Competitive electronic and steric effects in the spin-state modulation of a 3D-Hofmann framework and its extension towards the nanoscale

Abstract

Modulating the spin-crossover behaviour in porous coordination polymers is not only an excellent way to address the challenges associated with high-order data storage and processing, but the colour change involved in the process reveals their importance as thermal and optical sensors. Herein, we report the guest-induced regulation of the magnetic bi-stability in a Hofmann-type framework, {Fe(pina)[Pd(CN)₄]} (1, pina = N-(pyridin-4-yl)isonicotinamide) from ∼26 K (for 1·3.6MeCN, complex 1), to ∼40 K (for 1·stb.4H2O, stb = stilbene, complex 2) by a subsequent change of the guest molecules from CH₃CN to stilbene. While the electronic interactions stabilize the LS states through effective π⋯π stacking between the guest stilbene and the host layers, chemical pressure, i.e., the steric effect of the stilbene molecule, stabilizes the HS states, and the overall competition between them leads to the high thermal hysteresis. To explicate the spin crossover properties to be used as a potential thermal sensor, the guest-free nanoparticles of framework (1) were prepared using the reverse micelle method. Magnetic measurements reveal that the effective downsizing at the nanoscale results in a progressive downward shift with an incomplete gradual SCO behaviour, diminishing the hysteresis loop compared to the abrupt hysteretic transition found at the bulk phase. In addition, these nanoparticles are assimilated as free-standing thermochromic polymeric films as a proof of concept for thermal sensing device. The rationalization of the tunable SCO properties, along with the thermochromic behaviour, opens up a new strategy for the development of new π electron-responsive magnetic materials.