Influence of sample preparation, temperature, light, and pressure on the two-step spin crossover mononuclear compound

The three phases of the mononuclear two-step spin crossover compound [Fe(bapbpy)(NCS)2] (1) are characterized by powder X-ray diffraction, Mo¨ssbauer spectroscopy, Raman spectroscopy, calorimetry, and magnetic susceptibility measurements. Mo¨ssbauer and Raman spectroscopies confirm the results obtained from previous single crystal X-ray diffraction studies, as they show that the intermediate phase is characterized by a 1:2 high-spin/low-spin ratio and that it is not a mixture of the high-spin state (HS) and low-spin state (LS) phases. Sample preparation and sample history studies show that the spin crossover (SCO) properties of this compound are quite robust, as the two steps and hysteresis cycles are present both in the powder and in recrystallized samples and conserved when the sample is cooled and heated several times. Compound 2, a DMF solvate having the formula [Fe(bapbpy)(NCS)2] · 2DMF, shows SCO properties but no cooperative effects. The origin of the strong cooperativity found in compound 1 is discussed. Finally, external perturbations have been applied to compound 1. First, light irradiation at low temperatures promotes the LS f HS transition with reasonably good optical conversions [∼70%, T(LIESST) ) 56 K]. Second, the application of hydrostatic pressure increases both the temperature and the hysteresis width of the higher-temperature transition (HS T IP), while unexpectedly suppressing the lower-temperature transition (IP T LS).