Thermomechanical motion in single crystals triggered by de-solvation induced structural transformation: Molecular insights and actuation

Abstract

Stimuli-responsive molecular crystals have interesting applications as actuators and energy-harvesting materials. Herein, we report a nitrobenzoate stabilized nitropentaamminecobalt(III) complex (1H), exhibits thermo-mechanical motion ∼120 °C. During the process, the crystal converts collective molecular displacements within the lattice into a macroscopic movement due to dehydration-induced structural transformation (monoclinic-orthorhombic) forming a partially dehydrated species (1D) which was confirmed by single-crystal-to-single-crystal (SCSC) analysis. The dehydration process can be reversed when 1D was placed in moist air (RH > 80 %) reforming 1H. A three-state stability model analysis based on DFT calculations suggests that de-solvation of 1H leads to a large instability in the system, resulting in an intermediate state (1-int), which undergoes spontaneous relaxation through the movement of cations and anions forming 1D. Macroscopic transfer of thermo-mechanical energy from crystal to composite films was successfully established by fabrication of a hybrid polymer composite using 1H and polyvinyl di-fluoride (PVDF) that shows excellent actuating behavior upon heating. The present work provides a detailed experimental and theoretical molecular-level insights into thermo-mechanical motion and thereby brings a new dimension towards designing stimuli-responsive crystals as promising actuators.