Solid-state photochemical and photomechanical properties of molecular crystal nanorods composed of anthracene ester derivatives†

A series of 9-anthroate esters that can form photoresponsive molecular crystal nanorods is prepared and their properties are investigated. All crystal structures that can support a [4 + 4] photodimerization reaction lead to nanorods that undergo photomechanical deformations without fragmentation. In order to determine the molecular-level motions that give rise to the nanorod photomechanical response, the reaction of anthracene-9-carboxylic acid tert-butyl ester is studied in detail using X-ray diffraction and solid-state NMR techniques. The monomer crystal is well-aligned within the nanorod and reacts to form the photodimer crystal according to first-order kinetics. The solid-state reacted dimer crystal is a metastable intermediate that slowly converts into the low energy dimer crystal structure over the course of weeks. Based on single crystal X-ray diffraction studies and solid-state NMR data, this intermediate structure is likely composed of the [4 + 4] photodimer that has not yet undergone the ester group rotations and repacking is necessary to form the lower energy crystal polymorph that is produced directly by crystallization from solution. Our results show that the photomechanical response of these molecular crystal nanostructures is determined by nonequilibrium intermediate states and cannot be predicted based solely on knowledge of the equilibrium reactant and product crystal structures.