Light, Spin, and Flow in a Soft Nanocomposite: Coupling Between Magnetic, Mechanical, and Optical Properties via Hysteretic Spin Crossover at Room Temperature†

Abstract

We report the development of a multifunctional soft nanocomposite, [Fe(NH₂trz)₃]Cl₂/NaPSS·50H₂O (1), composed of 90 nm spin crossover (SCO) nanoparticles embedded in a hydrated sodium polystyrene sulfonate (NaPSS) matrix. This material exhibits a spin transition near room temperature with a wide thermal hysteresis, making it suitable for soft material applications. Rheological studies reveal rubber-like mechanical behaviour, with stiffness similar to silicone rubbers, and show that the linear viscoelastic properties change significantly with the spin-state. Stress relaxation experiments further demonstrate spin-state-dependent mechanical behaviour, with a transition from stretched exponential to power-law response across the SCO. Optical measurements, including diffuse reflectivity and temperature-dependent fluorescence, confirm strong coupling between the spin-state and the material’s optical properties. The addition of ethylene glycol enhances thermal stability, enabling better characterisation of a 22 K photoluminescent hysteresis. The fluorescence enhancement in the high-spin state is attributed to reduced photobleaching or polymer rearrangement. Overall, this nanocomposite combines magnetic, mechanical, and optical responsiveness, offering strong potential for use in smart materials and stimuli-responsive devices.