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 reveals 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. An irreversible chemical side reaction in these crystals results in plastic re...