1 research outputs found
Using EPR Spectroscopy as a Unique Probe of Molecular-Scale Reorganization and Solvation in Self-Assembled Gel-Phase Materials
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 poolsî—¸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