Alkyl Substitution on the Phase Transition and Optical Properties of Lead-Based Molecular Ferroelastic Materials

Organic–inorganic hybrid ferroelastic materials have attracted increasing attention due to their wide applications in sensing technologies, information storage, and flexible wearable devices. However, the complex relationship between the molecular structure and material performance requires further investigation. In this study, we synthesized two ferroelastic materials: (ETMP)PbBr₃ (1, ETMP = ethyl-trimethyl-phosphonium) and (PTMP)PbBr₃ (2, PTMP = propyl-trimethyl-phosphonium). Through systematic structural modulation, we successfully changed the molecular structures and tuned the properties. Both materials exhibit reversible ferroelastic phase transition behaviors with an Aizu notation of 6/mmmFmmm. By replacing the ethyl group with a propyl group, the phase transition temperature increased from 361 to 409 K (ΔT = 48 K). Meanwhile, the structural modification led to distinct optical and mechanical properties: 1 exhibits orange photoluminescence under UV irradiation, while 2 displays superior mechanical flexibility. This work provides an effective strategy for designing and developing multifunctional molecular ferroelastic materials and deepens the understanding of the structural–performance relationship.