Molecular Hopper Crystals and Electron Beam-Triggered Reversible Actuation

Molecular crystals with unusual morphologies characteristic of “hopper crystals” have rarely been explored; such structures are potentially useful for eliciting specific responses to external fields or stimuli. Upon simple reprecipitation and controlled growth, select members of a family of strongly zwitterionic molecules, assembling in non-centrosymmetric lattices, are shown to form microcrystals with novel “hopper” morphology. The molecular aggregation is monitored by their characteristic fluorescence enhancement; X-ray diffraction, microscopy, and surface potential mapping provide insight into the development of the unique morphology. Under optimized conditions of electron beam irradiation in a scanning electron microscope, the prototype microcrystal is found to exhibit smooth, prominent, and reversible actuation. Even though electrically triggered macromolecular actuators as well as mechanically responsive and photo/thermosalient molecular crystals are known, controlled bending/folding induced by electron beams are rare and have been demonstrated only in specialized nanostructures. The current observations with a simple small-molecule-based hopper microcrystal are analyzed by a detailed examination of the crystal lattice structure and asymmetric dipole distribution, together with the simulations of the electron beam interactions. An empirical model developed for the responses in the local electrostatic field provides a mechanistic understanding of the actuation process.