Dependence of the solid-state photomechanical response of 4-chlorocinnamic acid on crystal shape and size

The photochemical dynamics of crystals composed of 4-chlorocinnamic acid (4Cl-CA), whose photochemistry is dominated by an irreversible [2+2] photodimerization reaction, are studied using super(13)C solid-state NMR, powder X-ray diffraction, and optical and electron microscopy. We find that photoreaction leads to a new crystal phase, but prolonged irradiation leads to an amorphous solid. To investigate effect of crystal morphology on the photoresponsive behavior, molecular crystals with different shapes and sizes are prepared and compared under the same irradiation conditions. Microribbons with submicron thicknesses twist under irradiation, but no response is observed in larger crystals with thicknesses of 5-10 microns. Possible mechanisms to explain these differences are discussed, including differences in defect densities, optical properties, and heat dissipation. We posit that the dominant effect is the dependence of the torsion constant on crystal thickness, which leads to the thinner microribbons being more susceptible to deformation by an internal energy density. This work shows that photo-induced twisting can be observed in photoreactive systems different from the anthracene [4+4] photodimerization studied previously. Our results also suggest that shrinking crystal dimensions to the nanoscale can give rise to new types of photomechanical motion.