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

In this work, we have re-examined the photomagnetic properties of a series of binuclear iron(II) complexes of formula [{Fe(L)(NCX)(2)}(2)bpym], 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)}(2)bpym] (2), [{Fe(bt)(NCS)(2)}(2)bpym] (3) and [{Fe(bt)(NCSe)(2)}(2)bpym] ( 4) are close to the antiferromagnetic response of [{Fe(bpym)(NCS)(2)}(2)bpym] (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 2 versus 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.