Time Resolved CW-EPR Spectroscopy of Powdered Samples: Electron Spin Polarization of a Nitroxyl Radical Adsorbed on NaY Zeolite, Generated by the Quenching of Excited Triplet Ketones

Chemically induced dynamic electron polarization (CIDEP) generated in a faujasite zeolite (NaY) by the interaction between a stable free radical (4-oxo-TEMPO) and the triplet state of benzophenone was investigated by time-resolved electron spin resonance spectroscopy (TR-CW-EPR). The TR-CW-EPRs were performed by either pulling a long tube containing powdered zeolite through the EPR cavity during the laser irradiation, or by flowing a liquid transport medium (polydimethylsiloxane) for the zeolite powder, through a flat cell in the EPR cavity. CIDEP was observed for intermolecular triplet quenching (benzophenone triplets with 4-oxo-TEMPO) and intramolecular triplet quenching using a covalently linked TEMPO-benzophenone molecule. The identification of the polarized nitroxide structure was confirmed by employing both 14N and 15N 4-oxo-TEMPO isotopomers. The kinetics of the triplet quenching inside the zeolite were studied by diffuse reflection laser flash photolysis

Controlling the Extent of Spin Exchange Coupling in 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) Biradicals via Molecular Recognition with Cucurbit[<i>n</i>]uril Hosts

The binding interactions between two paramagnetic cobaltocenium guests and the hosts cucurbit[7]uril (CB7) and cucurbit[8]uril (CB8) were investigated using a combination of electronic absorption, NMR, and electron paramagnetic resonance (EPR) spectroscopies, mass spectrometry, and X-ray crystallography. Guest 1, (4-amido-2,2,6,6-tetramethylpiperidine-1-oxyl)cobaltocenium, forms very stable inclusion complexes with CB7 and CB8. However, CB7 interacts with 1 by including the organometallic cobaltocenium unit, while CB8 engulfs the TEMPO residue. The corresponding equilibrium association constant (K) values are 2.8 ± 0.3 × 106 M−1 for CB7•1 and 2.1 ± 1.0 × 108 M−1 for CB8•1. Biradical guest 2, 1,1′-bis(4-amido-2,2,6,6-tetramethylpiperidine-1-oxyl)cobaltocenium, forms a very stable ternary complex with two CB8 hosts, in which each 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) residue is encircled by a host molecule. The structure of this ternary complex was confirmed in the solid state using single-crystal X-ray diffraction. Binding of the TEMPO side arms by the CB8 hosts gradually decreases the observed level of spin exchange coupling between the two nitroxide groups. In the final 2:1 complex, no spin exchange coupling was observed, but the initial levels of spin exchange coupling could be regenerated in a reversible fashion by adding a competing guest, adamantyltrimethylammonium (AdTMA), to the solution. The binding interactions between 2 and CB7 are similar but the stabilities of the 1:1 and 2:1 complexes are much lower than those of the corresponding CB8 complexes

Hosting Ability of Mesoporous Micelle-Templated Silicas toward Organic Molecules of Different Polarity

Spin probe water solutions were adsorbed onto differently treated micelle-templated silicas (MTS) of different pore sizes to analyze the hosting ability of the MTS surface toward different organic molecules. The MTS synthesis was performed at 388 K by self-assembly of inorganic silica and micelles of cetyltrimethylammonium bromide (CTAB) to which different amounts of 1,3,5 trimethylbenzene (TMB) were added at different TMB/CTAB ratios to modify the pore size:  40, 65, and 80 Å pore diameter were obtained for TMB/CTAB ratio = 0, 2.7, and 13, respectively. As-synthesized MTS, calcined MTS, and octyldimethyl(C8) grafted MTS were used. These MTS were characterized by means of nitrogen sorption isotherms and TEM as homoporous silica with regular and reproducible structure. Different spin probes (nitroxides) were taken as models for different types of organic molecules, namely, neutral and charged molecules and surfactants. The computer aided analysis of the electron paramagnetic resonance (EPR) spectra of these probes provided information on the hosting ability of the differently treated solid surface in respect of the different structure and hydrophilicity of the probes. The spectral analysis allowed the depiction of the probable distribution and location of the different probes at the differently treated silica surfaces. For the as-synthesized MTS, void space became available for the probe adsorption in vicinity of the surface when TMB was used in the synthesis and then evaporated. For the calcined MTSs, the hydrophobic sites at the solid surface, namely, siloxanes, increased by increasing the TMB content in the synthesis mixture. The binding of the EPR probe with the surface of these MTS is favored when both hydrophilic and hydrophobic interactions occur, as found with surfactant probes bearing both a hydrophilic and a hydrophobic moiety. For the C8-grafted MTSs, the results provided a proof of the quality of grafting:  the surface is largely hydrophobic and favors self-aggregation of the surfactant probes, led by chain−chain interactions

