Hydrogen-Bonding-Driven Design of Organic-Inorganic Hybrid Ferroelastics with Reversible Photoisomerization

The development of photoresponsive ferroelastics, which couple light-induced macroscopic mechanical and microscopic domain properties, represents a frontier in materials science with profound implications for advanced functional applications. In this study, we report the rational design and synthesis of two new organic-inorganic hybrid ferroelastic crystals, (MA)(Me₄N)[Fe(CN)₅(NO)] (MA = methylammonium) (1) and (MA)(Me₃NOH)[Fe(CN)₅(NO)] (2), using a dual-organic molecular design strategy that exploits hydrogen-bonding interactions for tailoring ferroelastic properties. Specifically, 1 exhibits a two-step phase transition at 138 and 242 K, while the introduction of a hydroxyl group in 2 stabilizes its ferroelastic phase to a significantly higher temperature, achieving a phase transition at 328 K, 86 K above that of 1. This enhancement is attributed to hydrogen bonding between the hydroxyl group of Me₃NOH⁺ and the nitroprusside anion, which suppresses lattice dynamics and reinforces structural stability. Remarkably, 2 demonstrates a large spontaneous strain of 0.153, vastly exceeding the 0.021 of 1, and undergoes an 11% size change along the b-axis in response to thermal stimuli. Both compounds exhibit reversible, photoinduced nitrosyl-linkage isomerization, as confirmed by IR spectroscopy, transitioning between the ground state (N-bound nitrosyl) and the metastable state (O-bound nitrosyl). This integration of photoresponsive functionality with ferroelastic properties establishes a versatile platform for energy-efficient actuation, adaptive devices, and multifunctional sensing applications. These findings offer an innovative pathway for designing next-generation hybrid materials with enhanced tunable properties.