Impact of Molecular Flexibility on Binding Strength and Self-Sorting of Chiral π-Surfaces

In this work, we have explored for the first time the influence of conformational flexibility of pi-core on chiral self-sorting properties of perylene bisimides (PBIs) that are currently one of the most prominent classes of functional dyes. For this purpose, two series of chiral macrocyclic PBIs 3a-c and 4a-c comprising oligoethylene glycol bridges of different lengths at the 1,7 bay positions were synthesized and their atropo-enantiomers (P and M enantiomers) were resolved. Single crystal analysis of atropo-enantiomerically pure (P)-3a not only confirmed the structural integrity of the ethylene glycol bridged macrocycle but also illustrated the formation of pi-stacked dimers with left-handed supramolecular helicity. Our detailed studies with the series of highly soluble chiral PBIs 4a-c by 1- and 2-D H-1 NMR techniques, and temperature- and concentration-dependent UV/vis absorption and circular dichroism (CD) spectroscopy revealed that in pi-pi stacking dimerization of these PBIs chiral self-recognition (i.e., PP and MM homodimer formation) prevails over self-discrimination (i.e., PM heterodimer formation). Our studies clearly showed that with increasing conformational flexibility of PBI cores imparted by longer bridging units, the binding strength for the dimerization process increases, however, the efficiency for chiral self-recognition decreases. These results are rationalized in terms of an induced-fit mechanism facilitating more planarized pi-scaffolds of PBIs containing longer bridging units upon pi-pi-stacking.