title: Using EPR spectroscopy as a unique probe of molecular-scale reorganization and solvation in self-assembled gel-phase materials.
abstract: We describe the synthesis of spin-labeled bis-ureas which coassemble with bis-urea gelators and report on self-assembly as detected using electron paramagnetic resonance spectroscopy (EPR). Specifically, EPR detects the gel-sol transition and allows us to quantify how much spin-label is immobilized within the gel fibers and how much is present in mobile solvent pools-as controlled by temperature, gelator structure, and thermal history. EPR is also able to report on the initial self-assembly processes below the gelation threshold which are not macroscopically visible and appears to be more sensitive than NMR to intermediate-sized nongelating oligomeric species. By studying dilute solutions of gelator molecules and using either single or double spin-labels, EPR allows quantification of the initial steps of the hierarchical self-assembly process in terms of cooperativity and association constant. Finally, EPR enables us to estimate the degree of gel-fiber solvation by probing the distances between spin-labels. Comparison of experimental data against the predicted distances assuming the nanofibers are only composed of gelator molecules indicates a significant difference, which can be assigned to the presence of a quantifiable number of explicit solvent molecules. In summary, EPR provides unique data and yields powerful insight into how molecular-scale mobility and solvation impact on assembly of supramolecular gels.
authors.affiliation: Ilie Murgulescu Institute of Physical Chemistry, Romanian Academy , 202 Spl. Independentei, Bucharest 060021, Romania.
authors.forename: Agneta
authors.initials: A
authors.lastname: Caragheorgheopol
authors.forename: William
authors.initials: W
authors.lastname: Edwards
authors.forename: John G
authors.initials: JG
authors.lastname: Hardy
authors.forename: David K
authors.initials: DK
authors.lastname: Smith
authors.forename: Victor
authors.initials: V
authors.lastname: Chechik