Amplifiable Symmetry Breaking in Aggregates of Vibrating Helical Molecules

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.