Model for Photoinduced Bending of Slender Molecular Crystals

The growing realization that photoinduced
bending of slender photoreactive
single crystals is surprisingly common has inspired researchers to
control crystal motility for actuation. However, new mechanically
responsive crystals are reported at a greater rate than their quantitative
photophysical characterization; a quantitative identification of measurable
parameters and molecular-scale factors that determine the mechanical
response has yet to be established. Herein, a simple mathematical
description of the quasi-static and time-dependent photoinduced bending
of macroscopic single crystals is provided. This kinetic model goes
beyond the approximate treatment of a bending crystal as a simple
composite bilayer. It includes alternative pathways for excited-state
decay and provides a more accurate description of the bending by accounting
for the spatial gradient in the product/reactant ratio. A new crystal
form (space group P21/n) of the photoresponsive azo-dye Disperse Red 1 (DR1) is analyzed within the constraints of the aforementioned model.
The crystal bending kinetics depends on intrinsic factors (crystal
size) and external factors (excitation time, direction, and intensity).