Impact of the spin state switching on the dielectric constant of iron (II) spin crossover nanoparticles

A simple model makes it possible to relate the dielectric constant of spin-crossover nanoparticles to the indices of refraction and the absorption coefficients of solutions containing such nanoparticles. Using this model we show that in the visible spectral range, the switching from a diamagnetic (low spin state) to a paramagnetic (high spin state) state results in a noticeable change of the dielectric constants of the spin crossover nanoparticles containing 3, 5 and 7 iron (II) ions. Within the studied temperature range and for all the studied nanoparticles, the variation of their index of refraction is about 10%. Media whose optical dielectric constant can be tuned by different means open a wide range of possibilities in the design of optical or data storage components [1]. Among such media, optically bistable dispersions of Fe II spin-crossover nanoparticles (SCO NPs) are very promising [2–12]. Some of these SCO NPs present a large thermal hysteresis loop at room temperature. Within the hysteresis loop, the magnetic state of these NPs can be reversibly switched from a diamagnetic low spin (LS, S = 0) state toward a paramagnetic high spin (HS, S = 2) state. This spin state transition is due to electron spin pairing (S = 0) or unpairing (S = 2) within the Fe II ions. At the macroscopic scale the spin transition is associated with a phase transition. This phase transition can be induced by many means using thermal or pressure variations, laser or magnetic exci-tations. For most of the SCO NPs, the spin state transition results in marked absorption changes. For instance, a solution containing SCO NPs is almost uncolored when the particles are in the HS state and turns red or pink when the NPs are in the LS state. This pink color is related to a broad absorption band at around 520 nm, which is characteristic of the d–d transition of the Fe II ions in the LS state (1 A 1g → 1 T 1g) [13]. The d–d transition of the Fe II ions in HS state (5 T 2g → 5 E g) is expected at lower energy in the near infrared region (∼830 nm) [13]. This accounts for the absence of color of the solution containing the SCO NPs in the HS state. SCO NPs with a broad thermal hysteresis loop at room temperature and large absorption changes are therefore very good candidates for data storage. Indeed as previously shown, the spin state transition results in a change in the dielectric constant of the SCO NPs [14–19]. This should result inVers une génération de Gel-Coats Intelligents : thermo- et/ou photosensible