Characterization of Starburst Dendrimers by EPR. 3. Aggregational Processes of a Positively Charged Nitroxide Surfactant

The aggregation characteristics of aqueous solutions of a positively charged nitroxide surfactant (CAT16) in the presence and absence of half-generation polyamidoamine starburst dendrimers (n.5-SBDs) have been investigated by electron paramagnetic resonance (EPR). Computer simulation of the EPR spectra allowed the convenient extraction of several parameters that were related to the supramolecular structure of the aggregates formed by CAT16 and SBDs. From examination of the EPR spectra as a function of variation of the concentration of CAT16, the concentration of SBDs, and the ionic strength and application of the EPR parameters available from simulation of the spectra, a paradigm for the structure and dynamics of the aggregates formed by CAT16 in the presence and absence of SBDs under various conditions is deduced. A study of the fluorescence quenching of pyrene in the presence of CAT16 is compared to a previous investigation of the interaction of SBDs with cationic surfactants. It is concluded that at low SBD concentration, for earlier generation SBDs (G < 3.5), whose size is smaller than or comparable to the size of the CAT16 micellar aggregates, the SBDs act as “guests” that bind to the micelles that serve as “hosts”. In contrast, at low SBD concentration of the later generation SBD (G > 3.5), the size of the SBD is now larger than that of the micelles so that the latter can serve as “guests” for the former. A bilayer aggregate of the surfactant on the SBD is proposed. Finally, at high concentration of the later generation SBD, it is proposed that because of the large number of sites compared to the number of surfactants, an aggregate in which two or more SBDs are bridged by bilayers is formed by the surfactant and coexists with CAT16 micelles

Characterization of the External Surface of Silicalites Employing Electron Paramagnetic Resonance^†^,^‖

Sensitive and structurally specific methods for investigating silicalite external surface have been developed using electron paramagnetic resonance (EPR). The absorption of an EPR silent probe ortho-methyldibenzyl ketone (oMeDBK) (4 in Scheme ) on the external surface of a series of monodisperse silicalite crystals was studied using an initially coadsorbed EPR active nitroxide probe. The displacement of the initially adsorbed nitroxide probe by coadsorbed oMeDBK shows that the adsorbate molecules first adsorb on stronger binding sites characterized by slow rotational motion of the probe (broad EPR lines) and after the stronger sites are saturated, the displaced EPR probe molecules adsorb on weaker binding sites characterized by fast rotational motion of the probe (narrow EPR lines). The transition point from slow to fast rotational motion provides a quantitative measurement of the stronger binding sites on the silicalite external surface area and the external surface area of silicalite crystals. The adsorption strength is molecular structure-dependent, and polar functional groups provide significant contribution to the binding strength. Sequential adsorption of 14N and 15N spin-labeled nitroxides shows the presence of the dynamic exchange between the adsorbates on the strong binding sites and those in solution or on the weak binding sites, while concurrent coadsorption of 14N and 15N spin-labeled nitroxides provides another sensitive means of studying the molecular structural dependence of the binding strength

Interaction between Encapsulated Excited Organic Molecules and Free Nitroxides: Communication Across a Molecular Wall

Communication between two molecules, one confined and excited (triplet or singlet) and one free and paramagnetic, has been explored through quenching of fluorescence and/or phosphorescence by nitroxides as paramagnetic radical species. Quenching of excited states by nitroxides has been investigated in solution, and the mechanism is speculated to involve charge transfer and/or exchange processes, both of which require close orbital interaction between excited molecule and quencher. We show in this report that such a quenching, which involves electron–electron spin communication, can occur even when there is a molecular wall between the two. The excited state molecule is confined within an organic capsule made up of two molecules of a deep cavity cavitand, octa acid, that exists in the anionic form in basic aqueous solution. The nitroxide is kept free in aqueous solution. 1H NMR and EPR experiments were carried out to ascertain the location of the two molecules. The distance between the excited molecule and the paramagnetic quencher was manipulated by the use of cationic, anionic, and neutral nitroxide and also by selectively including the cationic nitroxide within the cavity of cucurbituril. Results presented here highlight the role of the lifetime of the encounter complex in electron–electron spin communication when the direct orbital overlap between the two molecules is prevented by the intermediary wall

