Critical evaluation of the application of direct energy transfer in probing the morphology of porous solids

Stable Aqueous Colloidal Solutions of Nd3+: LaF3 Nanoparticles, Promising for Luminescent Bioimaging in the Near-Infrared Spectral Range

Two series of stable aqueous colloidal solutions of Nd³⁺: LaF₃ single-phase well-crystallized nanoparticles (NPs), possessing a fluorcerite structure with different activator concentrations in each series, were synthesized. A hydrothermal method involving microwave-assisted heating (HTMW) in two Berghof speedwave devices equipped with one magnetron (type I) or two magnetrons (type II) was used. The average sizes of NPs are 15.4 ± 6 nm (type I) and 21 ± 7 nm (type II). Both types of NPs have a size distribution that is well described by a double Gaussian function. The fluorescence kinetics of the ⁴F_3/2 level of the Nd³⁺ ion for NPs of both types, in contrast to a similar bulk crystal, demonstrates a luminescence quenching associated not only with Nd-Nd self-quenching, but also with an additional Nd-OH quenching. A method has been developed for determining the spontaneous radiative lifetime of the excited state of a dopant ion, with the significant contribution of the luminescence quenching caused by the presence of the impurity OH- acceptors located in the bulk of NPs. The relative quantum yield of fluorescence and the fluorescence brightness of an aqueous colloidal solution of type II NPs with an optimal concentration of Nd³⁺ are only 2.5 times lower than those of analogous Nd³⁺: LaF₃ single crystals.

Concentration Dependent Dimensionality of Resonance Energy Transfer in a Postsynthetically Doped Morphologically Homologous Analogue of UiO-67 MOF with a Ruthenium(II) Polypyridyl Complex

A method is described here by which to dope ruthenium(II) bis(2,2'-bipyridine) (2,2'-bipyridyl-5,5'-dicarboxylic acid), RuDCBPY, into a UiO-67 metal-organic framework (MOF) derivative in which 2,2'-bipyridyl-5,5'-dicarboxylic acid, UiO-67-DCBPY, is used in place of 4,4'-biphenyldicarboxylic acid. Emission lifetime measurements of the RuDCBPY triplet metal-to-ligand charge transfer, (3)MLCT, excited state as a function of RuDCBPY doping concentration in UiO-67-DCBPY are discussed in light of previous results for RuDCBPY-UiO-67 doped powders in which quenching of the (3)MLCT was said to be due to dipole-dipole homogeneous resonance energy transfer, RET. The bulk distribution of RuDCBPY centers within MOF crystallites are also estimated with the use of confocal fluorescence microscopy. In the present case, it is assumed that the rate of RET between RuDCBPY centers has an r(-6) separation distance dependence characteristic of Förster RET. The results suggest (1) the dimensionality in which RET occurs is dependent on the RuDCBPY concentration ranging from one-dimensional at very low concentrations up to three-dimensional at high concentration, (2) the occupancy of RuDCBPY within UiO-67-DCBPY is not uniform throughout the crystallites such that RuDCBPY densely populates the outer layers of the MOF at low concentrations, and (3) the average separation distance between RuDCBPY centers is ∼21 Å.

Locations and reorientations of multi-ring-fused 2-pyridones in ganglioside G(M1) micelles

Fluorescent multi-ring-fused 2-pyridones, with chemical resemblance to other biologically active 2-pyridone systems, were solubilized in spherical micelles formed by the ganglioside G(M1) and studied with respect to their spatial localization and rotational mobility. For this, electronic energy transfer between the multi-ring-fused 2-pyridone (donor) and BODIPY-FL-labeled G(M1) was determined, as well as their fluorescence depolarization. From the obtained efficiency of energy transfer to the acceptor group (BODIPY-FL), either localized in the polar or in the nonpolar part of the ganglioside, it has been possible to estimate the most likely localization of the multi-ring-fused 2-pyridones. The center of mass of the studied multi-ring-fused 2-pyridones are located at approximately 33 Å from the micellar center of mass, which corresponds to the internal hydrophobic-hydrophilic interfacial region. At this location, the reorienting rates of the multi-ring-fused 2-pyridones are surprisingly slow with typical correlation times of 35-55 ns. No evidence was found for the formation of ground and excited state dimers, even when two monomers were forced to be near each other via a short covalent linker.

Multiply doped nanostructured silicate sol-gel thin films: spatial segregation of dopants, energy transfer, and distance measurements

Physical and chemical strategies that place designed molecules in spatially separated regions of surfactant-templated mesostructured silicate thin films are used to prepare films containing rhodamine 6G (R6G), lanthanide complexes, and both simultaneously. Fluorescence and photoexcitation spectra of R6G in amorphous and structured thin films show that it is located inside the surfactant micelles of structured thin films. A silylated ligand that binds lanthanides condenses to form part of the silica framework and causes the lanthanide to localize in the silica. Luminescence and photoexcitation spectra show that energy transfer from the metal complex to R6G occurs in the films. R6G quenches Tb emission in a concentration-dependent manner. Energy transfer efficiency is calculated using the Tb luminescence lifetime, and this quantity is used to calculate the distance between Tb and R6G with the aid of Forster theory.

