Evidence for Kinetic Nucleation in Helical Nanofiber Formation Directed by Chiral Solvent for a Perylene Bisimide Organogelator

The self-assembly behavior of an achiral perylene bisimide (PBI) organogelator that bears two 3,4,5-tri- dodecyloxybenzoylaminoethyl substitu- ents at the imide positions has been in- vestigated in chiral solvents (R)- and (S)-limonene in great detail by circular dichroism (CD) spectroscopy and atomic force microscopy (AFM). CD spectroscopic studies on dilute solu- tions revealed a preferential population of one-handed helical assemblies in chiral solvent with an enantiomeric excess close to 100 %, whereas AFM images of more than 100 nanofibers of the organogel obtained from more con- centrated solutions were found to con- sist of both handed helices with an enantiomeric excess of only 20 %. This discrepancy is attributed to the fast ge- lation process at high dye concentra- tion that evidently proceeds through non-equilibrated nuclei in a kinetic rather than thermodynamic self-assem- bly process. Under these conditions the chiral induction from the homochiral solvent may not be adequate in effec- tively populating only one-handed heli- ces.