A facile route for preparation of monodisperse nanoparticles of one-dimensional Fe(ii)-4-amino-1,2,4-triazole coordination polymers with hysteretic spin-crossover near room temperature

Spin-crossover (SCO) occurring in a transition metal complex is the switchable process between two spin states in response to external perturbations such as changes in temperature, pressure or photoexcitation. Such a material has potential applications in switching devices. In this study, we designed a one-dimensional spin-crossover polymer, with a formula of [Fe(NH ₂-Trz)₃](doe)₂ where NH₂-Trz = 4-amino-1,2,4-triazole and doe = sodium dodecyl sulfonate, and fabricated its monodisperse nanoparticles using a facile precipitation route. The nanoparticles aggregate from plate-shaped nanocrystals and their size dimensions fall in the range of 200-300 nm. The SCO polymer undergoes a thermal reversible spin transition near room temperature which is important for practical applications. The spin transition shows ca. 13 K hysteretic loops and is associated with a color change. The SCO polymer nanoparticles were pressed under ca. 8 MPa static pressure to give a good transparent disc with a thickness of ca. 1 mm; such a disc displays a pink-violet color at ambient temperature and a yellowish color above 45 °C. This SCO material has potential applications in the field of thermochromic devices.