HIV-TAT Enhances the Transdermal Delivery of NSAID Drugs from Liquid Crystalline Mesophases

Sodium diclofenac (Na-DFC) and celecoxib (CLXB) are common nonsteroidal anti-inflammatory (NSAID) drugs which suffer from poor bioavailability and severe side effects when consumed orally, and their transdermal delivery might present important advantages. In this study, the drugs were solubilized in cubic and lamellar mesophases as transdermal delivery vehicles, and a cell-penetrating peptide, HIV-TAT (TAT), was examined as a skin penetration enhancer. SD-NMR, ATR-FTIR, and EPR measurements revealed that, in the cubic mesophase (which is rich in water content), TAT populates the aqueous cores and binds water, while in the dense lamellar system (with the lower water content) TAT is bound also to the glycerol monooleate (GMO) and increases the microviscosity and the order degree. TAT secondary structure in the cubic system was found to be a random coil while once it was embedded in the closely packed lamellar system it transforms to a more ordered compact state of β-turns arranged around the GMO headgroups. TAT remarkably increased the diffusion of Na-DFC and CLXB from the cubic systems by 6- and 9-fold enhancement, respectively. TAT effect on drug diffusion from the lamellar systems was limited to an increase of 1.3- and 1.7-fold, respectively. The dense packing and strong binding in the lamellar phase led to slow diffusion rates and slower drug release in controlled pattern. These effects of the chemical composition and vehicle geometry on drug diffusion are demonstrated with the impacts of TAT which can be specifically utilized for controlling skin delivery of drugs as required

Sponge Mesoporous Silica Formation Using Disordered Phospholipid Bilayers as Template

Lecithin/dodecylamine/lactose mixtures in ethanol/aqueous media led to the formation of sponge mesoporous silica (SMS) materials by means of tetraethoxysilane (TEOS) as silica source. SMS materials show a “sponge-mesoporous” porosity with a pore diameter of about 5−6 nm, in accordance to the length of a lecithin bilayer. SMS synthesis was developed to create a new class of powerful biocatalysts able to efficiently encapsulate enzymes by adding a porosity control to the classical sol−gel synthesis and by using phospholipids and lactose as protecting agents for the enzymes. In the present study, the formation of SMS was investigated by using electron paramagnetic resonance (EPR) probes inserted inside phospholipid bilayers. The influence of progressive addition of each component (ethanol, dodecylamine, lactose, TEOS) on phospholipid bilayers was first examined; then, the time evolution of EPR spectra during SMS synthesis was studied. Parameters informative of mobility, structure, order, and polarity around the probes were extracted by computer analysis of the EPR line shape. The results were discussed on the basis of solids characterization by X-ray diffraction, nitrogen isotherm, transmission electron microscopy, and scanning electron microscopy. The results, together with the well-known ability of ethanol to promote membrane hemifusion, suggested that the templating structure is a bicontinuous phospholipid bilayer phase, shaped as a gyroid, resulting of multiple membrane hemifusions induced by the high alcohol content used in SMS synthesis. SMS synthesis was compared to hexagonal mesoporous silica (HMS) synthesis accomplished by adding TEOS to a dodecylamine/EtOH/water mixture. EPR evidenced the difference between HMS and SMS synthesis; the latter uses an already organized but slowly growing mesophase of phospholipids, never observed before, whereas the former shows a progressive elongation of micelles into wormlike structures. SMS-type materials represent a new class of biocompatible materials and open a bright perspective for biomolecule processing for pharmaceutical, biocatalysis, biosensors, or biofuel cell applications

Reversible Oxygenation of a Diphenylmethyl Radical Rendered Supramolecularly Persistent

Reversible Oxygenation of a Diphenylmethyl Radical Rendered Supramolecularly Persisten

Photochemistry of 4-Chlorophenol and 4-Chloroanisole Adsorbed on MFI Zeolites: Supramolecular Control of Chemoselectivity and Reactive Intermediate Dynamics

The phototransformation of aryl chlorides adsorbed on MFI zeolites is markedly different from that observed in solution and other solid surfaces such as silica. The formation of radical cations and the constraints imposed by the channels shift the reactivity from the C−Cl bond to the O−R bond. Irradiation generates kinetically stabilized intermediates that can be characterized using conventional steady-state spectroscopic techniques, and these intermediates can be used as ordinary chemical reagents