Using Two-Photon Excitation to Control Bending Motions in Molecular-Crystal Nanorods

Molecular-crystal nanorods composed of 9-anthracenecarboxylic acid can undergo reversible bending due to molecular-level geometry changes associated with the photodimerization of the molecules in the crystal lattice. The use of highly focused near-IR femtosecond laser pulses results in two-photon excitation of micrometer-scale regions and is used to induce transient bends at various locations along the length of a single 200-nm-diameter nanorod. Bending can be observed in nanorods with diameters as small as 35 nm, and translational motion of a single nanorod could be induced by sequential bending of longer segments. A kinetic model is presented that quantitatively describes the bending and relaxation dynamics of individual rods. The results of this work show that it is possible to use laser excitation conditions to control the location, rate, and magnitude of photodeformations in these nanorods. The ability to control the motion of these ultrasmall photomechanical structures may be useful for manipulating objects on the nanoscale.