Anomalous Piezochromic Luminescence in Covalent Triazine Frameworks via Molecular Insertion: Blueshifted and Enhanced Emission.

Piezochromic materials typically exhibit pressure-induced redshifted and quenched emission due to enhanced intermolecular π-π stacking and molecular planarization. Consequently, achieving blueshifted and enhanced emission in π-conjugated systems remains a significant challenge. Here, we report anomalous piezochromic luminescence in covalent triazine frameworks (CTFs) via molecular insertion. Upon introducing methanol into the nanopores of CTFs, a blueshift in emission from 507.0 to 485.5 nm, accompanied by enhanced intensity, is observed under compression up to 1.22 GPa, distinctly contrasting the redshifted and quenched emission typically observed in compressed pristine CTFs and other crystalline porous materials (CPMs). Combined experimental and theoretical analyses reveal that methanol can weaken the interlayer π-π stacking and intralayer conjugation of CTFs by forming weak interactions with CTFs, such as hydrogen bonding, to realize the interlayer slip and intralayer distortions of CTFs, which results in the blueshifted and enhanced emission. This strategy also proves effective with other molecular insertions, offering a general approach to achieving anomalous piezochromic luminescence in CTFs. Our findings establish molecular insertion as a robust method for engineering pressure-responsive luminescent materials and provide valuable insights for the design of advanced optical sensors and stimuli-responsive systems.