Hansen Solubility Parameters Clarify the Role of the Primary and Secondary Hydroxyl Groups on the Remarkable Self-Assembly of 1:3,2:4-Dibenzylidene Sorbitol

Experimental results are reported to explain important aspects of why the classic gelator 1:3,2:4-dibenzylidene sorbitol (DBS) and five of its derivatives do or do not form gels in 23 solvents covering a wide range of properties. The conclusions from those results are based principally on changes in Hansen solubility parameters (HSPs plotted in Hansen space) and the morphologies of the self-assembled-fibrillar networks (SAFiNs). Factors in addition to H-bonding by its primary or secondary hydroxyl group are at play: although each hydroxyl group aids gelation, removal of either alters gelation ability and affects the location of dichroic absorbance differences parallel and perpendicular to the axes of fibers constituting the SAFiNs. Solvent-specific insights provide intriguing results. For 5-deoxy-DBS, for example, in toluene, removal of the secondary hydroxyl produced macroscale, spherulitic objects (comprised of highly branched nanofibers); removal of the primary hydroxyl group led to fibers that are ∼100 μm long and ∼10 μm wide. Insights into DBS self-assembly are found in quadrant analyses of the polar and hydrogen-bonding Hansen solubility parameters, which provide evidence that both the 5-OH and 6-OH groups are essential for the unique gelation ability of DBS (as defined by the broad spectrum of solvents that it gelates). Coupling the HSPs and modifications of the individual molecular synthons represents a straightforward methodology to understand the mechanisms of self-assembly. Also, measurements have been made using polarization modulation infrared linear dichroism microscopy (μPM-IRLD) to determine the anisotropy of IR absorbances at several wavelengths, including most notably those within the regions of hydroxyl, C–C, and CC stretches. The simultaneous measurement of the absorbances at two orthogonal polarization directions, A(0°) and A(90°), at the same fiber location, provides information about the orientations of the functional groups of each of the DBS derivatives relative to the fiber axes.