Deciphering the Stimuli-Responsive Behavior of TPPE-Incorporated Flexible Metal-Organic Frameworks

Abstract

Aggregation-induced emission luminogens (AIEgens) hold great promise for the design of highly sensitive and stimulus-responsive luminescent metal-organic frameworks (LMOFs). However, the underlying response mechanisms, particularly the evolution of energy state and conformation of AIE linkers under external stimuli, remain insufficiently understood. Here, we investigate a three-dimensional MOF, [Cd2(D-Cam)2(TPPE)]n (D-MOF) and demonstrate that conformational locking of the flexible tetraphenyl-pyridylethylene (TPPE) ligand enables the retention of high-pressure-induced green fluorescence at ambient conditions. Upon compression, structural contraction and conformational rearrangement of TPPE enhance π-conjugation, leading to a remarkable 140 nm pressure-induced fluorescence red-shift from cyan blue (452 nm) to yellow-green (592 nm), accompanied by a transition in emission intensity from strong to nearly quenched. Single-crystal X-ray diffraction (SCXRD) analysis further confirmed that the conformational change arose from variations in the torsion angles of the phenyl and pyridyl rings of TPPE, resulting in diminished molecular symmetry. This work establishes an effective strategy to “harvest” high-pressure emission states through conformational locking, offering guidance for the rational development of luminescent materials with tunable photophysical properties.