Experimentally Mapping the Molecular Pathways and Intermolecular Forces that Drive Elasticity in Flexible Crystals

Elastically flexible molecular crystals are an emerging class of advanced materials, driving innovation in fields ranging from electronics to medicinal applications. At the fundamental level, bulk material elasticity is governed by the nature of the intermolecular forces found in thematerial, whicharenotdirectlyprobedusingstructural(crystallographic)measurements. Low-frequency vibrational spectroscopy, on the other hand, directly interrogates these in- teractions, and is applied here to directly measure strain in the intermolecular coordinate in mechanically-bent crystals of copper (II) acetylacetonate (Cu(acac)2). This represents the first instance of a direct measurement of the modulation of intermolecular forces through bending strain, enabling identification of the particular coordinates involved in bulk elastic phenomena without the need for large-scale instrumentation such as synchrotrons. From such observations, it is possible to derive a thermodynamic picture of the forces that govern elastic crystals, which highlights the delicate interplay between stabilizing and destabilizing interactions within the material that promote the elastic phenomena in these crystals.