine 6G were adsorbed onto silicas with different pore sizes ranging from 22 to 150 Å. Ground-state diffuse reflectance absorption spectra revealed the formation of different forms of adsorbed sulforhodamine 101 depending on concentration and on the pore size of the silica. For low loadings (0.001 to about 0.025 μmol of dye/g of silica) the absorption spectra are broad, hypsochromically shifted in relation to the monomer spectra, and quite different from the ethanolic solution spectra. For high loadings (0.050 to about 0.20 μmol g-1) they are similar to the solution spectra with a small shift of about 7 nm. For rhodamine 6G spectra are much more “solution type” in the entire range of concentrations under study (0.001 to about 0.20 μmol g-1). The “weighed” fluorescence quantum yields (∑ifiφFi) determined for sulforhodamine 101 were 0.10 ± 0.03, 0.35 ± 0.05, and 0.50 ± 0.10 for low loadings and for 25, 60, and 150 Å silicas, respectively. For high loadings ∑ifiφFi = 0.70 ± 0.10. These values for ∑ifiφFi can be compared with a value of about 0.70 obtained for rhodamine 6G in all silicas (low loadings) and a unitary φF for high loadings. X-ray photoelectron spectroscopic studies present clear evidence for multiple occupation of the pores for all silicas under study. Surface concentration is quite independent of dye loading, although it changes from silica to silica. These studies indicate that sulforhodamine 101 forms nonplanar conformers in small pore size silicas as compared to large pore silica samples where the amount of conformers being formed is reduced. Rhodamine 6G samples exhibit very little conformer formation, but their φF is still slightly dependent on pore size. Both rhodamines exhibit smaller fluorescence quantum yields when compared to the case of adsorption onto microcrystalline cellulose, this effect being more relevant in the sulforhodamine 101 case.

Preparation, characterization, and optical properties of the host-guest nanocomposite material zeolite-silver iodide

Zeolites NaZSM-5 and NaY as hosts for producing nanoscale silver iodide guest were studied with the goal of investigating the optical properties of the prepared host-guest nanocomposite materials. The host-guest nanocomposite materials of NaY–AgI and (NaZSM-5)–AgI were prepared by a heat diffusion method. The nanocomposite materials prepared were characterized by X-ray diffraction (XRD), differential thermal analysis (DTA), X-ray photoelectric spectroscopy (XPS), and adsorption. The properties of the diffuse reflectance absorption spectra and the surface photovoltage spectra of the prepared nanocomposite materials were investigated. The quantum confinement effect of the channels of zeolites made the prepared host-guest nanocomposite materials show some interesting optical properties. This makes the investigated nanomaterials suitable for use as quantum electronic and optoelectric materials.

Photovoltaic Properties and Quantum Size Effect in Nanocrystalline CeO<sub>2</sub>

Different size. nanocrystallines CeO2 were synthesized and their diffuse reflectance absorption spectra have heen measured. The absorption band in the region from 300 to 450 nm was assigned to the O2p-Ce2 4f transition. It was found that a strongly red-shifted broad continuum of the absorption band occured as the decrease of the partical size. We have also measured the surface photovoltage as function of wavelength by SPS technique. And the absorption band was resolved to two peaks with different photovoltaic properties. Photovoltaic quantum size effect was observed by FMSPS measurement.

Photoluminescence of Sulfide Phosphors with the MB 2 S 4 Composition

Photoluminescence emission spectra and diffuse reflectance absorption spectra were measured on phosphors composed of ternary sulfide hosts with the , calcium ferrite, spinel, and thiogallate structure types each activated by Eu2+, Ce3+, Ho3+, and Mn2+. Fourteen of the sixteen materials gave a luminescence characteristic of the activator ion but only, , and were intense emitters. The emission spectra can be accounted for by localized states of the activator ions in the sulfide hosts.

Hydrothermal crystallization of porphyrin-containing layer silicates

Porphyrins and metalloporphyrins have been demonstrated as organic templates in the hydrothermal crystallization of layered silicate smectite clays. Two water-soluble free-base porphyrins, tetrakis(N-methyl-4-pyridiniumyl)porphyrin (TMPyP) and tetrakis(N,N,N-trimethyl-4-aniliniumyl)porphyrin (TAP), along with metallo derivatives like Fe{sup III}TAP, were utilized as their chloride salts. Aqueous gels consisting of a 0.0166:0.20:1.00:1.52 porphyrin:LiF:Mg(OH){sub 2}:SiO{sub 2} molar ratio heated at reflux temperature crystallized synthetic porphyrin-containing hectorite clays in just 2 days. Results of x-ray powder diffraction indicated that the organic macrocycles are intercalated parallel between the clay layers. Microanalysis data and visible diffuse reflectance (DR) absorption spectra reveal that the porphyrin is incorporated intact. XRD and DR spectra also reveal that the free-base forms of the porphyrins dominate in hydrothermal crystallization products, while in ion-exchanged products the dication forms predominate. Small-angle neutron scattering (SANS) was used to follow the crystallization of these systems and to analyze interactions that exist when porphyrin is reacted with silica sol and magnesium hydroxide alone. Guinier analysis of the scattering curves yields structural information about the growth process of the clay system. Random network structures are observed for the porphyrin-silica system.