Impact of Torsional and Conformational Flexibility on Pedal Motion and Thermal Expansion in Pyridyl Bisimine Cocrystals

The impact of intermolecular interactions and molecular motion on solid-state properties is an active field of interest for chemists and materials scientists. For example, cocrystallization has been shown to modify and/or enhance the solid-state behaviors of a molecule when compared to the single-component solid. Here, we describe a series of cocrystals containing bis(pyridin-4-ylmethylene)benzene-1,4-diamine (BPDI) and ditopic or tritopic hydrogen-bond-donor molecules that are conformationally flexible. The components in all the cocrystals self-assemble through hydroxyl-pyridine heterosynthons to afford one-dimensional chains due to the conformations of the donor molecules. BPDI is torsionally flexible, and in cocrystals with ditopic hydrogen-bond donors, the molecule is almost planar, whereas in cocrystals with tritopic hydrogen-bond donors, BPDI is significantly twisted. This twisting in BPDI affects crystal packing and affords higher thermal expansion coefficients. Cocrystallization of BPDI with resveratrol, another torsionally flexible molecule, induces molecular pedal motion in BPDI and results in larger expansion behavior in the cocrystal.