Protonation induced symmetry-breaking and fluorescence switching in C3-symmetric self-assemblies with application in optical information encryption

Achieving simultaneous symmetry breaking and fluorescence switching in self-assembled systems through external stimuli remains challenging. In this work, we designed two achiral C3-symmetric molecules (BTCNPh and BTCNPy) containing three cyanostilbene units connected through amide bonds, terminated with phenyl or pyridyl groups respectively. Both molecules exhibited cyan fluorescence in DMSO/H₂O mixtures without circular dichroism (CD) signals. Upon HCl addition, the pyridyl groups in BTCNPy underwent protonation-triggered transformation to orange fluorescence accompanied by intense but randomly oriented CD signals. DFT calculations revealed that protonation-induced energy gap narrowing caused fluorescence red-shift, while enhanced intermolecular repulsion promoted staggered stacking for supramolecular chirality emergence. The controlled generation of chiral assemblies with stimuli-responsive fluorescence was successfully applied to information encryption. This study demonstrates a feasible strategy for manipulating symmetry-breaking and optical properties through external stimuli, providing some insights into the origin of supramolecular chirality.