Amplifiable Symmetry Breaking in Aggregates of Vibrating Helical Molecules.

Abstract

Symmetry breaking in the self-assembly of achiral constituents is of vital importance for the origin of molecular homochirality and developing advanced chiral materials. Here, we report a unique mode of spontaneous symmetry breaking in the aggregates of aza[4]helicenes with an achiral vibrating helical conjugated structure. The achiral molecules initially form clustered aggregates with a slight chiral bias of the P and M isomers, and subsequently the chiral imbalance is amplified by the conversion of the P and M conformations to favor a more thermodynamic stable π-π stacking (from PM to PP or MM stacking). The dynamical P/M transformation not only promotes the evolution of optical activity following the initial spontaneous symmetry breaking but also favors the healing of chirality after the majority is eliminated by heating. Notably, the aggregates reveal prominent circularly polarized luminescence with the absolute dissymmetry factor approaching 0.01. This work provides additional insights into the pathway of chiral symmetry breaking and illustrates a unique route to develop optically active materials from achiral helical molecules.