Hydrophobically Modified Amphiphilic Block Copolymer Micelles in Non-Aqueous Polar Solvents. Fluorometric, Light Scattering and Computer-Based Monte Carlo Study

The micellization behavior of a hydrophobically modified polystyrene-block-poly(methacrylic acid) diblock copolymer, PS-N-PMA-A, tagged with naphthalene between blocks and with anthracene at the end of the PMA block, was studied in 1,4-dioxane-methanol mixtures by light scattering and fluorescence techniques. The behavior of a single-tagged sample, PS-N-PMA, and low-molar-mass analogues was studied for comparison. Methanol-rich mixtures with 1,4-dioxane are strong selective precipitants for PS. Multimolecular micelles with compact PS cores and PMA shells may be prepared indirectly by dialysis from 1,4-dioxane-rich mixtures, or by a slow titration of copolymer solutions in 1,4-dioxane-rich solvents with methanol under vigorous stirring. In tagged micelles, the naphthalene tag is trapped in nonpolar and fairly viscous core/shell interfacial region. In hydrophobically modified PS-N-PMA-A micelles, the hydrophobic anthracene at the ends of PMA blocks tends to avoid the bulk polar solvent and buries in the shell. The distribution of anthracene tags in the shell is a result of the enthalpy-to-entropy interplay. The measurements of direct nonradiative excitation energy transfer were performed to estimate the distribution of anthracene-tagged PMA ends in the shell. The experimental fluorometric data show that anthracene tags penetrate into the inner shell in methanol-rich solvents. Monte Carlo simulations were performed on model systems to get reference data for analysis of time-resolved fluorescence decay curves. A comparison of experimental and simulated decays indicates that hydrophobic traps return significantly deep into the shell (although not as deep as in aqueous media). The combined light scattering, fluorometric and computer simulation study shows that the conformational behavior of shell-forming PMA blocks in non-aqueous media is less affected by the presence of nonpolar traps than that in aqueous media.

Kinetics of diffusion-assisted reactions in microheterogeneous systems

This review is focused on the basic theory of diffusion-assisted reactions in microheterogeneous systems, from porous solids to self-organized colloids and biomolecules. Rich kinetic behaviors observed experimentally are explained in a unified fashion using simple concepts of competing distance and time scales of the reaction and the embedding structure. We mainly consider pseudo-first-order reactions, such as luminescence quenching, described by the Smoluchowski type of equation for the reactant pair distribution function with a sink term defined by the reaction mechanism. Microheterogeneity can affect the microscopic rate constant. It also enters the evolution equation through various spatial constraints leading to complicated boundary conditions and, possibly, to the reduction of dimensionality of the diffusion space. The reaction coordinate and diffusive motion along this coordinate are understood in a general way, depending on the problem at hand. Thus, the evolution operator can describe translational and rotational diffusion of molecules in a usual sense, it can be a discrete random walk operator when dealing with hopping of adsorbates in solids, or it can correspond to conformational fluctuations in proteins. Mathematical formulation is universal but physical consequences can be different. Understanding the principal features of reaction kinetics in microheterogeneous systems enables one to extract important structural and dynamical information about the host environments by analyzing suitably designed experiments, it helps building effective strategies for computer simulations, and ultimately opens possibilities for designing systems with controllable reactivity properties.

Various aspects of the constraints imposed on the photochemistry of systems in porous silica

This manuscript briefly reviews the photochemistry of organic molecules on porous silica (or SiO2). To gain an understanding of the chemistry on silica, data are displayed and discussed with respect to studies in homogeneous solution. In particular, the exact dimensionality of kinetic processes on porous SiO2 is a matter for debate. Hence, units of concentration of an adsorbate on the surface are expressed as moles per nanometer squared and as moles per liter, in order to compare with solution. Many studies show that organic molecules adsorb to SiO2 via the surface silanol (or surface hydroxyl OH) groups. The adsorption is heterogeneous, due to various clusters of silanol groups and to charge transfer (CT) sites. Photophysical studies clearly show these effects. The photo-induced reactions on SiO2 may be described by 'fractal' approaches, but a 'Gaussian' approach is often more useful to the photochemist. Photo-induced reactions occur via movement of the reactants on the surface, as in the case of the Langmuir-Hinshelwood (LH) mechanism or, as in the case of the Eley-Rideal (ER) mechanism, by bombardment of a surface bound excited state by a gaseous reactant, such as O2. Quenching of excited singlet states by O2 produces excited triplet states, which in turn are quenched to give singlet molecular oxygen. At room temperature the O2 quenching process on silica occurs by both mechanisms to approximately the same extent. However, the LH mechanism is dominant at lower temperatures and the ER mechanism is dominant at higher temperatures. Some quenchers, including carbon tetrachloride and tetranitromethane only quench by the LH mechanism giving rise to static quenching and chloro or nitro derivatives of the excited state. Photo-induced electron transfer between excited arenes and amines occurs readily, but the ionic products are short-lived compared to solution. This is due to the limited diffusion of the products on the surface, which in turn promotes back-electron transfer. Photoionization of arenes occurs on SiO2 via a two-photon process and gives very long-lived ions compared to solution. This is due to trapping of the photo-produced electrons by the SiO2 itself. Finally, the effects of co-adsorbants, including solvents, surfactants, and polymers, in photoreactions at the SiO2 surface are considered. The review ends with suggestions for future studies.