Influence of the Solvent and the Enantiomeric Purity on the Transition between Different Supramolecular Polymers

The self-assembly of two enantiomerically pure hexaA (p-phenylene vinylene))-substituted benzenes having 24 stereocenters was studied in pure methylcyclohexane (MCH) and in a mixture of MCH/tol- uene (4:1). Irrespective of the solvent a cooperative supramolecular polymeri- zation mechanism was determined for these star-shaped molecules by using temperature-dependent CD and UV/ Vis spectroscopy. Quite remarkably, a transition from one helical supramolec- ular state (A) to a second more ther- modynamically stable supramolecular helical assembly (B) was observed. The rate of the A!B transition was strong- ly dependent on the nature of the sol- vent; being faster in the solvent mix- ture than in pure MCH. By using size exclusion chromatography we could relate the increased rate to a decreased stability of the supramolecular A state in the solvent mixture. Next, we mixed the two enantiomerically pure hexa- substituted benzene derivatives in a so- called majority-rules experiment, which lead to the anitcipated chiral amplifica- tion in the A state. More importantly it appeared that the A!B transition was significantly hampered in these mixed systems. Furthermore, the absence of chiral amplification in the B state re- vealed the formation of separated enantiomerically pure assemblies. Therefore, by using a wide variety of spectroscopic and chromatographic techniques we determined the influ- ence of solvent and enantiomeric purity on the transition between differ- ent supramolecular states.