Preparation, characterization and magnetic behavior of a spin-labelled physical hydrogel containing a chiral cyclic nitroxide radical unit fixed inside the gelator molecule

Abstract

An optically active amphiphilic nitroxide radical compound [(S,S,R)-5a], which contains a paramagnetic (2S,5S)-2,5-dimethyl-2,5-diphenylpyrrolidine-N-oxyl radical group fixed in the inner position together with a hydrophobic long alkyl chain and a hydrophilic (R)-alanine residue in the opposite terminal positions, was found to serve as a low-molecular-weight gelator in H₂O to give rise to a spin-labelled physical hydrogel. Characterization of the hydrogel was performed by microscopic (SEM, TEM and AFM) techniques, XRD and SAXS measurements, and IR, UV and CD spectroscopies. The gel–sol transition temperature was determined by EPR spectral line-width (ΔH_pp) analysis. Measurement of the temperature dependence of relative paramagnetic susceptibility (χ_rel) for the hydrogel and sol phases was achieved by means of the double-integration of VT-EPR spectra.