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Formation of organogels from covalently linked polyethylene glycols and cyclodextrins and their encapsulation/release properties were monitored with EPR spectroscopy using spin labels and spin probes; changes in molecular tumbling rates provided information on spin probe/label location inside the gel network, and molecular properties of the gel.

Electron spin-echo envelope modulation (ESEEM) spectroscopy was used to investigate intramolecular and intermolecular complexes of cyclodextrins (CDs) with a nitroxide group. The interaction with solvent molecules (D(2)O) was followed through the (2)H modulation depth. Competition experiments with adamantane-type guests were used to confirm complexation. The shielding of the nitroxide group from solvent upon inclusion into a CD cavity made this technique more sensitive to complexation than cw EPR spectroscopy. ESEEM analysis of a series of CDs mono and bis spin-labeled on the primary rim of the cavity showed that only one compound formed a self-inclusion complex. This suggests that significant linker length/flexibility is required for formation of inclusion complexes in functionalized CDs. DEER (double electron-electron resonance) experiments confirmed that the self-inclusion complex of the spin-labeled CD was intra- rather than intermolecular.

Formation of inclusion complexes between several cyclodextrin derivatives and TEMPO and DOXYL-based spin probes was studied by EPR spectroscopy. Competition between alkyl chains and nitroxide functionalities for cyclodextrin cavities leads to different types of complexation. Long alkyl chains in amphiphilic spin probes interact preferentially with cyclodextrins, and TEMPO units in such molecules are unaffected by complexation. DOXYL-type spin probes however form stronger complexes with cyclodextrins; this complexation changes hyperfine splitting and tumbling rate of the nitroxide group. Comparison of EPR spectra of free cyclodextrin and cyclodextrin-based polymeric nanocapsules made it possible to assess the tumbling of the spin probe inside the cyclodextrin units without the contribution of the tumbling of the whole complex.

Complexation of beta-cyclodextrin with flexible nitroxide biradicals linked by a polyethylene glycol chain was monitored by EPR spectroscopy. The EPR spectra of the uncomplexed biradicals show an exchange interaction due to the flexibility of the polyethylene glycol chain. Complexation with cyclodextrin leads to the disappearance of the exchange interaction in the EPR spectra. The complexation can be reversed by the addition of competing guests (e.g., adamantane derivatives). At high concentration, the inclusion complexes precipitate, and differential scanning calorimetry (DSC) of the precipitates proved the formation of complexes. Elemental analysis data revealed that the complexes contain several cyclodextrin units per biradical but that the composition was not stoichiometric.

EPR spectra of cyclodextrins labelled with TEMPO derivatives (SL-CDs) are sensitive to complexation with large guest molecules. We used SL-CDs to explore the behaviour of concentrated PEG/PPG solutions. The relationship between rotational correlation times and solvent viscosity showed significant deviations from the Debye-Stokes-Einstein equation, probably due to self-aggregation of alkylene glycols in concentrated solutions. The data fit the fractional Debye-Stokes-Einstein equation well. We have also studied complexation of SL-CDs with adamantane-functionalised DAB dendrimers. The strength of binding increases with dendrimer generation; formation of supramolecular aggregates at high concentrations was observed with the generation 3 dendrimer.

In this work, we explored by means of electron paramagnetic resonance (EPR) spectroscopy the accessibility of a series of spin probes, covering a scale of molecular weights in the range of 200–60,000 Da, in a variety of hydrogels: covalent network, ionotropic, interpenetrating polymer network (IPN) and semi-IPN. The covalent gel network consists of polyethylene or polypropylene chains linked via isocyanate groups with cyclodextrin, and the ionotropic gel is generated by alginate in the presence of Ca2+ ions, whereas semi-IPN and IPN gel networks are generated in a solution of alginate and chitosan by adding crosslinking agents, Ca2+ for alginate and glutaraldehyde for chitosan. It was observed that the size of the diffusing species determines the ability of the gel to uptake them. Low molecular weight compounds can diffuse into the gel, but when the size of the probes increases, the gel cannot uptake them. Spin-labelled Pluronic F127 cannot be encapsulated by any covalent gel, whereas spin-labelled albumin can diffuse in alginate gels and in most of the IPN networks. The EPR spectra also evidenced the specific interactions of spin probes inside hydrogels. The results suggest that EPR spectroscopy can be an alternate method to evaluate the mesh size of gel systems and to provide information on local interactions inside gels. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The linker length influences the response of the probes to environmental polarity. • The linker length determines the probe behaviour in supramolecular systems. • CT excited state in short-chain probes, LE excited state in long-chain probes. • Applications in measuring micropolarity and distinguishing between molecular hosts. Abstract Bis-dansyl fluorescent probes bearing aliphatic diamine linkers of increasing length (2DA 1.n , n = 2, 4, 6, 8, 10, 12) were synthesized, and their emissive characteristics in solvents and in cyclodextrin solutions were discussed. Dual emission from locally excited (LE) and intramolecular charge transfer (CT) excited states was observed. The LE/CT relative magnitude is sensitive to solvent polarity and linker length. In water, the CT or LE state is favoured for short chain or long chain probes, respectively. The LE/CT relative magnitude also depends on the CD type: 2-hydroxypropyl-β-CD favours the CT state, while β- and γ-CD stabilise the LE state. [ABSTRACT FROM AUTHOR]

