The largest CPL enhancement by further assembly of self-assembled superhelices based on the helical TPE macrocycle

Abstract

Helical self-assemblies from organic luminophores generally display significant enhancement of circularly-polarized luminescence (CPL) activity, but their CPL dissymmetry factor (g_lum) is still small. Whether further assembly of helical self-assemblies can further boost the CPL g_lum value is not known. In this paper, helical tetraphenylethylene (TPE) tetracycle tetramine enantiomers were synthesized and showed CPL activity with g_lum at the 10⁻³ level. After the helical enantiomer self-assembled into superhelices together with 4-dodecylbenzenesulfonic acid, the CPL signal was enhanced by more than 60-fold both in solution and in the solid state. Unprecedentedly, the superhelices could further self-assemble together with one enantiomer of tartaric acid (TA) to become longer and wider bundles, which showed a CPL |g_lum| of up to 0.61 and gave the largest CPL enhancement (more than 200-fold) among CPL active materials. But another TA enantiomer aroused no CPL change, showing great potential for chiral recognition of conventional chiral molecules through the CPL spectrum. Moreover, due to the phenyl tilt of the TPE unit, the helical sense of the superhelices is opposite to that of the helical monomer, which was confirmed by theoretical calculations. Meanwhile, thanks to the AIE character of the TPE unit and complete immobilization of its propeller-like conformation, the helical nanofibers displayed a Φ_f of 95%. This finding demonstrates that the CPL dissymmetry factor can be consecutively magnified by formation of superhelices and further assembly of helical self-assemblies, providing a new method for preparing highly active CPL materials.