Using EPR Spectroscopy as a Unique Probe of Molecular-Scale Reorganization and Solvation in Self-Assembled Gel-Phase Materials

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• 作者：Agneta Caragheorgheopol ; William Edwards ; John G. Hardy ; David K. Smith ; Victor Chechik
• 刊名：Langmuir
• 出版年：2014
• 出版时间：August 5, 2014
• 年：2014
• 卷：30
• 期：30
• 页码：9210-9218
• 全文大小：409K
• ISSN：1520-5827

文摘

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鈥搒ol 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鈥攁s 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.