The present study reports on the changes in dansyl moiety fluorescence due to the interactions with the β-cyclodextrin cavity observed by steady-state fluorescence spectroscopy. First, the interaction of a guest molecule, dansyl-TEMPO (DT), which has in its structure a dansyl group as a fluorescent moiety and a paramagnetic fragment (TEMPO), with β-cyclodextrin was studied. Analysis of the EPR spectral parameters provided additional information on the host–guest interaction, suggesting that the cyclodextrin cavity accommodates dansyl rather than a TEMPO moiety. Second, the affinity between 1-adamantol or adamantane end-capped polyethylene glycols –A2Pn– (as guests) and dansyl-β-cyclodextrin (D-β-CD, as a host) was investigated. In the presence of adamantol, the intensity of fluorescence of D-β-CD decreases and a hypsochromic effect was noticed. The same behaviour was observed in the presence of A2Pn. Using Benesi–Hildebrand and Stern–Volmer equations and also the sensitivity factor made possible to obtain quantitative data on the interaction between the dansyl moiety and the cyclodextrin cavity. The formation of inclusion complexes in the solid state between D-β-CD and A2Pn was evidenced analysing the DSC curves of the pure solids, physical mixture and prepared solid complexes. In addition, the use of D-β-CD for fluorescent labeling of a covalently based network hydrogel and diffusion of dansyl derivatives into a gel are shown as possible further application of this study. [ABSTRACT FROM AUTHOR]

Two isomers of permethylated β-cyclodextrin, labelled with paramagnetic TEMPO moieties attached on the same rim of the cavity, were synthesised and their host–guest properties investigated by EPR spectroscopy. [ABSTRACT FROM AUTHOR]

Molecular probes are often used to describe local interactions and to characterize inhomogeneities in complex systems. In this study, two series of monomolecular probes (paramagnetic or fluorescent) and a series of dual molecular probes are used to evaluate local changes and the regions targeted by these probes in three pluronic F127 systems, in the absence and in the presence of 2‐hydroxypropyl‐β‐cyclodextrin, which have been previously characterized. All molecular probes used in this study have as a common structural unit the short oligoethylene chains that link (2,2,6,6‐Tetramethylpiperidin‐1‐yl)oxyl (TEMPO) and/or pyrene moieties. Using these probes, changes in hydrophobicity inside the pluronic micelles are evidenced and the micelle‐to‐gel phase transition is characterized. Phase transition in F127/2‐hydroxypropyl‐β‐cyclodextrin systems is reported by mono and dual molecular probes bearing fluorescent and/or paramagnetic moieties attached to oligoethylene oxide chains. The probes target different regions in F127 micelles, as reported by the hyperfine coupling constant. The temperature variation of the fluorescence and paramagnetic spectral features is sensitive to the phase transition. [ABSTRACT FROM AUTHOR]

[Display omitted] • Inclusion complexes formed by four β-phosphorylated nitroxides with cyclodextrins were evidenced by EPR spectroscopy. • Binding constants were evaluated from the ratio free/complexed β-phosphorylated nitroxide components in EPR spectra. • EPR parameters of β-phosphorylated nitroxides are highly sensitive to the size of the cyclodextrin cavity. • EPR parameters of β-phosphorylated nitroxides as analytical markers for the composition of cyclodextrin solutions. The isotropic hyperfine coupling constants corresponding to nitrogen and phosphorous atoms of β-phosphorylated nitroxides are sensitive to the polarity sensed by these probes, and to local interactions determining a change in the molecular conformation of the probe. The β-phosphorylated nitroxides selected for this study exhibit very different solvent effects. In this article are presented results regarding the interaction of two cyclic and two non-cyclic β-phosphorylated nitroxides with cyclodextrins of different cavity size. In order to find the minimum concentration of cyclodextrin that determines a change in the EPR spectrum of the β-phosphorylated nitroxides, series of cyclodextrin solutions were prepared. The binding constants were of the 10–102 orders of magnitude. The reversibility of the association process was evidenced by competitive experiments with 1-adamantanol. The sensitivity of β-phosphorylated nitroxides to the interaction with cyclodextrins as a function of cavity size and concentration is translated in EPR spectra with different patterns. Starting from this observation, we exemplified that it is possible to discriminate between solutions containing one cyclodextrin and a mixture of cyclodextrins. As such, β-phosphorylated nitroxides can be used as analytical markers to provide information on the composition of cyclodextrin solutions. [ABSTRACT FROM AUTHOR]