Dynamic pedal motion and thermal expansion in an ionic co-crystal with 1,2-bis(4-pyridyl) ethylene.

An ionic co-crystal material (Bpe-BPh4) composed of 1,2-bis(4-pyridyl) ethylene (Bpe) molecules and sodium tetraphenylborate (NaBPh) is synthesized and isolated. Single crystal X-ray diffraction reveals that the molecular structure of the co-crystal contains a protonated HBpe cation and a [BPh] anion. Interestingly, variable-temperature single crystal X-ray experiments show that the material exhibits temperature-dependent pedal motion with dynamic disorder of the Bpe moiety, leading to thermal expansion (TE) behaviour within the Bpe-BPh molecules, which confers responsiveness to thermal stimuli. A Thermal Expansion Visualizing (TEV) method was employed to further investigate the TE process. This discovery provides valuable guidance for designing advanced materials with tunable thermal properties.