Using EPR spectroscopy as a unique probe of molecular-scale reorganization and solvation in self-Assembled gel-phase materials

TY - JOUR

T1 - Using EPR spectroscopy as a unique probe of molecular-scale reorganization and solvation in self-Assembled gel-phase materials

AU - Caragheorgheopol, Agneta

AU - Edwards, William

AU - Hardy, John G.

AU - Smith, David K.

AU - Chechik, Victor

PY - 2014/7/13

Y1 - 2014/7/13

N2 - 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.

AB - 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.

UR - http://www.scopus.com/inward/record.url?scp=84905695277&partnerID=8YFLogxK

U2 - 10.1021/la501641q

DO - 10.1021/la501641q

M3 - Article

AN - SCOPUS:84905695277

SN - 0743-7463

VL - 30

SP - 9210

EP - 9218

JO - Langmuir

JF - Langmuir

IS - 30

ER -