Mechanically Responsive Crystals: Analysis of Macroscopic Strain Reveals "hidden" Processes

Mechanical response of single crystals to light, temperature, and/or force - an emerging platform for the development of new organic actuating materials for soft robotics - has recently been quantitatively described by a general and robust mathematical model (Chem. Rev. 2015, 115, 12440-12490). The model can be used to extract accurate activation energies and kinetics of solid-state chemical reactions simply by tracking the time-dependent bending of the crystal. Here we illustrate that deviations of the macroscopic strain in the crystal from that predicted by the model reveal the existence of additional, "hidden" chemical or physical processes, such as sustained structural relaxation between the chemical transformation and the resulting macroscopic deformation of the crystal. This is illustrated with photobendable single crystals of 4-hydroxy-2-(2-pyridinylmethylene)hydrazide, a photochemical switch that undergoes E-to-Z isomerization. The irreversible isomerization in these crystals results in amorphization and plastic deformation that are observed as poor correlation between the transformation extent and the induced strains. The occurrence of these processes was independently confirmed by X-ray diffraction and differential scanning calorimetry. An extended mathematical model is proposed to account for this complex mechanical response.