1H NMR investigation of the composition, structure, and dynamics of cholesterol-stilbene tethered dyad organogels

The self-assembly of stilbene-cholesterol tethered dyads into fibrous networked structures induces the thermoreversible gelation of several classes of organic solvents. Herein, the gel and solution phases of the gelator, p-alpha-cholesteryl-p'-octoxystilbenoate (1), are investigated in 1-butanol-d(10) (gel forming) and cyclohexane-d(12) (nongel forming) by variable-temperature H-1 NMR spectra, T-1, and T-2 relaxation measurements. All of the NMR data are consistent with detection of 1 in the gel phase by a fast exchange on the NMR time scale between residual 1 dissolved in the gel liquid and 1 in a "mobile" (NMR detectable) gel solid. This conclusion is significant since in previous NMR studies on other gel systems detection of gelator in the gel phase was considered only within "mobile" regions of the gel solid. The decrease in peak integrals of 1 in the gel phase by declining temperatures reflects a convolution of (1) a decrease in concentration of 1 dissolved within the gel liquid as it condenses onto the gel solid strands and (2) a transition in the gel solid from "mobile" to more "rigid" (NMR silent) regions. The relaxation measurements provide evidence for the existence of a sol aggregate of 1 in 1-butanol-d(10) observed above the temperature required for gel formation, T-g. This aggregate is likely the incipient precursor structure to the gel, but its aggregate length is not yet long enough to nucleate the growth of the interpenetrating solid network required for solvent gelation. The absence of regioselectivity in the temperature dependence of the 1H relaxation rate behavior of 1 along the gel --> sol --> solution trajectory implies involvement of both the stilbene and cholesterol substructures of 1 in forming the sol and gel aggregates. However, one caveat is that, in the gel phase, the internal phenyl ring of the stilbene moiety of 1 may have greater mobility than in other regions of 1.