Synthesis and characterization of a dual-stimuli-responsive cobalt(iii) complex: comparison of photo-/thermo-mechanical behaviour in crystal and polymer composites†

Multiple stimuli-responsive molecular crystals have gained significant attention recently owing to their wide-range of applications in diverse fields such as optical switches, storage devices, and mechanical actuators. In this study, a nitropentaammine cobalt(III) complex stabilized by 4-nitro cinnamate (C-1) was synthesized and characterized using various spectroscopic and analytical techniques, including single-crystal X-ray diffraction (SCXRD) study. The complex C-1 exhibits dual-responsive behaviour under UV light (365 nm) and heat, showing bending, cracking, splitting, and displacement. These photo-/thermo-mechanical responses are attributed to nitro-to-nitrito linkage isomerization within the crystalline phase. The energy optimization of C-1 and its corresponding nitrito isomer through density functional theory (DFT) study reveals that the nitro isomer is energetically more stable than nitrito by 24 kcal mol⁻¹. This indicates that upon irradiation of UV light/heat, the nitro compound gains energy and is converted to the nitrito isomer. The photo-/thermo-mechanical behaviour of C-1 was successfully transferred to a polymer composite, which was fabricated via a solution casting method using C-1 and agarose (1 : 1 weight ratio) and shows actuating behaviour, with the extent of actuation varying for both the external stimuli.