Atypical photomagnetic properties in a series of binuclear iron(ii) spin crossover complexes

Abstract

In this work, we have re-examined the photomagnetic properties of a series of binuclear iron(II) complexes of formula [{Fe(L)(NCX)₂}₂bpym], where L is bpym = 2,2′-bipyrimidine and bt = 2,2′-bithiazoline and X = S, Se, by light irradiation at 647.1–676.4 nm and at 830 nm and we confirm that after optical excitation the magnetic behaviors of [{Fe(bpym)(NCSe)₂}₂bpym] (2), [{Fe(bt)(NCS)₂}₂bpym] (3) and [{Fe(bt)(NCSe)₂}₂bpym] (4) are close to the antiferromagnetic response of [{Fe(bpym)(NCS)₂}₂bpym] (1) characterized by two iron(II) metal ions in the HS electronic configuration. This hypothesis of quantitative HS–HS photoconversion is also supported by the reflectivity experiments and the infrared spectroscopies performed on 3. We have also investigated the relaxation process of 2–4 by using a SQUID magnetometer. Atypical magnetic behavior is found for compounds 3 and 4. More precisely, while the magnetic response of 2versus time continuously decreases whatever the temperature, the signal of 3 increases below 17 K, becomes stable at around 18–20 K and decreases at higher temperature. We thus propose a direct HS–HS→LS–HS relaxation process for 2 and an indirect HS–HS→LS–HS→LS–LS relaxation process for 3 and 4. The agreement found between the experimental T(LIESST) and the simulated T(LIESST) curves, calculated using the measured kinetic parameters, guarantees the validity of our approach.