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.