Large Local Internal Stress in an Elastically Bent Molecular Crystal Revealed by Raman Shifts

The structural dynamics involved in the mechanical flexibility of molecular crystals are not well understood yet. Here, we report an elastically bending lipidated molecular crystal that shows systematic shifts in characteristic vibrational frequencies across the bent crystal region - revealing the nature of structural changes during bending and the local internal stress distribution. The elastic flexibility is rendered by intermolecular N-H∙∙∙O hydrogen-bonded chains along with strong yet flexible alkyl-chain hydrophobic interactions in this crystal structure. The blue shifts in the bond stretching modes (such as C=O and C-H modes) in the inner arc region and red shifts in the outer arc region of the bent crystals observed via micro-Raman mapping are counterintuitive to the bending models based on intermolecular hydrogen bonds. Correlating these shifts with the trends observed from high-pressure Raman studies on the crystal reveals the local stress difference between the inner arc and outer arc regions of the bent crystal to be ~2 GPa, more than an order of magnitude higher than the previously proposed value in elastically bending crystals. High local internal stress can have direct ramifications on the properties of molecular piezoelectric energy harvesters, actuators, semiconductors, and flexible optoelectronic materials.