Photomagnetic properties in a series of spin crossover compounds

The spin crossover (SC) compounds [Fe(PM-AzA)2(NCX)2] and [Fe(PM-FIA)2(NCX)2] (with PM-AzA = N-2'-pyridylmethylene-4-(phe-nylazo)aniline, PM-FIA = N-2'-pyridylmethylene-4-(2-amino)fluorene, and X = S, Se) have been prepared. The SC regimes have been deduced from variable-temperature magnetic susceptibility data. The enthalpy and entropy changes associated with the SC have been evaluated from DSC measurements. A cooperativity factor, C, has been defined, and its values for the different compounds have been deduced from the spin crossover curves. At 10 K, the light-induced excited spin state trapping (LIESST) effect has been observed within the cavity of the SQUID magnetometer. The critical temperatures Tc(LIESST) have been determined for [Fe(PM-AzA)2(NCS)2] and [Fe(PM-F1A)2(NCX)2], and the role of cooperativity has been analyzed. A linear correlation has been found between the Tc(LIESST) and C values. The kinetics of HS-->LS relaxation have been investigated; a thermally activated mechanism at elevated temperatures and a nearly temperature independent relaxation behavior at low temperatures have been found. Finally, the magnetic behavior recorded under light irradiation in the warming and cooling modes has revealed the occurrence of the light-induced thermal hysteresis (LITH) effect.