Mutual Monomer Orientation to Bias the Supramolecular Polymerization of [6]Helicenes and the Resulting Circularly Polarized Light and Spin Filtering Properties

Abstract

We report on the synthesis and self-assembly of 2,15- and 4,13-disubstituted carbo[6]helicenes 1 and 2 bearing 3,4,5-tridodecyloxybenzamide groups. The self-assembly of these [6]helicenes is strongly influenced by the substitution pattern in the helicene core that affects the mutual orientation of the monomeric units in the aggregated form. Thus, the 2,15-substituted derivative 1 undergoes an isodesmic supramolecular polymerization forming globular nanoparticles that maintain circularly polarized light (CPL) with glumvalues as high as 2 × 10-2. Unlike carbo[6]helicene 1, the 4,13-substituted derivative 2 follows a cooperative mechanism generating helical one-dimensional fibers. As a result of this helical organization, [6]helicene 2 exhibits a unique modification in its ECD spectral pattern showing sign inversion at low energy, accompanied by a sign change of the CPL with glumvalues of 1.2 × 10-3, thus unveiling an example of CPL inversion upon supramolecular polymerization. These helical supramolecular structures with high chiroptical activity, when deposited on conductive surfaces, revealed highly efficient electron-spin filtering abilities, with electron spin polarizations up to 80% for 1 and 60% for 2, as measured by magnetic conducting atomic force microscopy.