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