Organic cation conformational flexibility governs mechanical response in organic–inorganic hybrid materials

Abstract

Organic–inorganic hybrids, that couple the structural design flexibility of organics and the rigidity of inorganic lattices, are gaining attention as next-generation stimuli-responsive materials for adaptive actuation applications. Understanding and controlling mechanical responses in organic–inorganic hybrids is vital for the development of smart materials. Herein, we designed and introduced five- and six-membered ring cations with distinct conformational rigidities to obtain two hybrid metal-halide crystals, (Hmpy)PbI₃ (1, Hmpy = 2-hydroxymethyl-pyrrolidinium) and (Hmpi)PbI₃ (2, Hmpi = 2-hydroxymethyl-piperidinium). Compound 1 exhibits a moderate reversible deformation of 5% with pronounced shape-locking. In contrast, compound 2 shows a large reversible deformation up to 17%. Structural and variable-temperature Raman analyses establish the adaptability of organic cations as the governing factor for ferroelastic strain modulation, operating through controlled ring dynamics and lattice slippage mechanisms. These results establish a clear structure-mechanics relationship: conformational rigidity promotes shape-locking, while enhanced conformational flexibility enables greater actuation freedom. Decoding the structural code behind mechanical response offers a rational basis for designing adaptive crystals with shape memory function.